Direct Current Electrical Potential Measurement of the Growth of Small Cracks

Abstract

This paper demonstrates the successful application of the direct current electrical potential difference method for continuous in-situ monitoring of the growth of short (50 to 5000 μm) fatigue cracks in a variety of metallic alloys exposed to various environments. Crack initiation site and crack shape must be known a priori. Instrumentation includes a constant current power supply, relay circuit, 104 gain amplifier, and computer analog to digital board with controlling software. A closed-form analytical model accurately relates crack size to measured potential, as a function of crack shape and probe position local to the growing crack. Analytical calibrations are experimentally confirmed for small through-thickness edge cracks, surface semielliptical and semicircular cracks, and corner cracks in steels, nickel-based super-alloys, titanium and aluminum alloys. Micron-level average crack advance resolution, long-term stability, continuous measurement, simplicity, compatibility with aggressive environments, and programmed stress intensity loading are attributes of the electrical potential method. Crack length versus load cycles and growth rate versus stress intensity range data demonstrate the power of this method for studies of the effects of microstructure, environment (elevated temperature, high purity gases, vacuum and aqueous solutions) and loading variables on the growth kinetics of small and short fatigue cracks. Additional work is required for electrical potential monitoring of fatigue cracks sized below 75 μm, cracking not associated with a defined initiation site and thermal-mechanical fatigue.