7 Fatigue Basics You Should Know

1. What is fatigue?

Fatigue is the phenomenon that when materials (metals) are subjected to cyclic stress or strain, their structural performance will decline and eventually lead to failure.

Fatigue failure is one of the most common forms of failure.

According to the data provided by the literature, fatigue failure parts account for 60~70% of the failure parts in various machinery.

In principle, fatigue fracture failure belongs to low stress brittle fracture failure, and it is difficult to observe obvious plastic deformation in fatigue, because it is mainly local plastic deformation, and mainly occurs in the inherent defects of the structure.

Although frequency has some influence on fatigue failure, in most cases, fatigue failure is mainly related to the number of cycles.

7 Fatigue Basics You Should Know 1

According to the stress characteristics that cause fatigue failure, it can be divided into:

Mechanical fatigue caused by mechanical stress and thermal fatigue caused by thermal stress (alternating thermal stress);

The cycle times can be divided into:

High cycle, low cycle and ultra-high cycle fatigue;

According to load properties it can be divided into:

Tension compression fatigue, torsion fatigue and bending fatigue;

According to the working environment of the workpiece, it can be divided into:

Corrosion fatigue, low temperature fatigue and high temperature fatigue.

Generally, the strength of materials and structures before fatigue damage is defined as “fatigue limit”.

2. Types of fatigue

1. Impact fatigue

It refers to fatigue caused by repeated impact load.

When the number of impacts N is less than 500~1000, that is, the parts are damaged, the fracture form of the parts is the same as that of one impact;

When the number of impacts is greater than 105, the part fracture belongs to fatigue fracture, and has typical fatigue fracture characteristics.

In the design calculation, when the number of impacts is greater than 100, the strength is calculated by a method similar to fatigue.

2. Contact fatigue

Under the action of cyclic contact stress, the parts will produce local permanent cumulative damage.

After a certain number of cycles, the process of pitting, shallow or deep peeling on the contact surface is called contact fatigue.

Contact fatigue is a typical failure form of gears, rolling bearings and camshafts.

3. Thermal fatigue

The fatigue of materials or parts caused by cyclic thermal stress generated by temperature cycle is called thermal fatigue.

The cyclic change of temperature leads to the cyclic change of material volume.

When the free expansion or contraction of the material is constrained, cyclic thermal stress or cyclic thermal strain will be generated.

There are mainly two kinds of thermal stress:

The thermal expansion and cold contraction of the parts are subject to the external constraints of the fixed parts, resulting in thermal stress;

Although there is no external constraint, the temperature of each part of the two pieces is inconsistent, and there is a temperature gradient, which leads to the inconsistent thermal expansion and contraction of each part, resulting in thermal stress.

In addition to generating thermal stress, temperature alternation will also cause changes in the internal structure of the material, reducing the strength and plasticity.

The temperature distribution under thermal fatigue condition is not uniform, where the temperature gradient is large, the plastic deformation is serious and the thermal strain concentration is large;

When the thermal strain exceeds the elastic limit, there is no linear relationship between the thermal stress and the thermal strain.

At this time, the solution of the thermal stress must be treated as an elastoplastic relationship.

The thermal fatigue crack starts from the surface and extends inward, and the direction is perpendicular to the surface.

The thermal stress is proportional to the thermal expansion coefficient.

The larger the thermal expansion coefficient is, the greater the thermal stress is.

Therefore, the matching of materials shall be considered when selecting materials, that is, the difference of thermal expansion coefficient of different materials shall not be too large.

Under the same thermal strain conditions, the greater the elastic modulus of the material, the greater the thermal stress;

The greater the temperature cycle change, that is, the greater the upper and lower limit temperature difference, the greater the thermal stress;

The lower the thermal conductivity of the material, the steeper the temperature gradient and the greater the thermal stress during rapid acceleration or cooling.

4. Corrosion fatigue

The fatigue caused by the combined action of corrosion medium and cyclic stress (strain) is called corrosion fatigue.

The corrosion damage caused by the joint action of corrosion medium and static stress is called stress corrosion.

The difference between the two is that stress corrosion only occurs in a specific corrosion environment, while corrosion fatigue will occur in any corrosion environment and under the combined action of cyclic stress.

For stress corrosion cracking, there is a critical stress intensity factor KISCC.

When the stress intensity factor KI ≤ KISCC, stress corrosion cracking will not occur;

However, there is no critical stress intensity factor for corrosion fatigue.

As long as there is cyclic stress in the corrosion environment, fracture will always occur.

The difference between corrosion fatigue and fatigue in the air is that in the process of corrosion fatigue, except for stainless steel and nitrided steel, the surfaces of mechanical parts are discolored.

The number of cracks formed by corrosion fatigue is large, that is, multiple cracks.

The S-N curve of corrosion fatigue has no horizontal part.

Therefore, for the corrosion fatigue limit, it must be pointed out that it is the value under a certain life, that is, there is only conditional corrosion fatigue limit.

The factors that affect the corrosion fatigue strength are more and more complex than those in air.

For example, when the fatigue test frequency is less than 1000HZ in air, the frequency basically has no effect on the fatigue limit, but the corrosion fatigue has an effect in the entire range of frequency.

3. Fatigue life

When a material or mechanical component fails, the total life usually consists of three parts:

1. Crack initiation life

A large number of engineering practices show that the crack initiation life of mechanical components accounts for the vast majority of the fatigue life (even up to 90% of the total life) in the actual service process.

2. Stable crack growth life

In most cases, when the depth of a microcrack reaches this size (about 0.1mm), it will steadily expand along the section of the material or component.

3. Instability extends to fracture life

4. Fatigue form of metal materials

Fatigue of metal materials mainly includes the following:

General plastic deformation;

Plastic deformation under low cycle fatigue;

Plastic deformation under high cycle fatigue;

Micro plastic deformation of crystal size under ultra-high cycle fatigue.

5. Factors affecting the fatigue strength of materials and structures

1. Average stress

With the increase of average stress (statistical stress), the dynamic anti-fatigue stress of materials decreases.

For forces with the same attribute, the larger the average stress σm, the smaller the stress amplitude σa for a given life.

2. Stress concentration

Due to the requirements of working conditions or processing technology, parts often have steps, small holes, keyways, etc., which cause sudden changes in the section, resulting in local stress concentration, which will significantly reduce the fatigue limit of materials.

However, experiments show that the degree of reduction of the fatigue limit is not in direct proportion to the stress concentration factor.

However, if we want to accurately predict the fatigue behavior of mechanical components, we must estimate the crack initiation life of high stress areas or manufacturing defects.

3. Residual stress

The literature research points out that it is meaningful to discuss the influence of residual stress on metal fatigue strength only under high cycle fatigue, because the residual stress will relax greatly under high strain amplitude of low cycle fatigue, so it does not show much effect under low cycle fatigue.

The surface residual compressive stress is beneficial to the parts bearing axial load and the fatigue crack originates from the surface, but it is necessary to pay attention to the residual stress relaxation problem caused by the yield of the residual tensile stress in the core area after the superposition of external load.

The effect of residual stress on notch fatigue strength of parts is very significant, which is due to the existence of stress concentration in residual stress and the greater influence of residual stress on fatigue crack growth.

However, the stress concentration of residual stress is not only related to notch geometry, but also related to material properties.

4. Size effect

The fatigue limit σ-1 value of the material is usually measured with a small sample, the diameter of the sample is generally 7~12 mm, and the section of the actual component is often larger than this size.

The test shows that the fatigue limit decreases with the increase of specimen diameter.

Among them, the steel with high strength drops faster than the steel with low strength.

5. Member surface state

The surface of the component is the place where the fatigue crack core is easy to produce, and the surface stress of the component bearing the alternating bending or alternating torsional load is the largest.

The roughness of the component surface and the machining tool mark will affect the fatigue strength.

The surface damage (tool mark, wear mark, etc.) itself is a surface notch, which will produce stress concentration, reducing its fatigue limit.

The higher the material strength, the more significant the notch sensitivity, and the greater the impact of the machined surface quality on the fatigue limit.

6. Environmental factors

The fatigue performance of metal materials is also affected by the surrounding liquid or gas environment.

Corrosion fatigue refers to the response of metal materials under the interaction of corrosive medium and cyclic load.

It is usually used to describe the fatigue behavior of materials in aqueous environment.

Corrosion fatigue, low temperature fatigue, high temperature fatigue, and different air pressure and humidity environments are all fatigue phenomena under the joint action of materials and environmental factors.

In atmospheric environment, the cycles of failure of the same material are also far lower than those in vacuum environment.

The crack initiation life in vacuum environment is much longer than that in atmospheric environment.

When the working environment pressure of the workpiece is close to Pcr (the air pressure at the life inflection point is defined as the critical air pressure), the fatigue life of the material becomes extremely sensitive.

The fatigue life of materials in atmospheric environment (generally lower than that in vacuum environment) will decrease with the increase of temperature and accelerate the crack growth.

The environmental humidity has a great influence on the durability of high-strength chromium steel.

Water vapor (especially at room temperature) has an adverse effect on the fracture resistance of most metals and alloys, which depends on the stress level, load ratio, amplitude and other loading conditions.

There is a strong interaction between microstructure and environment.

The gas environment significantly affects the fracture morphology and dislocation slip mechanism.

There is an interaction between environment and crack closure, especially in the near threshold region.

The degree of environmental impact depends on the morphology of the crack surface, especially in the depth direction.

At low temperatures, the strength of the metal increases while the plasticity decreases.

Therefore, the high cycle fatigue strength of smooth specimens at low temperature is higher than that at room temperature, while the low cycle fatigue strength is lower than that at room temperature.

For notched specimens, the toughness and plasticity are reduced more.

Notches and cracks are sensitive to low temperature, that is, the critical fatigue crack length at fracture will decrease sharply at low temperature.

The generalized high temperature fatigue refers to the fatigue phenomenon higher than normal temperature.

However, in general, although the working temperature of some parts is higher than the room temperature, it is not too high.

Only when the temperature is higher than 0.5Tm (Tm is the melting point expressed by thermodynamic temperature), or above the recrystallization temperature, there is a fatigue phenomenon of creep and mechanical fatigue, which is called high temperature fatigue.

7. Load type

The order of fatigue limit under different loads is: rotating bending < plane bending < compression load < torsion load.

In corrosive medium, the effect of loading frequency on crack propagation is obvious.

At room temperature and test environment, the conventional frequency (0.1~100HZ) has almost no effect on the crack growth of steel and brass.

In general, if the test loading frequency is lower than 250HZ, the influence of frequency on the fatigue life of metal materials is small.

8. Material defects

Cracks usually originate on the surface, such as the weld (eyelet), cast steel (loose) or sub surface (large inclusions change the local strain field), but rarely on the interior.

The crack initiation also depends on the number, size, nature and distribution of inclusions, as well as the loading direction of external forces.

In addition, the bond strength between inclusion and matrix can not be ignored.

The microcrack is the most dangerous defect in the material with a life of one million cycles, and the micro curve controls the life of the material with a life of one billion cycles.

Because the probability of defects in the material under micro size is much greater than that on the material surface, the probability of crack initiation in the material under ultra-high cycle fatigue loading is naturally greater than that on the surface.

Brittle materials do not have stress reduction or work hardening.

Once there is a notch, fracture may occur under the condition of small nominal stress.

Experience shows that when there is a notch, the fatigue limit of the metal decreases, and the worse the plasticity, the greater the impact of the notch on the fatigue limit.

9. Processing method

It is pointed out in the literature that the preparation process of fatigue test specimen should be the most important link leading to the dispersion of test data.

For example, turning, milling, straightening and other machining methods are all related to the final preparation quality of the specimen.

It is precisely because the preparation method and heat treatment factors will affect the fatigue performance of materials, especially the heat treatment, so even the same batch, size and morphology of tests are difficult to completely repeat the previous fatigue test results.

It can be seen that the production and processing factors of the workpiece will cause the actual fatigue life of the parts to deviate from the expected life value calculated by analysis.

10. Material Properties

The high cycle fatigue strength (when N>106) is related to the hardness of the material, while for medium and low cycle fatigue, toughness is an important indicator.

Under high stress conditions, high-strength steel has low fatigue performance due to poor toughness, while under low stress conditions, it has good fatigue resistance.

On the contrary, low strength steel is in the middle.

Generally speaking, the higher the elastic modulus, the lower the crack growth rate.

The influence of grain size on crack growth only exists in two extreme cases: △ K → △ Kth and △ Kmax → △ KC, which have no obvious influence on the characteristics of medium speed crack growth.

The fracture toughness KIC (or KC) is related to the propagation rate.

It is generally believed that the increase of material toughness will reduce the crack growth rate.

6. Discreteness of fatigue test data

The test equipment and sample itself are the fundamental reasons for the dispersion of fatigue test data (or results).

According to the analysis and introduction in the literature, when determining the fatigue life of components, the nominal load has a 3% error compared with the actual load, which will cause 60% error in the fatigue life, and the extreme situation may cause 120% error in the life.

For fatigue testing machine, 3% error is completely allowed.

However, it is also mentioned that in the static failure test, even for the casting materials and glass with large strength dispersion, there is no serious dispersion like the fatigue life.

The discreteness of fatigue test results is related to material properties, including: inherent properties of materials;

The preparation process of the test and the external environment of the test.

Among them, the test preparation process is the most important link leading to data dispersion, especially heat treatment.

Inclusions and second phase particles in materials are the essential reasons for the dispersion of test data, and its mechanism is still unclear.

7. Development of structural fatigue design methods

Safe life method:

The design stress is lower than the fatigue limit, and it is considered that there is no defect in the structure.

Fail safe method:

The design stress is related to the residual strength in the case of plane defects, and this design method allows for acceptable defects.

Safety crack method:

Definitely predictable propagation cracks are allowed.

Local failure method:

It can solve some problems in metal fatigue analysis and is widely used in France.

The rise of ultra-high cycle fatigue test technology in the 1990s fully shows that some micro defects (such as slag inclusion, porosity, large grain formed by forging, etc.) also have an important impact on the fatigue life of materials.

For steel materials, in the absence of fatigue test data of materials, an approximate S-N curve can be drawn from the tensile strength limit of materials.

It is an estimation method with high accuracy to associate fatigue limit with tensile strength and elongation at break of the specimen.

In the fatigue analysis of materials and structures, it is necessary to give priority to drawing conclusions from tests rather than blindly believing in elastic-plastic calculations.

Only in this way can the reliability of data be ensured.

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