Timing belt failure is usually the result of stripped teeth or delamination/unraveling of fiber cores. Because of the high tensile strength of the fibers used in the manufacture of timing belts, failure from breakage is not common.
Materials Used in Timing Belt Manufacturing
A timing belt is usually made of rubber with fiberglass or Twaron/Kevlar. Because rubber degrades in high temperatures and after contact with motor oil, the lifespan of a timing belt is substantially decreased in engines that run hot or that have a tendency to leak.
Fiberglass Reinforcement Cords
The fiberglass reinforcement cords are extremely sensitive to water and antifreeze. In off-road applications, care must be taken to ensure that water can sufficiently drain away from the belt to prevent premature failure. Newer belts are made of a temperature-resistant material to help them better withstand engine heat.