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Fatigue occurs when materials rupture under repeated or fluctuating loads at stress levels far below their static breaking strength. Fatigue failure often begins at microscopic cracks or imperfections that propagate until the material can no longer carry the load. This process can result from continuous bending, vibrations from unbalanced equipment, or other cyclic stresses applied over thousands or millions of cycles.

Fatigue failure displays brittleness even in normally ductile materials, producing sudden rupture rather than gradual deformation. This occurs because repeated loading creates and propagates microscopic cracks that weaken the material's structure. Unlike static loading, cyclic stresses prevent the material from exhibiting its typical ductile behavior, leading to catastrophic failure at lower stress levels.

Steel exhibits a distinct endurance limit—a stress level below which failure does not occur regardless of loading cycles. Aluminum and other nonferrous metals show no clear endurance limit; their stress at failure continuously decreases with increasing loading cycles. This fundamental difference means steel components can be designed for indefinite cyclic service at sufficiently low stresses, while aluminum requires cycle-life considerations.

Surface conditions significantly impact endurance, with machined and polished specimens demonstrating higher endurance limits than rolled, forged, or corroded components. Surface imperfections act as stress concentration sites where fatigue cracks initiate and propagate. Smooth, well-finished surfaces delay crack initiation, extending the material's fatigue life under cyclic loading.

High-stress applications require fewer loading cycles to cause rupture, while lower stress levels demand increasingly more cycles before failure occurs. This relationship is illustrated in stress versus loading cycles diagrams, which show an inverse correlation between applied stress and cycle count. As maximum stress decreases, the number of cycles needed for rupture increases until reaching the material's endurance limit.

Fatigue results from fluctuating loads such as continuous bending of thin steel rods or wires at the same location, or vibrations produced by unbalanced pump impellers. Any repetitive or varying load applied over thousands or millions of cycles can initiate fatigue failure. These conditions are common in rotating machinery, structural members, and components subjected to cyclic environmental stresses.

Fatigue is a critical design consideration because materials rupture under repeated loads at stress levels far below their static breaking strength. Engineers must account for cyclic loading when designing machines and structures subjected to repetitive or varying stresses. Understanding fatigue properties through stress versus loading cycles curves enables designers to select appropriate materials and stress limits for safe, reliable long-term operation.