不同强度钢筋考虑屈曲的低周疲劳损伤模型

Low-cycle fatigue damage model of buckled steel bar with different strength

钢筋混凝土柱受力后期的重要非弹性特征之一是纵向钢筋受压屈曲,以及反复拉压之后断裂,但可考虑钢筋屈曲的低周疲劳损伤模型很少,且少数考虑屈曲的疲劳损伤模型无法直接用于不同强度钢筋的疲劳损伤计算和断裂分析。对长径比为6.25、9.375、12.0、15.0的HRB400钢筋、HRB500钢筋试件分别进行考虑屈曲的拉压相等循环加载、拉压不等循环加载试验,测量平均应力-平均应变(σˉs-εˉs)曲线和跨中横向屈曲位移。与HRB600钢筋的相应试验结果结合,形成系统性试验数据。基于试验数据分析屈服强度、长径比对屈曲钢筋极限变形能力的影响,考察传统C-M疲劳寿命模型、基于总平均应变幅εˉsa的修正C-M模型对屈曲钢筋的适用性,并分析误差原因;提出适用性较好的基于循环总平均应变幅εˉsa-cyc的修正C-M疲劳模型。结果表明:由于钢筋的εsu、εsult和fu等力学性能参数不同,不同强度钢筋试件屈曲后的低周疲劳受力性能存在差异;屈曲钢筋循环受力时的极限变形能力与低周疲劳损伤有关,仅根据单调受拉的极限拉应变εsu不能正确判断钢筋的断裂状态;基于总平均应变幅εˉsa的修正C-M模型无法合理考虑不同加载方式对屈曲钢筋低周疲劳寿命的影响,存在系统误差;基于εˉsa-cyc的修正C-M模型可合理考虑不同加载方式的影响,能直接用于不同强度、不同长径比钢筋,且误差较小。

One of the important inelastic characteristics of reinforced concrete columns during the strain-softening stage is the buckling and the fracture of longitudinal reinforcement after tension-compression cyclic loading.However,there are few low cycle fatigue damage models considering the influence of buckling,and a few fatigue damage models considering buckling cannot be directly used for fatigue damage calculation and fracture analysis of steel bar with different strength.In this paper,the specimens of HRB400 reinforcement and HRB500 reinforcement with slenderness ratios of 6.25,9.375,12.0 and 15.0 were subjected to tension compression equal cyclic loading and tension compression unequal cyclic loading considering buckling respectively.The average stress-strain(σˉs-εˉs)curves and mid-span transverse displacements of buckled specimens were measured.Combined with the corresponding test results of HRB600 reinforcement completed by the author,a systematic test data was constituted.Based on the test results,the effects of yield strength and slenderness ratio on the ultimate deformation capacity of buckled reinforcement were analyzed,the applicability of the traditional low cycle fatigue damage model(C-M model)and the modified C-M model based on the total average strain amplitudeεˉsa to the buckled reinforcement was investigated,and the errors were analyzed.A modified C-M fatigue damage model based on cyclic total average strain amplitudeεˉsa-cyc with good applicability was proposed.The results show that specimens with different strength have different low cycle fatigue performance due to the different mechanical properties of steel bar,such asεsu,εsult and fu etc.The ultimate deformation capacity of buckled steel bar under cyclic loading is related to low cycle fatigue damage,the fracture of steel bars cannot be correctly determined by the ultimate tensile strainεsu under monotonic tension.The modified C-M model based on the total average strain amplitudeεˉsa cannot reasonably consider the influence of different loading methods on the low cycle fatigue life of buckled reinforcement,and there are systematic errors.The modified C-M model based onεˉsa-cyc can reasonably consider the influence of different loading methods,and can be directly used for reinforcement of different strength and slenderness ratio with small error.