Relationship between KIc and plane-strain tensile ductility and microscopic mode of fracture

Abstract

In most structural steels, the critical plane-strain stress-intensity factor, K Ic , increases markedly with increasing test temperature. Because of this transition behavior with temperature and the inherently high fracture toughness of many steels, very thick specimens must be tested to determine valid K Ic values. The large size of these specimens and the cost of conducting the tests minimize the usefulness of this procedure as a research tool for analyzing the fracture behavior of steels under plane-strain conditions. Therefore, as part of a long-range program to obtain K Ic values from small specimens and to extend linear-elastic fracture mechanics to the region of elastic-plastic fracture mechanics, the Research Laboratory investigated the relationship between K Ic and ordinary tensile material properties for four steels ranging in yield strength from 80 to 250 ksi (552–1720 MN/m 2). The results showed that, for these steels, the variation of K Ic with temperature was similar to the variation of the plane-strain tensile ductility with temperature. Scanning electron micrographs showed that the increase in the plane-strain stress-intensity factor for unstable crack extension, K Ic , with increasing temperature could be related to changes in the microscopic mode of fracture at the crack tip. That is, at temperatures below the fracture-toughness transition temperature, the mode of fracture was cleavage, whereas at temperatures well above the transition-temperature region, the fracture mode was ductile tear. In the transition-temperature region, a gradual change in fracture mode from cleavage to ductile tear occurred at the tip of the fatigue crack in the K Ic specimens. Scanning electron micrographs of the fracture-initiation region in the plane-strain tensile-ductility specimens showed that the increase in plane-strain tensile ductility with increasing temperature for steels ranging in yield strength from 80 to 250 ksi was accompanied by a change in the microscopic mode of fracture. The change in the microscopic mode of fracture in the plane-strain tensile-ductility specimens was similar to the change observed in the crack-initiation region in the K Ic specimens. That is, the microscopic mode of fracture in the plane-strain tensile-ductility specimens gradually changed from cleavage at cryogenic temperatures to ductile tear at room temperature. Thus, it is suggested that the increase in K Ic with increasing temperature is caused by an increase in the plane-strain tensile ductility with increasing temperature and that this increase in ductility is related to a change in the microscopic mode of fracture from cleavage to ductile tear.