An experimental and numerical investigation into the damage mechanisms in notched composites

Investigations of the effect of size on the tensile strength of composite laminates containing circular holes show that there is a large difference both in failure stress and mechanism due to changes in test configuration. This is particularly true of the ply and laminate thickness, and hole diameter. Interrupted tests have been performed on open hole tensile specimens at different load levels to determine the progressive damage development, evaluated through non-destructive testing (X-ray and C-scanning). The tests were also analysed using a novel Finite Element Modelling technique. This was able to accurately predict the wide range of ultimate strengths measured with variation in test parameters, principally through incorporation of the sub-critical damage in the analysis. A significant damage mechanism was seen to be delamination at the hole edge which generally occurred at a lower stress for a smaller hole diameter to ply block thickness ratio. Delaminations allowed damage to join up through the thickness of the laminate and propagate. In ply-level scaled specimens, the delamination propagation was the ultimate failure mode of most of the specimens. In sub-laminate level scaled specimens, localised damage relieved stress in the 0° fibres at the hole edge, delaying the onset of fibre failure. Less damage was seen for larger holes, thus leading to a decreasing failure stress with increasing hole diameter.