Visualization of strain distribution around fatigue crack developed by low cycle fatigue test in high-strength steels using digital image correlation method for replica films

Strain distributions around the fatigue crack during the low cycle fatigue of high-strength ferrite + martensite dual-phase (DP) and tempered martensite (Mt) steels were visualized during cycles from the initial state to the crack initiation and crack propagation using a digital image correlation analysis applied to replica films. The replica film successfully transcribed the microstructure on the specimen surface. Although both the DP and Mt. steels exhibited a high tensile strength, the work-hardening rate of the DP steel was higher than that of Mt. steel. Although the numbers of cycles to failure (Nf) in the DP and Mt. steels were almost the same at the total strain amplitudes (et) of 0.005-0.008 and 0.02-0.03, the Nf in the DP steel was higher than that in the Mt. steel at the et of 0.008-0.02. The cracks were generated and propagated along the high-tensile-strain regions in both steels and were initiated when the accumulated strain along the loading direction exceeded a critical value regardless of the value of et or the steel applied. The increment of the accumulated strain in the Mt. steel was higher than that of the DP steel at the et of 0.01, whereas it was almost the same between those at the et of 0.03. The inhomogeneity of the strain distribution, which was quantitatively evaluated from the histogram of the strain distribution, increased as the number of fatigue cycles increased. The strain was rather inhomogeneously introduced in the Mt. steel in comparison to the DP steel at the et of 0.01, whereas almost no difference in the inhomogeneity of the strain distribution was detected between the steels at the et of 0.03. Thus, the inhomogeneous strain distribution may promote crack initiation and propagation, resulting in a low Nf at the et of 0.008-0.02 in Mt. steel.