Fatigue Endurance of Magnesium Alloys

Abstract

Magnesium alloys belong to the important light alloys. For wide utilization of Mg-alloys it is significant to know their properties, including the fatigue properties. The fatigue of construction materials is a degradation process of irreversible changes in material properties caused by cyclic loading. Gradual accumulation of damage under repeated mechanical stress of the specimen, part of fabrication, known as the fatigue process, is conditioned by a great number of factors. Investigation of characteristics of the fatigue process depends on the kind of mechanical stress, the characteristic of testing machines and equipments. The criterion of material resistance to repeated stress is still the fatigue limit, expressed by the stress amplitude σa or the plastic deformation amplitude eap under whose action material will endure an infinite number o cycles (fatigue limit σc), or at least a number of loading cycles determined in advance. The Wohler curve in coordinates σa – log N, or ea – log N, is constructed on the basic of long-term tests of a great number of specimens. According to the respect of the fracture mechanics regularities, there is the dependence of fatigue crack growth on stress intensity factor (da/dN = f (Ka)) and threshold value of stress intensity factor Kath. The importance of fatigue is tied foremost to safety of persons, which life is dependent on the reliability of given device operation (Necas & Kelemen, 2010). Cracks initiate and propagate from pre-existing flaws, material defects, or design features (fatter holes or sharp corners). In the fact, most fatigue is widespread, as hundreds, or even thousands of cracks are manifested in cyclic loading. The net effect of numerous fatigue cracks located in the same general area is that they synergistically interact reducing the structure’s residual strength. However, the single-crack concept is still important, because ultimately, catastrophic failure can occur when a single crack goes critical and in the process envelops other adjacent cracks in zippering effect.