Ductile crack propagation characteristics and mechanism of structural steel under high strain rate

In the present study, it was carried out to clarify the characteristics and the mechanism of ductile crack propagation of HT490 structural steel, with special emphasis on the influence of strain rate. At first, bend test was performed in various displacement rate, and subsequently deformation microstructure was observed by using optical microscope and scanning electron microscope. It was found that higher strain rate increased the resistance to ductile crack propagation extremely. The mechanism that higher strain rate increase the resistance to ductile crack propagation is considered as follows: Higher strain rate suppresses multiple slip and cross slip. Therefore, the number of slip systems acting on primary voids is limited and then equiaxed growth of primary voids is suppressed. And also, the cross point of multiple slips, which could be the site of secondary voids, is decreased and the coalescence of primary voids may be suppressed. They lead to decrease in the area mainly composed of equiaxed dimple, and to increase in the area mainly composed of elongated dimple and the area of shear lip. Consequently plastic deformation accompanying the formation of ductile fracture surface is increased, and then the resistance of ductile crack propagation become higher.