Insights into toughening particulate brittle composites

A conventional approach to the toughening of brittle materials has been to incorporate fibrous inclusions. These toughen the composite by acting as crack bridges, with energy being dissipated by frictional losses during pull-out of the fibre or by matrix failure on the scale of fibre surface roughness. In all cases, toughening has been achieved only where propagating macrocracks have interacted with an appreciable number of reinforcing inclusions. This paper considers the probability that toughening via crack-bridging in these fibrous and especially particulate toughened materials has not been optimised and proposes the idea that the frequency of interaction between crack and inclusion is strongly increased by un-bonding the inclusion from the matrix. Results from studies on cement-mortars are presented in support of this and show significant improvements in toughness associated with samples in which inclusions are deliberately un-bonded from the matrix in advance of the crack arriving.