Measurement of crack length in a specimen under load is often an integral part of many materials test set-ups, such as, fracture and fatigue monitoring systems. There are several methods which can be used to measure crack length, such as, visual examination, using traveling microscope, strain gauged displacement measurements to measure the compliance of the cracked body, ultrasonic, eddy current based techniques, etc. DCPD method offers many advantages, when on-line crack growth monitoring is required under elevated temperatures and at higher rates of testing. In this method, highly stable DC current of fixed magnitude is passed through the specimen under test. Typical currents required are approx= 100 amp. with a few parts per million stability. Based on the crack length, there is a change in the resistance of the material, leading to a variation in the potential drop across the specimen, which is amplified and accurately measured. Since DCPD technique can detect very small changes in the specimen resistance as a crack propagates, the method offers very high sensitivity and the increase in specimen resistance can be fairly accurately related to the actual crack length.
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