Characteristics of Composite MaterialsChapter 13

Unique Characteristics of CompositesComposite• A substance consisting of a minimum of 2 materials• At least one material serves as a solid reinforcement• The second is a binding material (matrix)

• Binding material begins as a liquid or slurry• Will saturate the binding material as it solidifies• Hold the reinforcement in a desired orientation

• Mechanical properties of the composite determined by:• Position of the reinforcement material• Shape of product• Manufacturing process used

• Reinforcement materials carry the load• Matrix material distributes the load and protects the

reinforcement material

• Composites can be used in a wide variety of service and performance conditions

• Depends on:• Selection of reinforcement materials • Proper placement of reinforcement materials• Selection of suitable matrix material

Structure of Composite Materials• Fiber reinforced plastic (FRP)• Most widely recognized• Consists of glass fibers and polymer resin• Glass fiber very susceptible to damage• Polymer resin has low tensile strength• Resin is tough, and protects glass fibers

• FRP composites made up of layers• Called “lay-up” or “buildup”

• FRP used in open molding (layup procedure)• A common low cost manufacturing process

• Layer of polymer resin is sprayed in a mold• Layer of glass fiber is added• Fiber is “wetted” with more resin• Another layer of glass is added• Process is repeated until desired buildup is achieved

• FRP is competitive with woods, metals and plastics• Material costs more• Lower cost comes from less expensive manufacturing

processes for composites

Reinforcements• Variety of reinforcement materials• Three categories:• Fillers• Fibers• Solids

Fillers• Added to resins to change their flow characteristics and

increase the volume• Dust like particles and short fibers

• Easy to handle – paste like• Modest mechanical properties• Typically used to fill in gaps and voids and improve surface

finish• Reduce cost• Referred to as an extender

Classes of FillersMineral fillers Organic fillers Specialty fillers

Calcium carbonate Wood flour Microspheres

Calcium sulfate Calcium carbonate Solid glass spheres

Talc Walnut shells (ground) Hollow glass spheres

Mica Corncobs (ground) Ceramic spheres

Thermoplastic spheres

Phenolic spheres

Fibers• Mainstay of reinforcement• Economical • Good mechanical properties• Dependent on type of glass fiber and orientation

• Ingredients:• Silica sand (primary)• Metal oxides (added)

• Size of fibers – 3.5 to 24 microns

E-glass• Most economical glass fiber• Sufficient strength• Low cost

• At least 50% silica oxide• Also can have aluminum, boron, calcium, limestone, and others

• Accounts for nearly 90% of the glass reinforcements• “E” refers to it’s outstanding electrical insulating

characteristics

Chemical Resistant Glass• Glass fiber has good chemical resistance• Over exposure to hot water will cause erosion to exposed

glass fibers• Corrosion resistant resin is added to slow the erosion• Result it a composite with good chemical resistance

• Selection of appropriate fibers based on chemical exposure is also important

Specialty Fibers• Higher tensile strength and stiffness than glass fibers• Made from:• Carbon• Boron• Aramid

• Much higher cost• Exceptional performance for customer willing to pay

• Hybrid – specialty fibers mixed with glass fibers• Helps control cost

Reinforcement Enhancements• Single strands of reinforcement are hard to handle and keep in

place when adding a binding matrix• Can result in a product with variable mechanical properties• Time consuming to manufacture

• To solve this problem glass fiber must go through further processing• Glass fiber gathered into bundles or strands

• Strand – collection of more than one continuous filament• Roving – bundle of untwisted glass strands packaged like thread on a

large spool

• Fabrics are popular reinforcement• Mat – nonwoven fabric made from fibers held together by a

chemical binder• Chopped mat – randomly distributed fibers cut to length

• Lower cost• Continuous-strand mat – swirls of unbroken fiber strands

• Stronger• Plain weave – simple fabric, over under weave

Protecting the glass fiber during processing• Glass fibers can be damaged by abrasion while being handled• Can cause breakage• Sizing – chemical mixture to coat and protect glass fibers

Solid Reinforcement• Creates stiff and lightweight composites• Also called laminate• Core material - the solid reinforcement material

• Sandwich construction• Face skin laminate (fiber reinforcement)• Core material• Reinforced back skin laminate

• Results in thicker, stiffer laminates

• Weight is always a concern• Core materials must be light and economical• Must be able to be bonded by the resin being used• Polystyrene foam – dissolved by resin

Balsa wood• Marine hulls and decks• Lightweight and rigid• 6 to 16 lbs per cubic foot• Laminate bonds to the end grain of the wood• Improved crush strength

Polyvinyl chloride (PVC) foam• Combination of strength and low weight• 4 to 30 lbs per cubic foot

Linear PVC foam• Mainly for marine industry• Allows deflection before failure

Polyurethane foam• In sheet stock or liquid that is foamed into place• Used in cavity of boat hulls• Add stiffness• Buoyancy• Low shear strength• Not structural applications• Also good thermal insulator (wall panels of refrigeration units)• 2 to 20 lbs per cubic foot

Honeycomb cores• Aerospace and transportation• Made from: paper, aluminum, glass reinforced phenolic• 1 to 6 lbs per cubic foot• Very stiff• Lightweight• High crush strength

Creating the matrix in the composite• Matrix generally a resin• Function to transfer the load to and protect the reinforcement

fiber• Most popular matrix materials are plastics• Thermoplastic (re-formable)• Thermosets (not re-formable) (primarily used)

Gel Coat• Resin formulated to provide a quality finish to the outer

surface of a composite• Improves durability• Provides color, gloss and environmental protection• Not paint

Resin Additives• Improve the chemical and physical properties of the matrix

resin• Many available additives:• Fire retardants/suppressants• UV inhibitors• Conductive additives

Composite Stock• Prepregs – sheets of fiber saturated with resin• Manufacturer molds prepregs into shapes

• Composite stocks usually made on site• Purchase resins, reinforcement and additives separately• Mix themselves