RESIDUAL STRESS-INDUCED DAMAGE IN BRAKE DRUMS

The braking system represents one of the vital subsystems of the complex mechanical system of a motor vehicle. Together with the steering system and the tires, it plays a decisive role in the active safety of motor vehicles and road users. Brakes, as the actuators of individual subsystems, have a particularly important role in considerations of the reliability of motor vehicle braking systems due to their operating conditions and their influence on vehicle safety. In motor vehicles, two basic types are used: drum (radial) brakes and disc (axial) brakes. On the rear wheels of heavy vehicles, drum brakes are most commonly applied. Because of their good characteristics, these brakes are also applied on the front wheels of motor vehicles, especially on vehicles with larger mass. Since the rear-wheel brakes also serve as the actuators of the parking (auxiliary) brake, the importance of drum brakes for the reliable and safe operation of motor vehicle braking systems is evident. Based on the failure modes, effects, and criticality analysis (FMECA) of drum brake components in motor vehicles, it has been determined that the brake linings and the brake drum exhibit the highest criticality. Criticality represents a relative measure of the severity of failure mode consequences and the frequency of their occurrence. The concept of residual stresses refers to internal stresses that exist in materials and structures independently of any external loads. In metals, particularly in steels, residual stresses may arise due to [5]: non-uniform chemical composition, – unequal plastic deformations during transformations at room or elevated temperatures, – unequal plastic deformations caused by nonuniform heating or cooling in different zones of the cross-section,- structural deformations associated with volume changes in the tructure, for example during steel hardening processes, non-uniform plastic deformations in the crosssection of a machine element induced by service loading at room or elevated temperatures. With regard to residual stresses in brake drums, it is important to note that brake drums are manufactured from a special type of cast iron, which is thermally conductive and wear-resistant.