The most widely used gage is the bonded metallic strain gage. The metallic strain gage consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction. The cross sectional area of the grid is minimized to reduce the effect of shear strain and Poisson strain. The grid is bonded to a thin backing, called the carrier, which is attached directly to the test specimen. Therefore, the strain experienced by the test specimen is transferred directly to the strain gage, which responds with a linear change in electrical resistance. Strain gages are available commercially with the most common nominal resistance values of 120 Ω, 350 Ω, and 1,000 Ω.
In practice, the strain measurements rarely involve quantities larger than a few millistrain. Therefore, to measure the strain requires accurate measurement of very small changes in resistance. The gage factor is a strain gage's sensitivity to strain, expressed quantitatively as ratio of fractional change in electrical resistance to the fractional change in length. For example, suppose a test specimen undergoes a substantial strain of 500 me (microstrain). A strain gage with a gage factor of 2 exhibits a change in electrical resistance of only 2*(500*10-6 ) = 0.1%. For a 120 Ω gage, this is a change of only 0.12 Ω. To measure such small changes in resistance, and compensate for the temperature sensitivity, strain gages are almost always used in a bridge configuration with a voltage or current excitation source.
The general Wheatstone bridge circuit, illustrated below, consists of four resistive arms with an excitation voltage, VEX, that is applied across the bridge. The output voltage of the bridge, VO, is equal to:
Figure 1. Wheatstone Bridge Circuit
From this equation, it is apparent that when R1 /R2 = R4 /R3, the voltage output VO is zero. Under these conditions, the bridge is said to be balanced. Any change in resistance in any arm of the bridge results in a nonzero output voltage. Therefore, if we replace R4 in Figure 1 with an active strain gage, any changes in the strain gage resistance unbalance the bridge and produce a nonzero output voltage that is a function of strain.