Digital image correlation of laser-ablated platinum nanoparticles on the surface of a polycrystalline metal (nickel-based superalloy Rene´88DT) was used to obtain the local strain behavior from an in-situ scanning electron microscope tensile experiment at room temperature. By fusing this information with crystallographic orientations from electron backscatter diffraction (EBSD), a subsequent analysis shows that the average maximum shear strain tends to increase with increasing Schmid factor. Additionally, the range of the extreme values for the maximum shear strain also increases closer to the grain boundary, signifying that grain boundaries and triple junctions accumulate plasticity at strains just beyond yield in polycrystalline Rene´88DT. In-situ experiments illuminating microstructure-property relationships of this ilk may be important for understanding damage nucleation in polycrystalline metals at high temperatures.

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Fractures from tests on 2014‐T6511 and 2024‐T3 test coupons under specially designed programmed loading reveal voids with distinct fatigue markings. These ‘fatigue voids’ appear to form as a consequence of the separation of noncoherent secondary particulates from the matrix in early fatigue. The process of their formation is through the initiation, growth and coalescence of multiple interfacial cracks around the particulate. Such voids become visible on the fatigue fracture surface if and when the crack front advances through them. In vacuum, each fatigue void is the potential initiator of an embedded penny‐shaped crack. The one closest to the specimen surface is likely to become the dominant crack, indicating that fatigue voids appear to be the likely origins of the dominant crack in vacuum. In air, the dominant crack forms at the notch surface and grows much faster, giving less opportunity for multiple internal cracks to spawn off from the innumerable internal fatigue‐voids. Thus in air, fatigue voids do not appear to affect the fatigue process at low and intermediate growth rates. At high crack growth rates involving considerable crack tip shear, slip planes with particulate concentration offer the path of least resistance. This explains the increasing density of fatigue voids with growth rate. Very high growth rates signal the onset of a quasi‐static crack growth component that manifests itself through growing clusters of microvoid coalescence associated with static fracture. Fatigue voids are likely to form in other Al‐alloys with secondary noncoherent particulates. They have nothing in common with microvoids associated with ductile fracture.

Fatigue crack growth behavior was studied on C(T) and SE(T) specimens from Al-Cu alloy 2014 using specially-designed load sequences. The experiments were organized to induce microscopic mixed-mode fatigue crack growth while at the same time reducing or eliminating fatigue crack closure. This was achieved by switching between high-amplitude, low-stress ratio and low-amplitude, high-stress ratio cycles. Retardation effects observed under these conditions are attributed to local crack branching and mixed-mode conditions induced by variable amplitude loading.

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Fractographic measurements of fatigue crack growth rate for small cracks reveal stress-ratio effects even when fatigue crack closure is absent. These effects are restricted to low fatigue crack growth rates and become significant with increase in net stress levels. To characterize the effect, experiments and analyses were conducted on notched coupons of an Al alloy at stress levels producing inelastic conditions on initial loading. As a reference, fatigue crack growth rates were obtained for a long crack tested under fully elastic loading well below yield stress. The results indicate that for fatigue growth associated with low applied stress intensity range, minor changes in stress ratio can cause substantial variation in crack growth rate. A model is proposed for the small crack fatigue growth rate as a function of applied stress intensity and stress ratio. The model is based on crack growth rates obtained under several stress levels with crack size as small as 0.03 mm.

The understanding of fatigue variability in turbine engine materials is vital to permit a reduction of US Air Force sustainment costs through fatigue life extension and/or inspection interval extension. Additionally, the US Air Force is currently moving further towards the use of probabilistic damage tolerance design methods. These probabilistic models require not only a good understanding of the variability in specimen data, but also an understanding of the microstructural sources of variability to allow scaling to component analysis. The objective of this work was to study the fatigue variability of a common turbine engine alloy Ti-6Al-4V. Typical testing consisted of smooth bar fatigue tests at multiple stress ratios and stress levels in order to generate a fully populated stress-life curve. These tests, however, typically do not consist of many repeats. The approach of this work was to conduct a statistically significant number of repeated fatigue tests at several loading conditions. A similar approach has been performed on several other turbine engine material systems often revealing bimodal life distributions consisting of a number of low life specimens that may fail due to a separate mechanism. This paper discusses the Ti-6Al-4V life distributions and sources of variability. Crack propagation using small crack growth data was used to predict the lower tail of the life distributions.

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