Lecture 13 - ENGR 4250 Advanced Materials Engineering...

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ENGR 4250 – Advanced Materials Engineering Lecture 13 Chapter 8: Mechanical Failure Dr. A. Keshavarz - ENGR 4250U
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Failure Design Goal: Minimize possibility of failure Mechanisms: Fracture, Fatigue & Creep Dr. A. Keshavarz - ENGR 4250U
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Oil Tanker: Brittle Fracture – Crack Propagation Dr. A. Keshavarz - ENGR 4250U
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Fracture Fundamentals Def: Separation in two or more pieces – Stress, constant, low temp. Stress: tensile, comp., shear or tor . Uni-axial tensile stresses Two modes: Ductile & brittle – Based on plastic deformation ability Dr. A. Keshavarz - ENGR 4250U
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Fracture Fundamentals - 2 Two steps: crack formation & propagation – Imposed stress Mode: depends upon mechanism of crack propagation : ductile ; brittle – Ductile: • extensive plastic deformation in vicinity of crack • Slow process as crack length is extended: stable • Further extension only on increase in stress • Gross deformation: crack surfaces Dr. A. Keshavarz - ENGR 4250U
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Fracture Fundamentals - 3 Modes: ductile; brittle Brittle: • Very little plastic deformation • Extremely rapid crack propagation: unstable • Once started extension is spontaneous without increase in stress • Insignificant deformation: crack surfaces Dr. A. Keshavarz - ENGR 4250U
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Fracture Fundamentals - 4 Ductile fracture preferred Brittle fracture occurs very suddenly and catastrophically without warning Consequence of: spontaneous and rapid crack propagation Ductile fracture occurs slowly • Plastic deformation warning of imminent failure • Preventive measures possible • More strain energy is required: tougher Dr. A. Keshavarz - ENGR 4250U
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Ductile vs Brittle Failure Very Ductile Moderately Ductile Brittle Fracture behavior: Large Moderate % AR or % EL Small • Ductile fracture is usually desirable! Adapted from Fig. 8.1, Callister 7e. • Classification: Ductile: warning before fracture Brittle: No warning Dr. A. Keshavarz - ENGR 4250U
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Ductile failure: --one piece --large 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. Example: Failure of a Pipe Brittle failure: --many pieces --small deformation Dr. A. Keshavarz - ENGR 4250U
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Dr. A. Keshavarz - ENGR 4250U
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• Evolution to failure: • Resulting fracture surfaces (steel) particles serve as void nucleation sites. 50 mm From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 11.28, p. 294, John Wiley and Sons, Inc., 1987. (Orig. source: P. Thornton, J. Mater. Sci ., Vol. 6, 1971, pp. 347-56.) 100 mm Fracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission. Moderately Ductile Failure necking σ void nucleation void growth and linkage shearing at surface fracture Dr. A. Keshavarz - ENGR 4250U
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Fracto-graphic Studies Microscopic examination: SEM Mode: Ductile/brittle Cup & cone/no deformation Dr. A. Keshavarz - ENGR 4250U
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Ductile Failure Dr. A. Keshavarz - ENGR 4250U
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Ductile Failure Surface Dr. A. Keshavarz - ENGR 4250U
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Brittle Failure Dr. A. Keshavarz - ENGR 4250U
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Brittle Fracture
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This note was uploaded on 06/02/2011 for the course ECONMICS ECN204 taught by Professor Brenan during the Spring '08 term at Ryerson.

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Lecture 13 - ENGR 4250 Advanced Materials Engineering...

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