An experimental investigation into the tensile strength scaling of notched composites

An extensive experimental program has been performed to investigate the effect of scaling on the tensile strength of notched composites. Hole diameter, ply and laminate thickness, were investigated as the independent variables, whilst keeping constant ratios of hole diameter to width and length, over a scaling range of 8 from the baseline size. In most cases strength decreased as specimen size increased, with a maximum reduction of 64%. However the reverse trend of strength increasing with in-plane dimensions was found for specimens with plies blocked together. As well as the variation in strength, three distinct failure mechanisms were observed: fibre failure with and without extensive matrix damage, and complete gauge section delamination. Despite these differences, similar sub-critical damage mechanisms were seen in all specimens, with the extent of the damage determining the failure stress and mechanism. Damage propagated across the gauge section via delamination at the hole, which was controlled by the ply thickness to hole diameter ratio. This same mechanism can explain both the increasing and decreasing strengths observed. Simple analytical criteria for determining notched strength were found to be accurate for fibre failure in the absence of extensive sub-critical damage, but could not account for those conditions where delamination propagated across the width prior to failure.