Damage Extension in Carbon Fiber/PEEK Crossply Laminates under Low Velocity Impact

Abstract

Low-velocity impact in carbon fiber/PEEK crossply laminates has been studied by test and analysis. Emphases of the study were focused on the material properties which may control the damage extension of transverse crack and delamination. It was found that, considering the thermal residual stress and the crack constraining effect, extension of transverse cracks could not be predicted by the Strength of Materials approach. The impact-induced delamination could be characterized by the crack arrest concept of fracture mechanics. The delamination resulted from a Mode II-dominated unstable fracture, which occurred under displacement-controlled conditions and seemed to be arrested at a constant interlaminar fracture energy. It was found that the thermoplastic APC-2 composite exhibits the same damage modes as epoxy composites under low velocity impact. Both the matrix-controlled damage and the fiber-controlled penetration may become the dominant failure mode, depending on the stacking sequence of the laminate. The residual stress in the thermoplastic laminates is as high as half of the transverse strength of the unidirectional material. The crack constraining effect tends to increase the in situ transverse strength of the lamina as the lamina thickness decreases. Considering the residual stress and crack constraining effect, the transverse crack extension cannot be predicted by the Strength of Materials approach. The crack arrest concept of fracture mechanics seems to be a useful approach to predict the extension of impact-induced delamination. The delamination resulted from a Mode II-dominated unstable fracture, which occurred under displacement-controlled conditions and seemed to be arrested at a constant interlaminar fracture energy. By assuming the delamination arrest at about the time of maximum impact load, the delamination arrest toughness could be evaluated from the test data of [05/905/05] laminates. The delamination arrest toughness is also found to be close to the Mode II-propagation toughness of the material.