Tensile tests are to be carried out when evaluating the strength of metallic materials. In order to detect small changes in metal cracks or shape, you can measure the AC resistance of sample (called AC potential method). Since the resistance of metal is very low, the resulting voltage is also very weak, so it’s useful for a lock-in amplifier. 

Figure 1 is a block diagram of the measurement.By comparing with reference samples without stress, errors caused by factors such as temperature changes can be avoided and the first minute cracks or changes in shape can be detected.

Figure 1.Tension test for metallic materials (AC potentiometric method)

Since the output impedance of the signal is very low, the S/N can be improved by using transformer boost. Because the transformer can insulate the grounding ring, it can prevent common-mode mixing of the reference signal.

Before adding stress, the current and phase flowing through the reference sample should be adjusted, and the measurement can be started after the output of the phase-locked amplifier is adjusted to 0.