Brake disks of rolling stock are exposed to heavy thermal and mechanical loadings during braking, and lots of
thermal cracks occur on the surface of the disks. These thermal cracks may cause serious accidents, deterioration of
brake performance and increase of maintenance costs due to frequent exchange of friction materials. Therefore,
engineers and researchers have tried to develop materials with high resistance to thermal cracks. Under thermal
shock loading as in brake discs, thermal cracks occur after several thermal cycles. So, crack propagation
characteristic decides the lifetime of brake discs. Sakamoto and Hirakawa [1] developed forged steel discs mounted
on wheels for Shinkansen trains. They used fracture toughness and stress intensity factors as parameters to select
optimal candidate steels. They showed that thermal crack initiation is not influenced much by material
characteristics. Yamabe et al. [2] developed brake discs for trucks with high thermal strength. They used grey cast
iron with nickel. And they showed that cerium inoculation is effective to increase the number of graphite in
microstructure and that thermal fatigue strength is proportional to the graphite number. They used a pin-on-disc type