The Influence Of Loading Rate On Fracture Behavior Of Notched Specimens Of Different Organizations Alloy Steel

In this paper, two microstructures with coarse and fine grain size are obtained by heat treatment for a HSLA steel (WCF62). The tension test is carried out at different loading rate within the range of l-500mm/min at -100℃, and the mechanical properties of the two microstructure at different strain rates are measured. The single and double notched specimens with different grain size are loaded by four point bending (4PB) mode at different loading rates in the range of l-500mm/min at -100℃, and some single notched specimens are also loaded by three point bending (3PB) impact mode (v=2000, 4700mm/s) at -100℃. By experimental observation, parameter measurement and finite element method (FEM) calculations of the distribution of stress, strain and strain rate ahead of the notches at different loading rates. The effects of loading rate on fracture behavior of the notched specimens of the two microstructures with different grain sizes are investigated.It is found that for the two microstructures with different grain sizes, with increasing loading rate the fracture modes change from ductile fracture through ductile-brittle transition fracture then to brittle cleavage fracture, and the critical events of cleavage fracture change from mainly crack-propagation-controlled mechanism through a mixed controlling mechanism of the propagation and nucleation then to nucleation-controlled mechanism. The loading rates causing the changes for the fine grain size microstructure are higher than those for the coarse grain size microstructure. The cleavage fracture stress σf and the notch toughness are related with fracture mode and critical event. When the fracture mode is ductile-brittle transition fracture and the critical event is mainly crack-propagation -controlled mechanism the σf and the notch toughness are high. When the fracture mode is brittle cleavage fracture with Xf=0 and the critical event is nucleation-controlled mechanism the σf and the notch toughness are low. When the fracture mode and critical event do not change σf is a constant and does not change with loading rate, but the notch toughness decreases lightly with increasing loading rate. The σf of the fine grain microstructure is higher than that of the coarse grain microstructure when the fracture mode and critical event are identical. The notch toughness of fine grain microstructure is higher than that of the coarse grain microstructure at the same loading rate When the critical event is nucleation-controlled mechanism the critical nucleation strain εpc deceases slightly with increasing loading rate, and the ε pc of the fine grain microstructure is higher than that of the coarse grain microstructure at the same loading rate. The mechanisms of the changes in fracture mode, critical event, σf and notch toughness with loading rate are analyzed.