Abstract
Dynamic crack initiation with crack tip loading rates K˙ of approximately 2 ‧ 106 MPa√ms− in high-strength 42CrMo4 steel was investigated. To this end, a recently developed split Hopkinson pressure bar with four-point bending was utilized. V-notched and precracked Charpy specimens were tested. The tests were performed at temperatures of –40 °C and 20 °C. The loading of the specimen was determined by analyzing the strain in the incident and transmission bars. Furthermore, strain gauges at the specimen’s surface were applied to measure the crack tip loading. High-speed photography complemented the analysis of the specimens loading and the detection of the crack initiation. Fracture surface analysis by means of scanning electron microscopy enabled the measurement of the fracture surface topography and, consequently, stretch zone height and width. Hence, the macroscopically measured dynamic crack initiation toughness was correlated with the toughness at microscopic scale. It was observed that the resistance against dynamic crack initiation decreased with decreasing temperature. Microscopically, a decrease in toughness was analogously observed. Non-metallic inclusions resulted in crack path deflection with localized shear zones. After a small stable crack extension, cleavage fracture was observed.
Highlights
The dynamic fracture toughness behavior of different materials can be studied by utilizing the split Hopkinson pressure bar (SHPB) [1]
The calibration factor was determined statically (≈ 6.5 kN/V). This strain gauge can be applied as a load cell to the strain gauges at the outer fiber of the specimen
Force measurements by strain gauges at the bars and at the specimen were applied in combination with high-speed photography in order to detect the onset of crack growth
Summary
The dynamic fracture toughness behavior of different materials can be studied by utilizing the split Hopkinson pressure bar (SHPB) [1]. Both the incident and the transmission bars are used to analyze forces and displacements. Pulse shaping is applied to increase the rise time of the originally rectangular pulse generated by the impact of the striker bar to the incident bar. Ductile fracture consists of void nucleation, void growth and coalescence and is strongly affected by nonmetallic inclusions. It is known that elongated inclusions, e. The growing crack interacts with the voids and will be deflected towards these voids [7]
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