Why can repeated stress cause fatigue failure in a plastic object?

Repeated stress causes fatigue failure in plastic objects through a progressive process of structural degradation, even when the applied loads remain well below the material's tensile strength.1

This failure occurs in three distinct phases:

• Crack Initiation: Each cycle of loading and unloading induces microscopic damage within the material. Over time, this cumulative damage leads to the formation of a minuscule crack.2
• Crack Propagation: With continued stress cycles, these cracks gradually grow and spread inward through the material.2
• Fracture: As the crack reaches a critical size, the remaining unaffected portion of the plastic becomes too weak to support the applied load, resulting in a sudden and complete fracture.2

Because plastics are often poor conductors of heat, energy dissipated during these stress cycles can also influence material behavior. Designers must account for these fatigue characteristics, as the number of cycles required to initiate and propagate a crack varies, necessitating statistical analysis to predict the lifespan of a plastic component under cyclic loading.13