Lec 15 - ENGR ENGR 4250 Advanced Materials Engineering...

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ENGR 4250 ENGR 4250 – dvanced Materials Engineering Advanced Materials Engineering cture 15 Lecture 15 Chapter 8: Mechanical Failure ontd Contd. ….
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Briefly define and discuss the mechanism of recovery and re- crystallization in cold worked metals and comment on their relative treatment temperatures. (4) How do most imperfections in solid crystal structure of metals originate? (1) Which defects allow metals to deform plastically instead of failing under excessive loads? (1) Write down two MOST CHARACTERISTIC properties of metals, ceramics, polymers and composites (4) hy do ceramic point defects usually occur in pairs? (1) Why do ceramic point defects usually occur in pairs? (1) Write down any two of the four factors which promote the solubility of solids (2) Why liquid polymer flow properties are so different from those of other material groups in liquid form? (2)
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a. Rank these microstructures (by letter) from the hardest to softest. (2) b. What kind of steel yields the structures shown in figures B & D. (2) c. What is the difference in the heat treatment that yielded the two structures shown in B&D? (1) d. What would be the composition of iron-carbon alloy that yields the structure shown in A? (2)
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racture Fracture 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 Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined - ductile or brittle uctile fracture ost metals (not too cold): Ductile fracture - most metals (not too cold): ¾ Extensive plastic deformation ahead of crack ¾ Crack is “stable”: resists further extension unless applied stress increased 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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Example: Failure of a Pipe Ductile failure: ne piece --one piece --large deformation Brittle failure: --many pieces --small deformation 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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Ductile vs Brittle Failure Very Ductile Moderately Ductile Brittle Fracture behavior: • Classification: Adapted from Fig. 8.1, Callister 7e. arge oderate R r % L mall Large Moderate % AR or % EL Small • Ductile fracture is usually Ductile: warning before acture Brittle: No arning desirable! fracture warning
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Moderately Ductile Failure • Evolution to failure: necking void nucleation void growth and linkage shearing at surface fracture σ Resulting 0 mm 0 mm • Resulting fracture surfaces 50 mm 50 mm (steel) particles erve as void From V.J. Colangelo and F.A. Heiser, nalysis of Metallurgical Failures nd 100 mm Fracture surface of tire cord wire aded in tension Courtesy of F serve as void
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Lec 15 - ENGR ENGR 4250 Advanced Materials Engineering...

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