ANALYSIS OF THE INFLUENCE OF FRETTING ON THE FATIGUE LIFE OF INTERFERENCE FITTED JOINTS

The present paper deals with the fatigue behaviour of shaft hub interference fitted joints. The investigation is carried out by means of a down-scaled ad-hoc specimen, which has been developed by the authors in order to perform accelerated fatigue tests. The specimen is similar to those suggested by standard ISO 1143, but it consists of two parts, joined together by shrink fitting. In a previous work, the authors compared the performances of a plain specimen manufactured according to ISO 1143 and those of the shrink fitted specimen. Both the plain and the shrink fitted specimens were made of C40 EN 10083 low carbon steel. The outcome of that research was that shrink fitting determines a degradation of the fatigue response of the assembly, with respect to those of the plain specimen. Moreover, such decrease cannot be predicted by means of FEA alone, since it is partly due to fretting phenomena. In fact, fretting takes place on the mating surface between the shaft and the hub, and especially in the vicinity of the end of the contact. The present paper deals with the observation of failed and survived specimens by means of optical and SEM microscopes, in order to determine the actual tribological characteristics of the contact surface. For instance, the amplitude of the sliding zone observed experimentally is corn pared with that given by FEA for different choices of the contact formulation. Since fretting is often associated with the presence of secondary fatigue cracks, which do not propagate, the authors set up an experimental method for locating the secondary cracks prior to sectioning the specimen for microscope observation.