Strengthening of Composite T-joints using novel ply design approaches

Author(s): 
L. Burns, A.P. Mouritz, D. Pook, S. Feih (RMIT University)

CHALLENGE - T-shaped composite joints are used extensively in aircraft, ship, civil infrastructure, automotive and other applications to stiffen and connect structural components. The materials used in composite T-joints often have brittle-like properties with low failure strain, making them susceptible to delamination and other fracture modes, particularly in the radius bend region. The objective of this study is to evaluate three novel design approaches for the ply stacking sequence of the T-joint radius bend to increase the strength without compromising other key design parameters such as stiffness or weight when compared to the joint with a conventional quasi-isotropic laminate design. 

SOLUTION - This study determines the optimum stacking sequence for each design approach numerically and validates the predicted improvement experimentally for bending and tension load cases. 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 DoE and numerical optimisation programs were developed using the software modeFRONTIER v4.3 using MOSA algorithm.

BENEFITS - It is shown numerically that the new designs are able to reduce the maximum interlaminar tensile stress generated within the radius bend region of a composite T-joint under an externally applied load. 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.