Effects of hole layout, braking torque and frictional heat on crack initiation from small holes in one-piece brake discs

Brake discs have some holes in the flange for promoting heat dissipation as well as refreshing the pad surfaces. One-piece brake discs are desirable even for large sizes in order to reduce the production cost. However, cracks exceptionally occurred from some holes in the flange during braking under extremely severe test conditions for one-piece brake discs. On the other hand, no cracks were observed under the same condition for two-piece brake discs consisting of a hub and a flange. The objective of the present work is to show way cracks occur in the case of one-piece brake discs.

Cyclic braking tests under extremely severe conditions were carried out using large one-piece brake discs having some hole in their flange. When the number of braking cycles was beyond 300, some tiny cracks occurring around holes were found. They grew in the radial direction of the brake discs. In order to examine whether braking torque is a driving force for crack initiation or not, the time interval was altered 10 times longer than the normal condition. The experimental results showed that the number of total cracks around holes after 300 times of braking decreased as the braking interval was extended. The crack length also decreased. These results indicate that the braking torque itself does have less direct effect on crack initiation. Additional test results indicated that heat cycles due to braking caused thermal cyclic strain around holes and O-compression strain cycles acting discs beyond the yielding stress in compression lead compression tension cyclic stress. Such cyclic stress causes crack initiation around holes.