by Source: Baidu Library fatigue and fatigue fracture PPT, shared by gt250611585.
Fatigue fracture is one of the main forms of metal component fracture.
Based on the fatigue fracture failure analysis of metal components, the subject of fatigue has been formed and developed.
Since the publication of W ö hler’s classic fatigue works, people have fully studied the fatigue properties of different materials under various loads and environmental conditions.
Although most engineers and designers have noticed the fatigue problem and accumulated a large number of experimental data, there are still many equipment and machines with fatigue fracture.
Fatigue design has developed from infinite life design to finite life design.
The life estimation of parts, components and equipment has become an important part of fatigue strength.
Fatigue has developed from an ancient concept to a new subject – fatigue strength, which combines material science, mechanics and engineering design.
Basic forms and characteristics of fatigue fracture 1 basic forms of fatigue fracture failure there are many forms of fatigue fracture failure of mechanical parts: according to different forms of alternating load, it can be divided into tension compression fatigue, bending fatigue, torsion fatigue, contact fatigue, vibration fatigue, etc; According to the total cycles of fatigue fracture (NF), it can be divided into high cycle fatigue (NF > 10 ⁵) and low cycle fatigue (NF < 10 ⁴); 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, cold and hot fatigue and corrosion fatigue.
However, there are only two basic forms, namely, cutting 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) Cutting fatigue failure cutting fatigue initial crack is caused by shear stress.
The mechanical conditions of fatigue initial crack initiation caused by shear stress are as follows: shear stress / notch cutting strength ≥ 1; Normal stress / notch normal breaking strength < 1.
The characteristics of cut-off fatigue: the stress-strain field at the origin of fatigue crack is a plane stress state; The plane of the initial crack forms an angle of about 45 º with 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, under the conditions of unidirectional compression, tension and torsion, Ratio of maximum shear stress to maximum normal stress (i.e.
soft coefficient) are 2.0, 0.5 and 0.8 respectively.
Therefore, for this kind of materials, the surface layer of parts is relatively easy to meet the above mechanical conditions, so it is mostly damaged in the form of cutting.
For example, the fatigue initial cracks of aluminum, nickel, copper and their alloys are mostly formed and expanded in this way.
Small and medium-sized and thin-walled parts made of low strength and high plasticity materials have large stress amplitude and high strength Loading frequency and higher temperature conditions will be conducive to the occurrence of this failure form.
(2) Normal fracture fatigue failure the initial crack of normal fracture fatigue is caused by normal stress.
The mechanical conditions of initial crack are: normal stress / notch normal breaking strength ≥ 1, shear stress / notch breaking strength < 1.
The 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 roughly perpendicular to the stress axis, and the crack propagates along the non crystallographic plane or not strictly 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 met at the internal crack of the specimen, but when the surface processing is rough or has deep stress concentration phenomena such as notch, knife mark, corrosion pit and microcrack, positive fracture fatigue crack is also easy to occur on the surface.
High strength and low plasticity materials, 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 is sudden, highly localized and sensitive to various defects.
The stress causing 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.
(1) Although the sudden fatigue fracture of fatigue fracture goes through the three-dimensional process of fatigue crack initiation, subcritical propagation and instability propagation, the fracture has strong abruptness because there is no obvious plastic deformation and other obvious signs before the fracture.
Even the plastic material with a large amount of plastic deformation under static tension will show the fracture characteristics of macro brittleness under the action of alternating stress, so the fracture is sudden.
(2) The fatigue fracture stress is very low, and the maximum stress amplitude in the cyclic stress is generally much lower than the strength limit and yield limit of the material.
For example, for rotary bending fatigue, the breaking stress after 10 cycles of stress is only 20 ~ 40% of the static bending stress; For symmetric tension compression fatigue, the stress level of fatigue failure is lower.
For steel members, 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, but often takes a long time to complete.
The initiation and propagation of fatigue initial crack are the result of multiple stress cycle damage accumulation.
In engineering, the stress cycle cycle (N0) of a visible initial crack on the specimen or the ratio of N0 ￠ to the total life NF ￠ of the specimen (N0 / NF) are usually used as parameters to characterize the fatigue crack initiation and incubation period of materials.
The incubation period of fatigue crack initiation is related to the size of stress amplitude, specimen shape and stress concentration, material properties, temperature and medium.
(4) Fatigue fracture is more sensitive to material defects than metal fatigue loss, which is sensitive to various defects of materials..
