Large Chamber SEM Views Stress Fractures on Z06 Corvette Brake Rotor

Ever seen a cracked brake rotor before? If you are a racing fan it is likely you have. We recently scanned a Z06 Corvette drilled brake rotor with the Large Chamber Scanning Electron Microscope (LC-SEM) at the WKU NondestructiveAnalysis (NOVA) Center. Due to our microscope’s unique size and positioning system we were able to place the entire rotor within the chamber without cutting or destroying the rotor itself. In fact, our chamber is large enough to accommodate any sample up to 650lbs or 1.5 meters in diameter.

Brake Rotors from this Z06 Corvette were scanned

The owner of this Z06 stock form car extensively used it as a track car and ran it on run-flat street tires. The rotor has suffered serveral serious stress fractures on the surface due to natural wearing and exposure to extreme heating conditions. All rotors eventually warp, crack, and cause vibrations after varying periods of time as a result of internal stress and uneven temperatures in regions of the rotor. A significant observation in drilled rotors is the stress fractures occurring primarily around the holes. A drilled hole interrupts the grain structure of the metal rotor and when extreme thermal conditions are generated these holes expand and contract rapidly, allowing reformation in the metallic structure. When the brake pad slams against the turning rotor, stress is directly applied to the rotor and fatigues the drilled holes. The images show how the cracks form radially along the outer edge holes and suggest that less stress is applied to the inner radius holes due to fewer fractures.

Taking a closer look into a drilled hole revealed detailed images of the iron grains pulling directly apart. Had these fractures occurred instantaneously we might expect to see two rigid walls. However, the images suggest these fractures slowly pulled further apart over extended time due to the alignment of the grains normal to the fracture walls. The apparent decrease in diameter between the fracture walls as it protrudes away from the drilled hole simultaneously supports the likelihood that the crack forms over time.