考虑强度非均匀分布的焊接节点疲劳损伤内聚力模型

Welded joints are locations which are the most susceptible to fatigue damage in steel structures. The fatigue behavior of welded steel joints is characterized by the complex failure mechanisms. There has been a lack of a reliable and generalized model for the analysis of the fatigue behavior of welded steel joints. Cohesive zone model has been widely adopted for analysis of fatigue problems in mechanical and aerospace engineering applications. This is because the cohesive zone model avoids the singularity at the crack tip which has been a great challenge in linear elastic fracture mechanics when solving fatigue crack problems. Therefore, the fatigue analysis method of welded joints based on cohesive zone model is a promising candidate, after addressing the challenges including the non-uniform fatigue strength distribution, various fracture mechanisms and fatigue damage accumulation at different zones within the joints. The following tasks will be conducted in this project. 1) The non-uniform stress field and non-uniform strength distribution of welded joints are identified. The fatigue damage accumulation and failure criterion during the process of crack propagation are determined. Traction-separation law, loading-unloading path and fatigue damage evolution law are investigated. An improved cohesive zone model for fatigue analysis of welded joints is proposed. 2) The physical meaning of the numerical stability approach of the cohesive zone model is discussed. Essential parameters of this approach are determined. The efficiency and accuracy of the numerical stability approach is improved. 3) The cohesive zone model and the numerical stability approach are combined to build the fatigue analysis method of welded joints. Extensive fatigue experimental tests to achieve the crack sizes and fatigue lives will be conducted to validate the proposed method. The proposed model and method contribute to the knowledge and understanding of the fatigue behavior of the welded steel joints, and can promote development of engineering practice.

焊接节点是钢结构中最易发生疲劳破坏的部位，破坏机理复杂，至今尚未建立起可靠完善的疲劳分析方法。基于内聚力模型的疲劳分析方法，避免了线弹性断裂力学应力场奇异性的问题，高效便捷，已在机械、航空等领域得到了重要应用。由于焊接节点区域强度分布不均匀、断裂失效机制和疲劳损伤演化复杂等原因，基于内聚力模型的疲劳分析刚起步。本课题主要开展三方面研究：1）揭示焊接节点区域疲劳损伤演化及强度非均匀分布特征，确定裂纹非均匀应力场，提出内聚力-张开位移法则和疲劳损伤演化法则，确定加卸载模式，完整构建焊接节点疲劳损伤内聚力模型；2）深入剖析数值计算稳定性方法的物理意义，确定其影响因素及其变化规律，提出有较高计算效率和精度的方法；3）通过疲劳试验手段获取疲劳裂纹尺寸以及寿命，由其验证基于以上模型和方法的疲劳分析体系的准确性和适用性。最终形成系统化的焊接节点疲劳分析新体系，具有重要的理论和应用价值。

土木工程领域，大量的焊接钢结构破坏事故是由疲劳裂纹扩展引起。焊接节点的疲劳破坏是焊接钢结构最常见的失效形式之一。实际工程中存在大量承受循环荷载的钢结构焊接接头，已经存在或将会出现疲劳裂纹，严重危害着此类结构的安全和寿命。因此，焊接接头的疲劳分析方法对于此类结构的设计、运行和维护有着极其重要的意义。本项目围绕焊接接头疲劳破坏的特征，深入研究了焊接接头力学性能非均匀性和疲劳损伤演化过程。.项目的主要工作及成果如下：.（1）建立了疲劳损伤内聚力模型，深入揭示了疲劳损伤演化的能量耗散本质。内聚力模型中疲劳裂纹尖端处假设存在“材料单元”，高周疲劳裂纹扩展时，疲劳损伤累积过程可看作裂纹尖端处“材料单元”发生低周疲劳破坏过程。“材料单元”的低周疲劳破坏的驱动力为外荷载作用下塑性变形产生的塑性功，低周疲劳抗力为滞回能。疲劳损伤即为外力功与滞回能的比值。.（2）构建了基于硬度测试技术的焊接接头力学性能非均匀分布确定方法，探讨了该方法的适用范围及其精度。经与薄板拉伸试验结果对比，力学性能指标最大预测误差为13%。当应变小于10%时，预测得到应力-应变曲线与薄板拉伸试验结果相互吻合。.（3）提出了焊接接头精细化有限元模拟方法，明确了该方法的适用范围及其精度。五区模式能准确表达焊接接头力学性能的非均匀分布，研究表明有着较高精度和效率，综合性能最优。对比了精细化有限元模拟与焊接接头拉伸试验得到的荷载、位移和应变，结果表明预测的荷载和位移，精度高，最大误差在3%以内。.通过上述试验研究与理论分析工作，对焊接节点力学性能非均匀性和疲劳损伤演化过程有了较为深入的了解，为焊接节点疲劳分析方法后续进一步研究及工程应用奠定了理论基础。