Load transfer within the bolted joint of a laminate made from ultra-high molecular weight polyethylene fibres

The mechanism of load transfer within the bolted joint of a laminate sheet made from ultra-high molecular weight polyethylene (UHMWPE) plies is investigated both experimentally and by an analytical model. The nature of load transfer and the active failure mechanisms are obtained as a function of joint geometry and of the lateral clamping force on the faces of the laminate (by pre-tensioning of the bolt). A combination of X-ray tomography and optical microscopy reveal that the dominant failure mechanism in the clamped joint is shear failure involving splits of the 0° plies and sliding at the interface between the 0° and 90° plies. A simple analytical model is developed for this shear failure mechanism and, upon noting the competing failure mechanisms of bearing failure, bolt shear and of tensile failure of the 0° plies, a failure mechanism map is constructed in terms of the geometry of the bolted joint, for the case of no pre-tension of the bolt. The analytical model for shear failure suggests that the enhancement in joint strength with increased pre-tensioning of bolt is due to the fact that the shear strength of the UHMWPE increases with increasing hydrostatic pressure.