Heterogeneous local straining behavior under monotonic and cyclic loadings in a friction stir welded aluminum alloy

Local strain evolution under monotonic and fatigue loadings was investigated in a friction stir welded (FSWed) Al-Mg-Sc alloy. The microstructure of the weld exhibited strong heterogeneity. Digital image correlation (DIC) technique was employed to evaluate local straining behavior in the weld seam in tensile testing. Onset of initial yielding was found locally near regions of lowest hardness. However, the local straining development didn't take place homogeneously in the stir zone (SZ) of the weld and constraints by surrounding microstructure affected further straining behavior in the weld. Local cyclic straining behavior during fatigue loading was monitored in the same material using small strain gages. Fully reversed (R = -1) fatigue tests were conducted at a constant stress amplitude of sigma(a) = 220 MPa. Cyclic plastic strain found mainly on the advancing side of SZ in early fatigue phase contributed to initiation of fatigue cracks. Since fatigue crack initiation and the following propagation were also influenced by microstructure, hardness distribution or local straining evolution in tensile testing alone could not provide accurate prediction of fatigue fracture behavior of the weld.