Study On The Low-cycle Fatigue Damage Characterization And Life Prediction Of Metals Based On The Energy Dissipation Theory

It is found that the existing energy dissipation theory during metal fatigue hasmany deficiencies, especially an integrated study is lacked. Lacking in experimentalcondition and theory, the researchers only studied the energy dissipation duringfatigue from their most interested subjects. However, with the appearance of advancedexperiment apparatus, it is conceivable to achieve the integrated research. On thegroundwork of former, this paper takes up the integrated problem based on theanalysis of thermal dissipation and surface micro-topography of metal, and furtherperfects the theory of energy dissipation during low cyclic fatigue(LCF).Through analyzing the energy dissipation process during LCF, it is found that themechanical energy is dissipated in various forms of energy. The energy mainly are theheat energy dissipated in the environment and the stored energy consumed by thedeformation of the microstructures, which mainly is heat energy. Through analyzingthermal dissipation and stored energy during the fatigue damage, it is found that thethermal dissipation elevates the temperature of the specimen above that of theenvironment; thermal conduction takes place between the material volume units andthermal exchange between the specimen and the environment, which bring on localtemperature field on the specimen. The energy stored in materials during fatiguechanges the energy state of the materials, and the energy state reflects the change ofthe surface microstructures, which is directly related to the damage state. Manystudies show that the change of temperature, which can reflect the dissimilar course ofthe fatigue behaviors of materials during low cycle fatigue, is self-similar with thechange of thermal dissipation. The change of surface micrograph, which can reflectthe dissimilar state of the fatigue behavior of materials, is coincident with the changeof the stored energy.In this paper, the test method and test of the energy dissipation during LCF wereintroduced. The temperature response and the micrograph change of T2pure copperand LY12CZ Aluminum Alloy under LCF were measured synchronously, usinginfrared thermograph instrument and remote high power objective microscopy. Thetemperature variation on the surface of specimen changed with stress levels. DuringLCF damage, temperature response of specimen surface was related to stress level，and the temperature field of specimen surface could reflect the fatigue damageindifferent stages. The evolution of Lüders Band was simultaneously recorded during fatigue, which was reflected the material plastic deformation in early stage of fatiguedamage. The temperature has one relation on the basis of energy with the surface. Theresults showed that there exits obvious relationships between the measuredtemperature and the microscopic shape. The temperature and the microscopic occur todissimilar change with the difference of loading mode and specimen shape.For describing the heat energy dissipated during LCF, the expression, usingcorrelative thermal theories to calculate the thermal energy dissipated per unit volumeand per cycle with MATLAB Fourier Transform, was established. Because ofdisregard of other energy dissipated, the quantitative relation between thermaldissipation and plastic strain energy was uncertain, and the given expression was alsolimitative.The feature extraction was achieved utilizing image gray level histogram andgray co-occurrence matrix(GLCM) method. The surface morphology changes wasdescribed and characterized quantitatively. There existed obvious relationshipbetween the measured temperature and the microscopic shape. It reflected thevariation of thermal dissipation and energy storage in the fatigue.Based on the analysis on the mechanical energy dissipation during LCF, thecyclic hysteretic energy is very high at its earlier cycle, and gradually it rises steadilyat the steady cycle, but it decreases quickly before ruptures. The cumulative plasticstrain energy increases with the increase of the loading frequency and stress level,which is linear with fatigue life on semi-log coordinate. According to the Fourier’sLaw of the thermal conduction, the governing equation of the energy dissipation, theequation with temperature of fatigue life prediction was derived. The predicted liveswere found to be in good agreement with the experimental results. Subjected to actualexperiment means and technical level, it is difficult to quantitatively analyze therelationship between the thermal dissipation and stored energy. Therefore, thequantitative analysis of the energy dissipation during LCF remains to be furtherstudied.