The automobile brake drum is an important functional part of the automobile, is also the main part to measure the safety performance of the automobile, but also is the first part to be checked in the daily maintenance, relevant data show that the failure of the brake drum mainly has five forms: cracking, cracking, falling bottom, excessive wear, abnormal wear.

After the observation of the sample crack by professionals, it can be found that: The crack extension direction is straight, the crack extension in the direction of the graphite and almost in the same direction from one to another graphite, graphite even in dendritic, have obvious characteristics of brittle fracture but not when the crack extends to the phase change of the whole area crack to buckle, and ending at most and crack as a whole to show large Angle edge of one of the graphite. Therefore, it can be inferred that when the local part of the failed brake drum reaches a temperature higher than the phase transition, it is rapidly cooled, and the matrix in this part becomes hardened. The brittleness large and contains a lot of residual phase transformation stress, when hit by frequent complex stress (due to the braking action of the normal compressive stress and tangential friction stress and brake thermal stress produced by heating effect after a period of time, the site of some of the stress concentration place (carbide sharp corners or the place such as the tip of the graphite) is greater than the synthetic stress fracture strength of the particle material, I have the first crack. As the extension of crack and extended to a certain plastic without phase transformation matrix organization, matrix by plastic deformation, relaxation of stress, coupled with the tip of the crack due to plastic deformation, the deformation reinforcement, so the extension of crack path twists and turns, the degree of stress concentration at the same time as the bending of the path and slowly lose eventually vanish into the matrix cracks. In order to further verify this inference, we also conducted the following thermal fatigue tests simulating its working conditions.