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Fatigue and Fatigue Fracture of Metal Components

2022-08-09

Fatigue fracture is one of the main forms of fracture of metal components. Since the publication of Wöhler's classic fatigue work, the fatigue properties of different materials when tested under various loads and environmental conditions have been fully studied. Although fatigue problems have been noticed by most engineers and designers, and a large amount of experimental data has been accumulated, there are still many equipment and machines that suffer from fatigue fractures.
 
There are many forms of fatigue fracture failure of mechanical parts:
 
*According to the different forms of alternating loads, it can be divided into: tension and compression fatigue, bending fatigue, torsional fatigue, contact fatigue, vibration fatigue, etc.;
 
*According to the size of the total cycles of fatigue fracture (Nf), it can be divided into: high cycle fatigue (Nf>10⁵) and low cycle fatigue (Nf<10⁴);
 
*According to the temperature and medium conditions of the parts in service, it can be divided into: mechanical fatigue (normal temperature, fatigue in the air), high temperature fatigue, low temperature fatigue, cold and heat fatigue and corrosion fatigue.
 
But there are only two basic forms, namely, shear fatigue caused by shear stress and normal fracture fatigue caused by normal stress. Other forms of fatigue fracture are the composite of these two basic forms under different conditions.
The fractures of many shaft parts are mostly rotational bending fatigue fractures. During rotational bending fatigue fracture, the fatigue source area generally appears on the surface, but there is no fixed location, and the number of fatigue sources can be one or more. The relative positions of the fatigue source zone and the last fracture zone are generally always reversed by an angle relative to the direction of rotation of the shaft. From this, the rotation direction of the shaft can be deduced from the relative position of the fatigue source region and the last fracture region.
 
When there is a large stress concentration on the surface of the shaft, multiple fatigue source regions can appear. At this point the last fracture zone will move to the inside of the shaft.