在役桥梁拉吊索腐蚀-疲劳损伤与破断机理分析

Analysis on Corrosion-fatigue Damage and Fracture Mechanism of Cables/Hangers in Service Bridges

拉吊索(悬索桥的吊索、斜拉桥的斜拉索以及中、下承式拱桥的吊索)是拉索桥梁的重要承重构件,其服役可靠性直接影响这些桥梁的安全性。以拉吊索病害特性为导向,通过有限元分析、索体钢丝的腐蚀-疲劳模拟试验,结合理论分析研究拉吊索的损伤与破断机理,以期为拉吊索的设计、养护以及检测提供参考。首先通过有限元分析发现若下锚固区发生0.001 13 rad的转角,该处产生的弯曲应力就有18.8 MPa,研究下锚固区索体钢丝的病害不能忽视弯曲应力的影响,弯曲应力也是造成长拉吊索发生破断病害的因素之一;接着以近5 a中国西南地区雨水中形成酸雨的离子浓度的平均值为基准值,在盐雾腐蚀箱中模拟酸雨环境,将直径7 mm抗拉强度为1 860 MPa的镀锌钢丝在盐雾腐蚀箱中模拟酸雨腐蚀环境与交变应力耦合作用下索体钢丝的腐蚀-疲劳试验,研究其腐蚀-疲劳损伤机理。研究表明:服役的桥梁拉吊索在腐蚀环境和交变应力耦合作用下发生腐蚀-疲劳损伤,腐蚀-疲劳致使构件腐蚀加剧,塑性降低,脆性增强,发生脆性破断,若在设计、检测、评估分析中对结构的腐蚀-疲劳损伤考虑不充分就会有重大安全隐患;弯曲应力也是造成长拉吊索发生破断病害的因素之一;在相同的腐蚀环境下,随着时间的增加,交变应力工况试件的腐蚀程度最大,其次是静态应力工况,无应力工况下的腐蚀程度最小;试件发生腐蚀-疲劳损伤后,其破断应力约为无腐蚀试件的60%,断后伸长率降低得更多,约为无腐蚀试件的40%,断口表现为脆断;发生腐蚀-疲劳损伤的拉吊索钢丝,若有复杂空间应力的作用,更易发生脆断。

Cables/hangers(suspension cable of suspension bridge, stayed cable of cable-stayed bridge and hangers of middle and lower arch bridges) are the important bearing components of the cable-stayed bridges. Its service reliability directly affects the safety of these bridges. In order to provide reference for design, maintenance and detection of the cables/hangers, the damage and breakage mechanism of the cables/hangers are analyzed by finite element analysis, corrosion-fatigue simulation test of the cable steel wires and theoretical analysis, guided by the disease characteristics of the cables/hangers. First, through FE analysis, it is found that if the rotation angle of 0.001 13 rad occurs in the lower anchorage area, the bending stress will be 18.8 MPa. The influence of bending stress cannot be neglected in the study of the diseases of cable steel wires in the lower anchorage area, and bending stress is also one of the factors causing the breakage of long stayed cable. Then, taking the average ion concentration of acid rain formed in the rainwater of southwestern China in recent 5 years for example. In order to simulate the acid rain environment in salt fog corrosion chamber, the corrosion-fatigue test on galvanized steel wires with diameter of 7 mm and tensile strength of 1 860 MPa is carried out in salt fog corrosion chamber to study the corrosion-fatigue damage mechanism of the cable steel wires under the coupling action of acid rain corrosion environment and alternating stress. The result shows that(1) Corrosion-fatigue damage of cables/hangers in service bridges occurs under the coupling action of corrosive environment and alternating stress. It results in the corrosion intensification, the plasticity reduction, the brittleness enhancement and brittle fracture of components. If the corrosion-fatigue damage of the structures is not considered sufficiently in the design, detection and evaluation analysis, there will be significant safety risks.(2) In the same corrosion environment, with the increase of time, the corrosion degree of alternating stress of the specimen is the greatest, followed by static stress, and the least under non-stress condition.(3) After corrosion-fatigue damage of the specimen, the breaking stress of the is about 60% of that of the non-corrosive one, and the elongation of the cables/hangers decreases more after breaking, which is about 40% of that of the non-corrosive one, and the workpiece is brittle fracture.(4) The steel wires of the cables/hangers with corrosion-fatigue damage is more prone to brittle fracture if they have complex spatial stresses.