What is High Cycle Fatigue, HCF?

High Cycle Fatigue

High cycle fatigue (HCF) refers to material fatigue failure with relatively high number of load cycles that occurs in components where stress is relatively low and deformation is primarily elastic.

Examples of high cycle fatigue include:

rotating parts – shafts, gears, disks, and turbine blades in engines and machinery; aircraft wings under aerodynamic loads;

automotive springs and connecting rods; bridges – girders, trusses and connections in bridges subjected to heavy traffic;

offshore structures – structures exposed to wave loading;

industrial buildings – crane girders; lifting equipment – cranes, hoists, winches.

The arbitrary classification between high cycle fatigue and low cycle fatigue (LCF) is considered to be roughly 10,000 cycles to 100,000 cycles.

In general, high cycle fatigue is caused by alternating stresses in the elastic or slightly inelastic range, while low cycle fatigue is normally in the inelastic or plastic range which exhibits large strains.

Characteristics of High-cycle Fatigue

Low stress level: The applied alternating stress is far below the material's yield strength, causing only elastic deformation macroscopically, which can be fully recovered upon unloading.

Long fatigue life: The number of stress cycles before fracture is extremely high, usually between 10⁵ and 10⁹, or even higher.

Fracture mechanism: At microscopic defects, inclusions, or stress concentration points in the material, repeated elastic stress causes dislocation movement, forming microcracks. The cracks slowly and steadily propagate in the direction perpendicular to the principal stress. When the crack reaches a critical size and the remaining cross-section cannot withstand the load, sudden brittle fracture occurs.

Fracture Morphology:

The macroscopic fracture surface is flat and smooth, with the fatigue propagation region occupying the vast majority of the area, and the instantaneous fracture region being very small.

Microscopically (SEM) a large number of fine, parallel fatigue striations can be observed (traces of crack growth from each cycle).

Engineering applications and typical cases:

Rotating machinery: engine crankshafts, drive shafts, gears.

Elastic elements: springs, suspensions.

Aerospace structures: wings, engine blades.

Civil structures: bridges, tracks (bearing long-term cyclic loads from vehicles).