The Role of Grain Orientation and Martensitic Transformation during Propagation of Short Fatigue Cracks in Austenitic Stainless Steel

Abstract

During high-cycle-fatigue loading of metastable austenitic steel AISI304L, the elastic anisotropy between neighboring grains causes the occurrence of stress peaks at grain boundaries, which again act as crack nucleation sites. This is in particular the case at twin boundaries. Cyclic crack tip plasticity leads to a transformation from  austenite to ´ martensite when different slip bands are activated, alternating during their operation. By means of in-situ fatigue testing in a scanning electron microscope (SEM) in combination with electron back-scattered diffraction (EBSD), the distributions of grain size, geometry, and crystallographic orientation relationship were correlated with the local occurrence of slip, martensite formation and fatigue-crack initiation and propagation. It was shown that the extent of martensite formation ahead of a propagating crack increases with increasing crack length and eventually, due to its higher specific volume, gives rise to transformation-induced crack-closure effects. The variation in the crack-propagation rate depending on the local microstructure was simulated by means of a short crack model, where the displacement fields within the crack, the adjacent plastic zone and the grain boundaries in combination with the martensite volume increase strain are superimposed by means of a boundary-element approach.

In situ SEM fatigue testing technology for metallic materials: a review

“…In previous works, the faster crack growth rate of small crack than long crack had been explained by the lack of crack closure effect during the small crack regime. 111,112 Elber 113 proposed the crack closure mechanism while making adjust- ments to the application range of the stress intensity factor in the 1970s. By doing so, he aimed to enhance the assessment of different load ratios' effects on fatigue crack propagation.…”

“…010)[100] and(111)[011 ˉ] orientations propagate along the (1 ˉ1 ˉ1) and (1 ˉ11 ˉ) single slip planes, respectively, perpendicular to the loading direction (Fig.4(a), (c), (d) and (f )). In contrast, the cracks in the (011)[100] propagate alternately along the (11 ˉ1 ˉ) and (111) slip planes, forming a "Z" shaped propagation path, as shown in Fig.4(b) and (e).…”

A new approach to model the fatigue anisotropy due to non-metallic inclusions in forged steels

“…The effect of the grain size and orientation [11][12] and the effect of the crystallographic texture [13] have been the object of numerous studies. Stainless steels [2], rolled aluminium and titanium alloys [11] have been the closely studied in order to establish the links between the microstructural orientation and the anisotropic fatigue resistance.…”