Analysis of crack propagation resistance of Al–Al2O3 particulate-reinforced composite friction stir welded butt joints

This work is devoted to the analysis of fatigue crack propagation resistance of particulate metal-matrix composites butt joints obtained by friction stir welding. Two different aluminum alloy matrices reinforced with alumina particles were examined. Tests were conducted on both parent material and welded joint for comparison. Fatigue crack propagation was carried out both within the weld nugget and in the thermo-mechanically altered zone at the side of the weld. The comparison between parent material and joint showed that the welding process affects fracture toughness and fatigue crack growth rate differently depending on the material. The analysis of crack path roughness helped to understand those differences in the fatigue crack growth rate. Therefore, roughness-induced crack closure arguments have been introduced to discuss data obtained under different testing conditions (parent material/joint, R-ratio, crack location, crack growth regime). Both the classical Elber's approach and more recent approaches based on partial crack closure concept (adjusted compliance ratio, ACR, and 2/pi methods) were considered. The results showed that, using partial crack closure, all of the data collapse within a reasonable scatterband.