The finite element simulation and testing of stitched laminated composites

Laminated composites have found a wide range of applications in a wide variety of industries. Due to their high specific strength and drapability carbon fibre reinforcement polymers have been used in structural applications_in the aerospace industry where reduced weight can significantly improve the efficiency and performance of the component and aircraft, respectively. However, because of their weak interlaminar strength, the damage tolerance capacity of laminated composites is dependent on the relatively weak mechanical properties of the matrix material, the existence of manufacturing flaws and the fibre-matrix interfacial properties to name a few. As a result, due to such loading scenarios as low velocity impacts, manufacturing flaws and material imperfections, laminated composites are prone to delaminate and thereby lose their collective mechanical properties. This is particularly problematic since delaminations are usually hard to detect and costly to repair. For this reason researchers have worked in earnest to develop laminated composites with increased damage tolerance. Methods for this include the development of toughened epoxy matrices, three-dimensional (3D) fibre architectures and the mechanical insertion of 3D reinforcement in the form of stitching and pinning. Although research in the field of 3D reinforced laminated composites is continuously growing, the prohibitive costs involved in the manufacturing processes and the reduced inplane mechanical properties associated with 3D laminates, these materials have not found wide acceptance by engineers. Stitching and pinning on the other hand have been found to significantly improve the delamination toughness of laminated composites whilst reducing the inplane mechanical properties by 10- 20%. For this reason, 3D reinforcement in the form of stitching will form the foundation of this thesis. In order to test the effectiveness of 3D reinforced laminates, researchers have adapted conventional 2D unidirectional laminated composites tests. These include the mode I double cantilevered beam (DCB) test and the mode II end notch flexure (ENF) test. However, due to the low flexural modulus and added strength of the 3D reinforcement, the delaminating sub-beams have been found to fail in bending prior to crack propagation. To overcome this issue; some researchers have created new innovative tests whilst others have chosen to bond hick reinforcing tabs to either side of the stitched test specimen thereby increasing the flexural rigidity. Results have shown that 3D stitched laminates are far superior in resisting delamination than 2D laminates. With the development of 3D laminated composites has come the necessity for analytical and numerical models to predict their behavioural response due to damage. At present there are only a few good models designed for this purpose. These models have accredited 3D reinforcement with a significant improvement in delamination resistance and thereby validating experimental findings. All stitch models replace a discrete bridging traction with that of a uniformly distributed traction acting across the entire delamination area. Most available stitch models assume an elastic stretching and a debonding phase, stitch rupture and stitch pullout phase. These simple models have been found to be accurate in their prediction of stitch behaviour. Unfortunately the majority of developers and users of stitch models are universities and scientists undertaking research work. This leaves a yawning gap between researchers and the industries which use these computational tools. In this thesis, new finite element beam and plate schemes are developed for the analysis of stitched laminated composites. The stitches are modelled using discrete rod elements capable of tension only loads adhering to the independent and interconnected force­displacement relationships proposed by Jain and Mai. Mode I DCB and mode II ENF finite element schemes are proposed where the single-step virtual crack closure technique (VCCT) is used to calculate the strain energy release rates at the crack tip. To validate the finite element modelling, mode I and mode II delamination toughness tests are carried out on stitched woven laminated composites of varying stitch distributions. The results of both the finite element analysis and experiments indicate that the mode I and mode JI delamination toughness is significantly improved by stitching. The finite element analysis reveals that the discrete stitch model approach is effective in predicting mode I and mode II delamination toughness response. Furthermore; for the same stitch density, but varying stitch distributions, tests show that the delamination toughness results can be vastly different under mode I loading conditions. This implies that there is some optimum stitch distribution where delamination toughness response can be maximised whilst reducing such factors such as inplane damage and saturated bridging zone length. In this thesis, a protocol for the implementation of finite stitch element model is created in a commercial software package for the purposes of making stitching accessible and reliable for engineers wishing to use the technology but lack confidence in the material system. The protocol and model are validated against mode I and mode II experiments and were found to perform well for unstitched and stitched TDCB and TENF models. For further verification, a third test is devised named the L-Joint test, a specimen representative of a real world stringer/stiffener-panel structure. Designed to produce a mixed mode loading condition, the L-Joint test produced consistent load vs. displacement curves for the purposes of comparison. The L-joint specimen was modelled using 3D brick elements in MSC.Nastran. The results of the unstitched L-Joint finite element analysis showed good agreement with experiment; however, the stitched results were unsatisfactory. Experimental observation showed that multiple delaminations formed in the specimen at the elbow of the L-shaped joint. To account for this development, and reduce computational expense, a 2D plane strain L-Joint model was formed modelling the thin adhesive layer between adjacent laminae using a nonlinear stress-strain relationship. The load vs. displacement results of this analysis agreed well with experiments indicating that this protocol is highly effective at predicting the response of delaminating stitched laminated composites under mode I, mode II and mixed mode loading scenarios