Studies of the basic structural changes that occur when a metal is subjected to cyclic stress have found it convenient to divide the fatigue process into the following stages:
- Crack initiation includes the early development of fatigue damage which can be removed by a suitable thermal anneal.
- Slip-band crack growth involves the deepening of the initial crack on planes of high shear stress. This frequently is called stage I crack growth.
- Crack growth on planes of high tensile stress involves growth of well-defined crack in direction normal to maximum tensile stress. Usually called stage II crack growth.
- Ultimate ductile failure occurs when the crack reaches sufficient length so that the remaining cross section cannot support the applied load.
In general, larger proportions of the total cycles to failure are involved with the propagation of stage II cracks in low-cycle fatigue than in long-life fatigue, while stage I crack growth comprises the largest segment for low-stress, high-cycle fatigue. If the tensile stress is high, as in the fatigue of sharply notched specimens, stage I crack growth may not be observed at all.
An overpowering structural consideration in fatigue is the fact that fatigue cracks usually are initiated at a free surface. In those rare instances where fatigue cracks initiate in the interior there is always an interface involved, such as the interface of a carburized surface layer and the base metal.
Fatigue has certain things in common with plastic flow and fracture under static or unidirectional deformation. The work of Gough has shown that a metal deforms under cyclic strain by slip on the same atomic planes and in the same crystallographic directions as in unidirectional strain. Whereas with unidirectional deformation slip is usually widespread throughout all the grains, in fatigue some grains will show slip lines while other grains will give no evidence of slip.
Slip lines are generally formed during the first few thousand cycles of stress. Successive cycles produce additional slip bands, but the number of slip bands is not directly proportional to the number of cycles of stress. In many metals the increase in visible slip soon reaches a saturation value, which is observed as distorted regions of heavy slip. Cracks are usually found to occur in the regions of heavy deformation parallel to what was originally a slip band. Slip bands have been observed at stresses below the fatigue limit of ferrous materials. Therefore, the occurrence of slip during fatigue does not in itself mean that a crack will form.
A study of crack formation in fatigue can be facilitated by interrupting the fatigue test to remove the deformed surface by electro polishing. There will generally be several slip bands which are more persistent than the rest and which will remain visible when the other slip lines have been polished away. Such slip bands have been observed after only 5 percent of the total life of the specimen. These persistent slip bands are embryonic fatigue cracks, since they open into wide cracks on the application of small tensile strains. Once formed, fatigue cracks tend to propagate initially along slip planes, although they later take a direction normal to the maximum applied tensile stress. Fatigue-crack propagation is ordinarily transgranular.
An important structural feature, which appears to be unique to fatigue deformation, is the formation on the surface of ridges and grooves called slip-band extrusions and slip-band intrusions. Extremely careful metallography on tapered sections through the surface of the specimen has shown that fatigue cracks initiate at intrusions and extrusions.
W. A. Wood, who made many basic contributions to the understanding of the mechanism of fatigue, suggested a mechanism for producing slip-band extrusions and intrusions. He interpreted microscopic observations of slip produced by fatigue as indicating that the slip bands are the result of a systematic buildup of fine slip movements, corresponding to movements of the order of 10-7cm rather than steps of 10-5 to 10-4 cm, which are observed for static slip hands.
Such a mechanism is believed to allow for the accommodation of the large total strain (summation of the micro strain in each cycle) without causing appreciable strain hardening. The notch would be a stress raiser with a notch root of atomic dimensions. Such a situation might well be the start of a fatigue crack. This mechanism for the initiation of a fatigue crack is in agreement with the facts that fatigue cracks start at surfaces and that cracks have been found lo initiate at slip-band intrusions and extrusions.
Extensive structural studies of dislocation arrangements in persistent slip bands have brought much basic understanding to the fatigue fracture process.
The stage I crack propagates initially along the persistent slip bands. In a polycrystalline metal the crack may extend for only a few grain diameters before the crack propagation changes to stage II. The rate of crack propagation in stage I is generally very low, on the order of angstroms per cycle, compared with crack propagation rates of microns per cycle for stage II. The fracture surface of stage I fractures is practically featureless.
Failure to observe striations on a fatigue surface may be due to a very small spacing that cannot be resolved with the observational method used, insufficient ductility at the crack tip to produce a ripple by plastic deformation that is large enough to be observed or obliteration of the striations by some sort of damage lo the surface. Since stage II cracking does not occur for the entire fatigue life, it does not follow that counting striations will give the complete history of cycles to failure.
Stage II crack propagation occurs by a plastic blunting process that is illustrated in Fig. 1.
Fig.1 Plastic blunting process for growth of stage II fatigue crack
At the start of the loading cycle the crack tip is sharp (Fig. 1 a). As the tensile load is applied the small double notch at the crack tip concentrates the slip along planes at 45° to the plane of the crack (Fig. 1b). As the crack widens to its maximum extension (Fig. 1c) it grows longer by plastic shearing and at the same time its tip becomes blunter. When the load is changed lo compression the slip direction in the end zones is reversed. The crack faces are crushed together and the new crack surface created in tension is forced into the plane of the crack (Fig. 1e).
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