INPLANE AND INTERLAMINAR SHEAR PROPERTIES OF CARBON/EPOXY LAMINATES

Carbon/epoxy laminates of three different lay-ups have been tested to measure both in-plane shear and interlaminar shear (ILS) properties by the Iosipescu shear method. A reasonably pure state of shear strain is achieved in the region between the roots of two V-notches in all the laminates. However, the assurance of proper shear failure modes is extremely difficult for some lay-ups on certain shear planes. It is found in the tests of ILS properties that delamination or ILS cracking initiates at load levels much lower than the ultimate so that uncertainty arises as to which stress should be used as the measure of the material property. It is shown that the ILS stiffnesses deteriorate gradually in a non-linear fashion even after the onset of delamination. It is also found that the in-plane shear properties increase with increase in number of reinforcing fibre directions. The ultimate ILS stresses of specimens with fibres in the length direction (if ultimate loads are used) seem to be independent of material lay-up while the ILS moduli steadily decrease with increase in number of reinforcing fibre directions.