Numerical Analysis and Optimisation of an Automotive Disc Brake to Improve Thermal Stability

Friction foundation brakes are required on all road vehicles, even on fully electric vehicles where they are necessary to supplement the regenerative braking effort. Although disc friction brakes are a reliable and safe means of slowing a vehicle, they do suffer from noise and vibration problems, especially in the form of hot judder which is a severe vibration experienced by the vehicle occupants due to non-axisymmetric thermal expansion of the disc surface under severe braking conditions. Reduction of hot judder propensity in a disc brake system by optimising its thermo-mechanical performance is the major focus of this thesis. However this optimisation must be conducted in a weight-efficient manner since, even with the advent of electric vehicles, there is still impetus to reduce total vehicle mass which can extend driving range and reduce road damage. A coupled thermo-mechanical analysis was first developed to evaluate the performance of the selected baseline disc for the given braking condition known to induce hot judder. Parametric and topology optimisations were then introduced with the view of improving thermal performance whilst minimising mass. In the parametric optimisation, the geometry of the existing disc was parametrised into shape variables and the influence of each design variable on thermo-mechanical performance of the disc was studied using response surface methodology. Based on these surrogate models, an adaptive surface response optimisation was used to derive optimum values of the chosen geometric parameters, yielding design solutions with reduced propensity of hot judder as measured by a reduction in thermal disc thickness variation (DTV) of up to 64%. In the topology optimisation, a baseline ground structure was formulated to enable new conceptual vane geometries (straight and curved) for both improved thermal performance and lower disc mass to be derived. This approach allowed novel disc designs not previously considered to evolve and the potential performance benefits of this more radical optimisation strategy were demonstrated. Furthermore, CFD analysis was utilised to enable detailed flow patterns within the vane geometries to be predicted which allowed for the estimation of convective heat transfer coefficients (HTC) for the novel design concepts. An existing thermal flow rig was also used to measure the cooling characteristic of the baseline disc and this, together with the CFD simulation results, was used to validate the thermal FEA model of the disc by correlating the experimental and predicted cooling curves. Finally, it was shown that the validated FEA models for the conceptual designs with the CFD derived HTCs for the vane surfaces did indeed predict better judder performance and increased rate of cooling compared with the baseline disc. Based on these results, a design guideline using currently available software tools for the development of a weight efficient brake disc with reduced risk of hot judder was proposed. Although individually these techniques are well-established, their combined use in the minimisation of hot judder is novel and hence represents a substantial contribution to knowledge and understanding of this challenging area of automotive friction brake design.