Extremely low cycle fatigue assessment method for structural steel members

为了评估焊接接头的极低周疲劳强度，应估计通常产生裂纹的焊缝脚趾处的应变（局部应变）。为了通过有限元分析获得局部应变，必须使用非常精细的单元来模拟准确的焊趾形状。然而，在工程实践中使用细网是不现实的。因此，在本研究中，建立了焊接趾处荷载应变与构件公称应变之间的关系，该关系可由设计阶段通常使用的梁模型获得。即，考虑焊缝趾部轮廓的细网格模型计算的应变范围与不模拟焊缝趾部轮廓的1mm网格模型计算的应变范围相关。采用1mm网格模型进行三维有限元分析得到的应变与梁单元模型得到的应变具有相关性。在此基础上，建立了kcal应变范围与标称应变范围之间的关系。最后，提出了基于名义应变的极低周疲劳评估方法。在该方法中，通过梁分析得到的名义应变可以转换为具有建立的相关性的局部应变。然后，利用所提出的疲劳强度曲线和Miner的线性累积损伤规律对极低周疲劳寿命进行评估。为了验证所提出的评估方法，制作了钢墩试件并进行了大周期变形试验。试验结果表明，该方法与试验结果的相关性较好，可用于桥墩疲劳寿命的预测

In order to assess the extremely low cycle fatigue strength of welded joints, the strain at weld toe where cracks usually initiates (local strain), should be estimated. For obtaining the local strain by FEA, the use of very fine elements is inevitable to simulate the accurate weld toe shape. It is not practical, however, to use fine mesh in engineering practice. Therefore in this study, the correlation between the load strain at the weld toe and the nominal strain of the member, which can be obtained by beam model conventionally used in design phase, was established. Namely, the strain ranges computed with fine mesh model taking into account the weld toe profile were correlated to the strains obtained with the 1mm mesh size model, which does not model the weld toe profile. The strain by three-dimensional FE analysis with 1mm mesh size model was correlated with the strain by beam element model. Based on these results, the relationship between the kcal strain range and the nominal strain range was established. Finally, the extremely low cycle fatigue assessment method based on the nominal strain was proposed. In the method, the nominal strain obtained by beam analysis can be converted to the local stain with the established correlation. Then, the extremely low cycle fatigue life can be evaluated by using the proposed fatigue strength curves and Miner's linear cumulative damage rule. In order to vat the proposed assessment method, steel pier specimens were fabricated and subjected to large cyclic deformations. After the test, it was indicated that correlation between the test results and the lives estimated by proposed approach was good enough, and the proposed assessment method can be used to predict the fatigue life of the pier