13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2. Heat source identification associated with the fatigue crack propagation In order to identify the heat source associated with the crack propagation a cracked specimen which geometry is given in fig. 1, is subjected to a cyclic loading with a stress intensity factor range of about 20MPa m and a stress ratio of 0.1. The loading frequency is about 100Hz and the material is a C40 mild steel with an ultimate tensile strength UTS=600 MPa. The mechanical and thermal properties of this steel are summarized in table 1. The temperature field at the specimen surface was measured with an infrared camera (CEDIP Jade III MWR) whose spectral range is in the near infrared domain. The acquisition frequency and the aperture time of the camera were respectively 5Hz and 1100μs. In order to reduce the effect of the emissivity of the surface on the temperature determination, the specimen was covered with a fine coat of mat black paint. Table 1. Mechanical and thermal properties of C40 steel Material properties density Yield stress Young Modulus Poisson ratio Thermal expansion Heat capacity Heat conduction Heat diffusivity Notation y E C k a Unit kgm-3 MPa GPa K-1 JK-1kg-1 WK-1m-1 m2s-1 Value 7800 350 210 0.29 1.2x10-5 460 52 1.4x10-5 Figure 1. Cracked specimen geometry Figure 2. Temperature field near the crack front after 680s Fig. 2 and fig. 3 show respectively the temperature variation field near the crack tip at time 680s and the temperature evolution according to time at a distance of 5mm from crack front. The temperature evolution on fig. 3 exhibits a superposition of a high frequency evolution of the temperature due to the thermoelasticity and a low frequency signal evolution corresponding to the dissipated power in the reverse cyclic plastic zone. Fig. 2 shows that the temperature distribution remains heterogeneous due to the highly localized form of the heat source in the reverse cyclic plastic zone. The total increase of the temperature in the specimen between the beginning and the end of the test corresponds to the heat source related to the plastic dissipation in the reverse cyclic plastic zone. This total increase in the temperature at a distance of 5mm from the crack front is about 2.5°C.
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