Sensitivity analysis of a directional potential drop sensor for creep monitoring

Recent research efforts indicated that directional potential drop (PD) measurements could be exploited for in-situ creep monitoring. The present work investigates the sensitivity of such measurements with a square-electrode configuration to geometrical and material variations caused by creep. This measurement technique is based on a modification of the conventional PD technique as it measures simultaneously two resistance values in orthogonal directions. Under uniaxial stress condition, the ratio of the lateral and axial resistances is roughly proportional to the applied strain. Experimental tests showed that small anisotropic changes in the resistance ratio caused by directional effects of creep can be distinguished from potentially far larger isotropic changes caused by non-directional reversible and irreversible thermal effects. The sensitivity of the square-electrode PD sensor to geometrical and material variations was analyzed separately and the analytical predictions were validated by experimental tests in both cases. The directional PD technique was found to exhibit high sensitivity that allows the detection of elastic and plastic strains as low as 0.05%. Additional experimental results from an accelerated 400-hour creep test are presented to demonstrate the feasibility of this technique to monitor creep degradation in 304 stainless steel at 600 °C.