Impact performance and shear strength of composite materials for aerospace applications

Abstract

This PhD research involved both shear and impact evaluations of composite materials for aerospace applications. Firstly, to fully understand the shear strength of composite materials for aerospace applications, the Iosipescu shear test was performed and the different specimen geometries were compared to obtain accurate out-of-plane shear properties. A multi-scale Finite Element Model (FEM) was devised in collaboration with AVIC The First Aircraft Institute (FAI) with suitable failure criteria to simulate the failure of thick laminate Carbon Fibre Reinforced Plastics (CFRP) under shear loading. The thick-sectioned laminates are modelled at a sub-laminate level whilst the structural failure is predicted at a ply level which incorporates shear nonlinearity for the thick composite structure. Based on this multi-scale approach, a user-defined FORTRAN subroutine (VUMAT) has been written for ABAQUS/EXPLICIT solver and is used to model the shear nonlinearity and intra-laminar failure. In addition, a cohesive zone model is used to predict the inter-laminar delamination. The modelling has been employed to predict the failure processes for Iosipescu shear test specimens with different fibre orientations. The results show that both the failure mode and the load-displacement trace for finite element simulations agree closely with the experimental findings. This demonstrates the validity of this multi-scale, nonlinear, three-dimensional model for thick laminates. In particular, for the Iosipescu shear test, the effect of the fibres being aligned along the length of the specimen or out-of-plane is investigated as well as different dimensions of the specimen. This research has recently been published in Reference [1]. Secondly, for the impact evaluations of composites, low and high velocity impact performance of composite sandwich structures with foam cores were investigated. In particular, the idea of multi-layering the core by foam layers of different density and its effect on the energy absorption under low and high velocity impact is of interest. In this thesis, composite sandwich panels made of Glass Fibre Reinforced Polymers (GFRP) for skins and polyvinyl chloride (PVC) foam for the cores are used. Low and high velocity impact of the sandwich composite structure samples, using a drop weight tower and gas gun facility were performed. By stepwise grading the foam core with different densities, it was found that there is a better energy absorption if you have a lower density on the impact side, and higher density at back surface. Rigid and compliant impact by using steel and high-density polyethylene (HDPE) was performed in the experiments. The sharp edge of a projectile cuts the composite face skin to produce more damage on the back face skin, but the graded foam core of lower density on the front can help to absorb more impact energy by spreading the load. Mechanisms of failure have been established such as core crushing, skin/core cracking, delamination and fibre breakage, and suggestions for modelling these failure processes is proposed. A paper is in draft on this research in collaboration with researchers at Royal Melbourne Institute of Technology (RMIT) [2]. Both of the shear and impact experiments were validated by using Digital Image Correlation (DIC) system. By introducing this method, the full field strain on specimen during the experiments was recorded to determine the full structural response under static and dynamic loading. These studies were part of a research programme sponsored by the Aviation Industry Corporation of China (AVIC) Centre for Structural Design and Manufacture at Imperial College London.