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Fatigue has traditionally been associated with the failure of metal components which led to the term metal fatigue. In the nineteenth century, the sudden failing of metal railway axles was thought to be caused by the metal crystallising because of the brittle appearance of the fracture surface, but this has since been disproved. [ 1 ]
In aluminium (a face-centred cubic material), cracks grow close to low index planes such as the {100} and the {110} planes (see Miller Index). [3] Both of these planes bisect a pair of slip planes. Crack growth involving a single slip plane is term Stage I growth and crack growth involving two slip planes is termed Stage II growth. [17]
In true corrosion fatigue, the fatigue-crack-growth rate is enhanced by corrosion; this effect is seen in all three regions of the fatigue-crack growth-rate diagram. The diagram on the left is a schematic of crack-growth rate under true corrosion fatigue; the curve shifts to a lower stress-intensity-factor range in the corrosive environment.
Fretting decreases fatigue strength of materials operating under cycling stress. This can result in fretting fatigue, whereby fatigue cracks can initiate in the fretting zone. Afterwards, the crack propagates into the material. Lap joints, common on airframe surfaces, are a prime location for fretting corrosion.
mild steel cracks in the presence of alkali (e.g. boiler cracking and caustic stress corrosion cracking) and nitrates; copper alloys crack in ammoniacal solutions (season cracking); high-tensile steels have been known to crack in an unexpectedly brittle manner in a whole variety of aqueous environments, especially when chlorides are present.
Crack closure is a phenomenon in fatigue loading, where the opposing faces of a crack remain in contact even with an external load acting on the material. As the load is increased, a critical value will be reached at which time the crack becomes open .
PSB structure (adopted from [7]). Persistent slip-bands (PSBs) are associated with strain localisation due to fatigue in metals and cracking on the same plane. Transmission electron microscopy (TEM) and three-dimensional discrete dislocation dynamics (DDD [8]) simulation were used to reveal and understand dislocations type and arrangement/patterns to relate it to the sub-surface structure.
With static fatigue materials experience damage or failure under stress levels that are lower than their normal ultimate tensile strengths. [2] The exact details vary with the material type and environmental factors, such as moisture presence [3] and temperature. [4] [5] This phenomenon is closely related to stress corrosion cracking. [1]