ESTIMATION OF THE TOTAL LIFE IN FRETTING FATIGUE WITH SPHERICAL CONTACT Carlos Navarro, Jaime Domínguez Dpto. de Ingeniería Mecánica y de los Materiales ESI de Sevilla, Avda. de los Descubrimientos s/n, 41092 Sevilla ABSTRACT This paper proposes a method for estimating the total fatigue life in fretting fatigue. It compares the fatigue life of some specimens obtained in a series of fretting fatigue tests with the estimates of total fatigue life using a model that separately takes into account the initiation and propagation of the crack. These fatigue tests were conducted on an aluminum alloy (Al 7075-T6) and the contact pads used were spherical. A discussion is carried out regarding the results to decide the relative importance of the initiation and propagation phases, the model that should be used for short cracks growth analysis and the multiaxial fatigue criteria to be used for the initiation of the crack. INTRODUCTION Fretting is a phenomenon that can be observed everyday in service in many mechanical systems elements such as riveted and bolted joints, shrink-fitted couplings, metal ropes and cables, etc. [1]. It appears in systems where service loads cause relative displacement of very small amplitude between contact surfaces. This generates friction forces that add to the load normal to the contact surface and the bulk load of the system. When these forces vary cyclically, cracks initiate and propagate by fatigue mechanisms and this is usually referred to as fretting fatigue. An important issue in the design of mechanical systems that suffer fretting fatigue is the prediction of the endurance under the design loads and many efforts and resources are dedicated to it. This is a difficult problem and there are different approaches that are aimed at solving it. There is a general agreement that the fretting fatigue process has two different phases: the fatigue crack initiation and crack propagation, which are due to different mechanisms. Some methods to estimate the fretting fatigue life are based on the initiation mechanism, and use the data obtained in tests with smooth specimens under completely reversed loading, combined with various multiaxial fatigue criteria. This method gives better results when the initiation phase dominates over propagation. Another approach is to use linear elastic fracture mechanics. In this case the initiation life is supposed to be small compared to propagation [2]. Another method is to combine initiation with propagation using a fixed crack initiation length [3]. This paper uses a different method, which combines initiation and propagation defining a non-arbitrary crack initiation length, to estimate the life obtained in a series of fretting fatigue tests.