Aluminum extrusion fatigue strength requirements?

Fatigue failure often appears without warning. Many buyers focus on yield strength and miss fatigue. This gap leads to cracks, downtime, and high replacement cost.

Aluminum extrusion fatigue strength depends on alloy, temper, surface quality, and load pattern. In most industrial uses, fatigue strength is far lower than static strength and must be checked early in design.

Many projects fail not because aluminum is weak, but because fatigue behavior is ignored. Understanding fatigue early helps avoid redesign, delays, and safety risks.

What is the typical fatigue strength for extrusions?

Fatigue strength is not a single fixed number. It changes with alloy, temper, surface finish, and stress cycles. Designers often expect a clear value, but aluminum does not behave like steel in fatigue.

Typical aluminum extrusion fatigue strength ranges from 30 MPa to 100 MPa at 10 million cycles, depending on alloy and temper. There is no true endurance limit for aluminum.

This means fatigue damage keeps building as cycles increase, even at low stress.

Why aluminum has no endurance limit

Steel often shows a flat fatigue curve. Below a stress limit, it can survive infinite cycles. Aluminum does not behave this way.

For aluminum extrusions:

• Each stress cycle causes small damage
• Micro cracks grow slowly over time
• Failure can happen even under low stress

This makes cycle count critical.

Typical fatigue ranges by alloy family

Below is a general comparison used in early design. These are not guaranteed values. They help with screening only.

Alloy	Temper	Approx fatigue strength at 10^7 cycles (MPa)	Common use
6063	T5	30 to 50	Architectural, light frames
6061	T6	60 to 95	Structural, machinery
6082	T6	70 to 100	Heavy duty frames
7075	T6	90 to 130	Aerospace, high load

Surface condition can reduce these values by 20 percent or more.

Role of extrusion quality

Fatigue starts at weak points. In extrusions, these often include:

• Die lines
• Surface scratches
• Sharp corners
• Weld seams in hollow profiles

Good die design and process control reduce these risks. Smooth surfaces improve fatigue life more than increasing wall thickness in many cases.

Stress ratio matters

Fatigue strength depends on stress ratio. Fully reversed loading is more severe than one direction loading.

Designers must define:

• Maximum stress
• Minimum stress
• Mean stress

Ignoring this leads to unsafe assumptions.

Early design mistake to avoid

Many buyers only ask for tensile strength reports. This does not predict fatigue life. Fatigue strength is usually much lower than yield strength.

How does load cycling affect extrusion lifespan?

Fatigue failure is driven by repeated load, not one-time overload. Many extrusions fail under loads far below their rated strength because of cycling.

Load cycling reduces extrusion lifespan by creating micro cracks that grow with each cycle until sudden fracture occurs. Higher cycles and stress ranges shorten life sharply.

Understanding load patterns is more important than peak load.

What counts as a cycle

A cycle is one complete load change. This includes:

• Start and stop of machines
• Wind vibration
• Thermal expansion and contraction
• Repeated lifting or motion

Even small stress swings count.

S-N curve basics

Fatigue behavior is shown with an S-N curve:

• S = stress amplitude
• N = number of cycles to failure

For aluminum:

• High stress leads to fast failure
• Low stress leads to long life but not infinite

Designers often target a specific cycle life such as 2 million or 10 million cycles.

High-cycle vs low-cycle fatigue

There are two common fatigue zones.

Low-cycle fatigue

• High stress
• Plastic deformation
• Cycles usually below 100,000
• Common in seismic or shock loads

High-cycle fatigue

• Lower stress
• Elastic deformation
• Millions of cycles
• Common in machinery frames and supports

Most aluminum extrusions work in high-cycle fatigue.

Load direction and profile shape

Extrusions handle fatigue better when:

• Load paths are smooth
• Stress is evenly distributed
• No sudden section change exists

Poor designs include:

• Sharp notches
• Thin webs near holes
• Abrupt thickness change

Practical design adjustments

To extend fatigue life:

• Increase fillet radius
• Avoid sharp corners
• Use uniform wall thickness
• Reduce stress concentration

Small geometry changes often double fatigue life.

Hidden cycling sources

Some buyers only consider mechanical load. They forget:

• Temperature cycles
• Assembly stress
• Residual stress from straightening

These combine with service loads.

Real failure pattern

Fatigue cracks often start silently. They grow slowly. Then failure happens suddenly. There is often no visible warning until final break.

Which alloys offer superior fatigue resistance?

Not all aluminum alloys behave the same under fatigue. Alloy choice has a strong impact on service life.

6000 and 7000 series alloys offer better fatigue resistance than 3000 series, with 6061-T6 and 6082-T6 being common balanced choices for extrusions.

However, strength alone does not guarantee fatigue performance.

Why alloy chemistry matters

Fatigue resistance depends on:

• Grain structure
• Precipitation hardening
• Impurity control

Heat treatable alloys usually perform better.

Common extrusion alloys compared

Alloy	Fatigue behavior	Advantages	Limits
6063-T5	Low to moderate	Good surface, easy extrusion	Lower fatigue strength
6061-T6	Moderate to high	Good balance of strength and cost	Slightly harder to extrude
6082-T6	High	Stronger than 6061	Less surface quality
7075-T6	Very high	Excellent fatigue	Cost, corrosion risk

Why 6061-T6 is widely used

6061-T6 is often selected because:

• Stable fatigue data
• Good machinability
• Acceptable corrosion resistance
• Broad supplier availability

It is not the strongest, but it is predictable.

Role of temper

Temper changes fatigue behavior.

• T5: cooled from extrusion, lower fatigue
• T6: solution treated and aged, higher fatigue

A temper upgrade can raise fatigue strength without changing profile.

Welding impact

Welding reduces fatigue strength sharply.

• Heat affected zones soften
• Microstructure changes
• Cracks often start near welds

Designers should avoid welding in high fatigue zones or increase section size locally.

Surface treatment effects

Some treatments help, others hurt.

• Anodizing: may slightly reduce fatigue if thick
• Shot peening: can improve fatigue
• Polishing: improves fatigue

Surface control is critical.

Cost vs fatigue tradeoff

Higher fatigue alloys cost more. But replacement cost and downtime often cost more than material upgrade.

Are there standards for fatigue strength testing?

Fatigue testing must follow standards. Without standard methods, data cannot be compared or trusted.

Yes, aluminum extrusion fatigue testing is covered by ASTM, ISO, and EN standards that define specimen shape, load control, and cycle counting.

These standards guide both testing and design validation.

Why standards matter

Fatigue data varies widely. Standards ensure:

• Repeatable testing
• Comparable results
• Clear load definition

Buyers should always ask which standard was used.

Common fatigue standards

Below are widely used references.

Standard	Scope	Typical use
ASTM E466	Axial fatigue	Base material testing
ASTM E468	Presentation of fatigue data	Reporting format
ISO 1099	Axial fatigue	International reference
EN 1999	Aluminum design	Structural applications

Specimen vs real profile

Standard tests use smooth specimens. Real extrusions include:

• Corners
• Holes
• Weld seams

This means real fatigue strength is often lower than test values.

Component testing

For critical projects, component testing is recommended.

• Uses real profile
• Includes welds and joints
• Reflects real stress state

This is common in transport and heavy machinery.

Safety factors

Design standards apply fatigue safety factors. These account for:

• Manufacturing variation
• Surface damage
• Load uncertainty

Ignoring safety factors leads to premature failure.

Buyer checklist

When reviewing fatigue data, always confirm:

• Load ratio used
• Cycle count target
• Failure definition
• Specimen geometry

Many datasheets omit these details.

Design codes vs material data

Material fatigue data supports design codes. Design codes control final allowable stress.

Engineers must follow the design code, not only supplier data.

Conclusion

Fatigue strength controls long term safety in aluminum extrusions. Alloy choice, surface quality, load cycles, and standards all matter. Early fatigue planning reduces failure risk, redesign cost, and downtime.