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Unformatted text preview: 1 1 Lecture 21 Fatigue 2 2 Terminology for fatigue 3 “Tensiontotension” 4 “Zerototension” 5 “Tensile mean stress” 6 “Completelyreversed” 2 7 “Compressive mean stress” 8 Fatigue strength: Maximum stress amplitude (range) for a specified life time Fatigue limit Stress amplitude (range) below Endurance limit ( σ e ) : which fatigue does not occur Fatigue life time ( S N curves) 9 Basquin’s Law Use for fullyreversed (mean stress = 0), highcycle fatigue σ ar N f a = C 1 σ ar = AN f B or σ ar is fullyreversed stress amplitude N f is fatigue life (cycles to failure) a & C 1 are material constants (empirical fits to S/N curves) 10 Examples AISI 1015 steel (0.15% C): a = 0.14, C 1 = 886 MPa, σ Y = 227 MPa If σ m = 0, find fatigue life for σ a = 100 MPa Δσ = 400 MPa σ a = 226 MPa If σ m = 0, find fatigue strength for N f = 10 7 cycles N f = 10 5 cycles VK250 Lecture 21 1 C RACK INITIATION IN FATIGUE • Fatigue cracks nucleate under cyclic loading because of local yield (microscopic or macroscopic) • Lowcycle fatigue : Plastic strains are significant • Highcycle fatigue: Elastic strains dominate • Yield...
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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
 YALISOVE

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