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Unformatted text preview: ME 382 Lecture 26 1 C RACK INITIATION IN FATIGUE Fatigue cracks nucleate under cyclic loading because of local yield (microscopic or macroscopic) Low-cycle fatigue : Plastic strains are significant High-cycle fatigue: Elastic strains dominate Yield formation of slip bands parallel to shear stresses Low-cycle fatigue: macroscopic yield responsible for slip bands High-cycle fatigue: local stress exceeds yield at stress concentrations & microstructural defects (inclusions, scratches, etc. ) Stage 1: Repeated cycling nucleation of cracks along slip bands Stage 2: Cracks turn and grow perpendicular to applied stress ME 382 Lecture 26 2 Cracks then follow Paris law if stresses are elastic If a design subjected to cyclic loads, one must avoid stress concentrations to reduce possibility of fatigue crack initiation Basis of empirical design against fatigue is that fatigue life is dominated by crack initiation. (Alternative to fatigue design using Paris Law)...
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This note was uploaded on 09/08/2011 for the course MECHENG 382 taught by Professor Thouless during the Fall '08 term at University of Michigan.
- Fall '08