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

本研究的目的是提供对材料在高应变率和低温下变形和断裂的基本认识。应变速率和温度是影响金属力学行为的最重要的参数。从宏观和微观角度研究了应变速率和温度对流变应力的影响。位错热激活理论成功地解释了这些对屈服应力的影响。另一方面，温度和加载速率对断裂韧性的影响在实验上和理论上都不清楚。在这项研究中，实验技术已经开发了调查在高应变率和低温下使用动态测试系统和液氦低温恒温器的材料的力学行为。在4K ~ 298 ℃温度范围内测定了低碳钢的屈服应力和断裂韧性 ...更多信息 K和从准静态到冲击加载条件的各种应变率。断裂韧性随温度的降低和加载速率的增加而降低。随着加载速率的增加，脆韧转变向高温区移动。断裂韧性随屈服应力的增加而降低。这一事实表明，裂纹尖端附近的塑性变形与断裂韧性密切相关。针对裂纹尖端附近的位错运动进行了一些研究;位错从裂纹尖端的发射，裂纹尖端附近的应力场被位错塞积所屏蔽等等。韧性转变已经使用一个简单的模型进行，其中从裂纹尖端发射的位错被认为是由热激活引起的。过程计算结果似乎大致再现了实验观察到的脆韧性转变。根据本模型，增加加载速率等效于降低温度。根据本模型讨论了现有的实验数据。少

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