13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- singularity in cracked plates, which usually can be neglected for mode I loading, should be taken into consideration for mixed mode loadings. However, despite all these studies the role of the corner singularities in fatigue and fracture phenomena remains largely unknown. Fig.1 Generation of coupled fracture modes due to Poisson’s effect and redistribution of stresses on free surfaces when a crack is subjected to shear (a) and anti-plane loading (b) One more three-dimensional effect, which the two-dimensional numerical and analytical solutions are incapable to predict or analyse, is the existence of the coupled fracture modes. To illustrate these coupled fracture modes let us consider a through-the-thickness crack loaded in shear or anti-plane loading. In these cases additional local fracture modes are generated due to Poisson’s effect and/or the redistribution of stresses on the free surfaces as illustrated: in Fig. 1 (a) – mode II loading and (b) – mode III loading in the case of a zero notch angle [16]. In many papers (see among others Refs [19, 20]) it was highlighted that modes II and III cannot exist independently and the presence of one of these modes always generates a coupled singular state. The coupled modes induced by the primary modes II and III for through-the-thickness cracks have been known for a rather long period of time but the first systematic studies for semi-infinite cracks were conducted numerically using a careful FE modelling in [10, 21]. Recently the first coupled fracture mode (see Fig. 1a), which was called the out-of-plane or KO-mode, was investigated for finite geometries representing welded joints as well as for sharp and round notches of arbitrary opening angle. It was demonstrated that the KO-mode has many interesting and previously unknown features, which can influence mixed-mode fracture, crack initiation and fatigue growth phenomena. A recent experimental and numerical study has been performed to investigate the fatigue crack growth tests in mixed-mode 2 + 3 on maraging steel and Ti-6Al-4V. The 3D evolutions of the crack fronts -measured by SEM after interrupted tests- have been analyzed [22]. A 3D finite element model of tested specimens has been prepared, with a refined mesh around the crack front. It has been shown that the profiles of stress intensity factors and energy release rate vary along the crack front. In particular due to the coupled Mode 2 and Mode 3 loadings K2, K3 and G have been found with an asymmetrical profile. When the inclination angle of the crack plane relative to the horizontal axis surface is equal to 45° the energy release rate is ten times higher near one free surface than on the other inducing a strong asymmetry in crack growth and showing the non Compression Tension Free Surface (a) (b)
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