A computational investigation of failure modes in hybrid titanium composite laminates
High strength, low weight Fibre Metal Laminates (FMLs) are increasingly finding use in military and commercial aircraft applications by combining benefits of both fibre reinforced composite and metal components such as fracture toughness and fatigue resistance. Glass fibre reinforced composite laminates combined with aluminium (GLARE) was recently selected for the upper fuselage skin structure of Airbus A380 aircraft. Hybrid Titanium Composite Laminates (HTCLs) made from thin titanium sheets and carbon Fibre Reinforced Polymer (FRP) are more suitable in high strength environments at elevated temperatures and may therefore be used in future aerospace applications. Currently, the combination of titanium and FRP is mainly applied to surgical implants or biomedical devices. In order to improve design and performance of FMLs, it is essential to understand their failure mechanisms under various loading conditions. Major failure modes in composite laminates are delamination, matrix cracking and fibre fracture. Delamination can be initiated by stress concentrations at free edges, interfacial voids or transverse matrix cracks. The latter is referred to as Matrix Cracking Induced Delamination (MCID) An additional critical failure mode in FMLs is debonding between composite and metal sheets. These failure modes cause stress redistribution and can lead to complete loss of load carrying capability resulting in structural collapse. Virtual testing of FMLs can not only reduce physical testing, it also offers cost effective design flexibility and certification. FML structures can be virtually optimised to suit specific applications before time consuming and expensive manufacturing. Therefore, effective and accurate models are required to describe failure mechanisms and their interaction. The Phantom Node Method (PNM) is a numerical concept to simulate failure initiation and evolution in heterogeneous materials such as FMLs within the Finite Element Method (FEM) It includes the well established Cohesive Zone Method (CZM) to approximate nonlinear fracture processes in interfaces or cracks. Elemental locality and the use of standard shape functions make PNM conceptually simpler and hence easier to implement and modify compared to similar enrichment methods such as X FEM. This research project aims to develop, validate and use computational methods within FEM in order to investigate failure modes in composite laminates and HTCLs. Thereby, standard simulation tools as well as novel conceptual methods such as PNM are to be applied. Experimental investigations provide important modelling properties as well as data to validate numerical results. A novel Top Surface Analysis (TSA) technique using Digital Image Correlation in a