13th International Conference on Fracture June 16-21, 2013, Beijing, China 1 Evaluation of an ultrasonic device to test fretting-fatigue in very high cycle regime Pedro F. Filgueiras1,*, Claude Bathias2, Ernani S. Palma1 1 Department of Mechanical Engineering, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, Brazil 2 Laboratoire Energétique Mécanique Electromagnétisme, Université Paris Ouest, Ville d’Avray 92410, France * Corresponding author: pedroffilgueiras@gmail.com Abstract Traditional fatigue tests are not suitable for very high cycle fatigue (VHCF) problems, because of the enormous time spent to reach this regime. Many mechanical components may, however, be subjected to VHCF. Fatigue strength beyond certain limit is usually estimated using statistical approaches or establishing a threshold for crack nucleation. Fretting may take place whenever two fixed parts are under mechanical vibration and variable bulk load. It can have catastrophic effects for parts under fatigue, mainly in VHCF, virtually eliminating crack initiation period. The objective of the present work was to evaluate an ultrasonic device to accelerate very high cycle frettingfatigue tests. Axial load was imposed by a piezoelectric transducer to round cross-section specimens. Fretting was induced by forcing at right angle cylinder pads, which were replaceable and could be specified for each case. It was possible to choose, at the same time, both fretting amplitude and bulk load. Normal load was imposed by an adjustable spring. Results confirmed that fatigue strength decreased significantly when specimens were subjected to fretting. Besides this, worn surfaces showed many characteristics evidencing the occurrence of fretting. Upgrades are necessary to decrease the influence of the apparatus on fretted amplitude and to facilitate surfaces alignment. Keywords very high cycle fatigue, fretting-fatigue, ultrasonic fatigue testing 1. Introduction The study of materials behaviour under Very High Cycle Fatigue (VHCF) is of real interest of researchers on different engineering fields and several industrial sectors. Many mechanical components can be subjected to VHCF once that their loading histories easily exceed the assumed high cycle fatigue limit, usually established between 106 and 108 cycles, depending basically on the material in analysis. For example, the crankshaft of a car equipped with 30in. diameter tires, considering an average 1:3 reducing factor, will attain 108 cycles after 80,000km in service. To reach the same number of cycles, a 1.02m diameter train wheel will take around 100,000km and a aircraft turbine running at 10,000rpm will take about 170h of service. The transportation industry exemplified on the three cases above is perhaps where the study of VHCF is of importance. During their normal lives, vehicles’, trains’, and turbines’ parts can run up to 109, 1010, and 1011 cycles, respectively[1]. The time spent to reach such a high number of cycles using conventional fatigue testing machines is too long, making the study of VHCF in traditional ways prohibitive. The 100Hz testing line in Fig. 1 illustrates this problem: vertical layers delimit the span of cycles usually attained by each kind of craft’s parts during their lives, while horizontal shadows divide the ordinate in subparts, giving the reader an idea of time length. Beyond the usual limit of high cycle fatigue regime, fatigue properties are usually estimated based on statistical approaches, being a horizontal asymptote usually assumed for steels and other metals, while for aluminium alloys a steady decreasing in fatigue strength is supposed[2]. Nevertheless, the increasing necessity of full understanding of engineering phenomena made mandatory the
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