Experiment Research On Thermal Effect Mechanism During The Fatigue Testing

Energy dissipation is inevitable accompanying the non-equilibrium irreversible changes in the process of materials' fatigue failure, which is represented as the heat dissipation on the macroscopy.Temperature is the most important parameter of studying heat dissipation. The theory of determining fatigue limit and experiment method have been discussed, as well as the heat phenomenon and application in the fatigue of materials have been recommended in thisarticle.Based on energy theory, the infrared thermographie sensor (FLR ThermaCAM^TM E65) and the high-frequency fatigue testing machine (PLG-100) have been utilized in the fatigue test to Q235 steel and 45# steel. The law of temperature changes of materials was systematically studied during the fatigue test of materials. Based on the test result, the relation curve of temperature changes by load characteristics was given, Moreover, the mechanism of temperature rising has been discussed as well.The temperature oscillation that the load is lower than fatigue limit results from the thermoelastic effect, and the temperature rising comes from non-elasticity and non-plasticity effect (such as the viscous-ness effect). On the other hand, the cumulated plasticity work when the load is above fatigue limit leads to the fatigue damage and changes thermogenic mechanism. A certain relationship between the temperature rising at stable stage and the corresponding service load under any cases (above or below the fatigue limit) can be achieved and represented as two lines. The intersection of the two lines is the fatigue limit of this material. Compare to fatigue limit acquired from the traditional method, the error of this method can meet the requirements of the project error. Using this new method, the fatigue limit of Q235 steel and steel No. 45 were measured through single axle test in this paper. The error is within 1.5% compared with the reference result achieved from the methods introduced by other papers.In the test, the viscous-ness coefficient of two kinds of materials was acquired by the relationship between temperature and loaded amplitude with different frequency, and the different contribution of plasticity and viscous-ness effect of material were also discussed. Furthermore, a novel method using the starting point of plastic energy dissipation as the material fatigue limit was proposed with the parameter 'temperature'.Theoretical analysis and testing result demonstrate that the method of measuring the fatigue limit of materials by infrared thermographic technology has such advantages as efficiency, accuracy, convenience, economy, etc.