Principles of Strain Measurement
Strain-initiated resistance change is extremely small. Thus, for strain measurement a Wheatstone bridge is formed to convert the resistance change to a voltage change. Suppose in Fig. 3 resistances (Ω) are R1, R2, R3 and R4 and the excitation voltage (V) is E. Then,the output voltage e o (V) is obtained by the following equation:
Suppose the resistance R1 is a strain gage and it changes by ΔR due to strain. Then, the output voltage is,
If R1 = R2 = R3 = R4 = R in the initial condition,
Since R may be regarded extremely larger than ΔR,
Thus obtained is an output voltage that is proportional to a change in resistance, i.e. a change in strain. This microscopic output voltage is amplified for analog recording or digital indication for strain measurement.