L. (When compressed, it is conversely reduced, i.e. L-L). The thickness D reduces to D-D. (When compressed, it is conversely extende, i.e D+D).The amount indicated by the following equation is called normal or longitudinal strain.
External force applied to an elastic material generates stress as shown below, which subsequently generates strain in the deformation of the material. At this time, the length L extends to L+L. (When compressed, it is conversely reduced, i.e. L-L). The thickness D reduces to D-D. (When compressed, it is conversely extende, i.e D+D).The amount indicated by the following equation is called normal or longitudinal strain.
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 : strain
L: original length
D: original thickness
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Example: when a material of 100mm length deforms by 0.1mm long, it generates strain as follows:
=L/L=0.1mm/100mm=0.001=1000µm/m
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A strain gauge is constructed by bonding a fine electric resistance wire or photographically etched metallic resistance foil to an electric insulation base using an appropriate bonding material, and attaching gauge leads. |
The strain generated in the specimen is relayed through the base to the fine wire or foil, where expansion or contraction occurs. As a result, the fine wire or foil experiences a variation in resistance. This variation is exactly proportional to the strain.
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: strain
R: gauge resistance
k: gauge factor as shown on package |
Normally, this resistance change is very small and requires a Wheatstone bridge circuit to convert it to voltage output.
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Ua: voltage output
Ue: exciting voltage R1: gauge resistance R2 ... R4: fixed resistance |
Assuming the value R such that R = R1 = R2 = R3 = R4, the active gauge resistance varies to R+R due to strain. Thus, the voltage output due to strain is given as follows:
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