Effect of an adhesive layer on the mode I delamination in unidirectional CFRP bonded joints

Several toughening mechanisms, such as fibre bridging, can occur during delamination of fibre reinforced composites. In structures made of composite bonded joints, delamination can occur either in the composite or in the adhesive layer. In the first case composite toughness might be affected by the presence of the adhesive, especially if the crack propagates nearby the bond layer. The interaction of an adhesive layer with the propagation of sub-surface delamination cracks within the adherent remains a mechanism which is not well understood. In this work, the mode I delamination behavior of Asymmetric Double Cantilever Beam bonded joint specimens is investigated and compared with bulk composite specimens in order to assess the influence of an adhesive layer. Fracture resistance measurements show that the bond layer considerably affects the steady state fracture toughness of the composite with a very significant reduction of the bridging contribution. This effect is extended to two different types of epoxy adhesive: different curing cycles and different elasto-plastic properties. The extent of mode II due to the asymmetric position of the crack starter and to the adhesive layer is assessed both experimentally and numerically, and found negligible for the considered configurations. Besides, it is demonstrated that the change of compliance of a specimen due to the presence of an adhesive layer has almost no effect on the delamination behavior. Using an inverse identification methodology, the parameters describing the bridging traction relation are determined for both joint and bulk specimens. It is found that the rate of decay of bridging tractions is significantly higher for joints compared to bulk specimens. Those identified tractions-separation relations are subsequently implemented in cohesive zone models to simulate delamination of each configuration. Load displacement and crack growth are successfully predicted, demonstrating that this method, which consists in identifying a cohesive model for a specific crack position offset in the presence of an adhesive layer, can be used for prediction purposes. Moreover, it is shown that the identified tractions based on the steady state strain measurements predict the strains at the transient phase of bridging development. The isolated fibres and bundles of fibres participating to bridging are quantitatively measured in both bulk and joint specimens, and results show that mostly isolated fibres participate to bridging in the case of joints, whereas they tend to congregate to form clusters in bulk specimens, which require much more energy to break. A 3D finite element model show that the elasto-plastic properties of the adhesive create a local perturbation of the stress field around the crack tip. A shielding effect of the adhesive layer has been ascertained, preventing stresses from spreading continuously. The changes of amplitude and the spatial repartition of stresses in the process zone compared to bul