: To learn the use of strain gauges for measuring strain in bending.
Model beam
Strain Gauges (cemented to beam, Gauge Factor = 2.05)
Strain meter (gives direct strain reading)
45volt DC battery
Pressure cell and Hydraulic Jack
Vernier Caliper
Measuring Tape
Connecting wires
It Is The Ratio Of Change In Length To Original Length.
=
It Is The The Chsnge In Angle Between Two Two Planes Which Are Originally At At Right Angle .
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Optical strain gauge
Electrical resistancr strain gauge
Semiconductor strain gauges:
Thin-film strain gauges:
Diffused semiconductor strain gauges:
Bonded resistance gauges
These Are Very Fine Metal Grid Having Some Paper Base And Are Cemented On Structure Component To Measure The Normal Surface Strain In Any Desired Direction.
These gauges use a light beam, two fine gratings, and a photocell detector to generate an electrical current proportional to strain. A photoelectric gauge can be as short as 1/16 inch but its usage proves to be extremely costly and delicate. In 1938, the first bonded, metallic wire-type strain gage was introduced. The metallic foil-type strain gage is constructed of a grid of wire filament of approximately 0.001 in thickness, bonded directly to the strained surface by a thin layer of epoxy resin. When a load is applied to the surface, it gets strained and experiences a change in length. This resulting change in length is conveyed to the resistor and the corresponding strain is measured in terms of the electrical resistance of the foil wire, which varies linearly with strain. Other types of Strain Gauges are described below. FAISAL SARDAR [2009-CIV-122 ] Page 2
In the year 1970, the first semiconductor strain gages were developed for the use in automotive industry. Semiconductor strain gauges exhibit following key features: 1) Unlike other strain gauges, semiconductor strain gages are based upon the piezoresistive effects of silicon or germanium and measure the change in resistance with stress as opposed to strain. 2) The semiconductor bonded strain gage is a wafer with the resistance element diffused into a substrate of silicon. 3) No backing is provided for the wafer element and bonding it to the strained surface needs extra care since only a thin layer of epoxy is used to attach it. 4) Size of a semiconductor strain gauge is much smaller and the cost cos t much lower than for a metallic foil sensor.
Thin-film strain gage is more advanced form of strain gauge as it doesn’t necessitate adhesive bonding. A thin film strain gauge is constructed by first depositing an electrical insulation, usually a ceramic onto the stressed metal surface, and then depositing the strain s train gage onto this insulation layer. Techniques used to bond the materials molecularly are:
Vacuum deposition
Sputtering method
A further improvement in strain gage technology was introduced with the advent of diffused semiconductor strain gages since they purge the need for bonding agents. Main features are listed below: FAISAL SARDAR [2009-CIV-122 ] Page 3
By eliminating bonding agents, errors due to creep and hysteresis also are eliminated.
The diffused semiconductor strain gage employs photolithography masking techniques and solid-state diffusion of boron to molecularly bond the resistance elements.
Diffused semiconductors are frequently used as sensing elements in pressure transducers.
Following are the chief characteristics of bonded resistance strain gauges:
They are reasonably inexpensive.
They can pull off overall accuracy
of
better than ±0.10%.
They are available in a short gauge length and have small physical size.
These strain gauges are only moderately affected by temperature changes.
They are extremely sensitive and have low mass.
Bonded resistance strain gages can be employed to measure both static and dynamic strain.
The use of strain gauges is based on the fact that the resistance of a conductor changes when the conductor is subjected to strain Let R = Resistance of conductor L = Length of conductor A = X-sectional area of conductor ρ = Resistively of conductor
Change in Length = ΔL Change in Resistance = ΔR FAISAL SARDAR [2009-CIV-122 ] Page 4
Construction of electrical resistance strain gauges involves bringing together the optimum combination of electrical resistance material and backing plate. For a good strain gauge, some of the most important features are listed below: Length of Filament = 2-25 mm, For Tension Test = 10-25 Mm For Comp Test = 2-4 Mm Diameter of Wire = 0.025mm Range of Gauge Resistance = (30-3000) ohm Commonly used values are (100, 120, 350, 1000) ohm Safe Current = 25-50 mA Range of voltage = 35-50 Volts (we use 45volts
Metals for strain gauge should be very much sensitive with change in length. Proportional limit for strain gauge material should should be more than the elastic limit of structural component
It Is Ratio B/W Rate Of Change Or Resistance And Rate Of Change Of Length Denoted By
Ka = Range of gauge Factor
= 1.9 – 4
Gauge Factor which we use in lab = 2.05 A value of axial sensitivity is provided by manufacturer.
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The simple method of measuring the changes in resistance is by means of wheat stone circuit method, in which four arms of bridge contain resistance R1, R2, R3, R4. Potential diff b/w pt A and pt. B is measured with help of galvanometer and power is supplied using battery or adapter. According Kirchhoff’s law for floe of current in circuit and ohm’s law for potential diff, we have a relation of
If resistance of wheat stone bridge circuit is replaced by strain gauge called as active strain gauges. At time when load is zero, strain gauge reading is also zero. So L.H.S=R.H.S Now turn the 2nd reading. The difference of two reading is change in resistance; finally strain can be computed by using
This method is known as null method for measurement of change of resistance.
Rate of change of temp is directly proportional to the rate of change of length which is proportional the rate of change of resistance. (ΔT/T) α (ΔL/L) α (ΔR/R) Effect of temperature can’t be ignored even if a single degree changes in position.
The simplest solution to compensate temperature is installing a dummy gauge.
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It is a strain gauge having a same sensitivity and less an active strain gauge. It is cemented or pasted on a separate piece of metal as structural component and put in a position close to structural component. Dummy form one arm of Wheat Stone Bridge; so that any change in resistance duet to temp of the active gauge accrue in Dummy gauge and compensation leaves the balance bridge unordered.
Ra /R /R3 = Rd /R4 Humidity can affect result of gauges in two ways Due to corrosion. Due to short circuiting of filaments.
After cementing the strain gauges to structural component, apply the few coats of water proofing agent like
Measure the beam dimensions with the help of vernier ve rnier calliper.
Paste the strain gauges with the help of glue at the required locations and note the location of the gauges.
Complete the circuit by using wires and also attach the dummy strain gauge to the circuit.
Switch the strain meter on and check the setting of gauge factor and the voltage applied to the circuit. Take zero loads reading for each gauge and proving ring.
Apply the load in three equal increments increme nts over the range.
Unload and check the gauge zeros. If large difference between initial and final zero are detected then repeat the experiment of the gauges concerned.
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