MAT_101_Winter08_Lecture_18

MAT_101_Winter08_Lecture_18 - Loading Rate Increased...

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Loading Rate • Increased loading rate. .. --increases σ y and TS --decreases %EL •Why? An increased rate gives less time for disl. to move past obstacles. initial height final height sample σ ε σ y σ y TS TS larger ε smaller ε (Charpy) • Impact loading: --severe testing case --more brittle --smaller toughness Adapted from Fig. 8.11(a) and (b), Callister 6e. (Fig. 8.11(b) is adapted from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials , Vol. III, Mechanical Behavior , John Wiley and Sons, Inc. (1965) p. 13.)
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Temperature • Increasing temperature. .. --increases %EL and K c Ductile-to-brittle transition temperature (DBTT) ... BCC metals (e.g., iron at T < 914C) Impact Energy Temperature F C m e t a l s ( . g , u, N i ) High strength materials ( σ y >E/150) polymers More Ductile Brittle Ductile-to-brittle transition temperature Adapted from C. Barrett, W. Nix, and A.Tetelman, The Principles of Engineering Materials , Fig. 6-21, p. 220, Prentice-Hall, 1973. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.
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Ductile to Brittle Transition (Steels) DBTT (carbon content in steel) DBTT and Fracture Nature Mid-carbon steel
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Design Strategy: Stay Above the DBTT! • Pre-WWII: The Titanic • WWII: Liberty ships Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.1(b), p. 262, John Wiley and Sons, Inc., 1996. (Orig. source: Earl R. Parker, "Behavior of Engineering Structures", Nat. Acad. Sci., Nat. Res. Council, John Wiley and Sons, Inc., NY, 1957.) Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.1(a), p. 262, John Wiley and Sons, Inc., 1996. (Orig. source: Dr. Robert D. Ballard, The Discovery of the Titanic .) • Problem: Used steel with a DBTT ~ room temp.
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Fatigue Fatigue = failure under cyclic stress. tension on bottom compression on top counter motor flex coupling be a r i n g specimen • Stress varies with time. --key parameters are S and σ m σ max σ min σ time σ m S • Key points: Fatigue. .. --can cause part failure, even though σ max < σ c . --causes ~ 90% of mechanical engineering failures. Adapted from Fig. 8.16, Callister 6e. (Fig. 8.16 is from Materials Science in Engineering , 4/E by Carl. A. Keyser, Pearson Education, Inc., Upper Saddle River, NJ.)
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Fatigue Design Parameters Fatigue limit , S fat : --no fatigue if S < S fat • Sometimes, the fatigue limit is zero!
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MAT_101_Winter08_Lecture_18 - Loading Rate Increased...

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