Residual Tensile Strength of CFRP Panels with Dual Stepped-Lap Repairs: Effects of Ligament Spacing, Patch Lay-Up, and Repair Arrangement

On-site repair of carbon fibre-reinforced polymer (CFRP) structures is widely employed when component removal and autoclave-based processing are impractical. However, existing repair guidelines provide limited guidance on the interaction between closely spaced repairs. The objective of this research is to investigate the coupled effects of ligament spacing, patch lay-up, and repair arrangement on the residual tensile strength of dual stepped-lap repaired composite panels. The methodology consists of a shell-based finite element model combining Hashin intralaminar damage and a mixed-mode bilinear cohesive zone model, which was validated against published experiments on damaged, stepped, and wet lay-up repaired laminates, predicting the measured strengths within 6.4%. Subsequently, fifteen repair configurations were analyzed, comprising four ligament spacings, three patch lay-up strategies, and two repair arrangements. Residual strength varied non-monotonically with ligament spacing for repairs arranged transverse to the loading direction. For the baseline patch, the strength retention factor increased from 0.57 for closely spaced repairs to a maximum of 0.85 at 120 mm, and decreased to 0.59 at 180 mm as the repairs approached the panel free edges. The compliant ±45° patch was the most sensitive to spacing (0.42–0.80), whereas the stiffness-matched patch varied only between 0.48 and 0.61. In contrast, in-line repairs showed limited sensitivity to spacing, retaining 0.72–0.77 of the intact strength. In conclusion, the results demonstrate that repair orientation relative to the principal load path governs the interaction between adjacent repairs more strongly than separation distance alone.