基于损伤力学的桥式起重机疲劳寿命分析

Assessing and analysis of the fatigue life of the bridge-type crane based on the damage mechanics

为了评估桥式起重机金属结构的前期寿命,保证起重机的工作安全,需要建立对应的损伤理论模型并结合结构疲劳合算点的疲劳应力谱来评估寿命。通过Q235B钢的常温静拉伸疲劳试验获得不同循环应力(220~300 MPa)下的疲劳寿命,在双对数坐标下通过拟合得出lg(1-D)与lg(1-N/Nf)的线性关系,继而确定损伤模型中的初始损伤值D0和损伤指数q,从而建立20℃下的Q235B钢损伤退化模型。以动态仿真计算桥机的疲劳危险点应力-时间历程,通过雨流计数法建立桥式起重机应力谱。通过ANSYS仿真分析桥式起重机金属结构受力状态,计算各危险点在不同应力情况下的损伤累积,以临界损伤为界(Dc=0.2),评估桥式起重机金属结构的前期寿命。通过上述方法评估某厂桥机的前期寿命为37.72 a。研究表明,损伤力学结合应力谱评估前期寿命的方法是可行的,可为起重机日常维护管理提供参考。

The present paper is intended to establish a corresponding damage theoretical model and then combine it with the fatigue stress spectrum of the structural fatigue accounting point so as to estimate the earlier stage life fatigue of the metal structure of the bridge type crane and ensure its working safety. For the said purpose,we have done 5 different groups of steel Q235B fatigue experiments on the stress control under the different circular stress conditions (220 300 MPa) at the temperature of 20℃,so as to test and determine the fatigue life of the crane in correspondence with the different stress amplitude experimental data. Through the above said fatigue experiments,we have managed to establish the initial damage parameter D0 and the damage index q in the damage model through the linearity of combining lg (1-D) and lg (1-N/Nf). And,then,under the double-logarithmic coordinate axis,we have found and confirmed the corresponding damage indexes in the different degrees of stress,with the biggest cyclic stress σmax,which is involved in the damage index. And,so,the different degrees of stress tend to have an effect on the damage model. Last of all,it would be possible for us to gain the damage degeneration model that can reflect the stress levels gained. Thus,the dynamic simulation model can be turned over to calculate the stress-time history of the fatigue accounting points,so as to establish the bridge crane stress spectrum through the rain flow counting model and the frequency of the amplitude to get the mean value. On the other hand,the stress state of the metal structure of the bridge crane can be analyzed through ANSYS simulation to calculate and work out the damage data accumulation of each dangerous point under the different stress conditions. And,when the damage data worked out can reach the critical damage value (Dc= 0. 2) in the circulation process,the number of cycles can be taken as the fatigue earlier stage life of the bridge crane. In so doing,the earlier stage of the fatigue life of the 20/5 t bridge crane has actually been worked out in the factory concerned as 37. 72 years with the above said method. And,therefore,as compared with the latest discard qualification for the ladle crane (GB/T 36697—2018),the error of the calculated life estimation as has been done in this paper proves to be 9. 7%. And,so,the testing results tend,as has been said,to demonstrate that the method of the damage mechanics combined with the stress spectrum we have gained can be said feasible and adaptable,which can be said qualified enough to provide for an essential reference to the routine crane maintenance and management.