The Significance of Local Plasticity for the Crack Initiation Process During Very High Cycle Fatigue of High Strength Steels

Abstract

The present paper deals with experimental results on the fatigue damage of grade SAE 318 LN austenitic-ferritic duplex steel and a modified SAE 4150 tempering steel in the very high cycle fatigue (VHCF) regime. Particular attention was paid to the relation- ship between the crystallographic orientation of individual grains and grain patches that exhibit slip band formation, fatigue crack initiation and growth. Therefore, electrolytically polished bulk specimens have been fatigued under fully reversed loading in an ultrasonic fatigue testing machine while the surface has been observed in-situ by an optical microscope. The specimens were carefully analyzed by means of scanning electron microscopy (SEM) in combination with automated electron back-scatter dif- fraction (EBSD). In case of the austenitic-ferritic duplex steel under VHCF loading conditions, slip band formation is limited to the softer austenite grains. Once being formed, the bands generate high stress concentrations where they impinge the austenite- ferrite (γ−α) phase boundaries, eventually, leading to the crack initiation. In case of the tempering steel crack initiation is caused by slip band formation within the martensitic laths structure. It was found that in both materials microcracks are sensitive to changing crystallographic orientations when crossing a grain or phase boundary.