恒静载降K法测定焊缝疲劳裂纹扩展门槛值研究  

Fatigue Crack Growth Threshold Testing for a Weld Using the Constant Mean Load K Reduction Method

作　　者：刘秀国[1] 邓冲 邓彩艳[1,2] 梁行 张涛 牛得田 Liu Xiuguo;Deng Chong;Deng Caiyan;Liang Hang;Zhang Tao;Niu Detian(School of Materials Science and Engineering,Tianjin University,Tianjin 300350,China;Tianjin Key Laboratory of Advanced Joining Technology,Tianjin 300350,China;National Innovation Center of High Speed Train,Qingdao 266111,China)

机构地区：[1]天津大学材料科学与工程学院,天津300350 [2]天津市现代连接技术重点实验室,天津300350 [3]国家高速列车技术创新中心,青岛266111

出　　处：《天津大学学报（自然科学与工程技术版）》2024年第1期79-86,共8页Journal of Tianjin University：Science and Technology

基　　金：国家高速列车技术创新中心研发计划资助项目(CXKY-02-06(2020));国家自然科学基金资助项目(51875402,51771135).

摘　　要：焊接残余拉应力会显著降低焊接接头疲劳性能,因此,焊接结构疲劳寿命评估参数的测定应对焊接残余应力予以充分考虑.为测得考虑焊接残余应力影响条件下的疲劳裂纹扩展门槛值,本文提出通过设定恒定平均外载以等效残余应力作用,进而逐级降低裂尖应力强度因子范围(ΔK)测得焊缝疲劳裂纹扩展门槛值的方法,即恒静载降K法.通过与ASTM E647-15等标准推荐测试方法所得结果对比,探究了恒静载降K法测量值适用性;通过扫描电镜(SEM)、电子背散射衍射(EBSD)等手段观察,对比分析了不同测量方法近门槛值载荷下的裂尖材料疲劳断口和变形行为.结果表明:采用恒静载降K法测量焊缝疲劳裂纹扩展门槛值最低,采用恒最大应力强度因子(K_(max))降载法的测量值次之,采用恒应力比(R)降载法的测量值最高.对比不同裂纹尖端K_(max)设定对门槛值测量的影响,发现随着K_(max)增大,门槛值测量值先快速下降,随后趋于平稳;采用恒静载降K法测定过程中K_(max)呈逐渐上升趋势,相应的门槛值测量值最低.疲劳断口显示,随着外加平均载荷增大,在近门槛值阶段断口形貌由疲劳辉纹转变为微区解理断裂;基于EBSD表征计算的裂尖材料几何必需位错密度表明,随着近门槛值外载的平均载荷增大,裂纹尖端位错密度增加.Tensile welding residual stress significantly degrades the fatigue performance of welded joints.Hence,the effect of welding residual stress should be fully considered when determining the fatigue life assessment parameters of welded structures.This study proposed to reduce the crack-tip stress intensity factor range(ΔK)step by step by applying a constant mean external load equivalent to the effect of residual stress,which can be referred to as the constant mean load K reduction method.The applicability of the measurement value obtained through the proposed method was explored and compared with the results obtained based on the recommended test methods in ASTM E647-15 and other standards.The fatigue fracture surface and deformation behavior for crack-tip materials under near-threshold load with various measuring techniques were compared and studied using scanning electron microscope(SEM)and electron backscattering diffraction(EBSD).The results show that the fatigue crack growth threshold measurement value for a weld was the lowest with the constant mean load K reduction method,the higher measurement value was obtained based on the constant maximum stress intensity factor(K_(max))load reduction method,and the highest was obtained using the constant external load stress ratio(R)load reduction method.When the effect of different crack-tip K_(max) settings on threshold measurement value was compared,it was discovered that when K_(max) increases,the threshold measurement value decreases significantly at first and then tends to remain unchanged.During the determination process,the K_(max) value under the constant mean load K reduction method continuously increased,and the corresponding threshold measurement value was the lowest.The fatigue fracture surface shows that,with the increase in applied mean load,the fracture surface morphology changed from fatigue striation to micro-cleavage fracture at the nearthreshold stage.Based on EBSD analysis,the geometrically required dislocation density for the crack-tip material ind

关 键 词：焊缝 疲劳裂纹扩展门槛值 疲劳裂纹扩展速率 恒静载降K法 

分 类 号：TG405[金属学及工艺—焊接]