All-carbon hybrid laminates: tensile behaviour and numerical modelling

One of the main advantages of fiber reinforced composites is the design freedom, which can be better exploited using fiber-hybridization. Interlayer hybridization, where low-elongation (LE) material is sandwiched between high-elongation (HE) material, can be used to overcome the lack of ductility of fiber reinforced composites. The synergetic effects of hybrid fiber reinforcement can provide, among other features, the pseudo-ductility, i.e. a pseudo-ductile response by the combination of brittle composites. Hybridization tends to improve composite properties but induces complex failure mechanisms. Different types of hybrid laminates have been proposed by combining e.g. glass/carbon and aramid/glass. The disadvantage of coupling glass or aramid fibers with carbon fibers in a hybrid composite is the reduction in the stiffness of the laminate due to the limited amount of carbon fiber plies. A solution to achieve pseudo-ductility and limit the reduction in composite stiffness is to use all-carbon fiber hybrid reinforcements. The pseudo-ductility of all-carbon hybrid laminates has often been studied in quasi-static tension and their damage mechanisms are reasonably well-understood for UD thin-ply laminates (ply thickness 20-40 μm). In contrast, only a few studies have been devoted to thick-ply laminates (ply standard thickness 100-200 μm). In this context, the presentation is dedicated to an overview of our recent research to understand the mechanical behavior of all-carbon thick-ply hybrid laminates. Several stacking sequences have been considered to assess the effect on the tensile quasi-static pseudo-ductile behavior and on the damage evolution. Among the stacking sequences, a quasi-isotropic all-carbon interlayer hybrid laminate has been dedicated to the description of the tension-tension fatigue response in terms of: the fatigue life in a wide range of maximum strain levels in the pseudo-ductile range and beyond it, the evolution of the fatigue damage by the stiffness retention and the damage observation by X-ray micro-CT. Furthermore, the notch insensitivity of quasi-isotropic all-carbon thick-ply hybrid laminates with an open hole was demonstrated with the similar pseudo-ductile behavior of the notched and unnotched specimens. Finally, a finite element model is presented to predict the damage evolution in unidirectional (UD) all-carbon hybrid composites subjected to quasi-static tensile loading. The model exploits translaminar cohesive elements governed by a unimodal Weibull strength distribution in the LE plies to simulate fragmentation and interlaminar cohesive elements to simulate delamination at the LE and HE interfaces. The model had good agreement with experimental data for the tensile behaviour of thin- and thick-ply all-carbon hybrid laminates.