MECHANICS OF COMPOSITE MATERIAL

DEPARTMENT OF MECHANICAL ENGINEERING

GANDHI INSTITUTE FOR EDUCATION AND TECHNOLOGY, BANIATANGI, BHUBANESWAR

Composite materials – Introduction

• Definition: any combination of two or more different materials at the macroscopic level.

OR

• Two inherently different materials that when combined together produce a material with properties that exceed the constituent materials.
• Reinforcement phase (e.g., Fibers)
• Binder phase (e.g., compliant matrix)

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• Advantages
• High strength and stiffness
• Low weight ratio
• Material can be designed in addition to the structure

Applications

• Straw in clay construction by Egyptians
• Aerospace industry
• Sporting goods
• Automotive
• Construction

Types of Composites

MMC’s CMC’s PMC’s

Metal Matrix Composites Ceramic Matrix Comp’s. Polymer Matrix Comp’s

Costs of composite manufacture

• Material costs -- higher for composites
• Constituent materials (e.g., fibers and resin)
• Processing costs -- embedding fibers in matrix
• not required for metals Carbon fibers order of magnitude higher than aluminum
• Design costs -- lower for composites
• Can reduce the number of parts in a complex assembly by designing the material in combination with the structure
• Increased performance must justify higher material costs

Types of Composite Materials

There are five basic types of composite materials: Fiber, particle, flake, laminar or layered and filled composites.

A. Fiber Composites

• 1-D gives maximum strength in one direction.

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• 2-D gives strength in two directions.

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• Isotropic gives strength equally in all directions.

In fiber composites, the fibers reinforce along the line of their length. Reinforcement may be mainly 1-D, 2-D or 3-D. Figure shows the three basic types of fiber orientation.

Composite strength depends on following factors:

• Inherent fiber strength, Fiber length, Number of flaws
• Fiber shape
• The bonding of the fiber (equally stress distribution)
• Voids
• Moisture (coupling agents)

B. Particle Composites

• Particles usually reinforce a composite equally in all directions (called isotropic). Plastics, cermets and metals are examples of particles.
• Particles used to strengthen a matrix do not do so in the same way as fibers. For one thing, particles are not directional like fibers. Spread at random through out a matrix, particles tend to reinforce in all directions equally.

• Cermets

(1) Oxide–Based cermets

(e.g. Combination of Al2O3 with Cr)

(2) Carbide–Based Cermets

(e.g. Tungsten–carbide, titanium–carbide)

• Metal–plastic particle composites

(e.g. Aluminum, iron & steel, copper particles)

• Metal–in–metal Particle Composites and Dispersion Hardened Alloys

(e.g. Ceramic–oxide particles)

C. Flake Composites - 1

• Flakes, because of their shape, usually reinforce in 2-D. Two common flake materials are glass and mica. (Also aluminum is used as metal flakes)

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C. Flake Composites -2

• A flake composite consists of thin, flat flakes held together by a binder or placed in a matrix. Almost all flake composite matrixes are plastic resins. The most important flake materials are:

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1. Aluminum
2. Mica
3. Glass

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C. Flake Composites -3

Basically, flakes will provide:

• Uniform mechanical properties in the plane of the flakes
• Higher strength
• Higher flexural modulus
• Higher dielectric strength and heat resistance
• Better resistance to penetration by liquids and vapor
• Lower cost

D. Laminar Composites - 1

Laminar composites involve two or more layers of the same or different materials. The layers can be arranged in different directions to give strength where needed. Speedboat hulls are among the very many products of this kind.

D. Laminar Composites - 2

• Like all composites laminar composites aim at combining constituents to produce properties that neither constituent alone would have.

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• In laminar composites outer metal is not called a matrix but a face. The inner metal, even if stronger, is not called a reinforcement. It is called a base.

D. Laminar Composites - 3

We can divide laminar composites into three basic types:

• Unreinforced–layer composites

(1) All–Metal

(a) Plated and coated metals (electrogalvanized steel – steel plated with zinc)

(b) Clad metals (aluminum–clad, copper–clad)

(c) Multilayer metal laminates (tungsten, beryllium)

(2) Metal–Nonmetal (metal with plastic, rubber, etc.)

(3) Nonmetal (glass–plastic laminates, etc.)

• Reinforced–layer composites (laminae and laminates)
• Combined composites (reinforced–plastic laminates well bonded with steel, aluminum, copper, rubber, gold, etc.)

D. Laminar Composites - 4

• A lamina (laminae) is any arrangement of unidirectional or woven fibers in a matrix. Usually this arrangement is flat, although it may be curved, as in a shell.

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• A laminate is a stack of lamina arranged with their main reinforcement in at least two different directions.

E. Filled Composites

• There are two types of filled composites. In one, filler materials are added to a normal composite result in strengthening the composite and reducing weight. The second type of filled composite consists of a skeletal 3-D matrix holding a second material. The most widely used composites of this kind are sandwich structures and honeycombs.

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F. Combined Composites

• It is possible to combine several different materials into a single composite. It is also possible to combine several different composites into a single product. A good example is a modern ski. (combination of wood as natural fiber, and layers as laminar composites)

Forms of Reinforcement Phase

• Fibers
• cross-section can be circular, square or hexagonal
• Diameters --> 0.0001” - 0.005 “
• Lengths --> L/D ratio
• 100 -- for chopped fiber
• much longer for continuous fiber
• Particulate
• small particles that impede dislocation movement (in metal composites) and strengthens the matrix
• For sizes > 1 μm, strength of particle is involves in load sharing with matrix
• Flakes
• flat platelet form

Fiber Reinforcement

• The typical composite consists of a matrix holding reinforcing materials. The reinforcing materials, the most important is the fibers, supply the basic strength of the composite. However, reinforcing materials can contribute much more than strength. They can conduct heat or resist chemical corrosion. They can resist or conduct electricity. They may be chosen for their stiffness (modulus of elasticity) or for many other properties.

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Types of Fibers

The fibers are divided into two main groups:

• Glass fibers: There are many different kinds of glass, ranging from ordinary bottle glass to high purity quartz glass. All of these glasses can be made into fibers. Each offers its own set of properties.
• Advanced fibers: These materials offer high strength and high stiffness at low weight. Boron, silicon, carbide and graphite fibers are in this category. So are the aramids, a group of plastic fibers of the polyamide (nylon) family.

Fibers - Glass

Fiberglass properties vary somewhat according to the type of glass used. However, glass in general has several well–known properties that contribute to its great usefulness as a reinforcing agent:

• Tensile strength
• Chemical resistance
• Moisture resistance
• Thermal properties
• Electrical properties

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There are four main types of glass used in fiberglass:

• A–glass
• C–glass
• E–glass
• S–glass

Fibers - Glass

• Most widely used fiber
• Uses: piping, tanks, boats, sporting goods
• Advantages
• Low cost
• Corrosion resistance
• Low cost relative to other composites:
• Disadvantages
• Relatively low strength
• High elongation
• Moderate strength and weight
• Types:
• E-Glass - electrical, cheaper
• S-Glass - high strength

Fibers - Aramid (kevlar, Twaron)

• Uses:
• high performance replacement for glass fiber
• Examples
• Armor, protective clothing, industrial, sporting goods
• Advantages:
• higher strength and lighter than glass
• More ductile than carbon

Fibers - Carbon

• 2nd most widely used fiber
• Examples
• aerospace, sporting goods
• Advantages
• high stiffness and strength
• Low density
• Intermediate cost
• Properties:
• Standard modulus: 207-240 Gpa
• Intermediate modulus: 240-340 GPa
• High modulus: 340-960 GPa
• Diameter: 5-8 microns, smaller than human hair
• Fibers grouped into tows or yarns of 2-12k fibers

Fibers -- Carbon (2)

• Types of carbon fiber
• vary in strength with processing
• Trade-off between strength and modulus
• Intermediate modulus
• PAN (Polyacrylonitrile)
• fiber precursor heated and stretched to align structure and remove non-carbon material
• High modulus
• made from petroleum pitch precursor at lower cost
• much lower strength

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Fibers - Others

• Boron
• High stiffness, very high cost
• Large diameter - 200 microns
• Good compressive strength
• Polyethylene - trade name: Spectra fiber
• Textile industry
• High strength
• Extremely light weight
• Low range of temperature usage

Fibers -- Others (2)

• Ceramic Fibers (and matrices)
• Very high temperature applications (e.g. engine components)
• Silicon carbide fiber - in whisker form.
• Ceramic matrix so temperature resistance is not compromised
• Infrequent use

Fiber Material Properties

Steel: density (Fe) = 7.87 g/cc; TS=0.380 GPa; Modulus=207 GPa

Al: density=2.71 g/cc; TS=0.035 GPa; Modulus=69 GPa

Fiber Strength

Matrix Materials

• Functions of the matrix
• Transmit force between fibers
• arrest cracks from spreading between fibers
• do not carry most of the load
• hold fibers in proper orientation
• protect fibers from environment
• mechanical forces can cause cracks that allow environment to affect fibers
• Demands on matrix
• Interlaminar shear strength
• Toughness
• Moisture/environmental resistance
• Temperature properties
• Cost

Matrices - Polymeric

• Thermosets
• cure by chemical reaction
• Irreversible
• Examples
• Polyester, vinylester
• Most common, lower cost, solvent resistance
• Epoxy resins
• Superior performance, relatively costly

Matrices - Thermosets

• Polyester 

Polyesters have good mechanical properties, electrical properties and chemical resistance. Polyesters are amenable to multiple fabrication techniques and are low cost.

• Vinyl Esters

Vinyl Esters are similar to polyester in performance. Vinyl esters have increased resistance to corrosive environments as well as a high degree of moisture resistance.

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Matrices - Thermosets

• Epoxy

Epoxies have improved strength and stiffness properties over polyesters. Epoxies offer excellent corrosion resistance and resistance to solvents and alkalis. Cure cycles are usually longer than polyesters, however no by-products are produced.

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Flexibility and improved performance is also achieved by the utilization of additives and fillers.

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Matrices - Thermoplastics

• Formed by heating to elevated temperature at which softening occurs
• Reversible reaction
• Can be reformed and/or repaired - not common
• Limited in temperature range to 150C
• Examples
• Polypropylene
• with nylon or glass
• can be injected-- inexpensive
• Soften layers of combined fiber and resin and place in a mold -- higher costs

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Matrices - Others

• Metal Matrix Composites - higher temperature
• e.g., Aluminum with boron or carbon fibers
• Ceramic matrix materials - very high temperature
• Fiber is used to add toughness, not necessarily higher in strength and stiffness

Important Note

Composite properties are less than that of the fiber because of dilution by the matrix and the need to orient fibers in different directions.

MANUFACTURING PROCESSES �OF COMPOSITES

• Composite materials have succeeded remarkably in their relatively short history. But for continued growth, especially in structural uses, certain obstacles must be overcome. A major one is the tendency of designers to rely on traditional materials such as steel and aluminum unless composites can be produced at lower cost.

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• Cost concerns have led to several changes in the composites industry. There is a general movement toward the use of less expensive fibers. For example, graphite and aramid fibers have largely supplanted the more costly boron in advanced–fiber composites. As important as savings on materials may be, the real key to cutting composite costs lies in the area of processing.

• The processing of fiber reinforced laminates can be divided into two main steps:

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• Lay–up
• Curing

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• Curing is the drying and hardening (or polymerization) of the resin matrix of a finished composite. This may be done unaided or by applying heat and/or pressure.

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• Lay–up basically is the process of arranging fiber–reinforced layers (laminae) in a laminate and shaping the laminate to make the part desired. (The term lay–up is also used to refer to the laminate itself before curing.) Unless prepregs are used, lay–up includes the actual creation of laminae by applying resins to fiber reinforcements.

• Laminate lay–up operations fall into three main groups:

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1. Winding and laying operations
2. Molding operations
3. Continuous lamination

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• Continuous lamination is relatively unimportant compared with quality parameters as not good as wrt other two processes. In this process, layers of fabric or mat are passed through a resin dip and brought together between cellophane covering sheets. Laminate thickness and resin content are controlled by squeegee rolls. The lay–up is passed through a heat zone to cure the resin.

A. Winding Operation

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The most important operation in this category is filament winding. Fibers are passed through liquid resin, and then wound onto a mandrel. After lay–up is completed, the composite is cured on the mandrel. The mandrel is then removed by melting, dissolving, breaking–out or some other method.

B. Molding Operations

Molding operations are used in making a large number of common composite products. There are two types of processes:

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1. Open–mold

(1) Hand lay–up

(2) Spray–up

(3) Vacuum–bag molding

(4) Pressure–bag molding

(5) Thermal expansion molding

(6) Autoclave molding

(7) Centrifugal casting

(8) Continuous pultrusion and pulforming.

1. Hand Lay-up

Hand lay–up, or contact molding, is the oldest and simplest way of making fiberglass–resin composites. Applications are standard wind turbine blades, boats, etc.)

2. Spray-up

In Spray–up process, chopped fibers and resins are sprayed simultaneously into or onto the mold. Applications are lightly loaded structural panels, e.g. caravan bodies, truck fairings, bathtubes, small boats, etc.

3. Vacuum-Bag Molding

The vacuum–bag process was developed for making a variety of components, including relatively large parts with complex shapes. Applications are large cruising boats, racecar components, etc.

4. Pressure-Bag Molding

Pressure–bag process is virtually a mirror image of vacuum–bag molding. Applications are sonar domes, antenna housings, aircraft fairings, etc.

5. Thermal Expansion Molding

In Thermal Expansion Molding process, prepreg layers are wrapped around rubber blocks, and then placed in a metal mold. As the entire assembly is heated, the rubber expands more than the metal, putting pressure on the laminate. Complex shapes can be made reducing the need for later joining and fastening operations.

6. Autoclave Molding

Autoclave molding is similar to both vacuum–bag and pressure–bag molding. Applications are lighter, faster and more agile fighter aircraft, motor sport vehicles.

7. Centrifugal Casting

Continuous pultrusion is the composite counterpart of metal extrusion. Complex parts can be made.

Centrifugal Casting is used to form round objects such as pipes. ��8. Continuous Pultrusion and Pulforming

Pulforming is similar to pultrusion in many ways. However, pultrusion is capable only of making straight products that have the same volume all along their lengths. Pulformed products, on the other hand, can be either straight or curved, with changing shapes and volumes. A typical pulformed product is a curved reinforced plastic car spring. (shown in figure.)

B. Closed–mold

(1) Matched–die molding: As the name suggests, a matched–die mold consists of closely matched male and female dies (shown in figure). Applications are spacecraft parts, toys, etc.

(2) Injection molding: The injection process begins with a thermosetting (or sometimes thermoplastic) material outside the mold. The plastic may contain reinforcements or not. It is first softened by heating and/or mechanical working with an extrusion–type screw. It is then forced, under high pressure from a ram or screw, into the cool mold. Applications are auto parts, vanes, engine cowling defrosters and aircraft radomes.

Material Forms and Manufacturing

• Objectives of material production
• assemble fibers
• impregnate resin
• shape product
• cure resin

Sheet Molding Compound (SMC)

• Chopped glass fiber added to polyester resin mixture

• Question: Is SMC isotropic or anisotropic?

Manufacturing - Filament Winding

• Highly automated
• low manufacturing costs if high throughput
• e.g., Glass fiber pipe, sailboard masts

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Prepregs

• Prepreg and prepreg layup
• “prepreg” - partially cured mixture of fiber and resin
• Unidirectional prepreg tape with paper backing
• wound on spools
• Cut and stacked
• Curing conditions
• Typical temperature and pressure in autoclave is 120-200C, 100 psi

Manufacturing - Layups

compression

molding

vacuum bagging

Material Forms

• Textile forms
• Braiding or weaving
• Tubular braided form
• can be flattened and cut for non-tubular products

Fabric Structures

Woven: Series of Interlaced yarns at 90° to each other

Knit: Series of Interlooped Yarns

Braided: Series of Intertwined, Spiral Yarns

Nonwoven: Oriented fibers either mechanically, chemically, or thermally bonded

Woven Fabrics

Basic woven fabrics consists of two systems of yarns interlaced at right angles to create a single layer with isotropic or biaxial properties.

Physical Properties

• Construction (ends & picks)
• Weight
• Thickness
• Weave Type

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Components of a Woven Fabric

Basic Weave Types

Plain Weave

Basic Weave Types

Satin 5HS

Basic Weave Types

2 x 2 Twill

Basic Weave Types

Non-Crimp

Braiding

A braid consists of two sets of yarns, which are helically intertwined.

The resulting structure is oriented to the longitudinal axis of the braid.

This structure is imparted with a high level of conformability, relative low cost and ease of manufacture.

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Braid Structure

Types of Braids

Triaxial Yarns

A system of longitudinal yarns can be introduced which are held in place by the braiding yarns

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These yarns will add dimensional stability, improve tensile properties, stiffness and compressive strength.

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Yarns can also be added to the core of the braid to form a solid braid.

Fabric effects on material properties

Resin transfer molding (RTM)

• Dry-fiber preform placed in a closed mold, resin injected into mold, then cured

Material Forms

• Pultrusion
• Fiber and matrix are pulled through a die, like extrusion of metals -- assembles fibers, impregnates the resin, shapes the product, and cures the resin in one step.
• Example. Fishing rods

Pultrusion

Manufacturing

• Tube rolling - tubular products
• Examples
• fishing rods
• golf clubs
• oars
• Prepreg tape typically used wrapped in 2 directions or spiral wrapped