CHARACTERISTICS OF FATIGUE
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• In metal alloys, when there are no macroscopic or microscopic discontinuities, the process starts with dislocation movements, which eventually form persistent slip bands that become the nucleus of short cracks.
• Macroscopic and microscopic discontinuities as well as component design features which cause stress concentrations (holes, keyways, sharp changes of direction etc.) are common locations at which the fatigue process begins.
• Fatigue is a process that has a degree of randomness (stochastic), often showing considerable scatter even in well controlled environments.
• Fatigue is usually associated with tensile stresses but fatigue cracks have been reported due to compressive loads.^[7]
• The greater the applied stress range, 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, metallurgical microstructure, presence of oxidizing or inert chemicals, residual stresses, scuffing 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 fatigue failure.
• In recent years, researchers (see, for example, the work of Bathias, Murakami, and Stanzl-Tschegg) have found that failures can 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.^{[citation needed]}
• High cycle fatigue strength (about 10⁴ 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, to achieve frequencies up to 250 Hz.
• Low cycle fatigue (loading that typically causes failure in less than 10⁴ cycles) is associated with localized plastic behavior in metals; thus, a strain-based parameter should be used for fatigue life prediction in metals. Testing is conducted with constant strain amplitudes typically at 0.01–5 Hz.