Fracture toughness of Fe–Si single crystals in mode I: Effect of loading rate on an edge crack (–110)[110] at macroscopic and atomistic level

Abstract

This paper is devoted to an experimental and 3D atomistic study of the influence of loading rate on fracture toughness in dilute Fe–Si alloys and in bcc iron. We analyze new and previous experimental results from fracture tests performed at room temperature on bcc iron–silicon single crystals with edge cracks (1¯10)[110] (crack plane/crack front). The specimens of single edge notch-type were loaded in tension mode I under different loading rates. The ductile–brittle behavior at the crack front was monitored online via optical microscopy together with external force and prolongation of the specimens. About 30% decrease in fracture toughness was monitored in the new experiment under the highest loading rate. The nanoscopic processes produced by the crack itself were studied at room temperature via 3D molecular dynamics (MD) simulations in bcc iron under equivalent boundary conditions as in experiments to reveal (explain) the sensitivity of the crack to loading rate. For this purpose, this MD study utilizes the self-similar character of linear fracture mechanics. The results show that the emission of blunting dislocations from the crack is the most difficult under the highest loading rate, which leads to the reduced fracture toughness of the atomistic sample. This is in a qualitative agreement with the experimental (macro) results. Moreover, MD indicates that there may be some synergetic (resonant) effect between the loading rate and thermal activation that promotes dislocation emission.