Is there an exact cycle count that separates high-cycle from low-cycle fatigue?

It's a conventional guideline rather than a hard physical boundary — commonly cited as roughly 10^4 to 10^5 cycles. Below that range, the stress amplitude needed to fail in so few cycles is usually high enough to cause meaningful plastic strain each cycle (low-cycle behavior); above it, stress amplitudes are typically low enough that strain stays elastic (high-cycle behavior). The real, physical dividing line is whether plastic strain per cycle is significant, not the cycle count itself — the count is just the usual symptom.

Do all metals have a true endurance limit for high-cycle fatigue?

No. Many ferrous alloys (plain carbon and low-alloy steels) show a fairly distinct S-N curve plateau, treated as an endurance limit below which life is effectively infinite. Most non-ferrous metals — aluminum, copper, and their alloys among them — do not show a true plateau; their S-N curves keep sloping gently downward indefinitely, so fatigue design for these materials specifies an allowable stress at a defined finite life (e.g., 10^8 cycles) rather than a true infinite-life limit.

Why does low-cycle fatigue use strain instead of stress as the controlling variable?

Once a material yields every cycle, stress amplitude stops varying much with additional damage — it saturates near the material's cyclic yield strength regardless of how much plastic strain is actually being imposed. Strain amplitude, particularly the plastic strain component, keeps increasing with more severe loading and correlates far better with the actual cycle-by-cycle damage (dislocation motion, microstructural degradation) that drives crack initiation in this regime.

Can a single component experience both high-cycle and low-cycle fatigue?

Yes, and this combination — often called high-cycle fatigue superimposed on low-cycle fatigue, or HCF/LCF interaction — is a real design concern in turbomachinery. A turbine blade sees relatively few large low-cycle stress excursions from engine startup and shutdown, plus millions of small high-cycle vibratory stress cycles superimposed on top from blade resonance during steady operation. Each mechanism has to be evaluated, and their combined effect assessed, rather than treating the component as purely one regime or the other.

Does the Goodman diagram apply to low-cycle fatigue?

The modified Goodman criterion is a mean-stress correction built for the high-cycle, stress-life framework, and it assumes the material response stays elastic. It is not the appropriate tool for low-cycle fatigue, where the analysis needs to account for cyclic plasticity directly — strain-life methods (Coffin-Manson, or a combined strain-life equation with both elastic and plastic terms) are used instead.

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Concept Explainer · Materials Science

High-Cycle vs. Low-Cycle Fatigue

Not "how many cycles until failure" — that's the symptom. The real split is whether the material plastically deforms every single cycle.

Fatigue itself is cyclic-stress-driven crack growth — a completely different mechanism from creep, which is steady-stress deformation. But fatigue itself splits into two regimes that behave, and are analyzed, very differently. High-cycle fatigue (HCF) runs at low stress amplitudes, well within the material's elastic range, and takes a large number of cycles — typically more than 10⁴–10⁵ — to initiate and grow a crack to failure. Low-cycle fatigue (LCF)runs at high stress amplitudes that push the material past yield on every cycle, generating a small amount of plastic strain each time, and it fails in far fewer cycles as a direct result. The dividing question isn't "how many cycles will this take" — that's the outcome. It's "is the strain each cycle elastic, or does it include a plastic component," and that determines which design method actually applies.