Glossary

Cyclic loading refers to the process of repeatedly applying and removing stresses or strains to a material or structure over time — a pattern that causes the stress state to vary in a regular, cyclic manner. This can involve tension and compression, bending, torsion, or any combination of these forces.

When a material is subjected to cyclic loading, even if each individual load is below its ultimate tensile strength, the repeated stress fluctuations can lead to a progressive form of structural damage known as fatigue. Over thousands or millions of cycles, microscopic cracks can initiate at points of stress concentration — such as notches, holes, surface defects, or inclusions — and slowly propagate through the material. Eventually, this crack growth can cause sudden, brittle failure without significant plastic deformation, a phenomenon known as fatigue failure.

Cyclic loading is most often seen in mechanical components and structures that experience repeated motion or vibration, such as:

- Rotating shafts, gears, and crankshafts in engines and machinery.

- Aircraft wings and fuselage skins, which flex during flight.

- Bridges and rail tracks, which undergo loading from vehicles and trains.

- Bolts, fasteners, and springs, which experience repeated tightening, loosening, or oscillation.

Engineers characterize cyclic loading using terms like stress amplitude, mean stress, and stress ratio (R = σ_min / σ_max), and they study fatigue behavior through S–N curves (stress vs. number of cycles to failure). Materials tested under cyclic conditions exhibit a fatigue limit or endurance limit — a stress level below which the material can theoretically withstand an infinite number of cycles without failing (though not all materials, such as aluminum, possess one).