ON THE ACTUAL CONTRIBUTION OF CRACK DEFLECTION IN TOUGHENING PLATELET-REINFORCED BRITTLE-MATRIX COMPOSITES

The effectiveness of crack deflection as a single toughening mechanism is evaluated in the case of brittle-matrix reinforced by high aspect-ratio platelets. Microstructural parameters characterizing size, morphology, orientation and distribution of the second phase including aspect ratio, volume fraction and interparticle distance, were experimentally obtained by means of image analysis techniques on a model composite. Introducing ''true'' microstructural parameters in the fracture mechanics formulation of the strain energy release rate for a deflected crack, it is demonstrated that no appreciable toughening can be obtained due to the geometrical local misalignment of the crack front even in the case of increasing aspect-ratio or volume fraction of second phase. It is concluded that crack deflection processes may play an important role only as precursors for other operative mechanisms, being, in the general case, the deflection of the crack path only phenomenologically related to toughening.