13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 291MPa, and 319MPa for the type B. Fortunately, using the traditional method, the fatigue limits of the type A and B are 274MPa and 310MPa, respectively. The low values of errors, 6.2% and 2.9%, confirm the reliability of the infrared thermographic method in predicting fatigue parameters of materials with different heat treatments. To verify that the fatigue limit of the type B is higher, two fatigue tests, with the same cyclic stress 400MPa, were carried out to compare their fatigue life. The life of the type A is 141606 cycles, and 300748 cycles for the type B, confirming our results. It is different for the variation of energy accumulation and heat dissipation during different fatigue process. The above fatigue tests can be divided into three phases. At lower stress, i.e. 100MPa~300Mpa, the temperature increases slowly, and the intrinsic dissipation is practically null. The internal microstrucutre evolution is reversible under the elastic stress. However, when the stress is close to the fatigue limit, the local stress may be beyond the yield limit due to the stress concentration in micro-scale. Consequently, the slip band begins to form, and numerous microcracks initiate here. Fatigue damage sets to accumulate continuously. If the stress is higher than 450MPa, all the three damage indicators, related to the final failure, increase drastically. Figure. 4 Fatigue limit by the infrared thermographic method: (a) the type A; (b) the type B 4. Conclusions [1] The infrared thermographic method enables us to qualitatively and quantitatively evaluate fatigue behavior of materials with different heat treatments. (b) (a)
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