Experimental and numerical investigation of composite tension absorber joints for improved aircraft crashworthiness

This thesis investigates the use of specially-designed “tension absorber” joints in composite vehicular structures for the absorption of energy in a crash situation through a process referred to here as “extended bearing failure”. The specific targeted application is future narrow-body composite aircraft fuselages which require an innovative energy absorption strategy due to the limited height available below the cargo floor for traditional crush beams. However, tension absorbers could be applied in any structure requiring energy-absorption capability in a crash or overload situation. Through a combined experimental-numerical approach, the work aims to provide fundamental information on the effects of geometric and material parameters such as stacking sequence, pin diameter, laminate thickness and loading rate, and an assessment of whether state-of-the-art numerical simulation is capable of providing genuinely predictive capability for such a complex problem. To make the results as useful as possible the chosen material is IM7/8552 carbon/epoxy, one of the most widely-characterised materials in the literature. Thus the results can be used by other researchers to test out modelling approaches without the need for further material testing. Besides the results in the published papers, videos provided as supplementary information contain complete three-dimensional (3D) maps of internal specimen damage, obtained from computed tomography (CT). The chosen performance parameters are ultimate bearing strength (UBS), mean crushing stress (MCS) and mass-specific energy absorption (SEA). Diameter-to-thickness (D/t) ratio is found to be an excellent predictor of UBS and SEA for both quasi-static and dynamic loading rates, with small D/t values giving best results, provided the thickness is sufficient to avoid global bending of the specimen. Concerning the effects of stacking sequence, it is found that the most important factor in maximising SEA is having small changes in orientation at ply interfaces. This is even more important than 0° content. Laminates with a high SEA tend to have a low UBS. Highest UBS was for quasi-isotropic laminates. Increased loading rate results in increased UBS but decreased SEA. The implemented model is a physically-based, three-dimensional damage model which uses in-situ ply strengths, stress-based fibre failure criteria, Puck’s criteria for matrix damage, a non-linear law for in-plane shear, a cohesive zone model for delamination, a crack-band model to mitigate mesh sensitivity, and frictional contact between the pin and the laminate, and between plies once they delaminate. The developed model is found to accurately predict the global response in terms of strength and energy absorption and can forecast the effects of changing geometry and material parameters. Critically, comparison with CT scans shows that it also captures the key mesoscale damage mechanisms.