ch09 - 2/17/2012 Chapter 9: Mechanical Failure ISSUES TO...

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2/17/2012 1 Chapter 9 - 1 ISSUES TO ADDRESS. .. How do cracks that lead to failure form? How is fracture resistance quantified? How do the fracture resistances of the different material classes compare? How do we estimate the stress to fracture? How do loading rate, loading history, and temperature affect the failure behavior of materials? Ship-cyclic loading from waves. Computer chip-cyclic thermal loading. Hip implant-cyclic loading from walking. Adapted from Fig. 22.30(b), Cal ister 7e. (Fig. 22.30(b) is courtesy of National Semiconductor Corporation.) Adapted from Fig. 22.26(b), Cal ister 7e. Chapter 9: Mechanical Failure Adapted from chapter-opening photograph, Chapter 9, Cal ister & Rethwisch 3e. (by Neil Boenzi, The New York Times .) Chapter 9 - 2 Fracture mechanisms • Ductile fracture – Accompanied by significant plastic deformation • Brittle fracture – Little or no plastic deformation – Catastrophic Chapter 9 - 3 Ductile vs Brittle Failure Very Ductile Moderately Ductile Brittle Fracture behavior: Large Moderate % AR or % EL Small • Ductile fracture is usually more desirable than brittle fracture! Adapted from Fig. 9.1, Cal ister & Rethwisch 3e. • Classification: Ductile: Warning before fracture Brittle: No warning Chapter 9 - 4 Ductile failure: -- one piece -- large deformation Figures from V.J. Colangelo and F.A. Heiser, Analysis of Metal urgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., 1987. Used with permission. Example: Pipe Failures Brittle failure: -- many pieces -- small deformations Chapter 9 - 5 • 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 Metal urgical 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 necking s void nucleation void growth and linkage shearing at surface fracture Chapter 9 - 6 Ductile vs. Brittle Failure Adapted from Fig. 9.3, Cal ister & Rethwisch 3e. cup-and-cone fracture brittle fracture
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2/17/2012 2 Chapter 9 - 7 Brittle Failure Arrows indicate point at which failure originated Adapted from Fig. 9.5(a), Cal ister & Rethwisch 3e. Chapter 9 - 8 Inter granular ( between grains) 304 S. Steel (metal) Reprinted w/permission from "Metals Handbook", 9th ed, Fig. 633, p. 650. Copyright 1985, ASM International, Materials Park, OH. (Micrograph by J.R. Keiser and A.R. Olsen, Oak Ridge National Lab.) Polypropylene (polymer) Reprinted w/ permission from R.W. Hertzberg, "Defor-mation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.35(d), p. 303, John Wiley and Sons, Inc., 1996. 4 mm Intra granular ( within grains) Al Oxide (ceramic) Reprinted w/ permission from "Failure Analysis of Brit le Materials", p. 78.
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ch09 - 2/17/2012 Chapter 9: Mechanical Failure ISSUES TO...

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