高速铁路钢桥的低温脆性断裂评估

基于断裂力学理论,提出采用损伤容限法的高速铁路钢桥低温脆性断裂评估方法。采用Euoro-code3确定正交异性板钢桥面的有效宽度和等效疲劳应力幅,应用断裂力学的Paris公式计算疲劳裂纹扩展速率,按照同时考虑脆性断裂和塑性屈服断裂影响的R6破坏模式确定设计应力强度因子,根据裂纹的长度、冲击功和转变温度确定含裂纹钢板在不同温度下的断裂韧性,并考虑板厚对冲击功的影响,桥梁结构中焊接残余应力、列车速度、钢板弯曲成型等因素对钢桥低温脆性断裂的影响。该方法适用于无试验条件确定含裂纹钢板低温断裂韧性情况下的低温脆性断裂评估,也可用于钢桥的疲劳强度降低程度、使用寿命内检查次数和焊缝修补次数的确定。应用此方法对欧洲某高铁钢桥的钢横梁下翼缘进行-40℃条件下的低温脆性断裂评估结果表明,当列车速度大于150km.h-1且钢横梁、钢吊杆等局部构件活载应力在70MPa以上时,等效降温已经达到-5℃以上,此时必须考虑列车速度对其脆性断裂的影响,而对应变速率小于0.002 5s-1的构件可以不考虑列车速度对脆性断裂的影响。

Assessment of Brittle Fracture Induced by Low Temperature of High-Speed Railway Steel Bridges

Based on fracture mechanics theory and damage tolerance method,an assessment method of brittle fracture induced by low temperature of high-speed railway steel bridges was proposed.The effective width and equivalent fatigue stress range of orthotropic steel deck were determined based on Euorocode 3.The crack propagation rate was calculated by using Paris law of fracture mechanics.The intensity factor of design stress was defined according to R6 failure mode considering the influences of both the brittle fracture and plastic yield fracture.The fracture toughness of steel plate with cracks under different temperature was determined based on crack length,Charpy impact test energy and associated transition temperature.At the same time,the influences of plate thickness on impact energy as well as the effects of the welding residual stress in bridge structure,train speed and the brake forming of steel plate on the brittle fracture induced by low temperature of steel bridge were taken into consideration.The method can be applied to the assessment of brittle fracture induced by low temperature under such circumstance that the low temperature fracture toughness of the cracked steel plate is defined without any test condition.It can also be used to determine the reduced fatigue strength,the maintenance frequency in service life and weld repair times of steel bridge.The method was used to assess the low temperature brittle fracture of the transverse steel girder bottom flange of a European high-speed railway steel bridge under the condition of-40℃.The results show that,when the train speed is greater than 150 km·h-1 and the live load stress of transverse steel girder or hanger is more than 70 MPa,the equivalent temperature reduction reaches over-50℃.Here the influence of train speed on brittle fracture must be taken into account.If the strain rate of the member is smaller than 0.0025 s-1,the influence of strain rate on brittle fracture can be ignored.