Dr.
Abu Shaid Sujon
Lecturer
Department of Mechanical and Production Engineering
Islamic University of Technology
Composites
Department of Mechanical and Production Engineering
Why composite?
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What is composite material ?
A composite material is a structural material that consists of two or
more constituents combined at a macroscopic level with significantly
different properties to produce a material with characteristics different
from the individual components and are not soluble in each other
Key Characteristics:
➢ Distinct phases with an interface between them.
➢ Properties Of the composite are superior to those Of the individual
constituents.
➢ Engineered materials designed for specific performance
requirements.
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Why composites?
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Constituents:
Matrix: Continuous Phase (e.g.Polymers metal ceramics )
Reinforcement: Discontinuons phase (e.g., fibers, particles).
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Constituents:
Matrix: Continuous Phase (e.g.Polymers metal ceramics )
Role of the Matrix:
➢ Binds the fibers together.
➢ Transfers stress to the fibers.
➢ Provides shape and environmental protection.
➢ Contributes to interlaminar shear strength and
toughness.
➢ Types : MMC, PMC, CMC
Reinforcement: Discontinuons phase (e.g., fibers, particles).
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Constituents:
Matrix: Continuous Phase (e.g.Polymers, metal, ceramics )
Reinforcement: Discontinuons phase (e.g., fibers, particles).
Role of the Reinforcement (Fibers/Particles):
Provides strength and stiffness in the direction of the
reinforcement.
•Carries the primary load.
•Types: Continuous fibers (unidirectional, woven),
discontinuous fibers (chopped, short), particles.
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CFRC
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Challenges while compounding fibers
Starting After 2 minutes After 2-3 minutes
Not good
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During the compounding process of PMC
This was the best so far
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During the compounding process of PMC
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Micro-ct analysis of basalt fiber distribution in MABS
composites
Figure: Three-dimensional volumetric model of 10% basalt fiber reinforced BF/MABS composites
using micro-CT scanning.
Journal Paper J4
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Microscale morphology (SEM)
5% BF content 15% BF content
10% BF content 20% BF content
Journal Paper J4
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Introduction to Reinforcements
➢Reinforcements are not limited to long fibers.
➢Can be:
➤ Particles
➤ Flakes
➤ Whiskers
➤ Short/Long fibers
➤ Continuous fibers
➤ Sheets
➢Fibrous form is preferred for mechanical reasons:
➢Greater strength and stiffness
➢Better stress transfer
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Sources of Reinforcing Fibers
➢Natural Fibers:
❑Cotton, flax, jute, hemp, sisal, ramie (cellulose)
❑Silk, wool, hair (protein)
➢Synthetic (Man-made) Fibers:
❑Glass Fiber – Common & economical
❑Aramid Fiber – Kevlar (DuPont), Twaron (Teijin)
❑Gel-spun Polyethylene – Spectra, Dyneema
❑Ceramic & Carbon-based Fibers – High temp & high strength
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What Makes Fibers Effective?
Small Diameter
➤ Reduces internal flaws
➤ Increases usable strength
High Aspect Ratio (l/d)
➤ Efficient stress transfer
➤ Enables matrix load sharing
High Flexibility
➤ Small diameter + low modulus
➤ Supports forming & weaving
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The Size Effect – Graphical View
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Flexibility of Fibers – Concept & Equation
M= bending R = radius of
moment (force curvature (how tight
causing bending) the bend is)
E = Young’s I = second moment
modulus (material of area (depends
stiffness) on fiber diameter)
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Comparing Fiber Flexibility Across Materials
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Fiber Spinning Techniques
➢Wet Spinning
Polymer solution coagulates in a chemical bath.
➢Dry Spinning
Solvent evaporates post-extrusion.
➢Melt Spinning
Molten material extruded & solidified.
➢Dry-jet Wet Spinning (for aramids)
Solution exits spinneret into air gap → bath.
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Drawing and Chain Alignment
•Post-spinning stretching → improved alignment.
•"Skin effect": surface chains align more.
•Draw ratio = initial/final diameter
•High draw → high modulus
•Effects:
➢↑ Stiffness, strength
➢↓ Moisture absorption
➢↑ Crystallinity (but limited)
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Glass Fibers: Overview
Generic term: includes many oxide-based
fibers.
Types:
• E-glass: Electrical insulation + mechanical strength
• C-glass: Chemical resistance
• S-glass: High silica → High strength, high temperature
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Glass Fibers: Overview
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Atomic-Level Structure of Glass
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Atomic-Level Structure of Glass
Glass: amorphous network of SiO₂
No long-range order (Fig. 2.6a)
Metal oxides (e.g., Na₂O) disrupt network (Fig. 2.6b)
Isotropic mechanical and thermal behavior
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Introduction to Carbon Fibers
Made from polymer precursors (e.g., PAN,
pitch, rayon)
Consist of graphitic layers: hexagonal
carbon lattice
Highly anisotropic: high modulus in-plane
(a-axis), lower along thickness (c-axis)
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Carbon Fibers: A Versatile Material
Carbon is a light element (density = 2.268 g/cm³).
Exists in various crystalline forms, but we focus on
the graphitic structure.
Graphite has carbon atoms in hexagonal layers,
unlike the 3D structure of diamond.
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Extremely strong
Introduction to Carbon Fibers Bond In plane
Excellent stiffness
in fiber axis
bonded strongly in-plane, Hexagonal
which gives very carbon
high stiffness structure
weakly bonded
out-of-plane
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Anisotropy of Graphite
Graphite is highly anisotropic.
Theoretical Young's modulus in the layer plane:
~1000 GPa.
Young's modulus along the C-axis: ~35 GPa.
High in-plane modulus due to strong covalent
bonds between carbon atoms.
Low c-axis modulus due to weak van der
Waals bonds between layers.
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Dr. Abu Shaid Sujon
Lecturer
Department of Mechanical and Production Engineering
Islamic University of Technology
Chapter 3
Matrix material
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Introduction to Matrix Materials in Composite
➢Definition: Matrix is the continuous phase in a composite.
➢Functions of the Matrix:
➢Binds the fibers together
➢Protects fibers from environmental and mechanical damage
➢Transfers stress between fibers
➢Affects processability and performance
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Types of Matrix Materials
Three main types:
• Polymer matrix (PMCs)
• Metal matrix (MMCs)
• Ceramic matrix (CMCs)
• Each has unique advantages and limitations depending
on application
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Thermal Behavior of Polymers – Glass Transition &
Melting
➢Glass Transition Temperature (Tg):
▪A reversible change from rigid/glassy to rubbery state
▪Below Tg → Polymer is hard and glassy
▪Above Tg → Polymer becomes soft and rubbery
➢Melting Temperature (Tm):
▪Applies to semicrystalline polymers (e.g., HDPE)
▪Sharp phase transition: crystalline structure melts
➢Behavioral Differences:
▪Amorphous polymers: No Tm, only Tg
▪Semicrystalline polymers: Exhibit both Tg and Tm
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Thermal Behavior of Polymers – Glass Transition &
Melting
▪Amorphous polymers: No Tm,
only Tg
▪Semicrystalline polymers: Exhibit
both Tg and Tm
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Examples of Semicrystalline Polymers
Polymer Name Abbreviation Key Characteristics Typical Applications
High-Density High stiffness, waxy texture, chemical
HDPE Containers, piping, fuel tanks
Polyethylene resistance
Low-Density Flexible, good impact resistance, lower
LDPE Plastic bags, films, squeeze bottles
Polyethylene crystallinity
Lightweight, fatigue resistant, good chemical
Polypropylene PP Automotive parts, textiles, food containers
resistance
Polyethylene Transparent (amorphous) or strong Bottles, clothing fibers (e.g., Dacron),
PET
Terephthalate (semicrystalline), recyclable packaging
Nylon (Polyamide-
PA Strong, abrasion-resistant, hygroscopic Gears, ropes, textiles, carpets
6,6 or PA-6)
Polyoxymethylene Mechanical gears, bearings, automotive
POM High stiffness, dimensional stability, low friction
(Acetal) parts
Polytetrafluoroethyle Extremely low friction, high thermal and
PTFE Non-stick coatings, seals, gaskets
ne chemical stability
Polybutylene High crystallinity, good dimensional stability,
PBT Connectors, switches, appliance housings
Terephthalate electrical insulator
Polyetheretherketon High strength, chemical and thermal resistance, Aerospace, medical implants, structural
PEEK
e semicrystalline components
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Examples of Amorphous Polymers
Polymer Name Abbreviation Key Characteristics Typical Applications
Packaging, disposable utensils,
Polystyrene PS Transparent, brittle, low impact resistance
insulation
Poly(methyl
PMMA Excellent optical clarity, weather-resistant Lenses, acrylic glass, signage
methacrylate)
High impact strength, transparent, high Tg
Polycarbonate PC Safety helmets, eyewear, CDs, glazing
(~150 °C)
Polyvinyl Chloride
PVC Rigid, flame retardant, chemically resistant Pipes, window frames, credit cards
(rigid form)
Acrylonitrile Tough, impact-resistant, good dimensional Automotive parts, toys (e.g., LEGO),
ABS
Butadiene Styrene stability enclosures
High-temperature stability, electrically Aerospace interiors, electrical
Polyetherimide PEI
insulating, transparent connectors
Transparent, high thermal resistance, Sterilizable medical devices, filtration
Polysulfone PSU
hydrolytically stable membranes
Polyphenylene Good dimensional stability, hydrolysis Electronic housings, automotive
PPO
Oxide resistance components
Laboratory equipment, fluid-handling
Polyether sulfone PES Tough, flame-resistant, transparent
systems
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Polymer Matrices – Thermoplastics, Thermosets,
and Copolymers
➢Thermoplastics
✓Soften/melt on heating; re-harden on cooling
✓Suitable for molding and reshaping
✓Examples: HDPE, Polystyrene, PMMA
✓Amorphous structure lacks regular order
➢Thermosets
✓Harden irreversibly on curing (cross-linking)
✓Do not soften on reheating
✓Strong, rigid, heat-resistant
✓Examples: Epoxy, Phenolic, Polyester
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Polymer Matrices – Thermoplastics, Thermosets,
and Copolymers
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Copolymers – Polymer Chains with Two Monomers
• Copolymers are polymers made from more than one type of
monomer
•Copolymer – Two or more monomers
•Random Copolymer – Irregular sequence
•Block Copolymer – Long sequences of each monomer
•Graft Copolymer – Branches from a backbone
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Molecular Weight (MW)
➢MW = DP × (MW)ₘ
❑DP = Degree of Polymerization
❑(MW)ₘ = Molecular weight of repeating unit
➢Higher MW:
A higher MW usually results in
❑ Strength stronger polymers but also makes
❑ Strain-to-failure Elongation at break them more viscous and harder to
process.
❑ Viscosity → more difficult to process
➢Real polymers are a distribution of molecular weights
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Crystallinity and Its Effects on Polymer Behavior
➢Polymers: Can be amorphous or partially crystalline
➢100% Crystallinity: Practically unattainable in polymers
➢Typical Crystallinity Range: 30%–90%
➢Factors Influencing Crystallinity: ➢ Examples:
❑Polymer Type & Chain Structure ❑ Linear HDPE → up to 90% crystalline
❑Molecular Weight ❑ Branched polyethylene → ~65%
❑Crystallization Temperature ➢ Impact of Crystallinity:
❑Molecular Architecture ❑ Crystallinity → Stiffness & Strength
❑ Linear chains → higher crystallinity
❑ Branched chains → lower crystallinity
q
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Stress–Strain Behavior and Elastic Modulus in
Amorphous Polymers
Stress–Strain Characteristics
❑Fig. 3.5a: Glassy polymer shows Hookean
behavior (linear elastic)
❑Fig. 3.5b: Elastomer shows nonlinear elastic
behavior
❑Elastomers exhibit:
➢ Large elastic range
➢ Easy reorganization of tangled chains under
stress
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Stress–Strain Behavior and Elastic Modulus in
Amorphous Polymers
Temperature Dependence of Elastic
Modulus
above the flow
• Fig. 3.6: log(E) vs. Temperature
temperature Tf,
• Three regions based on temperature: polymer becomes
➢ Glassy Region (below Tg): Hard & a viscous fluid
stiff (E ≈ 5 GPa)
➢ Rubbery Region (between Tg and
Tf): Soft and flexible
➢ Fluid Region (above Tf): Polymer
flows; modulus drops sharply
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Thermal and Fire Response of Polymer Matrix
Composites
Fire Resistance /
Thermal Expansion of Flammability
Polymers ➢ Limiting Oxygen Index (LOI):
➢Polymers expand more than Minimum O₂ to sustain
metals and ceramics combustion
➢Thermal expansion is nonlinear ➢ Influencing Factors:
➢Highly sensitive to composition
and temperature • Matrix type (primary control)
➢Linear Expansion Coefficients: • Fire-retardant additives
• Epoxy: 50–100 × 10⁻⁶ K⁻¹ • Reinforcements (type and
• Polyester: 100–200 × 10⁻⁶ content)
K⁻¹
➢ Fire Resistance Order
(increasing):
Polyester < Vinyl Ester < Epoxy
< Phenolic
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Properties and Applications of Epoxy Matrix
Resins
What is Epoxy?
➢Thermoset polymer with reactive epoxide groups
➢Common formulation: DGEBA (Diglycidyl ether of bisphenol A)
➢Used with curing agents (e.g., DETA)
Key Properties
• Strong, stiff, and adhesive to glass fibers
• Low shrinkage (~3%) on curing
• Good moisture and chemical resistance
• Heat resistance up to 120°C (can exceed 180°C with special grades)
• Tunable via additives:
• Plasticizers – improve flexibility
• Accelerators – reduce curing time
• UV stabilizers – improve outdoor durability
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Properties and Applications of Epoxy Matrix
Resins
➢ Curing and Handling
▪Requires curing agents (amines, anhydrides)
▪Curing generates cross-links → irreversible hardening
▪B-stage epoxy prepregs: partially cured for later
molding
➢Applications
▪Aerospace, automotive, marine, electronics
▪Structural adhesives and high-performance composite
matrices
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Thermoset Resin Curing and Viscosity Behavior
➢Fig. 3.8: Viscosity (η) vs. time at
two temperatures (T₁ > T₂)
➢ Initial drop due to heat of
reaction
➢ Cross-linking causes
exponential rise in viscosity
➢Gel point → liquid to gel transition
➢After tgel becomes solid-like
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Quick Quiz?
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