Effects of loading rate on fracture behavior of low-alloy steel with different grain sizes

Abstract

Four-point bend (4PB) tests of notched specimens loaded at various loading rates, for low alloy steel with different grain sizes, were done, and the microscopic observation and finite-element method (FEM) calculations were carried out. It was found that for the coarse-grained (CG) microstructure, an appreciable drop in notch toughness with a loading rate of around 60 mm/min appeared, and further increasing the loading rate leads to a slight additional decrease in notch toughness. For the finegrained (FG) microstructure, the effect of loading rate was not apparent. The change in toughness resulted from a change of the critical event controlling the cleavage fracture with increasing loading rate. For the CG microstructure with a lower cleavage-fracture stress ( f ), with an increasing loading rate, the critical event of cleavage fracture can be changed from the propagation of a pearlite colony-sized crack or a ferrite grain-sized crack, through the mixed critical events of crack propagation and crack nucleation, then to crack nucleation. This change deteriorates the toughness. For the FG microstructure with a higher cleavage-fracture stress, the critical event of cleavage fracture is the crack propagation and does not change in the loading-rate range from 120 to 500 mm/min. The measured f values do not change with loading rate, as long as the critical event of cleavage fracture does not change. The higher notch toughness of the FG microstructure arises from its higher f and the critical plastic strain ( pc ) for initiating a crack nucleus, and the fracture behavior of this FG steel is not sensitive to loading rate in the range of this work.

Effects of loading rate, notch geometry and loading mode on the local cleavage fracture stress of a C–Mn steel

“…It is considered to be nearly independent of test temperature, and is taken as a fracture parameter connecting the macroscopic fracture toughness with the steel microstructure. The results in this work combined with previous studies (Wang et al 1999(Wang et al , 2001(Wang et al , 2003(Wang et al , 2004a(Wang et al , 2004b(Wang et al , 2005 by the authors show that the local cleavage fracture stress σ f is closely related to the cleavage fracture mechanism (critical events) in steels. The σ f values do not change with loading rate, notch geometry and loading mode, as long as the critical event of cleavage fracture does not change at various testing conditions.…”

“…By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ f is measured. Based on the results obtained and combining with previous studies (Wang et al 1999(Wang et al , 2001(Wang et al , 2003(Wang et al , 2004a(Wang et al , b, 2005 by the authors, the effects of loading rate, notch geometry and loading mode on the σ f are further fully investigated.…”

“…In previous works (Chen et al 1990a, b), σ f values estimated from Charpy V impact test are essentially the same as that measured by four-point bending (4PB) specimens tested at quasistatic loading, thus it is considered that the σ f is nearly independent of loading rate. In recent studies by the authors (Wang et al 2004a(Wang et al , 2005, 4PB tests of notched specimens loaded at various loading rates for low alloy steels with different grain sizes and a C-Mn steel were done, and the finite-element method (FEM) calculations were carried out. It was found that the measured σ f values do not change with loading rate, as long as the critical event of cleavage fracture does not change.…”

Effects of Material Microstructure on Cleavage Fracture

“…For steel 16Mnc (Figure 6.12M-b) in the vicinity of its notch root, the region left to the shaded strip, the ε p is higher than the ε pc , but the Q = σ yy /σ y is lower than the Q c = σ f /σ y = 1.30. Thus, a crack Ferrite grain-sized crack retained in front of survived notch root in fractured double-notched specimen of (a) 16Mnf (Wang, Wang, & Chen, 2003) and (b) WCF62f (Wang, Ren, & Chen, 2004). (Chen, Wang, Wang, Dong, et al, 2002).…”

Critical Event for Cleavage Fracture

“…High σ yy can occur in conditions such as very low temperatures (Chen, 2008) and high loading rates (Wang, Ren, & Chen, 2004). In this case, either the normal tensile stress σ yy ahead of the precrack tip is intensified to a very high level or the material's local fracture stress σ f is low.…”