Prediction of Crack Initiation for One-Piece Type Brake Disc for Motorcycles under Overload Condition

A prediction method was proposed for crack initiation in one-piece type brake discs under extreme braking conditions. Braking tests under extreme loading conditions were conducted by using sample discs. The variations of internal temperatures and surface strains at several locations were measured by using thermocouples and strain gages. In order to obtain the S-N curve of the disc material, specimens with a hole in their center were used for the fatigue test in which an alternative strain was cyclically applied. The numbers of strain cycles, when a 0.3mm crack initiated from the hole, were analyzed by the Weibull plot. The maximum and minimum strains at the hole edge were estimated by calculation considering the temperature variation with respect to time as well as the strain induced by friction due to braking pads. The number of cycles corresponding to the strain amplitude range was estimated by the rain-flow method. The fatigue life of the sample disc was predicted based on the cumulative damage principle as well as by shifting the master S-N curve. The predicted life due to crack initiation reasonably agreed with the experimental life as long as the shift factor for the master S-N curve was appropriately chosen.