Investigation of crack propagation mechanisms in ductile aluminum plates by numerical and experimental methods

In ductile metal plate tearing, cracks propagate in four different ways: i-) slanted, ii-) cup-cone, iii-) cup-cup, and iv-) a combination of these three. The crack propagation mechanism depends on the geometry and the material properties of the plate, as well as on the loading conditions. Several experimental studies in the literature reveal that strain hardening capacity affects the crack propagation mechanism. In high strain hardening capacity metals, a severe necking occurs at the crack tip before crack starts to propagate. The stress triaxiality reaches to considerably larger values in the center of the necking zone in comparison to that on the side faces of the plate, and therefore the crack initiates in the center and propagates in a cup-cup morphology. For low strain hardening capacity metals, on the other hand, the necking at the crack tip remains at low levels, plastic deformation localizes at shear bands and the crack propagates in either slanted or cup-cone morphology. Experimental results also show that the dominant crack propagation mechanism is cup-cup at low (quasi-static) loading rates, while it is slanted or cup-cone at high loading rates. The current literature focuses mainly on the mechanical/geometric properties of the plates; microstructure–crack morphology relationship is not yet fully clarified. This thesis aims to investigate the effects of microstructure on the crack propagation mechanism in detail. In ductile metals and metal alloys, cracks propagate predominantly by the growth and coalescence of voids nucleated by second phase particles whose size is in the order of micrometer. The fundamental hypothesis of this thesis is that, the volume fraction, the average size, the aspect ratio and spatial distribution of these particles/voids would affect crack propagation. In order to test this hypothesis, both numerical and experimental studies are performed. These studies verified the hypothesis, and the results showed that small, low aspect ratio and remotely distributed particles lead to cup-cup crack propagation, while large, elongated, and closely spaced particles favor slanted or cup-cone crack propagation. In addition, numerical calculations reveal that the orientation of elliptical nucleation sites, acting as particles, affects interactions between them and, therefore, the plastic deformation evolution in the fracture process zone. An orientation angle that allows smaller perpendicular distances between particles through the plate thickness or/and aligns particles on a shear band, promotes early localization of plastic deformation on a thin band. Consequently, this leads to a low level of plate tearing energy and, slanted or cup-cone crack propagation. In the case of an orientation angle that demotes interactions between particles, plastic localization is delayed leading to cup-cup crack propagation and a high level of plate tearing energy.