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Unformatted text preview: ME 382 Lecture 35 14/iv/06 1 C OMPOSITES Combine two (or more) materials combined to enhance (i) stiffness (ii) strength (iii) toughness Role of interfaces is very important For stiffness: Interface must be a strong bond Metal-matrix composites: Ceramics (fibers or particles) in metals Polymer-matrix composites: Stiff polymer fibers to epoxy matrix Glass, ceramic, C or B fibers to epoxy matrix For strength: Interface must be a strong bond Metal-matrix composites to increase strength above yield strength Polymer-matrix composites to increase strength For toughness: Generally interface should be relatively weak Brittle fibers in ceramics or polymers (energy absorbed by interface debond) Ductile metal in ceramics (energy absorbed by plasticity) - e.g. , WC/Co tools Other more sophisticated mechanisms to absorb energy at crack tip Density of composites Consider unit volume of composite Volume of fiber = f ; Volume of matrix = 1-f Mass of fiber = f ! f Mass of matrix = 1 ! f ( ) " m (in 1 m 3 of composite) Mass of 1 m 3 composite: f ! f + 1 " f ( ) ! m Density of composite: ! c = f ! f + 1 " f ( ) ! m Rule of mixtures Stiffness of composites Consider an area A c of composite with a force F c applied || to fibers (plates) Modulus of fibers: E f Modulus of matrix: E m Volume fraction of fibers: f ME 382 Lecture 35 14/iv/06 2 Area of fibers =...
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- Spring '08