18.8 : Fatigue

Fatigue occurs when materials rupture under repeated or fluctuating loads, even at stress levels far below their static breaking strength. It typically results in brittle failure, even for ductile materials. It is a critical consideration in designing machines and structural components subjected to repetitive or varying loads. The nature of these loadings can range from fluctuating loads like unbalanced pump impellers causing vibrations to repeatedly bending a thin steel rod wire back and forth in the same place, which can lead to fatigue failure.

The number of loading cycles needed to cause a specimen's failure can be experimentally determined for any maximum stress level. It leads to stress versus the number of loading cycles curve, providing significant insights into the material's fatigue properties. For example, typical stress versus the number of loading cycles curve for steel indicates that high-stress applications require fewer cycles to cause rupture. Still, as the maximum stress decreases, the number of cycles needed for rupture increases until reaching the endurance limit. It is crucial to note that fatigue failure often begins at a microscopic crack or imperfection, propagating until the material can no longer carry the maximum load. Machined and polished specimens tend to have a higher endurance limit than rolled, forged, or corroded components.