2013 TRAINING NOTES ROB MAXWELL
Ultrasonic Testing
Ultrasonic Testing
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Ultrasonic Testing
PART 1 – THEORY INTRODUCTION Ultrasonic is a term used in acoustics (the science of sound) when dealing with vibratory waves whose frequencies are beyond the limits of the hearing range of the average person. Ultrasonic waves are of the same nature as audible sound waves, i.e. they are stress waves and can only exist within the media. The energy of an ultrasonic wave is transferred from one point to another by vibrating the particles in the material through which they are being propagated. In this respect they differ from light and other forms of electromagnetic radiation which can travel freely through a vacuum. In other respects these two forms of energy obey similar laws of propagation1.
NATURE OF SOUND THE
SOUND SPECTRUM
When a block of metal is struck with a hammer, the sound of the blow is heard and the accompanying vibration of the block can be felt. The sound is conveyed by airborne waves. These audible oscillations2 are called sonic waves. The frequency of these waves are unlikely to exceed 5000 cycles/second. The term Hertz (Hz) is used to measure the frequency of vibration. In ultrasonics this frequency occurs many thousands of times per second. A more suitable way of stating frequency is to use the term kilo (k) and mega (M) to describe units of a thousand and a million respectively.
1 The movement of a wave through a medium. 2 A Cycle – i.e. the four seasons is a complete oscillation.
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Ultrasonic Testing Frequency Under 1 KHz 16 – 20 KHz 19 – 20 KHz 40 KHz
Uses Early underwater navigation. Upper limit to human hearing. Alarm systems.
Underwater signalling and cleaning. Testing of large grain material. 500 KHz (½ MHz) Upper limit to underwater signalling. 1 MHz – 5 MHz Common range for testing material. 5 MHz – 10 MHz Used on fine grain material.
TYPES OF WAVE AND PROPAGATION LONGITUDINAL
OR
COMPRESSION WAVES (NORMAL PROBE)
A longitudinal wave is a wave formed by individual particles oscillating in the direction of propagation. An ultrasonic wave is supported and propagated through a material by particles that oscillate around their point of equilibrium. To simplify this statement let us consider Fig 2. Shown in Fig 2, we have a model consisting of a series of identical pendula and for simplicity, this may be looked upon as representing a solid body prior to the application of ultrasonic energy. Position A and B illustrate the point where a pulse of ultrasonic energy will be applied. Once a pulse has been initiated pendulum A is forced towards pendulum B, transmitting its internal energy.
Compression Rarefaction
Fig 3 As each pendulum has to protrude somewhat from its neutral positions in order to strike the next one, the next pendulum will start its oscillation a fraction later than the preceding one.
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Ultrasonic Testing Consequently, the pendula do not oscillate in the same rhythm, but all of them with the same frequency. Longitudinal or compressed waves may be propagated through solids, liquids and gasses.
TRANSVERSE
OR
SHEAR WAVES (ANGLE PROBES)
Contrary to longitudinal waves the individual particles of the transverse wave oscillate vertically in the direction of propagation (Fig 4 & Fig 5). Fig 4 Fig 5 Fig 6 The use of the pendula, example serves well to illustrate the effect of transverse wave. Fig 6.
Fig 7 For this example it has been shown that a pre-requisite for the formation of transverse waves are molecules being ties firmly together. As this is only the case with solid bodies, then it will follow that transverse or shear waves are not supported in liquids or gas.
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Ultrasonic Testing RAYLEIGH WAVES (SURFACE) Rayleigh waves travel over the surface of a solid and bear a rough resemblance to waves on the surface of water. Reflections of Rayleigh waves from cracks in the surface or from discontinuities lying just beneath the surface may be seen on an oscilloscope screen. Rayleigh waves are also called surface waves since their depth of penetration is usually no more than one wavelength.
LAMB WAVES
Fig 9 Lamb waves occur in plate material, in two basic forms which in practice are usually mixed. Lamb waves consist of a mixture of zigzag reflected longitudinal and transverse waves with a mutual phase relationship in which some particles oscillate in a direction 90˚ to the plate surfaces and others of varying angles. The ability of Lamb waves to flow in thin plates make them applicable to a wide variety of problems requiring the detection of subsurface discontinuities. As lamb waves have a surface component, surface scale or dirt will affect the trace causing damping or reflection, and it is therefore necessary to ensure surface cleanliness and also to limit the couplant to the probe edge only.
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Ultrasonic Testing VELOCITY, FREQUENCY, WAVELENGTH VELOCITY The velocity of sound in a medium depends on the density and the elastic constants of the material. If follows that material with different densities will have different velocities, for example: Material
Air Aluminium Brass (70-30) Cast Iron Copper Gold Iron Lead
Comp velocity (m/s) 322 6400 4372 3500
Shear velocity (m/s)
4760 3240 5957 2400
2325 1200 3224 790
3130 2100 2200
Material
Oil Perspex Steel (Mild) Steel (Stainless) Water Tungsten Zinc Zirconium
Comp Velocity (m/s) 1440 2740 5960 5740 1480 5174 4170 4650
Shear Velocity (m/s) 1320 3240 3130
2880 2480 2300
Note: Comp velocity used for Longitudinal Probes, Shear velocity used for angle probes. The velocity of a surface wave for any given material is approximately 90% of the shear velocity. Velocity difference can cause problems if testing a material other than the material on which the calibration has been conducted. As an example, if having calibrated on a steel block, a probe is then placed onto a copper plate of 20mm thickness, the screen would indicate a thickness of 25mm. This can be proven by the following calculation: Actual thickness * (V Steel / V Copper) 20 mm * (5960 / 4760) = 25.04mm The velocity is different by a ratio of 1.25:1 If the thickness indicated is divided by the factor we arrive at 20mm – the true thickness (25.04mm / 1.25 = 20mm).
FREQUENCY The frequency of a wave form is the repetition rate, or the number of cycles or oscillations in a given time, and is usually expressed as cycles per second (Hertz). From Fig 11 it will follow that the higher the frequency (greater number of oscillations or cycles per second) then the crest of the waveform will come closer together, thereby producing a shorter wavelength. 7 | Page
Ultrasonic Testing Multiple units of frequency are expressed as kilohertz (KHz) which is equal to one thousand oscillations per second, it may also be written as MHz which is equal to one million oscillations per second.
WAVELENGTH Wavelength expressed as (λ) lambda is given as the distance between two successive crests in the waveform, this distance varies with frequency and velocity.
Fig 10
The wavelength formula is as follows:
Three examples follow to show the effect of frequency and velocity: 1. Find the wavelength of a 5MHz longitudinal probe in mild steel. Longitudinal velocity in mild steel Probe frequency λ = v/f is 5960/(5 * 106) =5960 / 5000000 8 | Page
=
= 5960 m/s 6 5 * 10 Hz
Ultrasonic Testing =0.001192m = 1.192mm
(0.001192 * 1000 < mm in a meter)
2. Find the wavelength for a 5MHz longitudinal probe in copper. Longitudinal velocity in copper Probe frequency
=
= 4760 m/s 6 5 * 10 Hz
λ = v/f is 4760/(5 * 106) =4760 / 5000000 =0.000952m = 0.952mm
3. Find the wavelength for a 5MHz angle probe in copper. Transverse velocity in copper Probe frequency
= =
2325 m/s 5 * 106 Hz
λ = v/f is 2325/(5 * 106) =2325 / 5000000 =0.000465m = 0.465mm Consider the three examples: Wavelength for 5MHz longitudinal probe in steel = 1.192mm Wavelength for 5MHz longitudinal probe in copper = 0.952mm Wavelength for 5MHz transverse probe in copper = 0.465mm We have proven that a 5MHz longitudinal probe has a shorter wavelength in copper than it has in steel and that a traverse or angle probe has a wavelength approximately half that of the longitudinal probe for the same frequency in the same material. An important point to remember is, that in theory, a probe can detect a defect whose diameter is approximately one tenth of its wavelength. However, in practice on third to one half seems to be the more acceptable figure.
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Ultrasonic Testing WAVE FORMATION AND PULSE SHAPE A wave train refers to a short group of waves, before or after which there are no waves. This formation is generally referred to as a pulse. Pulse takes several forms: It may start and drop rapidly (Fig 12) It may build and decay gradually (Fig 13) It may build up rapidly and decay exponentially (Fig 14) The type of wave illustrated in Fig 14 is referred to as a decayed train and this is probably the most commonly used type of pulse in ultrasonics. Almost all probes incorporate assisted damping to the crystal in the form of backing. This backing medium must have a higher acoustic impedance3 than the crystals. The reason for the damping in a single probe is the very fact that the crystal has to produce short, and above all sharp bursts of energy. Ideally the crystal motion must end abruptly from its previous pulse so that the reflected energy excites a relatively inactive crystal and not one that is already in a stage of oscillation. Fig 15 and Fig 16 show the effect of probe ringing time with assisted clamping.
3 The ability of a material to impede the passage of sound
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Fig 15
Fig 16
What type of pulse width can we expect to be displayed on the cathode ray tube time base? Pulse width will depend on the frequency of the probe used and is also a function of pulse energy. In other words one must apply an electrical pulse to the crystal that is wide enough to cause the transducer to reach maximum oscillations, at the same time remembering that an increase in pulse width will markedly reduce resolution. With a lower frequency probe the applied electrical pulse will be wider and the resolution will be inferior to that of a higher frequency probe that required a shorter pulse. The effect of pulse width with regard to resolution will be discussed later.
PRODUCTION OF ULTRASONIC ENERGY Ultrasonic energy is produced by a crystal or transducer that is subject to the phenomenon known as the PIEZO ELECTRIC effect.
DEFINITION The piezo electric effect is the property of certain crystals which when subjected to electrical energy convert this energy to mechanical energy (sound) and vice versa.
THE PIEZO ELECTRIC EFFECT
Electrical Current (IN)
Electrical Current (OUT)
Sound Waves
An electrical charge to a crystal produces mechanical energy (Ultrasound).
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(Conversely)
Mechnical pressure on the crystal produces an electrical charge at electrodes.
Ultrasonic Testing CRYSTALS Crystals used in the production of ultrasound and subject to the piezo electric effect are discussed later. A crystal may be one which has occurred naturally, as in the Quartz or it may be a crystal produced in polycrystalline form by a process of calcination and sintering at high temperatures. To produce various frequencies of ultrasonic energy the thickness of the crystal and the applied alternating current to the crystal are prime factors. For instance if a high frequency alternating charge is applied to a crystal that has been made to oscillate at the same frequency, then it will follow the applied field, causing the faces to vibrate in respect of each other at that frequency. It is possible however to use a 5MHz probe with the frequency switch positioned at the 2.5MHz position. The 5MHz probe will still oscillate but will be greatly reduced in output. The reason for this is the alternating current to the probe does not match the crystal thickness and therefore does not achieve maximum crystal oscillation. In other words the crystal is not operating at its resonant4 frequency. The fundamental resonant frequency (Ff) of the crystal is inversely 5 proportional to the crystal thickness.
Fundamental resonant Frequency (Ff) = velocity(v) / 2 * thickness(t) Example. What frequency will a 1mm crystal of Barium Titanate operate at? v = 4,400,00mm/sec Ff = v/2t Ff = 4,400,000 / 2 * 1 Ff = 2.2MHz
= 4,400,00 / 2 (220,000Hz / 100,000)
The frequency will produce maximum crystal oscillation i.e. maximum output.
4 Resounding / echoing 5 of course, related
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Ultrasonic Testing The cut of a crystal and the way it is mounted with the probe both have direct bearing on the type of crystal motion produced. Fig 17 Fig 17 (a) shows the direction of crystal motion for an x-cut crystal and Fig 17 (b) the direction of crystal motion for a Y-Cut crystal. The types of vibration that can be produced (torsional, longitudinal etc.) are achieved by the particular crystal chosen and the cut of the plate. Crystals for consideration are:
QUARTZ A naturally occurring compound which is hard and very stable both chemically and physically. It occurs as a six-sided prism with a pyramid attached to each end. If the point of the opposite corners are joined they provide the X-axis. The X-axis are the electrical ones and produce the ultrasonic vibration which is required. BARIUM TITANATE This is a pre-polarised crystal, or ceramic with the chemical composition of a crystal i.e. Barium carbonate is backed together with titanium dioxide at a temperature of around 1250˚C. The domains of the crystal or ceramic, when subjected to an intense electric field of about 24V/mm at 140˚C6 and allowed to cool, become orientated to the direction of the field and polarisation is achieved.
After polarisation the activity factor of the probe will drop about 50% in 24 hours and thereafter will remain fairly constant although slow deterioration will take place. 6 The Curie point - The temperature at which a phase change in the magnetic or ferroelectric properties of a substance occurs, especially the change from ferromagnetism to paramagnetism that occurs with increasing temperature.
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Ultrasonic Testing
The resultant crystal is fairly good but it should be appreciated that heating above Curie point will destroy the piezo effect. LEAD ZIRCONATE TITANATE The best and recently the most widely used of all the pre-polarised crystals. Sensitivity is excellent with very little ‘grass’. Now becoming the accepted crystal for Non-destructive Testing. LEAD METANIOBATE Less sensitive than Barium Titanate but has a high internal damping coefficient and is thereby capable of transmitting very narrow pulses. Having briefly discussed the merits of various crystals our next logical step will be to consider the various probes and their associated technicalities.
PROBES We can distinguish two main groups of probes, probes producing longitudinal or compression waves (L-Wave) vertically through the surface and probes producing transverse waves (SWave) which are transmitted into the specimen at an angle with respect to the surface.
SINGLE PROBE OR TRANSCEIVER A probe with only one crystal in its construction is known as a single probe, the mode of operation being pulse echo. In simple terms, the single crystal produces and also receives the ultrasonic energy. This is possible because the electrical charge is fed to the crystal in the form of rapid pulses with a number of micro seconds delay between each pulse. Each electrical charge produces a pulse of ultrasonic energy, which in turn are passed into a specimen via the coupling medium. Probe
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Fig 20a
Fig 20b
The energy travels through the specimen and continues to do so until it reaches an interface in Fig 20a, this is the backwall. Reflection takes place and ultrasonic energy returns towards the crystal Fig 20b. The returning mechanical energy excites the crystal during the nonproductive intervals and thereby produces an electrical charge which is fed back into the circuit to provide the signals for the cathode tube display.
COMBINED DOUBLE PROBES The combined double probe is constructed as its name suggests, by the use of two crystals in one probe. One crystal continually transmitting whilst the other crystal is continually receiving the reflected energy. Fig 21 shows such a probe.
Fig 21 It will be noted that the acoustic barrier is corrugated on the inside to reduce the effect of cross talk chatter, also, a layer of cork is interposed between the perspex faces. This cork is known as the acoustic separator. The use of a penetrating oil as a couplant with this type of probe should be discouraged, as it is merely a matter of time before the oil, through a process 15 | P a g e
Ultrasonic Testing of penetration and saturation, breaks down the acoustic separation, giving rise to ‘standing echoes’ on the cathode ray tube.
The probe, by the use of two crystals, eliminate the dead zone and thereby allow for the detection of defects close to the surface. Single or twin probes may be longitudinal or transverse in ultrasonic output.
BEHAVIOUR OF SOUND IN MATERIALS THE
ULTRASONIC
BEAM
DEAD ZONE The dead zone is a zone where it is not possible to detect defects. The dead zone is shown as the transmission signal at the start of the time base. Its depth can be seen on a calibrated time base as the amount of time base occupied by the transmission signal. The dead zone is the ‘ringing time’ of the crystal and is minimised by the damping medium behind the crystal. The dead zone increases when the frequency is decreased, therefore a 5MHz single probe will have a smaller dead zone than a 2.5MHz. NEAR OR FRESNEL ZONE The ultrasonic beam remains parallel and has the same diameter as the crystal over a distance known as the near zone.
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Ultrasonic Testing
Fig 24 Within the near zone there exists varying intensities of waves at the edge of the crystal (Fig 24) giving rise to unreliable signal amplitudes. This means that signal height from the same size defect may increase when positioned further from the crystal. The formula used to calculate the near zone is:
D2f / 4v
Example Calculate the near zone for a 20mm crystal size, longitudinal velocity, 2.5MHz probe in Steel (velocity = 5960m/s) D2f = 20 * 2.5 * 106 = 400 * 2.5 * 1,000,000 = 1,000,000,000Hz =1,000Mhz 2
D2f / 4v
= 0.0419m
4v = 4 * 5960 = 23,840
= 41.9mm
Water (velocity = 1480m/s) D2f = 20 * 2.5 * 106 = 400 * 2.5 * 1,000,000 = 1,000,000,000Hz =1,000Mhz 2
D2f / 4v
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= 0.1689m
4v = 4 * 1480 = 5,920
= 168.9mm
Ultrasonic Testing
FAR OF FRAUNHOFER ZONE Beyond the near zone is the far zone. In the far zone the beam diverges resulting in a decay in sound intensity as the distance from the crystal is increased, just as a beam of light from a torch gets weaker the further it travels. In the far zone, large and small reflectors follow different laws. Large reflectors (larger than the beam) follow the Inverse Law. The amplitude is inversely proportional to the distance i.e. If the distance is doubled the amplitude is reduced by half (6 dB). Small reflectors (smaller than the beam) follow the Inverse Square Law. The amplitude is inverse proportional to the square of the distance i.e. If the distance is doubled the amplitude is reduced by quarter (12 dB).
BEAM SPREAD, CRYSTAL SIZE, FREQUENCY When an ultrasonic beam is produced it is propagated in a rectilinear fashion (straight line) from which it diverges only slightly. This divergence is a function of frequency and crystal diameter. If the frequency is increased for a certain diameter of crystal the solid angle decreases and if the frequency is decreased for the same crystal diameter then the solid angle increases. This means that when using lower frequencies we must choose probes with larger crystal diameters if we wish to reduce the effect of an increased beam spread. The half beam spread formula is as follows: ASin (kλ / D)
- ASin is the inverse Sin
Where: D= The constant k is used to calculate the beam spread intensity. k = 1.22 for extreme edges of the beam. k = 1.08 for 10% (20 dB) edge k = 0.56 for 50% (6 dB) edge. Example: Calculate the beam spread of ultrasonic waves travelling through mild steel, the waves are generated from a 10mm, 5MHz crystal. 18 | P a g e
Ultrasonic Testing
Wavelength in steel is v / f
=5960 / (5 * 106)
=0.001192m
=1.192mm
Extreme edge of beam: k = 1.22
Half Beam Spread =ASin (k * λ/ D) 8.36˚
=ASin (1.22 * 1.192 / 10) =ASin (0.1454)
=
After considering the above example we will now calculate the half beams spread for a 2.5MHz longitudinal probe using the same size crystal. Wavelength in steel is v / f
=5960 / (2.5 * 106)
=0.00238m =2.38mm
Extreme edge of beam: k = 1.22 Half Beam Spread =ASin (k * λ/ D) 16.88˚
=ASin (1.22 * 2.38 / 10)
=ASin (0.2903)
=
Note the considerable increase in beam spread.
REFLECTION
AND
REFRACTION
Ultrasonic high frequency vibrations react in many ways as that of light. They can be focused into a beam, refracted and reflected. It is this ability which makes it possible to utilise sound energy as a means of flaw detection. REFLECTION When a beam of sound energy strikes a boundary, it is normally reflected at the same angle as the incident beam. Therefore it can be said that the angle of incidence equals the angle of reflection. REFRACTION When sound waves pass from one medium to another a change in wavelength takes place due to the differing acoustic velocities of each medium. As a result, the angle at which the sound enters the second medium does not equal that of the first medium. This is known as refraction. This refraction can be compared with the action of a beam of light when passing from air into water.
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Ultrasonic Testing
SNELL’S LAW Snell’s Law determines the angular relationship between the incident and refraction beam on transmission between the media of different acoustics velocities. Snell’s Law is used when calculating the angle to which the perspex wedge must be machined in order to produce a given refracted angle in the test material. The formula can also be used to find the angle of refraction that a given probe will produce when examining materials other than steels e.g. copper and aluminium. Example: Calculate the incident angle used in the machining of the Perspex wedge to produce a 70˚ refracted angle in steel. Velocity in Perspex (longitudinal) = 2740m/s Velocity in Steel (transverse) = 3240 m/s Incident angle used:
= Sin-1(V1 x Sin(˚)) / V2) = Sin-1(2740 *Sin(70) / 3240) = Sin-1(2740 *0.9397 / 3240) = Sin-1 (0.7947) =53.63˚ CRITICAL ANGLES As the incident angle is increased from the normal the refracted wave is predominantly longitudinal, although the shear mode exists at an insignificant strength. When the incident angle reaches the first critical angle (27.4˚) the longitudinal component will be totally internally reflected through 90˚. At this point only transverse waves exist in the second medium Calculation of First Critical Angle – Perspex to steel 20 | P a g e
Ultrasonic Testing
V Perspex (long) V Steel (long) Sin B Sin A
= 2740 m/s = 5960 m/s = Sin(90˚) = 1 = Sin-1(V1 * Sin B / V2) = Sin-1(2740 * 1 / 5960) = Sin-1(0.4597) = 27.37˚
When the incident angle reaches the second critical angle (57.7 ˚) the transverse wave is totally internally reflected through 90˚.
Calculate the Second Critical Angle – Perspex to Steel V Perspex (long) V Steel (trans) Sin B Sin A
= 2740 m/s = 3240 m/s = Sin(90˚) = 1 = Sin-1(V1 * Sin B / V2) = Sin-1(2740 * 1 / 3240) = Sin-1(0.8457) = 57.74˚ Angle A as previously stated is the angle of the perspex wedge and it will be appreciated from Fig 28 that a wedge angle between 28.7˚ and 56˚ will produce transverse waves of refracted angles from 35˚ to 80˚. RESOLUTION The effect of pulse width with regard to resolution will now be discussed. First of all it is necessary to enlarge on the fact that a pulse energy is made up of several waves produced from oscillations of the crystal during a given number of micro seconds. We will now consider the pulse width in steel for a crystal that has produced a pulse of energy for a period of two microseconds (2 µ sec). The width of the pulse will be approximately 12mm. Fig 30 shows two defects separated by 3mm. 21 | P a g e
Ultrasonic Testing
This pulse width of 12mm would not be able to resolve these two defects because their echoes would overlap. Therefore this pulse width would not satisfactorily resolve defects closer than approximately 6mm. Defects around this 6mm separation band would give an indication on the back of the main signal on the cathode ray tube. Defects below this value would be lost in the main signal envelope. Fig 31 and Fig 32 show the pulse from the 3mm separation and the 6mm separation. Fig 31 (3mm Separation)
Fig 32 (6mm separation)
To determine the pulse width: Pulse width = (velocity x number of waves) / frequency On summarising, good resolution demands a very short pulse so that the reflections of one defect lying close to another is not lost in the received signal of the first. It should be further noted that the higher the frequency, the shorter the pulse width. Also the higher the frequency the shorter the wavelength, thereby giving greater sensitivity to small defects. The two combinations give good defect detectability and good resolution. Fig 33 Fig 33 shows two examples of resolution. ABSORPTION OR ATTENUATION Attenuation or weakening of the echoes, is a combination of absorption and scattering and
leads to energy loss, both in the material under test and in the probe itself. Absorption of the wave can refer to loss through heat due to internal friction. The absorption factor is greater with high frequencies die to more rapid particle movement. Other factors which attenuate the acoustic intensity of the beam are find inclusions, micro porosity, the crystal structure and composition. Scattering of ultrasound is similar to the effect of fog or smoke on light. It occurs when the frequency of the probe chosen has a wavelength which approaches the grain size of the material under test. Due to grain size irregularity, scattering is multi-directional.
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Ultrasonic Testing THE FLAW DETECTOR PULSE GENERATOR (MASTER TIMER) To regulate the output of the transmission pulse and to supply the time base circuit.
TIME BASE (DEPTH RANGES) Which by virtue of the relationship between the distance travelled by ultrasonic waves in unit time (velocity) can be used as a distance of depth scale when locating defects or measuring thickness.
TRANSMITTER To provide short pulses of electrical energy to excite the transmitter probe.
RECEIVER / AMPLIFIER To pick up and magnify the signal coming from the receiver probe. The energy then being applied (as a voltage) to the ‘Y’ plates.
ATTENUATOR Controls the relationship of volts in / volts out across the ‘Y’ plates thereby controlling signal heights. Does not affect amplifier linearity.
DISPLAY UNIT For presenting visually the transmitted and received signals in their proper time sequence with indication of relative amplitude. Two types of display may be utilised a Cathode Ray Tube (CRT) which are now superseded by the digital display. The CRT is a highly evacuated tube and housed within this tube are an electron gun used to produce a pencil beam of electrons, a deflection system and a luminescent screen coated with special phosphor to render the movement of the electron beam as visible light.
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Ultrasonic Testing In the electron gun the beam is initiated by heating a coated cathode by a small filament, electrons are emitted and these in turn are attracted by a positively charged anode. Between the cathode and anode we have the grid, its function being to limit electron flow. The anode assembly can be produced in the form of apertures, their function being to accelerate the electron flow. The electrons are now in the form of a fine beam which will impinge on the luminescent screen and surrender its energy in the form of visible light. Located at the front of the electron gun there is the deflection system. Application of an electrical potential to the ‘X’ deflector plates will cause the beam to be deflected in the horizontal plane. When an electrical potential is applied to the ‘Y’ plates, deflection will occur in the vertical plane. We now have a pointer that has no mass, in fact, weightless. This pointer can be deflected in two dimensions having the ability to faithfuly reproduce high frequency signals as visible light. The Pulse Generator provides the initiating pulse, in other words triggers off the time base circuit and causes the spot to commence its movement across the face of the tube and to trace out the green base line; it also triggers off the transmitter, causing a large pulse of energy to be sent to the probe. The frequency with which the pulse generator performs these functions is known as the Pulse Repetition Frequency (PRF). The PRF could vary between 50 and 2000 pulses a second. The greater the rate of the PRF then the brighter becomes the green line of the time base. Due to this simultaneous triggering, it is assumed that the transmission pulse is located on the left hand side of the screen. Sometimes it is necessary to delay this pulse for various forms of testing. The Pulse Transmitter circuit is the part of the equipment which delivers the current to the crystal. High tension supplies to this circuit are required of 1 to 2 kV and it is into this circuit that we introduce frequency selection. The time base circuit is required to obtain a linear transverse of the spot from left to right in the horizontal sweep. During this sweep there will be deflections in the vertical plane from reflections within the specimen and these must be displayed on the screen directly proportional to time. Sweep times must be variable and depend on the depth range, therefore sweep times from 20 micro seconds to 1 millisecond may be required. If we compare this period of time to the PRF rete we can see that the spot has some time to spare and, in fact, has a small amount of time to wait at the left hand side before commencing further excursions. The receiver accepts and amplifies the returning electrical signals and these signals are so minute that amplification rates from 10,000 to 100,000 times as required. Another requirement of the amplifier is that it has to accommodate various frequencies and has to have adequate bandwidth.
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Ultrasonic Testing A further requirement of the amplifier is that it should have the ability to reproduce all signals on the CRT whether large or small, at an amplitude or height which is in direct proportion to the energy at the receiver crystal. That is to say, that the input/output relationship should be linear. These are but a few of the functions of an ultrasonic unit. They have been included to enable the student to have some appreciation of the electronics of ultrasonics. The standard flaw detector utilises the pulse-echo (A-Scan) presentation.
A-SCOPE PRESENTATION A form of cathode ray tube in rectangular co-ordinates, in which pulse amplitude is represented as a displacement along one axis, and time is represented as a displacement along the other axis.
B-SCOPE PRESENTATION A form of cathode ray tube display, in rectangular co-ordinates, in which the travel time of an ultrasonic pulse is represented as a displacement along one axis, and probe movement (generally rectilinear) is represented as a displacement along the other axis. In the display, reflected pulses are shown as bright marks on a dark background, or vice versa.
C-SCOPE PRESENTATION The line-by-line presentation of flaw data obtained by scanning the major surface of the material line-by-line (non-intersecting lines) so that discontinuities are shown in terms of probe position at the moment of detection. The presentation may be on a cathode ray tube screen or recorded on paper or film i.e. a two-dimensional presentation.
D-SCOPE PRESENTATION A two –dimensional graphical projection on to a plane normal to the test surface and normal to the projection of the beam direction on the test surface, showing the apparent size and position of reflections in the volume inspected by scanning an area of test surface.
DECIBEL NOTATION (dB) The decibel is a unit of comparison. It is the measurement of changes in sound intensities and has a logarithmic base. It is possible to calculate the dB difference between signals from two reflectors whose size ratio is known. e.g. If two reflectors, equidistant from the probe, have a size ratio of 10:1, what would be the dB difference in their echo heights? Formula: dB = 20 * log( h1/h2) where h1 & h2 are the echo heights. = 20 * log (10/1) =20 * 1 Difference = 20 dB 25 | P a g e
Ultrasonic Testing
Ratio of 2:1 = 20 * log (2/1) =20*0.30 Difference = 6 dB By transmitting the formula, it is possible to determine the ratio of sizes of two reflectors whose dB difference is known. e.g. Tow reflectors, equidistant from the probe, have a dB difference of 14 dB between their echo heights. What is their ratio of reflective areas? dB = 20 *log (h1/h2) 14 = 20 *log (h1/h2) (14/20) = log (h1/h2) Antilog 0.7 = (h1/h2) 5 = (h1/h2) Ratio = 5:1
Note: Log(5) = 0.7
CALCULATIONS OF dB RATIO’S – PRACTICE 1. The signals from two defects equidistant from the probe have heights of 10mm and 40mm. What is the decibel difference between the two signals? 2. Express a ratio of 5:1 in decibels. 3. What is the ration of signal amplitudes if the dB difference between the echoes is 40 dB? 4. Two reflectors equidistant from the probe had a difference of 8 dB between their echo heights, what is the ratio of their surface area? Check answers HERE
The table below gives approximate dB and amplitude ratio equivalents. Amplitude Ratio dB
Amp Ratio
dB
Amp Ratio
dB
Amp Ratio
dB
Amp Ratio
dB
Amp Ratio
1
1.12
6
2
11
3.55
16
6.31
21
11.22
2
1.26
7
2.24
12
3.98
17
7.8
22
12.59
3
1.41
8
2.51
13
4.47
18
7.94
23
14.13
4
1.59
9
2.82
14
19
8.91
24
15.85
5
1.78
10
3.16
15
20
10
25
17.78
26 | P a g e
5.62
Ultrasonic Testing
PART 2 - FACTORS INFLUENCING ULTRASONIC TESTING SURFACE CONDITION The surface condition of the specimen is of prime importance. There are many ‘Schools of thought’ on this subject and over-exuberant claims are constantly made by people who state that more than a fair degree of roughness is quite acceptable. More often than not these claims are made by people with little or no practical experience and, unfortunately, their comments are taken as read by management. When the ultrasonic operator attempts to perform his task of testing a specimen, and eventually requests some surface preparation, he is then considered as over critical and a burden to production costs. Surface at all times should be of reasonable nature, and a guide to the degree of irregularity that can be tolerated can be stated as approximately one tenth of a wavelength. Beyond this figure, coupling conditions deteriorate. The theoretical critical roughness is given by the equation: Rc = (λ1v2) / ( 2(v2-v1)) = (λ2v1) / (2(v2 –v1)) Whereλ1 V1 λ1 V2
is is is is
the the the the
wavelength in couplant velocity of sound in the couplant wavelength in the test piece velocity of sound in the test piece
An ultrasonic test applied to a specimen with a rough surface beyond the limits defined above would bear little relationship to a further test applied to the same specimen after preparation. The reason for this is as follows. Probe control wold be much easier, making the point of reflection of defects easier to locate and measure. A smaller dead zone, would be created because less gain would be required, due to improved coupling conditions. The use of a higher frequency probe being made possible by the improved surface. All of the aforementioned would make a far superior test. In some industries an unprepared surface is quite acceptable. The applied test is only required to find defects which are of certain dimensions. The size of defect, the surface condition and the probe frequency are all compatible. The coupling be water which is ejected onto the surface via an irrigated probe. General comments on good surfaces in the following conditions will now be made.
LOOSE SCALE Loose scale on a good surface should be removed. One layer of tightly adhering scale does not normally produce any significant hazards, but heat treatments do e.g. double stress relief or repeated tempering. These operations can create a double, tightly adhering scale, which presents an almost impenetrable acoustic barrier causing little or zero transmission.
PEENING Peening on the surface of a casting should be regarded with the greatest suspicion as invariably this has been done to remove sand or to tighten or blend surface holes. These surface holes could very well be gas tails leading to cavities, or shrinkage.
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Ultrasonic Testing SLIGHT RUST Slight rust on the surface should be wire brushed, if this is not done it will mix with the couplant and become quite a solid mass under the movement of the probe. In certain instances the use of a thin polythene sheet enhances the coupling conditions. To use this method it is first necessary to apply the coupling medium, one which has a high viscosity (grease, polycell, etc.) to the surface of the specimen. The polythene sheet is then placed on the couplant and smoothed over until all traces of air bubbles have been removed from below the sheet. When this has been accomplished, a thin smear of couplant on the exposed surface of the polythene is all that is necessary to provide a very good surface condition. An added advantage of this method is the fact that it reduces probe wear, and presents a cleaner trace on the cathode ray tube.
ACOUSTIC IMPEDANCE, TRANSMISSION AND REFLECTION At any interface between two media of differing acoustic impedance acoustic mismatch causes part of the energy to be reflected back and the remainder to be transmitted through the interface. The specific Acoustic Impedance is derived from the acoustic impedance of velocity and density. z p v
= = =
pv density of material velocity of sound through the material
For ultrasonic wave’s incident at normal angle to the interface, the percentage of energy transmitted is: (4z2 * z1 / (z2 + z1)2) * 100% Where z1 and z2 are acoustic impedance’s in materials 1 and 2 the percentage of energy reflected is: ((z2 - z1)2 / (z2 + z1)2) * 100% If the energy in a material arrives at a boundary in air, the value of z 2 is small compared to z1, so that reflection is practically complete, therefore when it is required to transmit ultrasonic energy through an interface it is necessary to use a COUPLANT. The nearer the acoustic impedance of the couplant to that of the two solids the greater will be the transmission.
COUPLANT The couplant used in ultrasonic inspection appears to be very much a matter of personal preference; some operators use grease to eliminate the air between the transducer and the specimen, while others prefer polycell, water or oil. Glycerine is a thick syrupy liquid, soluble in water and is very good, but expensive.
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Ultrasonic Testing Many are the mediums used, but it is worth knowing that in terms of efficiency some are much better than others. We shall consider some examples in the respect using the following formula. Efficiency of couplant is a function of acoustic impedance of couplant and test piece. Acoustic Impedance z = Density * Velocity of Sound Transmission = 4*z1*z2*100 / (z2 + z1)2 % z1 = A1 Test piece Steel = 45 * 105 g/cm2/sec z2 = A1 Couplant Oil = 1 * 105 g/cm2/sec Water = 1.5 * 105 Glycerine = 2.46 * 105 Polycell = 1.8 * 105 (Approx.) Example Oil Couplant = 4* z1*z2*100 / (z2 + z1)2 = 4*45*1*100 / (45 + 1)2 =18,000 / (46)2 =18,000 / 2116 =8.5% Water = 4* z1*z2*100 / (z2 + z1)2 = 4*45*1.5*100 / (45 + 1.5)2 =27,000 / (46.5)2 =27,000 / 2162.25 =12.5% Glycerine = 4* z1*z2*100 / (z2 + z1)2 = 4*45*2.46*100 / (45 + 2.46)2 =44,280 / (47.46)2 =44,280 / 2252.45 =19.7% Assuming compression waves and a very smooth surface Reflection R =
(z1 – z2)2*100 / (z1 + z2)2
% reflection + % transmission = 100% Example of reflection at a water to steel interface: R = (z1 – z2)2*100 / (z1 + z2)2 = (45 – 1.5)2 * 100 / (45 + 1.5)2 = (43.5)2 * 100 / (46.5)2 29 | P a g e
Ultrasonic Testing
= 1892.25*100 / 2162.25 = 189225 / 2162.25 = 87.5%
MODE CONVERSION AND SPURIOUS INDICATIONS When a wave is incident upon a face of other than 90˚, mode conversion may take place i.e. the transformation of a shear wave to a compression wave. Mode conversion may occur in the most simple of geometric shapes, a classic example of such an occurrence can be given from the IIW calibration block. Fig 34. Fig 34 (i) shows a compression probe placed at a convenient position on the IIW calibration block so as to obtain multiple reflections of 100mm. Fig 34 (ii) shows the end elevation, or through section, and the subsequent transformation of waves from compression (C) to shear (S) and back again to compression. Fig 34 (iii) shows the corresponding cathode ray presentation. The effect of mode change can be observed on a weld when examination of the root run is undertaken using a 60˚ probe. Fig 35 shows a root concavity of smooth contour, and the effect of mode change is shown. It may well be that the conversion from shear to compression may occur only once, that is from the concavity to the dressed surface of the weld, then back again for conversion back to shear, in other cases it may return to the probe as a compression wave after many reflections of only a few degrees separation. The net product of such an occurrence is to indicate the effect on the screen at a beam path equal to the total beam paths, in this case, well into the parent material. Fortunately we can damp down these signals by placing an oily finger at position x thereby confirming mode conversion. In any event, such a suspect area as indicated by the mode conversion in the parent material would have been investigated from the other side of the weld.
The golden rule in ultrasonic inspection is to predetermine any areas where mode conversion may occur by careful preparation of the geometry of the specimen. This enables the technician to pre-calculate beam path distance etc. from a drawing, thereby appreciating most of the variables that can happen before he starts the ultrasonic examination.
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Ultrasonic Testing Spurious indications are, as the name suggests, signals that occur and present irrelevant indications such as signals ghosting across the screen through electrical disturbances. The occurrence of a signal on the time base may be due to a build-up of couplant in front of an angle probe on flat or curved surfaces although it is fair to state that these signals are more predominant from a curved surface with the probe positioned for a traverse scan Fig 36. Fig 36 Other such indications happen when a combined double compression probe is used on a rough surface with the attenuator set to give quite a high sensitivity level. Spurious indications will be displayed as very strong signals in group formation occupying a position on the time base of between 15m to 20mm. This position and size of group depends on the degree of surface roughness and the size of the probe. The reason for such spurious indications are due to reflection grating, which, in simple language, means that sound waves are reflected from one crystal to another across the
surface producing indications on the cathode ray tube at positions equal to the distances travelled. Transmission of the beam still takes place under these conditions and good back wall indications are obtained. To the less experienced ultrasonic technician these signals may be misinterpreted as inclusions or lamina defects. Fig 37 illustrates the conditions described and the associated cathode ray tube display. Fig 37 One further example that is worthy of mention is the reflections obtained from a probe that has a refracted angle around 45˚.
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Ultrasonic Testing Indications can be obtained from such a probe when the smallest amount of surface irregularity is present on the incident surface. Fig 38
Fig 38 shows a situation where the signal heights from the surface irregularity could quite conceivably be greater than that of the defect for the indicated probe position. For the technician with little experience in this respect it is suggested that practical tests are carried out to appreciate just how strong such indications can be.
CALIBRATION OF COMPRESSION WAVES SINGLE PROBES Before the time base is calibrated, the graticule scale does not represent any distance. The scale is divided into ten vertical lines, running the full height of the screen and fifty small lines at the bottom of the display. Once the time base is calibrated to a known distance, each line has then a definite meaning. For example, when the time base has been calibrated to measure 0-50mm; each of the small lines represents 1mm. Before calibration is attempted, two factors must be known; the time base distance required and the thickness of the available calibration block. Calibration is achieved by placing a probe on the calibration block and aligning the repeated echoes to the corresponding graticules. In order to do that, one must first identify the meaning of each line. As stated above, when calibration for 0-50mm; the small lines represent 1mm, the large lines must then each represent 5mm (10 lines, screen 50mm, 50 / 10 = 5mm). If the calibration block is 20mm thick, the first echo would be 20mm and the second echo would be 40mm. The same method is used for any calibration required, by dividing the time base by the thickness of the calibration block and placing the echoes in their respective positions. For example, a time base calibration to 100mm using a 25mm thick calibration block would have four echoes, placed at 25, 50, 75 and 100mm. The same time base (100mm) using a 20mm thick calibration block, would have 5 echoes placed at 20, 40, 60, 80 and 100mm; and so on.
32 | P a g e
Ultrasonic Testing The key, then, to any calibration is to divide the required time base by the thickness of the calibration block and place the echoes in the correct position. An important point to remember is that the distance from echo to echo, is the thickness of the calibration block being used. For that reason it is not possible to calibrate the time base, using only one echo!
CALIBRATION
OF
SHEAR WAVE PROBES
Unlike compression waves, shear waves do not make use of a known thickness, but use a radius to achieve calibration. The main reason for this is to enable calibration of different angles from only one radius. The method of calibration is similar to that used for compression, but the probe must be
positioned to give the maximum response from the radius. At that point the beam is radial and is travelling the radial distance. For example, the calibration block known as the V1 block has a radius of 100mm and when the echo is maximised the distance travelled by the beam is 100mm. Therefore repeat echoes each indicate an additional 100mm. The echoes are repeated from slots, as indicated in Fig 41 above. I there were no slots in the calibration block, the beam would not show repeat echoes, which is the case when using a calibration block known as the V2.
V1 Calibration Block (above) & V2 Calibration Block (below)
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Ultrasonic Testing
It will be noted that the V2 block has two radii of 25mm and 50mm. You will recall that, as stated in the section on calibration of compression waves, it is not possible to calibrate using only one echo. With the V2 calibration block, echoes from 25mm and 50mm can be used to calibrate the time base for a required distance. Some practice is needed to be confident when calibrating shear waves and it must be restated tha the calibration is achieved, not only when the leading edge of the echo is on the correct graticule, but also when the echo is maximised. As an example an explanation of a calibration will now be given
It is required that a time base, reading from 0 – 100mm is to be used. The calibration block, which is available, is the V2. This has two radii of 25mm and 50mm. 1. Using the delay control, place the initial pulse at 0 on the time base. 2. Apply couplant to the calibration block and place the shear wave probe on the block, with the beam projected to the 25mm radius. 3. Using the delay and calibrate controls obtain two echoes. The distance between the first and second echoes is 75mm. Not, as one would expect, 25mm. The reason for this is that this block does not have and slots, like the V1. The sound beam does not repeat at 25mm, but reflects form the flat surface and is then projected to the 50mm radius. The second echo is then at the distance travelled from the 25mm radius to the 50mm radius. Add these together and you get 75mm. 4. With the delay control place the first echo on the graticule which represents 25mm, and the second echo on the graticule which represents 100mm. (First echo 25m, second echo is 25mm + 75mm = 100mm). 5. Turn the probe round so that the beam is projected at the 50mm radius, and make any adjustment using the calibrate control, to place the leading edge of the echo on the graticule which represents 50mm. it is important that the second echo is placed on 50mm as an additional check on the accuracy of the calibration. 34 | P a g e
Ultrasonic Testing
The time base is now calibrated to measure 0 – 100mm.
PROBE INDEX POINT The V1 block has many dimensions, which can be used for a great number of time base calibrations. It has one other important function, making use of the slot at the centre of the
radius. When the sound beam is reflected, as it passes from the probe shoe into the block, the exact point on the probe where this occurs is called the index point. With the echo from the 100mm radius maximised, the part of the probe which is aligned with the slot is the point at which the index point is to be marked on the probe.
GEOMETRY Before any attempt is made to examine a particular weld or component by ultrasonics, it is necessary to appreciate the geometric considerations involved in that inspection. In many instances the geometric configuration may be quite simple while in others it will be complex and require careful thought as to the correct technique to apply so that the possibility of incorrect interpretation through mode change etc. will be avoided With the qualification schemes for the approval of non-destructive testing personnel now operating it becomes increasingly apparent that a critical and precise approach in the methods of ultrasonic testing and recording of such information are of the highest priority.
If the ultrasonic technician has a constant system of recording and also a graphic presentation, all the ultrasonic examinations he makes will be consistent and of the highest integrity. 35 | P a g e
Ultrasonic Testing Such a system will now be discussed and will serve to introduce a new word into the field of NDT; the word being Ultragraph.
THE ULTRAGRAPH The Ultragraph should show three important aspects of a section of defective weld. 1. A cross section or side elevation of the defect(s) at various points indicating their size. 2. A plan view or radiograph type of presentation indicating the length of the defect. 3. A plot of the root contours in the case of a full penetration butt weld showing excess penetration and lack of penetration.
The Ultragraph is accompanied by a recording sheet which contains all the relevant plotting information obtained At the time of the test. One of the main advantages of this system is the fact that all plotting is done on a drawing board in warm and pleasant surroundings after the scanning techniques have been completed and the plotting data compiled. This is far removed and much more desirable than attempting to attain accuracy with greasy plastic slides and a chinagraph pencil that has a point, after a little use, of between 1 and 2 mm. it is quite ludicrous to lay any claim to accuracy under these conditions, furthermore the accumulation of such errors would very quickly exceed any acceptable plus or minus tolerance on defect size as set by the qualification examining body. Let us now proceed to a simple weld configuration in a pipe to pipe weld joint. It will be noted that the root gap is 3mm and that the land is also 3mm. The preparation is a U prep and a decrease in angle is introduced at 37mm to reduce the amount of weld volume required. Form this drawing the beam path distance, half and full skip distance can be measured. The choice of probe angle will respect to the junction zone can be selected remembering that the ultimate angle would be one of 90˚.
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Ultrasonic Testing Beam path, half and full skip distances can be calculated mathematically. These are as follows, complete with examples. (These are not related to the above dimensions). FULL AND HALF SKIP DISTANCE
The skip distance factor for any probe is found by 2t TAN(a). This is twice the material thickness multiplied by the tangent of the angle of refraction (obtained from trigonometric tables, calculator). Example: Probe angle 45˚, plate thickness 20mm 2t *TAN(a) 2t = 20mm x 2 = 40mm TAN(45) = 1 40 x 1 = skip distance = 40mm (½ skip / 2) To calculate half skip distance t*a 20mm x 1 = 20mm ½ skip BEAM PATH To calculate beam path distance (BP) BP = t / COS(a) The beam path (BP) is the distance travelled from the index point of the probe to the first reflection point of the material. To obtain this distance it is necessary to divide the cosine of the angle of refraction into the thickness of the material. Example : BP = t / COS(a)
Probe = 45˚ t = 20mm BP = 20 / COS(a) = 20 / 0.707 37 | P a g e
Ultrasonic Testing = 28.2mm
The foregoing formula can be deduced from the following: CB / AC = TAN(a) Where: CB is the half skip distance AC is the thickness t of the material Example: CB / AC
=
20mm / 20mm
=
1 (TAN of 45˚)
Calculation of the Beam Path and Surface distances mathematically provides a very reliable means to accuracy, however, the student who does not wish to use this method can use the geometric approach i.e. produce a drawing and determine the various distances by physically measuring the drawing. Many complex shapes are examined by ultrasonics and parts of the component can be most difficult to examine either because of accessibility or geometric configuration. Such a problem is illustrated in Fig 50. It can be seen that with the concave weld profile it is impossible from the stub side to examine the toe of the weld (an area where we may observe toe cracking) the changing contour of the header section makes any inspection from that face most difficult and almost impossible to plot under actual working conditions. Another important point which should be borne in mind when the examination of a cylindrical object is contemplated using angle probes, is the IRRADIATION FACTOR, from Fig 51 you will observe the effect of a curved surface and will appreciate that irradiation is a function of diameter and angle. IRRADIATION FACTOR Formula Irradiation Factor = (1-SIN(a))/2 Example: Where a = 45˚ = (1-SIN(a))/2 38 | P a g e
Ultrasonic Testing = (1-SIN(45))/2 = (1 - 0.707)/2 = 0.293 / 2 Irradiation Factor = 0.146 Irradiation Factor * Diameter = Irradiation Depth The factors for various angles are as follows: 20˚ 0.32 9
25˚ 0.28 9
30˚ 0.25 0
35˚ 0.21 3
40˚ 0.17 9
45˚ 0.14 6
50˚ 0.11 7
55˚ 0.09 0
60˚ 0.06 7
65˚ 0.04 7
70˚ 0030
75˚ 0.01 7
80˚ 0.00 8
Having taken an initial look at weld profile let us now discuss the method of testing such a weld, bearing in mind our geometric considerations.
PART 3 - TESTING METHOD OF TESTING The next important step will be to inquire as to the nature of the parent material and the weld metal (mild steel 2¼% chrome, 1% moly, etc.) and to any heat treatment operation carried out on the metal. Attention should be paid to the overburden or cap of the weld. This should be ground off, (where possible) and the surface polished. It is also necessary to polish the surface on each side of the weld to ensure good coupling. Measuring from the centre point of the weld, the area requiring polishing is approximately twice the thickness of material under inspection. The parent material should be scanned on both sides using longitudinal waves to establish that it is free from laminations or other defects. Precise readings of the thickness during this operation should be noted from both sides and any areas of reduction marked for special attention later. Next, scan the weld area noting any areas offering response. Mark the centre line of weld circumferentially and then draw in the half skip positions circumferentially as pre-determined from the drawing. This is the point where the probe index is beamed directly into the centre line of the root. Finally, draw in the lines which represent the outer limits of scanning. The use of a back or front stop is highly recommend for the initial root scan as this will provide a guide for the probe and allow the technician to direct most of his attention to the CRT. The method of root interpretation to be in accordance with the instructions received in the school.
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Ultrasonic Testing TEST SENSITIVITY First signs of ‘grass’ from weld area up to the maximum beam path distance, or as recommended by the specification. The body movement of the probe during examination of the weld should be smooth and well controlled with no greater lateral movement than half the crystal width. The probe should be transversed from the limit of scan line through the centre line – moved laterally a half crystal width and transversed back to the limit of scan, this sequence should be followed until the circumference has been internally scanned. This operation should be carried out from both sides.
TRANSVERSE FLAWS Materials prone to transverse cracking in weld and heat affected zones should be scanned in the appropriate direction again using the techniques leant in the school. NB: Angle probe irradiation factors are used here. It will be appreciated that the difficult part of an examination of a weld is the interpretation of the time base presentation. To state that a rigid code or set pattern of Trace Formation can be followed, would be extremely misleading. The number of variables are quite considerable, for example the shape of the Trace Pulse from a quartz transducer will vary from that of Barium Titanate. Equipment characteristics will be evident when a known flaw is scanned with two ultrasonic detectors. The variations may be small, but in some cases the difference will be very noticeable. The choice of probe angle and its frequency, all have an effect on the resolution of the pulse presentation. Good resolution is of great importance, it enables the technician to observe reflecting facets of the flaw displayed on the time base in a well-defined form, without this ability the facets of group porosity for instance would take up a broad trace with insufficient break up of peaks to display the various time intervals, from index point to reflecting facets of the pores. A signal of high amplitude (signal pulse height) does not necessarily mean that the flaw is a large one and conversely a small display of amplitude may be from a large defect not presenting face to the angle of ultrasonic propagation, further investigation using other angles would give conclusive results. From a series of carefully controlled probe movements much information can be gained and used to plot the flaw position within the weld. The welding technique must be known, and the type of flaw that may occur in that technique fully understood. It should also be noted that similar time base presentations can present themselves from different flaws. Accurate interpretation of the time base presentation is very much a matter of experience, but there are basic probe movements and trace formations that result from certain flaws.
BUTT WELD EXAMINATION In the examination of butt welds these fall into the following categories: 40 | P a g e
Ultrasonic Testing
1. 2. 3. 4. 5.
LACK
Lack of fusion side wall Porosity & piping Slag spots & linear inclusions Lack of penetration Cracks – longitudinal & transverse
OF FUSION SIDE WALL
The time base presentation for lack of side wall fusion tends to be quite sharp, and providing the probe angle selected is at right angles, only a little movement of the signal along the time base will be observed when the probe scans the flaw region. Some variations of this will be experienced and will depend upon the extent of the lack of side wall fusion. Detection of this type of defect on thin wall tubing is best carried out from a back stop position using the minimum number of skips. To provide adequate coverage of fusion area, the test should be set up for this type of flaw only, and the probe carefully transversed around the tube using the back stop to maintain a constant weld to index point relationship. Any lack of side wall fusion would present itself at a position on the time base as previously calculated. This signal would tend to be sharp and reasonably high in amplitude. A scan from each side is necessary. If a natural specimen containing this flaw is available for “Setting up” this would prove to be of immense value.
POROSITY & PIPING The time base presentation of a single pore is sharp with clean rising edges, the following movements will provide reasonable confirmation. a. Orbit the probe around the flaw, and observe the time base presentation. If the signal height remains constant, i.e. no reduction in amplitude, this then indicates a spherical reflector. b. Obtain maximum amplitude and rotate the probe on its axis, reduction of signal height will be rapid. c. A lateral scan will produce an equal rise and fall of the signal, with little signal width on the time base presentation. As the probe is traversed toward the pore, the leading edge of the beam makes contact, the signal presents itself fairly quickly although the gain to maximum is steady and fall off rapid. The probe movement will depend on the angle and depth of the flaw. The movement of the signal along the time base for this operation will be fairly long, again depending on the probe angle depth. Much the same conditions exist for a rounded isolated pipe. In the case of group porosity, there are present many reflecting faces at different beam path lengths. The time base presentation is a broad one, and its width depends on the shortest and the longest reflected travel path taken by the Ultrasonic beam. The left and right hand edges are clean but any signals superimpose themselves across the area and the time base presentation has a somewhat jagged appearance across the top. 41 | P a g e
Ultrasonic Testing
Orbiting the probe produces much the same information. Rotation of the probe causes a large number of maximum signals to be observed, which rise and fall rapidly.
SLAG SPOTS & LINEAR INCLUSIONS Slag inclusions present themselves in various ways, for a small inclusion time base presentation similar to that for a small pore could be observed. Slightly larger inclusions produce a signal that is forked, indicating irregularity of the reflecting face. Orbiting the probe for this type of flaw produces a time base presentation where a number of maximum amplitudes can be observed. Rotation of the probe produces a slower fall of than for a gas pore. Linear inclusions maintain a good amplitude when the probe is in lateral movement. If the amplitudes are a number of maximum and minimum, the whole length of the scan (deducing of course beam spread in the horizontal plane at that depth) must be taken as the flaw length. This is necessary owing to the fact that it was not possible to separate the flaws due to beam overlap.
LACK
OF
PENETRATION
Lack of root penetration being a root condition where unfused edge or edges provide good ultrasonic reflectors. In the case of a butt weld where for various reasons the specified root gap has closed and an unsuccessful attempt has been made to penetrate through the time base presentation in this case is a signal of quite some amplitude, singular and very sharp, with clean edges indicating a constant root condition; the type of signal to be expected from a machined face. Some linearity may be observed when the probe is in lateral movement. Orbiting the probe in an arc, using the flaw as a central point, produces a time base presentation where a fairly quick fall off of maximum amplitude occurs both on the right and left hand arc from the right angle probe position. Variations of the above occur and depend on wall thickness and probe angle. Pipe curvature must also be considered. Rotation of the probe produces a fairly quick rise and fall and does not indicate a great deal of movement, along the time base. Traversing the defect produces a very characteristic time base presentation when the leading edge of the ultrasonic beam contacts the defect. A signal is observed to the right hand side of the previously calculated root position. Further traversing of the probe produces the maximum amplitude and this should present itself as near limits to root position on the time base, depending on the height of the lock of penetration. Further traversing will produce a steady fall off of signal. The significant points to make are that the probe movement will be quite substantial and the signal travel along the time base will have been extensive, just how extensive would be a feature of the height of the flaw, and probe angle. 42 | P a g e
Ultrasonic Testing The observed signal from such an area with complete lack of penetration when traversed is one that is extremely smooth with gradual build up to maximum and gradual fall off and occupies a considerable amount of time base.
CRACKS Longitudinal and transverse cracks are good ultrasonic reflectors, if we were to observe the face of the crack magnified only several times we wold see multiple facets, in fact a quite jagged saw tooth formation. These are the faces of reflection that present themselves to the ultrasonic beam. Cracks have random orientation, varying height and length, thus, their detection by ultrasonics is relatively simple. The time base presentation will not always be a simple matter of interpretation, small fine cracks may produce a presentation surprisingly similar to a fine slag inclusion, much again depends on probe angle and orientation of flaw. In general a crack produces signals of quite high amplitude and because the facets are so close to one another, these reflections superimpose themselves on this signal. Many high intensity blips (half cycles) can be observed both on the left hand rising edge and on the right hand flank. Slight rotational probe movement will cause these high intensity blips to move up and down the signal rapidly. A crack that has height and various orientations can produce signals at different time intervals, their peaks being ragged. Orbiting of the flaw may cause loss of signal amplitude, to what extent depends on flaw orientation. Traversing of the flaw produces the following: when the leading edge makes contact the signal amplitude rises quickly, the amplitude fall off is fairly rapid, time base travel is dependent on flaw orientation, height and probe angle. Lateral scanning of this type of flaw will probably reveal other facets of reflection at longer or shorter beam path lengths. On the time base presentation these show that the flaw orientation has changed to some degree, and a number of maximum amplitudes may be observed. Other signals not placed in the above categories but worthy of mention are as follows:
TWIN PEAKING FROM ARGON ARC ROOT During the welding process a slight notch effect is formed, in some instances this is quite pronounced, the effect on the time base presentation is to produce twin peaking, with the root signal in the middle of the peaks. Amplitude of signal depends on the degree of notching or shrinkage and the probe angle. A single notch effect may be observed, this will probably be due to weld position and other variables.
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Ultrasonic Testing ROOT CAVITIES Root cavities that have a smooth contour are difficult to detect and special care when scanning the root should be exercised. In case of suspect indicators a miniature combined double probe proves of great assistance. In the case of a cavity having sharp rising edges the signal on the time base presentation will be displayed to the left of the calculated root position. The most likely place for root cavitation is around the six o’clock position in a vertical weld – horizontal pipe, although root cavities can occur at any point on the circumference.
ROOT PRESENTATION
WITH INTERNAL
MISMATCH
At some time or other through counter boring of a pipe the root landings may be slightly mismatched, if this is welded, the welder will have no alternative but to weld the root at an angle while still achieving root fusion. When an ultrasonic test is applied it is possible to present good root reflection from one side of the weld, but from the other side, root reflections are practically non-existent. This present a problem to the technician who after having done a compression test of both parent metals has found uniformity of thickness and no such mismatch is evident from external dimensions. It is fair to state the above is a rare occurrence, but nevertheless can happen. Although an effort has been made to establish some time base presentations it must be fully understood that variations of the above frequently happen, and the ultimate interpretation of the trace is the responsibility of the technician. Every effort should be made to confirm the type of defect with probes of different angles, also precise measurements should be taken from the time base and related to weld geometry. At all times the screen should be calibrated to the smallest scale, in keeping with the beam path. Time base presentation of a flaw at 80mm beam path and occupying 5mm of the time base, presents easier to observe characteristics on the 100mm scale than it would on the 250mm range. In summing up it will be self-evident that experience is most important, so too, are the characteristics of the ultrasonic detector and the probes. This highlights the need for a technician to have his own equipment, thus eliminating some of the variable involved.
FLAW IDENTIFICATION AND SIZE ESTIMATION The object of inspection is to prove suitability for service and unless flaws can be identified and their size estimated with accuracy, a rational approach to acceptance or rejection is not possible. Flaws cannot be identified from the initial appearance of the trace because this does not provide all the information needed.
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Ultrasonic Testing Moreover, the same flaw may produce somewhat different echo signal shapes on the supposedly identical equipment. Even when the sensitivity level has been methodically set, the height of any one signal relative to others cannot be directly related to flaw size until the orientation of the flaw is known. It is therefore important to determine the attitude of the flaw in relation to the beam axis before attempting to estimate its size.
FACTORS AFFECTING THE DETERMINATION
OF
THE TYPE
OF
FLAW
a. Accuracy and reliability of equipment, particularly the probes is essential to accurate location of the flaw. b. Knowledge of the welding process and procedure enables a flaw to be provisionally identified from its location. c. The determination of flaw shape depends mainly on the fact that the intensity of the reflected beam is chiefly a function of the angle or incidence of the primary beam upon the flaw.
PROCEDURE FOR DETERMINING THE TYPE
OF
FLAW
Having first fixed the most favourable beam angle and probe position to ensure maximum signal response, the type of flaw is ascertained by a series of controlled movements of the probe. During manipulation of the probe, the flaw intercepts differing intensity levels in the beam, giving rise to a succession of echo signals of varying height and range. With experience, the tester comes to recognise typical signal permutations and to visualise the shape and texture of the surfaces which give rise to them. For instance, when the probe is orbiting at a constant stand-off distance a steady peak at constant range means that the signal comes from an isolated reflector e.g. an inclusion or a pore. If the same thing happens when the probe is moving laterally the reflector must be an elongated flaw such as an infused root. Here, a traversing scan would show up a sharp peak over a short distance. If, however, the flaw extends through the thickness of the weld, a lateral scan will show a sharp peak over a short distance, and a traversing scan a constant response over an appreciable length of scan. In a planner flaw, e.g. a crack. Lateral and traversing scan will both produce a fairly steady signal over the extent of the flaw in each direction of scanning. If the crack is ragged e.g. a hot tear, the indications will be significantly different due to the fact that the surface of this type of flaw is made up of many small planes of varying attitude which scatter ultrasonic waves, while its edges diffract them. Since the signal is monitoring the range of the flaw, there will be small shifts of position accompanied by fluctuations of signal height, each indicating a change in direction of some portion of the crack. Some idea of the attitude of the crack will be obtainable by rotating the probe. Although it is easy to recognise a ‘line’ flaw, it is often difficult to decide whether such a flaw is continuous or intermittent. If intermittent it is even more difficult to determine the extent of the gaps between the flaws, chiefly because of the beam spread and consequent overlapping on successive flaws. The echo height usually fluctuates, the extent of the rise and fall being a function of the length of the gap. It can be shown, however, that a continuous but ragged flaw 45 | P a g e
Ultrasonic Testing has a similar effect due to random reflection of the beam. Unless the gaps are so small as to be insignificant, it may be necessary to differentiate between continuous and intermittent flaws. The following procedure is recommended: a. Choose a probe so as to reduce to a minimum the probe flaw distance, this reduces the effect of beam overlap. b. Eliminate other variables which affect echo height such as poor coupling, jerky manipulation of the probe, and poor surface condition. The use of a straight edge is recommended to ensure constant probe-to-flaw distance. c. Watch the screen closely and note where the echo signal rises and falls. It may be of advantage to do a trial run to obtain a general impression of the situation. A) Position of probe index for maximum signal B) C) position of probe index for 6dB or 20dB drop in signal
If echo height is being affected by random reflections from a ragged surface it can be detected by slight oscillation and swivelling of the probe opposite the point of echo fall. There will then be several points where the incidence of the beam is most favourable, and echo height will increase sharply. If, however, the flaw is intermittent, the same movements will result in rapid decline of echo height. With practice, the study of echo permutations leads to a process of mental integration which enables the test to classify each characteristic with little conscious effort. If necessary such classification can be confirmed by means of the flaw location slide, provided that the solid angle of the beam has been studied beforehand in at least two mutually perpendicular planes.
FACTORS AFFECTING ACCURACY
IN
THE ESTIMATION
OF
FLAW SIZE
a. Reliability of the equipment as a whole is the first essential; this implies adequate servicing. b. The resolving power of the probes determines the accuracy with which individual flaws can be separated, particularly near the root of the weld. c. Inadequate acoustical coupling caused, for instance, by rough or pitted surfaces and probe rock or tilt as a result of uneven surface grinding, lead to inaccuracy in determining the position and limits of a flaw. d. The increase in relative sensitivity of probes with increase in probe angle. e. Decrease in sensitivity with increase in path length. f. Lack of care in scribbling or pencilling fiducial marks on probe or work piece, particularly if coupled with error in locating the probe index.
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Ultrasonic Testing PROCEDURES FOR THE ESTIMATION
OF
FLAW SIZE
Estimation of size follows logically from determination of flaw type and can well be carried out at the same time. The recommended procedure is to place the probe with the beam axis at right angles to the line of the weld and positioned for maximum response from the flaw. The signal is adjusted to a convenient amplitude and the probe traversed towards the weld until the signal height has fallen by 6dB or 20dB, (i.e. one-half or one-tenth of its former value). At this point, the edge of the flaw will be intercepting the lower boundary of the beam as shown in Fig 53. The probe-to-weld distance and the range on the screen are carefully measured. The probe is now traversed away from the weld until 6dB or 20dB drop signal amplitude is again obtained. The upper boundary of the beam is then touching the lower edge of the flaw (Fig 53) probe position and flaw range are again carefully noted.
Finally, the measurements of distance and range made at positions B and C are transferred to the flaw location slide as shown in Fig 53 to determine the size of the flaw in section. If the length of a flaw needs to be known, it may be measured in a similar way by moving the probe laterally until a fall of 6dB or 20dB is obtained on either side of the position corresponding to maximum response. At these positions the ends of the flaw will be intercepting the beam boundary and its length can be calculated from the measurement of lateral probe shift and flaw range, referred to as the ‘horizontal’ beam profile previously determined from the IIW block. When the surface texture of a flaw, such as a lamination in parent material, causes fluctuation in echo height, the 6dB and 20dB drop should be reckoned from the height of the signal nearest to the edge of the flaw.
CHOICE
OF
PROBE
Without taking into account the special probes used for specific testing problems, the number of probes listed by the manufacturer is rather bewildering. A few brief directives will therefore be given to facilitate selection. For testing fine grained materials by the contact method it has been found that testing frequency wave lengths from 3 to 1mm in steel permit, under favourable conditions, detecting of flaws of approximately 0.5mm and larger i.e. sizes where the designated flaw usually begins to be applicable. With these frequencies the most common diameter of the standard probe for daily use is 8 to 10mm. Using this standard probe a skilled operator will be able to solve a great many problems. When, however, skill alone cannot give the answer, other frequencies will have to be used: where scattering in the material in the form of ‘grass’ masks the display on the screen, lower 47 | P a g e
Ultrasonic Testing frequencies such as 1 and 0.5MHz may have to be used, although in general this lengthens the close-range interference zone by a factor of 2 and 4 respectively, while proportionally reducing the detectability of small flaws. Even lower frequencies may have to be used to make penetration at all possible in materials with strong acoustic attenuation, such as castings, plastics and wood. Frequencies above 10MHz are more rarely required because excessive sensitivity to minute flaws is by no means always desirable, and particularly also because these thin crystals, when used in the direct contact methods are not very resistant even if fitted with a protective layer. This high frequency range is mainly reserved for indirect coupling where fragile crystals such as lithium sulphate can be used. Diameter of probe and frequency determine the form of the sound beam which in the case of the standard probe can be characterised by a near field in steel of 30 to 130mm, and an angle of divergence of 10˚ to 30˚. A long near field means long range, i.e. high sensitivity for small flaws at great depth if the attenuation of sound in the material is neglected. The length of the near field determines the decrease in sensitivity with distance: to bring this decrease to a minimum the length of the near field should where possible, be not less than approximately one third of the maximum flaw distance. In the case of large specimens the condition cannot be fulfilled because, although the length of the near field increases with the square of the diameter of the probe, the standard probe constitutes a practical limit since most test pieces lack sufficiently large contact faces. On the other hand, higher frequencies, owing to the strongly increasing attenuation are ruled out. Thus, while for long ranges, probes of maximum distance are used, the smaller probes of e.g. 10mm and less at frequencies from 2 to 15MHz are used only for testing ranges up to 100mm. Compared with the larger probes, the smallest probes in this range have the advantage of more accurate lateral flaw location because of their narrower beam in the near field. They are used perforce where the coupling surface of the test piece is too small for the larger probes. Accurate lateral flaw location usually makes a large angle of divergence undesirable. However, when the problem is the detection of smooth obliquely orientated flaws which might easily be overlooked completely if a pencil-like beam is used, and if the specimen cannot be scanned for such flaws by oblique beaming at all possible angles, it is preferable not to attempt an exact flaw location but rather to use a wide-angle beam e.g. a 10mm diameter probe at 2MHz with an angle of divergence of 20˚. The recommendation of a definite frequency and a definite probe which have been used successfully with a definite instrument for solving a given testing problem does not necessarily lead to similarity favourable results when an instrument of different manufacture is used, it is therefore always advisable first to try out the most suitable probe. The reason for this is that a particular instrument with a probe, may for instance have a substantially higher sensitivity at 0.5MHz compared with that at 1MHz than the other instrument. Thus while in the case of the first instrument a given test should preferably be carried out at 0.5MHz, the result 48 | P a g e
Ultrasonic Testing in this case of the second instrument may be better when using 1MHz. At both ends of the frequency range viz. the high and the low, the amplification of the instrument or the sensitivity of the probes usually decreases. Unfortunately no standardised data concerning this point is as yet available. When either the shape of a flaw or the proximity of small discontinuities in the same plane, e.g. inclusions given rise to multiple echo signals, the signals from the extremities of the region concerned must be used for size estimation. These may be very much smaller than those obtained from more favourably orientated parts of the flaw surface and are often found at closely similar ranges. In these cases, special care must be taken in the observation of echo signals. The soundest practice is to take the 20dB drop from the last echo seen. High-resolution probes should be used wherever practicable. When the procedure is employed on curved plate or on pipework the geometry of the method is essentially unaltered for measurements in the vertical plane. The length of the defect is that as marked on the surface, and particularly with thicker sections may not be the actual size. With undressed welds it is advisable to use some other datum related to the weld centre line. The possibility of error through accidental displacement of the probe makes it advisable (at least for newcomers) to use a probe guide such as a magnetised steel strip.
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Ultrasonic Testing
PART 4 – SI UNITS OF MEASUREMENT Base Quantities Length Mass Time Electric Current Thermodynamic temperature Luminous intensity Volume Mass Energy
Measurement metre kilogram second Ampere kelvin candela Litre Tonne Electron Volt
m Kg sec A K cd l t eV
Derived Units Frequency Force Pressure and Stress Work and Energy Wavelength Power Quantity of Electric V.M.F and Potential Difference Electric Capacitance Electric Resistance Electric conductance Magnetic Flux Magnetic Flux Density Inductance Luminous Flux Illumination
Measurement Hertz Newton Pascal Joule Lambda Watt Coulomb Volt
Hz N Pa J λ W C V
Farad Ohm Siemens Weber Tesla Henry Lumen Lux
F Ω S Wb T H lm lx
Prefixes 1012 109 106 103 50 | P a g e
Name tera giga mega kilo
Symbol T G M K
Symbol
Symbol
Prefixes 10-2 10-3 10-6 10-9
Name centi milli micro nano
Symbol c m µ n
Ultrasonic Testing 102 10 10-1
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hector deca deci
H da d
10-12 10-15 10-18
pico femto atto
p f a
Ultrasonic Testing
PART 5 - ANSWERS CALCULATIONS OF dB RATIO’S – ANSWERS Question 1 First calculate ratio 40:10 Formula: dB = 20 * log( h1/h2) =20*log(4/1) =20*log(4) =20*0.60 Difference = 12 dB
= 4:1
Question 2 Formula: dB = 20 * log( h1/h2) =20*log(5/1) =20*log(5) =20*0.69 Difference = 14 dB Question 3 dB = 20 *log1(h1/h2) 40 = 20 *log1(h1/h2) (40/20) = log1(h1/h2) Antilog 2 = (h1/h2) 100 = (h1/h2) Ratio = 100:1 Question 4 dB = 20 *log1(h1/h2) 8 = 20 *log1(h1/h2) (8/20) = log1(h1/h2) Antilog 0.4 = (h1/h2) 2.5 = (h1/h2) Ratio = 2.5:1 Back to Questions
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Note: Log(100) = 2
Note: Log(2.5) = 0.4
Ultrasonic Testing
Part 6 – Sample Questions Which wave is able to follow a surface around a curve? A surface wave Which wave is only able to travel through a thin section of material? A lamb wave Longitudinal waves are also called: Pressure waves or Compressional waves As frequency increases, sound tends to: Scatter more from large or course grain structure Surface or Rayleigh waves travelling along the surface of relative thick solid material, penetrate to a depth of approximately? One wavelength Shear waves do not propagate in: Gases or Liquids What would make a good couplant?
Water, grease, oil or Glycerine In order to have a reasonable chance at detecting a discontinuity, the reflective surface of the discontinuity must have a dimension that is at least as long as ________ wavelength. One-half Sound can propagate as: Longitudinal, shear or surface waves Which type of transducer will have better resolution? A highly damped transducer In the transverse or shear wave, the particles: Oscillate at a right angle to the direction of wave propagation When sound travels through a medium: Its intensity diminishes with distance, It spreads perpendicular to the primary direction of wave travel, Its speed remains constant The terms used to describe a technique’s ability to locate flaws are: Sensitivity and resolution
The ability to locate a small discontinuity is called: Sensitivity 53 | P a g e
Ultrasonic Testing
The ability to locate discontinuities that are close together within the material is called : Resolution Resolution generally increases: With an increase in transducer frequency When a single element transducer is operating in the pulse echo mode, the element: Cannot start receiving reflected signals until the element has stopped ringing from its transmit function. The combined effect of scattering and absorption is called: Attenuation Acoustic impedance is identified by the letter: Z The sound that emanates from a piezoelectric transducer originates: From most of the active surface When an ultrasound wave passes through an interface between two materials at an oblique angle, and the materials have different indices of refraction, it produces: Reflected and Refracted waves Artificial flaws can be produced by using: Side drilled holes, Flat bottom holes or EDM notches The active element of most acoustic transducers used today is: Piezoelectric ceramic When a wave encounters an interface at an oblique angle, what takes place at the interface due to the different sound velocities of the two materials? Refraction The measure of how a signal from a defect compared to other background reflections is called: Signal to noise ratio Which of the following could influence the behaviour of a transducer? Electrical construction, Material Construction or Mechanical Construction Immersion transducers have: An acoustical impedance matching layer Contact transducers often incorporate a: Wear plate 54 | P a g e
Ultrasonic Testing What is the material called that is used to improve the transmission of ultrasonic sound energy from the transducer into the part? Couplant Contact transducers are used for direct contact inspections, and are generally: Hand manipulated What is used to support the active element and dampen the transducers characteristics? Backing material with a similar impedance of the transducer Some transducers are specifically fabricated to be: Better transmitters, better receivers and used in high temperature applications A calibration block produced by the International Institute of Welding is called: An IIW block Less damped transducers will exhibit: A narrow frequency range The area in front of a transducer where there are extensive fluctuations in the sound field is called the: Near field zone The sound energy or ultrasonic beam is more uniform in the: Far field zone Couplant displaces the__________ and makes it possible to get more sound energy into the test specimen. Air What part of the ultrasonic machine generates short, large-amplitude pulses of controlled energy? Pulser Beam spread is greater when using: Low frequency transducers Which ultrasonic technique require the probe to be held firmly against component? Contact Inspection When performing a contact inspection on a curved surface, what pieced of equipment can be used to improve coupling? A shaped shoe
What type of transducer contains two independently operating elements in a single housing? Dual element transducer 55 | P a g e
Ultrasonic Testing
Angle beam transducers and wedges are typically used to introduce a: Shear wave into the part Which type of transducer can be used to scan wide areas when high sensitivity is not required? Paint brush transducer The total resistance that the cable presents to the electrical current passing through it is called: Impedance Which type of screen presentation displays the amount of received ultrasonic energy as a function of time? A-scan Which type of screen presentation displays a profile or cross-sectional view of the test specimen? B-Scan Which type of screen presentation displays a plan-type view of the location and size of the test specimen features? C-Scan In angle-beam testing, when the geometry of the part is relatively uncomplicated and the orientation of a flaw is well known, the length of a crack can be determined by a technique known as: Tip diffraction The act of evaluating and adjusting the precision and accuracy of the measurement equipment is called: Calibration What is the correct formula for calculating the wavelength when the wave velocity and frequency are known?
The number of cycles per second is the definition of: Frequency Which type of calibration block is used to determine the resolution of angle beam transducers per requirements of AWS and AASHTO? An RC block 56 | P a g e
Ultrasonic Testing DACC stands for: Distance Amplitude Correction Curve
Mode conversion, occurs when a sound wave encounters an interface between materials of different acoustic impedance and: The incident angle is not normal to the interface The material of the reference standard used to setup for a flaw inspection: Should be the same material being inspected What is used to establish a general level of consistency in measurements, and to help interpret and quantify the information contained in the received signal? Reference standards Within a given material, the speed of sound: Is constant Describes a shear wave? A wave in which particles vibrate in a direction perpendicular to the direction of propagation What is another name for a calibration standard? Calibration Block What does the gain control do on the ultrasonic testing equipment? Controls the amplifier circuit What relates to the ability of transducer to differentiate between two or more reflectors that are closely spaced in a lateral plan perpendicular to the axis of a sound beam propagation? Spatial resolution True Or False? A transducer's frequency is determined by the thickness of the crystal True True Or False? If a crystal produces a higher frequency sound beam it produces a higher sensitivity, resolution and depth of penetration False A planar discontinuity like a surface connecting crack or lack of fusion are best detected by using what? Shear wave transducer What is the best transducer to use when first scanning a large object? Paintbrush transducer
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Ultrasonic Testing What is used between the transducer face and test object to ensure efficient sound transmissions? Couplant
What should you use to ensure a part has been demagnetized? Gauss meter
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Ultrasonic Testing Multi-Choice Questions 1. What is the most common manual ultrasonic testing technique in NDT? a. Immersion b. Through Transmission c. Tandem d. Pulse Echo 2. In which area of the ultrasonic sound beam do the edges of the wave diverge? a. The Fresnel Zone b. The Fraunhoffer Zone c. Inside the damping material d. In the Dead Zone 3. What is the purpose of a couplant? a. To enable sound energy to pass from the probe to the test piece b. To protect the test piece c. To lubricate the test piece d. All the above 4. At an interface, when a compression wave in the second medium reaches a refracted angle of 90˚, the incident angle in the first medium is described as… a. The reflected angle b. The first critical angle c. The second critical angle d. The probe angle 5. Which circuit in an ultrasonic flaw detector controls the timing of the pulses and the creation of the timebase? a. The pulse generator b. The sweep circuit c. The receiver amplifier d. The suppression or reject circuit 6. Which of the following reference reflectors does the DGS or AVG system usually use? a. Vee notch b. Square notch c. Transverse hole d. Flat bottom hole 7. Which of the following are acceptable sizing methods for use with angle probes? a. 6 dB drop b. 20 dB drop c. Maximum Amplitude d. All of the above
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Ultrasonic Testing 8. Which component of the ultrasonic test equipment is responsible for the production of the ultrasonic vibrations? a. The Probe b. The Flaw detector c. The OPT d. The calibration block 9. What effect on beam spread does a change of probe, to one with the same diameter but a lower frequency, have? a. The half angle increases b. The half angle decreases c. a and b d. Probe frequency does not affect the beam spread 10. Which of the following statements best describes attenuation? a. The resistance of a material to the passage of ultrasound b. The difference in sound intensity of two signals on the CRT c. The increase or decrease in amplifier gain on the flaw detector d. The loss of intensity of sound as it passes through a material 11. At an interface, when a shear wave in the second medium reaches a refracted angle of 90˚, the incident angle in the first medium is described as… a. The reflected angle b. The first critical angle c. The second critical angle d. The probe angle 12. Which circuit in an ultrasonic flaw detector controls the voltage or charge across the Xplates in a CRT? a. The pulse generator b. The sweep circuit c. The receiver amplifier d. The suppression or reject circuit 13. When scanning a surface with 0˚ probe, the distance between the consecutive scans is known as the...? a. Gap b. Pitch c. Overlap d. All the above 14. Which of the following is the preferred sizing method used for assessing cracks? a. 6 dB Drop b. 20 dB Drop c. Maximum Amplitude d. All the above 60 | P a g e
Ultrasonic Testing
15. Which type of display does an analogue flaw detector use? a. LCD b. CRT c. VDU d. Plasma 16. What effect on beam spread does a change of probe, to one with the same frequency but larger diameter, have? a. The half angle increases b. The half angle decreases c. a and b d. Probe diameter does not affect beam spread 17. Which of the following materials would have the highest attenuation? a. A fine grained material b. A coarse grained material c. A medium grained material d. All of the above would have the same attenuation 18. At an interface, when the interface angle in the first medium is between 0˚ and the first critical angle, the refracted sound in the second medium will be… a. A compression wave b. Shear waves c. Surface waves d. a and b 19. The control on the flaw detector which controls the voltage or charge on the Y-plates is called the…. a. Range control b. Delay control c. Gain or attenuator control d. Focus control 20. Which of the following sizing methods may be used with 0˚ probe? a. 6 dB drop b. Equalisation c. Maximum amplitude d. All of the above 21. Which of the following is a sizing methods used for assessing porosity? a. 6 dB drop b. Equalisation c. Maximum amplitude d. All of the above 61 | P a g e
Ultrasonic Testing
22. Which of the following would accurately describe sound? a. Audible mechanical vibrations b. Ultrasonic mechanical vibrations c. Infrasonic mechanical vibrations d. All of the above 23. What are ‘k’ factors? a. Constants used in calculating beam spread b. Variables used in calculating beam spread c. Constants used in the near zone formula d. Variables used to calculate near zone length 24. What effect does beam spread have on sound attenuation? a. More beam spread, more attenuation b. Less beam spread, less attenuation c. Less beam spread, more attenuation d. a and b 25. At an interface, when the incident angle in the first medium is between the first critical angle and the second critical angle, the refracted sound in the second medium will be…. a. Compression waves b. Shear waves c. Surface waves d. a and b 26. What is the term used to describe an amplifier which accepts a wide array of signal frequencies and is good for defect interpretation but less sensitive at defect detection? a. Receiver amplifier b. Broad band amplifier c. Narrow band amplifier d. Signal amplifier 27. Which of the following sizing methods does not work accurately on defects which are smaller than the ultrasonic beam? a. 6 dB drop b. Maximum amplitude c. DGS d. AVG 28. Which of the following is a sizing method used for assessing length of a defect? a. 6 dB drop b. 20 dB drop c. Maximum amplitude d. All of the above 62 | P a g e
Ultrasonic Testing 29. Which two properties of a material, when combined, have the most measurable effect on the sound wave velocity? a. Grain size and surface condition b. Density and elasticity c. Acoustic impedance and resonant frequency d. All of the above 30. Which law do we use to compare sizes of small reflectors, located in the Fraunhoffer zone, using their signal amplitudes? a. Snell’s law b. The inverse Law c. The inverse square Law d. b and c 31. What unit is used to quantify difference in sound intensities? a. Hz b. mm c. dB d. m/s 32. At an interface, when the incident angle in the first medium has reached the second critical angle, the refracted sound in the second medium will be… a. All reflected b. Compression and shear waves c. Shear waves d. Surface waves 33. Which of the following calibration blocks would you use to check the dead zone on a single crystal 0˚ probe? a. Block No. 1 b. Block No. 2 c. A5 d. b or c 34. Which of the following calibration blocks may be used to calibrate the CRT to a range of 0100mm, when using angle probes? a. V1, A2, IIW or Block No. 1 b. V2, A4 or Block No. 2 c. A5 or IOW Block d. a or b
35. Which scanning system shows time or distance on the horizontal axis and amplitude on the vertical axis of the display? 63 | P a g e
Ultrasonic Testing a. b. c. d.
A-Scan B-Scan C-Scan All of the above
36. The wavelength of ultrasound in a material may be calculated by: a. Dividing sound frequency by material velocity b. Multiplying sound frequency by material velocity c. Dividing material velocity by sound frequency d. Multiplying material velocity by sound frequency 37. What is the difference in signal heights of two large reflectors in the far zone of the ultrasonic beam when one is twice the beam path of the other? (Assume large to be larger than the width of the ultrasonic beam) a. 2dB b. 4dB c. 6dB d. 12dB 38. What is the difference in dBs, between a signal at 100% screen height and one at 25% screen height on the CRT? a. 20dB b. 14dB c. 12dB d. 6dB 39. At an interface, when the angle in the first medium has exceeded the second critical angle, the refracted sound in the second medium will be.. a. All reflected b. Compression and shear waves c. Shear waves d. Surface waves 40. Which calibration block could you use to check probe angles? a. A2 b. A5 c. V2 d. Any of the above 41. Which of the following methods could be used to set test sensitivity when scanning with angle probes? a. Grass b. Reference reflectors c. DAC Curves d. Any of the above
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Ultrasonic Testing 42. Which scanning system shows a cross-sectional view of a test piece on the display? a. A-Scan b. B-Scan c. C-Scan d. All of the above 43. Calculate the wavelength of sound waves in steel when using a 5MHz compression probe (Steel velocity is 5960) a. 0.84mm b. 0.00084mm c. 1.192m d. 1.192mm 44. What is the relationship between damping, pulse length & resolution? a. Damping controls pulse length b. Pulse length controls resolution c. Damping controls resolution d. All of the above 45. What would be the ratio of signal heights of two signals with a difference of 14dB between then? a. 14:1 b. 20:1 c. 5:1 d. 2:1 46. What is the first critical angle for a perspex to steel interface? a. 27.4˚ b. 35 ˚ c. 57.7 ˚ d. 80 ˚ 47. What is the thickness of the perspex insert in the IOW block? a. 23mm b. 25mm c. 50mm d. None of the above 48. How could you increase the sensitivity of an angle probe inspection? a. Use a smaller reference reflector to plot the DAC Curve b. Lower the gain setting c. Add reject or suppression d. Any of the above
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Ultrasonic Testing 49. Which scanning system shows a plan view of a test piece, on a printout rather than a display? a. A-Scan b. B-Scan c. C-Scan d. All of the above 50. A material velocity of 5000m/s and a probe frequency of 2.5MHz will produce a wavelength of 2mm. What happens when we change the probe for a 5MHz one? a. The velocity remains the same and the wavelength halves b. The velocity remains the same and the wavelength doubles c. The velocity halves and the wavelength remains the same d. The velocity doubles and the wavelength remains the same 51. What is PRF? a. Pulse rate frequency b. Probe rate frequency c. Probe repetition frequency d. Pulse repetition frequency 52. The piezo electric effect can perform which one of the following? a. Convert electrical energy into mechanical energy b. Convert electrical energy into signals c. Convert sound to mechanical energy d. All of the above 53. What is the second critical angle for a perspex to steel interface? a. 27.4˚ b. 35 ˚ c. 57.7 ˚ d. 80 ˚ 54. What is the thickness of the V2 calibration block? a. 12.5mm b. 20mm c. 25mm d. Any of the above 55. Which scanning pattern requires an angle probe to be manipulated through a small arc whilst maintaining the sound focused on a fixed reflector? a. Orbital b. Swivel c. Lateral d. Depth 56. What are the advantages of A-Scan system 66 | P a g e
Ultrasonic Testing a. b. c. d.
Portability and less set-up time High persistence screen allows a permanent record to be made Printout provides an instant permanent record More skill required to set-up
57. Which of the following best describes what we term ‘infrasound’? a. Sound waves with a frequency above 20KHz b. Sound waves with a frequency below 16KHz c. Sound waves with a frequency above 20Hz d. Sound waves with a frequency below 16Hz 58. What happens if an excessive PRF is set on the flaw detector? a. The equipment overheats b. Ghost echoes appear on the CRT c. The piezo electric effect is lost d. Grass appears on the CRT 59. Crystals used in ultrasonic testing, which produce the correct mode of vibration, are.. a. X Cut b. Y Cut c. Square d. Round 60. What is the practical range of shear wave probe angles for use on steel? a. 29˚ to 56˚ b. 35˚ to 80˚ c. 29˚ to 80˚ d. 35˚ to 56˚ 61. What is an alternative name for the A4 calibration block? a. Dutch block b. Sultzer block c. Kidney block d. Road block 62. Which scanning pattern requires an angle probe to be moved sideways along a fixed line? a. Orbital b. Swivel c. Lateral d. Depth
63. What are the advantages of B-Scan systems? a. Portability and less set-up time b. High persistence screen allows a permanent record to be made 67 | P a g e
Ultrasonic Testing c. Printout provides an instant permanent record d. More skill required to set-up 64. What range of sound wave frequencies does ultrasound exist in? a. All above 20KHz b. Above 20Hz and below 16KHz c. Above 16Hz and below 20KHz d. All below 16Hz 65. What is meant by transit time? a. The time taken, for a pulse of energy, from leaving the probe to travel though a material and return b. The time between pulses of energy leaving the probe c. The time between pulses of energy returning to probe and the signal appearing on the CRT d. The time taken for the piezo electric conversion of mechanical vibrations to electrical signals 66. Which of the following formulae determines the fundamental frequency of a piezo electric crystal? a. Ff = t 2V b. F = λx V c. Ff = V 2t d. F = t x V 67. A a. b. c. d.
probe which transmits sound into a material at 90˚ to the test surface is known as a… 90˚ Surface wave probe Normal probe a and b
68. What is the tolerance on dimensions of calibration blocks? a. ± 1mm b. ± 0.1mm c. ± 0.2mm d. There is no tolerance on calibration block dimensions 69. Which scanning pattern requires an angle probe to be rotated on the spot? a. Orbital b. Swivel c. Lateral d. Depth
70. What are the advantages of a C-Scan system? 68 | P a g e
Ultrasonic Testing a. b. c. d.
Portability and less set-up time High persistence screen allows a permanent record to be made Printout provides an instant permanent record More skill required to set-up
71. What are the range of frequencies of sound that the human ear can detect? a. 16KHz to 20KHz b. 16Hz to 20Hz c. 16Hz to 20KHz d. 16KHz to 20Hz 72. What is the clock interval? a. The time taken, for a pulse of energy, from leaving the probe to travel though a material and return b. The time between pulses of energy leaving the probe c. The time between pulses of energy returning to probe and the signal appearing on the CRT d. The time taken for the piezo electric conversion of mechanical vibrations to electrical signals 73. Which piezo electric crystal is the best transmitter of ultrasound? a. Quartz b. Lithium Sulphate c. Lead Zirconate Titanate d. Barium Titanate 74. Which probe would not have a dead zone? a. A compression probe b. A combined double probe c. An angle probe d. A soft nose probe 75. What is the tolerance on the amplifier accuracy check described in BS EN 12668: Part 3: 2000: clause 3.2.2.4? a. ± 20 dB b. ± 6 dB c. ± 2 dB d. ± 1 dB 76. Which scanning pattern requires that an angle probe is moved back and forth in the direction of the beam and perpendicular to the major axis of a defect? a. Orbital b. Swivel c. Lateral d. Depth
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77. What are the disadvantages of an A-Scan system? a. Less portability and more set-up time b. Low persistence screen allows a permanent record to be made c. No indication of defect depth or orientation d. More skill required to interpret signal 78. In a. b. c. d.
which probe would you find an acoustic barrier? A compression probe A combined double probe An angle probe A soft nose probe
79. What is the tolerance on the timebase linearity of a flaw detector, according to BS EN 12668: Part 3: 2000? a. ± 1% of the whole timebase b. ± 1% of each large division on the timebase c. ± 2% of the whole timebase d. ± 2% of each large division on the timebase 80. What does the horizontal base line or x-axis on the flaw detector display represent? a. Time taken for the sound wave to travel through the material b. The distance travelled by the sound wave c. The amplitude of sound energy returning to the probe d. a and b 81. What is an alternative term used to describe longitudinal waves? a. Compression waves b. Shear waves c. Surface waves d. Plate waves 82. Which piezo electric crystal is the best receiver of ultrasound? a. Quartz b. Lithium Sulphate c. Lead Zirconate Titanate d. Barium Titanate 83. Which of the following are not scanning patterns? a. Root scan b. Transverse scan c. Depth scan d. a and b
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Ultrasonic Testing 84. What are the disadvantages of a C-Scan system? a. Less portability and more set-up time b. Low persistence screen allows a permanent record to be made c. No indication of defect depth or orientation d. More skill required to interpret signal 85. Which probe would be best for use on a rough surface? a. A compression probe b. A combined double probe c. An angle probe d. A soft nose probe 86. Which of the following is true? a. The higher the signal noise ratio the less noise on the screen b. The lower the signal noise ration the less noise on the screen c. The signal noise ratio does not affect the clarity of the screen d. The signal noise ratio is the same on all flaw detectors 87. What is another name for the Dead Zone? a. Fresnel one b. Fraunhoffer Zone c. Damping d. Ringing Time 88. What is an alternative name used to describe transversal waves? a. Compression wave b. Shear wave c. Surface wave d. Plate wave 89. What are the advantages and limitations of modern ceramic crystal materials? a. Good wear resistance but poor piezo electric properties b. Good receivers of ultrasound but soluble in water c. Good generators of ultrasound but low mechanical strength d. All of the above 90. What is the formula for calculating the depth of a defect when the angle of probe (β) and sound beam path length (bp) are known? a. bp Cos β b. bp x Cos β c. bp x Sin β d. bp Sin β
91. What term is used to describe a system that sends out pulses of ultrasound then listens out for returning echoes? 71 | P a g e
Ultrasonic Testing a. b. c. d.
Pulse echo Through Transmission Tandem Immersion
92. Which transducer uses a low frequency magnetic field to generate ultrasonic vibrations? a. Soft nosed probe b. Wheel type probe c. Delay line probe d. Magnetostrictive coil 93. What is the thickness of the A2 calibration block? a. 12.5mm b. 20mm c. 25mm d. 50mm 94. What is another name for the Near Zone? a. Fresnel Zone b. Fraunhoffer Zone c. Damping d. Ringing time 95. What is an alternative term used to describe Rayleigh waves? a. Compression waves b. Shear waves c. Surface waves d. Plate waves 96. What is the law of reflection? a. The angle of incidence = the angle of reflection b. The angle of refraction = the angle of reflection c. The angle of incidence = the angle of refraction d. Sin α Sin β = V1 V2 97. What is the ratio of beam path to depth with a 60˚ probe? a. 1.414 : 1 b. 2 : 1 c. 2.9 : 1 d. None of the above
98. Which of the following techniques uses two probes on opposite sides of the test piece? a. Pulse echo b. Through transmission c. Tandem 72 | P a g e
Ultrasonic Testing d. Immersion 99. What do we mean by the “Bandwidth” of a probe? a. The probe’s diameter b. The centre operating frequency of the probe c. The range of frequencies of sound the probe transmits d. The range of voltages that the probe will operate at 100. If the smallest range an analogue A-Scan flaw detector can be calibrated to is 0 – 10mm then what is the best accuracy that can be read on the CRT? a. 0.5mm b. 0.4mm c. 0.2mm d. 0.1mm 101. What is another name for the Far Zone? a. The Fresnel zone b. The Fraunhoffer zone c. Damping d. Ringing time 102. What is an alternative term used to describe Lamb waves? a. Compression waves b. Shear waves c. Surface waves d. Plate waves 103. What is Snell’s Law? a. The angle of incidence = the angle of reflection b. The angle of refraction = the angle of reflection c. The angle of incidence = the angle of refraction d. Sin α Sin β = V1 V2 104. What is the ratio of beam path to depth with a 45˚ probe? a. 1.414 : 1 b. 2 : 1 c. 2.9 : 1 d. None of the above
105. Which of the following techniques uses two probes, both on same surface of the test piece? a. Pulse echo b. Through transmission c. Tandem d. Immersion 73 | P a g e
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106. What are the advantages of the pulse echo technique? a. Defects perpendicular to the surface may be located easily b. Probe frequencies up to 25MHz may be used c. Thicker or highly attenuative materials may be tested as the sound only has to travel one way through the material d. Defects can be located with great accuracy and access to only one surface is required 107. Which waves can propagate in any medium? a. Compression waves b. Shear waves c. Surface waves d. Plate waves 108. When thickness checking, using the multiple back wall echo method, the 8 th repeat signal from the test piece is on 60mm on the graticule. The CRT is calibrated 0 to 100mm range, what is the actual thickness of the test piece? a. 60mm b. 48mm c. 7.5mm d. 6.0mm 109. What is the ratio of beam path to depth with a 70˚ probe? a. 1.414 : 1 b. 2 : 1 c. 2.9 : 1 d. None of the above 110. How would you determine the angle of beam produced in a different material, by a probe that produces a 45˚ angle in steel? a. Use a block of the test material containing reference reflectors at known depths, to plot the beam angle b. Calculate the angle using Snell’s Law c. a and b would both be used together d. either a or b could be used 111. What effect does probe frequency have on crystal ringing time? a. Higher frequency = shorter ring time b. Higher frequency = longer ring time c. a and b d. Frequency does not affect ringing time 112. Which probe would have the best resolving power? a. 10mm diameter, 0˚, single crystal, 5MHz Broad Band probe b. 10mm diameter, 45˚, single crystal, 5MHz Broad Band probe c. 10mm diameter, 0˚, single crystal, 2.5MHz Narrow Band probe 74 | P a g e
Ultrasonic Testing d. 10mm diameter, 45˚, single crystal, 2.5MHz Narrow Band probe 113. What are the advantages of the through transmission technique? a. Defects perpendicular to the surface may be located easily b. Probe frequencies up to 25MHz may be used c. Thicker or highly attenuative materials may be tested as the sound only has to travel one way through the material d. Defects can be located with great accuracy and access to only one surface is required 114. What are boundary waves? a. Surface waves b. Plate waves c. Love waves d. All of the above 115. A thickness reading of 25mm is obtained on a copper plate, which has an acoustic velocity of 4768 m/s. The calibration block used to calibrate the CRT is of steel, acoustical velocity 5960 m/s. What is the actual thickness of the copper plate? a. 25mm b. 20mm c. 31.25mm d. 16mm 116. What is the ratio of surface distance to depth with a 70˚ probe? a. 1 : 1 b. 1.73 : 1 c. 2.75 : 1 d. None of the above 117. A change in wave form, together with a change in sound velocity, due to reflection or refraction is called… a. A spurious indication b. A mode change c. Pulse repetition frequency d. Diffraction
118. In which zone of the ultrasonic beam do the signal amplitudes become unreliable for comparing defect sizes? a. Far zone b. Fraunhoffer zone c. Dead zone d. Near zone 75 | P a g e
Ultrasonic Testing
119. Which probe would have the best penetrating power? a. 10mm diameter, 0˚, single crystal, 5MHz Broad Band probe b. 10mm diameter, 45˚, single crystal, 5MHz Broad Band probe c. 10mm diameter, 0˚, single crystal, 2.5MHz Narrow Band probe d. 10mm diameter, 45˚, single crystal, 2.5MHz Narrow Band probe 120. What are the advantages of the tandem technique? a. Defects perpendicular to the surface may be located easily b. Probe frequencies up to 25MHz may be used c. Thicker or highly attenuative materials may be tested as the sound only has to travel one way through the material d. Defects can be located with great accuracy and access to only one surface is required 121. What is acoustic impedance a product of? a. Material density and velocity of sound through it b. Sound velocity through material and its elasticity c. Material density and elasticity d. Material elasticity and specific gravity 122. Which of the following methods of setting test sensitivity can only be used with 0˚ probes? a. Back wall echo b. Grass level c. Using reference reflectors (e.g. transverse holes) d. DAC Curves 123. What is the ratio of surface distance to depth with a 45˚ probe? a. 1 : 1 b. 1.73 : 1 c. 2.75 : 1 d. None of the above 124. A series of high and low intensity waves radiating out from the tips of a narrow reflector as sound passes it, is called… a. A spurious indication b. A mode change c. Refraction d. Diffraction 125. What is the effect on sound beam near zone length, in the same material, when changing to a probe with the same diameter bit a lower frequency? a. The length increases b. The length decreases c. a and b d. Probe frequency has no effect on near zone length 76 | P a g e
Ultrasonic Testing
126. Which probe would have the least beam spread? a. 10mm diameter, 0˚, single crystal, 5MHz Broad Band probe b. 10mm diameter, 45˚, single crystal, 5MHz Broad Band probe c. 10mm diameter, 0˚, single crystal, 2.5MHz Narrow Band probe d. 10mm diameter, 45˚, single crystal, 2.5MHz Narrow Band probe 127. A change in beam angle or direction in the vertical plane at an interface, due to differences in material velocity, is called? a. A spurious indication b. A mode change c. Refraction d. Diffraction 128. An unwanted signal on the flaw detector CRT is called… a. A spurious indication b. A mode change c. Refraction d. Diffraction 129. What is the effect on sound beam near zone length, in the same material, when changing to a probe with the frequency but with a larger diameter? a. The length increases b. The length decreases c. a and b d. Probe diameter has no effect on near zone length 130. Which of the following combinations of characteristics would be used to asses, by calculation, the length of the dead zone? a. Crystal diameter, probe frequency and sound velocity b. K factor, sound wavelength and crystal diameter c. Crystal thickness and fundamental frequency d. None of the above
131. Which probe would have the shortest near zone? a. 10mm diameter, 0˚, single crystal, 5 MHz broad band probe b. 10mm diameter, 45˚, single crystal, 5 MHz broad band probe c. 10mm diameter, 0˚, single crystal, 2.5 MHz narrow band probe d. 10mm diameter, 45˚, single crystal, 2.5 MHz narrow band probe 132. What is the main advantage of a combined double probe over a single crystal probe of the same diameter? 77 | P a g e
Ultrasonic Testing a. b. c. d.
Better near surface resolution Better long range penetration Lower in cost Shorter near zone
133. As the acoustic impedance ratio of two materials at an interface increases, what effect does this have on the amount of sound energy transmitted across the interface? a. More energy is transmitted b. Less energy is transmitted c. The amount of energy transmitted remains the same d. No energy will be transmitted across an interface where acoustic impedances differ 134. It is calculated that when sound is travelling from water to steel 87% of the energy is reflected at the interface. How much energy is reflected when the sound is travelling from steel into water? a. 23% b. 13% c. 3% d. 87% 135. What are the disadvantages of the pulse echo technique? a. Only defects at a pre-determined depth may be located b. The sound has to travel through the material both ways causing more attenuation c. There is no indication of defect depth d. Access to both sides of the test piece is required
136. What are the disadvantages of the through transmission technique? a. Access to both sides of the test piece is required b. No indication of defect depth c. Loss of a couplant could be mistaken for a defect d. All of the above
137. When constructing a DAC curve using transverse drilled holes, which size hole would give the best sensitivity for the job in hand? a. 5mm b. 3mm c. 1.5mm 78 | P a g e
Ultrasonic Testing d. The size stated in the test instructions 138. Which of the following calibration blocks could be used to construct a DAC curve? a. V1, A2, I.I.W or No. 1 Block b. V2, A4 or No. 2 Block c. A5 or I.O.W Block d. None of the above 139.
What is the ratio of surface distance to depth with a 60˚ probe?
a. 1 : 1 b. 1.73 : 1 c. 2.75 : 1 d. None of the above 140. Which formula is used to calculate the irradiation factor, when scanning the circumference of a pipe? a. Sinβ = ID OD b. Sinβ = OD ID c. Cosβ = ID OD d. Cosβ = OD ID
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Ultrasonic Testing Multi-Choice Answers 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
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D B A B A D D A A B C B B C B B D D C D C D A B B B A D B C
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
C D A D A C C C A D D B D D C A D A C A D A C D A A D B A B
61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90
C C B A A C C B B C C B D B D D D B C D A B D C D A D B C B
91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120
A D C A C A B B C D B D D A C D A C C D A B C D B C B D C A
121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150
A A A D B B C A A D C A B D B D D C B A
