The use of potential drop techniques for the evaluation of environment assisted cracking of austenitic alloys

The DC potential drop technique has been adapted for strain measurements. An AC potential drop technique has been developed for crack detection. It has been demonstrated that a DC with switched polarity can have a strain accuracy of ±0.12% after yielding and a multi-frequency AC system with up to 200 kHz frequency can have a sensitivity of 50 μm for crack detection. The techniques have been applied to environment assisted cracking tests of Ni-Cr-Fe alloys in 350 degrees C water and austenitic stainless steel in 288 degrees C water using both static and dynamic loading. Long term signal stability is maintained given: (1) rigid probe attachments; (2) local preamplification; and (3) adequate lead grounding and shielding. The AC potential drop technique is found to be more suitable for constant load than for dynamic loading and is also compatible with aggressive environments. During dynamic loading, sensitivity can be significantly reduced due to serrated yielding. An analytical model has been developed to predict the DC and AC potential drops in a round bar geometry. A sensitivity analysis has been made to determine test to test variation associated with variations in supplied current, specimen dimensions, and probe spacing. Multifrequency analysis shows that the measured data agrees with the prediction at frequencies up to about 100 kHz. Above 100 kHz induced signal in the probes results in increases in potential drop and phase angle. The induced signal is reproducible and therefore may be related to the crack mouth opening displacement