Efficient load distribution analysis and strength prediction of bolted composite joints at various loading rates

Mechanical fasteners are used extensively in composite aircraft structures, offering a cost-effective, reliable and repeatable joining method while also facilitating disassembly for maintenance and repair. However, current design practices rely extensively on experimentation and detailed numerical analyses which are time consuming and expensive and thus significantly increase development and production costs. Although three-dimensional finite element analysis can be used in-lieu of more expensive experimental tests, current state-of-the-art computing technology cannot facilitate analysis of large-scale sub-structures (e.g. panels, wing spars, etc.) with potentially thousands of fasteners. The work conducted herein addresses this issue through the development of highly efficient analysis methodologies for bolted composite joints loaded at various rates. Several highly efficient numerical methods are developed that can account for laminate-laminate friction, bolt-hole clearance, non-linear material behaviour, bolt failure and loading-rate effects. Validated against quasi-static and high-rate experimental data, these methods have significantly advanced the state-of-the-art, due to their ability to accurately predict the response of multi-row, multi-column joints in seconds. Such an increase in efficiency is owed to the implementation of a series of parameterised functions which model the local mechanical behaviour of the fastener and fastened material. This approach allows the statistical variations arising from manufacturing and material processing to be easily accounted for, a feat that is impractical using traditional modelling techniques and near-impossible experimentally. Furthermore, these methods have been developed into user-friendly, stand-alone joint analysis tools and a user-defined finite element which is straightforward to implement in commercial finite element software. The developed analysis methods were used to investigate the effects of loading rate and relative spacing between fasteners in multi-bolt joints. Although loading rate effects primarily manifest as changes in joint failure loads and energy absorption, the propagation of stress waves also leads to variations in load distribution of up to 1.7% in the joints considered. Furthermore, it was found that a strain shielded region exists in all multi-fastener arrays and consequently any bolts positioned within this region carry a reduced proportion of load. When load is evenly distributed across the joint, increased failure initiation loads are to be expected, however, the most effective method of augmenting joint strength is through the use of additional fasteners. Additionally, the use of a load-path visualisation methodology gave novel insight into the role of fibre orientation on load transfer in bolted multi-directional laminates. It was found that load is transferred in a trajectory that is strongly dependent on fibre orientation, which is important in the distribution of load in multi-fastener joints.