In this paper are presented the results of fatigue crack propagation tests on angled-slit, three point bend mixed-mode (I + 111) specimens manufactured from a low pressure steam turbine rotor forging. The path of crack propagation has been studied for two mixed mode (I + 111) loading conditions. It has been observed that crack growth occurs by a mode I mechanism and a model has been developed to correlate crack growth rates in mixed mode (I + 111) specimens with data from pure mode I fatigue tests. NOMENCLATURE a = crack length B = specimen thickness R = load ratio, Ph/Pmx S = loading span W = specimen width ,3 = angle of crack front with respect to the longitudinal axis of specimen K,, K,,, K,,, = mode I, I1 and 111 stress intensity factors, respectively

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Mode I, mode 111 and mixed mode (I + 111) fatigue crack growth threshold tests have been performed on a 3.5YoNiCrMoV steel at a range of mean stresses corresponding to load ratios R = 0.06 to 0.5. Angled slit three point bend specimens were used for mixed mode (I + 111) tests, circumferentially slit round bars for mode I11 tests and conventional three point bend specimens for mode I tests.Test results were divided into those specimens which had failed and those in which no extension of the initial slit had occurred. The results were compared with the response predicted by two existing mixed-mode threshold models. It was found that a model based on the magnitude and direction of mode I crack opening gave results which were in better agreement with the experimental results than a model based on branch crack formation. NOMENCLATURE E = Young's modulus AK,, AK,, , AK,,, = mode I, mode I1 and mode 111 stress intensity factor ranges, respectively AK,,,, = mode I fatigue threshold stress intensity factor range AKA = equivalent mode I stress intensity factor range for an angled slit specimen AK* = mode I stress intensity factor range for a crack in the direction of the maximum principal stress R = load ratio (rninimum/maximum load in a fatigue cycle), ! ? = orientation of initial slit to specimen axis 6 = crack opening displacement 4 =twist angle of a crack with respect to the initial slit p = shear modulus v = Poisson's ratio uy, T~ = yield stress and shear yield stress, respectively

Cold expansion of fastener holes is a recognized technique for enhancing the fatigue life of holed plates utilized in detachable joints. This method involves introducing compressive residual stress around the holes, which has proven to be effective in improving the structural integrity and longevity of such components. In this research, the role of using different plasticity models in finite-element simulations of cold expansion (CE) in determining residual stress distribution and predicting the fatigue life of lap joints was investigated. Finite-element simulations were conducted to analyze the distribution of residual stress in cold-expanded Al-alloy 2024-T3 plates utilized in double-shear lap joints. Three time-independent plasticity models, specifically multilinear kinematic hardening, multilinear isotropic hardening (M.L.I.H) based on monotonic tensile tests, and nonlinear combined hardening models derived from saturated hysteresis stress and strain loops, were employed in the simulations. These models allowed for an accurate determination of the residual stress distribution in the plates. In finite-element simulations, two CE sizes of 1.5% and 4.7% were employed to create residual stresses. In the simulations, after creating residual stress by CE, remote sinusoidal loads are applied to the joints corresponding to the previously conducted fatigue tests in order to obtain stress and mechanical strain distributions. In order to predict the fatigue life, four different multiaxial fatigue criteria (Smith–Watson–Topper, Glinka, Kandil–Brown–Miller, and Fatemi–Socie) were employed, using the stress and strain distributions obtained from the finite element simulations with the different plasticity models. The simulations yielded varying stress and strain distribution results for the multilinear kinematic and M.L.I.H models, while the results of the nonlinear combined hardening model fell between the other two models. Notably, the fatigue life prediction based on the nonlinear combined hardening plasticity model closely matched the experimentally observed fatigue lives, with an absolute percentage deviation of 24.8%.