The interfacial fracture observed in fragmentation tests performed with a model of fibre-reinforced composite is predicted using an elasto-plastic shear-lag method. The interfacial debonding propagation is significantly affected by the plastic deformation in the vicinity of the interfacial debonding tip. Hence, the matrix is assumed to be an elasto-plastic material that deforms according to J2 flow theory. The debonding process is predicted theoretically and compared with the experimental results obtained from the fragmentation tests using a model of glass-fibre reinforced vinylester composite. The theoretical prediction shows excellent agreement with the experimental results, which illustrates the importance of considering an effect of the plastic deformation.