The causes of brake drum cracking and cracking are verified
TIME：2019.12.26
The brake drums with the characteristics of cracking and cracking were selected from the brake drums found in the garage as samples, and the samples were sampled at the spot where the cracks were concentrated for experimental observation.The sampling method is: cut a 2cm piece of ingot on the working surface in contact with the drum wall and the brake lining where the cracks are concentrated (no high temperature is generated during cutting to prevent phase transition), mark the inner and outer wall of the ingot and number the corresponding sample.After preparation, the sample was metallographically polished and corroded (5% nitric acid alcohol solution corroded for 15 seconds at room temperature) to obtain the following structure.After preliminary observation, it is not difficult to find that there are one or two darker patches on the surface of each sample after corrosion, and the patches are usually perpendicular to the direction of crack extension and parallel to the direction of braking friction, and often in the middle of the crack.Therefore, it is not difficult to guess that the dark patch is due to the phase change structure formed by the local high temperature and rapid cooling caused by the uneven heat generation during the brake drum operation. The crack first emerges from the center of the phase change and extends vertically to both sides of the direction of the braking friction, and ends at the tissues without phase change.To prove the above conjecture, xingtai to far auto parts co., LTD., after the corrosion of the sample, in turn, taken in 400 x metallographic microscope and the metallographic photos are as follows: through observation of the specimen crack can be found: the extending direction of crack is straight, 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 and crack as a whole to show large Angle edge of one of the graphite.According to this, it can be inferred that the local failure drum is cooled by extreme speed when reaching higher than phase transition temperature, which makes the matrix of this part become 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, as the initial crack initiation.As the crack extension, 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 reduce the crack slowly vanish into the substrate.
