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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 σ m = 2σ (a/ρ t ) 1/2 occurs at the crack tip σ = magnitude of nominal applied stress ρ 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 = σ m / σ =2(a/ρ t ) 1/2...
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- Spring '10
- Stress, Fracture mechanics, brittle fracture