Lec 14 - ENGR 4250 ENGR 4250 – Advanced Materials...

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Unformatted text preview: ENGR 4250 ENGR 4250 – Advanced Materials Engineering Advanced Materials Engineering Lecture 14 Lecture 14 Chapter 8: Mechanical Failure Failure Design Goal: Minimize possibility of failure Mechanisms: Fracture, Fatigue & Creep Designed Failures racture Fracture Oil Tanker: Brittle Fracture – Crack ropagation Propagation Fracture Fundamentals • Def: Separation in two or more pieces tress constant low temp – Stress, constant, low temp. • Stress: tensile, comp., shear or tor . • Uni-axial tensile stresses – Two modes: Ductile & brittle – Based on plastic deformation ability Fracture Fundamentals - 2 • Two steps: crack formation & propagation posed stress – Imposed stress • Mode: depends upon mechanism of crack ropagation: ductile; brittle propagation: ductile; brittle – Ductile: xtensive plastic deformation in vicinity of crack • extensive plastic deformation in vicinity of crack • Slow process as crack length is extended: stable • Further extension only on increase in stress y • Gross deformation: crack surfaces Fracture Fundamentals - 3 • Modes: ductile; brittle rittle: – Brittle: • Very little plastic deformation • xtremely rapid crack propagation: unstable Extremely rapid crack propagation: unstable • Once started extension is spontaneous without increase in stress • Insignificant deformation: crack surfaces Fracture Fundamentals - 4 • Ductile fracture preferred rittle fracture occurs very suddenly and – Brittle fracture occurs very suddenly and catastrophically without warning • onsequence of: pontaneous and rapid crack 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 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!...
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This note was uploaded on 10/18/2010 for the course ENGR 3360 taught by Professor Ahmadbarari during the Fall '10 term at UOIT.

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Lec 14 - ENGR 4250 ENGR 4250 – Advanced Materials...

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