ch8 - Mechanical Failure of Materials Chapter 8 What you...

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1 Mechanical Failure of Materials Chapter 8 ¾ 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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2 Failure: General Classes ¾ 3 main classes: ± overload (simple) fracture ² material fails due to increasing stress ± fatigue fracture ² material fails due to sustained cyclic loading ± creep fracture ² material fails due to a sustained steady load at elevated temperature ¾ other failure modes (not considered as much in this course) ± stress-corrosion cracking ± corrosion-fatigue ± creep-fatigue
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3 Material Failures Occur in all Types of Devices and Structures ¾ Material failure may be due to… ± … poor design. ± … choosing the wrong material. ± … materials failing to meet specifications. ± … materials degradation. Ship-cyclic loading from waves. From Fig. 9.0, Callister (original Neil Boenzi, The New York Times .)
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4 Material Failures Occur in all Types of Devices and Structures ¾ Material failure may be due to… ± … poor design. ± … choosing the wrong material. ± … materials failing to meet specifications. ± … materials degradation. computer chip cyclic thermal loading Adapted from Fig. 18.11W(b), Callister 6e. (Fig. 18.11W(b) is courtesy of National Semiconductor Corporation.)
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5 Material Failures Occur in all Types of Devices and Structures ¾ Material failure may be due to… ± … poor design. ± … choosing the wrong material. ± … materials failing to meet specifications. ± … materials degradation. Hip implant cyclic loading from walking Adapted from Fig. 17.19(b), Callister 6e.
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6 Modes of Fracture ¾ Ductile: ± substantial plastic deformation ² large % RA , ε f , and % elongation (often > 20%) ¾ Brittle: ± insignificant plastic deformation ² low ε f , and % elongation (often 1%) Fig. 9.3 Callister Fig. 9.1 Callister
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7 Ductile Fracture ¾ Ductile fracture is induced by plastic deformation. ¾ It’s caused by damage accumulation. ± nucleation, growth, coalescence of voids ¾ After tensile instability starts, the damage is concentrated in the neck. ¾ Note the importance of particles.
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8 ALS: Failure Origins ¾ 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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9 An Example: Failure of a Pressurized Pipe 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.
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ch8 - Mechanical Failure of Materials Chapter 8 What you...

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