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Home / Engineering Guide / Metal Fatigue
Failure mechanics • testing • prevention

What Is Metal Fatigue? Why Metal Breaks After Repeated Stress

Metal fatigue is progressive, localized damage caused by repeated or fluctuating stress or strain. A small crack can start at a notch, weld toe, thread, scratch, pore or corrosion pit; grow a little with many load cycles; and finally leave too little sound material to carry the next load. That is why a part may fracture even when its nominal service stress stayed below the metal’s static yield strength.

Repeated load → hidden crack → fracture Metal gear components documented after fatigue failure
Fatigue-failed gear components. Martin Brown, NASA Glenn Research Center / NARA, public domain. Source.

The short engineering answer

Fatigue is not simply a metal “getting weak everywhere.” It is a local crack process controlled by the complete combination of load history, geometry, surface, manufacturing condition, environment and inspection capability.

Damage driverStress or strain range

Repeated variation usually matters more than one static load value.

Usual starting pointA local hot spot

Notches, welds, threads, pits, pores and scratches amplify local cyclic demand.

Failure sequenceInitiate, grow, break

The final fast fracture may be only the last fraction of the component’s damage history.

Best preventionControl the system

Design, process, assembly, duty cycle, environment and inspection must agree.

Mechanism in plain English

How does repeated stress break metal?

A fatigue crack normally develops through three practical stages. The boundaries are not perfectly sharp, and a pre-existing flaw can shorten or almost eliminate the initiation stage.

Stage 01Crack initiation

Microscopic cyclic plastic strain accumulates at a vulnerable surface, microstructural feature or defect. A tiny crack forms even though the entire part may still look elastic and unchanged.

Stage 02Stable crack growth

Each damaging cycle advances the crack by a small amount. Growth rate depends on crack size, stress range, stress ratio, load sequence, environment, residual stress and geometry.

Stage 03Final rapid fracture

When the remaining ligament can no longer carry the load—or the crack reaches a critical condition—the part separates rapidly. This final event is often overload of the reduced section.

Important diagnostic point

Calling a failure “fatigue” identifies the mechanism, not the root cause. The root cause is why the damaging cyclic load and the vulnerable location existed: poor radius, vibration, misalignment, weld detail, surface damage, corrosion, process variation, overload sequence, inadequate inspection—or a combination.

Why can fatigue happen below yield strength?

Yield strength comes from a monotonic material test and describes bulk behavior under a defined loading condition. A component is not a smooth coupon. A sharp radius, keyway, thread root, weld toe, pore, inclusion, scratch or corrosion pit can create much higher local stress and microscopic cyclic plastic strain than the nominal drawing stress suggests.

Repeated local slip can produce irreversible microstructural change at the hot spot. The entire part does not have to yield visibly for a fatigue crack to begin. This is why “the stress was below yield” is not a valid fatigue clearance by itself.

Does every repeated load cause fatigue?

No universal statement is safe. Whether damage accumulates depends on the metal and condition, stress or strain range, mean stress, number and sequence of cycles, temperature, corrosion, geometry, surface, residual stress and failure criterion. Some material systems and environments show practical endurance behavior; others continue to show declining strength at very high cycle counts.

A realistic assessment therefore defines the complete duty cycle and the actual production condition instead of relying on a single static strength value.

The language of a load cycle

Maximum stress alone is not enough

A fatigue calculation needs the alternating part of the load, the mean level and the number of repetitions. Fully reversed loading, tension–tension loading, start-stop torque, pressure pulsation, thermal cycling and random vibration can have very different crack-driving effects even if one maximum value matches.

The stress ratio R = σminmax helps describe the cycle. For many unwelded materials, a more tensile mean stress can reduce fatigue resistance. Welded structures are often assessed with detail-category methods that already account for high tensile residual stress, so the governing design rule must be followed rather than mixing methods.

Cyclic stress waveform and its key terms A sinusoidal stress cycle showing maximum, minimum, mean stress, stress range and stress amplitude. σmaxσminσmean time / cycles → stress range Δσ amplitude
Conceptual waveform. Real service histories may be irregular, multiaxial, thermal, impact-driven or frequency-dependent.
σmaxMaximum stress

The highest stress in one defined cycle.

σminMinimum stress

The lowest stress in the same cycle.

ΔσStress range

σmax − σmin; widely used in fatigue assessment.

σaStress amplitude

Half the stress range for a regular cycle.

σmMean stress

Average of maximum and minimum stress.

RStress ratio

σminmax, reported with test data.

NfFatigue life

Cycles to a defined failure or crack criterion.

RunoutTest stopped unfailed

Censored data—not proof of infinite life.

Fracture face of a high-strength bolt damaged by corrosion fatigue
Corrosion-fatigue fracture of a high-strength bolt. Wang Hui-li and Qin Si-feng, CC BY 4.0. Macroscopic bands are not automatically one service cycle per band.
Reading the fracture without over-reading it

What can a fatigue fracture surface reveal?

A fatigue surface may show one or more origins, a smoother progressive-growth region, curved progression or beach marks, ratchet marks from multiple origins and a rougher final-fracture area. Microscopic striations may be present in some alloys and growth conditions.

These features are useful, but none is guaranteed. Beach marks can reflect changes in loading or environment rather than every cycle. One microscopic striation should not automatically be equated with one service cycle. Corrosion, rubbing after fracture, oxidation, handling or cleaning can obscure the evidence.

Preserve failed parts before cleaning

Quarantine the assembly, photograph it in place, preserve orientation and mating parts, protect fracture faces and keep records of load, vibration, temperature, alarms and recent maintenance. Do not wire-brush, sandblast, polish, force mating faces together or remove corrosion products before a failure analyst reviews the evidence.

Stress-life data

How to read an S–N curve

An S–N or Wöhler curve relates a defined cyclic stress measure, S, to the number of cycles, N, associated with a defined failure criterion. Test laboratories run multiple specimens at several stress levels because fatigue results scatter—even among nominally identical specimens.

The curve is meaningful only with its conditions: alloy and heat, product form, orientation, heat treatment, hardness, surface finish, specimen geometry, notch, stress ratio, waveform, frequency, temperature, environment, failure criterion, runout policy and statistical basis.

Three limits that prevent bad decisions

Do not transfer a polished-coupon curve directly to a welded, corroded or differently sized component without a valid method. Do not extrapolate beyond the tested range. Do not interpret a runout as infinite life.

Generic S-N curve relating cyclic stress amplitude to fatigue life
Conceptual S–N curves. AndrewDressel, CC BY-SA 3.0. Not every alloy or component has a true endurance limit.
Low-cycle fatigue

Local cyclic plasticity matters

Strain-life methods are often preferred when severe mechanical or thermal cycling creates meaningful plastic strain. “Below 105 cycles” is a useful approximate context in some standards—not a universal boundary.

High-cycle fatigue

Nominal response is mostly elastic

Stress-life methods are common when cyclic strains are predominantly elastic. Geometry, mean stress, surface, defects and environment still control how coupon data transfer to a part.

Very-high-cycle fatigue

Internal origins can become important

At very large cycle counts, subsurface inclusions, pores and microstructure may govern some materials. The start of “VHCF” is application- and literature-dependent, not a fixed universal number.

Why identical-looking parts can live very different lives

Ten factors that control fatigue life

No single “fatigue strength” follows a metal everywhere. The component’s life emerges from the complete system below.

01

Stress or strain range

Larger cyclic ranges generally increase damage and accelerate crack growth. Local hot-spot range matters more than a remote nominal value.

02

Mean and maximum level

Tensile mean stress can reduce resistance in many unwelded applications; maximum stress also affects crack-tip conditions and overload interactions.

03

Geometry and load path

Small radii, holes, threads, keyways, abrupt thickness changes, poor stiffness transitions and eccentric loading concentrate cyclic demand.

04

Surface condition

Machining marks, grinding burns, scratches, dents, burrs, coating damage, roughness and unfavorable lay can create initiation sites.

05

Material and microstructure

Grade, heat, cleanliness, inclusion population, grain structure, hardness, heat treatment, texture and orientation all influence response.

06

Defects and discontinuities

Porosity, shrinkage, lack of fusion, laps, seams, decarburization and additive-manufacturing flaws can shorten initiation life or behave like initial cracks.

07

Residual stress

Welding, machining, forming, heat treatment, coating and peening can leave tensile or compressive stress that changes local fatigue behavior.

08

Size and stress gradient

Large components sample more material and may have different constraint, defect populations and gradients than small laboratory specimens.

09

Environment and temperature

Corrosion, humidity, salt, chemicals, hydrogen, lubrication, vacuum, temperature and dwell can change initiation and growth rates.

10

Load sequence and assembly

Overloads, resonance, start-stop events, preload, alignment, fit, contact, fretting and maintenance changes can alter the damage history.

Crankshaft section showing a fatigue fracture surface
Crankshaft fatigue fracture. Markus Schweiss, CC BY-SA 3.0. The image confirms a fatigue fracture, but not its exact service spectrum.
Shafts, welds and other common hot spots

Why a stronger metal does not always make a stronger detail

In a rotating shaft, the keyway, shoulder radius, oil hole, press fit, fretting zone or machining mark may govern long before the base metal’s tensile strength is reached. Alignment and balance determine the alternating bending load, so replacing a shaft with a higher-strength grade without correcting the load path can leave the same fatigue origin.

In an as-welded structure, toe and root geometry, attachment detail, undercut, misalignment, incomplete fusion, residual tensile stress, thickness and environment frequently dominate fatigue behavior. Many design systems therefore classify the weld detail and apply detail-category S–N curves, rather than crediting base-metal tensile strength directly.

Do not repair a crack by appearance alone

Grinding, stop-drilling, peening, weld repair or reinforcement can be appropriate only after the full crack extent, critical location, residual stress, load path, material condition and root cause are evaluated. Surface improvement does not erase an existing crack below the treated surface.

Failure identification

Metal fatigue versus other failure modes

A component can show more than one mechanism. Fatigue growth may end in final overload; corrosion may accelerate fatigue; elevated temperature may introduce creep-fatigue interaction.

Failure modePrimary driverTypical cluesKey distinction
FatigueRepeated or fluctuating stress/strainOrigin at a hot spot, progressive region, possible progression marks, final fast-fracture areaCan develop below nominal static yield; damage is cycle- and condition-dependent
Static overloadOne load exceeds immediate strength or stabilityGross plastic deformation, ductile tearing, buckling or rapid brittle fractureNo long progressive cyclic-growth history is required
Brittle fractureCrack, constraint, low toughness, temperature and high driving forceRapid cleavage-like fracture with limited gross deformation in susceptible conditionsCan be the final event after fatigue reaches critical size
Creep / creep-fatigueTime at temperature plus stress, with or without cyclingTime-dependent deformation, grain-boundary damage, dwell sensitivityTime and temperature become central, not just cycle count
Stress-corrosion crackingSusceptible material + tensile stress + specific environmentEnvironment-specific branching or intergranular/transgranular crackingMay occur under sustained stress; corrosion fatigue requires cycling
Wear / contact fatigueRepeated rolling or sliding contact, traction and lubrication conditionPitting, spalling, micropitting or subsurface-origin damageContact stress field and tribology govern the damage site
Interactive fatigue-risk triage

Which assessment route should your part start with?

Choose the closest project conditions. This tool does not calculate fatigue life; it identifies the level of engineering evidence likely needed before a sourcing, design or inspection decision.

Describe the component

Use actual service and production conditions where possible.

Engineering review recommended

Start with a detail-based fatigue assessment

Variable loading, a welded hot spot and meaningful failure consequence require more than a material datasheet or smooth-coupon S–N curve.

Assessment routeDefine the service spectrum, select the governing weld-detail or local-stress method, and validate the production-representative joint.
Evidence to requestDrawing, load history, weld procedure, geometry/misalignment records, material traceability, NDT capability and applicable design curve.
Immediate actionInstrument or reconstruct the real load path before changing material strength or weld size.

Planning aid only. It does not replace a code-based fatigue calculation, fracture-mechanics assessment, qualified inspection or competent engineering approval.

Life assessment methods

How engineers estimate fatigue life—and where each method stops

Method selection depends on whether the problem is mostly elastic, locally plastic, already cracked or too complex for coupon transfer.

Stress-life / S–N

Mostly elastic cycling

Uses stress range or amplitude versus cycles for a defined material or detail. Appropriate corrections and a valid statistical/design basis are needed for mean stress, size, surface, notch, environment and reliability.

Strain-life / ε–N

Local cyclic plasticity

Tracks total, elastic and plastic strain where notches, start-stop events or thermal cycling produce plastic response. Local multiaxial strain and cyclic material behavior must be represented.

Fracture mechanics

Known or assumed crack

Integrates crack-growth data with geometry, initial flaw, load history and critical size. Small cracks, closure, residual stress, overloads and environment can invalidate simple long-crack assumptions.

Component testing

System-level validation

Tests representative geometry, joints, assembly and spectrum. It can capture interactions that coupons miss, but the sample size, runout, failure criterion and production representativeness still matter.

Paris-type crack-growth relation

da/dN = C(ΔK)m

This is a useful mid-growth, linear-elastic correlation—not a law for the complete crack-growth curve. It should not be used blindly near threshold, for micro/small cracks, near rapid instability, under large-scale yielding or when the test and service conditions do not satisfy transferability assumptions.

Variable-amplitude damage

D = Σ(ni/Ni)

Palmgren–Miner linear damage is widely used as an approximation after a service history is cycle-counted, often by rainflow. Failure is frequently represented near D = 1, but real failure sums vary. Sequence, overloads, mean stress, crack closure, corrosion and residual-stress evolution are not fully captured by the linear rule.

A life estimate is only as good as its input spectrum

Record maximum and minimum loads, sequence, dwell, frequency, starts, stops, impacts, pressure, torque, acceleration, vibration, thermal gradients and unusual events. A spreadsheet built from one “typical load” can be more precise-looking and less accurate than a bounded test based on measured service data.

Design and production controls

How can metal fatigue be prevented?

The best time to manage fatigue is before the first production lot. The most effective solutions reduce the crack-driving stress and remove vulnerable hot spots rather than relying on inspection alone.

Define the real duty cycle

Capture load ranges, mean levels, sequence, frequency, starts, stops, thermal events, vibration, resonance, assembly boundary conditions and environmental exposure. Define required life and failure criterion.

Reduce cyclic demand

Change stiffness, load path, support, balance, speed, preload, damping or control logic to reduce the damaging stress/strain range and avoid resonant amplification.

Improve the hot-spot geometry

Use generous transitions, smooth load introduction, controlled holes and threads, adequate edge distance and reduced eccentricity. Evaluate where stiffness changes, not just where material is thin.

Control material and process

Specify grade, heat treatment, product form, cleanliness, orientation, hardness, defect limits, weld detail, special processes and production changes that affect fatigue-critical condition.

Protect surface and environment

Control roughness, lay, burrs, dents, grinding burns and coating damage. Use qualified corrosion protection, lubrication and surface-treatment processes appropriate to the actual material and load.

Validate and monitor

Combine representative testing, analysis, traceability, first-article checks, process capability and a qualified inspection plan tied to detectable and critical flaw sizes.

Detection is a probability, not a promise

How are fatigue cracks detected?

Inspection method depends on material, crack orientation and location, geometry, coatings, access, surface condition and the flaw size that must be found. A negative inspection result means no reportable indication was detected within the qualified capability, coverage and acceptance criteria of that procedure. It does not prove that the part is crack-free.

Inspection intervals should be connected to a damage-tolerance or code basis: the method’s reliably detectable size, expected crack-growth behavior under the real spectrum and environment, critical crack size, access and safety factor—not a generic calendar interval.

U.S. Air Force technician performing magnetic-particle inspection on a metal component
Magnetic-particle inspection in a U.S. Air Force NDI shop. U.S. Air Force photo by A1C Alexandria Lee, public domain. Source. MPI is limited to ferromagnetic materials and surface/near-surface discontinuities.
MethodBest suited toImportant limitationFatigue-program use
Visual / enhanced visualAccessible surface cracks, distortion, corrosion, fretting and leak evidenceSmall, tight, coated, hidden or subsurface cracks may be missedFrequent screening, documentation and targeting of further NDT
Liquid penetrantSurface-breaking discontinuities in nonporous materialsNeeds clean surface access; cannot detect a fully subsurface crackMachined parts, nonferromagnetic alloys and exposed weld surfaces
Magnetic particleSurface and near-surface discontinuities in ferromagnetic materialsNot for aluminum, copper or most austenitic stainless; orientation and magnetization matterSteel shafts, gears, fasteners and weld details
Eddy currentSurface/near-surface cracking in conductive materials, sometimes through thin coatingsProbe, geometry, conductivity, lift-off, crack orientation and calibration affect responseAircraft holes, fastener regions, tubes and localized scanning
Ultrasonic / phased arrayInternal and surface-connected flaws with suitable sound pathsNeeds access, coupling, qualified calibration and skilled interpretation; sizing uncertainty remainsThick sections, welds, shafts and encoded monitoring
Radiography / CTVolumetric discontinuities and complex internal geometryPlanar crack detectability depends strongly on orientation; safety, access and cost matterCastings, AM parts and selected high-value assemblies
Acoustic emission / monitoringDetecting active events or changes during loadNoise, source location and interpretation require qualification; another method may be needed to sizeLarge structures, proof tests and condition monitoring
Five practical component scenarios

Where fatigue risk hides in real production

These examples show why the corrective action is rarely just “buy stronger metal.”

Rotating shaftKeyway + misalignment + alternating bending

Check balance, bearings, alignment, fillet radius, keyway finish and actual bending spectrum before changing alloy or hardness.

Welded bracketAttachment toe + resonance

Measure vibration and operating modes, review detail category and stress range, then control toe geometry, stiffness transition and weld quality.

Process pipingThermal cycling + restraint

Model startup/shutdown gradients, supports, nozzle loads and dwell. Inspect the real hot spot and consider creep-fatigue or corrosion interaction where relevant.

Additive bracketPore/lack-of-fusion + rough surface + orientation

Control build orientation, process qualification, heat treatment/HIP, machining, NDT capability and representative fatigue data for the production route.

Fastener holePreload loss + burr + fretting/corrosion

Specify hole preparation, edge condition, fit, preload method, lubrication, locking, inspection and repair limits. A stronger bolt does not repair a damaged load-transfer joint.

Close-up of an aircraft fan-blade fracture surface with a discolored origin area and ratchet mark
NTSB close-up of a fan-blade fracture surface showing a discolored area and ratchet mark. National Transportation Safety Board, U.S. public domain. Source.
Failure analysis workflow

After a break, preserve the story in the evidence

A useful investigation separates the fracture mechanism from the systemic cause. It combines scene documentation, fractography, material verification, manufacturing history, load reconstruction and hypothesis testing.

  1. Make safe and quarantine: identify every affected serial, lot, sister component and mating part.
  2. Document before disturbance: orientation, fragments, alarms, operating state, corrosion, wear, fastener condition and maintenance changes.
  3. Preserve fracture surfaces: protect from contact, moisture, cleaning and coating removal.
  4. Collect the real history: load, strain, vibration, temperature, pressure, speed, starts, stops, impacts and abnormal events.
  5. Examine at multiple scales: visual and NDT, macrofractography, microscopy, chemistry, hardness, microstructure and dimensional checks.
  6. Reconstruct stress and process: geometry, assembly, residual stress, defects, surface, environment and changes.
  7. Test the competing hypotheses: reproduce the hot spot or failure mode with analysis, measurement and representative testing.
  8. Verify corrective action: remove the cause, not only the visible crack; update drawing, process, inspection and change control.
Buyer and OEM checklist

What should a fatigue-critical RFQ specify?

“Make it from Grade X” is not a fatigue requirement. Give the supplier enough information to control the real crack drivers and prove the quoted process represents production.

Complete duty cycle: maxima, minima, ranges, mean, sequence, cycles, frequency, dwell, overloads and abnormal events.

Boundary conditions: supports, preload, alignment, fits, assembly sequence, contact and resonance-avoidance requirements.

Environment: temperature, humidity, salt, chemicals, corrosion, hydrogen, lubrication, vacuum and cleaning exposure.

Material condition: exact specification, heat treatment, hardness, product form, orientation, cleanliness and traceability.

Defect limits: cracks, pores, inclusions, seams, laps, lack of fusion, decarburization and surface anomalies.

Critical geometry: radii, transitions, holes, threads, keyways, wall thickness, tolerances, alignment, runout and balance.

Surface requirements: roughness, lay, burr, scratch, gouge, recast, grinding-burn, coating and handling limits.

Weld and joint controls: detail, procedure, heat input, toe/root profile, penetration, undercut, misalignment and repair limits.

Validation basis: S–N, strain-life, crack-growth or full-scale method, sample size, runout, failure criterion and statistics.

NDT capability: method, procedure, coverage, calibration, sensitivity/POD basis, acceptance criteria and personnel qualification.

Production evidence: first article, special-process records, capability data, retained samples, audits and periodic revalidation.

Change control: written approval before changes to mill, process, machine, toolpath, heat treatment, weld, coating, NDT or repair.

Common test standards

Which fatigue standard answers which question?

This snapshot was reviewed on July 31, 2026. Always confirm the active licensed edition, governing design code and laboratory scope before placing an order.

StandardPrimary purposeProcurement note
ASTM E466-21Force-controlled, constant-amplitude axial fatigue of metallic specimens in a predominantly elastic regimeSpecimen test—not an automatic full-component life guarantee
ASTM E606/E606M-21Strain-controlled fatigue and cyclic stress–strain behaviorUseful when local plastic strain or thermomechanical cycling matters
ASTM E647-24Fatigue crack-growth rates from near-threshold toward instabilityResidual stress, R, small cracks, environment and closure affect transferability
ASTM E1049-85(2023)Cycle-counting methods, including rainflow, for irregular historiesIt describes methods; it does not guarantee which model fits a given service history
ASTM E2368-25Strain-controlled thermomechanical fatigue testingDefine temperature–strain phase, waveform, environment and specimen condition
ISO 1099:2017Axial force-controlled constant-amplitude metallic-specimen fatiguePublished edition; a replacement edition is progressing—verify current status
ISO 12106:2017Axial strain-controlled fatigue testingPublished edition with revision underway—verify current status
ISO 12107:2012Statistical planning and analysis of fatigue dataUse to define sample size, confidence, runout and reporting strategy
ISO 12108:2018Fatigue crack-growth testingPredominantly linear-elastic Mode I scope; verify revision status and suitability
ISO 12110-1/-2:2013Variable-amplitude testing principles and cycle countingConfirmed in 2024; scope exclusions still need review for the project
ISO 1143:2021Rotating-bar bending fatigue testingUseful for defined metallic specimens, not direct proof for every shaft detail
Standards do not remove engineering context

A purchase order should still define material condition, specimen or component geometry, stress/strain measure, R ratio, waveform, frequency, environment, failure criterion, runout, sample selection, statistical basis, reporting and how the results will be used.

Common mistakes

Metal-fatigue myths that create expensive failures

Myth“Below yield means fatigue-safe.”

Local cyclic plasticity at a notch, weld, scratch, pit or defect can accumulate while nominal stress remains below static yield.

Myth“Every material has one fatigue limit.”

Fatigue resistance depends on condition, life, mean stress, surface, size, geometry, statistics and environment.

Myth“A higher tensile-strength alloy fixes the weld.”

As-welded detail category, geometry, residual stress, defects and stress range often govern more than base-metal strength.

Myth“One runout proves infinite life.”

A runout is a test stopped without reaching the failure criterion. It is censored evidence at those conditions.

Myth“Miner’s sum always reaches exactly 1.”

The linear rule is an approximation and does not fully capture sequence, overload, corrosion, residual stress or scatter.

Myth“No NDT indication means crack-free.”

Every method has a detection capability, coverage, orientation sensitivity and probability of detection.

Myth“Every beach mark equals one cycle.”

Macroscopic progression marks can reflect changes in loading or environment; they are not a universal cycle counter.

Myth“Stop-drilling is a permanent repair.”

It may reduce crack-tip demand in selected cases, but only after crack extent, load path, critical size and root cause are assessed.

Frequently asked questions

Metal fatigue FAQ

What is metal fatigue in simple terms?

Metal fatigue is progressive localized damage caused by repeated or fluctuating stress or strain. A small crack starts at a vulnerable point, grows with additional cycles and eventually leaves too little sound material to support the load.

Can metal fatigue happen below yield strength?

Yes. A notch, weld toe, thread, scratch, pit, inclusion or crack tip can create much higher local cyclic stress and microscopic plastic strain than the nominal component stress. The whole part does not need to yield visibly.

What are the three practical stages of fatigue failure?

Crack initiation or small-crack development, stable cycle-by-cycle macrocrack propagation and final rapid fracture. A pre-existing defect can greatly shorten the initiation stage.

How many cycles cause metal fatigue?

There is no universal number. Severe cyclic plastic strain can fail a part in relatively few cycles, while mostly elastic cycling may require millions or more. Geometry, surface, defects, environment, mean stress and sequence change the answer.

What is the difference between S–N and strain-life analysis?

S–N methods relate cyclic stress to life and are common when response is predominantly elastic. Strain-life methods include elastic and plastic strain and are useful when local cyclic plasticity, start-stop or thermal cycling matters.

Is 10⁵ cycles the exact boundary between low- and high-cycle fatigue?

No. Some standards use approximately 10⁵ cycles as contextual guidance for strain-controlled applications, but the transition depends on material, local strain, geometry, temperature and the analysis framework.

When is Paris law valid?

A Paris-type relation is mainly a mid-growth, linear-elastic correlation for appropriate long-crack conditions. It is not universally valid near threshold, for micro/small cracks, near instability, under large-scale yielding or after complex load-sequence effects without qualification.

Does Miner’s rule predict exact remaining life?

No. It is a linear cumulative-damage approximation based on counted cycles and S–N data. Real damage can depend on sequence, overloads, mean stress, corrosion, residual stress, crack closure and scatter.

Why are welded joints often fatigue-critical?

Weld toe/root geometry, attachment details, undercut, misalignment, lack of fusion, thickness and high tensile residual stress create strong local crack drivers. Detail-category fatigue data often govern more than base-metal tensile strength.

Can NDT certify that a part is crack-free?

No. A qualified inspection can state that no reportable indication was detected within the capability, coverage and acceptance criteria of the method and procedure. Detection remains probabilistic and geometry-dependent.

Can a fatigue crack be repaired?

Sometimes, after engineering evaluation. The repair must address the full crack, critical size, load path, residual stress, material, inspectability and root cause. Replacement may be safer or more economical for critical components.

What information should I send for a fatigue review?

Provide the drawing, material and condition, manufacturing route, complete load/temperature/environment spectrum, required life, failure consequence, surface and defect limits, inspection plan, assembly/preload, production volume and any failed-part evidence.

Technical references

Authoritative sources used for this guide

These sources define testing scope, transferability limits, crack growth, welded-detail behavior and inspection boundaries. Standards may be revised; verify the current licensed edition for contractual use.

Continue the engineering review

Fatigue decisions improve when material behavior, manufacturing route, weld geometry and thermal history are considered together. These guides provide the next layer of context.

Turn a vague fatigue concern into an engineering brief

Review the load, hot spot and production condition together.

Send Oceanplayer your drawing, material condition, manufacturing route, service spectrum, environment, required life, inspection basis and failure consequence. We can help organize the information needed for a manufacturability discussion, sample validation or supplier review.