Apparent strength scaling in continuous fiber composite laminates

The tensile strength of the 0° plies within different cross-ply and quasi-isotropic laminates of varying size and stacking sequence has been investigated. For those lay-ups having failure confined to the gauge section, no size effect was observed in the strength of 0° plies. In laminates exhibiting a size versus strength relationship, failures were found to occur in the gauge section for the smallest laminates, but at the grip for the larger laminates. The occurrence of failure at the grips is not obvious for these lay-ups. A major purpose of this paper is to identify the failure initiation point, and the sequence of events that leads to complete fracture of 0° plies in unidirectional laminates, and the quasi-isotropic and cross-ply lay-ups. A statistically significant set of 3-point bend tests of unidirectional beams was conducted to obtain valid parameters (Weibull modulus and strength) for a Weibull-statistics-based strength model which would be used to predict the ultimate strength of 0° plies within different cross-ply and quasi-isotropic laminates. No significant difference between the maximum stress in bending or strength of the valid tensile tests was found. In contrast Weibull theory predicted unidirectional strength loss of over 30% for the range of volumes tested. Although the dramatic strength scaling observed here has been shown to arise from the test method, the authors are not ready to rule out scaling effects.