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

通过“海洋结构疲劳设计基础研究”项目（58460080）的研究，提出了环境疲劳断裂韧性的概念，适用于室温至77K的低温环境。很明显，该值决定了材料在低温下的疲劳开裂行为。疲劳特征应力强度Ke和Kv可作为控制应力强度，区域2和区域3分别开始疲劳裂纹扩展。由于疲劳断裂韧性与Ke、Kv之间的相关关系，还可以提出一种覆盖整个应力强度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阈值应力强度因子。提出的特征应力强度Kv对应于静态断裂模式开始叠加到疲劳裂纹扩展的K值。另一个疲劳特征应力强度Ke为循环塑性区尺寸等于材料晶粒直径处的K值。这表明，Ke和阈值应力强度Kth均与< δ >yROO<d>呈线性关系，其中< δ >y为屈服应力，d为晶粒直径，由实验可知。脆性材料（如硅玻璃）的疲劳裂纹扩展与静态加载条件下的疲劳裂纹扩展基本相同。这种行为可以解释为Kv值较低，这意味着该材料的裂纹扩展主要受3区裂纹扩展控制。

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.