A better understanding of the thermomechanical loading of brake discs is important for controlling material fatigue and crack propagation in the disc. In the present study, full-scale drag braking experiments were performed on brake discs made from eight different grey cast iron alloys. The well-performing materials were also tested with an alternative brake pad material. A testing procedure with repeated drag brakings was used. The disc and pad temperatures were registered by thermocouples embedded at selected locations, and the disc surface temperatures by a thermocamera. Extensive analyses of the measured temperatures were performed. The results for the thermocouples at the mid-radius of the disc and at the end of brake applications indicatd that the two sides of the disc have opposite deviations from the mean temperature. The temperature deviations are generally temporally alternating, but also stationary variations can be found. The thermocamera gives the possibility of identifying the phenomena behind the temperature variations found from the thermocouples. Banding of the disc-pads contact with alternating one band and two bands of high temperatures is observed for the studied brake discs exposed to severe braking load cases. Moreover, it was found that hot-spot patterns develop on the disc surface, which are spatially fixed during each brake application. However, they may be either slowly migrating or fixed relative to the disc during consecutive brake applications. Thermal images show that small cracks do not affect hot-spot migration as a hot spot migrates over the crack. However, at a critical length of the crack, the heat becomes localized at the crack and increases its growth, thus limiting the life of the disc. The tests indicate that a combination of hot-spot migration, alternating bands and small temperature differences over the disc are significant factors to be considered when improving the lifespan of the discs.

Cast iron brake discs are commonly used in the automotive industry, and efforts are being made to gain a better understanding of the thermal and mechanical phenomena occurring at braking. The high thermomechanical loading at braking arises from interaction between the brake disc and the brake pads. Frictional heating generates elevated temperatures with a non-uniform spatial distribution often in the form of banding or hot spotting. These phenomena contribute to material fatigue and wear and possibly also to cracking. The use of advanced calibrated material models is one important step towards a reliable analysis of the mechanical behaviour and the life of brake discs. In the present study, a material model of the Gurson–Tvergaard–Needleman type is adopted, which accounts for asymmetric yielding in tension and compression, kinematic hardening effects, viscoplastic response and temperature dependence. The material model is calibrated using specimens tested in uniaxial cyclic loading for six different temperatures ranging from room temperature to 650 °C. A special testing protocol is followed which is intended to activate the different features of the material model. Validation of the model is performed by using tensile tests and thermomechanical experiments. An application example is given where a 10° sector of a brake disc is analysed using the commercial finitie element code Abaqus under a uniformly applied heat flux on the two friction surfaces. The results indicate that the friction surface of the hat side and the neck can be critical areas with respect to fatigue for the uniform heating studied.

The development of fatigue life assessment models for vehicle components exposed to thermomechanical fatigue supports the establishing of adequate maintenance intervals that neither cause unnecessary vehicle downtime, nor jeopardize the function of the components. In modern automotive applications, braking is closely related to safety and is commonly performed with disc brakes. Failure here may result in structural damage or even breakdown and loss of lives. In the present work, the cyclic response of grey cast iron is analysed and the fatigue life of brake discs made from this material is studied by use of four different fatigue life assessment models: the Smith-Watson-Topper model, the Coffin-Manson model, and two mechanism-based damage models. Results from isothermal and thermomechanical experiments on uniaxially loaded specimens are used for calibration of the models. Finally, the models are used to assess the life of a brake disc for a simulated brake dynamometer experiment. It is found that the fatigue model parameters that are calibrated using different sets of isothermal uniaxial test data show a substantial spread. A comparison with results from full-scale brake rig experiments shows that predictions by any of the models that have been calibrated using data from a well-designed thermomechanical test are in reasonable agreement with the estimated crack initiation phase for actual brake disc lives. It can be concluded that it is not sufficient to calibrate the studied fatigue life models using isothermal uniaxial tests for predictions of thermomechanical fatigue lives.

Design of the brake disc geometry for a given brake disc material provides an opportunity for improvement in the fatigue life of the brake disc. High thermomechanical loads at braking lead to substantial local plastification and also induce tensile residual stresses in certain areas of the brake disc. This contributes to shortening of the fatigue life of the brake disc by possible initiation and growth of cracks. In the present paper, a simulation approach for evaluation of brake disc designs with respect to thermomechanical performance is developed and applied. Brake disc performance is analysed using commercial finite element software by employing a constitutive model for grey cast iron implemented in a Fortran subroutine. The thermal loading consists of consecutive severe braking cycles at a constant brake power and a constant speed, with cooling between the brake cycles. Based on a previous experimental study, three different assumptions are made regarding the spatial distribution of the thermal load at braking. A standard commercial brake disc made from grey cast iron having straight vanes is used as the reference case. Geometrical modifications are introduced in the ventilation arrangement using a design-of-experiments approach, studying both straight cooling vanes and different pillar layouts. A preliminary assessment of the fatigue life of the brake discs is carried out. The results indicate that the introduction of different pillar arrangements instead of straight vanes make it possible to decrease the mass of the brake disc by up to 13% or to increase the fatigue life of the brake disc by about 50%.

The brake system is a critical component for any passenger vehicle as its task is to convert the kinetic and potential energy of the vehicle into heat, allowing the vehicle to stop. Heat energy generated must be dissipated into the surroundings in order to prevent brake overheating. Traditionally, a lot of experimental testing is performed to ensure correct brake operation under all possible load scenarios. However, with the development of simulation techniques, many vehicle manufacturers today are looking into partially or completely replacing physical experiments by virtual testing. Such a transition has several substantial benefits, but simultaneously a lot of challenges and limitations need to be addressed and understood for reliable and accurate simulation results. This paper summarizes many of such challenges, discusses the effects that can and cannot be captured, and gives a broader picture of the issues faced when conducting numerical brake cooling simulations.