Machine element life, S-N curve, and fatigue guide
Set up a fatigue approach for shafts, bolts, welds, and bearings under repeated loading.
This guide is prepared for quick engineering review. Critical design decisions still require the relevant standard, supplier data, and engineering approval.
Problem / Objective
A part may look safe under static load and still crack under repeated load. Fatigue analysis combines load cycle, stress range, and notch effect.
Assumptions
- Load cycle and mean stress are approximately known.
- Material S-N data is available.
- Surface, size, and notch effects are handled with correction factors.
Step by step method
- Define load cycle and stress range.
- Select material fatigue data.
- Add notch, surface, and size corrections.
- Evaluate fatigue safety factor separately.
- Write inspection or test plan for the critical area.
Common mistakes
- Assuming static safety factor is enough for fatigue.
- Forgetting notch effect at weld toes.
- Treating surface roughness as independent from fatigue strength.
Related calculators
Related glossary terms
Quick FAQ
Question: What is fatigue?
It is failure by crack initiation and growth under repeated loading.
Question: What does an S-N curve show?
It shows the relation between stress level and cycle life.
Question: Where is fatigue usually critical?
Notches, section changes, weld toes, and surface defects are common critical regions.
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Glossary
Fatigue
Fatigue is a core engineering term. Definition, usage notes, and a practical example.