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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
- Tensile strength, Fiberglass