Thermodynamic Entropy Characteristic Analysis And Life Prediction Model Of Metal Fatigue Damage Process

The fatigue problem of material components has always been a key issue of concern to the engineering community.Due to the suddenness of fatigue fracture,it often brings huge losses to the social economy and life safety.How to accurately assess fatigue damage and predict fatigue life is the key and difficult problem in the field of structural state monitoring and fault diagnosis.Traditional fatigue theory methods are time-consuming and consumables.Fatigue inherent complexity that makes many fatigue theory and fatigue methods can,within a specific range for use in specific conditions.There is a lack of means to quickly assess the state of fatigue damage and predict life.In recent years,advanced measurement methods such as infrared thermal image with non-destructive,real-time,full-field,non-contact advantages are used to record characteristic data(like temperature field)of metal material samples under different fatigue loads,and then to estimate materials/components.The method of fatigue characteristics has been developed and utilized to a certain extent,it shows better prediction accuracy.Directly uses temperature change as an indicator to evaluate fatigue performance,which can quickly predict material fatigue limit and S-N curve.However,it is difficult to fully reflect the fatigue damage degradation process.As an irreversible measure of damage process,thermodynamic entropy has the ability to reflect the order degree(integrity)of the system.Compared with the traditional fatigue analysis and evaluation method,it can better explain the physical mechanism of fatigue damage process and has unique advantages.In order to clarify the changes of thermodynamic entropy characteristics during the fatigue process of material samples,and the effects of loading conditions on the thermodynamic entropy characteristics such as thermodynamic entropy yield,cumulative entropy production and fatigue fracture entropy during fatigue process.In this paper,the axial tensile low cycle fatigue test was carried out using Q235 material samples containing central holes.The fatigue thermal imaging platform was built to monitor and collect the surface temperature characteristics of Q235 material samples during low cycle fatigue.According to the second law of thermodynamics and the Clausius-Duhem inequality,the energy dissipation caused by plastic deformation during low-cycle fatigue is combined with the surface temperature characteristics of the material to obtain the entropy characteristics.The results show that the entropy yield of the fatigue process is basically constant at the same loading amplitude,and the entropy yield increases with the increase of the loading amplitude and frequency.The accumulation process of thermodynamic entropy during fatigue is a quasi-linear accumulation process,and the cumulative rate of entropy production increases with the increase of loading amplitude and frequency.The fatigue fracture entropy under different loading conditions decreases with the increase of the loading amplitude,but has no obvious relationship with the frequency.By analyzing the change of loading conditions to the thermodynamic entropy characteristics of thermodynamic entropy yield,cumulative entropy production and fatigue fracture entropy during fatigue process,the relationship between fatigue fracture entropy and loading amplitude is obtained.Through the normalized fatigue cumulative entropy and cycle correlation analysis,the life prediction model is established,and the model is verified by experiments.Compared with the traditional life prediction model,the prediction results are good.At the end of the paper,the macro and microscopic features of the low-cycle fatigue fracture of Q235 material samples were analyzed and discussed,which provided a reference and basis for fatigue damage assessment.The method of this paper provides a new idea for the prediction of low cycle fatigue life of metal components.There is a certain error in the damage assessment by a single thermodynamic entropy feature,In order to improve the accuracy of evaluation and reduce the uncertainty of evaluation,the multi-feature fusion damage assessment based on thermodynamic entropy will be carried out in the subsequent research,which is also an important direction for future research on fatigue damage.