Initiation and Propagation of Delamination in a Centrally Notched Composite Laminate

Abstract

Interlaminar stresses occur at the free edge and notch to cause composite laminates to delaminate. Actually, delamination is an important failure mode and affected, for example, by fiber orientation and stacking sequence as discussed. In analysis, our de veloped mathematical model yields the interlaminar edge stresses and singularities around the hole to predict the onset of delamination. Furthermore, our work mainly aims to inves tigate experimentally the initiation and propagation of delamination subjected to tension and cyclic tension-tension (T-T) tests in centrally notched T300/5208 graphite/epoxy com posite laminates, including four layups: (a) orthotropic [=±45]4s, [90/0] 4s and (b) quasi- isotropic [0/90/±45]2s, [90/0/±45]2s. Experimental result and observation associated with the obtained maximum stresses and the adopted failure criterion are in good agree ment with our predicted delamination onset. Other important findings are highlighted as follows. The failure mode of each layup recorded by X-ray radiograph can be closely cor related to the prediction of our model. The delamination growth rate has also been mea sured. An approximately linear relationship between delamination area and stiffness re duction for the centrally notched quasi-isotropic lammates is established. The critical load for delamination can be estimated by the Hashin-Rotem failure criterion satisfactorily. As a benefit of nondestructive testing, we finally find that the residual strength of the laminate first increases as applied cycles increase and then decreases during fatigue tests. However, residual stiffness always decreases monotonically with the increase of applied cycles.