A Microscopic Failure Analysis of Medium Truck Brake Disks

Routine checkup of security vehicle, together with drivers’ complaints of abnormal vibrations during the braking process, indicated a severe damage of brake disks in terms of radial macro-crack and heavily micro-cracks on the friction surfaces. This failure occurred after a short period of service (less than 15000 km) and significant damage was also noticed in the brake disk that was removed after 6000 km. Although the characteristics driving in such vehicle are accompanied by high speed with high g-deceleration frequently, it is still an abnormal failure. The inquiry process indicates that some modifications had been made which considerably increased the total mass of the vehicle. This change significantly affects the forces acting on the brake disk and accordingly on the temperature evolution during the braking process. This change did not involve any adjustment to the braking system. Comprehensive microscopic examination (metallographic, fractographic together with chemical analysis and mechanical testing) of the gray iron brake disks enables to point out some additional sources which lead to this precursory failure. Possible solutions were suggested to prevent such damage which can be hazardous to the proper functionality of this safety component. Firstly, to replace the brake disk more often based on the fatigue crack initiation study. Secondly, to consider using new structural material such as AISI 4330 with a higher fatigue endurance limit and with great resistance to thermal fatigue crack initiation.