Re-imagining Brake Rotor Thermal Fatigue Testing to Relate to Field Use

The validation of brake rotors has remained, to this day, heavily reliant on “Thermal Abuse” or “Thermal Cracking” type testing, with many procedures so dated that most engineers active in the industry today cannot even recall the origin of the test. These procedures - of which there are many variants - all share the trait of greatly accelerating durability testing by performing repeated high power (high speed and high deceleration) brake applies to drive huge temperature gradients and internal stress, and often allowing the rotor to get very hot, to where the strength of the material from which the rotor is constructed is significantly degraded. There is little debate about whether these procedures work; by and large rotor durability issues in the field are extremely rare. However, without the connection to the duty cycle in the field, it is extremely difficult to interpret results (especially since many standards allow significant cracking before a failure is declared), and this can lead to significant re-testing and design modifications that not necessarily needed. The publication of SAE J2995 (based heavily on VDA 311) provided a versatile and powerful means of relating simple lab-based test duty cycles to severe field use. There is not a straightforward interpretation of J2995 for brake rotors; however, in combination with simple models relating braking power to rotor stresses, a potential path emerges to connect rotor thermal abuse to field use. The present study explores this path, with a series of inertia dynamometer-based studies designed to see if an S-N curve (relating braking power to rotor life) could be developed for rotor thermal abuse, and if this could then be used to relate the results to customer use. Initial results are promising, giving hope that rotor durability validation can transition eventually to a less empirical, and more engineered approach.