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.