On improvement of brake discs for heavy vehicles - Experimental testing and material modelling

Licentiatavhandling, 2013

Disc brakes are commonly used in motorcycles, passenger cars, commercial vehicles, heavy-duty vehicles, passenger trains and landing gear of airplanes. Depending on the application, different types of brake discs can be found on the market with numerous designs, geometries and materials. The brake disc material commonly used for heavy-duty vehicles still is a traditional material based on grey cast iron due to its favourable performance and low cost. Gradual improvements of the thermal and mechanical properties of grey cast iron alloys have been introduced over the years. However, the properties of grey cast iron are not as well known as those of steel or aluminium alloys. In the present work, Scania has initiated a collaboration project with Chalmers University of Technology aimed at a better understanding of the mechanisms that control the life of brake discs and at enhanced vehicle performance and reduced maintenance costs. Firstly, a state of the art survey has been carried out with a review of brake disc technology and mechanisms related to brake disc life. Secondly, a uniaxial thermomechanical fatigue experiment has been analysed using analytical and numerical methods. The aim was to simulate temperature-induced stress and strain cycles and to estimate the life of the specimens based on the Coffin-Manson and Smith, Watson & Topper (SWT) laws. A simple analytical model available from the literature was implemented in order better to understand the concepts, which then was applied in a detailed finite element model. Thirdly, full-scale brake dynamometer experiments have been carried out and analysed with discs made from eight different materials at controlled torque, speed and cooling conditions. Disc surface temperatures were recorded by use of a thermocamera and local temperatures by use of thermocouples embedded in the brake discs. Wear properties, crack propagation and temperature loading were studied. Detailed analyses were performed on temperature data to reveal characteristics of the tested brake discs. From the results of the experiments, the main parameters influencing the life of brake discs were identified. Spatial fixation of bands and/or hot spots was found to have a major impact on disc life.