Crack initiation at high loading rates applying the four-point bending split Hopkinson pressure bar technique

Dynamic crack initiation with crack-tip loading rates of K˙≈2·106MPam0.5s-1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{K} \approx 2 \cdot 10^6\,\mathrm {MPa\,m^{0.5}\,s^{-1}}$$\end{document} in a high strength G42CrMoS4 steel was investigated. To this end, a previously developed split Hopkinson pressure bar with four-point bending was utilized. V-notched and pre-cracked Charpy specimens were tested. The detection of dynamic crack initiation was performed by analyzing the dynamic force equilibrium between the incident and the transmission bar. High-speed photography of the tests and analysis of the dynamic stress intensity factor revealed that the vibration of the specimen had to be considered. The dynamic and static analyses of the tests lead to nearly the same results when a force equilibrium was achieved. Fracture-surface analysis revealed that elongated MnS inclusions strongly affected both the dynamic crack initiation and growth. Blunting of the precrack did not take place when a group of MnS inclusions was located directly at the precrack tip. Due to the direction of the elongated MnS inclusions perpendicular to the direction of crack growth, the crack could be deflected. The comparison with a 42CrMo4 steel without elongated MnS inclusions revealed the detrimental effect in terms of resistance to crack initiation.