Interfacial debonding and fibre pull-out stresses

Two current theories 11, 17] of interfacial debonding and fibre pull-out, which have been developed on the basis of fracture mechanics and shear strength criteria, respectively, are critically compared with experimental results of several composite systems. From the plots of partial debond stress, _d ^p , as a function of debond length, three different cases of the interfacial debond process can be identified, i.e. totally unstable, partially stable and totally stable. The stability of the debond process is governed not only by elastic constants, relative volume of fibre and matrix but more importantly by the nature of bonding at the interface and embedded fibre length,L. It is found that for the epoxy-based matrix composite systems, Gaoet al.'s model 17] predicts the trend of maximum debond stress, _d ^* , very well for longL, but it always overestimates _d ^* for very shortL. In contrast, Hsueh's model 11] has the capability to predict _d ^* for shortL, but it often needs significant adjustment to the bond shear strength for a better fit of the experimental results for longL. For a ceramic-based matrix composite, _d ^* predicted by the two models agree exceptionally well with experiment over almost the whole range ofL, a reflection that the assumed stable debond process in theory is actually achieved in practice. With respect to the initial frictional pull-out stress, _f, the agreement between the two theories and experiments is excellent for all range ofL and all composite systems, suggesting that the solutions for _f proposed by the two models are essentially identical. Although Gaoet al.'s model has the advantage to determine accurately the important interfacial properties such as residual clamping stress,q _o, and coefficient of friction, , it needs some modifications if accurate predictions of _d ^* are sought for very shortL. These include varying interfacial fracture toughness,G _ic with debond crack growth, unstable debonding for very shortL and inclusion of shear deformation in the matrix for the evaluation ofG _ic and fibre stress distribution. Hsueh's model may also be improved to obtain a better solution by including the effect of matrix axial stress existing at the debonded region on the frictionless debond stress, _o.