Fatigue Limit Evaluation Method Based On Temperature Evolution After Reconstruction Of Thermal Boundary Conditions

Compared with the traditional test method,the fatigue limit evaluation method based on temperature evolution has the advantages of fast and economy.Ideally,the temperature measurement results used for fatigue limit assessment should only depend on the heating of the material itself caused by energy dissipation.However,the temperature of the fatigue specimen will be affected by many external factors during the test.Maintaining a relatively stable experimental environment temperature is an important basis for ensuring the reliability of subsequent fatigue limit assessments.In order to solve the above problems,the main factors affecting the temperature change of the specimen were analyzed from the perspective of heat transfer.It has been clarified that the contact heat transfer between the two ends of the specimen and the fixture is the main way of heat exchange between the specimen and the outside.It is proposed that the influence of the temperature rise at the end of the specimen can be transformed into the boundary conditions at the virtual boundary,which overcomes the difficulty of setting the boundary conditions of the heat conduction model and realizes the accurate description of the temperature change process of the specimen.On this basis,it is further proposed to reconstruct the thermal boundary conditions of the temperature change process of the specimen according to the superposition principle of the boundary value problem of linear partial differential equations.A data processing method was finally proposed to eliminate the adverse effect of the temperature rise at the end of the specimen during the test.Numerical simulation was performed to verify the effectiveness of the proposed data processing method.It was shown that the temperature evolution after reconstruction of thermal boundary conditions only depends on the heat production of the material itself,which is equivalent to the temperature data obtained under the ideal conditions where there is no the temperature rise at the end of the specimen.Compared with other existing methods for controlling the temperature rising,the proposed method has the advantages of good effect,wide application range,low cost,and easy implementation.The newly proposed data processing method was used in the actual temperature measurement of AZ31 B magnesium alloy during high cycle fatigue test.Using infrared thermal imaging technology as the temperature measurement approach,a dedicated image processing program was written to perform secondary processing on the raw infrared thermal image data obtained during the experiment.The results show that the temperature increase at the end of the specimen has a significant effect on the temperature change process of the AZ31 B magnesium alloy fatigue specimen in the actual test.The temperature evolution after reconstruction of thermal boundary conditions was used to study the fatigue behavior of AZ31 B magnesium alloy.The temperature evolution curve indicates that AZ31 B has undergone cyclic hardening during fatigue.By comparing the results of two different orientations,it is found that the temperature evolution of AZ31 B during the fatigue process has obvious anisotropy.The temperature evolutions under different load levels were summarized.An obvious inflection point can be found in the temperature rise vs.load curve,and the load corresponding to the inflection point is close to the fatigue limit of the material.A new fatigue limit evaluation method was proposed,which is based on a statistical analysis of temperature data.The proposed new method was used to evaluate the fatigue limit of AZ31 B magnesium alloy.The results show that the new method can accurately assess the fatigue limit of AZ31 B,and has a wider applicability than the classic Risitano method.In view of the uneven distribution of the heat source in the welded joint,a modified thermal boundary condition reconstruction method based on the two-dimensional heat conduction model was proposed which is based on the original one-dimensional method.The two-dimensional method was used to process the surface temperature evolution of the AZ31 B magnesium alloy manual TIG welded joint specimen in the high-cycle fatigue test.According to the processed temperature data,the fatigue limit was evaluated by using the new method proposed in this paper.And,the result is satisfactory.