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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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- Fall '08
- Strain, stress amplitude, High-cycle fatigue