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Unformatted text preview: ME 382 Lecture 27 1 E MPIRICAL FITS TO S-N CURVES ( FOR σ M = 0) (continued) Low-cycle fatigue: • Used for analyses where nominal stress exceeds yield or for detailed calculations where one computes effects of plasticity around notches • Effects of plastic strain dominate fatigue behavior • Plastic-strain range = total strain range - Δσ / E i.e. ε pl a = ε a − σ a / E • If σ a / E << ε pl a then ε pl a N f b = C 2 or Δ σ pl N f b = C 2 ' Coffin-Manson Law where, b , C 2 are empirical material constants; b ≈ 0.5 Example: • Coffin-Manson Law for a 1015 steel : ε pl a N f 0.59 = 0.50 • Assume paper clip made from this steel of diameter d . • Estimate cycles to failure if bent back and forth & compare with experiments (i) Fold paper clip back and forth to same radius of curvature, R p (iii) Repeat until clip fails; count number of complete cycles until failure. ME 382 Lecture 27 2 (iv) Estimate maximum strain from diameter of wire and radius of curvature (For bending: ε = y/R ⇒ ε m ax = d /2 R p ) (v) Use as plastic strain in the Coffin-Manson equation given above....
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