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Fatigue occurs when the materials, under repeated loads for thousands or millions of cycles, rupture at a stress level much lower than the material's breaking strength.

Fatigue failure displays brittleness, even in normally ductile materials.

Fatigue can occur due to fluctuating loads, such as continuous bending of thin steel rods or wires at the same spot or vibrations produced by unbalanced pump impellers.

The stress versus the number of loading cycles diagram for steel indicates that high-stress applications require fewer cycles to cause rupture. However, as the maximum stress decreases, the number of cycles needed for rupture increases.

The stress level decreases until the endurance limit at which failure does not occur, even for an indefinitely large number of loading cycles.

For nonferrous metals, like aluminum, stress at failure decreases with the increase in loading cycles.

Fatigue failure often starts at microscopic imperfections, increasing until the material fails under load. Surface conditions significantly impact endurance, with machined specimens proving more durable.