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Unformatted text preview: Chapter 9 Failure Introduction Simple fractures, fundamentals of fracture mechanics, brittle fracture of ceramics, impact fracture testing, ductile-to-brittle transition, fatigue and creep Fundamentals of Fracture Simple fracture = separation of a body into two or more pieces in response to an imposed stress that is static and at temperatures that are low relative to the melting temp of the material Applied stress can be tensile, compressive, shear or torsional Engineering materials experience two types of fractures: brittle and ductile o Brittle has no plastic deformation (elastic to fracture) o Ductile does (elastic region plastic region then fracture point) Ductility depends on percent elongation or percent reduction in area Ductile materials experience slow stable cracks while brittle cracks/fractures occur spontaneously and unstably (rapid crack propagation) More strain energy is required to induce a ductile fracture (tougher) Ductile Fracture Ductile materials experience necks down to a point then fractures, moderately ductile materials experience necking then fracture Brittle Fracture No appreciable deformation ; rapid crack propagation Direction of crack motion is perpendicular to direction of applied stress Cleavage = successive and repeated breaking of atomic bonds along specific crystallographic planes o Transgranular or transcrystalline fracture b/c fracture cracks pass through the grains o Grainy or faceted texture on macroscopic level Intergranular = crack propagation along grain boundaries Principles of Fracture Mechanics Stress concentration o Theoretical stress = E/10 o Fracture strengths for brittle materials are a lot lower than predicted by theoretical calculations based on atomic bond energies b/c there are flaws and cracks on the microscopic level that always exist and expand as stress is applied these flaws are called stress raisers o max stress m = 2 (a/ t ) 1/2 occurs at the crack tip = magnitude of nominal applied stress t = radius of curvature of crack tip a = length of surface crack or half of length of internal crack o stress concentration ratio K t = m / =2(a/ t ) 1/2...
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This note was uploaded on 01/02/2012 for the course ENME enme382 taught by Professor Bruck during the Spring '10 term at Maryland.
- Spring '10