The Dynamic Friction Tester - The Response to Macrotexture and Microtexture

Abstract

The Dynamic Friction Tester (DFT) is a well known stationary friction measuring device which is used to assess the state of skid resistance of road, airfield and pedestrian surfaces. The Permanent International Association of Road Congresses (PIARC) exponential decay model for the friction curve can be recast in the form + Δ = ∗ Δ which shows that the difference in the friction coefficient evaluated at two slip speeds + Δ and ( ), is a function of the Speed Constant ( ). Thus determining for a given surface, in order to fit the PIARC model friction-speed curve to friction data, becomes important. This is currently attempted by calculating as a linear function of a measure of the surface macrotexture. The DFT has been found to give higher friction values at speed on some surfaces than what is predicted by the PIARC exponential model for the IFI friction - speed curve and it is clear that the texture based Speed Constant ( ) is too low a value to enable a good fit to the friction data for those surfaces. An alternative approach to calculating the value of is given and this is found to give a better fit of the exponential model to the data. Stationary friction measuring devices like the British Pendulum Tester and the DFT record the coefficient of friction of rubber slider(s) in the contact patch, as they sliding across a pavement surface. Although in the past half century much has been learnt about the mechanics of how friction is generated within the contact patch between sliding rubber and the pavement surface, this is seemingly divorced from the operation of these devices. The thesis thus opens with an introduction to the development of the ideas around friction as it occurs with sliding rubber on rough surfaces, then moves onto the application of the DFT to measuring the co-efficient of friction of various surfaces and evaluating the information obtained from these measurements.