Chapter3b_fail

Chapter3b_fail - Chapter 3b Failures Fatigue and Creep...

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Chapter 3b Failures Fatigue and Creep
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Required Reading • Fatigue - Callister, p. 227 -237 • Creep - Callister, p. 238-240 • Corrosion – Callister, for information read briefly, p. 640-648
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Fatigue • 90% of all failure in metals occurs by fatigue • There is very little plastic deformation. Surface brittle-like, even for ductile material • and insidious • Crack initiation crack propagation catastrophic by fracture of remaining material • Fatigue occurs perpendicular to the direction of applied stress with σ ( fatigue limit) ~ .5UTS • Considered 2 ways: S-N (S tress vs N umber of cycles) and an extension of fracture mechanics
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Typical SN curve p. 231 Stress S Cycles N
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Design Approaches with Fatigue • Infinite-life design: this criterion is based on keeping the stress at some fraction of the fatigue limit in the S-N curve. It is the oldest criterion and has largely been replaced. • Safe-life design: this is based on the assumption that a part is initially flaw free and has a finite life in which to develop a critical crack. The aviation sector historically used a safe-life that was ¼ of the life demonstrated in a full scale fatigue test.
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• Fail-safe design – here the view is taken that fatigue cracks will occur and the structure is designed so that cracks will not lead to failure. This has been routinely used in aviation. Multiple load paths and crack stoppers are built into the design. Rigid regulation and criteria for inspection are required.
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• Damage-tolerant design – this is an extension of the fail-safe approach with the assumption that flaws and cracks exist. Fracture mechanics is used to determine whether or not the cracks will grow large enough to cause failure before they are detected. Materials with high fracture toughness are used along with extensive nondestructive examination in damage-critical areas.
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Typical appearance of a fatigue failure
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Log (da/dN) vs. Log Δ K for an edge crack da/dN=f(a, σ ) W. Smith, Foundations of Materials Science and Engineering, 2 nd . Ed., McGraw Hill p. 260 σ 2 > σ 1
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Using this curve, the power law (Paris law) relationship can be determined da/dN = C( Δ K) m C and m are constants for the material and depend on the environment, frequency and stress ratio R.
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Chapter3b_fail - Chapter 3b Failures Fatigue and Creep...

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