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3-Chapter_08--Mechanical Failure - CHAPTER 8 MECHANICAL...

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Chapter 8- 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? 1 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. CHAPTER 8: MECHANICAL FAILURE
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Chapter 8- • Ductile fracture is desirable! Because Ductile: warning before fracture Brittle: No warning DUCTILE VS BRITTLE FAILURE Very Ductile Moderately Ductile Brittle Fracture behavior: Large Moderate %AR or %EL: Small
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Chapter 8- 2 DUCTILE VS BRITTLE FAILURE
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Chapter 8- 3 Ductile failure: --one piece --large deformation Brittle failure: --many pieces --small deformation EXAMPLE: FAILURE OF A PIPE
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Chapter 8- 4 • Evolution to failure: necking void nucleation void growth and linkage shearing at surface fracture σ • Resulting fracture surfaces (steel) particles serve as void nucleation sites. 50 μ m 100 μ m MODERATELY DUCTILE FAILURE
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Chapter 8- 5 Inter granular ( between grains) Intra granular ( within grains) Al Oxide (ceramic) 316 S. Steel (metal) 304 S. Steel (metal) Polypropylene (polymer) 3 μ m 4 mm 160 μ m 1 mm BRITTLE FRACTURE SURFACES
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Chapter 8- 6 TS >> TS engineering materials perfect materials IDEAL VS REAL MATERIALS (at Room T) σ TS ε E/10 E/100 0.1 perfect mat’l-no flaws carefully produced glass fiber typical ceramic typical strengthened metal typical polymer
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Chapter 8- • DaVinci observed (500 yrs ago!)... the longer the wire, the smaller the load to fail it. • Reasons: Flaws cause premature failure. Larger samples are more flawed! IDEAL vs REAL MATERIALS
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Chapter 8- 7 Elliptical hole in a plate: Stress distrib. in front of a hole: Stress concentration factor: BAD! K t >>3 NOT SO BAD K t =3 σ σ o 2a K t = σ max / σ o Large K t promotes failure: FLAWS ARE STRESS CONCENTRATORS! ρ t σ max 2 σ o + 1 2 σ o a ρ t a ρ t for sharp cracks
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Chapter 8- 8 Avoid sharp corners!
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