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Creep and Fatigue Behavior of Materials
Published in Snehanshu Pal, Bankim Chandra Ray, Molecular Dynamics Simulation of Nanostructured Materials, 2020
Snehanshu Pal, Bankim Chandra Ray
During cyclic loading, the fatigue failure process consists of crack initiation, crack growth, and sudden fracture [14]. Most commonly, fatigue cracks are initiated at the surfaces or fatigue cracks have originated internally only in case of interfaces existence. Persistent slip bands (PSBs) are formed by dislocation movement, which causes to fatigue crack initiation. Materials move along the slip planes subjected to cyclic loading, and then, PSBs form at the surfaces above or below the surfaces of the metal. The PSBs that are raised above the surface of the material are slip-band extrusions (ridges), and the PSBs that are formed below the surface of the material are slip-band intrusions (grooves); these are shown in crack formation diagram (refer to Figure 3.7). These slip-band intrusions and extrusions also tend to crack initiation. Crack propagation takes place along the slip planes, followed by crack initiation, and later, the crack propagates perpendicular to an applied tensile stress. Crack propagation takes place through plastic blunting process, which is shown in Figure 3.4 [4]. Sharp crack tip is observed during initial stage of loading, which is shown in Figure 3.4a. When tensile load is applied, short double notch is concentrated along the slip planes of 45°, as shown in Figure 3.4b. Crack tip becomes blunter as crack grows longer through plastic shearing; this is shown in Figure 3.4c. During cyclic loading, the load is changed from tension to compression; then, the slip direction is also reversed at the end zone (refer to Figure 3.4d). Eventually, the new crack surface is formed during tension, as shown in Figure 3.4e, and this is partly folded by buckling phenomenon, leading to the formation of re-sharpened crack tip. The re-sharpened crack is then ready to advance and also to be blunted in the next stress cycle.
Fracture, fatigue, and creep of metals
Published in Gregory N. Haidemenopoulos, Physical Metallurgy, 2018
Crack initiation in smooth specimens, takes place at surface discontinuities, intrusions and extrusions, which are created at the surface of the metal, along zones of localized plastic deformation, called persistent slip bands (PSB). Fatigue cracks initiate either directly from these surface discontinuities or from the interaction of intrusions with inclusions and grain boundaries.
Predictive modelling of dry sliding wear in sealed plasma-sprayed Mo coating using response surface methodology
Published in Tribology - Materials, Surfaces & Interfaces, 2018
S. S. Manjunatha, M. Manjaiah, S. Basavarajappa
As the load increases, the wear volume loss also increases due to the formation of fatigue cracks on the coating surface (Figure 3). The fatigue is one of the primary reasons for the failure of structural components. The dislocations in the material play a major role in the fatigue crack initiation phase. It has been observed that after a large number of loading cycles dislocations pile up and form structures called persistent slip bands (PSB). The PSBs are areas that rise above or fall below the surface of the component due to movement of material along slip planes. This leaves tiny steps on the surface that serve as stress risers where fatigue cracks can initiate [24]. The lamellae, which is fractured during the prior wear regime, can be removed during this stage. Actually, the higher applied loads may cause the decohesion of the interlinked splats and detachment from the coating. It commonly occurs in plasma-sprayed coating, where the inter splat cohesion is very weak [21]. In addition, the abrasive action of the separated particles entrapped in the sliding contact area may contribute to the wear of material. The tensile and shear stresses in the sliding contact region increases due to increase in the applied load. The subsurface cracks are propagated drastically due to increase in contact stresses [25,26]. The drastic crack propagation leads to the delamination of the coating, resulting in the de-lamination as seen in Figure 4. From the SEM micrographs, it is observed that the wear volume loss of coating occurs mainly due to ploughing, fatigue cracks and delamination.