13th International Conference on Fracture June 16-21, 2013, Beijing, China 9 properties can be chosen. Nevertheless, as one can see in Fig. 3, given a bulk stress at the rubbing point, the higher the fretting amplitude (Xi) desired, the higher must be the amplitude imposed at specimen’s bottom (X(0)), therefore improving the maximum bulk stress over the specimen. As a result, a more elevated heat rate will be generated, rising specimen’s temperature, leading to the implications discussed earlier. So, summarizing, maximum fretting amplitude is limited: the higher the desired amplitude at a place, the lower the bulk load that can be chosen at this same point. 4. Conclusions The ultrasonic machine is a promising device to accelerate fretting-fatigue experiments, which can be carried out more than 200 times faster than in regular fatigue testing machines currently in use. Fretting could be reproduced, decreasing fatigue strength by a factor of 1.5 in the whole VHCF region. Surfaces under fretting presented many patterns characteristic of this phenomenon: presence of two distinguishable areas of stick and slip contact, formation of oxidized regions and debris in the slip zone, and crack initiation near the stick-slip regions bound or near the stick regions-region out of contact bound. Moreover, superficial factors brought crack nucleation to surface, unlike to happen in plain VHCF. The mechanism of the machine is simple, of easy maintenance and operation. Its components are lighter and cheaper than usual axial fatigue machines. In contrast to other fretting-fatigue devices usually found in literature, choice of fretting displacement amplitude and bulk stress in specimen is decoupled within certain limits. On the other hand, the necessity of extremely well polished specimens with very strict geometric tolerances is inherent to the ultrasonic testing machine, making specimen manufacture expensive. The use of a cooling system is mandatory in the ultrasonic machine device, especially on frettingfatigue tests. The current system, using compressed air, was responsible for blowing away oxide debris out of the fretting contact. These debris play, nevertheless, a very important role in fretting and their forced expulsion changes contact conditions. Further work must be done to understand the consequences of this fact to fretting-fatigue phenomenon. The main problems are, however, linked to the apparatus that supports fretting pads. Looseness and compliance caused excessive vibration, letting pads developed small displacements. Therefore, surface and volume worn as well as superficial temperature raised in some tests. Also, alignment was found to be very hard using the current fixture system. It is advisable to revamp the support apparatus, aiming to facilitate the operation, to allow the use of other types of contact configurations, and to decrease its influence on fretting amplitude. Acknowledgments The financial support of the Coordination of Improvement of Higher Level Personnel (CAPES) and National Council for Scientific and Technological Development – CNPq is greatly appreciated. The authors are grateful to Vallourec & Mannesmann do Brasil, for providing and manufacturing the specimens. References [1] C. Bathias and P. C. Paris, Gigacycle Fatigue in Mechanical Practice, New York: Marcel Dekker, 2005.
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