Evolution law of thermal strain during the adhesive repair process of edge-cracked Al alloy and its influence on mechanical properties

To repair 1.5 mm-thick 7B04 aeronautical Al alloy specimens with edge cracks, this study conducted single-sided patching tests using J-349-1 adhesive and CCF300/QY8911 pre-cured composite patches. The deformation behavior throughout the entire adhesive curing cycle was monitored in-situ via strain gauges. Combined with tensile and fatigue tests, the evolution law of thermal strain under different curing systems, patch layups, and materials, as well as the influence of residual thermal strain on mechanical properties, were systematically analyzed. The research results show that the curing system has a significant influence on the evolution process of thermal strain. The residual thermal strain increases with the increase in curing temperature, and the two-stage curing system can effectively reduce the residual thermal strain. By comparing the residual thermal strain and the tensile/fatigue property test results under different curing systems, it is found that the higher the residual thermal strain, the lower the tensile and fatigue properties of the repaired parts. Among the three systems, the 70°C/2h + 90°C/0.5h curing system exhibits the highest mechanical properties. The layup of the composite patch has no influence on the evolution process of thermal strain. However, because the hybrid layup [0/45/0/90/0/45] patch has better load transfer efficiency, its tensile properties after repair are higher than those of the orthogonal layup [0/90] patch. In addition, the difference in thermal expansion between the composite material and the Al alloy specimen plays a much greater role than the curing shrinkage of the adhesive, and serves as the main source of residual thermal strain.