Doubt on Fatigue Strength S-N diagrams

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SUMMARY

The discussion centers on the relationship between fatigue strength and yield strength in S-N diagrams, specifically for steel components. It is established that the fatigue strength (Sn) is plotted against the number of cycles, with a common practice of setting Sm at 0.9 Sut. The contributor, Abhishekdas, confirms that if the allowable tensile stress (Sta) based on the tensile yield strength (Sy) is lower than the fatigue strength, then Sta becomes the governing factor in design considerations. This highlights the importance of yield strength in assessing failure modes before reaching fatigue limits.

PREREQUISITES
  • Understanding of S-N (fatigue strength vs number of cycles) diagrams
  • Knowledge of tensile yield strength (Sy) and ultimate tensile strength (Sut)
  • Familiarity with the concept of allowable tensile stress (Sta) and yield factor of safety (FSy)
  • Basic principles of material fatigue and failure analysis
NEXT STEPS
  • Research the calculation of allowable tensile stress (Sta) in engineering design
  • Explore the significance of yield factor of safety (FSy) in material selection
  • Study the implications of fatigue strength (Sn) in component life prediction
  • Investigate advanced S-N curve analysis techniques for different materials
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Mechanical engineers, materials scientists, and design engineers involved in fatigue analysis and failure prevention in structural components.

Abhishekdas
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A standard way of plotting an S-N (fatigue strength vs no of cycles) graph is to start off by taking a value of Sm (for N=1000) cycles and joining it to the point depicting Se(Endurance Limit). Now we commonly take Sm= 0.9Sut for steels. Now I was wandering, before you actually get to use the component for 1000 cyces at 0.9 Sut the component will yield at around 0.5Sut(before you reach Sm), as Sy(steel)≈.5Sut. SO should yielding be the cause of failure for values of stress rather than fatigue?
Thank you in anticipation.
 
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Abhishekdas: Yes, you are correct. I.e., if the allowable tensile stress (Sta) based on tensile yield strength (Sta = Sy/FSy) is less than the fatigue strength (Sn), then Sta governs over Sn, in which case you would use Sta, where FSy = yield factor of safety.
 

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