Mechanical response and damage evolution mechanisms of single and double-sided patch-repaired open-hole CFRP laminates

This work investigated the influence of adhesively bonded patch repair on the mechanical performance and damage evolution behavior of carbon fiber reinforced polymer (CFRP) laminates with open-hole damage. A few specimens were designed, including standard open-hole plates, pre-damaged open-hole plates, and single- and double-sided patch-repaired specimens with two patch diameters (15 mm and 20 mm). Quasi-static tensile tests combined with digital image correlation (DIC) technique were conducted to systematically examine the global mechanical response, strain field evolution, and damage and failure mechanisms of different repair configurations. The results indicated that patch repair could effectively restore the load-carrying capacity of open-hole damaged laminates, with double-sided patch repair exhibiting significantly superior strengthening efficiency compared to single-sided repair. The specimen with 20 mm double-sided patches achieved a maximum failure load of 6.50 kN, representing an increase of 22.0% compared to the standard open-hole plate. The corresponding failure displacement reached 9.24 mm, which is 16.3% higher than that of the baseline specimen, demonstrating the best overall mechanical performance. DIC results reveal that patch repair substantially modifies the strain distribution around the central hole. Double-sided patch configurations effectively suppress local strain concentration and enhance the uniformity of load transfer. Damage evolution analysis shows that adhesive debonding is the primary initial failure mode in patch-repaired structures, followed by fiber fracture initiation, crack propagation, and final catastrophic failure. Increasing patch size delays the onset of interfacial debonding and improves structural damage tolerance. The present study provides valuable insights for the design and engineering application of adhesively bonded patch repair in composite structures.