Growth of interface defects and its effect on crack deflection and toughening criteria

The role of interface defects in crack deflection at planar interfaces has been studied by the direct observation of interface and crack growth in laminates of poly(methyl methacrylate) and a finite element analysis. It was observed that crack deflection occurred by the growth of interface defects ahead of the growing primary crack. It was also observed that the growth of these defects is not sufficient to ensure toughening through crack deflection, but that a further condition, that any growing interface defects must not kink out of the interface, must be also satisfied. A finite element analysis based on the observed deflection mechanism suggests that initial crack deflection is possible when the interfacial fracture energy is less than 64% of that of the bulk material, which is consistent with experimental values. However, the analysis also predicts that this is somewhat dependent on layer thicknesses and global loading states for relatively longer interface defects. Copyright (C) 1996 Acta Metallurgica Inc.