Concrete consists of an aggregate of particles that are bonded together by a cement. Limitations of concrete: üweak and brittle material ülarge thermal expansions - changes in temperature ümay crack when exposed to freeze-thaw f reeze-thaw cycles
Three reinforcement strengthening techniques: 1.reinforcement with steel wires, rods, etc 2.reinforcement with fine fibers of a high modulus material 3.introduction of residual compressive stresses by posttensioning.
Reinforced concrete - Reinforce with steel, rod or mesh - increases strength - even if cement matrix is cracked
Prestressed concrete - remesh under tension during setting of concrete. Tension release puts concrete under compressive force - Concrete much stronger under compression. - Applied tension must exceed compressive force
Post tensioning – tighten nuts to put under tension
Adapted from Fig. 16.3, is Callister 7e. 7e. (Fig. 16.3 is from R.H. Krock, ASTM Krock, ASTM Proc , Vol. 63, 1963.)
10 0 vol% tungsten
For equal volume of fibers: • Upper limit: isostrain conditions, higher modulus Lower limit: isostress conditions, lower modulus m odulus
Composite Survey: Fiber-I Particle-reinforced
Fibers very strong Provide significant significant strength improvement to material Ex: fiber-glass
Continuous glass filaments in a polymer matrix Strength due to fibers Polymer simply holds them in place
• • •
fiberglass-reinforced composites are utilized extensively? 14 üinexpensive to produce ücomposites have relatively high specific strengths üchemically inert in a wide variety of environments Limitations of these composites? care in handling the fibers, susceptible to surface damage lacking in stiffness in comparison to other fibrous composites limited maximum temperature use
Glass fibers 15
Glass fibers are amorphous (noncrystalline) and isotropic (equal properties in all directions) and are a long, three-dimensional network of silicon, oxygen, and other atoms arranged in a random fashion.
an inorganic, synthetic, multifilament material are the most common of all reinforcing fibers for polymeric (plastic) matrix composites strong, low in cost, nonflammable, nonconductive (electrically), and corrosion resistant.
Glass fibers 16
low tensile modulus relatively relatively high specific gravity (among (am ong the commercial fibers) sensitivity to abrasion with handling (which frequently decreases tensile strength) relatively relatively low fatigue resistance high hardness (which causes excessive wear on molding dies and cutting tools) • •
Glass fibers 17
S-2 hollow glass fiber S-2 glass
Glass fibers 18
E-glass has the lowest cost of all commercially available reinforcing fibers, which is the reason for its widespread use in the fiber-reinforced plastics (FRP) industry.
S-glass, originally developed for aircraft S-glass, components and missile casings, has the highest tensile strength among all fibers in use. However, the compositional difference and higher manufacturing cost make it more expensive than E-glass.
Composite Survey: Fiber-II Particle-reinforced
Whiskers - Thin single crystals - large length to diameter ratio graphite, SiN, SiC high crystal perfection – extremely strong, strongest known very expensive Fibers polycrystalline or amorphous generally polymers or ceramics Ex: Al2O3 , Aramid, E-glass, Boron, UHMWPE –
• • •
Metal – steel, Mo, W
Fiber Alignment 20 Adapted from Fig. 16.8, Callister 7e. 7e.
-- Ceramic Ceramic:: Glass w/SiC fibers formed by glass slurry E glass glass = 76 GPa; E SiC SiC = 400 GPa.
fibers: γ ’ (Ni3Al) (brittle) From W. Funk and E. Blank, “Creep deformation of Ni3Al-Mo in-situ composites", Metall. Trans. AVol. AVol. 19(4), pp. 987-998, 1988. Used with permission. permission.
fracture surface From F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Science, Reprint ed., CRC Press, Boca Raton, FL, 2000. (a) Fig. 4.22, p. 145 (photo by J. Davies); (b) Fig. 11.20, 11.20, p. 349 (micrograph by H.S. Kim, P.S. Rodgers, and R.D. Rawlings). Rawlings). Used with permission of CRC Press, Boca Raton, FL.
Composite Survey: Fiber-IV 22
Particle-reinforced Fiber-reinforced • Discontinuous, random 2D fibers • Example: Carbon-Carbon -- process: fiber/pitch, then burn out at up to 2500ºC. -- uses: disk brakes, gas turbine exhaust flaps, nose cones.
Structural C fibers: very stiff very strong
C matrix: less stiff view onto plane less strong fibers lie in plane
• Other variations: -- Discontinuous, random 3D -- Discontinuous, 1D
Adapted from F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Science, Reprint ed., CRC Press, Boca Raton, FL, 2000. (a) Fig. 4.24(a), p. 151; (b) Fig. 4.24(b) p. 151. (Courtesy I.J. Davies) Reproduced with permission of CRC Press, Boca Raton, FL.
Composite Survey: Fiber-V 23
Particle-reinforced Fiber-reinforced Structural • Critical fiber length for effective stiffening & strengthening: fiber strength in tension
fiber length > 15
fiber diameter σ f d τc
shear strength of fiber-matrix interface
• Ex: For fiberglass, fiber length > 15 mm needed more efficiently! efficiently! • Why? Longer fibers carry stress more Shorter,, thicker fiber: Shorter
transverse loading the fibers carry less of the load isostress c= m = f = c= mV m + f V f
1 E ct
V m E m
V f E f
Composite Strength Particle-reinforced
• Estimate of E of E c TS for discontinuous fibers: c and TS for -- valid when fiber length
σ f d τ c
-- Elastic modulus in fiber direction:
E c = E mV m + K E Ef V f efficiency factor : -- aligned 1D: K = 1 (aligned ) -- aligned 1D: K = 0 (aligned ) -- random 2D: K = K = 3/8 (2D isotropy) -- random 3D: K = K = 1/5 (3D isotropy)
Values from Table 16.3, Callister 7e. 7e. (Source for Table Table 16.3 is H. Krenchel, Fibre Reinforcement , Copenhagen: Akademisk Forlag, 1964.)
-- TS in in fiber direction:
(TS )c = (TS )mV m + (TS )f V f
Fiber forms 27
Woven fabric fabr ic
Fiber forms 28
X-mat bidirectional unidirectional
Composite Production Methods-I Pultrusion
Continuous fibers pulled through resin tank, then preforming die & oven to cure Precision machine to impart the final shape
Performs of desired shape, Establishes the resin/fiber ratio
Adapted from Fig. 16.13, Callister 7e. 7e.
advantages are: üProcess may be automated üHigh production rates ü Variety of shapes,constant cross-sections, long pieces may be produced The disadvantage – shapes are limited to those having a constant cross-section.
Composite Production Methods-II Filament
Ex: pressure tanks Continuous filaments wound onto mandrel to form a hollow (usually cylindrical) shape
Adapted from Fig. 16.15, Callister 7e. 7e. [Fig. 16.15 is from N. L. Hancox, (Editor), Fibre Composite Hybrid Materials, The Macmillan Company, New York, 1981.]
Filament Winding 32
advantages are: üProcess may be automated ü Variety of winding patterns are possible üHigh degree of control over winding uniformity and orientation The disadvantage – variety of shapes is limited
Adapted from T.G. T.G. Nieh, "Creep rupture of a silicon-carbide reinforced aluminum composite", Metall. Trans. AVol. AVol. 15(1), pp. 139-146, 1984. Used with with permission. permission.
Summary 35 • Composites are classified according to:
-- the matrix material (CMC (CMC,, MMC MMC,, PMC PMC)) -- the reinforcement geometry (particles, fibers, layers).
• Composites enhance matrix properties: -- MMC: enhance σ y , TS , creep performance -- CMC: enhance K c c -- PMC: enhance E , σ y , TS , creep performance • Particulate-reinforced: -- Elastic modulus can be estimated. -- Properties are isotropic. • Fiber-reinforced: -- Elastic modulus and TS can TS can be estimated along fiber dir. dir. -- Properties can be isotropic or anisotropic. • Structural: -- Based on build-up of sandwiches in layered form.