STP1332

    An Evaluation of the Adjusted Compliance Ratio Technique for Determining the Effective Stress Intensity Factor

    Published: Jan 1999


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    Abstract

    In a common interpretation of crack closure, the crack is visualized as “peeling” open as stress is applied. This interpretation has led to the assumption that the driving force for crack propagation, ΔKeff, exists only when the crack tip is fully open (above K-opening). The current opening load method of closure measurement in accordance with ASTM Standard Test Method for Measurement of Fatigue Crack Growth Rates, Appendix X2 (E 647) is based on this concept. However, based on a comparison with “closure free” data, evidence of significant crack-tip cyclic strain below K-opening will be presented to show that the exclusion of this additional driving force can give misleading values of ΔKeff. This is so particularly in the near-threshold regime where opening loads are typically high. The new analysis technique for estimating ΔKeff is referred to as the adjusted compliance ratio (ACR) method and is based on an interpretation of crack closure as a stress redistribution (or load transfer) on a relatively compliant crack wake. The ACR method is evaluated using the results of fatigue crack growth tests on 6013-T651, 2324-T39 and 7055-T7751 aluminum alloys using both compact tension C(T) and middle crack tension M(T) specimen geometries. The results of this study indicate that ΔKeff obtained by the ACR method is relatively independent of measurement location, crack length and specimen geometry. Comparison of ΔKeff data obtained by both methods with “closure free” crack growth data obtained at high stress ratios indicates that the ACR method is superior to the ASTM opening load method in the near-threshold regime for estimating the “true” driving force for crack propagation.

    Keywords:

    fatigue, fatigue threshold, crack propagation, effective stress intensity, crack closure


    Author Information:

    Donald, JK
    Director, Fracture Technology Associates, Bethlehem, PA

    Bray, GH
    Staff engineer and technical specialist, Alcoa Technical Center, Alcoa Center, PA

    Bush, RW
    Staff engineer and technical specialist, Alcoa Technical Center, Alcoa Center, PA


    Paper ID: STP14977S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP14977S


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