Research On High-Cycle Fatigue Property Evaluation And Thermal-Mechanical Response Analysis:Based On Intrinsic Dissipation

Fatigue property is one of the most important properties of engineering materials.Due to the complexity of fatigue problems and the diversity of materials,fatigue failure laws and damage mechanisms are still misunderstood,which has become the bottleneck restricting the further development of modern industry.Especially,the fatigue property evaluation by using traditional experimental methods always needs performing a lot of fatigue tests with a long period.This greatly increases the manufacturing cost,limits the speed of research and development,and reduce the production efficiency.In nature,fatigue ruptxure is the inevitable result of the material microstructxure evolution under cyclic loading to degradation and failure.Fatigue damage evolution is an irreversible thermodynamic process accompanied with energy dissipation.Experiments show that,most of the dissipated energy is converted into heat inducing temperature change.It is hence an important approach for fatigue research that,firstly,analyzing the thermal-mechanical response of the material under cyclic loading and obtaining the information on fatigue damage evolution,and then,analyzing the fatigue properties by combining constitutive theories and numerical models,finally,further revealing the fatigue damage mechanisms.For this reason,the rapid evaluation of fatigue property and the analysis of thermal-mechanical response,based on intrinsic dissipation,is becoming one of the most active and the most promising research directions.The main objective of this doctorate research is further developing and improving the fatigue intrinsic dissipation theory and the related experimental technique and numerical method,and applying to the rapidly estimation of high-cycle fatigue behavior for metal materials and the analysis of thermo-mechanical response for rubber materials,and thereby further revealing the fatigue damage mechanisms.The main works are as follows:1)An estimation method for fatigue intrinsic dissipation of metal materials was proposed.The thermodynamic state equation under cyclic loading was derived and analyzed in the framework of continuum thermodynamics and based on the internal state variable theory.Fatigue self-heating mechanisms and heat conduction problems with different dimensions for sheet specimens were particularly studied.Based on the double exponential regression of the one-dimensional temperature distribution,a calculation model of high-cycle fatigue intrinsic dissipation was formulated.Its feasibility and validity were analyzed according to the experimental data.2)An energy method was proposed to rapidly evaluate the high-cycle fatigue property of metal materials.This energy method takes intrinsic dissipation as the fatigue damage indicator,and extracts the part of intrinsic dissipation related to microplastic deformation after eliminating the one related to internal friction,and considers that fatigue failure occurs once the former accumulates to a threshold value.Systemic fatigue tests were carried out by using this energy method to research the high-cycle fatigue behavior of FV520B steel,as well as the load dependency of intrinsic dissipation.It was shown,by comparing with the traditional method,that the energy method can not only achieve high prediction precision,but also greatly shorten the fatigue experimental period and reduce the experimental cost.3)An intrinsic dissipation model of metal materials was formulated.By introducing the internal state variables,the model considers two types of micro-mechanisms,being irreversible thermodynamic processes,i.e.the recoverable microstructure motion inducing anelasticity and the unrecoverable microstructure motion inducing damage.The dislocation-point defect interaction model was improved to analyze the intrinsic dissipation mechanisms,either qualitatively or quantitatively.Besides,the plastic pre-strain effect on the intrinsic dissipation was experimentally studied,and a universal empirical formula was proposed.4)The viscoelastic thermal-mechanical response of carbon-filled rubbers under cyclic loading was experimentally studied,as well as its evolution.The study was focused on the pre-stretched and filler content effects on the fatigue-induced stress-softening,hysteresis and self-heating.The relationship between the mechanical dissipation corresponding to hysteresis and the intrinsic dissipation corresponding self-heating was analyzed theoretically and related to the experimental data.By formulating a rubber chain-carbon cluster interaction model,the intrinsic dissipation mechanisms and the fatigue damage mechanisms were analyzed.