Ch 8 2010 - Mechanical Failure ISSUES TO ADDRESS: How do...

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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. Mechanical Failure
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p_pg207 Structural Failure of Tank Barge I.O.S. 3301
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Fracture Mechanisms Ductile fracture Occurs with plastic deformation Brittle fracture Little or no plastic deformation Catastrophic
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Ductile vs Brittle Failure Very Ductile Moderately Ductile Brittle Fracture behavior: Large Moderate % AR or % EL Small • Ductile fracture is usually desirable! • Classification: Ductile: Warning before fracture Brittle: No warning
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Ductile failure: --One piece --Large deformation Example: Failure of a Pipe Brittle failure: --Many pieces --Small deformation
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• Evolution to failure: • Resulting fracture surfaces (steel) 50 mm Particles serve as void nucleation sites. 50 mm 100 mm Moderately Ductile Failure Necking F Void nucleation Void growth and linkage Shearing at surface Fracture
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Ductile vs. Brittle Failure Cup-and-cone fracture Brittle fracture
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Brittle Failure Arrows indicate pt at which failure originated
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Inter granular ( Between grains) Trans granular ( Within grains) 316 Stainless Steel 304 Stainless Steel 4 mm 160 mm Brittle Fracture Surfaces
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Flaws are Stress Concentrators! ρ t
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Flaws are Stress Concentrators! Results from crack propagation Griffith Crack where ρ t = radius of curvature σ o = applied stress σ m = stress at crack tip a = length of surface crack, ½ length of internal crack o t / t o m K a σ = ρ σ = σ 2 1 2
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r / h Sharper fillet radius Increasing w / h 0 0.5 1.0 1.0 1.5 2.0 2.5 Stress Conc. Factor, K t σ o = Engineering Fracture Design Avoid sharp corners! σ o r , Fillet radius w h
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Crack Propagation Cracks propagate due to sharpness of crack tip A plastic material deforms at the tip, “blunting” the crack. Deformed region Brittle Energy balance on the crack Elastic strain energy- Energy stored in material as it is elastically deformed This energy is released when the crack propagates Creation of new surfaces requires energy Plastic
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When Does a Crack Propagate? Crack propagates if above critical stress Where E = Modulus of elasticity γ = Specific surface energy 2 1 2 / s c a E π γ = σ i.e., σ m > σ c
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Fracture Toughness Composite reinforcement geometry is: f = fibers; sf = short fibers; w = whiskers; p = particles. Graphite/
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Ch 8 2010 - Mechanical Failure ISSUES TO ADDRESS: How do...

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