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 this technique is often insufficient for detection of small cracks initiating from a smooth surface. By using a multi-frequency 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 nickel-base alloys at 350 degrees C in water using both static and dynamic loading.