Experiments, modelling and analysis of fretting fatigue for Inconel 718 and Ti-6Al-4V under time-varying contact normal load at room and high temperature

This work has been developed within the scope of a joint PhD agreement between the University of Paris-Saclay (UPS) and the University of Brasilia (UnB). The scientific context of the thesis has been chosen to try to provide data and solutions for problems in the aeronautical sector, as Safran Aircraft Engines has been an important industrial partner for this and other thesis in both universities. In this setting, one of the motivations is to study the effect of a variable contact normal loading in the fretting fatigue problem. Within this context, to be more precise, the present thesis has three main goals. The first one is to design and construct a new fretting fatigue apparatus capable of performing different types of tests, where the loads and their respective actuators involved in the experiments can be independently controlled. The test rig will be designed so that fretting fatigue experiments can also be carried out at high temperatures. The second goal is to evaluate the effect of cyclic normal load in fretting fatigue strength for the titanium alloy Ti-6Al-4V at room temperature and for the Inconel 718 alloy at room and elevated temperature. These materials were specially chosen due to the great interest of the aeronautical industry. The third and final aim is to assess fretting life by means of a finite element model which considers wear effects and a multiaxial fatigue parameter.A new four actuators fretting fatigue apparatus was entirely designed to tackle the problem of imposing cyclic contact normal load. The cyclic normal load is now applied by two MTS servo-hydraulic actuators, which were installed perpendicularly to the fretting specimen. Further, the tangential and fatigue loads are applied by independent servo-hydraulic actuators. Due to these upgrades, the new fretting fatigue rig of the University of Brasilia can independently control all the loads involved in the fretting tests (bulk, normal and tangential loads). All these loads can not only vary with time in-phase or out-of-phase, but their waveforms can also be applied synchronously or asynchronously. Moreover, two ceramic igniters were placed next to the contact parts to conduct tests at elevated temperatures. The heating system is capable to reaches to reach temperatures up to 750°C and maintaining it stable within a range of +/- 10°C.In order to evaluate the influence of the cyclic normal load on the fretting fatigue strength, tests under the partial-slip condition with a constant and cyclic contact normal load were conducted at room temperature for the Ti-6Al-4V alloy and at room and 540°C for the Inconel 718 alloy. The experimental results indicate that cyclic normal load has a beneficial effect on fretting life for the specific conditions of tests here conducted. This behaviour was observed for the Ti-6Al-4V and Inconel 718 alloys at both tested temperatures. Besides that, an experimental campaign to assess the influence of time-varying normal load on the friction coefficient was also carried out. Based on this experimental evaluation, the presence of cyclic normal loads does not seem to influence the coefficient of friction under partial slip conditions. However, comparing the friction coefficient results of the Inconel 718 alloy at room and elevated temperature, a significant reduction was observed for both contact normal loading situations with the increase of the temperature.In addition, a finite element model (considering and neglecting wear effects) was used in conjunction with the Smith-Watson-Topper critical plane parameter and a non-local stress averaging approach to estimate fretting life. Concerning the life estimate approaches considered in this study, for the constant and cyclic normal load cases, both implemented methodologies, whether accounting or neglecting wear, provided satisfactory results, with the one disregarding wear being slightly more accurate.