Design of composite to metal bolted joints for lightweight structures

Composite materials are increasingly adopted in transportation vehicles, for which weight reduction is one of the greatest challenges. In the vehicles frame structures, joints represent the weakest point, due to the introduction of geometrical and material discontinuities, which cause highly stressed regions. The joint design is particularly crucial when composite materials are used. In the present thesis, the mechanical behaviour of hybrid composite-to-metal bolted joints subject to static and dynamic tensile loads was investigated. It was found that the first material damage occurred in the most stressed 0° oriented ply of the carbon/vinyl ester laminate, via a micro-buckling phenomenon. The experimental results, combined with microscopic analysis, showed that the fibre micro-buckling damage initiation corresponded to a significant drop in the joint bearing stiffness plot. The effects of bolt pre-tensioning moment, washer size and temperature on the static micro-buckling strength were experimentally examined. All of these parameters were shown to significantly affect the mechanical behaviour of the joint. The effects of bolt pre-tensioning and washer size were studied with a detailed three-dimensional finite element (FE) model. The model also allowed the determination of the most important phenomena involved in the load transfer and the stress distribution within the different composite plies. Two different failure criteria were proposed and implemented in the model to predict the first micro-buckling load; one of them proved to be suitable for this purpose and was consistent with the experimental results. FE simulations were also used to investigate possible design improvements, such as an increased overlap length or a larger bolt diameter. The numerical results revealed a higher micro-buckling strength in both cases. Finally, a brief experimental investigation on the effects of bolt pre-tensioning on the fatigue life of single bolted joints was performed; also in this case the clamping force was proven to have a strong influence on the dynamic strength.