STP738: An Assessment of A-C and D-C Potential Systems for Monitoring Fatigue Crack Growth

    Wei, RP
    Professor of Mechanics and research assistant, Lehigh University, Bethlehem, Pa.

    Brazill, RL
    Professor of Mechanics and research assistant, Lehigh University, Bethlehem, Pa.

    Pages: 17    Published: Jan 1981


    Abstract

    Electrical potential techniques for crack length measurements have been used for over 15 years to monitor subcritical crack growth. These techniques, based on the changes in electrical resistance of a specimen with crack extension, utilize either a d-c or, more recently, an a-c measurement system. The d-c and a-c systems are described, and the operating characteristics of two specific systems are discussed. A description of the empirical calibration of a specimen using these systems is included. Operating data are included for crack length measurements in a steel compact tension specimen. The d-c system, operated at a current of 10 A, had a crack length resolution of 0.013 mm. The a-c system, employing a lock-in amplifier as a tuned amplifier to provide noise rejection, can be operated at a much lower current of 0.75 A and provided a crack length resolution of 0.010 mm. By design, the a-c system would reject thermal (d-c) electromotive force introduced into the measurement circuit, whereas the d-c system had no such capability. Both systems, however, would be sensitive to changes in resistivity of the material with temperature. In materials of high magnetic permeability, alternating current is not uniformly distributed through the specimen thickness and tends to be concentrated near the specimen surface. The consequences of this “skin effect” in relation to crack length measurements are discussed. Operating procedures that assist in reducing noise and improving measurement sensitivity in both systems are described.

    Keywords:

    measurement techniques, fracture mechanics, fatigue crack growth crack length, electrical potential, instrumentation


    Paper ID: STP33456S

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP33456S


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