Fatigue & Fracture of Engineering Materials & Structures最新文献

We conducted a statistical fatigue analysis of an additively manufactured aluminum alloy (AlSi10Mg) with five different sizes of specimens to establish probabilistic S-N curves and assess an anomalous behavior of strength deterioration in high-cycle and very-high-cycle regimes. For the five specimen types, S-N data were obtained by ultrasonic axial cycling at R = −1. Based on the fitted Basquin equations, the S-N data were converted to equivalent stress amplitudes at the same life. In general, the stress amplitudes can be regarded as normal, log-normal, and Weibull distributions, with the log-normal slightly overestimating the fatigue performance, whereas the Weibull slightly underestimated it, and the normal being the most suitable. Finally, we compared the normal distribution of fatigue resistance with the control volume V₉₀ of specimen types and plotted the strength deterioration from 10⁶ cycles to the extrapolated 10¹⁰ cycles, indicating that the specimen size effect diminishes as fatigue life increases.

The present work is focused on the analysis of configurations in metallic components subjected to cyclic loading, in which fatigue crack nucleation is generated in zones where hardness gradients and/or residual stress gradients are present. In the first case, a gradient of resistance to the nucleation and propagation of small cracks is produced, whereas in the second case, gradients in load ratios are generated as the crack propagates. A fatigue crack propagation prediction approach based on fracture mechanics is proposed, through which the influence of the hardness gradient on the definition of the fatigue crack growth resistance curve and the effect of the compressive residual stress gradient on the quantification of the resulting driving force are accounted for. The approach is applied to a carburized steel with surface shot peening, for which fatigue failure results, including those in the presence of artificial semicircular planar defects with depths of 0.15, 0.2, and 0.3 mm, have been reported in the literature. The results of the estimations are analyzed and found to be highly satisfactory, and several important aspects are discussed.

Understanding small-crack behavior in Hastelloy C-276 is essential for very-high-cycle fatigue (VHCF) studies, as crack growth strongly depends on microstructural features and deformation modes. This research explores the influence of gradient bimodal grain structures and slip bands on VHCF-induced damage in Hastelloy C-276. The findings show that fatigue cracks typically nucleate on favorably oriented slip planes within coarse grains, with primary activity occurring on the {1 1 1} slip planes. Because a single-slip system cannot accommodate the deformation, multiple-slip systems are activated, deflecting short cracks and slowing their propagation. Notably, the gradient distribution of coarse and fine grains enhances the fatigue resistance of the alloy.

The present study quantifies microvoid characteristics formed at fracture in cylindrical notched specimens made of ASTM A36, A572, and A992 steels subjected to ultralow cycle fatigue (ULCF) loading. Microvoid size and circularity were evaluated at different locations across the fracture surface using SEM micrographs. The microvoid size distributions under ULCF were significantly positively skewed, prompting evaluation of multiple statistical descriptors such as mean, median, 80th percentile, and 90th percentile sizes. The mean and median microvoid sizes under ULCF loading were 1.8 to 3.7 times smaller than those under monotonic tension for ASTM A992 steel. Contrary to the existing notion, microvoids on ULCF fracture surfaces have higher circularity when compared to their monotonic counterparts, and the largest microvoids are concentrated at the periphery of the fracture surfaces. Finally, the experimentally evaluated microvoid sizes were used for calibrating a micromechanical ULCF fracture model, which yielded conservative estimates of ULCF cycles to failure.

In this study, true triaxial graded cyclic loading and unloading intermediate principal stress tests were performed on sandstones. The effects of graded cyclic increments (GCI) (1, 3, and 5 MPa) and minimum principal stresses (4, 5, 6, and 7 MPa) on the mechanical responses of sandstones during graded cyclic loading were examined. The experimental results reveal that during the cyclic tests, plastic hysteresis loops are clearly observed along all three principal stress directions, indicating memory characteristics. As the graded cycles proceed, the mechanical responses (elastic modulus, volumetric strain) exhibit a nonlinear attenuation trend; the strains are larger in the unloading than those in the loading. Both GCI and the minimum principal stress level are of great significance to rock behaviors under graded cyclic loading conditions. Overall, lower minimum principal stress levels result in stronger deformation responses and shorter fatigue life. Final failure morphology indicates more pronounced spalling under higher cyclic stress increments and minimum principal stresses. The dissipated energy of the rock mass increases exponentially with the graded cyclic amplitude, and the evolution of the damage variable follows a “slow-fast-stable” trend. On the above, sandstone is more prone to yield under lower minimum principal stresses and higher GCIs.

This study investigates the gigacycle fatigue behavior of Ti6Al4V titanium alloy—a critical aerospace material—following laser shock peening (LSP). Previous research conducted by the authors has shown that LSP can improve the fatigue life of samples with stress concentrations by as much as eight times; however, this study indicates a contradictory decline in fatigue performance under gigacycle conditions (N = 10¹⁰) when using ultrasonic testing (Shimadzu USF-2000). In contrast to the established LSP benefits, the treated specimens exhibited a substantial decrease in the fatigue limit when compared to the as-received material. Infrared thermography revealed significantly elevated levels of energy dissipation in LSP-processed specimens, whereas fractography showed a complete transition in failure origin: cracks invariably initiated centrally in LSP specimens versus near-surface initiation in untreated counterparts. These outcomes are attributed to tensile residual stresses in the specimen core and the microstructural alterations induced by LSP. The residual stresses are estimated by numerical simulation based on the Johnson–Cook and Fabbro models.

This study explores the impact of Ultrasonic Impact Treatment (UIT) on the fatigue performance of high-strength microalloyed steel commonly used in crankshaft applications. Building on prior observations of fatigue life improvement with UIT, this work focuses on unraveling the process–structure–performance relationships underpinning these enhancements. Microstructural analysis revealed significant grain refinement in the near-surface layers, with deformation depth increasing at higher impact energies. This led to increased surface hardness and the development of deeper compressive residual stresses, particularly in samples treated with higher impact energy. Rotating bending fatigue testing showed a substantial improvement in fatigue life for UIT-treated samples, with the endurance limit nearly doubling compared with untreated specimens. Fractographic analysis revealed a transition in crack initiation from surface defects in untreated samples to interior regions in UIT-treated samples, characterized by the formation of noninclusion-induced granular bright facets (GBFs). The observed fatigue enhancement is attributed to the synergistic effects of strain hardening and compressive residual stresses, which increase the surface crack threshold and promote interior crack initiation. This study provides new mechanistic insight into UIT-induced fatigue resistance and interior failure behavior in high-strength steels.