Research On Fatigue Life Prediction Method Based On Heat Dissipation Behavior Of Materials

Fatigue failure has always been one of the most common failure modes of materials and structures.Because of its suddenness,it often leads to catastrophic accidents,causing serious economic losses and casualties.With the in-depth study of fatigue problems and the development of advanced microscopic characterization technologies,a large number of models from phenomenological to considering material failure mechanisms have been proposed.However,as the service conditions of modern equipment are developing in the direction of high temperature,high load,long service life and complex environment.It is required that the study of fatigue problems should consider more complicated service conditions,such as very high cycle fatigue,multiaxial fatigue and variable amplitude fatigue.The focus and difficulty of metal fatigue research are mainly reflected in the disclosure of fatigue damage mechanisms and the establishment of accurate quantitative models.For a long time,the understanding of fatigue mechanisms relied on the study of material microstructure characterization,while fatigue life models are often developed from empirical formulas based on the macro continuum mechanics.There are few models based on fatigue damage mechanisms(such as crack initiation and small crack growth,etc.),which greatly limits the applicability of the model.Irreversible plastic deformation of materials or structures under fatigue loads will be accompanied by heat dissipation.And the level of heat dissipation is directly related to the magnitude of plastic deformation.In addition,local damage to materials or structures,such as the initiation and propagation of microcracks,will also increase the level of heat dissipation in this local area.Therefore,this irreversible heat dissipation behavior is closely related to the fatigue damage behavior of materials or structures,and can be used as a parameter to characterize fatigue damage.In recent years,the fatigue thermography method based on dissipated energy has successfully achieved rapid prediction of fatigue strength and S-N curve in the field of uniaxial fatigue.Based on the advantages of the fatigue thermography method and the wide application of full-field temperature measurement technology,it is of great significance to extend the fatigue thermography method to the application of more complex fatigue problems.In this research,the fatigue thermography method based on dissipated energy is applied to the research of very high cycle fatigue,multiaxial fatigue and variable amplitude fatigue,and some results have been obtained.The main research contents and conclusions of this study are as follows:(1)Introduce the fatigue assessment method based on energy dissipation theory.Firstly,under the framework of thermodynamics theory,combined with the first and second laws of thermodynamics,the thermo-mechanical coupling equation is given by deriving the energy conversion relationship in the fatigue process of material.Secondly,combining the state variables in the fatigue process of material to further describe the thermodynamic process,obtain the fatigue heat conduction equation in the form of the internal state variables of the material such as strain and temperature,and then clarify the inherent dissipation energy,thermoelastic source,internal coupling source and external heat source form.Thirdly,according to the material fatigue heat conduction equation,through further assumptions,the 2D,1D and 0D heat conduction models that can calculate the heat source from temperature are respectively derived.So far,the theoretical basis and calculation method of the dissipation energy as the fatigue damage parameter are given.(2)An evaluation method of microplastic strain in very high cycle fatigue regime is proposed.Firstly,the principle of ultrasonic fatigue test is introduced,and a series of ultra-high cycle fatigue tests are carried out on pure copper.Secondly,based on the characteristics of the ultrasonic fatigue test,a method for evaluating the distribution of plastic strain based on the dissipated energy is proposed by establishing the energy balance equation in the fatigue process.And then,the plastic strain in the very high cycle tests of pure copper was evaluated,and the results showed that the magnitude of the plastic strain was basically at the level of 1 με.Finally,the estimated plastic strain amplitude is correlated to the fatigue life according to the MansonCoffin equation.The results show that the plastic strain and fatigue life show a good linear correlation in the double logarithmic coordinate,indicating that the Manson-Coffin equation is also applicable to the very high cycle fatigue regime.(3)A multiaxial fatigue life model based on dissipated energy is proposed.The model is based on the idea of the quantitative thermography method and takes the dissipated energy as the multiaxial fatigue damage parameter.It is proposed that the material energy tolerance EC in multiaxial fatigue is related to the loading path,and then a new multiaxial fatigue life model is given by establishing the energy tolerance EC under any path.Then,a series of multiaxial fatigue tests were carried out on 316 L stainless steel to verify the life prediction ability of the model and compared with three classic critical plane models.The results show that the fatigue life predicted by the proposed model is basically within the range of error factor of 2 compared to the experimental life,and the prediction accuracy is higher than that of the three critical plane models.The fatigue failure mechanism of experimental 316 L stainless steel was analyzed in-depth,and a large number of uniformly distributed micro-cracks were found under the torsion path,resulting in a significantly higher fatigue life under the torsion path than other paths.The dissipated energy can be directly related to the local fatigue damage of the material,so the multiaxial fatigue model based on the dissipated energy has satisfactory life prediction ability.(4)A variable amplitude fatigue model based on dissipated energy is proposed.Firstly,a nonlinear damage accumulation model based on dissipated energy is proposed,and the continuous damage accumulation evolution is used to describe the contribution of variable amplitude fatigue process to material damage.Considering the influence of the previous load on the evolution of fatigue damage under subsequent loads,the influence factors of the loading sequence based on the dissipated energy are given.Then,a variable amplitude fatigue model based on the dissipated energy is proposed.Finally,three types of variable amplitude test data are used to verify the predictive ability of the proposed model,and the results reflect the good prediction ability of the model.Moreover,it also proves that the nonlinear accumulation of damage is more precise than the linear accumulation,and the prediction result considering the influence factors of the load sequence is more accurate.The advantage of this model is also reflected in the easy-to-obtain energy tolerance EC instead of the S-N curve that requires a lot of experiments to determine.Through the above research,the fatigue assessment method based on energy dissipation theory was successfully applied to the research of very high cycle fatigue,multiaxial fatigue and variable amplitude fatigue.The damage sensitivity of the dissipated energy makes the life model have a physical basis,thereby improving the accuracy of life prediction.In short,the fatigue assessment method based on dissipated energy provides a new idea based on the physical mechanisms for the study of fatigue problems.