Comparison of methods for determination of fracture toughness in a multi-directional CFRP laminate

Quasi-static tests were carried out on calibrated end loaded split (C-ELS) specimens to determine a critical initiation interface energy release rate or fracture toughness G_ic. A multi-directional (MD) carbon fiber reinforced polymer (CFRP) laminate with a delamination between a unidirectional (UD) fabric ply with fibers oriented mainly in the 0^?- direction and a plain balanced woven ply with tows oriented in the +45^?/ - 45^?- directions was considered. The G_ic values from the non-precracked (NPC) specimens containing an artificial delamination were evaluated by means of an experimental compliance method (ECM), a beam theory (BT) method, as well as a two-dimensional finite element analysis (FEA) together with the area J-integral. In addition, the displacement extrapolation (DE) method and the virtual crack closure technique (VCCT) were used to determine the stress intensity factors K_m (m = 1, 2) for each test. The stress intensity factors were normalized with a length scale L = 100 µm and used to calculate the phase angle ?. Finally, the obtained results were compared to critical initiation values which were obtained in a previous study from Brazilian disk (BD) tests for the same material and interface. The G_ic value is a necessary property for predicting propagation of a delamination along the investigated interface. The aim of this paper is to examine the differences which occur as a result of using various methods to determine this value. Work is in progress to determine fracture resistance curves or R-curves. These curves relate the energy required for a delamination to propagate G_iR to the delamination extension ?a and may be used to predict the delamination resistance to propagation.