13th International Conference on Fracture June 16-21, 2013, Beijing, China 2 acceleration of fatigue testing. One of the evolutions that made possible the evaluation of VHCF was the ultrasonic fatigue testing device, frequency first reached by Mason[3] apud Bathias & Paris[1], in which fatigue experiments are carried out at 15kHz or more. The 20kHz testing line in Fig. 1 shows, in comparison with the 100Hz one, how greatly fatigue experiments carried out in such a high frequency can be achieved quicker, becoming feasible and affordable. Figure 1 – Comparison of time spent to reach a certain number of cycles in 100Hz and 20kHz fatigue tests As tests in the VHCF regime became more accessible and frequent, several studies [1,4-6] showed that the infinite fatigue life assumption for steels and other metals did not represent faithfully the reality of this phenomenon. On the contrary, fatigue strength of ferrous materials tends to decrease as the number of cycles increases. Moreover, it was proved that even crack nucleation mechanism in VHCF regime is different from low and high cycle ones. For the latter, cracks typically arise from the surface, while for the former, subsurface initiation preponderates [1,4-7]. Fatigue properties of a component can be deteriorated by many external reasons; a very common factor is the fretting phenomenon, which occurs whenever two surfaces subjected to mechanical vibration are fixed against each other, i.e., there is no nominal displacement between them. This vibration will result in minute displacement between both parts, mostly in regions of geometric or loading discontinuities. This random or cyclic movement will induce wear of surfaces in contact and, under specific conditions, will trig fretting cracks, which has been remarked in minute displacement as small as 0.1µm[8]. Many practical situations can be subjected to this phenomenon, like, for example, bolted connections, interference assemblies, flanges, keyways, cables, and ropes. The effect of fretting can be catastrophic, once that it diminishes crack initiation period because of stress concentrations over the contact surfaces[9]. Virtually eliminating this span of life, fretting decreases a material’s fatigue strength to very low levels, dropping it to values as low as of its original value[8]. This effect is still more remarkable when a steel body is subjected to VHCF, where crack initiation period accounts for the most of its fatigue life, such as in high cycle fatigue[10]. Unlike plain fatigue experiments, just a few works have been done using ultrasonic frequency to accelerate fretting tests[11-13]. These authors criticize the technique of increasing the oscillatory movement’s amplitude in order to accelerate fretting experiments, inasmuch as it changes the contact conditions with simultaneous change in crack initiation and wear mechanisms. These studies conclude that many observed wear mechanisms exhibit the features described in some of the most accepted models for fretting. So ultrasonic testing seems to be a promising technique for
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