Study on a Unified Fracture Mechanics Parameter Controlling Crack Growth Behavior and its Evaluation Method

提出了裂纹能量密度作为一个参数，可以统一地描述各种裂纹行为，而不管断裂类型是脆-韧断裂、疲劳断裂还是蠕变断裂。本课题研究了用裂纹能量密度表示裂纹扩展速率的方法和数值分析评价裂纹能量密度参数的方法，并利用新研制的裂纹尖端变形观测装置，研究了基于裂纹能量密度的蠕变裂纹扩展起始条件。研究结果如下：从裂纹能量密度的角度考虑延性裂纹、疲劳裂纹和蠕变裂纹的裂纹扩展速率，可以用裂纹尖端裂纹能量密度增量和单位时间（疲劳时为1个周期）裂纹能量密度表示的断裂阻力以及裂纹能量密度分布梯度来统一描述它们的裂纹扩展速率。将不连续裂纹模型作为一种能够计算裂纹能量密度分布和裂纹尖端值的模型进行了扩展，使其能够处理蠕变问题，并提出了一种用路径无关积分计算该模型中裂纹能量密度的新方法。通过对一个标准问题的分析，证实了该模型的有效性。研制了一种炉外裂纹尖端变形观测装置，并进行了蠕变裂纹扩展实验。用上述方法求得的裂纹开始扩展时的裂纹能量密度值表明材料特有的裂纹能量密度临界值的存在。

The crack energy density was proposed as a parameter which may describe all kinds of crack behavior in a unified form regardless of fracture types such as brittle-ductile fracture,fatigue fracture and creep fracture. In this research project,the way of expressing crack growth rate by the crack energy density and the evaluation method of thes parameter by numerical analysis are studied and,moreover,the initiation condition of creep crack growth is studied based on the crack energy density by using a newly-developed equipment for observing the deformation around a crack tip. The results are as follows;1. By considering the crack growth rates of ductile,fatigue and creep cracks from the standpoint of crack energy density,it is made clear that their growth rates can be described in a unified form by the increments of crack energy density at crack tip and fracture resistance expressed by crack energy density per unit time(per one cycle in case of fatigue) and the gradient of crack energy density distribution.2. The discontinuous crack model proposed as a model,which enablesus to evaluate the distribution of crack energy density as well as the value at crack tip,is extended so as to be able to deal with a creep problem and a new method to evaluate the crack energy density in this model by a path-independent integral is developed. The availability of the model is confirmed through the analyses of a standard problem.3. An equipment for observing the deformation around a crack tip out of the furnace is developed and the experiment of creep crack growth is carried out. The value of crack energy density evaluated by the method above at the time when a crack starts to grow suggests the existence of the critical value of crack energy density peculiar to the material.