FATIGUE

The term fatigue is used to describe the failure of a material under a repeatedly applied stress. The stress required to cause failure, if it is applied a large number of times, is far below the necessary breaking strength. The fatigue property of a material, therefore, determines its behaviour when subjected to many thousands or even millions of cyclically changing stresses or cyclic load in which the maximum stress developed in each cycle is well within the elastic range of the material.

MECHANISM OF FATIGUE FAILURE -                                      

Fatigue failure almost invariably begins at irregularities on the surface of metals, which act as stress raisers and at points of high stress or stress concentration. The basic mechanism in fatigue is slip. Commercial metals are composed of aggregation of small crystals with random orientation. The crystals themselves are non isotropic. Some crystals in a stressed piece of metal reach their limit of elastic action sooner than others owing to their unfavourable orientation, which permits slip to occur. Also the distribution of stress from crystal to crystal within a piece of stressed metal is not uniform. When a piece is subjected to cyclic stress variation, the constituent particles tend to move slightly with respect to one another. This movement finally weakens some minute elements to such an extent that it ruptures and microscopic crack or a series of such cracks originate on the surface of piece. In the zone of failure, a stress concentration developer, and with successive repetition of stress the fracture spreads inward from the nucleus across the entire section. Ultimately the unaffected portion of the section is reduced to a small core, no longer, capable of sustaining the applied load, and the specimen breaks in to two parts. It is therefore, evident that fatigue failures are neither sudden nor hidden, but develops progressively. Thus it is clear that fatigue is a result of cumulative process involving slip. High temperature increases the mobility of atoms, facilitating greater slip and deformation before fracture.

High localized stress is also developed at abrupt changes in cross-section, at the base of surface scratches, at the root of a screw thread, at the edge of small inclusion of foreign substances and at a minute blow hole or similar internal defects.

CHARACTERISTIC OF FATIGUE FAILURE - Fracture caused by fatigue is of brittle nature, even in ductile materials. The regions corresponding to the progressive and final fractures can be easily identified in freshly broken sections. One region is smooth and polished, while the other is jagged and rough. While the fracture is progressing, the severed portions of the section rub and hammer against each other every time the alternation of loading closes the micro crack. This process ends up by smoothing out any roughness produced by the crack propagation. However, the roughness does not disappear in the core, because here the break occurs under a single load application in the last cycle.

FATIGUE STRENGTH - The stress at which a metal fails by fatigue is termed the fatigue strength. It is found that for most materials there is a limiting stress below which a load may be repeatedly applied for an indefinitely large number of times without causing failure. This limiting stress is called endurance or fatigue limit. The magnitude of endurance limit depends upon the kind of stress variation to which the material is subjected. The presence of stress raisers such as notches at the surface lowers the endurance limit. An increase in the tensile strength usually causes a corresponding increase in the endurance limit.

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