Deformatio and Fracture of Materials at High Strain Rates and Cryogenic Temperature

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The objective of this research is to provide the basic understanding on deformation and fracture of materials at high strain rates and cryogenic temperature. Strain rate and temperature are the most important parameters which affect the mechanical behavior of metals. The influences of strain rate and temperature on the flow stress have been investigated from macroscopic and microscopic standpoints. Successful explanation of these influences on the yield stress has been given by the thermal activation theory of dislocation. On the other hand, the effects of temperature and loading rate on fracture toughness have not been made clear experimentally or theoretically. In this research experimental techniques have been developed for investigating mechanical behavior of materials at high strain rates and cryogenic temperature using dynamic testing systems and liquid helium cryostats. Yield stress and fracture toughness for low-carbon steels were measured at the trmperature range of 4 K to 298 … More K and various strain rates from quasi-static to impact loading conditions. The fracture toughness was dropped with decreasing temperature and increasing loading rate. The brittle-to-ductile transition was shifted to higher temperature range by the increase of the loading rate. The fracture toughness decreases with increasing the yield stress. This fact denotes that the plastic deformation around a crack tip is tightly related to the fracture toughness. Several studies were presented in view of the dislocation motion around the crack tip; the emisson of dislocations from the crack tip, the stress field around the crack tip shielded by a dislocation pile-up and so on. In this research a computer simulation of the brittle-to-ductile trnsition has been carried out using a simple model where the emission of dislocations from the crack tip is thought to arise from a thermally activated process. The computaional result seems to roughly reproduce the brittle-to-ductile transition observed experimentally. According to the present model, the increasing loading rate is equivalent to the lowering temperature. The experimental data available have been discussed in the light of the present model. Less