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

Characterizing the Fatigue Damage in a Martensitic Spring Steel

“…[36] Plastic incompatibility can lead to a dislocation pairing along the active slip band and/or dislocation pileup at the grain boundary which provokes a slip step and subsequent crack initiation. [11,12,15,35] In this study, two types of intergranular crack initiation and early short crack propagation along PAGBs were observed. On the one hand, an impingement of slip bands on the PAGBs was found causing intergranular crack formation, as also described in Batista et al and Krupp et al [11,14] On the other hand, some intergranular crack initiation sites exhibited just one slip step at the PAGB which coincides with Motoyashiki et al [15] However, it should be noted that Motoyashiki et al observed a transgranular early short crack propagation in the depth direction after intergranularly crack initiation.…”

“…In the studied material, the subsequent crack initiation and early short crack propagation occurred intergranularly along the PAGBs which is in accordance with some another studies. [11,12,14,15] In general, intergranular crack initiation can be attributed to the effect of grain boundary precipitates or segregations and to the mutual superimposing mechanisms of elastic anisotropy and plastic incompatibility. Elastic anisotropy can locally cause a stress concentration at the grain boundary that may exceed the interfacial cohesion.…”

“…The crack initiation in martensitic steels seems to occur often at slip bands, [7][8][9][10][11] which mainly arise at microstructural interfaces and are oriented parallel to martensitic laths. [11][12][13] It seems that those slip bands do not cross microstructural interfaces with a high-angle boundary like packet boundaries or prior austenite grain boundaries (PAGBs). [10,13,14] Hence, the crack initiation and early short crack propagation in martensitic steels seem to occur mainly intergranularly at PAGBs [11,12,14,15] or block boundaries, [11,16] although in some studies, transgranular crack initiation and early short crack propagation were also observed.…”

“…[11][12][13] It seems that those slip bands do not cross microstructural interfaces with a high-angle boundary like packet boundaries or prior austenite grain boundaries (PAGBs). [10,13,14] Hence, the crack initiation and early short crack propagation in martensitic steels seem to occur mainly intergranularly at PAGBs [11,12,14,15] or block boundaries, [11,16] although in some studies, transgranular crack initiation and early short crack propagation were also observed. [9,13] In many studies, the preferential alignment of carbides along microstructural interfaces is cited as a possible reason for intergranular crack initiation and early short crack propagation.…”

“…[7,14,17,18] In addition, intergranular crack initiation can be explained by the impingement of slip bands on grain boundaries [14,19,20] or the anisotropic elastic and plastic properties of the grains. [11,12,15,[21][22][23] The number of slip bands formed and cracks initiated rises with increasing number of cycles and with increasing stress amplitude. [4,8,9,13,24] The subsequent short crack propagation seems to be strongly influenced by the local microstructure, whereby a short crack propagation parallel to the martensitic laths is often observed.…”