13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- (a) Along transversal axis (b) Along longitudinal axis Fig.1. Microstructure of S15C steel Table 1. S15C chemical compositions (mass %) C Si Mn Cu Ni Cr Fe 0.15 0.21 0.40 0.02 0.02 0.15 Re Table 2. Mechanical properties of S15C steel Mechanical properties Value Lower yield stress (MPa) 273 Tensile strength (MPa) 441 Young modulus (GPa) 207 Elongation (%) 40.2 Reduction of area (%) 65.8 Vickers hardness (HV) 161 Fatigue tests have been conducted at several frequencies: 20 kHz using an ultrasonic type machine, under displacement control; 140 Hz using an electro-magnetic type machine under stress control; 20 Hz, 2 Hz and 0.2 Hz using servo-hydraulic type machine under stress control. An air-cooling system and intermittent loading conditions were admitted to avoid temperature rising of specimen in the case of ultrasonic fatigue tests. For fatigue tests performed at 140 Hz, an air-cooling system has been also added for the same reason. All fatigue tests were performed in air, at room temperature, with a stress ratio R = -1. Configurations of fatigue specimens are presented in Fig. 2. Diameter of tested portion was fixed at 5 mm, whatever the testing method used. After machining, center portion of specimens are electro-polished to remove residual stresses. As a consequence, we avoid any size effect and residual stress effect on the S-N properties of S15C steel. One can see that servo-hydraulic and electro-magnetic specimens present a cylindrical shape at center rather than usual hour glass shape. It allows us to carefully attach a 4-strain gauge system in order to follow micro-plasticity behavior during fatigue test. Choice of this type of full-bridge system has been done in order to obtain accurate measures and to avoid heating influence on strain measures.
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