On crack propagation in homogeneous and composite materials under mixed mode loading conditions

Abstract

The direction of crack propagation under mixed mode loading conditions is investigated by FEM simulation in comparison with delamination experiments performed in the 4 point bending mode. Composites consisting of aluminum nitride attached to copper by active metal brazing were delaminated with use of a central notch for crack initiation. The main crack line propagated along the interface direction. However, SEM micrographs revealed that the crack line propagating along the interface is subjected to bifurcation. Small cracks digress from the main delamination line under kinking angles of 90° until they are stopped at the copper substrate. The observed behavior of crack propagation is studied by Linear Elastic Fracture Mechanics and by nonlinear Finite Element Analysis. In fact, the bifurcation angle of 90° can be considered as an indication for dominant influence of geometrical nonlinearities at the crack tip. Finite Element Analysis is performed in two stages starting from macroscopic level and then the analysis of crack tip behavior is continued on a microscopic level. On the macroscopic level, the whole experimental setup is simulated in order to derive an approximation of the stress distribution around the crack tip. Thereafter, the analysis is further extended on microscopic level, whereby local mesh refinement is continued until the element size approaches the dimension of the lattice constant. Thus, high stress values at the crack tip lead to considerable amounts of material rotation. Consequently, the observed direction of crack extension may be derived from the maximum hoop stress criterion.