Optimisation study of tapered scarf and stepped-lap joints in composite repair patches

Stepped-lap repairs of composite structures can offer an alternative that is easier to perform and less time-consuming to produce than the widely used tapered scarf repair. However, the design of stepped-lap joints must be carefully investigated in order to avoid generating stress concentration regions. This study investigates the influence of joint parameters on peak stresses in the adhesive bondline in tapered scarf and stepped-lap joints. Linear finite element analysis was performed to conduct a parametric study with focus on six joint design parameters: ply thickness, adhesive thickness, taper angle, stacking sequence, overply layup, and overply lap length. Results showed that tapered scarf and stepped-lap joints have a strong sensitivity to ply thickness, taper angle, and stacking sequence. The introduction of overplies provided protection and stiffness at joint tips, and a critical overply lap length was identified. The location of 0° plies in the composite laminates was highlighted as an important factor. The analysis was then extended to three-dimensional FE models for verification. In conclusion, results showed that high stress concentration in stepped-lap joints can be mitigated with the introduction of overplies and appropriate changes in joint design parameters to reduce stress peaks at joint tips and step corners.