| Fatigue fracture is one of the most common failure modes occurred in engineering structures and mechanical equipments. Fatigue failure often leads catastrophic consequences to the actual productions since there is no apparent sign before the sudden fatigue fracture. As a consequence, it is necessary to carry out investigations on the fatigue property assessment, the localized stress evaluation, the macro-and micro-fatigue failure mechanisms analysis. These research works are very helpful to suitably make periodic maintenance schedules and to ensure the reliability and safety of structures in service so that the possibility of fatigue failure can be reduced or avoided.For a long time, the traditional fatigue testing methods have played important roles in determining fatigue parameters of materials. However, the weaknesses of the traditional testing methods due to the long testing period, high cost, and low efficiency limit the increase of the economic benefit during the optimum structural design. Therefore, the development of a new method for fatigue assessment is in urgent need in order to meet the high requirements of modern design. From the macroscopic view, the evolution of the temperature field reflects the energy dissipation during fatigue process. The infrared thermographic technique, as a non-destructive, real time, full field and non-contact measuring method, makes it possible to record the evolution of the surface temperature signal for the purpose of fatigue investigation. Based on the infrared thermography and the fatigue energy theory, this paper is focused on the evolution of damage indicators:the temperature signal, the stress status and the intrinsic dissipation during fatigue process in order to carry out rapid prediction of fatigue parameters and evaluation of the stress status. The main purposes of this work are to develop a new quantitative model of the infrared thermogrpahy for rapid prediction of fatigue parameters of materials and components and to extend the application scopes of the quantitative model; moreover, the relationships between the heat treatment processes to microstructures and mechanical properties of materials are studied and the macro-and micro-fatigue fracture mechanisms are introduced in detail. The main works in this paper are listed as follow:1. Using the finite element method (FEM), the variations of the stress concentration factors around blind holes with different depths and diameters are studied for the purpose of size selection and anti-fatigue design of structures with blind holes. Evolutions of the intrinsic dissipation and the temperature signal are considered as fatigue damage indicators to assess the fatigue damage process and to determine the fatigue limit. Good accordance is confirmed between the two predicted the fatigue limits. A thermographic model is developed to predict the fatigue notch coefficient, and comparison between this model to FEM and TSA is made.2. The evolution of the surface temperature field is used to qualitatively analyze the processes of the nucleation, initiation and propagation of micro-fatigue cracks in the round specimen so as to conduct the damage status analysis and safety evaluation of structures in service. Based on the infrared thermography and the fatigue energy theory, a quantitative model of the infrared thermography for rapid prediction of the fatigue limit, residual fatigue life and S-N curve of materials is developed. Comparisons between the predicted results and the traditional results prove that the quantitative model not only can accurately predict fatigue parameters, but also can shorten the experimental period, reduce the cost and improve the efficiency.3. The effect of different heat treatment processes on microstructures and mechanical properties of FV520B steels is studied in order to provide guidance for proper selection of heat treatment processes, and to obtain the best microstructures and mechanical properties of the material. A new idea to assess the influence of the heat treatments on fatigue parameters is proposed by predicting the fatigue limits of different FV520B steels based on the infrared thermography. The variation of the superficial temperature field is utilized to analyze the irreversible damage of internal microstructures for the purpose of unifying the macro-and micro-failure mechanisms. The morphologies of the fatigue fracture surfaces of different FV520B specimens are investigated, and the macro-and micro-fatigue failure mechanisms are discussed in detail.4. Under the special conditions of fixed high tensile stress with variational stress ratios, the quantitative model of the infrared thermography is extended to assess fatigue performance parameters of the cruciform welded joints with two different dimensions for the purpose of overcoming the limitations of the traditional assessment methods on fatigue parameters of welded components. Comparisons between the predicted results and the traditional results show that the quantitative model of the infrared thermography works well on the fatigue behavior evaluation of heterogeneous welded joints. Based on the model, the influence of the high tensile stress on fatigue parameters of the cruciform welded joints is discussed in detail. Moreover, the morphologies of the fatigue fracture surfaces are analyzed, and different fatigue failure mechanisms of the welded joints are revealed. |
