Fundamental Studies on teh Environmental Fatigue Fracture Toughness and the Design Curve of Structural Materials

作者通过课题“海洋结构疲劳设计基础研究”(项目编号58460080)，提出了适用于室温至77K低温环境的环境疲劳断裂韧度的概念，明确了该值决定了材料的低温疲劳开裂行为。疲劳特征应力强度Ke和Kv可分别作为控制应力强度的区域2和区域3疲劳裂纹扩展的起点。由于疲劳断裂韧性与Ke和Kv之间的关系，还可以给出一种覆盖应力强度中心范围的疲劳裂纹扩展曲线的估计方法。该方法可用于确定低温环境下的门槛应力强度因子，所提出的特征应力强度Kv对应于静态断裂模式开始叠加疲劳裂纹扩展的K值。另一个疲劳特征应力强度Ke是K值，其中循环塑性区大小等于材料的晶粒度。这意味着K_E和门槛应力强度K_(Th)都与&lt；δ&gt；yROO&lt；d&gt；呈线性关系，其中，&lt；δ&gt；y是屈服应力和d颗粒直径，如石英玻璃等脆性材料中的疲劳裂纹扩展与静态加载条件下的基本相同。这一行为是由低的Kv值解释的，这意味着该材料的裂纹扩展主要受区3裂纹扩展的控制。

The concept of the environmental fatigue fracture toughness proposed by the author, through the research "Basic Study on Fatigue Design for Offshore Structure", Project No.58460080, has been developed for a low temperature environment from room temperature to 77K.It is cleared that this value governs the fatigue cracking behavioru of the material at low temperature. The fatigue characteristic stress intensities Ke and Kv, can be proposed as controlling stress intensities, where the region 2 and the region 3 fatigue crack propagation starts, respectively.Since the correlations between the fatigue fracture toughness and the Ke, Kv, a method of teh estimation of the fatigue crack growth curve, covering the entrire range of stress intensity can be also presented. This method can be utilized to determine the threshold stress intensity factor in a low temperature environment.The proposed characteristic stress intensitiy Kv corresponds to the K value where the static mode of fracture begins to superimpose to the fatigue crack propagation. The other fatigue characteristic stress intensity, Ke, is the K value where the cyclic plastic zone size equals to the grain diameter of the material. This implies that both of the Ke and threshold stress intensity, Kth, are linearly related to the <delta>yROO<d>, where, <delta>y is yield stress and d grain diameter, as indicated by the experiment.The fatigue crack propagation in the brittle material, such as silica glass, is fundamentally the same as that in static loading condition. This behaviour is explained by the low value of the Kv, meaning that the crack propagation of this material mainly controlled by the region 3 crack propagation.