Microstructure and mixed mode I/II fracture of AA2524-T351 base material and friction stir welds

Mixed mode I/II experiments for base material AA2524 indicate that (a) transition from a local opening mode of fracture to a local shear mode of fracture occurs in AA2524 at much lower levels of far-field shear loading than observed in AA2024, showing the increased potential for mode transition in the cleaner aluminum alloy AA2524 and (b) the measured critical crack opening displacement (vertical bar COD(crit)vertical bar) in AA2524 is isotropic and equal to the maximum measured value (LT orientation) in AA2024. Mixed mode I/II experiments in AA2524 friction stir weld specimens show that (a) changes in microstructure and particularly, reduction in constituent particle volume fraction result in virtual elimination of fracture along constituent particle bands, (b) AA2524 has an increased tendency to transition from local mode I to local mode II crack growth during stable tearing and (c) specific features observed during the fracture process (e.g. mode transition) are shown to be directly correlated to both metallurgical differences in the FSW microstructure and local variations in the measured CODcrit. (c) 2005 Elsevier Ltd. All rights reserved.