The effect of interfacial properties on the strength of discontinuous reinforced metal-matrix composites is systematically studied by theoretical modelling. The calculations were carried out within the framework of continuum plasticity theory using cell models and the finite element method. A wide range of inclusion aspect ratios, volume fractions, and interfacial strengths were investigated for perfectly plastic and hardening matrices. Interfaces were modelled either as strongly bonded, or as shearable but strong normal to the inclusions, or as debonding at the reinforcement ends but strong on the sides. Additionally, the effects of reinforcement arrangement and extensive damage to continuous fibre composites were addressed. Debonding at the ends of the inclusions was found to have the most deleterious effect on the strength of the composite. When debonding does not occur but interface sliding takes place freely, an amount of strengthening is seen which is a function of the inclusion volume fraction but is primarily independent of the inclusion aspect ratio. For extensively damaged continuous fibre composites, a weak interface yields a steady-state composite flow strength slightly higher than the volume fraction of the matrix times the yield strength of the matrix. This increases linearly with the interfacial shear strength up to the level for strongly bonded composites and can be estimated from the intact fibre aspect ratio, the matrix yield stress, the volume fraction, and the interfacial strength
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