Impact of composite patch stiffness, adhesive type, and material on the fatigue life of repaired aluminum alloy 6061-T61 plates

This study examines the combined effects of patch stiffness, adhesive type, and composite patch material on the fatigue life of repaired structures. Simulations were conducted with four repair configurations: Boron epoxy with FM-73 and 3 M AF163-2 K adhesives, and Graphite epoxy with FM-73 and 3 M AF163-2 K adhesives, with patch stiffness varying from 70 to 120%. Results show that increasing patch stiffness significantly enhances fatigue life, with the best performance achieved with a 120% stiffness patch combined with 3 M AF163-2 K adhesive, extending fatigue life to nearly 200,000 cycles. The study also demonstrates that patches with higher stiffness improve load distribution and reduce stress concentrations around cracks compared to those with lower stiffness. In variable loading tests, 120% stiffness patches show a greater delay in crack propagation and better overload absorption, validating the Willenborg model. Additionally, the analysis of applied stresses reveals that higher stiffness patches better maintain stresses beyond 10⁶ cycles, highlighting the importance of selecting appropriate patch stiffness to maximize repair durability.