Study on Fatigue Strength Design of Welded-joint under random loading

利用结构模型试件进行了恒定载荷和“风暴模型”块载荷作用下的疲劳裂纹扩展试验。对现有疲劳评估方法的可靠性和断裂力学在疲劳评估中的应用进行了研究，并利用“高精度柔度测量系统”和CCT试件对疲劳裂纹尖端应变的变化进行了详细的观测，以正确认识疲劳裂纹的张开和闭合现象。结果表明：1)基于恒载疲劳试验得到的S-N图的疲劳评估不能考虑过去加载历史的影响。因此，在某些加载模式下，这种评估的准确性被证明是非常低的。(例如，滞后显著的加载模式)2)基于块加载FATI…得到的S-N图的疲劳评估更多的GUE测试可以考虑一个块内的加载模式的影响。因此，这种评估的精度比上述方法高1)，但它甚至不能考虑不同块体的相互作用的影响，那么这种评估的高精度取决于块体加载模型对实际加载模式的定量反映。3)基于断裂力学的疲劳评估可以考虑过去加载历史的影响，因此这种评估的精度可以高于上述方法2)如果建立合适的疲劳裂纹扩展模型。此外，这一评估的高精度还取决于对疲劳裂纹扩展行为，特别是对疲劳裂纹张开/闭合现象的正确理解。4)成功地高精度地测量了载荷-应变滞后曲线，并引入柔度曲线和一个周期内柔度变化率，成功地实现了闭合载荷和RPG载荷的自动测量。较少

The fatigue crack propagation tests using structural model specimens are carried out under constant loading and the "Storm Model" block loading. Also study on the reliability of the current fatigue assessment methods and the application of fracture mechanics to fatigue assesment are made.Moreover, for the purpose of proper understanding of the fatigue crack opening/closure phenomenon, detailed observations of change in the strain near fatigue crack tip using "High-precision Compliance Measurement System" and CCT specimens are carried out.As a result, the following four main points are obtained.1) The fatigue assessment based on S-N diagram obtained from constant loading fatigue tests can not take into account the effect of past loading history. So the accuracy of this assessment turn out to be extremely lowered under some loading patterns. (for example, the loading pattern in which retardation is remarkable)2) The fatigue assessment based on S-N diagram obtained form block loading fati … More gue tests can take into account the effect of the loading pattern within one block. So the accuracy of this assessment is higher than the above mentioned method 1) but it even can not take into account the effect of interaction of different block, then further high accuracy of this assessment depend on how quantitatively the block loading model reflects the actually loading pattern.3) The fatigue assessment based on fracture mechanics can take into account the effect of past loading history, so the accuracy of this assessment could be higher than above mentioned method 2) if proper fatigue crack propagation model was established. Also further high accuracy of this assessment depend on proper understanding of the fatigue crack propagation behavior especially of the fatigue crack opening/closure phenomenon.4) Successful measurement of the hysteresis curve of load vs.strain with high accuracy was made and, introducing the curve of compliance and the change rate of compliance during one cycle we succeeded in the automatically measurement of the Closing load and RPG load. Less