Composite Materials

Introduction
A Composite material is a material system composed of two or more macro constituents that differ in shape and chemical composition and which are insoluble in each other. The history of composite materials dates back to early 20th century. In 1940, fiber glass was first used to reinforce epoxy.
Applications:
Aerospace industry
Sporting Goods Industry
Automotive Industry
Home Appliance Industry

Advanced Aerospace Application:
Lear Fan 2100 “all-composite” aircraft

Advanced Aerospace Application:
Boeing 767 ,777, 787 airplanes w/ the latest, full wing box is composite):

Sporting Goods

Automotive

Various applications

• Composites:
-- Multiphase material w/significant
proportions of each phase.
• Dispersed phase: -- Purpose: enhance matrix properties.
MMC: increase σy, TS, creep resist.
CMC: increase Kc
PMC: increase E, σy, TS, creep resist.
-- Classification: Particle, fiber, structural
Elyaf dokuma
Terminology/Classification

Composite Structural Organization: the design variations

Fig. 2 (a) Schematic diagram of an individual layer of honeycomb-like carbon called graphene and how this could be rolled in order to form a carbon nanotube; (b)–(d) HR-TEM images of single, double- and multi-walled carbon nanotubes (insets are their corresponding images).
Fig. 1 SEM image of the smallest working gear (carbon nanotube/nylon composite); inset exhibits the fractured surface.

Composite Survey

• CMCs: Increased toughness
Composite Benefits

Composite Survey: Particle-I

Composite Survey: Particle-II
Concrete – gravel + sand + cement
- Why sand and gravel? Sand packs into gravel voids
Reinforced concrete - Reinforce with steel rebar or remesh
- 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

• Elastic modulus, Ec, of composites:
-- two approaches.
• Application to other properties:
-- Electrical conductivity, σe: Replace E in the above equations
with σe.
-- Thermal conductivity, k: Replace E in above equations with k.
Composite Survey: Particle-III

Composite Survey: Fiber
Fibers themselves are very strong
Provide 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 and environmentally protects them

Fiber Loading Effect under Stress:

• Critical fiber length (lC) for effective stiffening & strengthening:
• Ex: For fiberglass, a fiber length > 15 mm is needed since this length
provides a “Continuous fiber” based on usual glass fiber properties
Composite Survey: Fiber
fiber diameter
shear strength of
fiber-matrix interface
fiber strength in tension

Fiber Load Behavior under Stress:

Composite Survey: Fiber
Fiber Materials
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
Wires
Metal – steel, Mo, W

Fiber Alignment
aligned
continuous
aligned random
discontinuous
Adapted from Fig. 16.8, Callister 7e.

Behavior under load for Fibers & Matrix

Composite Strength: Longitudinal Loading
Continuous fibers - Estimate fiber-reinforced composite strength for long continuous fibers in a matrix
Longitudinal deformation
σc = σmVm + σfVf but εc = εm = εf
volume fraction isostrain
Remembering: E = σ/ε and note, this model corresponds to the “upper bound” for particulate composites

Composite Strength: Transverse Loading
In transverse loading the fibers carry less of the load and are in a state of ‘isostress’
σc = σm = σf = σ εc= εmVm + εfVf
Remembering: E = σ/ε
and note, this model corresponds to the “lower bound” for particulate composites

An Example:
Note: (for ease of conversion)
6870 N/m2 per psi!
UTS, SI Modulus, SI
57.9 MPa 3.8 GPa
2.4 GPa 399.9 GPa
(241.5 GPa)
(9.34 GPa)

• Estimate of Ec and TS for discontinuous fibers:
-- valid when
-- Elastic modulus in fiber direction:
-- TS in fiber direction:
efficiency factor:
-- aligned 1D: K = 1 (aligned )
-- aligned 1D: K = 0 (aligned )
-- random 2D: K = 3/8 (2D isotropy)
-- random 3D: K = 1/5 (3D isotropy)
(aligned 1D)
Values from Table 16.3, Callister 7e. (Source for Table 16.3 is H. Krenchel, Fibre Reinforcement, Copenhagen: Akademisk Forlag, 1964.)
Composite Strength
(TS)c = (TS)mVm + (TS)fVf
Ec = EmVm + KEfVf

• Aligned Continuous fibers
• Examples:
From W. Funk and E. Blank, “Creep deformation of Ni3Al-Mo in-situ composites", Metall. Trans. A Vol. 19(4), pp. 987-998, 1988. Used with permission.
-- Metal: γ'(Ni3Al)-α(Mo)
by eutectic solidification.
Composite Survey: Fiber

• 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.
• Other variations:
-- Discontinuous, random 3D
-- Discontinuous, 1D
Composite Survey: Fiber
efficiency factor:
-- random 2D: K = 3/8 (2D isotropy)
-- random 3D: K = 1/5 (3D isotropy)
Ec = EmVm + KEfVf

Looking at strength:

• Stacked and bonded fiber-reinforced sheets
-- stacking sequence: e.g., 0º/90º or 0°/45°/90º
-- benefit: balanced, in-plane stiffness
Adapted from Fig. 16.16, Callister 7e.
Composite Survey: Structural

Composite Manufacturing Processes
Particulate Methods: Sintering
Fiber reinforced: Several
Structural: Usually Hand lay-up and atmospheric curing or vacuum curing

Open Mold Processes
Only one mold (male or female) is needed and may be made of any material such as wood, reinforced plastic or , for longer runs, sheet metal or electroformed nickel. The final part is usually very smooth.
Shaping. Steps that may be taken for high quality
1. Mold release agent (silicone, polyvinyl alcohol, fluorocarbon, or sometimes, plastic film) is first applied.
2. Unreinforced surface layer (gel coat) may be deposited for best surface quality.

Hand Lay-Up: The resin and fiber (or pieces cut from prepreg) are placed manually, air is expelled with squeegees and if necessary, multiple layers are built up.
Hardening is at room temperature but may be improved by heating.
Void volume is typically 1%.
Foam cores may be incorporated (and left in the part) for greater shape complexity. Thus essentially all shapes can be produced.
Process is slow (deposition rate around 1 kg/h) and labor-intensive
Quality is highly dependent on operator skill.
Extensively used for products such as airframe components, boats, truck bodies, tanks, swimming pools, and ducts.

A spray gun supplying resin in two converging streams into which roving is chopped
Automation with robots results in highly reproducible production
Labor costs are lower
SPRAY-UP MOLDING

Cut and lay the ply or prepreg under computer control and without tension; may allow reentrant shapes to be made.
Cost is about half of hand lay-up
Extensively used for products such as airframe components, boats, truck bodies, tanks, swimming pools, and ducts.
Tape-Laying Machines
(Automated Lay-Up)

Filament Winding
Ex: pressure tanks
Continuous filaments wound onto mandrel
Adapted from Fig. 16.15, Callister 7e. [Fig. 16.15 is from N. L. Hancox, (Editor), Fibre Composite Hybrid Materials, The Macmillan Company, New York, 1981.]

Filament Winding Characteristics
Because of the tension, reentrant shapes cannot be produced.
CNC winding machines with several degrees of freedom (sometimes 7) are frequently employed.
The filament (or tape, tow, or band) is either precoated with the polymer or is drawn through a polymer bath so that it picks up polymer on its way to the winder.
Void volume can be higher (3%)
The cost is about half that of tape laying
Productivity is high (50 kg/h).
Applications include: fabrication of composite pipes, tanks, and pressure vessels. Carbon fiber reinforced rocket motor cases used for Space Shuttle and other rockets are made this way.

Pultrusion
Fibers are impregnate with a prepolymer, exactly positioned with guides, preheated, and pulled through a heated, tapering die where curing takes place.
Emerging product is cooled and pulled by oscillating clamps
Small diameter products are wound up
Two dimensional shapes including solid rods, profiles, or hollow tubes, similar to those produced by extrusion, are made, hence its name ‘pultrusion’

Composite Production Methods
Pultrusion
Continuous fibers pulled through resin tank, then preforming die & oven to cure
Adapted from Fig. 16.13, Callister 7e.
Production rates around 1 m/min.
Applications are to sporting goods (golf club shafts), vehicle drive shafts (because of the high damping capacity), nonconductive ladder rails for electrical service, and structural members for vehicle and aerospace applications.

PREPREG PRODUCTION PROCESSES
Prepreg is the composite industry’s term for continuous fiber reinforcement pre-impregnated with a polymer resin that is only partially cured.
Prepreg is delivered in tape form to the manufacturer who then molds and fully cures the product without having to add any resin.
This is the composite form most widely used for structural applications

Manufacturing begins by collimating a series of spool-wound continuous fiber tows. Tows are then sandwiched and pressed between sheets of release and carrier paper using heated rollers (calendering).
The release paper sheet has been coated with a thin film of heated resin solution to provide for its thorough impregnation of the fibers.
PrePreg Process

The final prepreg product is a thin tape consisting of continuous and aligned fibers embedded in a partially cured resin
Prepared for packaging by winding onto a cardboard core.
Typical tape thicknesses range between 0.08 and 0.25 mm
Tape widths range between 25 and 1525 mm.
Resin content lies between about 35 and 45 vol%
PrePreg Process

The prepreg is stored at 0°C (32 °F) or lower because thermoset matrix undergoes curing reactions at room temperature. Also the time in use at room temperature must be minimized. Life time is about 6 months if properly handled.
Both thermoplastic and thermosetting resins are utilized: carbon, glass, and aramid fibers are the common reinforcements.
Actual fabrication begins with the lay-up. Normally a number of plies are laid up to provide the desired thickness.
The lay-up can be by hand or automated.
PrePreg Process