Effects of vanadium addition on strain distribution, crack initiation and propagation during low-cycle fatigue test in ferrite and martensite dual-phase steel

Abstract

Effects of vanadium (V) addition on strain distribution, crack initiation and propagation behavior during low-cycle fatigue test in ferrite and martensite dual phase (DP) steel were revealed. V addition effectively extended the low-cycle fatigue life, even though the tensile properties were approximately identical between V-added DP and DP steels. The stress amplitude suddenly decreased just before fracture in the V-added DP steel. The fatigue crack initiated from the surface and propagated inside, and the brittle fracture occurred in the crack propagation region in the V-added DP steel. The strain distribution introduced during the low-cycle fatigue test was more inhomogeneous for the V-added DP steel than that for the DP steel, and the fatigue crack was generated from the high-strain region. Considering that the number of cycles for crack initiation in the V-added DP steel was larger than that in DP steel, the inhomogeneous strain distribution in the V-added steel promoted crack nucleation but suppressed crack growth in the crack initiation stage. Cracks propagated independent of the strain distribution in the V-added DP steel in the crack propagation stage. The crack propagation rate in the V-added DP steel was remarkably higher than that in DP steel in the crack propagation stage. Thus, V addition effectively extended the number of cycles for crack initiation but caused brittle fracture and faster crack propagation.