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Plasticity induced closure often strongly influences the behaviour of fatigue cracks at engineering scales in metallic materials. Current predictive models generally adopt the effective stress‐intensity factor (ΔΚeff = Κmax–Κop) in a Paris law type relationship to quantify crack growth rates. This work describes a 3D finite element study of mode I fatigue crack growth in the small‐scale yielding (SSY) regime under a constant amplitude cyclic loading with zero T‐stress and a ratio Κmin/Κmax = 0. The material behaviour follows a purely kinematic hardening constitutive model with constant hardening modulus. Dimensional analysis suggests, and the computational results confirm, that the normalized remote opening load value, Κop/Κmax, at each location along the crack front remains unchanged when the peak load (Κmax), thickness (B) and material flow stress (σ0) all vary to maintain a fixed value of . Through parametric computations at various K levels, the results illustrate the effects of normalized peak loads on the through‐thickness opening–closing behaviour and the effects of σ0/E, where E denotes material elastic modulus. The examination of deformation fields along the fatigue crack front provides additional insight into the 3D closure process.

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Electron beam melting is a powder bed fusion (PBF) additive manufacturing (AM) method for metals offering opportunities for the reduction of material waste and freedom of design, but unfortunately also suffering from material defects from production. The stochastic nature of defect formation leads to a scatter in the fatigue performance of the material, preventing wider use of this production method for fatigue critical components. In this work, fatigue test data from electron beam melted Ti-6Al-4V specimens machined from as-built material are compared to deterministic fatigue crack growth calculations and probabilistically modeled fatigue life. X-ray computed tomography (XCT) data evaluated using extreme value statistics are used as the model input. Results show that the probabilistic model is able to provide a good conservative life estimate, as well as accurate predictive scatter bands. It is also shown that the use of XCT-data as the model input is feasible, requiring little investigated material volume for model calibration.

A B S T R A C T This paper describes the effects of a single overload event, within otherwise constant amplitude cycles, on the plasticity-induced closure process for mode I fatigue crack growth in the small-scale yielding (SSY) regime. The 3-D finite element (FE) analyses extend the initially straight, through-thickness crack front by a fixed amount in each load cycle, using a node release procedure. Crack closure during reversed loading occurs when nodes behind the growing crack impinge on a frictionless, rigid plane. A bilinear, purely kinematic hardening model describes the constitutive response of the elastic-plastic material. Extensive crack growth in the analyses, both before and after the overload, allows the crack to grow out of the initial and the post-overload transient phases, respectively. The work presented here shows that the large plastic deformation in the overload cycle reduces the crack driving force through enhanced closure. Further, the residual plastic deformations due to the overload cause a disconnected pattern of closure in the wake long after the crack front passes through the overload plastic zone. The computational studies demonstrate that the 3-D scaling relationship for crack opening loads established in our earlier work for constant amplitude cycling (with and without a T-stress) also holds before, during and after the overload event. For a specified ratio of overload-to-constant amplitude loading (R OL = K OL max /K max ), the normalized opening load (K op /K max ) at each location along the crack front remains unchanged when the constant amplitude peak load (K max ), thickness (B) and material flow stress (σ 0 ) all vary to maintain a fixed value of K = K max /σ 0 √ B. The paper concludes with a comparison of the post-overload response predicted by the 3-D analyses and by the conventional Wheeler model. a p = sum of the crack length at overload and the size of plastic zone due to overload a d = delay distance B = thickness C = Paris model constant C p = crack growth retardation factor da/dN = instantaneous crack growth per cycle E = elastic modulus E T = hardening modulus K I = mode I stress intensity factor K c = fracture toughness K max = maximum applied stress intensity factor in constant amplitude cycles K min = minimum applied stress intensity factor Correspondence: Robert H. Dodds.

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