Phy351 ch 9


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Phy351 ch 9

  2. 2. INTRODUCTION A composite material is a material system, a mixture or combination of two or more micro or macro constituents that differ in form and composition and do not form a solution.  Properties of composite materials can be better-quality to its individual components.  Examples: - fiber reinforced plastics - concrete - asphalt - wood etc. 2
  3. 3. EXERCISE 1 a. b. c. d. Give 2 examples of natural composite. Give 3 special properties of composite materials. Give 2 applications of composite. Distinguish between cement and concrete. 3
  4. 4. FIBERS FOR REINFORCED PLASTIC COMPOSITE MATERIALS  FIBER REINFORCED PLASTIC is a composite materials consisting of a mixture of a matrix of a plastic material such as a polyester or epoxy strengthened by fibers of high strength such as:     glass carbon arimid The fibers provide the high strength and stiffness and the plastic matrix bonds the fibers together and supports them. 4
  5. 5. GLASS FIBER FOR REINFORCING PLASTIC RESINS GLASS fiber are used to reinforce plastic matrices to form structural composites and molding compounds.  Glass fiber reinforced plastic composite materials have high strength-weight ratio, good dimensional stability, good temperature and corrosion resistance and low cost.  The most important types of glass used to produce glass fiber for composites are:  i. ii. ‘S” Glass = ‘S’ (high-strength) glasses used for military and aerospace application. ‘E” Glass = ‘E’ (electrical glasses) cheaper than S glass 5
  6. 6. CARBON FIBERS FOR REINFORCED PLASTICS CARBON fiber such as epoxy are characterized by having a combination of light weight, very high strength and high stiffness (modulus of elasticity).  Produced from polyacrylonitrile (PAN) and pitch.  Steps:  Stabilization: PAN fibers are stretched and oxidized in air at about 2000C.  Carbonization: Stabilized carbon fibers are heated in inert atmosphere at 1000-15000C which results in elimination of O,H and N resulting in increase of strength.  Graphitization: Carried out at 18000C and increases modulus of elasticity at the expense of strength  6
  7. 7. ARAMID FIBERS FOR REINFORCING PLASTIC RESINS  ARAMID fiber is the generic name for aromatic polyamide fibers.  Trade name is Kevlar. There are two commercial type:  Kevlar 29:- Low density, high strength, and used for ropes and cables.  Kevlar 49:- Low density, high strength and modulus and used for aerospace and auto applications.  Hydrogen bonds bond fiber together.  Used where resistance to fatigue, high strength and light weight is important. 7
  8. 8. COMPARISON OF MECHANICAL PROPERTIES  Carbon fibers provide best combination of properties.  Due to favorable properties, carbon and aramid fiber reinforced composites have replaced steel and aluminum in aerospace applications. 8 Figure 12.7: Stress-strain behavior of various types of reinforcing fibers.
  9. 9. Figure 12.8: Specific tensile strength (tensile strength to density) and specific tensile modulus (tensile modulus to density) for various types of reinforcing fibers. 9
  10. 10. MATRIX MATERIALS FOR FIBER REINFORCEDPLASTIC COMPOSITE MATERIALS  Two of the most important MATRIX plastic resins for fiber-reinforced plastics are: • •  unsaturated polyester epoxy resins The polyester resin are lower in cost but are usually not as strong as the epoxy resin. 10
  11. 11.  Fiberglass-reinforced POLYESTER resins:    Higher the wt% of glass, stronger the reinforced plastic is. Nonparallel alignment of glass fibers reduces strength. Carbon fiber reinforced EPOXY resins:     Carbon fiber contributes to rigidity and strength while epoxy matrix contributes to impact strength. Polyimides, polyphenylene sulfides are also used. Exceptional fatigue properties. Carbon fiber epoxy material is laminated to meet strength requirements. 11
  13. 13. QUESTION 2 a. Cite the general difference in strengthening mechanism between large-particle and dispersion-strengthened particle-reinforced composites. b. A undirectional Kevlar 49 fiber-epoxy composite contains 60% by volume of Kevlar 49 fibers and 40 % epoxy resin. The density of the Kevlar 49 fibers is 1.48Mg/m3 and that of the epoxy resin is 1.2 Mg/m3. i. ii. What are the weight percentages of Kevlar 49 and epoxy resin in the composite material? (Answer: 64.9%, 35.1%) What is the average density of the composite? (Answer: 1.37 Mg/m3) 13
  14. 14. EQUATION FOR ELASTIC MODULUS OF LAMELLAR COMPOSITE  Isostrain condition: Stress on composite causes uniform strain on all composite layers. Pc = P f + P m Pc = Load on composite Pf = Load on fibers Pm = load on matrix Known; σ = P/A Therefore; σcAc = σfAf + σmAm 14
  15. 15. Since length of layers are equal, σcVc = σfVf + σmVm Where; Vf and Vm = volume fractions Vc = 1 Therefore; σc = σfVf + σmVm 15
  16. 16. Since strains; εc = εf = εm Therefore;  c  f V f  mVm   c f m Ec = EfVf + EmVm (Rule of mixture of binary composites) 16
  17. 17. LOADS ON FIBER AND MATRIX REGIONS  Since σ = Eε and εf = εm  f Af E f  f Af E f Af EfVf     Pm  m Am E m  m Am E m Am E mVm Pf Pc = Pf + Pm  From above two equations, load on each of fiber and matrix regions can be determined if values of Ef, Em,Vf, Vm and Pc are known. 17
  18. 18. QUESTION 3 The composite consists of a continuous glass-foberreinforced-epoxy resin produced by using 60% by volume of E-glass fibers having a modulus of elasticity of Ef = 7.24 x 104 MPa and a tensile strength of 2.4 GPa and a harded eposy resin with a modulus of Em = 3.1 x 103 MPa and tensile strength of 0.06 GPa. Calculate the composite a. i. ii. iii. A modulus of elasticity (Answer: 44.64 GPa) The tensile strength (Answer: 1.46 GPa) The fraction of the load carried by the fiber for the following composite material stresses under isostrain conditions. (Answer: 0.97) 18
  19. 19. ISOSTRESS CONDITION  Stress on the composite structure produces an equal stress condition on all the layers. σc = σf + σm εc = εf + εm Assuming no change in area and assuming unit length of the composite εc = εfVf + εmVm 19
  20. 20. But; c   Ec , f   Ef , m   Em Therefore;  Ec  V f Ef  Vm Em 20
  21. 21. ELASTIC MODULUS FOR ISOSTRESS CONDITION  We know that  Ec  V f Ef  Vm Em 21 • Higher modulus values are obtained with isostrain loading for equal volume of fibers
  22. 22. Dividing by σ; V f Vm 1   Ec E f Em V f E m Vm E f 1   Ec E f E m Em E f Ec  E f Em V f E m  Vm E f 22
  23. 23. QUESTION 4 a. Calculate the modulus of elasticity for a composite material consisting of 60% by volume of continuous E-glass fiber and 40% epoxy resin for the matrix when stressed under isostress conditions (i.e. the material is stresses perpendicular to the continuous fiber). The modulus of elasticity of the E glass is 72.4 GPa and that of the epoxy resin is 3.1 GPa. (Answer : 7.3 GPa) 23
  24. 24. WOOD Wood is naturally occurring composite with polymeric material lignin and other organic compounds.  Nonhomogenous and highly anisotropic.  Consists of layers:   (a) Outer bark – provides protection  (b) Inner bark – moist and soft, carries food (c) Cambium layer – forms wood and bark cells (d) Sapwood – carries wood and sap. (e) Heartwood – dead, dark and provides strength (f) Pith – Soft tissue at the center (g) Wood rays      24
  25. 25. PROPERTIES OF WOOD  Moisture content: Water occurs in wood as absorbed in fiber walls or in cell fiber lumen.  150% for softwood and sapwood. Wood moisture content (wt%) = Wt of water in sample x 100 Wt of dry wood sample  Mechanical strength: Compressive strength parallel to the grain is 10 times higher than that perpendicular to the grain.  Wood in green condition is weaker than kiln-dried wood.  Shrinkage: Green wood shrinks if dried.  Shrinkage is more in transverse direction. 25
  26. 26. QUESTION 5  A piece of wood containing moisture weighs 165.3g and after oven drying to a constant weight, weighs 147.5g. What is its percent moisture content? (Answer: 12.1%) 26
  27. 27. REFERENCES     A.G. Guy (1972) Introduction to Material Science, McGraw Hill. J.F. Shackelford (2000). Introduction to Material Science for Engineers, (5th Edition), Prentice Hall. W.F. Smith (1996). Principle to Material Science and Engineering, (3rd Edition), McGraw Hill. W.D. Callister Jr. (1997) Material Science and Engineering: An Introduction, (4th Edition) John Wiley. 27