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Unformatted text preview: MECH 210 Page 1 of 3 MECH 210 080908 Fracture Mechanics Fracture is the separation of material into two or more separate pieces. Fracture proceeds in stages of crack initiation, crack propagation, and eventual failure. Crack initiation usually occurs at stress raisers. There are two modes of material fracture: brittle fracture and ductile fracture. Brittle fracture: fracture that occurs by rapid crack propagation and without appreciable macroscopic (plastic) deformation consuming only small amounts of energy. Ductile fracture: fracture that occurs by slow crack propagation accompanied by extensive gross plastic deformation consuming large amounts of energy. Fracture mechanics is the technique of fracture analysis used to determine the stress level at which preexisting cracks of known size will propagate, leading to fracture. Stress Concentration The fracture strength of a solid material is a function of the cohesive forces that exist between atoms. The theoretical strength of a brittle elastic solid has been estimated to be approximately E/10 where E is the modulus of elasticity of the solid. The actual strength, however, is usually many times lower than the theoretical value due to inherent defects produced in fabrication processes. These defects, acting as stress raisers, result in stress concentration and crack initiation where the local stress is significantly larger than the applied stress. With an internal crack of elliptical shape with the long axis oriented perpendicular to an applied stress o , the maximum stress m occurs at the crack tips is estimated as...
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- Fall '08