|
Fatigue of Metals: Part One |
|
|
|
|
|
Components of machines, vehicles, and structures are frequently subjected to repeated loads, also called cyclic loads and the resulting cyclic stresses can lead to microscopic physical damage to the materials involved. Even at stresses well below a given material’s ultimate strength, this microscopic damage can accumulate with continued cycling until it develops into a crack or other macroscopic damage that leads to failure of the component.
The process of damage and failure due to cyclic loading is called fatigue. Use of this term arose because it appeared to early investigators that cyclic stresses caused a gradual but not readily observable change in the ability of the material to resist stress.
It has been recognized since 1830 that a metal subjected to a repetitive or fluctuating stress will fail at a stress much lower than that required to cause fracture on a single application of load. Failures occurring under conditions of dynamic loading are called fatigue failures, presumably because it is generally observed that these failures occur only after a considerable period of service.
Fatigue has become progressively more prevalent as technology has developed a greater amount of equipment, such as automobiles, aircraft, compressors, pumps, turbines, etc., subject to repeated loading and vibration. Today it is often stated that fatigue accounts for al least 90 percent of all service failures due to mechanical causes.
A fatigue failure is particularly insidious because it occurs without any obvious warning. Fatigue results in a brittle-appearing fracture, with no gross deformation at the fracture. On a macroscopic scale the fracture surface is usually normal to the direction of the principal tensile stress.
A fatigue failure can usually be recognized from the appearance of the fracture surface, which shows a smooth region, due to the rubbing action as the crack propagated through the section, and a rough region, where the member has failed in a ductile manner when the cross section was no longer able to carry the load. Frequently the progress of the fracture is indicated by a series
of rings, or "beach marks", progressing inward from the point of initiation of the failure.
Three basic factors are necessary to cause fatigue failure. These are:
- maximum tensile stress of sufficiently high value,
- large enough variation or fluctuation in the applied stress, and
- sufficiently large number of cycles of the applied stress.
The KEY to METALS database brings global metal properties together into one integrated and searchable database. Quick and easy access to the mechanical properties, chemical composition, cross-reference tables, and more provide users with an unprecedented wealth of information. Click the buttons below to learn more from the Guided Tour or to test drive the KEY to METALS database.
|
|
|
|