Lecture26 - ME 382 Lecture 26 CRACK INITIATION IN FATIGUE...

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ME 382 Lecture 26 20/iii/06 1 C RACK INITIATION IN FATIGUE Fatigue cracks nucleate under cyclic loading because of local yield (microscopic or macroscopic) Low-cycle fatigue : Yield 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
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ME 382 Lecture 26 20/iii/06 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) T
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