Contact: Reza Hojjati
Background
Fretting fatigue is a phenomenon, in which two contact surfaces undergo a small relative oscillatory motion due to cyclic loading. Due to fretting, the fatigue lifetime is significantly reduced as compared to that when no fretting takes places, by a factor up to 2.7. This is because of the high stresses that are generated at the contact surfaces of the two bodies. Many failures in mechanical components due to fretting fatigue have been reported and investigated in literature, e.g. threaded pipe connections, riveted joints, blade-disk attachment in turbine, shrink-fitted shaft and aero engine splined couplings Fig1. In general, fretting fatigue failure process is divided into two main phases, namely crack initiation and crack propagation. The fraction of fretting fatigue lifetime spent in crack initiation and in crack propagation depends on many factors, e.g. contact stresses, amount of slip, frequency, environmental conditions, etc.., and varies from one application to another. Therefore, both crack initiation and propagation phases are important in analysing fretting fatigue. Although several studies on fretting fatigue has been reported in literature, most of the works were of experimental nature with a simple finite element model to calculate contact stresses and strains. However, in fretting fatigue experiments, it is very difficult to detect the crack initiation phase; the onset of crack growth and the crack growth rate because damage and cracks are always hidden between the two contact surfaces. Therefore, numerical modelling techniques for analysing fretting fatigue crack initiation and propagation phases are very desirable. Furthermore, these numerical models could be used to predict the fatigue lifetime of components subjected to fretting conditions.
Aim
The main aim of this project is to develop generic numerical modelling techniques in order to predict the response of materials and the damage initiation and propagation under fatigue loading in fretting conditions. The individual objectives of the projects are summarised as follows:
Methodology
The methodology of the project is based on integrating finite element models with Continuum Damage Mechanics and Fracture Mechanics in order to produce a numerical tool for fretting fatigue analysis which is illustrated Fig2. This numerical tool will help in understanding the fretting fatigue process and providing a reliable prediction of lifetime. Damage and fracture material parameters are required as input data in the finite element models. These parameters will be determined through experiments or from literature. The material type that will be used in this project is Steel 52, for which fatigue fracture parameters can be easily found in the literature. However, biaxial experiments are required to determine the crack initiation damage parameters, which are not available in literature. Furthermore, some fretting fatigue crack initiation and crack propagation experiments will be carried out in order to validate the finite element models and the numerical tool. The methodology is divided into two main work packages, namely, fretting fatigue crack initiation, which is illustrated in Figure 3, and fretting fatigue crack propagation, which is illustrated in Figure 4.
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 Fig. 3. Flow chart for fretting fatigue crack initiation (D is damage variable). |
 Fig. 4. Flow chart for fretting fatigue crack propagation (a is crack length and af is final crack length). |
