钢管K形节点足尺模型疲劳性能试验研究

Experimental study on fatigue performance of full-scale circular hollow section K-joint

为研究钢管K形节点的疲劳性能,进行了足尺模型的疲劳试验,采用梯度应变片测量焊缝区域应力变化,采用相控阵扫描成像检测系统(简称相控阵)检测初始缺陷和疲劳裂纹,对疲劳开裂后的钢管K形节点进行切片观测,研究节点相贯焊缝附近的热点应力分布、疲劳裂纹扩展规律和疲劳破坏模式等,并分析了采用名义应力法和热点应力法评定钢管K形节点疲劳寿命的合理性。结果表明:钢管K形节点的热点应力宜采用梯度应变片得到的梯度应力进行线性外推得到,最大热点应力位于主管和受拉支管相贯焊缝靠近主管侧的冠点处;节点两侧鞍点热点应力值最大误差率为6.64%,节点基本不受面外弯矩影响;采用相控阵检测能够对节点初始缺陷和疲劳裂纹进行定位、定性和定量分析;切片观测结果表明,钢管K形节点疲劳裂纹属于张开型I裂纹;疲劳裂纹的萌生、扩展和破坏阶段分别对应的加载次数为0~229.1万次、229.1~245.5万次和245.5~251.2万次,疲劳裂纹萌生于热点应力最大主管和受拉支管相贯焊缝靠近主管侧的冠点处,其疲劳破坏属于弹性范围内的脆性破坏;名义应力法由于名义应力幅放大系数偏大,评定所得的钢管K形节点疲劳寿命过于保守;基于热点应力法,CIDECT评定所得结果偏不安全,API评定结果具有1.11倍的安全富余,适用于评定本试验中钢管K形节点的疲劳寿命。

To study the fatigue performance including distribution of hot spot stress, regularity of fatigue crack development and fatigue failure mode of circular hollow section(CHS) K-joint, fatigue test on full-scale CHS K-joint model was conducted. Variation of stress around the intersection weld, initial imperfection and fatigue crack were monitored by gradient strain gauge and phased-array ultrasonic testing system, respectively, and slicing detection was also carried out on CHS K-joint with fatigue crack. Furthermore, the rationality of evaluation on fatigue life of CHS K-joint was also studied based on nominal stress and hot spot stress, respectively. It is found that hot spot stress of CHS K-joint is suitable to be obtained by linear extrapolation according to gradient stress measured by gradient strain gauge. The location of the maximum hot spot stress appears on the crown point of chord and tensile brace intersection welding seam near chord intersecting line. The maximum deviation of saddle points on both sides of CHS K-joint is 6.64%, namely test results of CHS K-joint is hardly affected by out-of-plane bending moment. Locational, quantitative and qualitative analyses on initial imperfection and fatigue crack can be achieved by phased-array ultrasonic testing system. Fatigue crack of CHS K-joint belongs to opening class I crack by slicing observation. Loading cycles corresponding to crack initiation, crack propagation and joint damage are 0-2.291 million, 2.291-2.455 million and 2.455-2.512 million, respectively. Fatigue crack firstly derives from the crown point of chord and tensile brace intersection welding seam near chord intersecting line with the maximum hot spot stress, namely, the fatigue failure of CHS K-joint belongs to brittle failure in the elastic range. Due to higher amplification coefficients within nominal stress range, the evaluation result of fatigue life on CHS K-joint is excessively conservative. As for the method adopting hot spot stress, the evaluation result acquired by CIDECT is unsafety. However, the evaluation result acquired by API possesses a safty factor of 1.11. In summary, API is suitable to be adopted as evaluation criteria for fatigue life of CHS K-joint in the fatigue test.