Fatigue Life Characterisation of Interference Fitted Joints

Abstract

Reversed bending fatigue tests to be performed on full scale assemblies, joined by interference fitting, are very complicated to set up, as well as very expensive to carry out, due to material costs and to low achievable testing frequencies. Moreover, along with the interference level and the shape of the hub edge, the method by which interference is created is deemed to influence the fatigue behaviour of the assembly. A quicker and cheaper way to obtain information about the fatigue behaviour of such assemblies could be performing fatigue tests on down scaled specimens, similar to those suggested by standard ISO 1143. Since no standardized specimen exists for characterising the fatigue behaviour of interference fitted shaft hub joints, the authors designed an ad-hoc specimen (“notched” specimen) and set up a convenient assembly procedure for performing rotating bending fatigue tests. The present investigation focuses on an interference fitted joint, made of EN 10083-2 C40 low carbon steel, whose hub bore edge is rounded. The rationale of the experimentation is retrieving the fatigue stress concentration factor by comparing the endurance limit results of the “notched” specimen to those of a plain specimen manufactured according to ISO 1143. Results were analysed according to standard ISO 12107. Furthermore, a three dimensional numerical model was developed, suitable for evaluating the local stress state of an axisymmetric interference fitted joint. The numerical model accounts for the effect of an external bending moment applied to the shaft. Experimental results are then compared to those given by finite elements analyses, in order to find out the relationship between the fatigue life and the local stress state of interference fitted components.