Lec 16 - ENGR ENGR 4250 Advanced Materials Engineering...

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ENGR 4250 ENGR 4250 – dvanced Materials Engineering Advanced Materials Engineering cture 16 Lecture 16 Chapter 8: Mechanical Failure ontd Contd. ….
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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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Fatigue at gue ynamic or fluctuating stresses Dynamic or fluctuating stresses Bridges, aircraft, machine components 90% of all metallic failures Initiation & propagation of cracks: brittle like
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Aloha Airline 1988
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Fatigue Failure of crankshafts
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Fatigue Fatigue = failure under cyclic stress. Adapted from Fig. 8.18, compression on top specimen pg , Callister 7e. (Fig. 8.18 is from Materials Science in Engineering , 4/E by Carl. A. Keyser, Pearson Education, Inc., Upper counter motor flex coupling bearing bearing • Stress varies with time. -- ey parameters are and σ max σ Saddle River, NJ.) tension on bottom key parameters are S , σ m , and frequency σ min time σ m S • Key points: Fatigue. .. --can cause part failure, even though σ max < σ c . --causes ~ 90% of mechanical engineering failures.
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Fatigue Design Parameters Fatigue limit , S fat : --no fatigue if S < S fat case for steel (typ.) unsafe S = stress amplitude Endurance limit 35-60% of TS e Ti alloys Adapted from Fig. 19(a) allister 7e S fat safe Fe, Ti alloys 8.19(a), Callister 7e. N = Cycles to failure 10 3 10 5 10 7 10 9 • Sometimes, the Sometimes, the fatigue limit is zero! ost non rrous alloys case for Al (typ.) unsafe S = stress amplitude Most non-ferrous alloys Adapted from Fig. 8.19(b), Callister 7e. safe N = Cycles to failure 10 3 10 5 10 7 10 9
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Fatigue Strength
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Scatter in Fatigue data Depends upon test and material parameters pecimen fabrication Specimen fabrication Metallurgical variables Specimen alignment in test instrument Mean stress Test frequency Precise control is impossible Significant design uncertainties
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Failure Probability Curves
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Fatigue High-cycle fatigue nly elastic strains – Only elastic strains – Range > 10 4 Low-cycle fatigue – High loads – Elastic strains + plastic strains – Range < 10 4
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Fatigue Stages Crack initiation Surface stress concentration • Scratches, sharp fillet, keyways, threads, dents • Dislocations slip steps at surface - discontinuities Propagation: incremental each marks Beach marks Striations thousands in one beach mark Final failure
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Lec 16 - ENGR ENGR 4250 Advanced Materials Engineering...

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