Why 10% More Load Halves the Service Life
It is one of the most uncomfortable numbers in fatigue analysis: for notched, unwelded regions of a component, the calculated service life is halved when the load increases by just 10%. Not by 10%, not by 20% — by half.
Anyone sizing components should keep this figure in mind. It explains why a seemingly small change in load can tip a product from the safe into the critical range, and why knowing the actual operating loads matters so much.
Where Does the Factor of 2 Come From?
The relationship between load and the number of load cycles that can be sustained is described by S-N curves (Wöhler curves). On a log-log scale these curves are straight lines — so the relationship between load and life is not linear, but follows a power law.
The slope of that line determines how sensitively the service life reacts to changes in load. For notched, unwelded components it typically lies in a range where 10% more load roughly halves the number of cycles that can be sustained. The reverse is equally true: 10% less load can double the service life.
What This Means for Your Design
Two consequences follow immediately:
The loads have to be right. A key aspect of any fatigue analysis is knowing what the component actually experiences over time in terms of forces, accelerations and temperatures. If that load assumption is 10% too low, the calculated service life is too optimistic by a factor of two.
Being too conservative is no solution either. Setting the loads too high to be safe over-sizes the product — more material, more weight, more cost. The lever works in both directions.
The Influencing Factors Behind It
A sound fatigue analysis accounts for more than just the magnitude of the stress. The assessment includes, among others:
- notch support effect through stress redistribution
- surface roughness
- mean stress effect
- technological size effect
- survival probability
Only these factors together give a realistic statement about safety and utilisation — presented as colour utilisation plots or in tabular form for the individual load cases.
Conclusion
The 10% rule is not an exotic edge case but a direct consequence of the S-N curve characteristic. It shows why care over load assumptions weighs so heavily in fatigue analysis: a small error in the load becomes a large error in the service life.
Want to know how this affects a specific component? Get in touch — we can support you with fatigue analysis.
