An Investigation of Thermal Conditions Leading to Surface Rupture of Cast Iron Rotors

This paper presents the results of an investigation into the thermal performance characteristics of solid rotor disc brakes. Using the results of mathematical and computer analyses, as well as road test experiments, relationships for predicting the temperature and thermal stress response of the rotor are developed for braking in a single stop. Special emphasis is given to determining the onset of surface cracking as a result of different braking conditions. A design criterion for avoidance of surface rupture in a single stop is developed.

Results obtained in the analytical and experimental study indicate that acceptable correlation between temperature prediction and measurement require an exact assessment of the distribution of braking energy between tire and brake and rotor and pad. Thermally induced surface rupture was found to occur when the theoretical compressive surface stress exceeded the yield strength of the material. Inspection of the rotor surface under the microscope showed that single brake applications from a speed of 60 mph produced surface rupture, caused mainly by the temperature gradient existing at the surface of the rotor. For higher rotor temperatures attained over longer periods of time during repeated braking, other factors such as fatigue properties and composition changes have significant effect upon rotor surface deterioration.