DOBSON+CH15 - COMPOSITES •  Combina0on of two ...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: COMPOSITES •  Combina0on of two or more individual materials •  Design goal: obtain a more desirable combina0on of proper0es (principle of combined ac/on) –  e.g., low density and high strength Carbon Fiber Boeing 787 Dreamliner Skis Aerospace composites- Desirable properties Low density Stiff Strong Corrosion resistant Abrasion resistant Creep resistant Plywood Composites: Terminology/Classifica0on • COMPOSITE: - - Mul0phase material that is ar0ficially made. • PHASE TYPES: - - Matrix - is con0nuous - - Dispersed - is discon0nuous and surrounded by matrix Composites: Terminology/Classifica0on Schema0c representa0on of various geometrical and spa0al characteris0cs of par0cles of the dispersed phase that may influence the proper0es of composites Concentra0on Distribu0on Size Shape Orienta0on Composites: Terminology/Classifica0on • Matrix phase: - - Purposes are to: Woven fibers - transfer stress to dispersed phase - protect dispersed phase from environment • Dispersed phase: - - Purpose: 0.5 mm - Increase yield stress, creep resistance, fracture toughness, etc - - Types: par0cle, fiber, structural Cross section view 0.5 mm Classifica0on of Composites The following schema0c provides one categoriza0on of composite composi0on and structure. Par0cle Reinforced Composites A)  Large- par0cle composites B) Dispersion- strengthened composites Large Par0cle Composites: polymeric materials to which fillers have been added Copper- tungsten •  Par0cle geometry varies composite •  Par0cles should be equiaxed •  Par0cles should be well dispersed •  A rule- of- mixtures provides an upper and lower bound to physical and mechanical proper0es. Ec (u ) = EmVm + E pV p Ec (l ) = Em E p E pVm + EmV p E – elas0c modulus V – volume frac0on c – composite phase m – matrix phase p – par0culate phase Par0cle Reinforced Composites Concrete is a large- par0cle composite in which both phases are ceramic. It is composed of 60- 80% aggregate filler (par0ally designed to reduce cost). Fillers of different size, sand and gravel, are employed to achieve good packing. Concrete: gravel + sand + cement + water - Why sand and gravel? Sand fills voids between gravel par0cles Reinforced concrete – Reinforce with steel rebar or remesh - increases strength - even if cement matrix is cracked Pre- stressed concrete - Rebar/remesh placed under tension during se]ng of concrete - Release of tension a^er se]ng places concrete in a state of compression - To fracture concrete, applied tensile stress must exceed this compressive stress Post- tensioning – 0ghten nuts to place concrete under compression threaded rod nut Classifica0on of Composites Fiber Reinforced Composites Design goal: produce materials of high strength and s0ffness with reduced weight. Applica0on: Aerospace, automo0ve, spor0ng goods Specific strength: ra0o of tensile strength to specific gravity Specific modulus: ra0o of modulus of elas0city to specific gravity Design parameters: Fiber length (sub- classifica0on) Fiber orienta0on Fiber concentra0on Fiber/matrix composi0on Composites: Fiber Length A critical length (lc) is required to realized enhanced mechanical properties. Below this length, fibers will have no influence on composite performance. * f σd lc = 2τ c The critical length is influenced by the fiber diameter (d) , the ultimate fiber strength (σ*f), and the shear strength between the fiber and its matrix ( c ). τ Composites: Fiber Orienta0on & Concentra0on •  Two extremes: (1) parallel alignment of longitudinal axis (2) completely random •  Continuous fibers (l >> lc) are typically aligned with the longitudinal direction. •  Discontinuous fibers (l << lc) can be aligned, partially aligned, or random. •  The greatest impact on mechanical properties is observed for a uniform orientation. Influence of Fiber Orienta0on & Concentra0on The mechanical properties of aligned fiber composites are highly anisotropic, i.e. they depend strongly on the direction in which they are measured. For stress, applied in the direction of fiber alignment, the longitudinal direction, the fibers are seen to enhance the modulus and strength beyond that of the matrix. Stage I: elastic deformation of both fiber and matrix Stage II: Fiber-elastic Matrix- plastic Polymer Matrix Composites Widely employed, polymer-matrix composites offer superior strength to weight ratios and are found in a variety of applications. Polymer-matrix composites can be categorized as follows: 1.  Glass fiber-reinforced polymer (GFRP) composites 2.  Carbon fiber-reinforced polymer (CFRP) composites 3.  Aramid (aromatic polyamide) fiber-reinforced polymer composites (Kevlar) The selection of both fiber and matrix determine the ultimate composite performance. Design considerations include: •  •  •  •  •  Strength Stiffness Thermal stability Chemical stability Ductility and toughness Metal Matrix Composites In these composites, the matrix is a ductile metal. The composites are designed for use at high temperatures and offer generally superior chemical resistance and oxidative stability over polymer matrix composites. Metalmatrix composites tend to be more expensive than others, and limits relatively their use. Both particulates and fibers are used as the dispersed phase. Note the composition of the fibers listed below. Ceramic Matrix Composites The primary aim of developing composites using ceramics as the matrix is to enhance the fracture toughness. Particulates, fibers, and whiskers have been employed as the dispersed phase. Several mechanisms by which enhancement of fracture toughness have been identified. These include transformation toughening (below) and crack deflection. Classifica0on of Composites Classifica0on of Composites: Structural Particle-reinforced Fiber-reinforced • Laminates - -- stacked and bonded fiber-reinforced sheets - stacking sequence: e.g., 0º/90º - benefit: balanced in-plane stiffness • Sandwich panels -- honeycomb core between two facing sheets - benefits: low density, large bending stiffness Skis face sheet adhesive layer honeycomb Structural Composite Produc0on Methods Pultrusion •  •  •  Con0nuous fibers pulled through resin tank to impregnate fibers with thermose]ng resin Impregnated fibers pass through steel die that preforms to the desired shape Preformed stock passes through a curing die that is –  precision machined to impart final shape –  heated to ini0ate curing of the resin matrix Fig. 16.13, Callister & Rethwisch 8e. Composite Produc0on Methods •  Filament Winding –  Con0nuous reinforcing fibers are accurately posi0oned in a predetermined pajern to form a hollow (usually cylindrical) shape –  Fibers are fed through a resin bath to impregnate with thermose]ng resin –  Impregnated fibers are con0nuously wound (typically automa0cally) onto a mandrel –  A^er appropriate number of layers added, curing is carried out either in an oven or at room temperature –  The mandrel is removed to give the final product Composite Benefits • CMCs: Increased toughness Force particle-reinf • PMCs: Increased E/ρ E(GPa) ceramics 10 3 10 2 PMCs 10 fiber-reinf 1 un-reinf 0.1 0.01 0.1 0.3 Bend displacement 10 -4 • MMCs: ε ss (s-1) Increased creep resistance 6061 Al 10 -6 10 -8 6061 Al w/SiC whiskers 10 -10 20 30 50 σ(MPa) 100 200 metal/ metal alloys polymers 1 3 10 30 Density, ρ [mg/m3] ...
View Full Document

This note was uploaded on 02/28/2012 for the course EMA 3010 taught by Professor Unknown during the Fall '08 term at University of Florida.

Ask a homework question - tutors are online