The reinforcement of metallic matrices with stiff ceramic particles improves significantly the elastic modulus as well as the wear and creep resistance. However, the incompatibility in the deformation modes between the ductile matrix and the brittle particles generates large stress concentrations, which lead to the early nucleation of damage during deformation. Many experimental investigations (see [1, 2] for instance) have shown that there are three dominant micromechanisms of damage in particle-reinforced metal-matrix composites which are: fracture of the particles, damage to the matrix, and damage at the particle/matrix interface. Damage is normally initiated by either reinforcement fracture or interface decohesion, and progresses homogeneously with deformation as the load released by interfacial damage or particle fracture is taken up by the surrounding matrix and the neighbouring particles. However, sooner or later damage is localized in a section of the composite, and the final fracture of the material occurs by the ductile failure of the metallic matrix. Damage nucleation in these composites often begins at the onset of plastic deformation and grows rapidly, resulting in a dramatic reduction of the tensile ductility (figure 12.1(a)) and of the fracture toughness (figure 12.1(b)); two critical properties from the point of view of engineering design.