Chapter 8 Slides

Chapter 8 Slides - Chapter 8 Failure Lectures begin Mar 27...

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1 Chapter 8 Chapter 8 - - Failure Failure Lectures begin Mar. 27 or Apr. 1, 08
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2 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? Welded ship fracture stress at low temperature Computer chip-cyclic thermal loading. Hip implant-cyclic loading from walking. Chapter 8: Failure Chapter 8: Failure
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3 Fracture mechanisms Fracture mechanisms Fracture – Ductile fracture • Requires much plastic flow and energy dissipation – Brittle fracture • Little or no plastic flow and often catastrophic failure • Fracture under repetitive loading – Fatigue – Can be ductile and brittle, or just brittle • Fracture under constant loading at high temperatures (>~0.6T M ) – Creep – At high temperature, thermal activation allows time- dependent plastic flow even at constant stress – ductile failure modes
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4 Very Ductile Moderately Ductile Brittle Fracture behavior: Large Moderate % AR or % EL Small Ductile: warning before fracture Brittle: No warning Ductile Ductile vs vs Brittle Failure Brittle Failure • Ductile fracture is usually desirable! • Classification:
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5 Ductile failure: --one piece --large deformation Examples: Ductile and Brittle Failure of a Pipe Examples: Ductile and Brittle Failure of a Pipe Brittle failure: --many pieces --small deformations
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6 • Evolution to failure: • Resulting fracture surfaces (steel) 50 mm particles serve as void nucleation sites. 50 mm 100 mm Moderately Ductile Failure Moderately Ductile Failure necking σ void nucleation void growth and linkage shearing at surface fracture
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7 Ductile fracture summary Ductile fracture summary Macroscopic neck appears – Stress increases in necked region and thus stress changes from homogenous to inhomogeneous Intense local flow tears around precipitates and inclusions to produce microvoids that elongate to produce macrovoids Load carrying cross-section continually decreases so load must be carried by strong work-hardening Finally work-hardening cannot support the load and rapid fracture occurs A cup and cone is characteristic of the slow (cup) and fast (cone) fracture ductile brittle
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8 Brittle Failure Brittle Failure Arrows indicate point where failure originates Dark area is a rough, energy-absorbing area – it is where the crack started, bright means little energy dissipation – easy propagation part
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9 Inter granular ( between grains) Intra granular ( within grains) Al oxide (ceramic) 316 SS (metal) 304 Stainless Steel (SS) (metal) Polypropylene (polymer) 3mm 4mm 160mm 1mm Brittle Fracture Surfaces Brittle Fracture Surfaces
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10 Little work is done in brittle fracture = very little energy is Little work is done in brittle fracture = very little energy is dissipated as crack passes through material dissipated as crack passes through material Fracture path is straight and can be
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This note was uploaded on 04/20/2008 for the course EML 3234 taught by Professor Hellstrom during the Spring '08 term at FSU.

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Chapter 8 Slides - Chapter 8 Failure Lectures begin Mar 27...

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