Short crack initiation and growth kinetics analysis in a microalloyed steel plate using rotating bending fatigue modified notched specimens

Abstract

Experimental microalloyed steel was tested employing samples with a suitable geometry for crack initiation and kinetics of short crack propagation monitoring through rotating bending fatigue constant loads. Trough the Finite Element Method (FEM) stress distribution was evaluated in pre-selected zones to establish the critical points for visual examination of the initiation cracks by optical microscopy. The experimental test loads were established between the fatigue and elastic limits. The short crack evolution was studied by means of the exponential crack growth law. It was shown that the initiation process of fatigue cracks is intensified gradually by the fatigue damage, featuring by the appearance of persistent slip bands, and the growth of short cracks mechanism is related to the coalescence of micro-cavities. Features of interaction between the surface cracks and steel microstructure are reported. The smoothing splines method was employ to analyze the kinetic crack growth data to establish a correlation between the crack length and crack growth rate. For the first time in this kind of steels, a relevant result is reported, the crack growth coefficient, Kg, that characterize their fatigue short crack growth under the influence of different constant loads.