4097978009

4097978009 - Why study failure ? ME4214 Design for Product...

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1 ME4214 Design for Product Safety FRACTURE MECHANICS (How do things break?) Dr. David Yuen Department of Mechanical Engineering The Hong Kong Polytechnic University 2 Why study failure ? • failure mechanisms -> design -> avoid in- service failure • As an engineer, you will be responsible for: – prediction/design of failure – assessment of failure cause and prevention of future failures 3 Fracture Mechanisms • Materials break if: – Fracture is energetically possible, and – A fracture mechanism is available. • It’s important to understand what fracture mechanisms are available to a material. 4 Case Study THE TITANIC • Why did it sink so quickly?
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5 Overview • Case Study – The Titanic • Fracture Mechanisms – Types of Fracture • Metals/Ceramics/Polymers • Composites – Brittle/Ductile Transition • Charpy Impact Test – Fatigue • Fatigue Life – Environmental Effects • Stress Corrosion • Creep 6 Fundamentals of Fracture •A simple fracture is the separation of a body into two or more pieces in response to an imposed stress that is static at low temperature. • Two fracture modes: ductile and brittle (tension in this class) • Two steps in fracture: crack formation and propagation • Materials and fracture modes: – metal alloys are generally ductile – ceramics are brittle – polymers maybe ductile or brittle 7 Ductile Fracture Very ductile Ductile Brittle Schematic of fracture when materials are: 8 Cup-and-cone fracture Five stages in cup-and-cone fracture: (a) Initial necking (b) Small cavity formation (c) Coalescence of cavities to form a crack (d) Crack propagation (e) Final shear fracture at 45 o relative to the tensile direction • Fractographic: microscopic examination using Scanning Electron Microscopy (SEM).
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9 Experimental observation of fracture appearance Cup-and-cone fracture in aluminum Brittle fracture in mild steel 10 SEM fractograph of ductile fracture Spherical dimples under tension Each spherical dimple is half a void Each parabolic dimple is half a void that is elongated along 45 o direction Parabolic dimples under shear 11 Brittle Fracture • It takes place with little deformation, and by rapid crack propagation. • Propagation is nearly perpendicular to the direction of tensile stress. • Fracture surface is nearly flat; it is shiny for amorphous materials. • Markings of brittle fracture: V shape or fanlike pattern . * see figures below Origin of crack Origin of crack 12 Cleavage • Cleavage: successive and repeated breaking of atomic bonds along a specific crystallographic planes. Transgranular fracture: cracks passing through grains. Intergranular fracture: crack propagation along grain boundaries. SEM: transgranular SEM intergranular
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13 Fracture Mechanisms • Fracture Mechanisms are different in different materials.
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This note was uploaded on 04/06/2010 for the course ME ME4214 taught by Professor Dr.yuen during the Spring '10 term at NYU Poly.

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4097978009 - Why study failure ? ME4214 Design for Product...

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