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Unformatted text preview: 1 1 Lecture 22 Fatigue 3 2 Δε plastic Δσ/ Ε Δσ σ ε ε pl a N f b = C 2 Low-cycle fatigue Δ σ pl N f b = C 2 ' or Coffin-Manson 3 High-cycle fatigue - effect of mean stress 4 Alternative plot of high-cycle fatigue data 5 Normalize data ...... 6 Example AISI 1015 steel a =0.14, C 1 =886 MPa, σ Y = 227 MPa, σ u = 415 MPa 1. Maximum value of stress range applied at a mean stress of a) 100 MPa and (b)- 80 MPa to give a life-time of N f = 10 6 cycles? 2. Find life-time if σ min = 0 MPa, σ max = 200 MPa? VK250 Lecture 22 1 E MPIRICAL FITS TO S-N CURVES ( FOR σ M = 0) 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. ! pl a = ! a " # a / E • If ! a / E << " pl a then ! pl a N f b = C 2 or ! " 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 : ! 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,...
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This note was uploaded on 09/15/2010 for the course MATSCIE 250 taught by Professor Yalisove during the Fall '08 term at University of Michigan.
- Fall '08