Optimization of multi-directional fiber architecture for resistance and ductility of bolted FRP profile joints

In view of the low structural efficiency of bolted joints in pultruded fiber-reinforced polymer (FRP) profiles with mainly unidirectional (UD) fiber architecture, off-axis plies were considered as an effective way to improve bolted joint performance. An experimental study of bolted joints was conducted with laminates composed of UD basalt fiber fabrics laid-up at different ply angles and stacking sequences to optimize the lay-up with regard to the integral structural performance of FRP laminates and bolted joints. Furthermore, 3D finite element models of the joints were established to obtain a better understanding of the progressive joint response and failure. The results showed that laminates with off-axis plies could significantly increase joint resistance due to a shift of the failure mode from sudden shear-out to progressive bearing failure. The change of failure mode thus also changed the response from brittle to pseudo-ductile. The experimental and numerical results agreed well in both the failure mode and joint resistance, with one exception, i.e. the applied Hashin-Rotem failure criterion was not able to capture fiber tie-action resistances in the 90°-plies. An optimum stacking sequence and proportion of 0°-, 45°- and 90°-plies was obtained, which can serve as reference for applications in optimized FRP pultrusion.