Experimental stress analysis

Jeppiaar Engineering College AE2352

AE2352 Experimental Stress Analysis

EXPERIMENTAL STRESS ANALYSIS

LTPC3003

OBJECTIVE To bring awareness on experimental method of finding the response of the structure to different types of load. UNIT I MEASUREMENTS & EXTENSOMETER 10 Principles of measurements, Accuracy, Sensitivity and range of measurements. Mechanical, Optical Acoustical and Electrical extensometers and their uses, Advantages and disadvantages. UNIT II ELECTRICAL RESISTANCE STRAIN GAUGES 10 Principle of operation and requirements, Types and their uses, Materials for strain gauge. Calibration and temperature compensation, cross sensitivity, Rosette analysis, Wheatstone bridge and potentiometer circuits for static and dynamic strain measurements, strain indicators. UNIT III PHOTOELASTICITY 10 Two dimensional photo elasticity, Concept of light – photo elastic effects, stress optic law, Interpretation of fringe pattern, Compensation and separation techniques, Photo elastic materials. Introduction to three dimensional photo elasticity. UNIT IV

BRITTLE COATING AND MOIRE METHODS

8

Introduction to Moiré techniques, brittle coating methods and holography. UNIT V

NON – DESTRUCTIVE TESTING

7

Fundamentals of NDT, Radiography, ultrasonic, magnetic particle inspection, Fluorescent penetrant technique, Eddy current testing, Acoustic Emission Technique. TOTAL: 45 PERIODS

TEXT BOOKS 1. Srinath, L.S., Raghava, M.R., Lingaiah, K., Garagesha, G., Pant B., and Ramachandra, K., “Experimental Stress Analysis”, Tata McGraw-Hill, New Delhi, 1984. REFERENCES 1. Dally, J.W., and Riley, W.F., “Experimental Stress Analysis”, McGraw-Hill Inc., New York, 2005, IV edition. 2. Hetyenyi, M., “Hand book of Experimental Stress Analysis”, John Wiley and Sons Inc., New York, 1972. 3. Pollock A.A., “Acoustic Emission in Acoustics and Vibration Progress”, Ed. Stephens R.W.B., Chapman and Hall, 1993

Department of Aeronautical Engineering

Mr.Haston Amit Kumar (Assistant Professor)

Jeppiaar Engineering College


JEPPIAAR ENGINEERING COLLEGE DEPARTMENT OF AERONAUTICAL ENGINEERING AE 2352 EXPERIMENTAL STRESS ANALYSIS IMPORTANT 15 QUESTIONS UNIT - I 1. 2. 3.

Explain in detail the Principles of Measurements. What are the types of Mechanical Extensometers? Explain with aid of neat sketch Huggenberger extensometer. What is Strain Gauge? Explain with a neat sketch the working principle of Acoustical strain gage and measurements of strain using this gage. Also mention its advantages and disadvantages. UNIT - II 1. What are the different types of electrical strain gauges? Derive the expression of strain experienced by electrical resistance type strain gages. (ii) Explain how the electrical resistance type gage can be used to determine modulus of Elasticity and Poisson’s ratio of engineering materials 2. Determine the principal stresses and principal strains with the help of a delta rosette mounted on an aluminum specimen with values of εA= 400 µm/m; εB= 200 µm/m; εC= 100 µm/m, E= 70 Gpa, γ= 0.3. Also determine the principal directions and shear stress. 3. (i) Explain Wheat stone Bridge and obtain the balance condition.

(ii) Derive an expression for the output voltage measured from wheat stone bridge circuit. UNIT – III

1. Sketch a circular polariscope. Explain the effects of a stressed model and the fringes obtained in it 2. Describe in detail how fringe sharpening is obtained using partial mirrors. 3. Explain any two compensation techniques used in photo elasticity. Why Tardy’s compensation method is preferred over other methods? UNIT – IV 1. Explain the brittle coating method of stress analysis. Drive expression for brittle stress 2. What are fiber-optic sensors? What is their application in experimental mechanics? 3. Write notes on (a) moiré method of strain analysis, (b) Holography





UNIT – V 1. State the uses and advantages of non-destructive testing procedures. Explain in detail any one NDT procedure of evaluating a given specimen. 2. Explain in detail the principle of ultrasonic pulse echo testing and also explain the sequence of test and its application. 3. Explain any four

(a)Eddy current testing (b)Acoustic emission technique (c)Radiography (d) Fluorescent penetrant technique (e)Magnetic particle inspection (f)Resonance test





JEPPIAARENGINEERING COLLEGE DEPARTMENT OF AERONAUTICAL ENGINEERING AE-2352 Experimental Stress Analysis Two Mark Questions UNIT 1 MEASUREMENTS & EXTENSOMETER

1. Define Measurement: The measurement of a given quantity is essentially an act or result of comparison between a quantity whose magnitude (amount) is unknown, with a similar quantity whose magnitude (amount) is known, the later quantity being called a standard. 2. What are the basic requirements for measurement? (i) The standard used for comparison purposes must be accurately defined & should be commonly acceptable. (ii) The standard must be of the same character as the measure and (ie, the unknown quantity or the quantity under measurement) (iii) The apparatus used and the method accepted for the purposes of comparison must be provable. 3. What are the methods of measurements? (i) Direct Method: In these methods, MEASURAND is directly compared against a STANDARD. (ii) Indirect Method: Measuring systems are used in indirect methods for measurement Purposes 4. What are the uses of measuring instruments? Measurements involve the use of ‘instruments’ as a physical means of determining quantities (or) variables. In simple cases, an instrument consists of a single unit which gives an output reading or signal according to the unknown variable (MEASURAND) applied to it. 5. What is dimensional measurement? Dimensional Measurements are characterized as determination of size of an object. These are the measurements of dimensions of an object. 6. Explain the measurements of length and displacement A displacement means the movements of a point with respect to another point. Generally length (or change in length) means it referring a distance between two reference points. Thus measurement of length includes both the dimensional measurement and the displacement measurement. 7. What are the ‘STANDARD’S for the measurement of an angle? The primary standards of angle may be taken either as angle subtended by a circle at its centre ie, 360° or as angle between two straight line intersecting in a manner such that all four angles so formed are equal, ie, each angle is 90° 8. How we can measure the Area of survey plats? Department of Aeronautical Engineering




Measurement of area of regular and standard geometrical figures can be obtained from the dimensions using standard mathematical relationships. There are many engineering applications which require the measurement of plane area. Graphical determination of the area of survey plots form maps, the integration of function to determine the area enclosed by a curve analysis of experimental data may require the use of measurement of plane area. 9. Give any two methods for measure an unknown force An unknown force may be measured by the following methods, 1. Balancing the unknown force against the known gravitational force either directly (or) indirectly using a system of levers. 2. Transferring the unknown force to a fluid pressure and then meaning the resulting fluid pressure. Hydraulic and Pneumatic load cells are used for transferring the force into pressure. 10. How we can measure the temperature changes? Temperature is not measured directly, but is measured through indirect means; change of temperature of a substance causes a variety of effects. These effects may be physical, chemical, electrical (or) optical and they may be used for the measurement of temperature through use of proper temperature sensing devices. 11. Tell some thing about ‘static characteristics’ and ‘static calibration’ in measurements: ‘Static characteristics’ of a measurement system are in general those that must be considered when the system or instrument is used to measure a condition not varying with time. All the static performance characteristics are obtained in one form or another by a process called’ static calibration’ 12. What is accuracy and tell about point accuracy? Accuracy is the closeness with which an instrument reading approaches the true value of the quantity being measured. Thus accuracy of a measurement means conformity to truth. ‘Point Accuracy’ is the accuracy of the instrument only at one point on it scale. 13. Give the Type of errors in measurements (i) Gross errors (ii) Random errors (iii) systematic errors - Instrumental errors - Environmental errors - Observational errors 14. Differentiate Gross error and Instrumental Error Gross errors mainly cover human mistakes in reading instruments and recording and calculating measurement results. Instrumental Error occurs due to the wrong adjustments of a measuring devices while it construction or misalignments of its pats.





15. Differentiate environmental and systematic errors Environmental errors causes due to the surrounding condition of instruments, but systematic errors causes because of the fault of the arrangement of measuring system during the whole measuring process. 16. What absolute static error? Give example Absolute static error is the difference between measured values of quantity and the true value of quantity. ie, Eo= X(m) – X(t) Example: an error of 2mm is negligible when the length being measured is of the order of ( 1000 mm) 1 m but the same error of 12 mm may be considerable when the measurement of 10mm length 17. What is relative static error? It is the ratio between absolute error to the true value of quantity ie, Er= Eo/Xt 18. What is static correction? Static correction is the negative value of absolute static error ie, Cs= -Eo 19. A meter reads 127.50V and the true value of the voltage is 127.43v. Determine (a) static error and (b) static correction for this instrument. Sol: (a) static error (E0) = Xm- Xt = + 0.07V (b) static correction (Cs)= – Eo= -0.07V 20. a thermometer reads 95.45 0c and the static correction given in the correction curve is – 0.08 0c Determine the true value of the temperature Sol: Cs= - ( Xm- Xt) ie, Xt= Cs + Xm = -0.08 + 95.45 = 95.37 °C 21. An inclined Limb manometer is used for measurement of flow rate reads 0.161 x 10- ³m³/s. The true value of flow rate is 0.159 x 10-³ m³/s. Determine (a) static error (b) static correction. Sol: Eo= Xm – Xt = ( 0.61- 0.15) x 10-³ = 0.002 x 10-³ m³/s 22. What do you mean by limiting errors? The limits of the deviations from the specified values of measurement datas are defined as ‘Limiting errors’ (or) ‘Guarantee error’ 23. Determine relative limiting error: The relative error (fractional error) is defined as the ratio of the error to the specified magnitude (nominal magnitude) of a quantity. Relative limiting error A Limiting error ( Er) = ____ = ____________________ As

A Normal Value

24. What is an uncertainty in a measurement? Un certainty of measurement is a parameter associated with the result of measurement, that characterizes the dispersion of values that could be reasonably attributed to the measurand. Department of Aeronautical Engineering




25. Define static sensitivity The static sensitivity of an instrument or an instrumentation system is the ratio of the magnitude of the output quantity to the magnitude of input (quantity being measured) 26. What is known as deflection factor? Deflection factor or inverse sensitivity is the reciprocal of the static sensitivity. 27. Define scale range. The scale range of an instrument is defined as the difference between the largest and the smallest reading of the instruments. 28. What is the frequency range in measurements? Frequency range is defined as frequency over which measurements can be performed with a specified degree of accuracy.




AE2352 Experimental Stress Analysis UNIT II

ELECTRICAL RESISTANCE STRAIN GAUGES

1. What is extensometer? Extensometer is an instrument used top measure minute deformation of material while it is subjected to a stress. 2. Classify the extensometers depending upon the magnification systems. 1. Mechanical Extensometers 2. Optical extensometers 3. Acoustical (Vibrating wire) extensometers 4. Electrical extensometers 5. Pneumatic extensometers. 3. What are the Basic Requirements of extensometer? 1. Very high magnification:The magnification required is usually greater than 1000: 1 2. Sensitivity: The relation between input and output should not be affected by the reversal in the direction of input and this requires that the movement should not have any friction. 3. Low input force: The input force required to cause displacement should be extremely small thus there is no defamation of the component due to the process of measurement. 4. Classify the mechanical extensometers depending upon the manner of obtaining the magnification. 1. Wedge magnification 2. Screw magnification 3. Compound magnification 4. Lever magnification 5. Give the disadvantages of a simple mechanical lever magnification For strain measurements on inner surfaces of small structures, the clearance may be in sufficient for the gauge and all its auxiliary equipment. 6. Give the magnification and gauge length of porter- Lipp strain gauge. Gauge length is 25 mm Magnification is 300 times 7. Give the advantages of porter- Lipp gauge 1. Very compact 2. Light weight 8. Give the magnification capacity & gaugelength of Huggen-Berger tensometer The magnification may vary from 300 to 2000 depending upon the model. The gauge length varies from 12.5 to 25 mm.





9. What is the measurement capacity of C.E. John extensometer? C.E. Johnson extensometer can measure strain up to 5 micro strains over a gauge length of 50 mm. 10. Give the minimum strain value that scratch gauge can be sense. The minimum strain that a strain gauge can sense is above 100 micro strains. 11. Give the advantages of scratch gauge 1. Compact in size 2. It can be attached to almost any surface with clamps or screws & adhesive bonding 3. It can measure scratch under all types of loading (static, shock, fatigue) 4. It can be used to record stress in all types of environments. 12. Tell something about the magnification capacity of Marten’s extensometer This instrument is extremely sensitive and by using telescope, regarding may be estimated to 1/10 mm which corresponding to a length charge of approximately 0.0002m. 13. What is the sensitivity of Tuckerman gauge? The sensitivity of tucker man gauge is 2 micro strains 14. For the following reading find the deformation sensitivity & strain sensitivity for Tuckerman optical gauge. Base length = 25 mm, d= 1/20, magnification factor = 5 solution Ds=d/m = 1/(20x5)= 0.01mm Ss= Ds/L = 0.01/25 = 4 x 10-4 strains 15. Give the formula to obtain fundamental frequency for a vibrating wire

Where A = Cross sectional Area of vibrating wire E = Young’s moduls of wire material L = Length of Vibrating wire M = Mass for unit length P = tensioning force in the wire L = incremention length of the vibrating wire. 16. Give the classifications of Electrical strain gauges (a) inductance (or) magnetic strain gauges (b) Electrical resistance strain gauges (c) The capacitance strain gauges. 17. How strain can be measured by using Electrical strain gauges? Electrical strain gauge is a device in which a change in length produces a change in some electrical characteristics of the gauges.





18. Give the working principle of inductance strain gauges? An electrical inductance gauge is a device in which the mechanical quantity to be measured produced a change in the magnetic field, and hence in the impedance of a current carrying coil. 19. Give the working principle of variable air gap gauges In this types of gauges the reluctance of the magnetic field is varied by changing the air gap. 20. Give the working principle of movable core solenoid gauges In this type of gauges the reluctance of the magnetic circuit is varied by changing the position of the iron core in the coil. 21. Give the working principle of eddy- current gauges In this types of gauges the losses in the magnetic circuit are varied by changing the thickness or position of the high- loss element inserted in the magnetic field. 22. Give the formula for the impedance of a coil to the passage of alternating current. Z=¥ (2pfl) ² + R² Where Z= impedance in ohms F= frequency in herzs L= inductance of the coil in henrys R= resistance component in ohms 23. How the displacement of material structures can be measured by using electrical resistance strain gauges? In the electrical resistance strain gauges the displacement (or) strain is measured as a function of the resistance change produce by the displacement in the gauging circuit. 24. What is known as bonded strain gauges? A thin paper sheet or metal sheet with strain gauge wire is bonded with an adhesive material to the structure under strictly. 25. What is the use of gauge factor in strain gauge measurements? Gauge factor is used to determine the sensitivity of strain gauges. 26. Give the requirements for strain gauges? Gauge factor should be high Wires used in a strain gauge should have high resistance. The wires should have a linear relationship between strain & resistance of the wire. 27. What is the resistance of material? Resistance is the property of a material that restricts the flow of charges through the material.





28. What is known as foil strain gauges? The foil strain gauge has metal foil photo- etched in a grid pattern of the electric insulator of the thin resin and gauge leads attached. 29. Give the formula for electrical capacity between parallel plates in capacitance strain gauges C= [8.86 x 10-3 KA (N-1)]/H Where c- capacitance in picofoarads K- dielectric constant of the medium between the two plates A- area of one sife of the plate, mm2 N- No of plates H- distance between plates in mm 30. What is the basic principle of unbonded metallic strain gauges the principle of unbonded metallic gauges is based on change in electrical resistance of a metallic wire due to change in tension of the wire. 31. What is photo-etching? Photo etching is the act of producing grid configuration on metal foil with the help of photo effect. 32. What is known as strip gauges? The number of strain gauges are arranged in a particular manner is called strip gauges. 33. What is known as strain rosettes? An arrangement of strain gauges in particular orientation or angle is known as strain rosettes 34. Give the quantities required for a good gauge material? High gauge factor High resistance Low temperature sensitivity High electrical stability Good corrosion resistance High yield point stability 35. Give some arrangements of strain gauges to obtain strain rosettes 1. Two gauge rosette 2. Rectangular rosette a) Three element b) Four element 3. Delta (or) Equiangular rosette 4. T. Delta rosette





36. Give the formula for strain measured by a strain gauge in particular angles

eq= ½ (ex + ey) + ½ (ex- ey) cos2q + ½ ¡xy sin2q Where eq is the strain value in q direction

37. Give the relation on between principal stress and principal strains

s1=[E/(1-v²)](e1+ve2) and s2=[E/(1-v²)](e2+ve1) where s1 and s2 are principal stresses e1 and e2 are principal strains

38.What are the methods are available for computing the strain rosette datas? 1. Analytical Solutions 2. Graphical Solutions 3. Semi – graphical (or) vectorial layout method 4. Nomographic Solutions 5. Geometrical Computers. 39. Give the advantages of strain Rosette analysis. (i)Extreme simplicity and speed of application. (ii)Possibility of allowing for transverse effects. (iii)No requirements for additional equipments. (iv) The possibility of training relativity unskilled persons to use the method. 40.Give the type of strain gauge circuits. 1. Wheat stone bridge circuits for static strain measurements. 2. Potentiometer circuits for dynamic strain measurements. 41. Define sensitivity of potentiometer: Sensitivity of the potentiometer circuit is defined as the ratio of the output voltage divided by the strain.




AE2352 Experimental Stress Analysis UNIT III PHOTOELASTICITY

1. Define Light Light is usually defined as the radiation that can affect the human eye. 2. Define White light: Light from a source that emits a continuous spectrum with equal energy for every wave length is called white light. 3. Define monochromatic light: Light of a single wave length is called monochromatic light. Example: sodium vapour lamp, Hg arc lamp. 4. What is wave front? The focus of points on different radial lines from the source exhibiting the same disturbance at a given instant of time, in a surface known as a wave front. 5. Define Ray? A line normal to the wave front, indicating the direction of propagation of the waves is called a ray. 6. What are longitudinal waves? The waves in which vibration are along the direction of their travel is known as longitudinal waves. 7. What are transverse waves? Transverse waves are waves in which vibrations are perpendicular to the direction of wave travels. 5. What is polarized light? The light having vibration only along a single straight line perpendicular to the direction of propagation of light is said to be polarized. 9. What is plane of polarization? The plane containing the direction of propogation of light, but containing no vibrations is called the plane of polarization. 10. What are the methods are available to obtain plane polarized light? (i) Refraction and Reflection for glass plate. (ii) By using Nichol Prism. 11. What are the disadvantages are available in Nichol prism when we use that to obtain plane polarized light? 1. Costly 2. Intensity is Poor.





12. What is the basic principle for photo elasticity? When polarized light enters a loaded transparent component, it is split into two beams both beams travel along the same path, but each vibrates along a principal direction and travels at a speed proportional to the associated principal stress. 13. What is Refractive index? The ratio of the velocity of light In air to the velocity in the medium is called the refractive index of the medium and is denoted by ‘n’. 14. What is wave length? The wave length of any given frequency in the distance traveled during one complete vibration. 15. Define Amplitude? Amplitude is a magnitude of the disturbance, the intensity of light may be taken as directly proportional to the square of the amplitude of the vibration. 16. Define phase The phase of vibration at any instant defines the stage of the cycle reached at that instant. 17. Give the general equation of motion of a transverse light wave propagating in Zdirection. E = a cos( -wt) Where E = magnitude of light vector t = amplitude of the wave. 18. What do you mean by Polariscope? Polariscope is an optical instrument that utilizes the properties of polarized light in its operation. 19. Give the types of polariscopes used in experimental stress analysis? (i) Plane Polariscope (ii) Circular Polariscope 20. Give the four different possible setups in circular polariscope? Setup Polariser- Analyzer Quarter wave plates Field

21. What is the standard setup in circular polariscope? The crossed – crossed setup is called the standard setup of the circular polariscope. Department of Aeronautical Engineering




22. State Stress optic law: The relative change in index of refraction is proportional to the difference of principal stresses. 23. What is known as isotropic point in a polariscope setup? In a particular point in a stressed model where that fringe order is zero and permanent block dots appear at these points such are called isotropic points. 24. What is known as singular points in a polariscope setup?

25. What are the properties of photo elastic materials? 1. Transparent to light used in the Polariscope. 2. Easily machinable by conventional means. 3. It should be force form residual stresses 4. They should have both mechanical and optical isotropy and homogeneity. 26. Give the most commonly used methods for compensation techniques 1. Babinet compensation method. 2. Babinet soleil compensation method 3. Tension or compression stop method. 4. Tardy method of compensation. 5. Senarment method of compensation 6. Photometric method. 27. What are the techniques used to determine the stresses at the inner layers of the body in 3D photo elasticity? 1. Locking in the stresses in the model 2. Multilayer reflection technique.




AE2352 Experimental Stress Analysis UNIT IV


1. Give some advantages of Brittle coating method. - Provides nearly whole field area - Is non destructive if the coating is sensitive enough. - Is simple to analyze 2. Give some advantages by using fiber optic sensors. - Light weight (very small in size) - Low power requirement - Resistant to electromagnetic interference - High sensitivity




AE2352 Experimental Stress Analysis UNIT V NON – DESTRUCTIVE TESTING

1. Define Non – destructive testing Non –destructive testing is a technique for revealing flaws and defects in a material or device without damaging as destroying the test sample. 2. Give some advantages of NDT: 1. Can be done directly on production items without regards to part cost or quantity available. 2. Can be done on 100% of production of on representative samples. 3. Give the limitations of NDT: 1. Results often must be interpreted by a skilled. Experienced technician. 2. In absence of contact with each other, different observers may disagree on meaning & significance of test results. 4. What is neutron Radiography? Radiographic inspection using a stream of neutrons known as neutron Radiography. 5. Give the advantages of Radiographic inspection. - The ability to detect internal flows - An ability to detect significant variations in composition. - Permanent recording of raw inspection datas.




AE2352 Experimental Stress Analysis PART B (16 MARK QUESTIONS) UNIT 1

MEASUREMENTS & EXTENSOMETER

4. 5.

3. 4. 5. 6. 7.

Explain in detail the Principles of Measurements. Write short notes on: (a) Accuracy (4) (b) Sensitivity (4) (c) Range (4) Write a short account of the various types of strain gauges. Give their special advantages and limitations. What are the basic characteristics of a strain gauge? Which factors should be considered What are the various types of Mechanical strain gauges? Explain Huggenberger tensometer in detail. What are the various types of optical strain gauges? Explain the Tuckerman gauge in detail. Explain the construction and working of Acoustical strain gauge. UNIT II ELECTRICAL RESISTANCE STRAIN GAUGES

1. What are the different types of electrical strain gauges? Describe a capacitance strain gauge and give its uses and limitations. 2. What is the necessity of temperature compensation? How this can be achieved? 3. What do you understand by a strain rosette? What are the different types of strain rosette configurations currently in use? Discuss their uses and limitations. 4. Discuss the various methods of calibrating a strain gauge. 5. What are the essential requirements of a balancing technique? Discuss the different ways in which you can balance a bridge. 6. Discuss the associated instrumentation for measuring i. Strain strains ii.Dynamic strains

UNIT III PHOTOELASTICITY

4. Sketch a circular polariscope. Explain the effects of a stressed model and the fringes obtained in it 5. Sketch a plain polariscope. Explain the effects of a stressed model and the fringes obtained in it. 6. Describe in detail how fringe sharpening is obtained using partial mirrors. 7. Explain any two compensation techniques used in photo elasticity. 8. What are the important properties of an ideal photoelastic material? Discuss the important photoelastic materials. 9. Explain the Tardy’s compensation method in detail. Why this method is preferred over other methods? Department of Aeronautical Engineering



AE2352 Experimental Stress Analysis UNIT IV


1. Explain the brittle coating method of stress analysis. Drive expression for brittle stress 2. What are fiber-optic sensors? What is their application in experimental mechanics? 3. Write notes on (a) moiré method of strain analysis, (b) Holography UNIT V NON – DESTRUCTIVE TESTING

1. State the uses and advantages of Non-Destructive Testing procedures. Explain in detail any one of the NDT procedure for evaluating a given specimen. 2. Write short notes on the following: a. Eddy Current Testing b. Radiography 3. Explain the process of Magnetic particle inspection with a neat sketch. 4. With a suitable sketch explain the process of Fluorescent penetrant technique. 5. Explain the brittle coating method in brief. What are the advantages and limitations of this method? 6. Briefly explain the following which are used in the experimental analysis: a. Fibre – optic sensors b. Thermography c. C – Scan



Experimental stress analysis

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