Showing papers on "Paris' law published in 2021"

Microstructural fracture mechanics: Stored energy density at fatigue cracks

Abstract: This paper addresses the mechanistic drivers of short fatigue crack growth using theory, computational crystal plasticity and experimental test and characterisation. The asymptotic theory shows that the crack tip stored energy density is non-singular and finite and can be related to stress intensity, but unlike the latter, it depends on the crystal Burgers vector and intrinsic slip strength. The computational methods allow the stored energy to be calculated accurately at crack tips and show that good agreement is obtained for static cracks with the theory. The experiments allow the crack tip stored energy to be measured, demonstrating intimate microstructural sensitivity, direct correlation with experimental crack growth variations and good quantitative agreement with both asymptotic theory and computational modelling. Hence a new microstructurally-sensitive fracture mechanics has been presented in the context of short cracks within crystalline materials.

Fatigue testing of a 14.3 m composite blade embedded with artificial defects – Damage growth and structural health monitoring

Abstract: Understanding fatigue damage growth of composite wind turbine blades is an essential step towards reliable structural health monitoring (SHM) and accurate lifetime prediction. This study presents a comprehensive experimental investigation into damage growth within a full-scale composite wind turbine blade under fatigue loading. The blade has artificial defects embedded to initiate damage growth. The damages are detected and monitored using Infrared (IR) thermography, Digital Image Correlation (DIC), and Acoustic Emission (AE). Steady damage growth and imminent structural failure are identified, demonstrating the effectiveness of these techniques to detect subsurface damages. New experimental observations include cyclic buckling of a trailing edge region and tapping and rubbing between the shear web and spar cap, both damages due to adhesive joint debonds. These observations highlight the necessity and the complexity of reliable modeling of nonlinear structural behavior on a large scale in order to predict local fatigue crack growth.

Effect of mesoscale phase contrast on fatigue-delaying behavior of self-healing hydrogels.

Abstract: We investigate the fatigue resistance of chemically cross-linked polyampholyte hydrogels with a hierarchical structure due to phase separation and find that the details of the structure, as characterized by SAXS, control the mechanisms of crack propagation. When gels exhibit a strong phase contrast and a low cross-linking level, the stress singularity around the crack tip is gradually eliminated with increasing fatigue cycles and this suppresses crack growth, beneficial for high fatigue resistance. On the contrary, the stress concentration persists in weakly phase-separated gels, resulting in low fatigue resistance. A material parameter, λtran, is identified, correlated to the onset of non-affine deformation of the mesophase structure in a hydrogel without crack, which governs the slow-to-fast transition in fatigue crack growth. The detailed role played by the mesoscale structure on fatigue resistance provides design principles for developing self-healing, tough, and fatigue-resistant soft materials.

Effect of deposition strategies on fatigue crack growth behaviour of wire + arc additive manufactured titanium alloy Ti–6Al–4V

Abstract: The influence of three deposition strategies on the fatigue crack growth behaviour of Wire + Arc Additive Manufactured (WAAM) Ti–6Al–4V has been investigated in the as-built condition. Test samples were prepared using single pass, parallel pass, and oscillation deposition strategies and tested with cracks propagating parallel and normal to the plane of deposition. Due to the higher local heat input, the oscillation build exhibited a significantly coarser columnar β grain structure as well as a coarser transformation microstructure, compared to the single pass and parallel pass builds, which were very similar. Among the three build methods, the lowest crack growth rates were found with the oscillation build. The crack growth data was found to broadly fall between that of a recrystallized α (mill-annealed) and β annealed wrought material, with the oscillation strategy build behaving more similarly to a β annealed microstructure. The fatigue crack growth rate was lower when cracks were propagated perpendicular to the build layers. For each build strategy, a greater microstructural influence on crack growth rate was found at lower levels of stress intensity factor range (

A systematic study on the effects of shot peening on a mild carbon steel: Microstructure, mechanical properties, and axial fatigue strength of smooth and notched specimens

Abstract: Used with the right combination of parameters, shot peening process can be regarded as a severe plastic deformation method that can greatly enhance the strength of mechanical components by inducing surface grain refinement, increasing the hardness, and generating compressive residual stresses in the top surface layer. The present experimental study explores the effects of gradual rise of shot peening kinetic energy on microstructure, mechanical properties, and fatigue behavior of AISI 1045 carbon steel by changing the process parameters over a wide range of Almen intensity (20-32A) and surface coverages (100-3000%). Microstructural observations, X-ray diffraction, microhardness and residual stress measurements were carried out to analyze the effects of grain refinement as the kinetic energy of the peening process increased. In addition to compression-tension axial fatigue tests, fatigue crack growth measurements were also performed on specimens in both smooth and notched configurations. The results demonstrated that increasing the kinetic energy of the process is an effective technique to achieve nanostructured surface layer combined with superior mechanical properties. Severe shot peening treatment significantly improved the fatigue strength of the treated specimens, while the continuous growth of surface coverage proved to have detrimental effects on fatigue behaviour.

Real-Time Acoustic Emission Monitoring of Wear-Out Failure in SiC Power Electronic Devices During Power Cycling Tests

Abstract: In the article, acoustic emission (AE) was applied to monitor the wear-out failure in discrete SiC Schottky barrier diodes devices with a Ag sinter die attach to successfully monitor the real-time progress of failure of Al ribbons for the first time. After eliminating background AE noise including the power on–off switch and the ambient noise via a noise filtering process, AE signals were successfully collected for the SiC devices during a power cycling test. Furthermore, AE monitoring was compared to the traditional failure monitoring method using forward voltage. Physics-of-failure analysis was performed, and fatigue cracks, and lift-off in Al ribbons were confirmed as the dominant failure modes for the discrete devices. Specifically, AE counts that correspond to one of the time-domain parameters of AE signals increased with power cycling, thereby corresponding to the observed fatigue cracks in Al ribbons leading to lift-off failure. Additionally, the AE count rate was highly correlated with the crack growth rate. Based on the relationship between an AE count rate and fatigue crack growth rate, the results indicate that AE monitoring can be used to understand the fatigue propagation in Al ribbons (i.e., failure mechanism) and also as an early warning before catastrophic lift-off fracture for power electronic devices.