Interfacial debonding and fibre pull-out stresses - Part II A new model based on the fracture mechanics approach

An improved analysis has been developed for the interfacial debond stress in a fibre pull-out model based on the concept of fracture mechanics where the debonded region is considered as an interfacial crack and its extension is dependent on a fracture energy criterion being satisfied. By evaluating the partial debond stress, σdp against debond length l, during progressive debonding, instability conditions are derived where the maximum debond stress, σd*, is determined for different embedded fibre length, L. Comparisons between theory and experimental fibre pullout results on several composite systems show that the present model gives excellent prediction of the maximum debond stress, σd*, for the whole range of L including even the very short L, whereas the previous Gao-Mai-Cotterell model, also developed on the basis of a fracture mechanics approach, always overestimates σd* for short L and gives a finite value for L=0. The initial frictional pull-out stress, σfr, after complete debonding predicted by the present model is basically the same as the Gao-Mai-Cotterell model and agrees well with experiments. The implications of stress distributions in the constituents for different composite systems are discussed on the basis of the proposed debond criterion.