On the fracture of brittle-matrix/ductile-particle composites

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Abstract
A mechanism for the toughening of brittle matrices with ductile particles is presented. It is shown that the constraints imposed on the particles by the rigid matrix suppress plastic deformation of the particles at crack tips so that the main contribution to composite toughness comes from ligament formation in the matrix and their fracture behind the advancing crack front. The incorporation of inherently tough second-phase particles into a brittle matrix does not therefore automatically lead to a large toughness improvement of the composite, if other requirements are not satisfied. As far as toughness is concerned, the most desirable composite is one consisting of a soft (low yield strength) particle strongly bonded to a brittle matrix and free from internal stresses. The mechanism of stress relaxation in a spherical particle embedded in a matrix of smaller thermal expansion coefficient is also described.