13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- initiation. With a flat surface, the spatiotemporal localization of the point where the temperature was the highest was possible. The highest temperature corresponds to the crack tip during the test. Then, the results are compared with the fractographic examinations for identification with each stage. 2. Material The studied material is a polycrystalline α iron whose chemical composition is given in Table1. The carbon content is 80ppm. The microstructure is ferrite with equiaxe grains. The ferrite grain size is included in 10 to 40 μm. No specifically orientation was observed by EBSD. Yield Stress is 240 MPa. Table 1: Chemical composition of studied material C P Si Mn S Cr Ni Mo Cu Sn Fe 0.008 0.007 0.005 0.048 0.003 0.015 0.014 0.009 0.001 0.002 Balance 3. Experimental procedure 3.1. Mechanical and Thermal Procedure Tests were performed on a piezoelectric fatigue machine designed by C. Bathias and co-workers [6]. For the reason of surface observation condition by IR camera, a new design of 1 mm flat specimen (Fig. 1) was used to carry out fatigue tests. Specimen, special attachment and piezoelectric fatigue machine constituted the resonance system working at 20kHz. The cyclic loading is tension-compression . The stress ratio is then Rσ = -1. For this test, the applied stress amplitude Δσ was 175 MPa, and the number of cycles to failure 3.2155x107 cycles Fig. 1: Design of flat specimen A CEDIP Orion infrared camera was used to record the temperature evolution during the test.The frequency of the camera was 50Hz and the aperture time was 100 μs. Before testing, both side surface of the flat specimen were electrolytic polished. One side surface of the flat specimen was etched and another side painted in black color to have the surface emissivity close to 1.
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