Chapter 8 - Overload simple fracture FatigueFracture...

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The fracture of a Al bicycle crank arm.
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Overload ሺsimpleሻ fracture Material fails due to increasing stress Fatigue Fracture Material fails due to sustained cyclic loading Creep Fracture Material fails due to a sustained steady load at an elevated temperature Stress‐corrosion cracking Corrosion fatigue Creep fatigue
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3 Ship-cyclic loading from waves. From Fig. 9.0, Callister (original Neil Boenzi, The New York Times .) Material failure may be due to… … poor design … choosing the wrong material … materials failing to meet specifications … materials degradation
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Fracture: crack formation and propagation in response to an imposed stress 2 Fracture Modes ሺclassified based on the ability of the material to experience plastic deformationሻ Ductile ሺstableሻ Exhibit substantial plastic deformation large % RA, e f , and % elongation ሺoften ൐ 20%ሻ High energy absorption Brittle ሺunstableሻ Little or no plastic deformation low e f , and % elongation ሺoften ൑ 1%ሻ Low energy absorption Ductility is a function of: Temperature Strain rate Stress rate Ductile Material Brittle Material
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6 Ductile fracture is induced by plastic deformation. It’s caused by damage accumulation. nucleation, growth, coalescence of voids After tensile instability starts, the damage is concentrated in the neck. Note the importance of particles. Indicates plastic deformation ሺfibrousሻ
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Occurs by rapid crack propagation Fracture surfaces will have their own distinctive patterns; for example : Steel chevron markings Lines or ridges originating from near the center of the cross section Amorphous materials shiny, smooth surface Cleavage : crack propagation corresponding to the successive and repeated breaking of atomic bonds along specific crystallographic planes Brittle transcrystalline steel fracture http://www.tescan.com/gallery‐gallery.php?obrൌ40&menuൌ2
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9 Defects are the key to brittle fracture Fig. 9.3(b) Callister
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10 Figures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., 1987. Used with permission. specimen still a single piece extensive deformation many pieces little deformation
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Crack orientation Crack geometry t t t a K 2 0 max
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13 When max stress exceeds the yield strength in a ductile material, plastic deformation occurs Consider an elliptical hole in a plate: Magnitude of stress diminishes with distance from the crack tip
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14 What shape are the window panels in airplanes? Answer:
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15 According to what we have so far: As r t 0 max  ሺfor any 0 That’s NOT POSSIBLE.
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Chapter 8 - Overload simple fracture FatigueFracture...

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