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AS 4100:2020 Fatigue in SDC Verifier – How Section 11 Is Implemented
Read Full ArticleHow SODIMEC verified a 198 t ladle lifting beam with SDC for Ansys
Read Full StoryAnalysis of the cylinder system of grab unloader
All ProjectsAS 4100:2020 Fatigue in SDC Verifier – How Section 11 Is Implemented
Read Full Article
Yurii Shumak
Last updated: 24 Mar 2026
Fatigue strength and fatigue limit are related — but they are not the same thing, and treating them as interchangeable leads to unsafe designs.
Fatigue strength is the stress level that causes failure after a specified number of load cycles. The fatigue limit — often called the endurance limit — is the stress level below which some materials can withstand very large numbers of cycles without fatigue failure in practical design terms. Many ferrous alloys and many titanium alloys show this behaviour; aluminium alloys generally do not.
That distinction matters directly in how you design. For steels, engineers often check whether the stress stays below a corrected endurance limit. For aluminium, they design to a target fatigue life at a defined number of cycles using S–N data — because there is no safe plateau to aim for.
This article covers the concepts, formulas, symbols, and real material data you need to use both correctly. If you want step-by-step calculations, see our companion guide: How to Calculate Fatigue Strength (Hand Calculations).
| Term | What it means | Best used for |
|---|---|---|
| Fatigue strength | Stress level that causes failure after a specified number of cycles | Finite-life design |
| Fatigue limit / endurance limit | Stress threshold below which some materials can survive very large numbers of cycles without fatigue failure | Infinite-life or endurance-based checks |
| S–N curve | Plot of stress amplitude versus number of cycles to failure | Reading fatigue life across materials |
| Basquin equation | Stress-life relationship used in the high-cycle regime | Estimating fatigue strength at a given cycle count |
| Goodman equation | Mean-stress correction model | Adjusting allowable stress amplitude when mean stress is present |
All fatigue strength values below are at 107 cycles unless otherwise noted.
| Material | Fatigue strength at 107 cycles | Fatigue limit / endurance limit | Design takeaway |
|---|---|---|---|
| Plain carbon steel | ~340 MPa | ~0.4–0.5 × UTS | Useful endurance-limit checks, but modifiers matter |
| High-strength steel | 700 MPa or higher | Often well-defined | Strong fatigue performance, but notch and surface sensitivity still govern |
| Aluminium alloy (7075-T6) | ~210 MPa | No distinct endurance limit | Design to finite life using S–N data |
| Aluminium alloy (6061-T6) | ~140 MPa | No distinct endurance limit | No safe plateau; every design needs a target service life |
| Austenitic stainless steel | 300–650 MPa | Grade-dependent; often no clean classical limit | Use grade-specific data, not generic stainless values |
| Duplex stainless steel | Grade-dependent | Plateau-like behavior may appear | Good option where corrosion fatigue matters |
| Ti-6Al-4V (annealed) | 450–590 MPa | Often defined | High strength-to-weight ratio, but route-sensitive |
| Ti-6Al-4V (EBM, as-built / stress-relieved) | 200–250 MPa | Process-dependent; use test data | Manufacturing route materially changes fatigue performance |
| Ti-6Al-4V (HIP post-processed) | 550–600 MPa | Process-dependent; use test data | Post-processing can transform fatigue performance |
Fatigue strength answers a practical question: how much cyclic stress can this material survive for N cycles?
If a material has a fatigue strength of 200 MPa at 10^6 cycles, that means 200 MPa is associated with failure at around one million cycles under the specified test conditions. Change the surface finish, the stress ratio, the loading mode, the geometry, or the environment, and the number changes.
That is exactly why fatigue test standards such as ASTM E466 tightly control specimen geometry, loading, and surface condition.
Fatigue strength is always a finite-life value. It decreases as the target cycle count increases, which is why it must always be quoted alongside a cycle count to be meaningful.
The fatigue limit is the stress threshold below which a material is treated as having no fatigue failure at very large cycle counts. For steels, this plateau typically appears clearly in the S–N curve somewhere between 106 and 107 cycles. For aluminium, it does not appear at all.
Important nuance for very-high-cycle fatigue work: The idea of a truly infinite fatigue life is debated in research for some metals. In very-high-cycle fatigue (VHCF) studies, even some steels have shown failures beyond 108–109 cycles under specific conditions. For standard design work, the endurance-limit concept remains useful when it is supported by the material system, test data, and the relevant standard — but it is not an unconditional guarantee.
The simplest way to remember both terms:
An S–N curve, also called a Wöhler curve, plots stress amplitude against the number of cycles to failure. It is the core map engineers use to interpret fatigue performance across the full life range.
As stress amplitude goes down, cycles to failure go up. For steels and titanium, the curve eventually flattens; that flat region is the endurance limit. For aluminium, the curve keeps descending, which means no safe stress threshold exists and every design must target a specific service life.
Key regions on an S–N curve:
There is no single universal fatigue-strength formula that works for every case. Engineers use material S–N data together with the loading condition, mean stress treatment, and design method. The most commonly used equations are below.
The lab test value must be reduced by several factors before it applies to a real component:
When a non-zero mean stress is present, the allowable stress amplitude is reduced:
For full worked examples using these formulas, see: How to Calculate Fatigue Strength (Hand Calculations).
Not all metals behave the same under cyclic loading. Generic statements like “the fatigue limit is X” are frequently wrong outside carefully controlled lab conditions. Here is what the data actually shows.
For many steels, a common engineering rule of thumb is that the endurance limit is about 0.5 × UTS for smooth, polished laboratory specimens — up to a practical cap near 700 MPa. But that shortcut is only a starting point.
The real-component gap: Notches, surface finish, loading mode, size, residual stress, corrosion, and reliability requirements all reduce what a real component can carry in service. A polished rotating-bending specimen is not a welded bracket, a drilled plate, or a corroding offshore detail. “Steel has a fatigue limit” is only partly true — the usable endurance level in a real structure is always lower than the lab value.
Alloy additions improve performance. Adding chromium and molybdenum refines the grain structure and raises fatigue strength. Sulfur and phosphorus inclusions act as crack initiation sites and should be minimised in fatigue-critical applications. Heat treatments such as quenching and tempering, and surface treatments such as shot peening, can raise the effective endurance limit significantly.
Aluminium is where many engineers make the most consequential mistake.
Aluminium alloys do not exhibit a conventional endurance limit. Published very-high-cycle fatigue research shows that aluminium alloys can still fail beyond 107 cycles and even beyond 109 cycles. In studies of 7075-T6 and 6061-T6, fatigue strength at 107 cycles was approximately 210 MPa for 7075 and 140 MPa for 6061, with no endurance limit observed up to 109 cycles.
So for aluminium, the right design question is not “what is the fatigue limit?” but rather “what stress amplitude is acceptable for the target service life?”
Many titanium alloys show fatigue-limit-type behaviour — one reason they are used in aerospace, biomedical implants, and high-performance motorsport. However, titanium is highly sensitive to alloy condition, manufacturing defects, and post-processing.
Published NIST data on Ti-6Al-4V produced by electron beam melting is instructive:
That is more than a twofold difference from the same alloy, differing only in processing route. The lesson is clear: with titanium, the processing route is not a side note — it is a core part of the fatigue property.
“Stainless steel” is too broad a category to discuss as a single fatigue class.
The correct approach: use grade-specific fatigue data, not a generic “stainless steel fatigue limit.”
The fatigue value in a datasheet or handbook is not the number your component automatically achieves. Every factor below reduces the effective fatigue strength relative to the laboratory specimen.
Fatigue cracks almost always initiate at or near the surface. Rougher surfaces create stronger local stress concentrations and shorten life. Open-access research on aluminium confirms that improving surface quality measurably raises both fatigue strength and the effective fatigue limit.
Surface treatments that help: shot peening introduces compressive residual stresses that oppose crack opening; nitriding and carburizing create a harder surface case that resists crack initiation — commonly applied to gear teeth and crankshafts.
Sharp transitions, holes, weld toes, cutouts, and notches raise local stresses significantly above the nominal applied stress. A smooth test coupon and a real bracket do not fail the same way, even from identical material. The fatigue notch factor kf accounts for the sensitivity of the material to these features in the Marin correction.
Axial, bending, torsion, and combined loading do not produce the same fatigue response. Mean stress and stress ratio (R = σmin/σmax) shift the result — a tensile mean stress reduces the allowable stress amplitude, while a compressive mean stress can improve fatigue life. ASTM E466 explicitly notes that geometry, surface condition, stress state, and testing conditions all affect fatigue resistance.
The practical approach depends on the material and the geometry:
Real fatigue design means combining material data with geometry, loading history, stress concentration factors, weld classification, and the applicable standard. That is the difference between quoting a material property and doing fatigue engineering.
Fatigue analysis always incorporates safety factors to account for material variability, load uncertainty, and scatter in test data. A component expected to see 200 MPa cyclic stress might be designed to withstand 250 MPa — a safety factor of 1.25 on stress. For critical applications such as aircraft structures, safety factors of 2.0–4.0 on life (not stress) are common.
Hand calculations are useful for quick estimates, understanding the mechanics, and sense-checking results. They are not enough when:
The SDC Verifier performs code-compliant fatigue checks per DNV-RP-C203, Eurocode 3, DIN 15018, and more. The built-in Weld Finder tool automatically detects welded plates and classifies weld details — no manual S–N category selection needed.
Explore the Fatigue Checks
Fatigue strength is the stress level at which a material fails after a specified number of loading cycles. It is a finite-life value — not an infinite-life guarantee — and must always be quoted alongside a cycle count to be meaningful. It decreases as the target cycle count increases.
Fatigue strength refers to failure at a specified cycle count — it is finite. Fatigue limit refers to a stress threshold below which some materials can survive very large numbers of cycles without fatigue failure in engineering practice. Not all materials have a fatigue limit: aluminium typically does not; most steels and titanium alloys do.
In most engineering contexts, yes — the terms are used interchangeably. Terminology can vary by author and material system, so the safest approach is always to define the term clearly in context, particularly in reports and standards submissions.
Plain carbon steel typically shows a fatigue strength of around 340 MPa at 107 cycles, with an endurance limit of roughly 0.4–0.5 × UTS. High-strength steels can exceed 700 MPa at 107 cycles. However, these are laboratory values for polished specimens — real components with notches, welds, and surface roughness will have lower usable endurance levels.
Usually no. Aluminium alloys typically do not exhibit a conventional endurance limit — their S–N curves continue to descend past 109 cycles in high-cycle fatigue research. For 7075-T6 and 6061-T6, no endurance limit was observed up to 109 cycles. Designers must therefore target a finite service life using S–N data, often using the fatigue strength at 5 × 108 cycles as a conventional reference value.
Many steels do, especially in controlled laboratory testing. But the usable endurance level in a real component is lower once you account for notches, size, surface finish, loading mode, and environment. The 0.5 × UTS rule is a starting point, not a component-level guarantee.
The most commonly used equation is the Basquin equation: σa = σf′ · (2Nf)b. For mean stress correction, the modified Goodman equation is standard: σa / Se + σm / UTS = 1. The corrected endurance limit Se is found using the Marin equation, which multiplies the lab value by factors for surface finish, size, load type, temperature, and reliability. There is no single universal formula — the right approach depends on material, loading, and design standard.
Common symbols: σf′ (fatigue strength coefficient, Basquin equation), Se (corrected endurance limit), SN or Sf (fatigue strength at N cycles), b (fatigue strength exponent). Eurocode 3 uses ΔσC for the fatigue class reference stress at 2 × 106 cycles.
No. Ultimate tensile strength (UTS) measures resistance to a single monotonic load. Fatigue strength measures resistance to repeated cyclic loads, which can cause failure at stress levels well below the UTS — sometimes as low as 30–50% of UTS for aluminium, or 40–50% for steel. The ratio of endurance limit to UTS is called the fatigue ratio or endurance ratio.
Titanium alloys offer competitive fatigue strengths (450–590 MPa at 107 cycles for Ti-6Al-4V annealed) at roughly 56% of steel’s density. Both materials show a defined endurance limit. However, titanium is strongly sensitive to processing route — NIST data shows Ti-6Al-4V produced by electron beam melting achieves 200–250 MPa as-built but 550–600 MPa after HIP post-processing. Processing is not a secondary consideration; it is part of the fatigue property.
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