TRANSITION IN FLEXURAL MICROBUCKLING MECHANISMS IN UNIDIRECTIONAL GLASS-FIBER-REINFORCED THERMOPLASTICS

A transition in the mechanism of flexural failure previously observed in low matrix modulus unidirectional glass fibre composites is semi-qu antitatively explained by considering the criterion for each of the fa ilure modes. The failure strength for cooperative fibre microbuckling is controlled by the shear modulus of the composite which is linearly related to the Young's modulus of the matrix, while the failure streng th for delamination splitting microbuckling is controlled by the compo site shear strength which is not as strongly dependent on the Young's modulus of the matrix. Because the critical failure stresses have diff erent dependencies on the matrix modulus, a transition from cooperativ e fibre microbuckling to delamination splitting microbuckling occurs a s the matrix modulus increases. Due to the stress gradient in the beam , the compressive failure behaviour in bending is not the same as in u niform compression. When the failure mode is cooperative fibre microbu ckling, the bending strength is higher than expected, especially in th e thin beams. In bending, the delamination splitting microbuckling mod e does not lead to abrupt splitting of the entire beam, but rather occ urs by gradual accumulation of surface damage.