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Lec 3 Fatigue Failure 031004_for_students (1)

Lec 3 Fatigue Failure 031004_for_students (1) - Theories...

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Chapter 6 Fatigue Failure  Theories
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Fatigue Failure Occurs when stresses are changing throughout the life of a part. _______________________ starts with a crack that propagates until a catastrophic failure occurs. This usually begins at a manufacturing defect or stress concentration that is subjected to tensile stresses for part of its load cycle.
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Fatigue-Failure Models Stress-Life - cyclic stresses are kept below fatigue strength or endurance limit. Most widely used. Strain-Life - complicated not often used. _________________ - sometimes used, gaining favor especially in the aircraft and aerospace industry.
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Fatigue Stresses -10000 -5000 0 5000 10000 15000 20000 25000 30000 0 2 4 6 8 10 Load Cycles Applied Stress - psi σ a σ a σ a σ m σ m Fully Reversed Repeated Fluctuating
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Fully Reversed Failure Criteria Most data comes from R. R. Moore rotating-beam test. Highly polished specimen of 0.3 inches in diameter is subjected to pure bending stresses that are alternated by rotating the specimen. Rotation is at 1725 rpm. Takes 1/2 day to reach 10 6 cycles.
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S-N Curve for Steel with Sut = 100 ksi 10000 100000 1.0E+3 10.0E+3 100.0E+3 1.0E+6 10.0E+6 100.0E+6 Cycles to Failure - N Fatigue Strength - Sf
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Lec 3 Fatigue Failure 031004_for_students (1) - Theories...

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