1. What is fatigue ?
Fatigue refers to the decline in the structural performance of materials, particularly metals, when subjected to cyclic stresses or deformations, ultimately leading to failure.
Fatigue failure is a predominant form of failure.
Research shows that fatigue failures are responsible for 60 to 70% of failures in various machines.
Fatigue fracture failure is classified as low-stress brittle fracture failure and it is challenging to detect significant plastic deformation during fatigue as it mainly results from local plastic deformation and occurs at structural weaknesses.
Although frequency can play a role in fatigue failure, it is typically linked to the number of cycles rather than frequency.
According to the characteristics of the stresses that cause fatigue failure, it can be divided into two categories:
- Mechanical fatigue caused by mechanical stress, and
- Thermal fatigue caused by alternating thermal stress.
Regarding cycle times, fatigue can be divided into:
- High cycle,
- Low cycle and
- Ultra-high cycle fatigue.
In terms of load properties, fatigue can be classified into:
- Tension-compression fatigue,
- Torsional fatigue and
- Flexion fatigue.
And based on the working environment of the part, fatigue can be divided into:
- Corrosion fatigue,
- Fatigue at low temperatures and
- High temperature fatigue.
It is important to note that the strength of materials and structures before fatigue damage is called “fatigue limit”.
2. Types of fatigue
1. Impact fatigue
Refers to fatigue caused by repeated impact loads.
When the number of impacts, N, is less than 500 to 1000, the parts may be damaged and the fracture shape of the parts will be similar to that of a single impact.
When the number of impacts exceeds 105, the fracture of the part is classified as fatigue fracture, showing typical characteristics of fatigue fracture.
In design calculation, if the number of impacts exceeds 100, the resistance should be calculated using a method similar to fatigue analysis.
2. Contact fatigue
Under the influence of cyclic contact stress, parts will suffer gradual and permanent damage at a local level.
After a certain number of cycles, the development of corrosion, surface scaling, or deep scaling on the contact surface is called contact fatigue.
Contact fatigue is a common failure mode in gears, bearings and camshafts.
3. Thermal fatigue
Materials or parts that fatigue due to cyclic thermal stress caused by temperature changes are called thermal fatigue.
Cyclic changes in temperature result in cyclic changes in the volume of the material.
When the material's ability to expand or contract freely is restricted, cyclic thermal stress or cyclic thermal deformation is generated.
There are basically two types of thermal stress:
The thermal expansion and contraction of parts are affected by the constraints of fixed parts, leading to thermal stress.
In the absence of external constraints, inconsistent temperatures between parts of two parts result in unequal thermal expansion and contraction, resulting in thermal stress.
Temperature fluctuations also cause changes in the internal structure of the material, reducing its resistance and plasticity.
Under conditions of thermal fatigue, the temperature distribution is not uniform, leading to severe plastic deformation, large temperature gradients and thermal strain concentrations.
When thermal deformation exceeds the elastic limit, the relationship between thermal stress and thermal strain is no longer linear and must be treated as an elastoplastic relationship.
Thermal fatigue cracks begin at the surface and extend inward, perpendicular to the surface.
Thermal stress is proportional to the coefficient of thermal expansion, with larger coefficients leading to greater thermal stress.
Therefore, material selection must consider the combination of materials without the differences in thermal expansion coefficients being too large.
Under the same conditions of thermal deformation, the greater the elastic modulus of the material, the greater the thermal stress.
The greater the variation in the temperature cycle, that is, the difference between the upper and lower limit temperatures, 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
Fatigue caused by the joint action of a corrosive medium and cyclic stress is called corrosion fatigue.
Damage caused by the combined action of a corrosive medium and static stress is called stress corrosion cracking.
The main difference between the two is that stress corrosion occurs only in specific corrosion environments whereas corrosion fatigue can occur in any corrosion environment under the influence of cyclic stresses.
For stress corrosion cracking, there is a critical stress intensity factor known as KISCC. If the stress intensity factor KI is less than or equal to KISCC, stress corrosion cracking will not occur. However, there is no critical stress intensity factor for corrosion fatigue and fracture will occur as long as cyclic stress exists in a corrosive environment.
The difference between corrosion fatigue and air fatigue is that, with the exception of stainless steel and nitrided steel, the surfaces of mechanical parts subject to corrosion fatigue become discolored. Furthermore, corrosion fatigue results in a large number of cracks rather than just one. The SN curve for corrosion fatigue does not have a horizontal portion.
It is important to note that the corrosion fatigue limit is only conditional and is based on a given service life. The factors that affect corrosion fatigue resistance are more complex than those that affect air fatigue. For example, although fatigue test frequency has no effect on the fatigue limit in air when it is less than 1000 Hz, it does have an impact on corrosion fatigue across the entire frequency range.
3. Tired life
When a material or mechanical component fails, the total service life generally consists of three parts:
1. Crack initiation life
A significant number of engineering studies have demonstrated that the crack initiation life of mechanical components accounts for a large portion, up to 90%, of the total fatigue life during actual service.
2. Stable crack growth life
In most cases, when the depth of a microcrack reaches approximately 0.1 mm, it will grow continuously along the portion of the material or component.
3. Instability extends into the life of the fracture
4. Fatigue form of metallic materials
Fatigue of metallic materials mainly includes the following:
- General Plastic Deformation;
- Plastic Deformation by Low Cycle Fatigue;
- Plastic deformation due to high cycle fatigue;
- Ultra-high cycle fatigue plastic microdeformation of crystal size.
5. Factors affecting the fatigue resistance of materials and structures
1. Medium stress
As the average stress (statistical stress) increases, the anti-fatigue dynamic stress of materials decreases.
For forces with equal characteristics, the greater the average stress σ i will be the smaller the stress amplitude σ a for a given life.
2. Concentration of stress
Due to the demands of working conditions or processing techniques, components often have features such as steps, small holes, keyways, etc. These characteristics cause abrupt changes in the cross section, leading to local stress concentration, which significantly reduces the fatigue limit of the material.
Experiments have shown that the reduction in fatigue limit is not directly proportional to the stress concentration factor.
To accurately predict the fatigue performance of mechanical components, it is necessary to estimate the crack initiation life in regions of high stress or manufacturing defects.
3. Residual stress
The literature review highlights that it is relevant to consider only the impact of residual stress on the fatigue strength of the metal under high cycle fatigue. This is because the residual stress relaxes greatly under the high strain amplitude of low-cycle fatigue and therefore has little effect on low-cycle fatigue.
Surface residual compressive stress is advantageous for components subject to axial loading and when fatigue cracking originates at the surface. However, it is important to be aware of the problem of residual stress relaxation caused by the yielding of residual tensile stress in the core region after external load is applied.
The effect of residual stress on the fatigue strength of components is highly significant. This is because residual stress contains stress concentration and has a greater impact on fatigue crack growth.
However, the residual stress concentration is not only linked to the notch geometry but also to the material properties.
4. Size effect
The fatigue limit value of a material, denoted as σ-1, is generally determined using a small sample, with a diameter typically ranging from 7 to 12 mm. However, the cross-section of the actual components is often larger than this size.
Tests have shown that the fatigue limit decreases as the sample diameter increases.
In particular, the fatigue limit drops more quickly for high-strength steels than for low-strength steels.
5. Limb surface state
The surface of a component is prone to producing fatigue cracks, and the surface tension of a component under alternating bending or alternating torsional load is greater.
The surface roughness of the part and the presence of machining tool marks can affect its fatigue resistance.
Surface damage, such as tool marks or scuff marks, act as a surface notch, causing stress concentration and reducing the fatigue limit.
The greater the strength of the material, the more sensitive it will be to notches and the greater the effect of the quality of the machined surface on the fatigue limit.
6. Environmental factors
The fatigue behavior of metallic materials is influenced by the surrounding liquid or gaseous environment. Corrosion fatigue” refers to the response of metallic materials to the combined effect of a corrosive medium and cyclic loads, typically in an aqueous environment.
Different environmental conditions, such as corrosion fatigue, low temperature fatigue, high temperature fatigue and variation in air pressure and humidity, can affect the fatigue behavior of materials. In atmospheric environments, the failure cycles of a material are typically shorter than in vacuum environments, and the crack initiation life in vacuum environments is longer.
When the part operates close to critical air pressure (Pcr), its fatigue life becomes highly sensitive. The fatigue life of materials in atmospheric environments, which is generally shorter than in vacuum environments, decreases with increasing temperature, accelerating crack growth.
Environmental humidity has a significant impact on the durability of high-strength chrome steel. Water vapor, especially at room temperature, can weaken the fracture toughness of most metals and alloys, depending on the stress level, loading rate, and other loading conditions.
There is a strong interaction between the microstructure and the environment, with the gaseous environment affecting the fracture morphology and the dislocation sliding mechanism. The environment also interacts with crack closure, particularly in the region close to the threshold. The impact of the environment depends on the morphology of the crack surface, especially in the depth direction.
At low temperatures, metal strength increases while plasticity decreases. As a result, the high-cycle fatigue strength of smooth samples is higher at low temperatures, but the low-cycle fatigue strength is lower. For notched samples, the toughness and plasticity decrease further. Low temperatures can be particularly damaging to notches and cracks, as the critical fatigue crack length at fracture decreases drastically.
“Generalized high temperature fatigue” refers to fatigue that occurs at temperatures higher than normal. Although some parts can operate at temperatures higher than room temperature, high-temperature fatigue is only observed when the temperature exceeds 0.5 times the melting point (Tm), or above the recrystallization temperature. At these elevated temperatures, both creep and mechanical fatigue occur, resulting in high-temperature fatigue.
7. Type of load
The order of fatigue limit under different loads is: rotary bending
In a corrosive environment, the impact of loading frequency on crack progression is evident.
At room temperature and in a test environment, conventional frequencies (0.1-100 Hz) have minimal impact on crack growth in steel and brass.
In general, if the test load frequency is less than 250 Hz, the influence of frequency on the fatigue life of metallic materials is minimal.
8. Material defects
Cracks typically originate at the surface, such as in the weld (eyelet), molten (loose) steel, or in the subsurface (large inclusions that alter the local strain field), but are rarely found in the interior.
The initiation of cracks also depends on the number, size, type and distribution of inclusions, as well as the direction of applied external forces.
The bond strength between the inclusions and the matrix should not be neglected.
Microcracks are the most dangerous defects in materials, with a useful life of one million cycles. Microstructures control the lifespan of materials, with a lifespan of one billion cycles.
Since the probability of defects in microsized materials is much higher than that on the surface of the material, the probability of crack initiation under ultra-high cycle fatigue loading on the material is naturally higher than that on the surface.
Brittle materials do not undergo stress reduction or work hardening.
If there is a notch, fracture may occur under low rated stress.
It was observed that when there is a notch, the fatigue limit of the metal decreases, with a greater impact on the fatigue limit in materials with lower plasticity.
9. Processing method
It has been emphasized in the literature that the process of preparing fatigue specimens is a critical factor that contributes to the variability of test results.
For example, the processes of turning, milling, straightening, and other machining methods impact the final quality of sample preparation.
This is because the preparation method and heat treatment factors can affect the fatigue performance of materials, especially heat treatment, making it difficult to obtain consistent results even with the same batch, size and morphology of tests.
It is clear that part production and processing factors will cause the actual fatigue life of parts to deviate from the expected life value calculated through analysis.
10. Properties of materials
Material hardness is a key factor in resistance to high cycle fatigue (when N > 10 6 ), while toughness is an important indicator for medium and low cycle fatigue.
High strength steel has low toughness and therefore poor fatigue performance under high stress conditions. However, it has good fatigue resistance under low stress conditions.
Low strength steel has moderate fatigue performance.
In general, the higher the modulus of elasticity, the slower the rate of crack growth.
The effect of grain size on crack growth is significant only in extreme cases (△ K → △ Kth and △ Kmax → △ KC) and has little impact on crack growth at medium speed.
The propagation rate is related to the KIC (or KC) fracture toughness.
It is widely accepted that increasing the toughness of the material will decrease the rate of crack growth.
6. Discretion of fatigue test data
The scatter of fatigue test data can be attributed to the testing equipment and the sample itself.
According to the literature, a 3% error in the nominal load in relation to the actual load can result in a 60% error in fatigue life and in extreme cases, a 120% error in fatigue life.
Although an error of 3% is acceptable in fatigue testing machines, it is noted that there is no significant dispersion in static failure tests, even for materials with large strength dispersion, such as foundry materials and glass.
Variability in fatigue test results is influenced by material properties, including inherent material properties and the test preparation process and external environment. The preparation process, especially heat treatment, is the most critical factor that leads to data dispersion.
Inclusions and second-phase particles in materials are also important contributors to data scattering, however, the mechanism behind this is still unknown.
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.
Failsafe method:
Design stress is related to residual strength in the case of plane defects, and this design method accommodates acceptable levels of such defects.
Security breach method:
Certainly, the propagation of cracks that can be predicted with certainty is permitted.
Local failure method:
Ultra-high cycle fatigue testing technology, which emerged in the 1990s, has demonstrated that even small microdefects, such as slag inclusion, porosity and large grains formed by forging, can significantly impact the fatigue life of materials.
For steel materials, when fatigue test data is not available, an approximate SN curve can be drawn based on the tensile strength limit of the material.
This estimation method, which associates the fatigue limit with the tensile strength and elongation at break of the sample, is highly accurate.
In fatigue analysis of materials and structures, it is essential to rely on test results, rather than just elastic-plastic calculations, to obtain accurate and reliable data.
