Fatigue

• Slow development of cracks in components:

1. Repeated or fluctuating stresses (pre-requisite)
2. Failure much lower than σy of material
3. Note fallacy of ‘safety factors’
4. Real ‘safety factor’ (margin of spare capacity) - must be based on all plausible modes of failure

• How fatigue cracks grow

1. Initiation
2. Propagation
3. Each load cycle produces a small striation
4. When the crack stops a beach mark is generated
5. Fracture (brittle or ductile) of residual material


Characteristics of fatigue

• In metals and alloys, 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 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, microstructure, 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 in metals; thus, a strain-based parameter should be used for fatigue life prediction in metals and alloys. Testing is conducted with constant strain amplitudes typically at 0.01–5 Hz.