13th International Conference on Fracture June 16-21, 2013, Beijing, China 8 Figure 10 –Influence of geometry on bulk load and fretting amplitude deviations Alignment was found to be another problem on the ultrasonic device. Position both pads such that their axis were parallel to each other and at the same height of the specimen, in order to avoid bending, was very difficult using the current fixture system. Complications came from the attachment process using Araldite® and from the loosing apparatus. It is advisable to revamp the support apparatus, aiming to facilitate the operation of the machine and to broaden its range of application, allowing the use of other types of contact configurations, e.g., line contacts as cylinder on flat or cylinder on cylinder, which are harder to align. 3.3.1 The overheating matter Temperature rise in ultrasonic fretting-fatigue tests can be caused by two different methods. The first one is due to high strain cycling, which generates high heat rates. Specimen overheating must be avoided, because it modifies its mechanical properties and, beyond certain limit, changes fundamental frequency of specimen, killing the test. That is why the cooling apparatus is so necessary. The problem is that, as observed during tests, air coming out in high velocity from the apparatus blew away slender oxide debris formed in fretting contact. Nevertheless, the role of these debris in fretting is very important[8] and their forced expulsion from the area between surfaces clearly changes contact conditions. It is necessary to dedicate further efforts to understand its consequences to fretting-fatigue behaviour. The second phenomenon is linked to wear caused by fretting, even more important at high frequencies. All other parameters being held constant, heat generated increases when wear volume per cycle raises. Superficial temperature, mainly at contact must remain in reasonable levels, in accordance with practical situations. Otherwise, high temperature tends to increase oxidation rates and to degenerate mechanical properties of materials in contact, affecting fretting conditions. In some tests, it was observed that superficial temperature raised when apparatus presented looseness and compliance, undergoing excessive vibration or when Araldite® softened because of high temperature. In both cases, it is probable that fretting pads were not properly fixed and was able to develop small displacements, increasing both surface and volume worn. It is recommended to rethink the apparatus that supports fretting pads, stiffening it, decreasing looseness, and principally changing how pads are attached to it, stop using gluing process. 3.3.2 Decoupling of fretting displacement amplitude and bulk stress Decoupling fretting displacement amplitude and bulk stress at a given point without any additional equipment is a great advantage of the machine in study, so almost any combination of these
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