On improvement of cast iron brake discs for heavy vehicles

Doktorsavhandling, 2015

Disc brakes are commonly used in motorcycles, passenger cars, commercial vehicles, heavy-duty vehicles, passenger trains and landing gears of airplanes. Depending on the application, a wide variety of brake discs have been developed, with a multitude of designs, geometries and materials. The brake disc materials used for heavy-duty vehicles are still mostly based on traditional grey cast iron, due to its favourable performance and low cost. Gradual improvements of the thermal and mechanical properties of grey cast iron for production of brake discs have been introduced over the years through fine-tuning of the alloying elements. Nevertheless, because of a material behaviour that comprises non-linear elasticity and non-symmetric yield stress in tension and compression, the mechanical properties of grey cast iron are not as well established as those of steel or aluminium alloys. The subject of the present work is a better understanding of the mechanisms that control the life of cast iron brake discs. In the end, the aim is an enhanced vehicle performance and reduced maintenance costs of heavy-duty vehicles. First, a state of the art survey has been carried out with a review of brake disc technology and mechanisms related to brake disc life. Second, full-scale brake dynamometer experiments have been carried out and analysed, with brake discs made from eight different grey cast iron alloys, at controlled torque, speed and cooling conditions. Crack propagation and disc temperatures have been studied. Disc temperatures were recorded by use of a thermocamera and embedded thermocouples. Detailed analyses were performed on temperature data to clarify characteristics of the tested brake discs. The main phenomena influencing the life of brake discs were identified, and the spatial fixation of bands and/or hot spots was found to have a major impact on disc life. Third, a constitutive model useful for grey cast iron was implemented for use with a commercial finite element software. The model was calibrated for temperatures relevant for braking applications using dedicated isothermal tests and, finally, validated against monotonic tensile tests, isothermal fatigue tests and thermomechanical fatigue tests. Fourth, a parametric study was performed to explore potential improvements of the cooling channels of a brake disc design when considering an established fatigue criterion. The thermal loading of the brake discs was chosen to represent the full-scale experiments. In the end, a response surface methodology was applied to optimize six types of cooling arrangements (one with straight vanes and five with different numbers of pillar rows) in order to minimize brake disc mass and/or maximize fatigue life. Finally, fifth, some fatigue life models useful for grey cast iron were calibrated and used to assess brake disc design. In the calibration process, results from both isothermal and thermomechanical experiments were exploited. The life models were used to study the reference brake disc.