Strengthening of composite T-joints using novel ply design approaches

This study assesses three different and novel design approaches to increase the failure stress and elastic damage limit of T-shaped structural joints made of carbon fibre-epoxy composite. The ply orientations to the laminate within the T-joint radius bend, which is highly susceptible to delamination failure, were designed using a numerical optimisation method and two design-of-experiment methods to increase the failure load. Finite element modelling and experimental testing proved that all three designs successfully tailored the ply orientations within the radius bend region to increase the delamination initiation load of the T-joint, while maintaining its original stiffness and weight. The new designs resulted in large improvements to the damage initiation load under both bending loading (up to ∼125% increase) and tensile loading (up to ∼85%) compared to a conventional composite T-joint with a quasi-isotropic ply design. The three new designs also increased the elastic damage limit and absorbed strain energy capacity of the T-joint. The improved designs lower and generate a more uniform interlaminar stress field within the radius bend region, and this increases the initiation load for delamination failure.