Why must engineers assign lower allowable stresses to parts that will undergo many loading cycles?

Engineers must assign lower allowable stresses to parts undergoing many loading cycles to prevent fatigue, a process of progressive fracture that occurs under cyclic loading.12

Even if a load is well below the material's yield stress, repeated application of that load causes microscopic damage that accumulates over time. This process involves three distinct phases: the initiation of cracks, the gradual propagation of those cracks through the material, and eventually, sudden failure. Because each stress cycle contributes to the growth of these cracks, the material's ability to support the load degrades.21

Consequently, the more cycles a part is expected to endure during its service life, the lower the stress level must be to ensure that these microscopic cracks do not reach a critical size that leads to abrupt rupture. Designing for fatigue is essential in engineering to ensure the safety and integrity of structures and machinery subject to vibrations or repeated forces.12