Characteristics of Composite Materials

1. Characteristics of Composite Materials Chapter 13

2. Unique Characteristics of Composites Composite • 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

3. 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

4. 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

5. 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”

6. 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

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

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

9. 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

10. Classes of Fillers

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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)

24. 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

25. Resin Additives • Improve the chemical and physical properties of the matrix resin • Many available additives: • Fire retardants/suppressants • UV inhibitors • Conductive additives

26. 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