13th International Conference on Fracture June 16-21, 2013, Beijing, China 6 3.2. Fretting evidences A brief analysis of results can show some indirect clues of the reproduction of fretting in tests. It was observed that somehow superficial factors highly influenced fatigue life of component. First of all, as one can see in Fig. 6, the drop in fatigue strength of material in analysis was remarkable. Another indication was that cracks nucleated on surface, as exemplified in Fig. 7, unusual behaviour in VHCF regime, in which subsurface initiation preponderates. Figure 7 – Crack nucleation on surface Fretted surfaces of tested specimens were analyzed and most of them showed fretting characteristics described by some of the most accepted fretting models in literature[8]. The presence of two clearly distinguishable areas can be seen in Fig. 8: smooth stick region and rough slip zone with scars in rubbing direction. Damaged surfaces were crossed vertically by a smooth and shinny section, which corresponds to stick zone, located about the centre of the contact ellipse. This stick zone is surrounded in its left and right sides by a rougher and deeper region, the slip zones. The formation of oxidized regions and debris, mainly when excessive fretting occurs, is also very characteristic. Debris could be found in great quantity far from fretted surfaces, because they were really slender and were blown away by cooling apparatus. Red-brownish spots, characteristic of steel oxidization, were found in almost every worn surface, predominantly in regions subjected to heavy wear, as expected[8]. An example is illustrated in Fig. 8, where bright white spots represent the oxidized regions. Figure 8 –Example of factors usually found in surfaces under fretting
RkJQdWJsaXNoZXIy MjM0NDE=