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.