IntermediateTopic 68 of 106Engineering Mechanics
Fatigue
Repeated cyclic loading causes cracks at stresses below yield; described by S-N curves.
In depth
Cyclic loading causes crack initiation and growth at stresses well below yield. The S-N curve gives life versus stress amplitude; many steels have an endurance limit. Mean stress, surface finish, size and notches reduce strength (Goodman or Gerber diagrams).
Key points
- Beach marks on fracture surfaces
- Shot peening extends life
- High-cycle vs low-cycle fatigue
ExampleAircraft wings and rail axles are inspected for fatigue cracks.
Fatigue causes most mechanical failures in machines and aircraft, so it must be designed for, even when the peak load is safe.
What is Fatigue?
Repeated cyclic loading causes cracks at stresses below yield; described by S-N curves.
Why is it important to understand fatigue?
Fatigue causes most mechanical failures in machines and aircraft, so it must be designed for, even when the peak load is safe.
What is an S-N curve?
A plot of stress amplitude against the number of cycles to failure. It shows the fatigue strength and whether there is an endurance limit for the material.
Why can a part fail below its yield strength?
Repeated loading makes small cracks grow with each cycle, so failure can happen far below the static strength.
How is Fatigue used in practice?
Typical applications include estimating the life of a rotating shaft under reversed bending, designing a suspension arm that sees millions of load cycles and predicting when a welded frame will crack under vibration.
What should I learn before Fatigue?
Study "Stress concentration" first (topic 67 of 106 in Engineering Mechanics). It covers the groundwork this topic builds on.
What are the key points of Fatigue?
Beach marks on fracture surfaces. Shot peening extends life. High-cycle vs low-cycle fatigue.
Can you give an example of Fatigue?
Aircraft wings and rail axles are inspected for fatigue cracks.
What comes after Fatigue?
The next topic is "Fracture mechanics", which builds directly on this one.
How difficult is Fatigue?
It is classed as Intermediate in Engineering Mechanics (topic 68 of 106). It is an intermediate topic. Work through the beginner topics in Engineering Mechanics first.