To investigate the mechanical response and fracture characteristics of adhesively bonded titanium alloy structures under mode I loading conditions, this study employed a co-curing method to fabricate repair specimens with single-sided carbon fiber reinforced polymer (CFRP) patches bonded to titanium alloy substrates. The effects of patch thickness, ply orientation, and surface treatment on mode I interfacial fracture mechanics were systematically examined using double cantilever beam (DCB) tests. Peak load and interlaminar fracture toughness were utilized as quantitative metrics to evaluate the overall repair performance. Furthermore, failure modes and fracture surface morphologies at both macroscopic and microscopic scales were analyzed to elucidate the underlying failure mechanisms of mode I static delamination in the titanium alloy/CFRP repaired specimens. The results reveal that increasing the thickness of the patch leads to a rising trend in both the bending stiffness of the specimen and the extent of fiber bridging. The mode I fracture performance of the repair interface improves significantly, with failure modes consistently evolving from adhesive failure of the glue film and cohesive damage to failure at the CRFP interface. For multidirectional laminates, the 0° ply at the bottom of the patch exhibits the strongest constraint on delamination paths, while the 45° ply effectively induces inter-ply crack migration, enhancing the toughening effect. Notably, the two-dimensional woven patch demonstrates the best repair performance. For surface-treated specimens, cohesive failure of the adhesive film is the predominant failure mode. Specifically, sulfuric acid anodization provides the most significant toughening effect, increasing fracture toughness by 3.8% and 1.9% compared to quartz sandblasting and 400# sandpaper abrasion, respectively, and by 19.2% compared to untreated specimens. These conclusions provide references for the optimized design and practical application of damage repair processes under mode I loading conditions for titanium alloy components.
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