Lack of deformation

Since initiation of fatigue fracture does not require a high stress, there is usually little or no deformation in a part or specimen that has fractured by fatigue. If the maximum stress did not exceed the yield strength (actually the elastic limit), there can be no gross plastic, or permanent, deformation, although the final rupture region may have some obvious macroscopic deformation. The typical fatigue fracture that occurs in most load-bearing parts, which have relatively low-stress, high-cycle loading.

Not only the fracture surface but the entire part should be examined for deformation. For example, if a unidirectional (one-way) bending fracture is observed, it is useful to carefully reassemble the pieces to determine if there was gross deformation in the part prior to fracture. Of course, the origin of the fracture would be on the convex side, which is the tension side in bending.

As pointed out at the beginning of this section, in a "true" highcycle fatigue fracture, there will be no deformation in the fatigue region, provided that there has been no postfracture damage to the fracture surface. If the final rupture region (stage 3) is ductile, the resulting deformation will prevent close realignment of the fractured pieces; however, if the final rupture region is a truly brittle fracture, there should be no gross deformation, except for postfracture damage. A partially ductile/brittle final rupture region probably will show some degree of deformation.
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