A multi-frequency AC potential drop technique for the detection of small cracks

DC potential drop techniques have been widely used to measure crack length. However, the sensitivity of the technique is often insufficient for detection of small cracks initiating from a smooth surface. By using a multifrequency ac potential drop technique, a resolution of 50 mu m has been achieved for multiple crack initiation with a probe spacing of 10 mm during slow strain rate tensile tests. The sensitivity is reduced for single crack initiation for a given probe spacing. For single crack initiation 50 mu m resolution is obtained by decreasing the probe spacing to 1.2 mm. Data reproducibility is found to be adequate when probe spacing and configurations are made consistently. The technique has been applied to environment-assisted cracking tests of Ni-base alloys at 350 deg C in water using both static and dynamic loading. Long-term signal stability is achieved given: (i) rigid probe attachments, (ii) preamplification, and (iii) 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 ac potential drop in a round bar geometry. The measured data agree with the prediction at frequencies up to approx 100 kHz. Above 100 kHz, an induced signal in the probes results in increases in potential drop and phase angle. A Nyquist analysis of the multifrequency data shows an increasingly inductive spectrum as the crack grows. The induced signal was reproducible and therefore may be related to the crack mouth opening displacement.