13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- The surface temperature of specimen under high frequency is very complex including the effects of elastic strain energy, plastic strain energy, inelastic internal friction and heat dissipation [8,9,18]. An advanced infrared thermometer was used to monitor the variations of temperature on the surface of specimen during test. The temperature changes of weld specimen were not evident due to the low stress level and the larger waist diameter. But for BM specimens, it was found that the cyclic loading at an ultrasonic frequency generated a substantial amount of heat, and the temperature of BM specimen increased significantly at the center of the gage length of specimen as shown in Fig.6. The temperature distributions in the gage-length section of the specimen during ultrasonic testing were heterogeneous along the axis of the specimen, with the highest temperature occurring at the high strain location. Fig.5 illustrates the temperature variation at the center of the base metal specimen for two different stress levels without air cooling. The temperature increased evidently at the beginning of the test, which followed by a stable state. Thus stabilization could be explained by a balance between the mechanical energy dissipated into heat and the energy lost. The temperature was higher for higher stress loading. Fig.6 also shows that the temperature at the center of the specimen increased very sharply (3.627×106~3.673×106cycles) just before the fatigue failure occurred. Observation for fracture surface confirmed that the fatigue crack initiation site was located just on the point A plotted in Fig.6, which declared a close relation between the rise of temperature and crack propagation. XUE et al. [9] discussed that the dissipated energy came from internal material damping and the local plastic deformation at the microscopic scale, although the stress in the specimen is lower than the elastic limit. When the micro crack originated and started to propagate under cyclic loading, the plastic zone at the crack tip generated heat until the specimen fractured. Thus, the fatigue crack propagation life could be estimated by detecting the temperature variation of specimen and this will be discussed later. 3.4 The varying resonance frequency of welded joint specimen Fig.7 Natural frequency profiles of tested at different stress level.
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