13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- (calculated numerically) is presented in Fig. 1. The stress is concentrated in the middle of the gauge part of the specimen and reduced toward the specimen ends. This geometry allows to systematically study the material response at all desired stress amplitudes with one single specimen. Figure 1. Ultrasonic fatigue plate specimen and distribution of stress along the specimen axis 2.2. Fatigue tests The experimental setup involves the mechanical testing machine shown in Fig. 2. It essentially involves 3 components [1]: • a piezoelectric system that transforms an electrical signal into a displacement • an ultrasonic horn which amplifies this displacement (usually 3 to 9 times) • a specimen screwed on the horn and free of stress at its bottom extremity. This dynamic loading system is designed, assuming an elastic behaviour, in order to have a longitudinal vibration mode at a frequency of around 20 kHz. In order to obtain the relation between the displacement on the horn edge and the input electrical signal, the testing machine is calibrated with a laser extensometer. During fatigue tests, an infrared camera (512x640 pixels) monitors the temperature field on the specimen surface. A pixel calibration is performed before each new test. The specimen surface is painted in matte black to have a uniform surface emissivity close to 1. Spatial resolution is about 0.1 mm/pixel. From the temperature measurements, the intrinsic dissipation was determined using a 1D heat diffusion model (see the following section).
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