鸟嘴式X形方管节点在支管荷载作用下的应力集中及疲劳性能研究

Stress concentrations and fatigue behavior of bird-beak SHS X-joints under brace loadings

为研究鸟嘴式X形方管节点的应力集中特性和疲劳性能,设计了8个节点试件,并开展支管轴力工况和支管面内弯矩工况下的疲劳试验。通过静力单调加载试验,测得焊趾附近的应变分布,并采用二次外推法得到热点应力值和应力集中系数(SCF),分析此类节点在典型支管工况下的关键热点区域;同时,基于有限元参数分析揭示节点几何无量纲参数对SCF的影响规律,并提出SCF计算式。在此基础上开展循环加载试验,得到节点的疲劳破坏模式、裂纹扩展过程、刚度退化及疲劳寿命等性能指标。研究表明:支管轴力工况下的SCF最大值位于主管鞍部或支管鞍部,支管面内弯矩工况下的SCF最大值均位于支管冠部;裂纹多萌生于SCF最大值区域,且在裂纹贯穿壁厚之后节点刚度急剧下降;裂纹达到极限长度时,支管轴力工况下的节点刚度退化率在14%以内,而支管面内弯矩工况下的节点刚度退化率最大达40%;国际管结构协会疲劳设计指南的S_(rhs)-N曲线可用于评估鸟嘴式节点的疲劳寿命;在同等名义荷载作用下,鸟嘴式方管节点的疲劳性能要显著优于相同几何尺寸的传统方管节点。

Eight specimens were fabricated and tested to investigate the stress concentrations and fatigue behavior of bird-beak X-joints under brace axial forces and brace in-plane bending.Static loadings were first implemented and the strain distribution near the weld toe was measured,from which the hot spot stress and stress concentration factor(SCF)can be obtained by the quadratic extrapolation method.The critical hot spot regions of the joints under such brace loadings can be determined.Finite element parametric analysis was further carried out to reveal the influence of non-dimensional parameters on SCFs,and the corresponding calculation formulae of SCF were then proposed.During the subsequent cyclic loading,the fatigue behaviors including the fatigue failure mode,crack propagation process,stiffness degradation and characteristic fatigue lives were all observed.The results show that,the maximum SCFs of bird-beak joints under brace axial forces occur at the brace saddle areas or chord saddle areas,while the maximum SCFs of the joints under brace in-plane bending always occur at the brace crown areas.The fatigue cracks commonly initiate at the critical areas with the maximum SCFs,and significant stiffness degradations are observed after the cracks run through the wall thickness.When the cracks reach the limit length,the accumulated rigidity degradation in the case of brace axial forces is found to be no greater than 14%,while the total degradations in the case of brace in-plane bending are up to 40%.The S_(rhs)-N design curves in the CIDECT fatigue design guide can be adopted for the fatigue life predictions of bird-beak X-joints.In the case of equal nominal brace loads,the fatigue behaviors of bird-beak X-joints are much better than those of conventional SHS joints with identical dimensions.