13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- damage status, and accordingly the fatigue limit can be fast determined. 3. Experiment investigations Material and specimen. The studied material is FV520B [7]. Its mechanical properties are greatly affected by heat treatments. Thus, two type specimens (Figure 1. Size of the specimen) were machined from two plates. One plate was just annealed (type A), and the other was treated with treatments in Ref.[9] (type B). Figure 1. Size of the specimen (unit: mm) Experimental procedures. Fatigue tests were carried out at room temperature without disturbance of the external heat resource. The testing system is composed of MTS810 system, infrared camera, lock-in module, computers and corresponding softwares [9]. Before tests, all the specimens were polished with fine grit papers, and then painted the specimens black to improve the heat radiation. The stress ratio is set as R=-1 with a frequency of 20Hz. The stepwise loading procedure was applied to the same specimen. To avoid fatigue damage accumulation, the stress was applied from 100MPa with steps of 50MPa until fracture. The thermal images on the hot-spot zone were recorded by the infrared camera to perform the subsequent data analysis. 3. Results and discussion Qualitative identification of the fatigue damage. Temperature evolution is mainly attributed to the thermoelastic effect, plastic effect and heat conduction. The fluctuating temperature is due to the thermoelastic effect. The temperature amplitudes increase with the increasing cyclic loading. However, the thermoelastic effect has no contribution to the average temperature [8]. Figure.2 exhibits the relative temperature evolution of the two type specimens, showing the almost same trend. The elastic stress controls mechanical responses of tested specimens when the stress is less than 300MPa. At this term, the specimen mainly takes place elastic deformation, and the temperature increment are mainly induced by non-plastic effects, i.e. viscosity effect etc. If the stress is higher than the fatigue limit, microcracks will initiate from the specimen boundaries, such as: the corner and the free surface. They initiate along the axial direction of 45o, and then coalesce to form a main crack. The main crack propagates perpendicular to the principal stress direction. The plastic strain energy accumulates with the fatigue evolution. Most of the mechanical energy is dissipated as heat energy heating up the specimen. There are，however, still some different between the two type steels. The temperature increment of the type B is generally less than that of the type A. That is attributed to the slow evolution of internal microstructures of the type B, indicating its better fatigue resistance.
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