This research examines the fracture characteristics of friction stir processing (FSP) 6061 aluminum alloy under modes-I, II, and 45° mixed-mode loading conditions. Friction stir processing, recognized as a novel technique in metal forming, significantly improves the material properties through microstructure evolution. The study utilized linear elastic fracture mechanics (LEFM) and essential work of fracture (EWF) theories to assess fracture parameters thoroughly. Furthermore, finite element analysis (FEA) was employed to evaluate the geometric parameters at various loading angles. Experimental fracture tests on butterfly and double-edge notched tensile (DENT) specimens demonstrated that FSPed samples exhibited consistently lower resistance to crack propagation compared to as-received samples. In modes-I, mixed-mode, and II, the critical fracture parameters diminished by approximately 21.0 %, 15.7 %, and 21.4 % for LEFM, and by 21.5 %, 29.0 %, and 27.1 % for EWF, respectively. Fractography by scanning electron microscopy indicated that although friction stir processing enhanced strength, the primary fracture mechanism transitioned to brittle cleavage, accompanied by a reduction in dimple regions. These results demonstrate the importance of careful FSP implementation in structural components where fracture resistance is crucial.
