13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 18.1 1.9 1.27 The VHCF testing samples were designed to resonate longitudinally at 20kHz with the ultrasonic fatigue testing system. The detailed geometrical size of the specimens is shown in Fig. 2. The surface of the specimens was polished to surface roughness 0.2 um. Surface roughness of the material was determined with Atomic force microscope (AFM). Surface roughness Ra for different regions were measured and found up to 80nm. Figure 2. Schematic representation of fatigue specimen 3. Results and Discussion 3.1- S-N Curve Fig. 3 shows the S-N curve for the material. The curve showed a higher decreasing slope at higher stress levels up to 106 cycles. This showed that at higher stress levels, the fatigue life was continuously decreasing. A semi-asymptote type curve was obtained and the curve became almost horizontal after 106 cycles. This was in agreement with earlier studies where a horizontal curve was reported for stainless steels [3, 10]. The material showed fatigue limit at around 500MPa and exhibited run out samples. It was found that the fatigue fracture for this material can occur beyond 107 cycles up to 109 cycles, though the difference in the fatigue strength was only around 50MPa. The little difference in fatigue strength between 106 to 109 cycles and lower slope of S-N curve may be attributed to the relatively lower UTS of the material [3]. It was found that owing to the lower slope, little difference in stress levels changed the failure cycles significantly. This showed that fatigue crack initiation and propagation at relatively lower stress levels required considerable increase in the fatigue cycles to yield a failure. On the other hand, this showed that for a very small increase in stress level, the fatigue cycles would reduce significantly. The difference in stress levels for fatigue life between 106 to 108 cycles was almost 30MPa. For engineering applications, where materials are designed to serve 107 to 108 fatigue cycles, only increase in 30MPa may reduce the fatigue life from 106 to 108 cycles, which is significant.
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