A component does not need to fail from one overload to break in service. In many real structures, failure happens because the load repeats.
Fatigue in engineering is the progressive initiation and growth of cracks caused by cyclic or fluctuating loading. Over time, those cracks can reach a critical size and cause sudden fracture. What makes fatigue dangerous is that it often develops under stress levels well below the material’s ultimate strength and, in some cases, below yield.
That is why fatigue is not just a materials topic. It is a design, verification, fabrication, inspection, and life-management problem.
If a structure is exposed to traffic, vibration, wave loading, rotating motion, pressure changes, thermal expansion, startup-shutdown events, or repeated lifting, fatigue must be considered explicitly. And if the structure is modeled withFEAorFEM in structural analysis, fatigue should be part of the verification workflow, not an afterthought.
What is fatigue in engineering?
Fatigue in engineering is the progressive accumulation of damage under cyclic stress or strain. In practice, that damage usually takes the form of a crack.
The process is simple in principle:
a local weak point experiences repeated loading
a small crack starts
the crack grows with each cycle
the remaining cross-section becomes too small
final fracture occurs rapidly
This is why engineers often distinguish betweenfatigueas the damage mechanism andfatigue failureas the final fracture event.
What is fatigue failure in engineering?
Fatigue failure in engineering is the fracture of a material or component after repeated or fluctuating loading causes a crack to initiate, grow, and eventually reach a critical size.
A static failure happens because one load event exceeds the capacity of the component. A fatigue failure happens because many cycles gradually damage the material, even when each individual cycle appears acceptable.
That distinction matters in engineering practice:
a beam may survive one heavy load without issue
the same beam detail may crack after years of repeated smaller loads
a welded bracket may pass a static check but still have poor fatigue life
Static strength tells you whether a structure can carry a load once. Fatigue assessment helps determine whether it can survive repeated loading over its intended service life.
The three stages of fatigue failure
1. Crack initiation
A crack begins at a location where local stress is higher than nominal stress or where the material already has a weakness.
Common crack initiation sites include:
weld toes and weld roots
holes and cutouts
keyways and thread roots
sharp corners and abrupt section changes
machining marks and scratches
corrosion pits
inclusions, porosity, and fabrication defects
In welded structures especially, fatigue almost never starts in a random place. It starts where geometry, fabrication quality, or load transfer is poor. That is why weld detail assessment matters so much, especially when using methods such asnominal stress and hot-spot stress evaluation for welds in FEAor reviewing localweld stresses.
2. Crack propagation
Once initiated, the crack grows incrementally during each load cycle. This phase often consumes most of the fatigue life.
At this point, the structure may still look intact from the outside. The damage is real, but not always visible without inspection.
If a crack or crack-like flaw is already known, engineers may use fracture mechanics and crack-growth methods to estimate how fast it will grow. One of the best-known relationships is the Paris law form:
$$
\frac{da}{dN} = C(\Delta K)^n
$$
where \( \frac{da}{dN} \) is crack growth per cycle, \( \Delta K \) is the stress-intensity-factor range, and \( C \) and \( n \) are material constants.
3. Final fracture
When the crack reaches a critical size, the remaining intact section can no longer carry the load. Failure then occurs rapidly.
This is why fatigue failures are often described as sudden. The final fracture is sudden. The damage process leading up to it usually is not.
Why fatigue matters in engineering
Fatigue is one of the most common structural and mechanical failure mechanisms because repeated loading is everywhere.
Typical fatigue-critical applications include:
bridges under traffic
offshore and marine structures under wave loading
crane and lifting structures under repeated operating cycles
rotating shafts, gears, and bearings
pressure components under pressure fluctuations
welded frames and supports under vibration
hot components exposed to thermal cycling
In all of these cases, the key question is not only whether the part is strong enough. The real question is whether it will remain crack-free and functional over the required service life.
Main types of fatigue
Fatigue is not one single case. The mechanism changes depending on load amplitude, strain level, temperature, environment, and detail type.
High-cycle fatigue (HCF)
High-cycle fatigue usually involves a very large number of cycles with primarily elastic response.
Typical examples:
rotating shafts
vibrating equipment
turbine components
machine parts under repeated moderate stress
High-cycle fatigue is commonly evaluated using stress-life, or S-N, methods.
Low-cycle fatigue (LCF)
Low-cycle fatigue involves fewer cycles but larger strain ranges, often with local plastic deformation.
Typical examples:
startup-shutdown events
severe thermal transients
pressure and temperature swings
highly loaded local details near yield
Low-cycle fatigue is typically assessed using strain-life methods.
Thermal fatigue
Thermal fatigue is caused by repeated heating and cooling, which creates expansion and contraction stresses.
Typical examples:
exhaust systems
heat exchangers
power plant components
restrained hot structural details
Corrosion fatigue
Corrosion fatigue occurs when cyclic loading and a corrosive environment act together. Corrosion accelerates crack initiation and can increase crack growth rates.
Typical examples:
offshore structures
marine equipment
pipelines
process equipment in aggressive environments
Marine structures are exposed to both cyclic wave loading and corrosive environments, making corrosion fatigue a critical design and inspection concern.
Fretting fatigue
Fretting fatigue occurs at contacting surfaces that experience small relative motion under cyclic loading.
Typical examples:
bolted joints
interference fits
clamped assemblies
blade roots and mechanical interfaces
What causes fatigue failure?
Fatigue failure is usually caused by a combination of repeated loading and a vulnerable local detail.
1. Cyclic loading
This is the fundamental requirement. Without repeated loading, there is no fatigue problem.
Sources include:
vibration
rotating bending or torsion
traffic loads
wave loading
pressure fluctuations
thermal cycling
repeated lifting or operational transients
2. Stress concentrations
Fatigue cracks do not usually start in the middle of a smooth, well-designed, evenly stressed surface. They start where local stress is amplified.
Common stress raisers include:
holes
notches
sharp corners
abrupt thickness changes
attachment terminations
misalignment
poor weld geometry
3. Welded detail quality
In structural engineering, fatigue frequently becomes a welded-detail issue rather than a base-metal issue.
Poor fatigue performance is often linked to:
undercut
poor toe transition
misalignment
attachment details that interrupt load flow
weld profile irregularities
local fabrication defects
If this is a recurring issue in your projects,weld fatigue challengesis the natural follow-on read.
4. Surface condition and defects
Surface scratches, corrosion pits, rough machining marks, inclusions, voids, and porosity all make crack initiation easier.
5. Environment
Temperature, moisture, corrosion, and aggressive process conditions can reduce fatigue resistance and accelerate crack growth.
6. Unrealistic load assumptions
Many fatigue problems are analysis problems before they become structural problems.
If the load history is wrong, the fatigue result will be wrong. Common mistakes include:
underestimating the number of cycles
ignoring dynamic amplification
simplifying duty cycles too aggressively
missing thermal transients or startup conditions
overlooking local vibration effects
Key fatigue terms engineers use
A fatigue article that never defines the basic cycle terms is not very useful, so here are the essentials.
Stress range
The difference between maximum and minimum stress during one cycle.
Stress amplitude
Half of the stress range.
Mean stress
The average stress in the cycle.
Stress ratio (R)
The ratio of minimum stress to maximum stress in a cycle.
Different cyclic stress histories produce different stress ranges, amplitudes, and mean stresses, all of which affect fatigue life.
These terms matter because fatigue life depends not only on peak stress, but on how the load repeats.
How engineers calculate fatigue life
There is no single universal fatigue formula. The correct method depends on the material, detail category, load type, number of cycles, strain level, and whether a crack already exists.
S-N curve method
The S-N curve relates stress level to the number of cycles to failure.
This is one of the most common methods for fatigue assessment, especially for:
high-cycle fatigue
welded structures
detail-class-based verification
components where stress-life data is available
In an S-N approach, higher stress ranges mean fewer cycles to failure.
Miner’s rule is widely used because it is practical, but it is still a simplification. Load sequence effects, environment, and nonlinear behavior can make real fatigue damage more complex than a linear summation suggests.
Crack-growth methods
If a crack or crack-like flaw already exists, the problem changes.
Now the key question is not when a crack will initiate, but how fast it will grow and when it will become critical.
That is where fracture mechanics and crack-growth methods become relevant.
Where FEA fits into fatigue analysis
FEA does not replace fatigue methodology. It provides the stress and strain results that fatigue assessment depends on.
A practical fatigue workflow often looks like this:
build the model
define realistic loads, constraints, and operating cases
identify hot spots and fatigue-critical details
extract stresses or strains for the governing cycles
apply the correct fatigue method or code-based evaluation
review life, damage, usage factor, or safety margin
This is where a software SDC Verifier becomes useful. The value is not only seeing contour plots. The value is connecting FEA results to fatigue verification workflows engineers actually use for real structures and real standards.
A few well-known failures made fatigue impossible for engineers to ignore.
de Havilland Comet
Repeated pressurization cycles contributed to fatigue cracking around stress concentrators in the fuselage. The case became a defining lesson in fatigue-sensitive detail design.
Alexander L. Kielland platform
A fatigue crack initiated at a welded detail and grew under offshore wave loading until it contributed to catastrophic structural failure. The case remains a classic example of how local detail quality can govern global structural safety.
Aloha Airlines Flight 243
Repeated flight cycles and accumulated damage led to fatigue cracking in fuselage lap joints. The event pushed the industry toward stronger aging-aircraft inspection and fatigue-management practices.
These cases matter for one reason: fatigue failures rarely begin as “big structural problems.” They usually begin as local detail problems that were underestimated, missed, or poorly managed.
How to identify fatigue failure
Engineers typically identify fatigue failure by combining service history, geometry review, and fracture evidence.
Common signs include:
repeated-load service conditions
a crack origin at a stress concentrator
beach marks or striations in the crack-growth region
limited overall plastic deformation before final fracture
a final rapid-fracture zone after long crack growth
Example of visible cracking in a steel structural detail. In service, cracks like this require immediate inspection and root-cause assessment; fatigue is one possible mechanism, but the image alone does not confirm it.
In service, early warning signs may also include:
unexpected vibration changes
recurring cracking at the same detail
local corrosion or surface damage at hot spots
NDT indications at weld toes, holes, or transitions
How to prevent fatigue failure
Fatigue prevention is mostly about eliminating easy crack starters and reducing damaging stress cycles.
Improve geometry
Better fatigue performance usually starts with better load paths.
Good practice includes:
smooth transitions
larger radii
fewer abrupt section changes
reduced eccentricity
cleaner attachment details
Design welded details carefully
For welded structures, fatigue often depends more on the detail than on the nominal plate strength.
Focus on:
weld profile quality
attachment termination details
alignment
toe transition quality
appropriate detail classification under the governing design standard
Reduce stress range
For fatigue, lowering the repeated stress range is often more valuable than only reducing the maximum static stress.
Improve surface condition
Machining quality, polishing, shot peening, and other surface treatments can improve fatigue resistance when correctly applied.
Control manufacturing quality
Fatigue-critical parts should be checked for:
undercut
lack of fusion
porosity
dimensional deviation
surface damage
misalignment after fabrication
Use inspection and monitoring
For critical assets, design is only one layer of protection. The rest is life management.
Fatigue is not just repeated stress. It is a crack-driven failure mechanism that develops over time and can destroy components that look safe in a purely static analysis.
If a structure or component sees repeated loading, the real engineering questions are:
where will a crack start?
how fast will it grow?
how many cycles can the detail survive?
what design, analysis, and inspection measures are in place to stop failure before it becomes critical?
That is the mindset required for real fatigue assessment.
If your workflow depends on FEA-based structural verification, fatigue should not sit outside the process. It should be built into it.
SDC Verifier helps integrate fatigue assessment directly into the FEA workflow engineers already use, making it easier to identify critical details, evaluate fatigue life, and verify designs against the standards that matter in practice.
Need to check fatigue directly in your FEA workflow?
Use SDC Verifier to identify fatigue-critical details, evaluate fatigue life, and verify structures against the standards used in real engineering practice.
Fatigue is the gradual accumulation of damage under repeated loading, usually leading to crack initiation, crack growth, and eventual fracture.
What is fatigue failure in engineering?
Fatigue failure is the fracture of a component after repeated loading causes a crack to initiate and grow over time.
What are the three stages of fatigue failure?
The three stages are crack initiation, crack propagation, and final fracture.
Can fatigue failure happen below yield strength?
Yes. That is one of the defining features of fatigue. Repeated loading can cause failure even when each individual cycle is below yield or well below ultimate strength.
What causes fatigue cracks to start?
Fatigue cracks usually start at stress concentrations such as weld toes, holes, notches, thread roots, corrosion pits, surface defects, or other local weaknesses.
What is the difference between high-cycle and low-cycle fatigue?
High-cycle fatigue usually involves many cycles with mostly elastic behavior. Low-cycle fatigue involves fewer cycles with larger strain ranges and often local plastic deformation.
How do engineers predict fatigue life?
Engineers use methods such as S-N curves, strain-life analysis, Miner’s rule for cumulative damage, crack-growth methods, and FEA-based stress evaluation.
How do engineers prevent fatigue failure?
They reduce stress concentrations, improve geometry and weld details, use realistic load histories, improve manufacturing quality, and apply inspection and residual life assessment where needed.
