2010-02-15 Chapter 08 Failure

2010-02-15 Chapter 08 Failure - Chapter 8 Mechanical...

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Chapter 8 - 1 ISSUES TO ADDRESS. .. How do flaws in a material initiate failure? How is fracture resistance quantified; how do different material classes compare? How do we estimate the stress to fracture? How do loading rate, loading history, and temperature affect the failure stress? Ship-cyclic loading from waves. Computer chip-cyclic thermal loading. Hip implant-cyclic loading from walking. Adapted from Fig. 22.30(b), Callister 7e. (Fig. 22.30(b) is courtesy of National Semiconductor Corporation.) Adapted from Fig. 22.26(b), Callister 7e. Chapter 8: Mechanical Failure Adapted from chapter-opening photograph, Chapter 8, Callister 7e. (by Neil Boenzi, The New York Times .)
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Chapter 8 - 2 Fracture mechanisms Ductile fracture Occurs with plastic deformation Brittle fracture Little or no plastic deformation Catastrophic Failure Modes include: Fracture Excessive deformation: component is no longer suitable for its purpose Buckling (instability)
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Chapter 8 - 3 Failure Mode I: Fracture Fracture is separation of a body into two or more pieces in response to applied stress. Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.1(b), p. 262, John Wiley and Sons, Inc., 1996. (Orig. source: Earl R. Parker, "Behavior of Engineering Structures", Nat. Acad. Sci., Nat. Res. Council, John Wiley and Sons, Inc., NY, 1957.)
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Chapter 8 - 4 Failure Mode II: Excessive Deformation If the shaft bends (flexes) excessive then gears will not mesh smoothly. This is considered as failure. www.rhin-o-tuff.com/art.html
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Chapter 8 - 5 Failure Mode III: Buckling and Instability Buckling Instability Structure may collapse because of its critical support members is unable to hold its shape even though material does not fail by fracture.
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Chapter 8 - 6 Tensile Strength (TS), Ductility, Toughness Important C Engineering tensile strain, ε E ngineering tensile stress, σ A B Material Tensile Strength (TS) Ductility Toughness Modulus A High Low Low High B Medium Medium High Medium C Low High Medium Low When Strength Ductility Toughness Many engineering applications require materials having high strength yet some ductility and toughness.
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Chapter 8 - 7 Ductile vs Brittle Failure Very Ductile Moderately Ductile Brittle Fracture behavior: Large Moderate % AR or % EL Small • Ductile fracture is usually desirable! Adapted from Fig. 8.1, Callister 7e. • Classification: Ductile: warning before fracture Brittle: No warning Important
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Chapter 8 - 8 Ductile failure: --one piece --large deformation Figures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., 1987. Used with permission. Example: Failure of a Pipe Brittle failure: --many pieces --small deformation Important
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Chapter 8 - 9 • Evolution to failure: • Resulting fracture surfaces (steel) 50 mm particles serve as void nucleation sites.
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2010-02-15 Chapter 08 Failure - Chapter 8 Mechanical...

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