【作者】 杨明；

【导师】 朱国仁；

【作者基本信息】 吉林大学 ， 机械设计及理论， 2022， 硕士

【摘要】 焊接接头由于焊接残余应力导致应力集中现象产生,在工况下易发生疲劳破坏。现阶段主要采用试验方法进行结构疲劳性能评估,但对于试样的需求量较大,整体评估花费周期长。半经验法如名义应力法和热点应力法对焊接接头几何形状较为敏感,对于部分无法获得名义应力或应力集中于焊根位置的接头无法进行评估。针对上述评估方法存在的局限性,本文提出一种考虑残余应力的应变能密度法,通过将应变能密度作为控制参量对焊接接头进行疲劳评估,对不同焊接工艺的碳钢T型焊接接头疲劳性能的疲劳寿命进行评估验证该方法的准确性。本文通过对Q345NQR2耐候钢不同焊接工艺单侧T型焊接接头采用应变能密度法进行疲劳寿命评估,考虑焊接残余应力对焊接接头疲劳寿命的影响。结合力学与疲劳试验对焊接接头相关性能进行分析研究,通过有限元方法获得两种焊接接头应变能-寿命疲劳曲线。与名义应力法进行对比分析,验证考虑残余应力的应变能密度法的准确性,主要研究内容和成果如下:(1)基于应变能密度法,通过有限元分析法验证了T型焊接接头整体应变能密度对网格尺寸的不敏感性,基于子模型技术对焊接接头焊根处和焊趾处应变能密度进行提取,结果表明不同网格类型及整体网格尺寸对子模型应变能密度存在一定程度影响。通过对比二维模型与三维模型计算结果发现,两种模型结果几乎相同。(2)对碳钢T型接头进行微观组织观察,通过拉伸试验获得接头最大拉力平均值。在不同应力水平下进行疲劳试验,获得疲劳寿命数据。基于疲劳断裂接头微观形貌可以看出不同焊接接头裂纹扩展模式,从微观角度可以观察到两种焊接接头均从焊根处断裂。对比分析不同疲劳评估方法对碳钢T型焊接接头疲劳寿命评估曲线,应变能密度法与名义应力法对接头疲劳寿命评估趋势一致,验证应变能密度法可以对碳钢T型接头进行疲劳寿命评估。(3)建立焊接接头热分析模型,通过添加不同热源模型对不同焊接接头进行焊接模拟数值分析。基于有限元分析法对焊接接头残余应力进行计算分析,残余应力主要以纵向残余应力存在于焊接接头内部,应力主要集中于焊缝区。温度场结果表明随焊接热源移动,焊缝区域表面节点温度急剧上升后下降,上升速率高于下降。考虑残余应力下焊接接头采用应变能密度法进行疲劳寿命评估,较不考虑残余应力评估曲线相比具有更窄的散射带。将三种疲劳评估方式进行对比研究,在疲劳寿命处于10~5-10~6区间,考虑残余应力的应变能密度法更接近实际试验值。更多还原

【Abstract】 Welded joints are susceptible to fatigue damage under working conditions due to stress concentration caused by welding residual stress.At this stage,the test method is mainly used for structural fatigue performance assessment,but the demand for specimens is large and the overall assessment period is long.Semi-empirical methods such as the nominal stress method and the hot spot stress method are sensitive to the geometry of the welded joint and cannot be evaluated for joints where nominal stresses are partially unavailable or where stresses are concentrated at the weld root location.In view of the limitations of the above assessment methods,this paper proposes a consideration of residual stress strain energy density method,through the strain energy density as a control parameter for fatigue assessment of welded joints,the fatigue life of carbon steel T-joints with different welding processes to assess the fatigue performance to verify the accuracy of the method.In this paper,the fatigue life of Q345NQR2 weathering steel single side T-joints with different welding processes is evaluated by strain energy density method,and the influence of welding residual stress on the fatigue life of welded joints is considered.Combined with mechanical and fatigue tests,the relevant properties of welded joints are analyzed and studied,and two strain energy-life fatigue curves of welded joints are obtained by finite element method.A comparative analysis with the nominal stress method is carried out to verify the accuracy of the strain energy density method considering residual stress.The main research contents and achievements are as follows.(1)Based on the strain energy density method,through the finite element analysis method to verify the overall strain energy density of the T-joints on the grid size insensitivity,based on sub-model technology to extract the strain energy density at the weld root and toe of the welded joint,the results show that different grid types and the overall grid size of the sub-model strain energy density has a certain degree of influence.By comparing the calculation results of the two-dimensional model and the three-dimensional model,it is found that the results of the two models are almost the same.(2)The microstructure of the carbon steel T-joint was observed,and the average maximum tensile force of the joint was obtained through the tensile test.Fatigue tests were conducted at different stress levels to obtain fatigue life data.Based on the microscopic morphology of the fatigue fractured joints,the crack propagation modes of different welded joints can be seen,and it can be observed from the microscopic angle that both kinds of welded joints fracture from the welding root.Comparative analysis of the fatigue life evaluation curves of carbon steel T-joints by different fatigue evaluation methods.The strain energy density method and the nominal stress method have the same trend of joint fatigue life evaluation.It is verified that the strain energy density method can be used to evaluate the fatigue life of carbon steel T-joints.(3)The thermal analysis model of the welded joint is established,and the welding simulation numerical analysis of different welded joints is carried out by adding different heat source models.Based on the finite element analysis method to calculate and analyze the residual stress in the welded joint,the residual stress mainly exists inside the welded joint as longitudinal residual stress,and the stress is mainly concentrated in the weld zone.The temperature field results show that with the movement of the welding heat source,the surface node temperature in the weld area rises sharply and then falls,and the rate of rise is higher than the fall.Fatigue life assessment of welded joints under consideration of residual stresses using the strain energy density method has a narrower scattering band compared to the assessment curve without consideration of residual stresses.The three fatigue assessment methods were compared and studied,and the strain energy density method,which takes into account residual stresses,was closer to the actual test values when the fatigue life was in the range of 10~5-10~6.更多还原