Investigation of FGM patch repairs for cracked aluminum structures: J-integral analysis

Functionally Graded Materials (FGMs) represent a highly advanced solution for the repair of cracked aircraft structures, offering superior performance compared to conventional homogeneous materials. FGMs contribute to more durable, lightweight, and structurally effective repairs, extending service life while maintaining safety and performance standards. This study investigates the effectiveness of functionally graded material (FGM) patches for reinforcing cracked aluminum alloy plates. Using three-dimensional finite element analysis implemented in ABAQUS with a USDFLD subroutine, spatial variations of the Young's modulus were applied according to linear, logarithmic, and exponential gradation laws. The fracture behavior was quantified through J-integral calculations under plane strain conditions. Results show that FGM patches significantly reduce the J-integral at the crack tip compared to unrepaired plates: up to 56% reduction for linear gradation, 52% for logarithmic, and 44% for exponential gradation. Linear gradation is most effective in mitigating longitudinal crack propagation, while logarithmic gradation efficiently reduces local principal stresses. Contour convergence and comparison with homogeneous patch and unrepaired plate benchmarks confirm the reliability of the numerical results. These findings provide mechanistic insight into stiffness-gradient-driven stress redistribution, guiding the optimal design of FGM repairs for enhanced durability.