Compliance-based testing method for fatigue crack propagation rates of mixed-mode I–II cracks

Mixed-mode I–II crack-based fatigue crack propagation (FCPI–II) usually occurs in engineering structures; however, no theoretical formula or effective compliance test methods have been established for FCPI–II to date. For mixed-mode I–II flawed components, based on the principle of mean-value energy equivalence, we propose a theoretical method to describe the relationship between material elastic parameters, geometrical dimensions, load (or displacement), and energy. Based on the maximum circumferential stress criterion, we propose a uniform compliance model for compact tensile shear (CTS) specimens with horizontal cracks deflecting and propagating (flat-folding propagation) under different loading angles, geometries, and materials. Along with an innovative design of the fixture of CTS specimens used for FCPI–II tests, we develop a new compliance-based testing method for FCPI–II. For the 30Cr2Ni4MoV rotor steel, the FCP rates of mode I, mode II, and mixed-mode I–II cracks were obtained via FCP tests using compact tension, Arcan, and CTS specimens, respectively. The obtained da/dN versus ΔJ curves of the FCP rates are close. The loading angle α and dimensionless initial crack length a0/W demonstrated negligible effects on the FCP rates. Hence, the FCP rates of mode I crack can be used to predict the residual life of structural crack propagation.