Numerical study on interfacial properties of advanced composites

Composites, which have been extensively used in broad areas are produced to optimise the material’s mechanical, chemical and physical properties. The interfacial properties often influence significantly the composite performance in all types of composites. The main aim of this research is to investigate the interfacial properties of various advanced composites, including carbon fibre-epoxy composites, carbon nanotube (CNT)-epoxy and graphene-polymer nanocomposites, and CNT-hybridized carbon fibre (CNT/CF) composite. In the study of a single carbon fibre pullout from a polymer matrix under the influence of residual thermal stresses, a finite element method was first applied to analyse the residual thermal stress components along the interface between the fibre and the matrix due to curing. Four typical carbon fibres, T300, AS4, T800HB-40B and IM7 with anisotropic properties embedded in epoxy matrix HY6010 were considered. A comparison study indicates that simplified isotropic fibres model would overestimate the compressive residual axial and interfacial radial stress by up to 30%. The cohesive zone model was then used to study the single carbon fibre pullout test. The numerical results indicate that the residual thermal stresses have a significant influence at the stage of frictional sliding after interfacial debonding, which leads a higher specific pullout energy. An interesting finding is that the interfacial shear strength is not a constant, which decreases with the increasing fibre embedded length. In addition, the specific pullout energy increases with the embedded length due to the effect of residual thermal stresses and interfacial friction. But the specific pullout energy does not depend on the fibre radius, which implies that reducing fibre size alone would not enhance the fibre bridging effect. In order to investigate the interfacial behaviour of CNT-epoxy nanocomposite, a numerical simulation of a single CNT pullout using cohesive zone modelling was carried out. The numerical results indicate that the debonding force increases almost linearly with the increase of interfacial crack initiation shear stress, and also slightly increases as the complete tangential separation increases. A saturated debonding force exists corresponding to a critical CNT embedded length. If the interface contains strong chemical bonding, the higher saturated debonding force can be achieved, and the force can approach to the saturated value more rapidly. Carbon nanotube (CNT)-hybridized carbon fibre (CF) composite is a new generation composite. To evaluate the bridging effect of this new reinforcing phase, a numerical method is proposed to theoretically investigate the pullout of a hybrid fibre. There are two finite element models developed in this method, which are applied to simulate a single CNT pullout from the matrix at microscale and the pullout of the hybrid fibre at macroscale. The bridging effect of the CNTs during the hybrid fibre pullout is simulated by spring elements in the macroscale finite element model, where the properties of spring elements are obtained from the microscale finite element simulation. The numerical results indicate that the apparent interfacial shear strength of the hybrid fibre and the specific pullout energy can be significantly increased due to the additional bonding of the CNT-matrix interface. A parametric study indicates that the bridging effect of the hybrid fibre can be further enhanced by improving the interfacial bonding between CNT and matrix and increasing the size or length of CNTs. In order to evaluate the interfacial properties of a flat monolayer graphene-polymer nanocomposite, a plane-strain numerical model is developed to simulate the graphene-polymer peel test, where opening mode of fracture is dominated. The cohesive parameters are characterized by matching the simulated peel force with the experiment result. The numerical result shows that the adhesive fracture energy is slightly dependent on the peel angle. The cohesive parameter determined from a 90° peel test may be used to define the cohesive law for other angles. This study systematically investigated the interfacial behaviours of four various advanced composites. A better understanding of the effect of nano-reinforcement in composite against fracture is gained. The outcome of the study will contribute new knowledge to the further development of the composites and improve their reliability.