# Mechanical EngineeringTribology Laboratory

## Effects of contact geometry, material, and the third body effect on fretting wear

### Fretting wear test rig

The first phase of this investigation was designing and building a test rig to study fretting wear. The finished testing machine can carry out tests in ball on flat, crossed cylinder, and flat on flat contact configurations. The fretting contact can be observed visually in the flat on flat and ball on flat configurations when a sapphire specimen is used for the upper test specimen. In the flat on flat configuration the test rig has been designed to carry out tests at up to 400 degrees Celsius.

Fig 1: Fretting Wear Test Rig

The test rig can record friction force and reciprocating specimen location in order to plot fretting loops which are used to calculate the frictional energy loss. It is also capable of observing gaseous cavitation and the movement of wear particles within a contact using high speed videography.

Fig 2: High speed video of gaseous cavitation in a fretting contact

### Experimental Testing

It is difficult to predict the fretting wear because the wear rate and wear coefficient of a material can vary widely depending on operating conditions. Unfortunately, the fretting wear rate varies greatly with displacement amplitude and normal force. Fretting wear is typically analyzed using the dissipated energy and Archard wear equations. In the dissipated energy equation wear is proportional to energy lost to friction while in the Archard equation wear is proportional to the product of the normal force and sliding distance. As seen below our experimental investigations have demonstrated that the dissipated energy equation more accurately describes the fretting phenomenon.
 (a) (b)

Fig 3: Fretting wear plotted against the Archard parameter and dissipated energy

A technique for creating fretting wear maps has been developed which allows engineers to quickly determine the fretting rate over a wide range of operating conditions. To create a fretting wear map the energy dissipation rate is calculated for a wide variety of conditions and a surface is fitted to the data. The surface is then multiplied by the dissipated energy wear coefficient converting it into a wear map. The procedure has been developed for Hertzian and flat on flat contact configurations. These differ slightly because both contact types behave differently. An example of a fretting map is shown below.

Fig 4: Example fretting wear map

### Numerical Modeling

A combined finite-discrete element model (FDEM) has been developed to model fretting wear. In this approach the finite element method is used to calculate the deformation of individual bodies while the discrete element method is used to calculate the interaction of different bodies. One of the FDEMÃ¢â‚¬â„¢s primary strengths is analyzing systems with large numbers of deformable bodies.

The FDEM can model wear through both incremental modification of the surface profile and through particle detachment from the surface. The FDEM can use either the dissipated energy or Archard equations to calculate the wear volume. It had been used to model a wide range of conditions with varying surface roughness and coatings. The model has been used to model test rig geometries and components in industrial machinery.

Fig 5: Wear of a Hertzian fretting contact

Fig 6: Detached third body particle in a sliding fretting contact

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