Chapter 8 - Mechanical Failure of Materials(Chapter 8 The...

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1 Mechanical Failure of Materials (Chapter 8) The fracture of a Al bicycle crank arm.
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2 What you should understand by the end of this chapter: The main modes of material failure. How failure is initiated by flaws. How fracture behaviour is quantified, and the basis for the field of fracture mechanics. What factors determine failure stress and ductility. How factors such as temperature, loading rate and loading history affect failure.
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3 Failure: General Classes 3 main classes: Overload (simple) 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 elevated temperature Other failure modes (not considered as much in this course) Stress-corrosion cracking Corrosion-fatigue Creep-fatigue
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4 Ship-cyclic loading from waves. From Fig. 9.0, Callister (original Neil Boenzi, The New York Times .) Material Failures Occur in all Types of Devices and Structures Material failure may be due to… … poor design. … choosing the wrong material. … materials failing to meet specifications. … materials degradation.
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5 Computer chip Cyclic thermal loading Adapted from Fig. 18.11W(b), Callister 6e. (Fig. 18.11W(b) is courtesy of National Semiconductor Corporation.) Material failure may be due to… … poor design. … choosing the wrong material. … materials failing to meet specifications. … materials degradation. Material Failures Occur in all Types of Devices and Structures
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6 Hip implant Cyclic loading from walking Adapted from Fig. 17.19(b), Callister 6e. Material Failures Occur in all Types of Devices and Structures Material failure may be due to… … poor design. … choosing the wrong material. … materials failing to meet specifications. … materials degradation.
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7 Modes of Fracture Ductile: substantial plastic deformation large % RA, ε f , and % elongation (often > 20%) Brittle: insignificant plastic deformation low ε f , and % elongation (often 1%) Fig. 9.3 Callister Fig. 9.1 Callister
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8 Ductile Fracture 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.
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9 ALS: Failure Origins What do you see on the mild steel specimen that suggests why it failed at this location? 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.
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Chapter 8 - Mechanical Failure of Materials(Chapter 8 The...

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