Identifying short-term variation of dynamic friction by means of its frequency response function

A challenging case study of dynamic friction was presented in a previous study (A. Cabboi, J. Woodhouse, Validation of a constitutive law for friction-induced vibration under different wear conditions, Wear 396–397 (2018) 107–125), concerning tests performed with a Polycarbonate pin sliding on a steel disc. Identifying and modelling the frictional frequency response for this system turned out to be rarely possible, since the measurements were affected by significant wear and by intermittent squeal occurrence. To shed light on the observed “capricious” behaviour, an “instantaneous” estimation of the frequency response of dynamic friction was developed, allowing the dynamic friction behaviour to be captured and tracked before, and for few cases during, squeal events. Each “instantaneous” frequency response of dynamic friction was fitted by a rate-and-state model, and variations of the model parameters for different sliding speeds, changing normal forces and at different wear stages were tracked. With direct relevance to squeal predictions, the model parameters identified through the proposed processing and fitting methodology could detect rapid transitions between velocity-strengthening and weakening behaviour. These transitions may occur at different sliding speeds, but they also occur during measurements carried out at a constant sliding speed. Based on the identified model parameters, a first qualitative attempt to predict squeal events by means of rate-and-state models is presented, and shown to give promising correlation with experimental results.