Lecture 11 - Mechanical Failure Mechanical Failure ISSUES...

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1 Mechanical Failure Mechanical Failure 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. Mechanical Failure Mechanical Failure Adapted from chapter-opening photograph, Chapter 8, Callister 7e. (by Neil Boenzi, The New York Times .)
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2 Fracture mechanisms Fracture mechanisms Ductile fracture Occurs with plastic deformation Brittle fracture Little or no plastic deformation Catastrophic Ductile Ductile vs vs Brittle Failure 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
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3 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 Example: Failure of a Pipe Brittle failure: --many pieces --small deformation • Evolution to failure: • Resulting fracture surfaces (steel) particles serve as void nucleation sites. 50 mm From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 11.28, p. 294, John Wiley and Sons, Inc., 1987. (Orig. source: P. Thornton, J. Mater. Sci ., Vol. 6, 1971, pp. 347-56.) 100 mm Fracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission. Moderately Ductile Failure Moderately Ductile Failure necking σ void nucleation void growth and linkage shearing at surface fracture
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4 Ductile vs. Brittle Failure Ductile vs. Brittle Failure Adapted from Fig. 8.3, Callister 7e. cup-and-cone fracture brittle fracture Brittle Failure Arrows indicate pt at which failure originated Adapted from Fig. 8.5(a), Callister 7e.
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5 Inter granular ( between grains) Intra granular ( within grains) Al Oxide (ceramic) Reprinted w/ permission from "Failure Analysis of Brittle Materials", p. 78. Copyright 1990, The American Ceramic Society, Westerville, OH. (Micrograph by R.M. Gruver and H. Kirchner.) 316 S. Steel (metal) Reprinted w/ permission from "Metals Handbook", 9th ed, Fig. 650, p. 357.
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This note was uploaded on 04/27/2009 for the course MEEN 222 taught by Professor Radovic during the Fall '08 term at Texas A&M.

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Lecture 11 - Mechanical Failure Mechanical Failure ISSUES...

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