Interface strength effects on the compressive-flexural/shear failure mode transition of composites subjected to four-point bending

Wang's microbuckling model 1] has been extended to oriented fibre composites loaded in four-point bending. The modified model shows that when any internal parameter (e.g. the interface strength) is changed, the resultant change of the interlaminar shear strength and that of the compressive strength are always of the same sense. Additionally, the degree of change of the shear strength is always larger than that of the compressive strength. As a consequence of this conclusion, a four-point bend test piece which normally fails in the flexural compressive mode may fail in the shear mode upon interface degradation. This was rationalized with the aid of the failure-mode transition diagram 2]. This diagram has been used to explain the change in bend failure mode resulting from a change of the external parameters, such as the span-to-thickness ratio, and the fibre fraction. Experiments were conducted to verify such a failure-mode transition behaviour for fibreglass composites of different interface conditions, when the flexural compressive failure mechanism was of the microbuckling type.