by Fatigue fracture is one of the main forms of fracture of metal components.
The fatigue discipline has been formed and developed on the basis of fatigue fracture failure analysis of metal components.
Since the publication of W ö hler’s classic fatigue work, people have fully studied the fatigue properties of different materials under various loads and environmental conditions.
Although most engineering technicians and designers have noticed the fatigue problem, and have accumulated a large number of experimental data, there are still many equipment and machines that have fatigue fracture.
Fatigue design has now developed from infinite life design to finite life design.
Life estimation of parts, components and equipment has become an important part of fatigue strength.
Fatigue has developed from an old concept to a new discipline combining material science, mechanics and engineering design – fatigue strength.
The basic forms and characteristics of fatigue fracture 1.
The basic forms of fatigue fracture failure There are many forms of fatigue fracture failure of mechanical parts: according to the different forms of alternating load, it can be divided into tension and compression fatigue, bending fatigue, torsion fatigue, contact fatigue, vibration fatigue, etc; According to the total number of cycles of fatigue fracture (Nf), it can be divided into high cycle fatigue (Nf ＞ 10 〞) and low cycle fatigue (Nf ＜ 10 〞); According to the service temperature and medium conditions of parts, it can be divided into mechanical fatigue (fatigue in normal temperature and air), high temperature fatigue, low temperature fatigue, hot and cold fatigue and corrosion fatigue.
However, 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 combination of these two basic forms under different conditions.
(1) Cut-off fatigue failure Cut-off fatigue initial crack is caused by shear stress.
The mechanical condition of initial fatigue crack initiation caused by shear stress is: shear stress/notch cutting strength ≥ 1; Normal stress/notch breaking strength ＜ 1.
Characteristics of cutting fatigue: the stress and strain field at the origin of fatigue crack is plane stress state; The plane of the initial crack is at an angle of about 45 º to the stress axis and propagates along its slip surface.
Because the cutting strength of single phase metal materials with face-centered cubic structure is generally slightly lower than the normal breaking strength, and the ratio of the maximum shear stress to the maximum normal stress (i.e., the softness coefficient) is 2.0, 0.5 and 0.8 respectively under the conditions of uniaxial compression, tension and torsion, the surface layer of the parts of this kind of materials is relatively easy to meet the above mechanical conditions, so it is mostly broken in the form of cutting.
For example, the initial fatigue cracks of aluminum, nickel, copper and their alloys are mostly formed and expanded in this way.
Small and medium-sized parts and thin-walled parts made of low strength and high plasticity materials, large stress amplitude, high loading frequency and high temperature conditions will be conducive to the occurrence of this failure form.
(2) Fracture fatigue failure The initial crack of fracture fatigue is caused by normal stress.
The mechanical conditions for the initial crack are: normal stress/notch breaking strength ≥ 1, shear stress/notch breaking strength ＜ 1.
Characteristics of normal fracture fatigue: the stress-strain field at the origin of fatigue crack is in plane strain state; The plane of the initial crack is approximately perpendicular to the stress axis, and the crack propagates along the non-crystallographic plane or loosely along the crystallographic plane.
The fatigue failure of most engineering metal components is carried out in this form.
In particular, the proportion of BCC metals and their alloys destroyed in this form is larger; The above mechanical conditions are easy to be satisfied at the internal crack of the test piece, but when the surface is rough or has deep notch, knife mark, corrosion pit, microcrack and other stress concentration phenomena, the normal fracture fatigue crack is also easy to occur on the surface.
Materials with high strength and low plasticity, large section parts, small stress amplitude, low loading frequency, corrosion and low temperature conditions are conducive to the initiation and propagation of normal fracture fatigue cracks.
2 General characteristics of fatigue fracture failure The fatigue fracture of metal parts in use has the characteristics of sudden, highly localized and sensitivity to various defects.
The stress that causes fatigue fracture is generally very low, and special patterns reflecting the macro and micro processes of each stage of fracture can often be observed on the fracture surface.
(1) Although the sudden fatigue fracture of fatigue fracture has gone through three stages of fatigue crack initiation, subcritical growth and unstable growth, the fracture has a strong burst because there is no obvious plastic deformation and other obvious signs before fracture.
Even plastic materials with a large amount of plastic deformation under static tension will show macroscopic brittle fracture characteristics under the action of alternating stress, so the fracture is sudden.
(2) The fatigue fracture stress is very low, and the maximum stress amplitude of the cyclic stress is generally far lower than the strength limit and yield limit of the material.
For example, for rotational bending fatigue, the stress after 10 cycles of stress is only 20~40% of the static bending stress; For symmetric tension and compression fatigue, the stress level of fatigue failure is still lower.
For steel components, the approximate calculation formula used in engineering design is: or (3) fatigue fracture is a process of damage accumulation.
Fatigue fracture does not occur immediately, and often takes a long time to complete.
The initiation and propagation of initial fatigue crack are the result of multiple stress cycle damage accumulation.
In engineering, the stress cycle number (N0) that produces a visible initial crack on the test piece or the ratio of N0 to the total life Nf of the test piece (N0/Nf) is usually used as a parameter to characterize the fatigue crack initiation and incubation period of the material.
The incubation period of fatigue crack initiation is related to such factors as the size of stress amplitude, the shape of specimen and stress concentration, material properties, temperature and medium.
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