Study on Crack Propagation Mechanism due to Thermal Stress around Small Holes in One-piece Brake Disc

摘要

Cyclic braking tests under extremely severe condition were carried out using two types one-piece brake discs. When the number of braking cycles was beyond 300 times, some cracks occurring around holes in the flange were found. In order to clarify the effect of decoration window for crack propagation mechanism in one-piece brake disc, overloading braking tests were conducted. The temperature distribution was measured using an infrared camera just after the disc had stopped and before stopped. The crack initiation mechanism for one-piece brake disc was explained as follows. Due to braking, temperature of the flange rises. As the temperature of the flange is distributed, thermal expansion in the radial direction is constrained. Therefore, thermal stress beyond the yield point of disc material occurs in the principal direction of stress. Then, disc is cooled during rotation for the next braking. As the temperature decreases, the thermal stain at holes decreases. The residual tensile stress occurs due to non-linear stress-strain response of material. The successive braking induces the compression-tension cyclic loading. In conjunction with the stress concentration at edges of holes, cracks initiate and grow.