Experimental Quantification of Barrier Effects for Microstructural Short Fatigue Crack Propagation in Martensitic Steel

In the scope of the underlying study, microstructurally short fatigue crack propagation was observed and analyzed in a fully martensitic microstructure of the 0.5C-1Cr-Mo alloy steel SAE4010. For this purpose, a three-stage examination procedure was developed to prepare macroscopic shallow notched specimens, define and analyze area of interest by EBSD and afterwards applying fully reversal load cycles with a frequency of 95Hz. To predefine the crack path, small rhombic notches were added by focused ion beam (FIB) milling in the area of interest. In the second stage, the crack growth was monitored in-situ by light optical microscopy. The final stage comprised the combination and correlation of the crack path, the crack growth data and the documented microstructure with its crystallographic properties. Based on essential literature results, which comprises statistical boundary plane orientations for a martensitic microstructure, it was possible to analyze the barrier effect of different martensitic boundary types as well as the effect of the twist angle and residual burgers vector on the crack propagation behavior. By an additional consideration of common evaluation parameters, a detailed analysis of the prediction behavior of slip system activity was done. The results suggest the application of a combined criterion (twist angle, residual burgers vector, resolved shear stress) to describe the general barrier effect against fatigue crack propagation in a martensitic microstructure. In detail, there was no clear difference in barrier effect between the observed boundary types.