Abstract
The critical strain energy release rate, Glc, and the critical stress intensity factor or fracture toughness, Klc, of alumina trihydrate (ATH) filled epoxy resin have been determined as a function of filler volume fraction, using tapered double cantilever beam and single edge notch geometries respectively. GIc showed a maximum at a filler volume fraction of approximately 0.18. The fracture toughness increased linearly up to a filler volume fraction of 0.55. It is proposed that the predominant toughening mechanism depends on the volume fraction of filler and that ATH particles are strong enough for crack pinning to constitute the main toughening mechanism in the intermediate volume fraction region. At low volume fractions and with unstable crack propagation, crack blunting is also important. When the volume fraction is very high, ductile responses by the resin matrix are less easy and particle fracture becomes common, reducing the scope for crack pinning. The value of the breakaway parameter in the crack pinning model of Green was estimated in terms of the particle size.
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