Insights into Toughening Particulate Brittle Composites
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Insights into Toughening Particulate Brittle Composites. I.J.Merchant, H.W.Chandler, R.J.Henderson T. Stebbings and D.E.Macphee Departments of Engineering and Chemistry, University of Aberdeen, Old Aberdeen AB24 3UE ABSTRACT 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. INTRODUCTION Despite the considerable amount of literature available on crack propagation and toughening mechanisms in composites [1-6], the exact sequence of events surrounding the interaction of propagating macrocracks and reinforcing inclusions still seems to be unclear. There appears to be universal agreement over the importance of matrix-inclusion interfaces whether or not these are bonded at the time of crack arrival does not appear to have been systematically studied. In light of results from our own studies on toughening of cement mortars [7,8], the emphasis of this paper is on engineering matrix-inclusion de-bonding prior to crack arrival. Normally, a de-bonded interface represents a weakness in composite materials. Matrix flaws, such as porosity and unbonded interfaces, attract cracks in a stressed composite, and provide an easy propagation path. The resulting fracture surface in a porous matrix is therefore characterised by 'flat' fracture paths. A propagating crack front interacting with a spherical pore in a solid is very different to a crack in a plate interacting with a throughthickness hole. As in the plate the crack is “attracted” by the pore but in three dimensions the crack front can extend around the pore maintaining the sharpness of the tip. This leads to the flat fracture seen in a confectionery bar (Figure 1) where it is clear that the crack has bisected the pores. It is this property of porous brittle matrices which is exploited in the present and novel approach to toughening. Attracting the propagating crack provides a means of optimising crack-bridging in a matrix provided suitable bridging inclusions have been distributed into Figure 1: Flat fracture in engineered porosity [7]. In other words, by inducing matrixfoamed confectionary. Inset inclusion de-bonding prior to crack arrival, a t
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