基于刚度下降的混凝土梁疲劳累积损伤模型的研究

A Fatigue Cumulative Damage Model of RC Beam Based on Stiffness Degradation

通过疲劳试验，引入剩余刚度和累积损伤值的概念，研究钢筋混凝土梁由于疲劳累积损伤而造成的刚度衰减规律。结果表明：荷载循环初期刚度衰减较快，且不稳定，这一阶段约占疲劳寿命的5％～10％；材料物理性能的变化规律基本稳定后，刚度衰减速率基本为一定值，剩余刚度与循环次数之间基本呈线性关系，这一阶段约占疲劳寿命的80％～90％；接近疲劳寿命时，刚度急剧下降，直至疲劳破坏，这一阶段约占疲劳寿命的3％～5％；在疲劳破坏时，混凝土梁的剩余刚度降为初始刚度的70％。基于钢筋混凝土梁疲劳的第二阶段约占疲劳寿命的85％，且在此阶段剩余刚度与循环次数之间基本呈线性关系，建立了钢筋混凝土梁累积损伤值D与循环寿命比N／Nf之间关系的数学模型。在工程应用中，可以以累积损伤值D-1作为失效判据，通过少量循环的疲劳试验确定混凝土梁刚度衰减速率k的大小，然后由D=3．317k（N／Bo）来预测钢筋混凝土梁的疲劳寿命。

The concepts of the residual stiffness and cumulative damage value are introduced through fatigue experiments to study the laws of stiffness degradation of RC beams caused by fatigue cumulative damages. The following conclusions can be drawn： The stiffness decreases quickly and unsteadily at the initial stage of load cycling and the stage occupies about 5%-10% of the fatigue life; when the physical property changes of materials are basically stablized, the stiffness degradation rate remains at a certain value, a basically linear relation between the residual stiffness and the cycling number is demonstrated,and the stage occupies about 80%-90% of the fatigue life,when the fatigue life is approaching its limit,the stiffness decreases sharply until to the fatigue failure, and the stage occupies about 3%-5% of the fatigue life; at the time of fatigue failure, the residual stiffness of the RC beam is reduced to 70 percent of the initial stiffness. In view that the second fatigue stage occupies about 85 percent of the fatigue life and there is a basically linear relation between the residual stiffness and the cycling number, a mathematical model describing the relation between the fatigue cumulative damage D and the cycle life ratio N/Nf of the RC beam is established. In engineering practice,D=1 may be regarded as the failure criterion, the stiffness degradation rate of the RC beam may be gained through a small number of cycled fatigue experiments, and then the fatigue life of the RC beam may be predicted by means of D=3. 317k（N/ B0）.