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Materials Engineering – Day 4

Materials Engineering – Day 4. Finish Fracture including the example problem Discussion of Fatigue Failure. You need to be able to:. State what is measured in a fatigue test and list two reasons fatigue is important to designers.

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Materials Engineering – Day 4

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  1. Materials Engineering – Day 4 • Finish Fracture including the example problem • Discussion of Fatigue Failure

  2. You need to be able to: • State what is measured in a fatigue test and list two reasons fatigue is important to designers. • Name two factors that increase fatigue life and two that decrease fatigue life. • Define the fatigue limit (endurance limit), state which materials exhibit this limit, and describe how a designer would use the information.

  3. Classic Example – Comet (first commercial Jetliner) http://en.wikipedia.org/wiki/De_Havilland_Comet

  4. Recent Classic Example – Aloha Airlines Flight 243

  5. What’s Driving Fatigue? Cyclic Stress. • The part is subject to stresses that vary cyclically. Part of the cycle is tension. • Cracks initiate and propagate. • When the crack reaches the critical length, the part fails in a way predicted by fracture mechanics (KI>KIC). • This failure stress can be below the yield strength.

  6. Gross Appearance of Fatigue Failure • Here is a conrod that failed due to fatigue. Note the fatigued area and the fast fracture area.

  7. How to recognize fatigue in a broken piece • Beach marks on the fracture surface x120 Indications of loading changes • Striations X700 (Show indiv. Cycles)

  8. Importance of Fatigue • Fatigue is a major killer. It is an automatic suspect in almost any failure. • Fatigue appears in subtle ways, eg. Thermal cycling. • The use of rational, accepted design procedures against fatigue is absolutely essential. (Subject of a later course.)

  9. Fatigue Testing We need something that does cycles. Here is the rotating beam test. (There are other tests as well.) We get cycles to failure (N) at a corresponding stress amplitude, S. We plot these on a curve.

  10. The S-N curves Curves are based on widely scattered data! Here is SN curve for a typical steel. Note: There is an endurance limit. To the right is SN curve for typical aluminum alloy. Sorry! No endurance limit.

  11. S-n curve from the Homework

  12. What’s done with the curves • The curves provide a starting point for rational fatigue design. In particular they are useful where “high-cycle” fatigue failure is possible. • If the cyclical stress is superimposed on a mean stress, this mean stress is also important and should be accounted for (R ratio). • You will be introduced to the methodology in a later course. • Be aware of the statistical scatter on these curves. You CAN get failure at a stress below the curve. There is always a probability of failure, but there are ways of making that probability acceptably small.

  13. Ways to Reduce Danger of Fatigue • Keep stresses below the endurance limit. • Avoid stress concentrations. They may not hurt much in the static loading case, but are deadly in the cyclic loading case. • Use surface treatments. • Carburizing or Nitriding makes the steel harder (i.e. stronger) on the surface plus it introduces residual compressive surface stresses. • Another method: shot peening. • Another method: polishing

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