In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.

Tensile strength measures the engineering stress applied (to something such as rope, wire, or a structural beam) at the point when it fails. It is an intensive property of the material, which not only depends on the type of material but also the preparation of the specimen and the temperature of the test.

Characteristics of fatigue

The process starts with dislocation movements, eventually forming persistent slip bands that nucleate short cracks.

Fatigue is a stochastic process, often showing considerable scatter even in controlled environments.

The greater the applied stress, the shorter the life.

Fatigue life scatter tends to increase for longer fatigue lives.

Damage is cumulative. Materials do not recover when rested.

Fatigue life is influenced by a variety of factors, such as temperature, surface finish, presence of oxidizing or inert chemicals, residual stresses, contact (fretting), etc.

Some materials (e.g., some steel and titanium alloys) exhibit a theoretical fatigue limit below which continued loading does not lead to failure.

In recent years, researchers (see, for example, the work of Bathias, Murakami, and Stanzl-Tschegg) have found that failures occur below the theoretical fatigue limit at very high fatigue lives (109 to 1010 cycles). An ultrasonic resonance technique is used in these experiments with frequencies around 10–20 kHz.

High cycle fatigue strength (about 103 to 108 cycles) can be described by stress-based parameters. A load-controlled servo-hydraulic test rig is commonly used in these tests, with frequencies of around 20–50 Hz. Other sorts of machines—like resonant magnetic machines—can also be used, achieving frequencies up to 250 Hz.

Low cycle fatigue (typically less than 103 cycles) is associated with widespread plasticity; thus, a strain-based parameter should be used for fatigue life prediction. Testing is conducted with constant strain amplitudes at 1–5 Hz.

(Pic- Fracture of an Aluminium Crank Arm. Dark area: slow crack growth. Bright area: sudden fracture)