1 | INTRODUCTION
The evaluation of the dynamic fracture behavior of engineering materials has an important significance for the assurance of the integrity and safety of structural components subjected to various dynamic loads, such as impact, explosion, or earthquake. Since the loading rate range of these dynamic loads is very wide, a more comprehensive understanding of the relationship between the fracture behavior and the loading rate is necessary.1,2
Some research has been carried out on the area of dynamic fracture. For the complexity of the dynamic fracture process, theoretical methods are limited and tests are the main means of dynamic fracture mechanics research. Three ranges are divided according to the loading rate\(\dot{K_{I}}\): low loading rate range (\(10^{-3}\ \text{MPa}\frac{\sqrt{m}}{s}\leq\dot{K_{I}}<10^{3}\ \text{MPa}\frac{\sqrt{m}}{s}\)), at which it is regarded as quasi-static loading and belongs to quasi-static fracture; medium loading rate range (\(10^{3}\ \text{MPa}\frac{\sqrt{m}}{s}\leq\dot{K_{I}}<10^{5}\ \text{MPa}\frac{\sqrt{m}}{s}\)), at which the influence of inertia effect should be considered; and high loading rate range (\(\dot{K_{I}}{\geq 10}^{5}\ \text{MPa}\frac{\sqrt{m}}{s}\)), at which it must consider the interaction between stress wave and crack in addition to the influence of inertia effect. According to different loading rates, different types of loading test devices can be selected. The experimental techniques for dynamic fracture often include the drop-weight impact test, instrumented Charpy impact test, and Hopkinson pressure bar (HPB) impact test.3-6
Chaouadi and Puzzolante7 examined the dynamic fracture toughness of ferritic steel with an instrumented Charpy impact test. The dynamic fracture toughness was greater than the quasi-static one. Similar conclusions were obtained in Foster et al.8and Prasad et al.9, where fracture toughness of the 4340 steel and Al-Li 8090 alloy increased with the increasing loading rate. Wu et al.10 performed experimental studies on the dynamic fracture behavior of FV520B steel under quasi-static and dynamic loading conditions, and the fracture toughness increased linearly with the increasing loading rate. Galvez et al.11 conducted experimental studies on the fracture behavior of high strength steel Armox500T, and the static and dynamic fracture toughnesses were quite similar. The fracture toughness without a marked loading rate effect was also determined for Al 7075-T651.12 Different from these insensitive ones, the irregular relation of the fracture toughness and the loading rate was observed experimentally for 685 homogeneous steel.13 When the loading rate was less than 1.8778 MPa.m0.5, the dynamic fracture toughness decreased with the increasing loading rate. However, when the loading rate was greater than 1.8778 MPa.m0.5, the dynamic fracture toughness rose due to the effect of thermal softening near the crack tip. Additionally, Wu et al.14 conducted an experimental study on fracture behavior of AISI 1045 steel. The fracture modes exhibited a transition from ductile to brittle fracture with the increasing loading rate, and the dynamic fracture toughness was less than the quasi-static one. A similar conclusion was also obtained in Lorentzon et al.15 for the ordinary C-Mn structural steel. According to the published reports, it was found that when the loading rate increased, some fracture toughnesses increased, some decreased, and other fracture toughness levels changed insensitively or irregularly.
AISI 5140 steel, also named 40Cr steel, has been widely used in engineering structures, and investigations on the dynamic fracture behaviors are related to many engineering problems. Xu and Li16 carried out the fracture test of 40Cr steel loaded by HPB and the fracture toughness of the steel increased with the increasing loading rate. However, for the same material, a conflicting result was obtained in Li et al.17, where the fracture toughness decreased with the increasing loading rate. It should be noted that in Refs. 16 and 17, the fracture toughnesses were both determined by the numerical-experimental method, but the dynamic constitutive relation of the steel was not considered in the numerical simulations. Besides, as described in the literature, the loading rate range tested may not be wide enough, resulting in inconsistent conclusions. Few results can be found on the fracture toughness and fracture mechanism of the steel over a wide range of loading rates.
In this paper, experimental studies were performed on the fracture behavior of AISI 5140 steel over a wide range of loading rates. True stress-strain relations were measured and the dynamic constitutive model was proposed. Fracture tests under quasi-static condition, instrumented Charpy impact condition, and HPB impact condition were carried out, and fracture toughnesses and characteristics were analyzed. Comparisons and discussions of the fracture behaviors under the above loading rates were conducted in terms of fracture modes and fracture toughness values. Finally, based on the fracture assessment method of the CEGB R6 procedure, the effects of the strain rate and the loading rate on the assessment curve were discussed.