1. CHAPTER 16: COMPOSITE MATERIALS
ISSUES TO ADDRESS...
• What are the classes and types of composites?
• Why are composites used instead of metals,
ceramics, or polymers?
• How do we estimate composite stiffness & strength?
• What are some typical applications? 1

2. Definition of Composite Materials
It contains two or more physically distinct and mechanically separable materials
It is made by dispersing one material in the other in a controlled way to achieve optimum properties
The properties of the composite are superior and possibly unique in some specific respects to the properties of individual components
Professor Derek Hull
University of Liverpool

3. TERMINOLOGY/CLASSIFICATION
• Composites:
--Multiphase material w/significant
proportions of ea. phase.
• Matrix:
--The continuous phase
--Purpose is to:
transfer stress to other phases
protect phases from environment
--Classification: MMC, CMC, PMC
metal
ceramic
polymer
• Dispersed phase:
--Purpose: enhance matrix properties.
MMC: increase sy, TS, creep resist.
CMC: increase Kc
PMC: increase E, sy, TS, creep resist.
--Classification: Particle, fiber, structural
Reprinted with permission from
D. Hull and T.W. Clyne, An Introduction to Composite Materials, 2nd ed., Cambridge University Press, New York, 1996, Fig. 3.6, p. 47. 2

4. COMPOSITE SURVEY: Particle-I
Particle-reinforced
• Examples:
Adapted from Fig , Callister 6e. (Fig is copyright United States Steel Corporation, 1971.)
Adapted from Fig. 16.4, Callister 6e. (Fig is courtesy Carboloy Systems, Department, General Electric Company.)
Adapted from Fig. 16.5, Callister 6e. (Fig is courtesy Goodyear Tire and Rubber Company.) 3

5. COMPOSITE SURVEY: Particle-II
Particle-reinforced
• Elastic modulus, Ec, of composites:
-- two approaches.
Adapted from Fig. 16.3, Callister 6e. (Fig is from R.H. Krock, ASTM Proc, Vol. 63, 1963.)
• Application to other properties:
-- Electrical conductivity, se: Replace E by se.
-- Thermal conductivity, k: Replace E by k. 4

6. Why Composites?

7. Mechanical Properties

8. Basic Principles of Reinforcement
Types of reinforcing elements
Forms of reinforcing elements
Direction of reinforcement
Load transferring ability
Volume fraction of reinforcements
Continuous vs. discontinuous reinforcement

9. Characteristics of fiber-reinforced Composites
Amount of fibers
Orientation of fibers
Types of fibers
Fiber aspect ratio
Fiber orientation effects
Strain rate effects
Type of matrix
Interfacial bonding conditions

10. Fibers Inorganic fibers Glass Graphite (carbon) Silicon carbide
Aramid
Thermoplastics
Metallic fibers
Stainless steel
Nickel alloy

11. Glass Fibers
Glass is an amorphous material that consists of a silica (SiO2) backbone with various oxide components to give specific compositions and properties.
Types: E-glass, S-glass, C-glass, quartz
E-glass: calcium aluminoborosilicate with 2% alkali; good strength and electrical resistivity; the least expensive one
S-glass: 40% higher than E-glass; high temp application
C-glass: soda limeborosilicate; use in corrosive environments
Quartz: low dielectric; use for protecting antennas and radomes

12. Properties of Glass Fibers

14. Use of Glass Fibers
Tensile strength is highly dependent on surface defects. The shorter the sample, the higher the value.
Moisture has a detrimental effect on strengths
Temperature has profound impact on strength and modulus. The higher the temperature, the lower the strength (E-glass will be lower than S-glass). The higher the temperature, the higher the tensile modulus (E and S are about the same)

15. Surface Treatments
Glass fibers are very brittle and susceptible to developing tiny cracks when processed lead to premature failure
Sizing effect – a thin temporary water soluble coating
Coupling agent – Silanes

16. Fiber Glass Manufacturing Processes

17. Carbon/Graphite Fibers
Three precursors: polyacrylonitrile (PAN), Rayon, and mesophase pitch fibers, stretched and heated at 350oC then 1000oC
CVD method: Pyrolytic deposition (via methane, benzene and naphthalene) at 1000oC
Carbon fibers heated to 2000oC with/without stretching (graphitization) graphite fibers

18. Structure of Graphite Fibers

19. Properties of Graphite Fibers

20. Manufacturing Process

21. Organic Fibers
Para-phenylene diamine and terephthaloyl chloride are mixed in an organic solvent to form poly paraphenyleneterephthalamide.
Developed from aromatic polyamides, also known as aramid fibers
Manufactured by E.I. Du Pont with a trade name – Kevlar
It is fully aligned and closely packed
The polymer is washed and then dissolved in sulfuric acid. 20 wt% polymer solution is then passed through an extruder and spinnerettes to develop a high degree of orientation (liquid crystal form; process patented)
Four types of aramid fibers: kevlar®, Kevlar 29 (high toughness), kevlar 49 (high modulus) and kevlar 149 (ultra-high modulus)
Kevlar fibers are less brittle than carbon or glass fibers, however, a combination of good strength, light weight and excellent toughness has led to the unique applications for aramid composites

22. Properties of Kevlar Fibers
Tensile Modulus (MPa)
Tensile Strength (MPa)
Elongation (%)
Density
(g/c.c.)
K-29 83
3.6 4
1.44
K-49
131
2.8
K-149
186
3.4 2
1.47

25. Inorganic Fibers
A class of short crystalline fibers, often called crystal whisker fibers
Made of aluminum oxide, beryllium oxide, magnesium oxide, potassium titanate, silicon carbide, silicon nitride, titanium boride, etc.
Boron and silicon carbide fibers are popular because they offer very high tensile strength and modulus.
Boron fibers are considered amorphous (crystal structure is small), but SiC has a much larger crystal structure.
Boron fibers have a surface structure like scale or corncob appearance. SiC fibers have a smoother surface than boron.
Boron composites have excellent tensile property retention with increasing temperature

26. Boron/Silicon Carbide
Boron trichloride interacts with the tungsten filament to form tungsten borides, simultaneous bonded with the deposited boron coated layers.
Silicon carbide, a structure similar to diamond, offers a low density, high stiffness, high strength and excellent thermal stability and thermal conductivity properties.
Growth from a melt --whisker form – defect free, single crystal rod, 0.1-1μm in diameter
CVD on fine carbon (30 μm) or tungsten (10 μm) fiber core to yield a large-diameter SiC monofilament fiber ( μm)
Developed from polycarbosilane (PCS) precursor, stretched and heated at 1300oC .

28. Properties of Boron and SiC Filaments
Silicon Carbide
Density (g/cc)
Tensile Strength (GPa)
3.60
3.90
Tensile Modulus (GPa)
441
400
Elongation (%)
0.9
N/A
CTE (10-6 cm/cm/oC)
2.5
Cost ($/lb)
320
100

29. Thermal Stability of Fibers
Carbon fibers can be used in high temperatures (above 2000oC) in which the oxidizing environments are absent.
The strength and modulus of silica based glass fibers decrease rapidly above 250oC and have a softening point around 850oC. Aramid fibers are worse than carbon and glass fibers at high temperatures. Aramid fibers can be attacked by UV lights as well.
Inorganic fibers (Al2O3 and SiC) can sustain much higher temperatures (above 2500oC)

30. Flexibility
The flexibility of a fiber, defined as k/M, can be expressed in moment, M.
Where E is the tensile modulus, d is the fiber diameter and k is the reciprocal of the radius of the curvature

32. Matrices
Thermosetting polymeric resins – epoxy, polyester, phenolics, polurethane, polyimides
Thermoplastic resins – polyamide (nylon), polypropylene (PP), poly ether ether ketone (PEEK)
Elastomers – silicone, neoprene (CR), NBR, SBR
Metal matrix
Ceramic matrix

33. Polymer Matrices Polymers Thermosets Thermoplastics Elastomers
Non-crystalline
Crystalline

35. The Rule of Mixtures Density c= Vmm +Vf f Thermal Conductivity
Kc= Vm Km+Vf Kf
Electrical Conductivity
c= Vmm+Vf f

36. Modulus of Elasticity (iso-strain)
When a stress is applied parallel to the fiber orientation direction, the modulus of the elasticity of the composite can be expressed as
Ec= Vm Em+Vf Ef (to be derived later)
Since the matrix contributes little to the stiffness of the composite, the modulus can be simplified as
Ec =Vf Ef

37. Modulus of Elasticity (iso-stress)
When a stress is applied perpendicular to the fiber orientation direction
The modulus of the composite is expressed as

38. Iso-Strain Loading Force is applied along the axis of the fibers
The total force is the sum of all forces carried by matrix and fibers
Fc = Fm +Ff
Since F =A, cAc= mAm+ fAf
c =mVm+fVf (assuming uniform cross-section, area fraction is the same as volume fraction)
Iso-strain condition, c = f = m Ec=VmEm+VfEf

39. Iso-Stress Loading
Force is vertically applied to the axis of the fibers (the strains are no longer equal – each component acts independently)
c=Vmm+Vff
Since c=m=f
The modulus of the elasticity of the composite is now expressed as

40. Example 1
Boron fibers (40 v%) are used to reinforce aluminum for an engineering application. Use the table provided below to calculate the density, modulus of elasticity, and tensile strength long the fiber orientation. Also estimate the modulus of elasticity if the load is to be applied perpendicular to the fibers.
Material
Density (g/cc)
Modulus (ksi)
UTS (ksi)
Fibers
2.36
55,000
400
Aluminum
2.70
10,000 5

41. From the rule of mixture
For density:
c= Vmm +Vf f
c= (0.6) (2.7)+(0.4) (2.36) = 2.56 g/cc
For modulus:
Ec= Vm Em+Vf Ef\
Ec= (0.6) (10 x 106) +(0.4) (55 x 106)= 28 x 106 psi
For tensile strength:
UTS = (0.6) (5,000) +(0.4) (400,000) = 163,000 psi
Loading perpendicular to fibers:

42. Example 2
E-glass fibers are used to reinforce nylon in an industrial application. If the nylon contains 30 vol% glass fibers, what fraction of the applied force is carried by the glass fibers? (The modulus of elasticity for E-glass fibers and nylon are 10.5 x106 psi and 0.4 x106 psi, respectively)

43. Assume equal strain (iso-strain) condition
Why? If the bonding is good, the composite should yield only one strain value.
c = f = m

44. How about particle reinforcement?
The formula will have to be modified.
Kc is an empirical constant. If its value is smaller than one, then the equal (iso) strain condition can’t be applied.
Kc and Ks are different constants, both are determined in the lab

45. Deformation Mechanisms in Composites
Parallel to fiber orientation:
Stage I: Strain is small, fibers and matrix both elongate (or deform) elastically
Fibers carry the load, EcVfEf
Stage II: Incompatibility of the lateral matrix and fiber deformation strains
Matrix deforms plastically; Fibers deform elastically
Stage III: Fiber deformed plastically before fracture (found in metallic fibers)
Both matrix and fibers deform plastically

46. Reinforcement with Discontinuous Fibers
Fiber midpoint offers the lowest shear stress and highest highest tensile stress
Fibers have to reach a critical aspect ratio (lc/df), equal strain condition

47. Characteristics of fiber-reinforced Composites
Amount of fibers
Orientation of fibers
Types of fibers
Fiber aspect ratio
Fiber orientation effects
Strain rate effects
Type of matrix
Interfacial bonding conditions

48. COMPOSITE SURVEY: Fiber-I
Fiber-reinforced
• Aligned Continuous fibers
• Examples:
--Metal: g'(Ni3Al)-a(Mo)
by eutectic solidification.
--Glass w/SiC fibers
formed by glass slurry
Eglass = 76GPa; ESiC = 400GPa.
(a)
From F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Reprint ed., CRC Press, Boca Raton, FL, (a) Fig. 4.22, p. 145 (photo by J. Davies); (b) Fig , p. 349 (micrograph by H.S. Kim, P.S. Rodgers, and R.D. Rawlings). Used with permission of CRC
Press, Boca Raton, FL.
(b)
From W. Funk and E. Blank, “Creep deformation of Ni3Al-Mo in-situ composites", Metall. Trans. A Vol. 19(4), pp , Used with permission. 5

49. COMPOSITE SURVEY: Fiber-II
Fiber-reinforced
• Discontinuous, random 2D fibers
• Example: Carbon-Carbon
--process: fiber/pitch, then
burn out at up to 2500C.
--uses: disk brakes, gas
turbine exhaust flaps, nose
cones.
(b)
(a)
• Other variations:
--Discontinuous, random 3D
--Discontinuous, 1D
Adapted from F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Reprint ed., CRC Press, Boca Raton, FL, (a) Fig. 4.24(a), p. 151; (b) Fig. 4.24(b) p (Courtesy I.J. Davies) Reproduced with permission of CRC Press, Boca Raton, FL. 6

50. COMPOSITE SURVEY: Fiber-III
Fiber-reinforced
• Critical fiber length for effective stiffening & strengthening:
fiber strength in tension
fiber diameter
shear strength of
fiber-matrix interface
• Ex: For fiberglass, fiber length > 15mm needed
• Why? Longer fibers carry stress more efficiently!
Shorter, thicker fiber:
Longer, thinner fiber:
Adapted from Fig. 16.7, Callister 6e.
Poorer fiber efficiency
Better fiber efficiency 7

51. COMPOSITE SURVEY: Fiber-IV
Fiber-reinforced
• Estimate of Ec and TS:
--valid when
-- Elastic modulus in fiber direction:
--TS in fiber direction:
efficiency factor:
--aligned 1D: K = 1 (anisotropic)
--random 2D: K = 3/8 (2D isotropy)
--random 3D: K = 1/5 (3D isotropy)
Values from Table 16.3, Callister 6e. (Source for Table 16.3 is H. Krenchel, Fibre Reinforcement, Copenhagen: Akademisk Forlag, 1964.)
(aligned 1D) 8

52. COMPOSITE SURVEY: Structural
• Stacked and bonded fiber-reinforced sheets
-- stacking sequence: e.g., 0/90
-- benefit: balanced, in-plane stiffness
Adapted from Fig , Callister 6e.
• Sandwich panels
-- low density, honeycomb core
-- benefit: small weight, large bending stiffness
Adapted from Fig ,
Callister 6e. (Fig is
from Engineered Materials
Handbook, Vol. 1, Composites, ASM International, Materials Park, OH, 1987. 9

53. COMPOSITE BENEFITS • CMCs: Increased toughness • PMCs: Increased E/r
• MMCs:
Increased
creep
resistance
Adapted from T.G. Nieh, "Creep rupture of a silicon-carbide reinforced aluminum composite", Metall. Trans. A Vol. 15(1), pp , Used with permission. 10

54. SUMMARY • Composites are classified according to:
-- the matrix material (CMC, MMC, PMC)
-- the reinforcement geometry (particles, fibers, layers).
• Composites enhance matrix properties:
-- MMC: enhance sy, TS, creep performance
-- CMC: enhance Kc
-- PMC: enhance E, sy, TS, creep performance
• Particulate-reinforced:
-- Elastic modulus can be estimated.
-- Properties are isotropic.
• Fiber-reinforced:
-- Elastic modulus and TS can be estimated along fiber dir.
-- Properties can be isotropic or anisotropic.
• Structural:
-- Based on build-up of sandwiches in layered form. 11

55. REVIEW Reading: 16.1 -16.14 Core Problems: Definition of composite
Longitudinal vs. Transverse Strengths
Rule of Mixtures
Efficiency factor
PMC, CMC, and MMC