Chapter8

Chapter8 - Chapter Outline: Failure How do Materials Break?...

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1 MSE 2090: Introduction to Materials Science Chapter 8, Failure How do Materials Break? Chapter Outline: Failure ± Ductile vs. brittle fracture ± Principles of fracture mechanics 9 Stress concentration ± Impact fracture testing ± Fatigue (cyclic stresses) 9 Cyclic stresses, the S—N curve 9 Crack initiation and propagation 9 Factors that affect fatigue behavior ± Creep (time dependent deformation) 9 Stress and temperature effects 9 Alloys for high-temperature use Not tested: in 8.5 Fracture Toughness 8.14 Data extrapolation methods
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2 MSE 2090: Introduction to Materials Science Chapter 8, Failure Fracture: separation of a body into pieces due to stress, at temperatures below the melting point. Steps in fracture: ¾ crack formation ¾ crack propagation Fracture Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined - ductile or brittle Ductile fracture - most metals (not too cold): ¾ Extensive plastic deformation ahead of crack ¾ Crack is “stable”: resists further extension unless applied stress is increased Brittle fracture - ceramics, ice, cold metals: ¾ Relatively little plastic deformation ¾ Crack is “unstable”: propagates rapidly without increase in applied stress Ductile fracture is preferred in most applications
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3 MSE 2090: Introduction to Materials Science Chapter 8, Failure Brittle vs. Ductile Fracture Ductile materials - extensive plastic deformation and energy absorption (“toughness”) before fracture Brittle materials - little plastic deformation and low energy absorption before fracture
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4 MSE 2090: Introduction to Materials Science Chapter 8, Failure Brittle vs. Ductile Fracture A. Very ductile , soft metals (e.g. Pb, Au) at room temperature, other metals, polymers, glasses at high temperature. B. Moderately ductile fracture , typical for ductile metals C. Brittle fracture, cold metals, ceramics. AB C
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5 MSE 2090: Introduction to Materials Science Chapter 8, Failure Ductile Fracture (Dislocation Mediated) (a) Necking (b) Formation of microvoids (c) Coalescence of microvoids to form a crack (d) Crack propagation by shear deformation (e) Fracture Crack grows 90 o to applied stress 45 O - maximum shear stress Cup-and-cone fracture
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6 MSE 2090: Introduction to Materials Science Chapter 8, Failure Ductile Fracture (Cup-and-cone fracture in Al) Scanning Electron Microscopy: Fractographic studies at high resolution. Spherical “dimples” correspond to microvoids that initiate crack formation. tensile failure shear failure
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7 MSE 2090: Introduction to Materials Science Chapter 8, Failure ¾ No appreciable plastic deformation ¾ Crack propagation is very fast ¾ Crack propagates nearly perpendicular to the direction of the applied stress ¾ Crack often propagates by cleavage - breaking of atomic bonds along specific crystallographic planes ( cleavage planes ).
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Chapter8 - Chapter Outline: Failure How do Materials Break?...

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