What is fatigue life? - Metal Fatigue Life Prediction (2023)

Fatigue is the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations. The idea of fluctuating stresses and strains is critical. The need to have fluctuating (repeated or cyclic) stresses acting under either constant amplitude or variable amplitude is critical to fatigue.

Fatigue life is the number of loading (stress or strain) cycles of a specified character that a specimen sustains before failure of a specified nature occurs. For some metals, notably steel and titanium, there is a theoretical value for stress amplitude below which the material will not fail for any number of cycles, called a fatigue limit, endurance limit, or fatigue strength.

The majority of engineering failures are caused by fatigue. There is very little warning or no warming at all before fatigue failure, therefore the consequences are often catastrophic.

In dealing with fatigue life problems, two different assumptions are made:

  1. The material is an ideal homogeneous, continuous, isotropic continuum that is free of defects or flaws.
  2. The material is an ideal homogeneous, isotropic continuum but contains an ideal cracklike discontinuity that may or may not be considered a defect or flaw, depending on the entire design approach.

The first assumption leads to either the stress-life or strain-life fatigue design approach. These approaches are typically used to design for finite life or infinite life. The second assumption leads to the damage-tolerant approach. The damage-tolerant approach is based on the ability to track the damage throughout the entire life cycle of the component, and it couples directly to nondestructive inspection.

General applicability of the stress-life method is restricted to circumstances where continuum free of defects or flaws assumptions can be applied. However, as weldments inherently contain discontinuities, some design guidelines for weldments offer procedures with a variety of joint types that generally follow the stress-life approach, with the severity of discontinuities taken into account by different stress categories. The advantages of this method are simplicity and ease of application. It is best applied in or near the elastic range, addressing situations in high cycle fatigue regime.

In the low cycle, inelastic regime of material behavior, the description of local events in terms of strain made more sense and resulted in the development of assessment techniques that used strain as a determining quantity. The general data presentation is in terms of ε-N (log strain vs. log number of cycles or number of reversals to failure). The failure criterion for samples is usually the detection of a small crack in the sample or some equivalent measure related to a substantive change in load-deflection response, although failure may also be defined by separation.

Many components have crack-like discontinuities induced during service or repair or as a result of processing, fabrication, or manufacturing. It is clear that in many instances, parts containing such discontinuities do continue to bear load and can operate safely for extended periods of time. The design philosophy, damage-tolerant, was thus developed. It is intended expressly to address the issue of cracked components. In the case where a crack is present, an alternative controlling quantity is employed. Typically, this is the mode I stress intensity range at the crack tip (ΔKI), determined as a function of crack location, orientation, and size within the geometry of the part. This fracture mechanics parameter is then related to the potential for crack extension under the imposed cyclic loads for either subcritical growth or the initiation of unstable fracture of the part. Property descriptions for the crack extension under cyclic loading are typically da/dN – ΔKI curves (log crack growth rate vs. log stress-intensity range).

The advantage of the damage-tolerant design philosophy is the ability to treat cracked objects in a direct and appropriate fashion. Use of the stress intensity values and appropriate properties allows the number of cycles of crack growth over a range of crack sizes to be estimated and fracture to be predicted. The clear tie of crack size, orientation, and geometry to NonDestructive Examination (NDE) is also a plus. Disadvantages are: computationally intensive stress-intensity factor determinations, greater complexity in development and modeling of property data, and the necessity to perform numerical integration to determine crack growth. In addition, the predicted lives are considerably influenced by the initial crack size used in the calculation, requiring quantitative development of probability of detection for each type of NDE technique employed.

In materials science, fatigue is the weakening of a material caused by repeatedly applied loads called cyclic loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate tensile stress limit, or the yield stress limit.

Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentration regions such as the surface, persistent slip bands (PSBs), and grain interfaces. This is the crack initiation phase Ni. Eventually a crack will reach a critical size, the crack will propagate suddenly, and the structure will fracture. This is the crack propagation phase Np. The total fatigue life Nf is the summation of the two phases:

Nf = Ni + Np

The shape of the structure and surface finish of the material will significantly affect the fatigue life, especially at the crack initiation phase. Square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets will increase the fatigue life of the structure. Smooth surfaces will also improve fatigue life. In the crack propagation phase however, the surface condition is insignificant. For machine components the crack initiation phase accounts for most of the life; for welded connections, small crack-like imperfections are generally present during welding process, the entire life is considered to be spent mostly (90% to 100%) in the propagation phase.

In fatigue life analysis, it is important to distinguish stress range Δσ, stress amplitude σa, and maximum stress σmax. It is the stress range (i.e. the fluctuation of the stresses), rather than maximum stress, that drives fatigue damage. Going hand-in-hand, the stress amplitude σaand mean stress σm are the important parameters in fatigue life analysis.

What is fatigue life? - Metal Fatigue Life Prediction (2)

The stress range Δσ is the difference between the maximum stress σmax and minimum stress σmin in a loading cycle:

Δσ = σmax – σmin = 2σa

The effect of mean stress is expressed by a stress ratio R:

R = σmin/ σmax = (σm – σa) / (σm + σa)

Referring to the figure below, the most commonly applied stress ratios in fatigue tests are R = -1 (fully reversed loading) and R = 0 (pulsation tension).

What is fatigue life? - Metal Fatigue Life Prediction (3)

(Video) Video #2.6 - Fatigue Life Prediction Methods (Mechanical Properties of Materials)

It should be noted that some fatigue curves (S-N curves or ε-N curves) are expressed by stress (strain) ranges; on the other hand, some use stress (strain) amplitude in the curves. Because the difference between stress (strain) range and stress (strain) amplitude is 2 times, close attention should be paid to which fatigue curve is being used.

The following figure shows definitions of the bilinear S-N curve for structural steel. The stress is expressed in terms of stress amplitude σa. However, the stress may also be given in terms of stress range Δσ. Both stress and fatigue life are in log scale.

What is fatigue life? - Metal Fatigue Life Prediction (4)

In this case, the primary (pre-knee) and secondary (post-knee) relationships are

What is fatigue life? - Metal Fatigue Life Prediction (5)

The parameters, such as m1, m2, C1 and C2 etc., are experimentally obtained from tests. In practice, these curves are codified to account for worst case detrimental effects that may arise due to variance in material quality, fabrication, workmanship, accuracy of analyses etc.

Note that horizontal dashed line in the S-N curve above represents the fatigue limit or Constant Amplitude Fatigue Limit (CAFL) for constant amplitude loading. The difference between Constant Amplitude (CA) loading and Variable Amplitude (VA) loading is as shown below:

What is fatigue life? - Metal Fatigue Life Prediction (6)

Constant Amplitude (CA) loading

What is fatigue life? - Metal Fatigue Life Prediction (7)

Variable Amplitude (VA) loading

It should be noted that not all constant amplitude loading result in CAFL. For instance, some materials do not exhibit a fatigue limit in a corrosive environment.

Mean Stress Effects

Fatigue life generally depends on the mean stress values. Various criteria have been proposed to deal with the mean stress effect on fatigue life, including the following:

Soderberg criterion

Soderberg criterion is very conservative. It is not bounded when using negative (compressive) mean stresses.

What is fatigue life? - Metal Fatigue Life Prediction (8)

Goodman criterion

Goodman criterion is usually a good choice for brittle materials. It is not bounded when using negative (compressive) mean stresses. For ductile materials however, because the Goodman method employs the ultimate tensile strength, the results are highly inaccurate and should not be used.

What is fatigue life? - Metal Fatigue Life Prediction (9)

Gerber criterion

Gerber criterion is usually a good choice for ductile materials. It is bounded when using negative (compressive) mean stresses.

What is fatigue life? - Metal Fatigue Life Prediction (10)

(Video) Understanding Fatigue Failure and S-N Curves

Morrow criterion

What is fatigue life? - Metal Fatigue Life Prediction (11)

where σf is true fracture strength, which is often not available, but can be taken as σ’f = fatigue strength coefficient, which is the stress intercept at Nf = 0.5 cycles. The σf = σ’f estimate is quite good for steels, but often gives grossly non-conservative life estimates for aluminum alloys.

The Morrow mean stress correction in the above form was included by Morrow into the elastic term of the strain-life equation given as
What is fatigue life? - Metal Fatigue Life Prediction (12)The morrow model predicts that the mean stress has a significant effect on longer lives, where the elastic strain amplitude dominates. The model also predicts that the mean stress has little effect on shorter lives, where the plastic strain is large. The prediction trend of the Morrow mean stress correction model is consistent with observations that the mean stress has greater impact at longer lives.

Smith-Watson-Topper (SWT) criterion

What is fatigue life? - Metal Fatigue Life Prediction (13)

An extension of the SWT parameter can be applied to the strain-life equation by replacing σa in SWT equation given above with the strain amplitude εa. The product of the maximum tensile stress σmax and the strain amplitude εa in the strain life model controls the influence of both the mean stress and the strain amplitude:

What is fatigue life? - Metal Fatigue Life Prediction (14)

This model gives good estimation of mean stress effect in the long life regime, but it is conservative in the low cycle fatigue region. SWT method provides good results in most cases, and for aluminum alloys it is more accurate than the Morrow equation.

Walker equation

Walker equation is similar to SWT but involves an additional material property γ. The equation is expressed as

What is fatigue life? - Metal Fatigue Life Prediction (15)

Values of γ varies from 0 to 1; for metals the γ values are in the range 0.4 to 0.8. If γ = 0.5, the Walker equation is reduced to SWT criterion. With the adjustable fitting parameter γ, Walker equation gives superior results. Relatively high strength aluminum allows have γ = 0.5; higher values of γ apply for relatively low-strength aluminum alloys. For both steel and aluminum alloys, there is a trend of decreasing γ with increasing strength, indicating an increasing sensitivity to mean stress.

For elastic-plastic fatigue model, the walker equation can be modified as

What is fatigue life? - Metal Fatigue Life Prediction (16)

The modified model combines the advantages of the Walker material parameter γ and the SWT parameter.

Manson-Halford criterion

Manson and Halford suggested that both elastic and plastic terms of the strain-life equation should be modified to account for the mean stress effects and maintain the independence of the elastic to plastic ratio from the mean stress.

What is fatigue life? - Metal Fatigue Life Prediction (17)

The Manson-Halford criterion tends to overestimate the mean stress effect on short lives, where the plastic strain dominates.

In the equations above,

σa = stress amplitude

(Video) Multiaxial Fatigue Life Prediction

εa= strain amplitude

σm = mean stress

σar = equivalent fully reversed (zero mean stress) stress amplitude resulting in the same fatigue life as the σa – σm combination

σu = material ultimate strength

σy = material yield strength

σf = true fracture strength

σmax = maximum stress

R = stress ratio

E = modulus of elasticity

σ’f = fatigue strength coefficient

b = fatigue strength exponent

ε’f = fatigue ductility coefficient

c = fatigue ductility exponent

Nf= number of cycles to failure

Fatigue life assessments are regulated by design codes in different sectors, the most widely used codes being as listed below:

ANSI/AISC 360, Specification for Structural Steel Buildings

This design code covers industrial building sector with highly cyclic live loading, such as bridge cranes, crane runways, monorails, lifting beams, lifting lugs (lifting eyes), manufacturing equipment, etc. AISC 360 also covers highway bridges, railway bridges and pedestrian bridges in transportation sector. AISC 360 deals with steel members, welded components, bolts and threaded parts.

ASME BTH-1, Design of Below-the-Hook Lifting Devices

ASME BTH-1 deals with steel members, welded components, bolts and threaded parts of below-the-hook lifting devices, such as lifting beams, spreader beams, lifting lugs (lifting eyes), hooks, slings and rigging hardware, etc.

EN 1993-1-9, Design of steel structures — Part 1-9: Fatigue

EN 1993-1-9 gives methods for the assessment of fatigue resistance of members, connections and joints subjected to fatigue loading. These methods are derived from fatigue tests with large scale specimens, that include effects of geometrical and structural imperfections from material production and execution. Fatigue strengths are determined by considering the structural detail together with its metallurgical and geometric notch effects. In the fatigue details presented in this design code the probable site of crack initiation is also indicated. The assessment methods presented in this code use fatigue resistance in terms of fatigue strength curves for (1) standard details applicable to nominal stresses; (2) reference weld configurations applicable to geometric stresses.

DNVGL-RP-C203, Fatigue design of offshore steel structures

This recommended practice presents recommendations in relation to fatigue analysis based on fatigue tests (S-N data) and fracture mechanics. DNVGL-RP-C203 is valid for carbon manganese steel (C-Mn) in air with yield strength less than 960 MPa. For carbon and low alloy machined forgings for subsea applications, the S-N curves are valid for steels with tensile strength up to 862 MPa in air environment. For steel (C-Mn) materials in seawater with cathodic protection or steel with free corrosion, the recommended practice is valid up to 690 MPa. This limit applies also to the carbon and low alloy machined forgings for subsea applications. This recommended practice is also valid for bolts in air environment or with protection corresponding to that condition of grades up to 10.9, ASTM A490 or equivalent. This recommended practice may be used for stainless steel.

API-2A-WSD, API Recommended Practice 2A-WSD:

Planning, Designing, and Constructing Fixed Offshore Platforms—Working Stress Design

This recommended practice is based on global industry best practices and serves as a guide for those who are concerned with the design and construction of new fixed offshore platforms and for the relocation of existing platforms used for the drilling, development, production, and storage of hydrocarbons in offshore areas.

Nontubular members and connections in deck structures, appurtenances and equipment; and tubular members and attachments to them, including ring stiffeners, may be subject to variations of stress due to environmental loads or operational loads. Where variations of stress are applied to conventional weld details, the associated S-N curves provided in AWS D1.1 are used. For service conditions where details may be exposed to random variable loads, seawater corrosion, or submerged service with effective cathodic protection, the fatigue life reduction factors are applied accordingly.

For tubular connections, this recommended practice provides S-N curves for welded joints (WJ) and cast joints (CJ) in air environment, seawater with cathodic protection, and seawater free corrosion conditions.

(Video) Failure Fatigue and Creep

ISO 19902, Petroleum and natural gas industries — Fixed steel offshore structures

This International Standard specifies requirements and provides recommendations applicable to the following types of fixed steel offshore structures for the petroleum and natural gas industries:

  • caissons, free-standing and braced;
  • jackets;
  • monotowers;
  • towers.

In addition, it is applicable to compliant bottom founded structures, steel gravity structures, jack-ups, other bottom founded structures and other structures related to offshore structures (such as underwater oil storage tanks, bridges and connecting structures), to the extent to which its requirements are relevant.

ISO 19902 provides S-N curves for tubular joints (TJ), cast joints (CJ) and other joints (OJ) in air environment, seawater with cathodic protection, and seawater free corrosion conditions.

EN 12952-3, Water-tube boilers and auxiliary installations — Part 3:

Design and calculation for pressure parts of the boiler

Water-tube boiler pressure parts shall be designed in accordance with the requirements of this European Standard. Boiler components are deemed to be exposed to cyclic loading if the boiler is designed for more than 500 cold start-ups. The design rules presented in Annex B, Fatigue cracking – Design to allow for fluctuating stress, apply to the design of pressurized components of boilers made from ferritic and austenitic rolled or forged steels. These rules allow for the fluctuating stresses occurring at the most highly stressed points as a result of internal pressure and differences in temperature and/or the addition of external forces and moments. The maximum shear stress theory shall be used in the determination of the decisive cyclic stress amplitude and the mean cyclic stress. Stress intensity, which is two times of shear stress, is also used in the analysis. The controlling stress range is divided into elastic range, partly elastic range, and fully plastic range.

Cyclic stress range and mean cyclic stress shall be increased to account for the notch effect (micro notch effect) associated with surface and weld influences. Here, the governing factor in each case is the final state following manufacture.

In the case of a load-cycle temperature t* ≥ 100 °C, the reduction in the fatigue strength caused by the temperature shall be taken into account by means of a correction factor Ct* for ferritic and austenitic alloys.

EN 13445-3, Unfired pressure vessels Part 3: Design

Clause 18, detailed assessment of fatigue life, specifies requirements for the detailed fatigue assessment of pressure vessels and their components that are subjected to repeated fluctuations of stress. The Tresca criterion is applied in this clause but use of the ‘von Mises’ criterion is also permitted.

At structural discontinuities, the following stresses are defined:

Nominal stress – stress which would exist in the absence of a discontinuity. Nominal stress is a reference stress (membrane + bending) which is calculated using elementary theory of structures. It excludes the effect of structural discontinuities (e.g. welds, openings and thickness changes). The use of nominal stress is permitted for some specific weld details for which determination of the structural stress would be unnecessarily complex. It is also applied to bolts. The nominal stress is the stress commonly used to express the results of fatigue tests performed on laboratory specimens under simple unidirectional axial or bending loading. Hence, fatigue curves derived from such data include the effect of any notches or other structural discontinuities (e.g. welds) in the test specimen.

Structural stress – linearly distributed stress across the section thickness which arises from applied loads (forces, moments, pressure, etc.) and the corresponding reaction of the particular structural part. Structural stress includes the effects of gross structural discontinuities (e.g. branch connections, cone/cylinder intersections, vessel/end junctions, thickness change, deviations from design shape, presence of an attachment). However, it excludes the notch effects of local structural discontinuities (e.g. weld toe) which give rise to non-linear stress distributions across the section thickness. For the purpose of a fatigue assessment, the structural stress shall be evaluated at the potential crack initiation site. Structural stresses may be determined by one of the following methods: numerical analysis (e.g. finite element analysis), strain measurement or the application of stress concentration factors to nominal stresses obtained analytically. Under high thermal stresses, the total stress rather than the linearly distributed stress should be considered.

Notch stress – total stress located at the root of a notch, including the non-linear part of the stress distribution. Notch stresses are usually calculated using numerical analysis. Alternatively, the nominal or structural stress is used in conjunction with the effective stress concentration factor Kf.

What is fatigue life? - Metal Fatigue Life Prediction (18)

Distribution of nominal, structural and notch stress at a structural discontinuity

A fatigue assessment shall be made at all locations where there is a risk of fatigue crack initiation. It is recommended that the fatigue assessment is performed using operating rather than design loads. In fatigue, welds behave differently from plain (unwelded) material. Therefore, the assessment procedures for welded and unwelded material are different. Plain material might contain flush ground weld repairs. The presence of such repairs can lead to a reduction in the fatigue life of the material. Hence, only material which is certain to be free from welding shall be assessed as unwelded.

ASME Section VIII, Rules for Construction of Pressure Vessels Division 2 —

Alternative Rules

ASME BPVC.VIII.2 contains mandatory requirements, specific prohibitions, and nonmandatory guidance for the design, materials, fabrication, examination, inspection, testing, and certification of pressure vessels and their associated pressure relief devices. Pressure vessels are containers for the containment of pressure, either internal or external. This pressure may be obtained from an external source or by the application of heat from a direct or indirect source as a result of a process, or any combination thereof. These vessels shall be designated as either a Class 1 or Class 2 vessel in conformance with the User’s Design Specification.

Class 1 Vessel – a vessel that is designed using the allowable stresses from Section II, Part D, Subpart 1, Table 2A or Table 2B.

Class 2 Vessel – a vessel that is designed using the allowable stresses from Section II, Part D, Subpart 1, Table 5A or Table 5B.

Fatigue provisions are in Part 5 – Design by Analysis Requirements, provides requirements for design of vessels and components using analytical methods. A fatigue evaluation shall be performed if the component is subject to cyclic operation. The evaluation for fatigue is made on the basis of the number of applied cycles of a stress or strain range at a point in the component. The allowable number of cycles should be adequate for the specified number of cycles as given in the User’s Design Specification.

ASME Section VIII, Rules for Construction of Pressure Vessels Division 3 —

Alternative Rules for Construction of High Pressure Vessels

The rules of ASME BPVC.VIII.3 constitute requirements for the design, construction, inspection, and overpressure protection of metallic pressure vessels with design pressures generally above 70 MPa (10 ksi). Pressure vessels within the scope of this Division are pressure containers for the retainment of fluids, gaseous or liquid, under pressure, either internal or external. This pressure may be generated by: an external source; the application of heat from direct source or indirect source; a process reaction; or any combination thereof.

Article KD-3 Fatigue Evaluation presents a traditional fatigue analysis design approach. In accordance with KD-140, if it can be shown that the vessel will fail in a leak‐before‐burst mode, then the number of design cycles shall be calculated in accordance with this Article.

Cyclic operation may cause fatigue failure of pressure vessels and components. While cracks often initiate at the bore, cracks may initiate at outside surfaces or at layer interfaces for autofrettaged and layered vessels. In all cases, areas of stress concentrations are a particular concern. Fatigue‐sensitive points shall be identified and a fatigue analysis made for each point. The result of the fatigue analysis will be a calculated number of design cycles Nf for each type of operating cycle, and a calculated cumulative effect number of design cycles when more than one type of operating cycle exists. The resistance to fatigue of a nonwelded component shall be based on the design fatigue curves for the materials used. Fatigue resistance of weld details shall be determined using the Structural Stress method, which is based on fatigue data of actual welds.

The theory used in this Article postulates that fatigue at any point is controlled by the alternating stress intensity Salt and the associated mean stress σnm normal to the plane of Salt. They are combined to define the equivalent alternating stress intensity Seq, which is used with the design fatigue curves to establish the number of design cycles Nf.

(Video) Fatigue (Strength-Number of Cycles) SN-DIAGRAMS in Under 10 Minutes!

ASME Section III — Rules for Construction of Nuclear Facility Components — Appendices

ASME BPVC.III.A XIII-3500 Analysis for Fatigue Due to Cyclic Operation covers fatigue life analysis. The design fatigue curves used in conjunction with XIII-3500 are those in Mandatory Appendix I. When more than one curve is presented for a given material, the applicability of each is identified. Where curves for various strength levels of a material are given, linear interpolation may be used for intermediate strength levels of these materials. The strength level is the specified minimum room temperature value. The maximum possible effect of mean stress is included in the fatigue design curves.


What is fatigue life? - Metal Fatigue Life Prediction? ›

Fatigue life is the number of loading cycles that a structure or component sustains before failure occurs. Because fatigue failure can occur at very low stresses with little warning, this mode of failure is often overlooked in the design and the consequences are frequently catastrophic.

What is fatigue life prediction? ›

A fatigue life prediction method has been proposed for multiaxial variable amplitude stress histories. It is based on a counting variable representative of the stress states and of their evolution versus time in order to identify and extract multiaxial cycles.

What is the meaning of fatigue life? ›

Fatigue life is defined as the number of loading (stress) cycles of a specified character that a specimen sustains before failure of a specified nature occurs. The number of cycles is related to engine speed. It can be converted to equivalent durability hours.

What are the phases in fatigue life? ›

The fatigue life of a component can be expressed as the number of loading cycles required to initiate a fatigue crack and to propagate the crack to critical size. Therefore, it can be said that fatigue failure occurs in three stages – crack initiation; slow, stable crack growth; and rapid fracture.

How do you calculate fatigue life? ›

Fatigue life calculations for variable loads are performed on the basis of appointed sinusoidal cycles set due to apply one of the counting cycles methods. The sinusoidal cycles in set are characterized by wide range of amplitude Sai and mean value Smi variability.

What can fatigue testing predict? ›

Fatigue testing utilizes cyclic loading to predict the life expectancy of material components and how it can react in real-world situations.

How do you increase the fatigue life of metals? ›

Surface rolling and shot peening are effective ways to improve the fatigue strength because they can form a deformable hardened layer at a certain depth on the surface of the sample and produce residual compressive stress on the surface.

What is an example of fatigue? ›

Fatigue has been described as “a feeling of weariness, tiredness or lack of energy.”1 In workplace settings, it is commonly associated with nonstandard schedules which disrupt or shorten sleep. Examples of nonstandard schedules include night shift work and extended work hours.

What causes fatigue in end of life? ›

Factors contributing to someone's experience of fatigue include: illnesses including cancer, heart and lung disease, kidney failure, hypothyroidism and neurodegenerative conditions such as motor neurone disease. treatments, especially radiotherapy and chemotherapy. medicines, such as opioids and antihistamines.

Are there 4 types of fatigue? ›

She spoke about different types of fatigue and how our symptoms might vary after we've experienced each kind. She listed six types of fatigue: social, emotional, physical, pain, mental, and chronic illness.

In what terms fatigue life is measured? ›

Solution: Fatigue life is measured in terms of the number of cycles of failure. The maximum stress is kept fixed.

What are three feelings that manifest fatigue? ›

What Are the Signs and Symptoms Associated With Fatigue?
  • Weakness.
  • Lack of energy.
  • Constantly feeling tired or exhausted.
  • Lack of motivation.
  • Difficulty concentrating.
  • Difficulty starting and completing tasks.

What is high cycle fatigue life? ›

HCF. High cycle fatigue is a type of fatigue caused by small elastic strains under a high number of load cycles before failure occurs. The stress comes from a combination of mean and alternating stresses.

What is the fatigue life of a structure? ›

Fatigue life is defined by a cyclic stress with load having unlimited number. Fatigue strength is the maximum strength where load acts to structure without failure for definite load. Fatigue life is depended some factors such as material characteristic, crack, structural geometry and so on.

What is the stress range in fatigue life? ›

The stress amplitude at which failure occurs for a given number of cycles is the fatigue strength. N is the number of cycles required for a material to fail at a certain stress range in fatigue life. The stress range, ΔS = Smax–Smin, and number of cyclic loading are defined in Figure 12.2.

What are the 2 methods of fatigue testing? ›

Fatigue tests include the following types, which are classified by the form of the applied load, environmental factors, etc.
  • Tension-compression fatigue tests. ...
  • Fatigue crack propagation testing machine. ...
  • Plane bending fatigue testing machine.
  • Torsional fatigue testing machine.
  • Rotating bending fatigue testing machine.

How do you test for metal fatigue? ›

A fatigue test works by subjecting the metal to a repeated loading and unloading force in a specialized fatigue testing machine. ATS' mechanical testing lab can test to common SAE and ASTM standards as well as fully custom specifications.

Why is the fatigue test important? ›

Fatigue test results are an important basis for assessing life expectancy and strength of materials, components or structures under cyclic load, or to ensure that the necessary safety criteria are met.

Can metal fatigue be repaired? ›

Heat treatment for metal fatigue

Typically, one of the most useful treatments for fractures caused by metal fatigue is welding. A welder must take care to eliminate any and all traces of the fracture in order to perform a quality repair.

What is an example of metal fatigue? ›

Metal fatigue occurs when the material is a metal. For example, a steel wire might be used to suspend weights that are less than the amount needed to cause the wire to break apart (its tensile strength). Over time, however, those weights might slowly cause defects to develop in the steel.

What causes metal fatigue failure? ›

In addition to bending, or radial stresses, other types of stresses can cause metal fatigue. There may be a defect caused by the manufacturing process or within the material itself. Increased metal fatigue also can occur due to corrosion, part rotation, temperature, wear, or structural design.

What are 4 symptoms of fatigue? ›

Symptoms of fatigue
  • chronic tiredness or sleepiness.
  • headache.
  • dizziness.
  • sore or aching muscles.
  • muscle weakness.
  • slowed reflexes and responses.
  • impaired decision-making and judgement.
  • moodiness, such as irritability.

What is the most common fatigue? ›

Most of the time fatigue can be traced to one or more lifestyle issues, such as poor sleep habits or lack of exercise. Fatigue can be caused by a medicine or linked to depression. Sometimes fatigue is a symptom of an illness that needs treatment.

What is fatigue life of a material? ›

Fatigue life is a term used in fatigue testing and refers to the deformation and failure behavior of materials under cyclic loading. A fatigue life test with constant amplitudes is referred to as S-N test. The tolerable stress amplitude is determined using the corresponding number of cycles.

Do you sleep a lot at end of life? ›

People often become more drowsy and sleep more towards the end of life. This is one of many signs that a person may have when they are in their last few days and hours of life, but not everyone will experience this.

Why do I suddenly burst energy before death? ›

Some argue that a rise of neurotransmitters could be released from the brain in response to deteriorating health. Others suggest that the body might be attempting to prepare for the final stages of life or that the surge is connected to the patient's emotional state.

How long before death is terminal agitation? ›

Terminal agitation is typically seen during the hours or days before death and can be distressing and overwhelming for caregivers.

What are 6 symptoms of fatigue? ›

Know when it's time to rest
  • yawning.
  • poor concentration.
  • tired or sore eyes.
  • restlessness.
  • drowsiness.
  • slow reactions.
  • boredom.
  • feeling irritable.

What is psychological fatigue? ›

The part of fatigue which is psychological in nature has also been called 'mental fatigue' [61]. Mental fatigue is a gradual and cumulative process and is associated with unwillingness to put in effort, reduced efficiency and alertness and impaired mental performance (Grandjean, 1988 as cited in: Lal & Craig, [61]).

How do you know if you're emotionally drained? ›

People experiencing emotional exhaustion often feel like they have no power or control over what happens in life. They may feel “stuck” or “trapped” in a situation. Lack of energy, poor sleep, and decreased motivation can make it difficult to overcome emotional exhaustion.

How do you prevent metal fatigue? ›

When designing and making a part there are a number of factors to consider in order to get maximum fatigue life.
  1. Avoid sharp corners. ...
  2. Avoid abrupt changes in cross-section. ...
  3. Make machined surface finishes as smooth as possible. ...
  4. Welding needs to be good quality with no inclusions, gas holes (porosity) or worm holes.

Which is the strongest predictor of emotional exhaustion? ›

Neuroticism and Extraversion correlated strongest with Exhaustion, and Neuroticism and Agreeableness correlated strongest with Depersonalization.

Where does sadness manifest in the body? ›

Along with the emotional baggage it carries, extreme sadness can cause distinctive physical sensations in the chest: tight muscles, a pounding heart, rapid breathing, and even a churning stomach. As you can see on the body map, survey respondents pinpointed the chest as a major spot for the manifestation of sadness.

What part of the brain controls how tired you are? ›

The hypothalamus, a peanut-sized structure deep inside the brain, contains groups of nerve cells that act as control centers affecting sleep and arousal.

What is the ASTM standard for high cycle fatigue? ›

ASTM D3479 describes the test on composites. The high cycle fatigue test is used for the determination of finite life fatigue strength and high cycle fatigue strength for tensile, compression, flexure and torsion loading.

How many cycles of fatigue do you need? ›

In the elastic range, the fatigue failure usually occurs after 105 cycles of stress (high cycle fatigue). For large stresses causing plastic deformation, much fewer cycles of stress are needed for failure (low cycle fatigue).

How many cycles is fatigue limit? ›

Fatigue limit is defined as the stress value below which fatigue failure occurs when the fatigue life is sufficiently high (e.g., 10–500 million cycles).

What is the factor that affects fatigue life? ›

The fatigue strength of materials is highly sensitive to various internal and external factors. External factors include the part's shape, size, surface finish, and service conditions, while internal factors include the material's composition, microstructure, purity, and residual stress.

What is a healthy stress level? ›

The stress level range is from 0 to 100, where 0 to 25 is a resting state, 26 to 50 is low stress, 51 to 75 is medium stress, and 76 to 100 is a high stress state. Knowing your stress level can help you identify stressful moments throughout your day.

What does stress level 20 mean? ›

0–25: Resting state. 26–50: Low stress. 51–75: Medium stress. 76–100: High stress.

What is the difference between fatigue life and fatigue strength? ›

The difference between fatigue strength and fatigue limit is in the number of cycles. It is considerably higher with fatigue limit. Therefore, engineers try to design their parts so that they are kept under the fatigue limit during work.

What is the fatigue life prediction using the stress life method? ›

The stress-life method works well in predicting fatigue life when the stress level in the structure falls mostly in the elastic range. Under such cyclical loading conditions, the structure typically can withstand a large number of loading cycles; this is known as high-cycle fatigue.

What is infinite life in fatigue? ›

Fatigue Life. Fatigue life, at a given alternating stress level and mean stress, is the number of cycles required to cause failure due to fatigue. Infinite Life. Number of cycles where the fatigue strength ceases to decrease. In other words, the number of stress cycles required to cause failure is practically infinite.

What is an example of fatigue stress? ›

Fatigue stress failures occurs as well in metals as in polymers. Some examples of fatigue stress failures: Fatigue stress failure in an aircraft engine. Fatigue cracking in a bridge girder.

How does fatigue analysis work? ›

It is observed that repeated loading and unloading weakens objects over time even when the induced stresses are considerably less than the allowable stress limits. This phenomenon is known as fatigue. Each cycle of stress fluctuation weakens the object to some extent.

What causes metal fatigue? ›

In addition to bending, or radial stresses, other types of stresses can cause metal fatigue. There may be a defect caused by the manufacturing process or within the material itself. Increased metal fatigue also can occur due to corrosion, part rotation, temperature, wear, or structural design.

What is an example of a fatigue material? ›

Fatigue occurs when a material is subject to alternating stresses, over a long period of time. Examples of where fatigue may occur are: springs, turbine blades, airplane wings, bridges and bones.

Are there two types of fatigue? ›

There are two main types of fatigue: physical and mental. A person with physical fatigue may find it physically hard to do the things they usually do, such as climbing the stairs. Symptoms include muscle weakness, and diagnosis may involve completing a strength test.

What is the fatigue life and fatigue limit? ›

The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure.

Does steel have an infinite fatigue life? ›

For many metals, including steel and aluminum, no damage occurs once a constant amplitude load drops below the endurance limit (infinite life). This behavior can be represented by a straight line in a logarithmic diagram called S-N curve.

What materials have infinite fatigue life? ›

Some materials, like titanium and most steels, have a clear fatigue limit. So below that limit, the fatigue life is indeed infinite. Unfortunately, a lot of other materials (aluminium, magnesium, copper, austenitic steel) do not show a very clear fatigue limit.


1. Using S-N curves to predict the fatigue of materials
(Taylor Sparks)
2. Introduction to Fatigue: Stress-Life Method, S-N Curve
3. Miner's Rule for Damage
(Simcenter Testing)
4. Fatigue FAILURE CRITERIA in Just Over 10 Minutes!
(Less Boring Lectures)
5. ANSYS Workbench | Fatigue Analysis | Fatigue Life | Damage & Safety Factor
6. Predicting Creep Life and Fatigue Life for Engineering Components in Service
(Engineers Academy)


Top Articles
Latest Posts
Article information

Author: Patricia Veum II

Last Updated: 25/10/2023

Views: 5803

Rating: 4.3 / 5 (64 voted)

Reviews: 95% of readers found this page helpful

Author information

Name: Patricia Veum II

Birthday: 1994-12-16

Address: 2064 Little Summit, Goldieton, MS 97651-0862

Phone: +6873952696715

Job: Principal Officer

Hobby: Rafting, Cabaret, Candle making, Jigsaw puzzles, Inline skating, Magic, Graffiti

Introduction: My name is Patricia Veum II, I am a vast, combative, smiling, famous, inexpensive, zealous, sparkling person who loves writing and wants to share my knowledge and understanding with you.