Thermal cracking in disc brakes

Abstract

Disc brakes are exposed to large thermal stresses during routine braking and extraordinary thermal stresses during hard braking. High- g decelerations typical of passenger vehicles are known to generate temperatures as high as 900°C in a fraction of a second. These large temperature excursions have two possible outcomes: thermal shock that generates surface cracks; and/or large amounts of plastic deformation in the brake rotor. In the absence of thermal shock, a relatively small number of high- g braking cycles are found to generate macroscopic cracks running through the rotor thickness and along the radius of the disc brake. The analysis herein shows that rotor failure is a consequence of low cycle thermo-mechanical fatigue. An analysis of the vehicle dynamics was used to find a heat flux equation related to braking forces. The heat flux equation was then used in a finite element analysis to determine the temperature profile in the brake. Once the brake temperature was obtained, a simplified shrink fit analysis was used to estimate the stresses that arise during hard braking. This approach shows that plastic deformation occurs due to the large thermal strains associated with high- g braking. The calculated strain amplitude was then used in a Coffin–Manson law to predict the number of high- g braking cycles to failure. Good agreement was obtained between reported braking cycles to failure and the proposed theoretical approach