In our daily lives, we discover that some tools fail only after repeated use, but others fail quickly. For example, cars and airplanes have limited lives. Why does this happen? According to "Mechanics of Materials" and the knowledge we have gained in Chapter 2, a material or component does not fail if the applied load is lower than the critical load, or if the stress intensity factor is lower than the fracture toughness of the material. Failure under cyclic loading-even though the maximum load is much less than the critical load under static load failure-is called fatigue failure.