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Within the past decade, critical plane approaches have gained increasing support based on correlation of experimentally observed fatigue lives and microcrack orientations under predominately low cycle fatigue (LCF) conditions for various stress states. In this paper, we further develop an engineering model for microcrack propagation consistent with critical plane concepts for correlation of both LCF and high cycle fatigue (HCF) behavior, including multiple regimes of small crack growth. The critical plane microcrack propagation approach of McDowell and Berard serves as a starting point to incorporate multiple regimes of crack nucleation, shear growth under the intluence of microstructural barriers, and transition to linear crack length-dependent growth related to elastic-plastic fracture mechanics (EPFM) concepts. Microcrack iso-length data from uniaxial and torsional fatigue tests of 1045 steel and IN718 are examined and correlated by introducing a transition crack length which governs the shift from nonlinear to linear crack length dependence of da/dN. This transition is related to the shift from strong microstructural influence to weak influence on the propagation of microcracks.Simple forms are introduced for both the transition crack length and the crack length-dependence of crack growth rate within the microcrack propagation framework (introduced previously by McDowell and Berard) and are employed to fit the 1045 steel and IN 718 microcrack iso-length data, assuming preexisting sub-grain size cracks. The nonlinear evolution of crack length with normalized cycles is then predicted over a range of stress amplitudes in uniaxial and torsional fatigue. The microcrack growth law is shown to have potential to correlate microcrack propagation behavior as well as damage accumulation for HCF-LCF loading sequences and sequences of applied stress states. NOMENCLATURE a = crack length b, b, = fatigue strength exponents in tension and torsion, respectively c, co = fatigue ductility exponents in tension and torsion, respectively C,, C, = fully elastic and plastic microcrack propagation law coefficients, respectively E, G = Young's modulus and shear modulus, respectively j,, jp = biaxiality exponents for fully elastic and plastic forms for da/dN, respectively d = grain size kd = transition crack length, where k is the scaling factor K', Kb = cyclic strength coefficients in pure tension and torsion, respectively M, m = exponents for fully elastic and plastic terms in da/dN, respectively mr = crack length exponent in da/dN law Nr = number of cycles to fatigue crack of prescribed length considered as "failure" R, = the ratio Ay&/Aymx R,, =the ratio Aa,/Az, Be, BP = biaxiality functions for fully elastic and plastic cases, respectively Ay-= maximum engineering shear strain range AyLx = elastic part of by,, AVp, . = plastic part of Ay, , n, nb = cyclic strain hardening exponents in pure tension and torsion, respectively be", AcP = ranges of uniaxial elastic and plastic strain, respectively Aa,, AT, = normal and shear stress ran...

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This paper explores the effects of microstructural heterogeneity on the cyclic crack tip opening and sliding displacements for stationary, microstructurally small transgranular surface cracks in a single phase metallic polycrystal using planar double slip crystal plasticity computations. Crack tip displacements are examined under plane strain conditions for stationary cracks of different lengths relative to grain size as a function of the applied nominal strain amplitude for tension‐compression and cyclic shear. Nominal strain amplitudes range from well below to slightly above the nominal cyclic yield strength for each type of loading condition. Results indicate the complex nature of the crack tip sliding and opening displacements as functions of nominal strain amplitude and orientation of the nearest neighbour grains, the influence of the free surface in promoting the cyclic opening displacement even for cracks in the first surface grain, the rather restricted limits of applicability of linear elastic fracture mechanics, and very interesting crack tip plasticity effects which include crack tip displacement ratcheting or progressive accumulation, even for completely reversed, proportional applied loading. Results are compared for cases with and without crack face friction.