1. Basic Concept of Composite Materials

A composite material is a material formed by combining two or more different materials to achieve mechanical, thermal, electrical, or chemical properties that are difficult to obtain from a single material.

A typical composite material consists of:

1. Matrix material

2. Reinforcement

3. Interface or interphase

4. Fillers

5. Additives

6. Coatings or surface-treatment layers

The matrix maintains the overall shape and transfers loads to the reinforcement.

The reinforcement improves strength, stiffness, wear resistance, heat resistance, or functional performance.

The interface transfers loads between the matrix and reinforcement and is a critical region that determines the actual performance of the composite.

2. Main Purposes of Composite Materials

The principal reasons for using composite materials include:

• High specific strength

• High specific stiffness

• Weight reduction

• Improved corrosion resistance

• Improved wear resistance

• Improved fatigue life

• Control of thermal expansion

• Adjustment of thermal conductivity

• Electrical insulation or electrical conductivity

• Improved vibration damping

• Integrated forming of complex shapes

• Reduction in part count

• Directional stiffness design

• Improved high-temperature structural performance

Specific strength is strength divided by density, while specific stiffness is elastic modulus divided by density.

In aircraft, robots, automobiles, semiconductor equipment, and high-speed rotating systems, specific strength and specific stiffness may be more important than absolute strength.

3. Basic Components of Composite Materials

3.1 Matrix Material

The matrix forms the continuous phase of the composite.

Its primary functions include:

• Maintaining the product shape

• Holding the reinforcement in position

• Transferring load

• Distributing impact and cracks

• Protecting the reinforcement from the environment

• Transferring shear load

• Forming the surface

• Providing processability and formability

Representative matrix materials include:

Polymer Matrices

• Epoxy

• Unsaturated polyester

• Vinyl ester

• Phenolic resin

• Polyamide

• Polypropylene

• PEEK

• PPS

• PEI

• TPU

Metal Matrices

• Aluminum

• Magnesium

• Titanium

• Copper

• Nickel

• Cobalt

Ceramic Matrices

• SiC

• Al₂O₃

• Si₃N₄

• Glass-ceramics

• Carbon

• Ultra-high-temperature ceramics

3.2 Reinforcement

The reinforcement is the principal component used to increase composite strength and stiffness.

Classification by form:

• Continuous fibers

• Short fibers

• Chopped fibers

• Woven fabrics

• Braided fibers

• Nonwoven mats

• Particles

• Flakes

• Whiskers

• Nanotubes

• Nanoparticles

Representative reinforcements include:

• Glass fibers

• Carbon fibers

• Aramid fibers

• Basalt fibers

• Ultra-high-molecular-weight polyethylene fibers

• Silicon carbide fibers

• Alumina fibers

• Boron fibers

• Natural fibers

• SiC particles

• Al₂O₃ particles

• Graphite

• Carbon nanotubes

3.3 Interface

The interface is the contact region between the matrix and reinforcement.

Its main functions include:

• Transferring load from the matrix to the fibers

• Controlling crack propagation

• Preventing moisture penetration

• Relieving stress caused by thermal-expansion mismatch

• Controlling fiber pullout behavior

• Determining fatigue life

If interfacial adhesion is too weak:

• Load transfer becomes inefficient.

• Strength decreases.

• Delamination may occur.

• Moisture penetration increases.

• Fatigue life decreases.

If interfacial adhesion is excessively strong:

• Cracks may pass directly through the reinforcement.

• Impact toughness may decrease.

• Brittle fracture may increase.

• Energy absorption may decrease.

Therefore, the optimum interface is not simply the strongest possible interface. It is one that balances load transfer with crack-relief behavior.

4. Expression of Composite Composition

Composite composition is generally expressed using mass fraction and volume fraction.

4.1 Mass Fraction

Mass fraction is the proportion of a component relative to the total mass.

Example:

• Carbon fiber: 60 wt.%

• Epoxy resin: 40 wt.%

Mass-based composition is convenient for production control and raw-material mixing.

4.2 Volume Fraction

Volume fraction is the proportion of a reinforcement or component relative to the total volume.

Example:

• Carbon-fiber volume fraction: 60 vol.%

• Resin volume fraction: 38 vol.%

• Void content: 2 vol.%

Because mechanical properties are directly influenced by volume fraction, volume fraction is generally more important in structural design.

4.3 Relationship Between Mass Fraction and Volume Fraction

The reinforcement volume fraction can be calculated as:

[
V_f =
\frac{W_f/\rho_f}
{W_f/\rho_f + W_m/\rho_m}
]

Where:

• (V_f): Reinforcement volume fraction

• (W_f): Reinforcement mass fraction

• (W_m): Matrix mass fraction

• (\rho_f): Reinforcement density

• (\rho_m): Matrix density

Because carbon fiber is denser than most polymer resins, 60 wt.% carbon fiber does not necessarily mean 60 vol.% carbon fiber.

5. Typical Composition Ranges of Composite Materials

High-performance aerospace prepreg laminates commonly have fiber volume fractions of approximately 55–65%.

General hand-lay-up GFRP may have fiber volume fractions of approximately 20–40%.

6. Classification by Reinforcement Form

6.1 Continuous-Fiber Composites

The fibers extend continuously through the component or over a significant structural length.

Characteristics:

• High directional strength

• High directional stiffness

• Low density

• Laminate design capability

• Strong anisotropy

• Potentially low interlaminar strength

Representative materials:

• CFRP

• GFRP

• AFRP

• SiC/SiC

• Carbon/carbon composites

6.2 Short-Fiber Composites

Short fibers are dispersed throughout the matrix.

Representative examples:

• PA66-GF30

• PBT-GF30

• PEEK-CF30

• PP-GF20

Advantages:

• Suitable for injection molding

• Suitable for mass production

• Complex shapes can be produced

• Can replace metal components

Limitations:

• Properties vary with fiber length and orientation.

• Weld-line strength may be reduced.

• Molding-direction anisotropy occurs.

• Strength is lower than that of continuous-fiber laminates.

6.3 Particle-Reinforced Composites

Ceramic, metallic, carbon, or inorganic particles are dispersed in a matrix.

Representative examples:

• Al-SiC

• Al-Al₂O₃

• Rubber-particle-toughened epoxy

• Silica-filled epoxy

• WC-Co

• Concrete

Main purposes:

• Improve wear resistance

• Reduce thermal expansion

• Increase stiffness

• Adjust thermal conductivity

• Reduce cost

• Reduce shrinkage

6.4 Flake- and Platelet-Reinforced Composites

Representative materials:

• Mica

• Graphite flakes

• Graphene

• Glass flakes

• Aluminum flakes

Effects:

• Improved gas-barrier performance

• Improved dimensional stability

• Improved thermal conductivity

• Improved electrical conductivity

• Improved corrosion resistance of coatings

6.5 Hybrid Composites

Hybrid composites use two or more reinforcement types together.

Representative combinations:

• Carbon fiber + glass fiber

• Carbon fiber + aramid fiber

• Glass fiber + natural fiber

• Carbon fiber + metal mesh

• Fiber + particles

Purposes of hybridization:

• Cost reduction

• Improved impact toughness

• Improved fatigue behavior

• Adjustment of electrical conductivity

• Control of failure mode

• Local stiffness control

7. Polymer-Matrix Composites — PMC

Polymer-matrix composites are the most widely used class of composites.

The matrix may be either a thermosetting or thermoplastic polymer.

Representative materials:

• GFRP

• CFRP

• AFRP

• Natural-fiber composites

• Short-fiber-reinforced engineering plastics

• Particle-filled polymers

8. Epoxy-Matrix Composites

8.1 Basic Composition

Epoxy composites consist of epoxy resin, curing agent, reinforcing fibers, and additives.

Representative composition:

In prepreg CFRP, fiber content may reach approximately 60–70 wt.%.

8.2 Epoxy Constituents

Representative components:

• Bisphenol-A epoxy

• Bisphenol-F epoxy

• Novolac epoxy

• Amine curing agents

• Anhydride curing agents

• Accelerators

• Rubber-based tougheners

• Silica

• Flame retardants

Role of each component:

Bisphenol-A Epoxy

• General-purpose performance

• High adhesion

• Good mechanical properties

• Relatively high viscosity

Bisphenol-F Epoxy

• Lower viscosity

• Good impregnation

• Suitable for precision molding

Novolac Epoxy

• High crosslink density

• High heat and chemical resistance

• Possible increase in brittleness

Rubber-Based Toughener

• Improves fracture toughness

• Improves impact resistance

• May reduce elastic modulus and glass-transition temperature

8.3 Properties

Advantages:

• High adhesion

• Low curing shrinkage

• Excellent mechanical properties

• High fatigue performance

• Low void content can be achieved

• High dimensional stability

Limitations:

• Requires curing time

• May be vulnerable to impact

• Service temperature limited by glass-transition temperature

• Moisture absorption

• Difficult recycling

• Curing conditions must be controlled

9. Unsaturated-Polyester Composites

9.1 Basic Composition

Representative composition:

9.2 Properties

Advantages:

• Relatively low cost

• Fast curing

• Suitable for large structures

• Good corrosion resistance

• Easily combined with glass fiber

Limitations:

• Curing shrinkage

• Styrene odor and volatile emissions

• Lower fatigue and adhesion performance than epoxy

• Long-term degradation from moisture and chemicals

• Potential for high void content

9.3 Main Applications

• FRP tanks

• Ducts

• Equipment covers

• Boats

• Pipes

• Large panels

• Chemical-processing equipment

10. Vinyl-Ester Composites

Vinyl ester combines epoxy-derived molecular structures with curing behavior similar to unsaturated polyester.

Representative composition:

Properties:

• Higher chemical resistance than unsaturated polyester

• High water resistance

• Good adhesion

• Good fatigue performance

• Faster curing than epoxy

• Shrinkage and volatile-organic-compound control required

Main applications:

• Chemical tanks

• Corrosive piping

• Cleaning equipment

• Marine structures

• Battery cases

• High-corrosion-resistance FRP

11. Thermoplastic-Matrix Composites

Representative matrix materials:

• PP

• PA

• PBT

• PPS

• PEEK

• PEI

• PC

• TPU

Advantages:

• Fast molding

• Reheating and reshaping possible

• High impact toughness

• Weldability

• Potential recyclability

• High productivity

Limitations:

• High melt viscosity

• Difficult continuous-fiber impregnation

• High-temperature processing equipment may be required.

• Cooling shrinkage and warpage

• Some materials absorb significant moisture.

12. Glass-Fiber-Reinforced Plastic — GFRP

12.1 Composition of Glass Fiber

Glass fibers are mainly composed of silica and metal oxides.

Typical E-Glass Composition

Typical S-Glass Composition

S-glass has higher strength and elastic modulus than E-glass.

12.2 Typical GFRP Fiber Content

12.3 Properties

Advantages:

• Economical

• High tensile strength

• Excellent electrical insulation

• High corrosion resistance

• Nonmagnetic

• Compatible with many forming processes

• Radio-frequency transparency

Limitations:

• Lower specific stiffness than carbon fiber

• Moisture-related interfacial degradation

• Fatigue and creep

• Possible fiber exposure

• Dust during machining

• Difficult recycling

12.4 Main Applications

• Tanks

• Piping

• Equipment covers

• Electrical-insulation components

• Profiles

• Marine structures

• Wind-turbine blades

• Grating

• Covers for food and chemical equipment

13. Carbon-Fiber-Reinforced Plastic — CFRP

13.1 Basic Composition of Carbon Fiber

The principal element in carbon fiber is carbon.

Representative precursor materials:

• PAN-based

• Pitch-based

• Rayon-based

PAN-based carbon fibers generally contain approximately 92–99% or more carbon.

Highly graphitized carbon fibers may contain more than 99% carbon.

13.2 Typical CFRP Composition

Fiber volume fraction is often approximately 50–65 vol.%.

13.3 Carbon-Fiber Grades

Standard-Modulus Grade

• High tensile strength

• Relatively economical

• General structural use

Intermediate-Modulus Grade

• Balanced high stiffness and strength

• Aerospace and precision machinery

High-Modulus Grade

• Very high elastic modulus

• Low thermal expansion

• Increased brittleness and cost

High-Strength Grade

• High ultimate strength

• Potentially improved impact and fatigue performance

13.4 Properties

Advantages:

• Very high specific strength

• Very high specific stiffness

• Low thermal expansion

• High fatigue performance

• Electrical conductivity

• High vibration damping

• Excellent dimensional stability

Limitations:

• High cost

• Sensitivity to impact damage

• Low interlaminar strength

• Potential galvanic corrosion

• Difficult electrical insulation

• Machining dust

• Difficult repair and inspection

13.5 Main Applications

• Aircraft structures

• Robot arms

• High-speed rotating components

• Drive shafts

• Precision-equipment frames

• Sporting equipment

• Automotive bodies

• Semiconductor-equipment beams

• Metrology structures

14. Aramid-Fiber-Reinforced Plastic — AFRP

14.1 Basic Composition

Aramid fibers have an aromatic polyamide structure.

Principal constituent elements:

• Carbon

• Hydrogen

• Nitrogen

• Oxygen

Representative types:

• Para-aramid

• Meta-aramid

14.2 Typical Composition

14.3 Properties

Advantages:

• High specific strength

• Excellent impact-energy absorption

• High toughness

• High fatigue performance

• Low density

• Electrical insulation

• High vibration damping

Limitations:

• Low compressive strength

• Moisture absorption

• Difficult machining

• Difficult surface finishing

• Ultraviolet sensitivity

• Interfacial adhesion requires careful control

14.4 Main Applications

• Ballistic protection

• Impact-absorbing panels

• Ropes

• Hose reinforcement

• Pressure vessels

• Protective equipment

• Hybrid laminates

15. Natural-Fiber Composites

Representative fibers:

• Flax

• Hemp

• Jute

• Kenaf

• Sisal

• Bamboo fiber

• Wood flour

Main constituents:

• Cellulose

• Hemicellulose

• Lignin

• Waxes

• Moisture

Representative composition:

Advantages:

• Low density

• Renewable source

• Low cost

• Good vibration damping

• Low tool wear during processing

Limitations:

• Moisture absorption

• Dimensional change

• Quality variation

• Limited heat resistance

• Mold and biological degradation

• Interfacial adhesion problems

16. Short-Fiber-Reinforced Engineering Plastics

16.1 Representative Compositions

PA66-GF30

• PA66: Approximately 70 wt.%

• Glass fiber: Approximately 30 wt.%

• Stabilizers and lubricants: Small amount

PBT-GF30

• PBT: Approximately 70 wt.%

• Glass fiber: Approximately 30 wt.%

• Flame retardants and stabilizers: Grade-dependent

PEEK-CF30

• PEEK: Approximately 70 wt.%

• Carbon fiber: Approximately 30 wt.%

PPS-GF40

• PPS: Approximately 60 wt.%

• Glass fiber: Approximately 40 wt.%

16.2 Effects of Increasing Reinforcement Content

General tendencies:

• Increased elastic modulus

• Increased tensile strength

• Reduced thermal deformation

• Reduced creep

• Reduced thermal expansion

• Reduced molding shrinkage

Limitations:

• Impact strength may decrease.

• Elongation decreases.

• Fiber-orientation anisotropy increases.

• Surface roughness increases.

• Mold wear increases.

• Weld-line strength decreases.

17. Sheet Molding Compound — SMC

SMC is a sheet-form molding material consisting of thermosetting resin, chopped glass fibers, and fillers.

Representative composition:

Properties:

• Suitable for compression molding

• Suitable for large panels

• High productivity

• Relatively uniform quality

• Can replace metal components

• Lower structural performance than continuous-fiber laminates

Main applications:

• Automotive panels

• Electrical enclosures

• Equipment covers

• Bathtubs

• Structural panels

18. Bulk Molding Compound — BMC

BMC is a bulk molding material made by mixing resin, short fibers, and fillers.

Representative composition:

Properties:

• Suitable for compression or injection molding of complex shapes

• High electrical insulation

• Good dimensional stability

• Heat resistance

• Lower structural strength than continuous-fiber composites

19. Metal-Matrix Composites — MMC

Metal-matrix composites are produced by dispersing ceramic particles, fibers, or carbon materials in a metallic matrix.

Representative matrices:

• Aluminum

• Magnesium

• Titanium

• Copper

• Nickel

Representative reinforcements:

• SiC

• Al₂O₃

• B₄C

• Carbon fiber

• Graphite

• SiC fiber

• Boron fiber

20. Aluminum-Matrix Composites

20.1 Al-SiC Particulate Composites

Representative composition:

In general industrial materials, SiC content is often approximately 10–30 vol.%.

20.2 Properties

As SiC content increases:

• Elastic modulus increases.

• Wear resistance increases.

• Thermal expansion decreases.

• Thermal conductivity changes.

• Hardness increases.

• Machinability decreases.

• Ductility decreases.

• Impact toughness decreases.

20.3 Main Applications

• Brake discs

• Cylinder liners

• Thermal-management substrates

• Precision stages

• Space structures

• Electronic packages

• Wear-resistant components

21. Al-Al₂O₃ Composites

Representative composition:

• Aluminum alloy: Approximately 70–95 vol.%

• Al₂O₃ particles or fibers: Approximately 5–30 vol.%

Properties:

• Increased hardness

• Improved wear resistance

• Reduced thermal expansion

• Increased stiffness

• May be less expensive than SiC reinforcement

• Increased cutting-tool wear

• Reduced ductility

22. Magnesium-Matrix Composites

Representative composition:

• Magnesium alloy: Approximately 70–95 vol.%

• SiC or Al₂O₃: Approximately 5–30 vol.%

Advantages:

• Very low density

• High specific stiffness

• Good vibration damping

• Improved dimensional stability

Limitations:

• Corrosion

• Oxidation and casting-safety concerns

• Interfacial reactions

• Low ductility

• High manufacturing cost

Main applications:

• Aerospace components

• Lightweight rotating parts

• Precision housings

• Sporting equipment

23. Titanium-Matrix Composites

Representative reinforcements:

• Continuous SiC fibers

• Boron fibers

• TiB particles

• TiC particles

Representative composition:

Properties:

• High specific strength at elevated temperature

• High stiffness

• Excellent heat and corrosion resistance

• Interfacial reactions must be controlled.

• Very high manufacturing cost

• Complex processing and joining

Main applications:

• Aircraft engines

• High-temperature rotating parts

• Space structures

• High-load lightweight components

24. Copper-Matrix Composites

Representative systems:

• Cu-W

• Cu-Mo

• Cu-graphite

• Cu-Al₂O₃

• Cu-Cr

Cu-W

Representative composition:

• Copper: Approximately 10–50 wt.%

• Tungsten: Approximately 50–90 wt.%

Properties:

• High thermal conductivity

• High heat and wear resistance

• Arc resistance

• High density

Cu-Graphite

• Copper: Approximately 50–90 wt.%

• Graphite: Approximately 10–50 wt.%

Properties:

• Electrical conductivity

• Self-lubrication

• Suitable for brushes and sliding electrical contacts

25. Ceramic-Matrix Composites — CMC

Ceramic-matrix composites are designed to retain the high-temperature capability of ceramics while reducing the brittle failure behavior of monolithic ceramics.

Representative materials:

• SiC/SiC

• C/SiC

• C/C

• Al₂O₃/Al₂O₃

• Oxide/oxide composites

26. SiC/SiC Composites

26.1 Basic Composition

Representative interphase layers:

• Carbon

• BN

• Multilayer interfacial coatings

26.2 Properties

• High elevated-temperature strength

• Low density

• High thermal-shock resistance

• Crack bridging

• Non-catastrophic fracture behavior

• Oxidation degradation of fibers and interphase

• High manufacturing cost

• Porosity control required

26.3 Main Applications

• Aircraft-engine hot-section components

• Combustor liners

• Turbine components

• Thermal-protection structures

• Nuclear-reactor components

27. Carbon/Carbon Composites — C/C

27.1 Basic Composition

Carbon/carbon composites consist of carbon fibers in a carbon matrix.

Representative composition:

27.2 Properties

Advantages:

• Maintains strength at extremely high temperatures

• Low density

• High thermal-shock resistance

• High friction performance

• Low thermal expansion

Limitations:

• Oxidizes in oxygen-containing atmospheres

• Very long manufacturing time

• High cost

• Porosity and internal-defect control required

• Protective coatings may be necessary

27.3 Main Applications

• Aircraft brakes

• Rocket nozzles

• Thermal-protection materials

• High-temperature furnace components

• Spacecraft components

28. Oxide-Matrix Composites

Representative materials:

• Al₂O₃ fiber/Al₂O₃ matrix

• Mullite fiber/aluminosilicate matrix

Properties:

• Stability in oxidizing atmospheres

• High service temperature

• Electrical insulation

• Lower oxidation sensitivity than SiC/SiC

• Relatively lower high-temperature strength

• Low thermal conductivity

• High manufacturing cost

29. Cemented Carbides and Cermets

29.1 WC-Co Cemented Carbide

Representative composition:

WC provides hardness and wear resistance, while Co provides toughness and bonding strength.

29.2 TiCN Cermet

Representative composition:

Properties:

• High hot hardness

• Excellent wear resistance

• Good machined surface finish

• May have lower toughness than cemented carbide

• Suitable for finishing steel

30. Laminated Composites

Laminated composites are produced by stacking fiber layers in different orientations.

Representative ply angles:

• 0°

• 90°

• +45°

• -45°

Function of each orientation:

0° Plies

• Carry tensile and compressive loads in the primary direction

• Provide high axial stiffness

90° Plies

• Carry transverse loads

• Control cracking and dimensional stability

±45° Plies

• Carry shear loads

• Provide torsional resistance

• Distribute impact and deformation

30.1 Representative Layups

Examples:

• [0/90]s

• [0/+45/-45/90]s

• [0/0/+45/-45/90]s

The symbol “s” indicates a symmetric laminate about the mid-plane.

Symmetric layups help reduce thermal distortion and warpage.

31. Sandwich Structures

A sandwich structure consists of two thin, strong face sheets separated by a thick, lightweight core.

Components:

• Upper face sheet

• Lower face sheet

• Core

• Adhesive layers

• Local inserts

31.1 Face Sheets

Representative materials:

• CFRP

• GFRP

• Aluminum

• Stainless steel

• Plywood

The face sheets mainly carry tensile and compressive loads.

31.2 Core Materials

Representative materials:

• Aluminum honeycomb

• Aramid honeycomb

• Polymer foam

• Balsa wood

• PMI foam

• PVC foam

• PET foam

Main functions of the core:

• Maintain spacing between the face sheets

• Carry shear load

• Prevent buckling

• Increase bending stiffness with low weight

31.3 Representative Proportions

The core may account for approximately 70–95% of the total thickness.

However, the face sheets may still account for a significant part of the total mass.

Example:

• CFRP face sheets: 1–2 mm each

• Honeycomb core: 10–50 mm

• Adhesive film: Approximately 0.1–0.3 mm at each interface

31.4 Main Failure Modes

• Face-sheet tensile failure

• Face-sheet compressive buckling

• Core shear failure

• Face-sheet/core debonding

• Local crushing

• Insert pullout

• Internal delamination caused by impact

32. Mechanical Properties of Composite Materials

32.1 Anisotropy

Composite properties vary with direction.

For unidirectional CFRP:

• Longitudinal elastic modulus: Very high

• Transverse elastic modulus: Low

• Longitudinal strength: High

• Interlaminar shear strength: Relatively low

Therefore, composites must not be treated as isotropic materials like most metals.

32.2 Rule of Mixtures

The longitudinal elastic modulus can be approximated as:

[
E_L = E_f V_f + E_m V_m
]

Where:

• (E_L): Longitudinal elastic modulus of the composite

• (E_f): Fiber elastic modulus

• (E_m): Matrix elastic modulus

• (V_f): Fiber volume fraction

• (V_m): Matrix volume fraction

Actual properties may be lower than theoretical values because of fiber orientation, interfacial adhesion, void content, fiber damage, and manufacturing defects.

32.3 Specific Strength and Specific Stiffness

CFRP may have a lower absolute elastic modulus than steel in some directions, but because its density is much lower, its specific stiffness can be higher.

This advantage is important in lightweight vehicles, robot arms, and aircraft structures.

32.4 Fatigue

Fatigue damage in composites generally develops through the accumulation of several damage mechanisms rather than a single crack.

Representative damage modes:

• Matrix cracking

• Fiber-matrix debonding

• Interlaminar delamination

• Fiber fracture

• Cracking around voids

Fatigue life is influenced by fiber direction, stress ratio, humidity, temperature, and prior impact damage.

33. Main Failure Modes of Composite Materials

33.1 Fiber Fracture

Fiber fracture occurs when the load in the fiber direction exceeds the fiber strength.

It is generally a critical and difficult-to-repair failure mode.

33.2 Matrix Cracking

Cracks form in the resin because of transverse tension, thermal stress, or cyclic loading.

Matrix cracking may not immediately cause complete failure, but it promotes moisture penetration and delamination.

33.3 Interfacial Debonding

Interfacial debonding is the loss of adhesion between fiber and matrix.

Causes include:

• Moisture

• Impact

• Thermal-expansion mismatch

• Inadequate surface treatment

• Cyclic loading

33.4 Interlaminar Delamination

Delamination is separation between laminate plies.

Typical causes:

• Impact

• Free-edge stresses

• Stress concentration around holes

• Nonuniform curing

• Moisture

• Assembly damage

33.5 Fiber Buckling and Microbuckling

Under compressive loading, fibers may locally bend or lose alignment.

Fiber misalignment and matrix shear strength strongly affect this failure mode.

33.6 Voids and Porosity

Voids may cause:

• Reduced interlaminar shear strength

• Reduced fatigue life

• Increased moisture absorption

• Nonuniform thermal conductivity

• Crack-initiation sites

High-quality aerospace composites often control void content below approximately 1–2%.

34. Thermal Properties

34.1 Thermal Expansion

Internal thermal stress develops because the reinforcement and matrix have different coefficients of thermal expansion.

Carbon fiber may have a very low or even negative coefficient of thermal expansion in the fiber direction.

Through laminate design, CFRP can be engineered to have a very low overall thermal expansion.

34.2 Thermal Conductivity

General trends:

• Carbon fiber: Directional thermal conductivity

• Glass fiber: Low thermal conductivity

• Metal matrix: Relatively high thermal conductivity

• Ceramic matrix: Wide variation depending on material

• Foam core: Very low thermal conductivity

Thermal conductivity may vary significantly with fiber direction.

34.3 Service Temperature

The service temperature of polymer-matrix composites is limited by the glass-transition and thermal-decomposition temperatures of the matrix.

General relative tendencies:

Actual continuous-use temperature depends on load, humidity, oxidation environment, and required service life.

35. Moisture and Chemical Environments

35.1 Moisture Absorption

Moisture primarily enters through the resin and interface.

Effects:

• Matrix softening

• Reduced glass-transition temperature

• Reduced interfacial adhesion

• Reduced interlaminar shear strength

• Dimensional change

• Reduced electrical insulation

35.2 Ultraviolet Radiation

Ultraviolet radiation can degrade the resin surface.

Possible effects:

• Discoloration

• Chalking

• Surface cracking

• Fiber exposure

• Reduced resin strength

Countermeasures:

• UV stabilizers

• Protective paint

• Gel coat

• Shading structures

35.3 Chemical Exposure

Vinyl-ester and epoxy composites generally provide good corrosion resistance, but they are not stable in every chemical environment.

Items to confirm:

• Chemical type

• Concentration

• Temperature

• Stress state

• Exposure duration

• Resin type

• Degree of cure

• Reinforcement type

36. Electrical Properties

36.1 GFRP

• High electrical insulation

• Radio-frequency transparency

• Suitable for electrical enclosures and antenna covers

36.2 CFRP

• Electrically conductive

• Can dissipate static electricity

• Can provide electromagnetic shielding

• May cause galvanic corrosion when contacting metals

36.3 Filled Composites

Conductive additives:

• Carbon black

• Carbon fiber

• CNT

• Graphene

• Metal powder

Electrically insulating and thermally conductive additives:

• AlN

• BN

• Al₂O₃

• MgO

37. Composite Manufacturing Processes

37.1 Hand Lay-Up

Fiber mats are manually placed while resin is applied.

Advantages:

• Low tooling cost

• Suitable for large parts

• Suitable for low-volume production

Limitations:

• Quality variation

• High void content

• Low fiber fraction

• Operator dependence

37.2 Vacuum-Bag Molding

The laminate is compacted using a vacuum bag to remove air and excess resin.

Effects:

• Reduced void content

• Increased fiber fraction

• Improved surface quality

• Reduced thickness variation

37.3 Autoclave Molding

Vacuum, external pressure, and heat are applied simultaneously.

Advantages:

• Low void content

• High fiber fraction

• Excellent structural performance

• High quality repeatability

Limitations:

• High equipment cost

• Long production time

• Part-size limitations

• High material cost

37.4 Resin Transfer Molding — RTM

A dry-fiber preform is placed in a mold and resin is injected.

Advantages:

• Good surface quality on both sides

• Closed-mold process

• Suitable for complex shapes

• Medium production rate

Limitations:

• Risk of incomplete impregnation

• Resin-flow analysis required

• Preform manufacturing required

37.5 Vacuum Infusion

Vacuum pressure is used to draw resin through the fibers.

Main applications:

• Boats

• Wind-turbine blades

• Large panels

• Tanks

37.6 Filament Winding

Continuous fibers are wound around a rotating mandrel.

Suitable components:

• Pressure vessels

• Pipes

• Shafts

• Tanks

• Rocket motor cases

Fiber angle can be adjusted to design circumferential and axial strength.

37.7 Pultrusion

Continuous fibers are impregnated with resin and pulled through a heated die.

Suitable shapes:

• Beams

• Channels

• Tubes

• Rods

• Ladder-type profiles

37.8 Compression Molding

SMC, BMC, or thermoplastic sheets are compression-molded.

Advantages:

• High productivity

• Suitable for large parts

• Good dimensional repeatability

• Relatively short cycle time

37.9 Injection Molding

Used for short-fiber-reinforced thermoplastics.

Main process variables:

• Fiber length

• Gate location

• Flow direction

• Weld lines

• Holding pressure

• Mold temperature

• Resin drying

38. Machining of Composite Materials

38.1 Machining Problems

Possible machining defects:

• Delamination

• Fiber pullout

• Burr formation

• Thermal damage

• Resin melting

• Tool wear

• Dust generation

• Dimensional instability

38.2 Cutting Tools

Representative tools and processes:

• Carbide tools

• Diamond-coated tools

• PCD

• Waterjet cutting

• Laser processing

• Ultrasonic machining

CFRP and GFRP are highly abrasive to cutting tools.

38.3 Hole Machining

Areas around holes are sensitive to stress concentration and delamination.

Countermeasures:

• Use backup plates.

• Use low-thrust drills.

• Use step drills.

• Use dedicated drill geometries.

• Apply appropriate feed rates.

• Add local reinforcement plies.

• Use inserts where required.

39. Joining Methods

39.1 Adhesive Bonding

Advantages:

• Distributes load over a wide area

• Joins dissimilar materials

• Minimizes hole machining

• Adds little weight

Limitations:

• Surface preparation required

• Curing time

• Temperature and humidity sensitivity

• Difficult inspection

• Vulnerable to peel loading

39.2 Mechanical Fastening

Representative methods:

• Bolts

• Rivets

• Inserts

• Pins

• Clamps

Considerations:

• Stress concentration around holes

• Bearing failure

• Net-section failure

• Delamination

• Fastening torque

• Galvanic corrosion

39.3 Hybrid Joining

Adhesive bonding is combined with bolts or rivets.

Advantages:

• Initial position fixing

• Improved fail-safe behavior

• Increased peel resistance

Disadvantages:

• Complex load sharing

• Increased manufacturing steps

• Reduced repairability

40. Composite Design Principles

40.1 Align Fibers with the Load Path

Fibers should be placed along the primary load direction.

Simply copying a metal-part geometry and replacing the material with CFRP may result in an inefficient design.

40.2 Avoid Abrupt Thickness Changes

Abrupt thickness transitions may cause:

• Resin-rich regions

• Wrinkles

• Voids

• Residual stress

• Delamination

• Cure distortion

40.3 Reinforce Free Edges and Corners

Interlaminar stresses increase near free edges.

Countermeasures:

• Tapers

• Distributed ply drop-offs

• Reinforcement patches

• Curved transitions

• Local woven-fabric layers

40.4 Avoid Concentrated Loads

Point loads and bolt loads are unfavorable for composites.

Countermeasures:

• Load-distribution plates

• Metal inserts

• Potting compounds

• Local laminate thickening

• Increased bonded area

40.5 Use Symmetric and Balanced Laminates

Symmetric laminates:

• Reduce curing warpage

• Reduce bending under temperature change

Balanced laminates:

• Use both +θ and -θ plies

• Reduce extension-shear coupling

41. Inspection of Composite Materials

41.1 Visual Inspection

Inspection items:

• Wrinkles

• Fiber distortion

• Voids

• Resin starvation

• Excess resin

• Surface cracks

• Core crushing

41.2 Ultrasonic Inspection

Detectable defects:

• Delamination

• Voids

• Bonding defects

• Foreign material

• Core separation

41.3 Thermographic Inspection

Internal defects are detected using differences in heating and cooling response.

Suitable defects:

• Delamination

• Impact damage

• Core debonding

• Moisture ingress

41.4 X-Ray Inspection

Suitable defects:

• Voids

• Fiber alignment

• Metal inserts

• Internal foreign material

• Thickness variation

41.5 Shearography

Laser shear interferometry is used to detect delamination and bonding defects.

42. Composite Repair

Representative repair methods:

• Scarf repair

• Stepped-laminate repair

• External patch

• Core replacement

• Resin injection

• Bolted reinforcement

Repair considerations:

• Extent of damage

• Moisture removal

• Surface preparation

• Restoration of fiber orientation

• Cure temperature

• Matching the original laminate configuration

• Post-repair nondestructive inspection

43. Composite Materials in Food Machinery

43.1 Applicable Materials

• GFRP

• CFRP

• Glass-fiber-reinforced PP

• Glass-fiber-reinforced PA

• Vinyl-ester FRP

• Food-contact epoxy composites

• Ceramic-filled polymers

43.2 Main Applications

• Equipment covers

• Tanks

• Ducts

• Waterproof panels

• Robot arms

• Lightweight frames

• Insulating plates

• Cleaning-equipment structures

• Rollers

• Pipes

43.3 Advantages

• Corrosion resistance

• Weight reduction

• Electrical insulation

• Integrated molding of complex shapes

• Low thermal conductivity

• Compatibility with surface coatings

43.4 Considerations

• Fiber exposure

• Resin wear particles

• Coating delamination

• Moisture penetration

• Cleaning chemicals and temperature

• Food-contact compliance

• Fiber fragments after fracture

• Microporosity and hygienic design

• Difficulty of metal detection

Food-contact applications require compliance verification for the complete composite material, not only for the base resin.

44. Composite Materials in Semiconductor and Precision Equipment

Representative materials:

• CFRP

• Al-SiC

• Silicon-carbide-particle-reinforced aluminum

• Ceramic-filled epoxy

• Glass-fiber composites

• Carbon/carbon

Main requirements:

• Low thermal expansion

• High dimensional stability

• Low vibration

• High specific stiffness

• Low outgassing

• Cleanliness

• Electrical insulation or grounding capability

• Precision machinability

45. Comparison of Major Composite Materials

46. Composite Selection by Component

46.1 Lightweight Structural Frames

Recommended materials:

• CFRP

• GFRP

• Aluminum-CFRP hybrids

• Sandwich panels

46.2 Corrosive Tanks and Piping

Recommended materials:

• Vinyl-ester GFRP

• Epoxy GFRP

• Thermoplastic-lined FRP structures

46.3 High-Speed Rotating Components

Recommended materials:

• CFRP

• Carbon-fiber-wrapped metal rotors

• SiC-fiber-reinforced metal-matrix composites

46.4 Wear-Resistant Components

Recommended materials:

• Al-SiC

• WC-Co

• Ceramic-particle-filled polymers

• Carbon-fiber-reinforced PEEK

• Polyurethane-fiber composites

46.5 High-Temperature Components

Recommended materials:

• SiC/SiC

• C/SiC

• C/C

• Oxide CMC

• Titanium-matrix composites

46.6 Electrically Insulating Structural Components

Recommended materials:

• GFRP

• G10 and FR-4

• Glass-fiber-reinforced PBT

• Alumina-filled epoxy

47. Drawing and Material-Specification Designation

It is not sufficient to specify only “CFRP,” “GFRP,” or “composite.”

The following should also be specified.

47.1 Matrix Material

Examples:

• Epoxy

• Vinyl ester

• PEEK

• PPS

• Aluminum 6061 matrix

• SiC matrix

47.2 Reinforcement

Examples:

• PAN-based carbon fiber

• E-glass

• S-glass

• Aramid fiber

• SiC particles

• Al₂O₃ fiber

47.3 Reinforcement Content

Examples:

• Carbon fiber: 60 vol.%

• Glass fiber: 30 wt.%

• SiC particles: 20 vol.%

47.4 Reinforcement Form

• Unidirectional fiber

• Woven fabric

• Braided fiber

• Short fiber

• Chopped-strand mat

• Particles

• Whiskers

47.5 Laminate Configuration

Examples:

• [0/90]s

• [0/+45/-45/90]2s

• Quasi-isotropic laminate

47.6 Manufacturing Process

• Autoclave

• RTM

• Vacuum infusion

• Filament winding

• Pultrusion

• Compression molding

• Injection molding

47.7 Void Content

Examples:

• Void content ≤ 2 vol.%

• Porosity ≤ 1%

47.8 Surface and Inspection Requirements

• Surface roughness

• Paint or gel coat

• Allowable delamination size

• Ultrasonic-inspection grade

• Damage criteria around holes

• No exposed fibers

48. Composite-Material Selection Procedure

48.1 Confirm the Function

• Is it a lightweight structure?

• Is it a corrosion-resistant component?

• Is it an electrical-insulation component?

• Is it a wear-resistant component?

• Is it a high-temperature component?

• Is vibration damping required?

• Does it contact food?

48.2 Confirm the Load

• Tension

• Compression

• Bending

• Shear

• Torsion

• Impact

• Fatigue

• Local contact

• Fastening load

48.3 Confirm Directionality

• Primary load direction

• Transverse load

• Shear load

• Torsion

• Multiaxial load

• Thermal-expansion direction

48.4 Confirm the Environment

• Temperature

• Moisture

• Ultraviolet exposure

• Acids

• Alkalis

• Solvents

• Fuel

• Vacuum

• Radiation

• Cleaning chemicals

48.5 Confirm Manufacturability

• Tooling

• Number of plies

• Forming pressure

• Cure temperature

• Production volume

• Part size

• Post-machining

• Inspection

• Repairability

48.6 Confirm Reliability

• Fiber orientation

• Void content

• Interlaminar strength

• Impact damage

• Fatigue

• Moisture degradation

• Thermal cycling

• Bonded-joint life

• Manufacturing variation

49. Final Summary

Composite materials are not simply materials made by mixing two substances.

Their actual performance is determined by:

• Matrix type

• Reinforcement type

• Reinforcement content

• Fiber length

• Fiber direction

• Stacking sequence

• Interfacial adhesion

• Void content

• Molding pressure

• Curing conditions

• Manufacturing defects

• Service environment

• Load direction

Polymer-matrix composites are the most widely used composites because of their low density, high specific strength, corrosion resistance, and formability.

GFRP provides good cost performance, electrical insulation, and corrosion resistance and is suitable for tanks, pipes, covers, and general structures.

CFRP provides high specific strength, high specific stiffness, and low thermal expansion and is suitable for aircraft, robots, precision machinery, and high-speed rotating components.

AFRP provides high toughness and impact-energy absorption and is suitable for ballistic protection, protective equipment, and impact-resistant components.

Short-fiber-reinforced thermoplastics combine injection moldability with structural performance and are useful for replacing metal parts.

Metal-matrix composites retain the toughness and thermal conductivity of metals while using ceramic reinforcements to improve stiffness, wear resistance, and thermal-expansion behavior.

Ceramic-matrix composites use fibers and interfacial layers to reduce brittle failure and retain structural performance at high temperatures.

Laminated composites allow strength and stiffness to be placed in required directions by controlling fiber orientation and stacking sequence.

Sandwich structures combine thin high-strength face sheets with lightweight cores to provide very high specific bending stiffness.

However, composite materials are sensitive to delamination, interfacial degradation, voids, fiber-orientation variation, and impact damage.

Therefore, composite selection should not be based only on generic names such as “CFRP,” “GFRP,” or “composite.”

The following must be specified and reviewed together:

• Matrix type

• Reinforcement type and grade

• Mass fraction and volume fraction

• Reinforcement form and length

• Fiber orientation

• Laminate configuration

• Interfacial treatment

• Manufacturing process

• Void content

• Thermal properties

• Environmental resistance

• Inspection method

• Repair method

• Supplier material grade

Selecting a composite material is not merely the process of choosing the strongest fiber.

It is the process of designing the combination, content, direction, and laminate structure of the matrix and reinforcement according to load direction, weight, environment, manufacturing process, joining method, inspection requirements, and service-life conditions.