Type of material fatigue
Strain fatigue (low cycle fatigue): high-stress level, few cycles.
The damage of materials due to strain fatigue is generally controlled by the allowable strain value.
Stress fatigue (high cycle fatigue): low stress level and many cycles. The material is damaged due to stress fatigue, which is generally controlled by the allowable stress value.
Secondary fatigue: when the stress level is lower than a certain value, the crack will stop growing.
Factors Affecting Fatigue Properties of Materials
1. Average stress
The fatigue properties of materials are described by the relationship between the applied stress S and the life N to failure.
Under fatigue load, the simplest load spectrum is constant amplitude cyclic stress.
When the stress ratio R=- 1, under the control of symmetrical constant amplitude cyclic load, the stress life relationship given by the test is the basic fatigue performance curve of the material.
The influence of the change of stress ratio R on fatigue performance is discussed below.
As shown in the figure above, the increase of stress ratio R indicates that the average cyclic stress Sm increases. And when the stress amplitude Sa is given, there are:
Sm = (1+R)Sa/(1-R)
In general, when Sa is given, R increases, and the average stress Sm also increases.
The increase of tensile part in cyclic load is unfavorable to the initiation and propagation of fatigue crack, which will reduce the fatigue life.
The general trend of the influence of average stress on the S-N curve is shown in the figure below.
The S-N curve when the average stress Sm=0 is the basic S-N curve.
When Sm>0, that is, under the action of tensile average stress, the S-N curve moves downward, indicating that the life under the action of the same stress amplitude decreases, or that the fatigue strength under the same life decreases, which has adverse effects on fatigue;
Sm<0, that is, when the compression average stress acts, the S-N curve moves upward, indicating that the life under the same stress amplitude increases, or the fatigue strength under the same life increases, and the compression average stress has a favorable effect on fatigue.
Under a given life N, the relationship between the cyclic stress amplitude Sa and the average stress Sm is studied, and the results shown in the figure above can be obtained.
When the life is given, the larger the average stress Sm, the smaller the corresponding stress amplitude Sa;
However, in any case, the average stress Sm cannot be greater than the ultimate strength Su of the material.
Su is the ultimate tensile strength of high-strength brittle materials or the yield strength of ductile materials.
The Sa-Sm relation of metal material N=10 ^ 7 is given in the figure, which is normalized with fatigue limit S-1 and Su respectively.
Therefore, the Sa-Sm relationship under the condition of equal life can be expressed as:
(Sa/S-1) + (Sm/Sn) = 1
This is the parabola in the figure, called Gerber curve, and the data points are basically near this parabola.
Another expression is the straight line in the figure, namely:
n(Sa/S-1) + (Sm/Sn) = 1
The above equation is called Goodman line, and all test points are basically above this line.
The straight line form is simple, and under a given life, the estimated Sa-Sm relationship is conservative, so it is commonly used in engineering practice.
2. Load form
The fatigue limit of materials has the following trend with different load forms:
S (bending)>S (stretching)>S (twisting)
Assuming the same level of applied stress, the volume of the high stress zone in tension and compression is equal to the volume of the entire test section of the specimen;
In the case of bending, the volume of the high stress zone is much smaller.
We know that fatigue failure mainly depends on the magnitude of applied stress (external cause) and the ability of materials to resist fatigue failure (internal cause), that is, fatigue failure usually occurs in high stress areas or material defects.
If the maximum cyclic stress Smax of the action in the figure is equal, because the material volume in the high stress area is large during tension and compression cycles, there is a high probability of defects and crack initiation.
Therefore, under the same stress level, the life under tension compression cyclic load is shorter than that under bending;
In other words, under the same service life, the fatigue strength under tension compression cycle is lower than that under bending.
The fatigue life decreases during torsion, and the volume has little effect.
It needs to be explained by the failure criteria under different stress states, and will not be discussed further here.
3. Size effect
The influence of different specimen sizes on fatigue performance can also be explained by the different volume of high stress zone.
When the stress level is the same, the larger the specimen size is, the larger the material volume in the high stress area is.
Fatigue occurs at the weakest part of the material in the high stress area.
The larger the volume, the greater the possibility of defects or weak parts.
Therefore, the fatigue resistance of large size components is lower than that of small size specimens.
In other words, under a given life N, the fatigue strength of large size components decreases;
Under a given stress level, the fatigue life of large size components decreases.
4. Surface finish
It is obvious from the fatigue locality that if the surface of the specimen is rough, the local stress concentration will be increased and the crack initiation life will be shortened.
The basic S-N curve of the material is measured by the standard specimen with good finish after fine grinding.
5. Surface treatment
Generally speaking, fatigue cracks always originate from the surface.
In order to improve the fatigue performance, in addition to the aforementioned improvement of the finish, various methods are often used to introduce compressive residual stress on the high stress surface of the component to achieve the purpose of improving the fatigue life.
If the cyclic stress is as shown in 1-2-3-4 above, and the average stress is Sm, then when the compression residual stress Sres is introduced, the actual cyclic stress level is the superposition of the original 1-2-3-4 stresses and – Sres, becoming 1 ‘- 2’ – 3 ‘- 4′, and the average stress is reduced to S’m, and the fatigue performance will be improved.
Surface shot peening, cold extrusion of parts and introduction of residual compressive stress on the surface of components are common methods to improve fatigue life.
The higher the material strength is, the lower the cyclic stress level is, the longer the service life is, and the better the life extension effect is.
Shot peening is better when there is stress gradient or notch stress concentration.
The surface nitriding or carburizing treatment can improve the strength of the surface material and introduce compressive residual stress on the surface of the material, both of which are beneficial to improving the fatigue performance of the material.
The test shows that the fatigue limit of steel can be doubled by nitriding or carburizing treatment. For notched specimens, the effect is better.
6. Influence of environment and temperature
The S-N curve of materials is generally obtained under room temperature and air environment.
Fatigue in corrosive medium environment such as seawater, acid and alkali solution is called corrosion fatigue.
The effect of corrosive medium is unfavorable to fatigue.
The corrosion fatigue process is a comprehensive process of mechanical and chemical actions, and its failure mechanism is very complex.
There are many factors affecting corrosion fatigue, and the general trend is as follows:
(1) The effect of load cycle frequency is significant.
When there is no corrosive environment, the frequency has little effect on the S-N curve of the material in a relatively wide frequency range (such as 200Hz).
However, in the corrosive environment, with the decrease of frequency, the time experienced by the same cycle number increases, and the adverse effect of corrosion has a relatively sufficient time to show, which has a significant impact on the decline of fatigue performance.
(2) In corrosive media (such as seawater), semi-immersion (or seawater splash zone) is more unfavorable than complete immersion.
(3) Corrosion resistant steel with good corrosion fatigue resistance;
The fatigue limit of many ordinary carbon steels decreases a lot, even disappears due to corrosion environment.
(4) The fatigue limit of metal materials generally increases with the decrease of temperature.
However, with the decrease of temperature, the fracture toughness of the material also decreases, showing low temperature brittleness.
Once cracks occur, instability fracture is easy to occur.
High temperature will reduce the strength of materials, may cause creep, and is also unfavorable to fatigue.
At the same time, it should be noted that the residual compressive stress introduced to improve fatigue performance will also disappear with the increase of temperature.