Abstract
Dynamic crack initiation with crack-tip loading rates of $$\dot{K} \approx 2 \cdot 10^6\,\mathrm {MPa\,m^{0.5}\,s^{-1}}$$ 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.
Highlights
The offset in time between the two measurements at the incident bar was defined by the distance between the laser measurement site and the position of the strain gauge
Using a pulse shaper and the strain gauge measurement, the end of the incident pulse interfered with the beginning of the reflected pulse
The beginning of the reflected pulse is of great importance for the calculation of F1 and, the analysis of the force equilibrium at the early part of loading
Summary
For the investigation of dynamic fracture toughness properties at high loading rates, the split Hopkinson pressure bar principle is used, e.g. Materials which exhibit linear-elastic behaviour under high loading rates are subject of intensive research [2, 4,5,6]. Investigation dealing with elasticplastic behaviour are rare [7]. Dynamic crack initiation can be detected by a strain gauge in the near-field of the crack tip [8]. The strain gauge can be utilised to determine the stress intensity factor at the crack tip assuming a static calibration of the strain gauge [9]
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