Chapter 1

1. Definition of Composite Materials

A composite material is a multiphase material that exhibits a significant proportion of properties not
found in its individual constituents. It consists of:

• A matrix (continuous phase): Holds everything together and protects the reinforcement.

• A reinforcement (dispersed phase): Provides mechanical strength and stiffness.

• An interface: The boundary where the matrix and reinforcement interact—critical for load transfer.

Key Characteristics of Composites:

• Heterogeneous at macroscopic scale.

• Tailored material properties.

• Anisotropic behavior (properties vary with direction).

• Typically stronger and lighter than traditional materials.

2. Classification of Composites

A. Based on Matrix Material

Type Matrix Material Common Examples

Reinforcement

Polymer Matrix Epoxy, polyester, Glass, carbon, aramid Fiberglass, carbon fiber
Composite (PMC) thermoplastics fibers composites

Metal Matrix Al, Mg, Ti, Ni SiC, B4C, Al₂O₃ fibers Aluminum-SiC
Composite (MMC) composites

Ceramic Matrix SiC, Al₂O₃ Ceramic fibers (SiC, SiC/SiC used in jet
Composite (CMC) carbon) engines

B. Based on Reinforcement Geometry

1. Particle-Reinforced (e.g., cermets)

2. Fiber-Reinforced

o Short fibers (discontinuous): Randomly oriented

o Continuous fibers (aligned): Superior load-bearing capacity

3. Laminar Composites: Layers bonded together, like plywood

C. Based on Scale
 • Macro-composites: Reinforcement visible to naked eye

• Micro-composites: Fiber/matrix not easily distinguishable

• Nano-composites: Nanoparticles (e.g., carbon nanotubes) in matrix

3. Metal Matrix Composites (MMCs)

Description:

• Metals like Al, Mg, Ti act as the matrix.

• Reinforcements: ceramic particles (SiC, Al₂O₃) or fibers.

Processing Techniques:

• Powder metallurgy

• Liquid metal infiltration

• Stir casting

• Diffusion bonding

Properties:

• High specific strength and modulus

• Retain strength at high temperatures (unlike PMCs)

• Wear, creep, and fatigue resistance

• Better thermal and electrical conductivity

Limitations:

• High cost of production

• Complex manufacturing processes

• Poor machinability

• Difficult recycling

Applications:

• Aerospace structural parts

• Automotive piston heads, engine blocks

• Thermal management components in electronics

4. Polymer Matrix Composites (PMCs)

Categories:

• Thermosets: Epoxy, polyester, phenolic

• Thermoplastics: Nylon, polycarbonate, PEEK

Fabrication Methods:

• Hand lay-up

• Resin transfer molding (RTM)

• Filament winding

• Pultrusion

• Compression molding

Properties:

• Lightweight (1.5–2.0 g/cm³)

• Resistant to corrosion and chemicals

• Easily moldable

• Electrically insulating

• Good damping characteristics

Limitations:

• Limited service temperature (~200°C)

• Susceptible to moisture absorption

• UV degradation

• Flammability (unless modified)

Applications:

• Aircraft and UAV bodies

• Wind turbine blades

• Sporting goods

• Automotive bumpers and dashboards

5. Ceramic Matrix Composites (CMCs)

Materials Used:

• Matrix: SiC, Al₂O₃, ZrO₂

 • Reinforcement: Continuous ceramic fibers or whiskers (SiC, carbon)

Features:

• Withstand extreme environments (up to 1600°C)

• High hardness and compressive strength

• Oxidation and corrosion resistance

• Fracture toughness improved over monolithic ceramics

Fabrication Methods:

• Chemical vapor infiltration (CVI)

• Sol-gel process

• Hot pressing

• Polymer infiltration and pyrolysis (PIP)

Applications:

• Gas turbine engines (nozzle, blades)

• Brake disks for aircraft and sports cars

• Re-entry heat shields in spacecraft

6. Reinforcing Fibers

Fibers are the primary load-bearing component in fiber-reinforced composites.

Types and Properties:

Fiber Type Density Tensile Strength Modulus Key Features

(g/cm³) (MPa) (GPa)

Glass (E- 2.5 ~3400 ~70 Inexpensive, corrosion-

glass) resistant

Carbon Fiber 1.75 ~4000–7000 ~230–600 High stiffness, fatigue

resistance

Aramid 1.44 ~3000 ~130 High impact strength, low

(Kevlar) density

Boron Fiber 2.6 ~3100 ~400 High stiffness, brittle

Natural ~1.2–1.5 ~200–900 ~10–50 Biodegradable, moisture-

Fibers sensitive
7. Matrix Materials and Their Properties

The matrix supports the fibers, distributes load, and protects them from environmental damage.

A. Thermosetting Polymers:

• Epoxy: Strong adhesion, chemical resistance, low shrinkage

• Polyester: Good for large structures, fast curing, inexpensive

• Phenolic: Flame-retardant, thermal stability, used in electrical applications

B. Thermoplastics:

• Nylon, PEEK, ABS: Re-moldable, better toughness, high impact resistance

• Ideal for automated manufacturing

C. Metal Matrices:

• Aluminum: Lightweight, high thermal conductivity

• Magnesium: Lowest density among structural metals

• Titanium: Corrosion resistance, high strength-to-weight ratio

D. Ceramic Matrices:

• Resistant to extreme heat, wear, and chemical attack

• Suitable for space and nuclear applications

8. Polymers and Their Properties in Composites

Epoxy Resin:

• High tensile and compressive strength

• Excellent adhesion to many materials

• Low shrinkage on curing

• Good thermal and chemical resistance

Applications: Aircraft bodies, electrical insulators, adhesives

Polyester Resin:

• Low cost and fast curing

• Less tough than epoxy but easier to handle

• Can be UV stabilized
Applications: Marine panels, bathtubs, vehicle components

Phenolic Resin:

• Thermally stable, flame-retardant

• Good dimensional stability

• Used in fire-prone environments

Applications: Electrical switchgear, brake pads, fireproof panels

9. Applications of Composites in Engineering

Aerospace:

• Fuselage sections, control surfaces, radomes, interior panels

• Use of carbon/epoxy or carbon/PEEK composites for weight savings

Automotive:

• Carbon fiber-reinforced plastics (CFRPs) for weight reduction

• Fiberglass in body panels, dashboards, and under-hood components

Marine:

• Composite hulls offer corrosion resistance and reduced weight

• Propellers made from composites reduce noise and increase fuel efficiency

Civil Construction:

• Composite rebars and rods for corrosion-resistant reinforcement

• Precast sandwich panels for thermal insulation

Defense:

• Kevlar for body armor and ballistic helmets

• Carbon fiber composites in missile and UAV structures

Electrical/Electronics:

• Circuit boards with epoxy/glass composites

• Composite housings and insulators

Medical:

• Lightweight prosthetics

• Dental restoratives (composite resins)

 • Radiolucent surgical tools (do not interfere with X-rays)

Certainly! Here are detailed diagrams to enhance your understanding of composite materials:

1. Structure of a Composite Material

This diagram illustrates the fundamental components of a composite material:

• Matrix: The continuous phase that binds and protects the reinforcement.

• Reinforcement: The dispersed phase (fibers or particles) that provides strength and stiffness.

• Interface: The region where the matrix and reinforcement interact, crucial for load transfer.

Source: ResearchGate

2. Classification of Composites

This hierarchical diagram categorizes composites based on matrix material and reinforcement type:

• Matrix Material:

o Polymer Matrix Composites (PMCs)

 o Metal Matrix Composites (MMCs)

o Ceramic Matrix Composites (CMCs)(SlideShare)

• Reinforcement Type:

o Fiber Reinforced

o Particle Reinforced

o Structural Composites(YouTube, Princeton University, The Wall Street Journal)

Source: ResearchGate

3. Fiber-Reinforced Composite (FRC) Diagram

This schematic showcases different fiber arrangements within the matrix:

• Continuous and Aligned Fibers: Provide high strength and stiffness in one direction.

• Discontinuous and Aligned Fibers: Offer moderate strength with easier manufacturing.

• Discontinuous and Randomly Oriented Fibers: Provide isotropic properties but lower strength.
4. Applications of Composites

This diagram illustrates the diverse applications of composite materials across various industries:

• Aerospace: Aircraft structures, satellite components.

• Automotive: Body panels, suspension parts.

• Construction: Bridges, building reinforcements.

• Marine: Boat hulls, propellers.

• Medical: Prosthetics, implants.(The Wall Street Journal)

 Chapter 2
1. Introduction to Composite Lamina Elastic Behavior

A composite lamina is a single ply or layer in a laminated composite, typically consisting of strong, stiff
fibers embedded in a relatively weaker and more ductile matrix. The combination gives rise to
anisotropic mechanical behavior, meaning the properties vary significantly with direction.
Understanding the elastic behavior is crucial for predicting how the lamina responds to mechanical
loads.

This elastic behavior is typically studied using two primary theoretical approaches:

1. Micromechanics – analyzes the role of individual constituents (fibers and matrix).

2. Macromechanics – treats the lamina as a homogenized orthotropic material.

2. Micromechanics – Theoretical Perspective

Micromechanics investigates how the internal structure and material properties of the constituents
influence the overall properties of the lamina.

2.1 Purpose and Philosophy

The micromechanical approach views the lamina as a composite of two or more distinct materials:

• Fibers: Provide strength and stiffness in the desired direction.

• Matrix: Transfers loads to fibers and protects them from environmental damage.

By understanding the individual behavior and distribution of these materials, one can theoretically
model how they collectively behave under mechanical loads.

2.2 Idealizations in Micromechanics

Micromechanical theories make the following assumptions:

• Fibers are continuous, aligned, and uniformly distributed.

• The matrix is isotropic and linearly elastic.

• There is perfect bonding between fiber and matrix.

• Both fiber and matrix deform elastically.

These simplifications allow analytical modeling of complex materials.

 2.3 Key Concepts (Theory-Focused)

a) Volume Fraction

This is the proportion of fiber or matrix in a given volume of composite.

• The theoretical significance lies in how the load-carrying ability is distributed:

o A higher fiber volume fraction results in better mechanical performance along the fiber
direction.

b) Load Sharing and Stiffness

Fibers, being stiffer, carry more load in the longitudinal direction. The matrix helps in distributing the
load, especially in the transverse direction and under shear. The theoretical model assumes uniform
strain in the longitudinal direction and uniform stress in the transverse direction to derive effective
properties.

c) Anisotropic Behavior

Unlike metals, which are typically isotropic, composite laminae are anisotropic. This means:

• In one direction (fiber direction), the stiffness is high.

• In the perpendicular direction, the stiffness is lower.

• Shear properties are influenced by both constituents.

This directional dependence must be captured in engineering models.

d) Elastic Constants

The micromechanics approach helps predict:

• Longitudinal modulus: Influenced mainly by fiber stiffness and content.

• Transverse modulus: Controlled largely by matrix behavior.

• Shear modulus and Poisson’s ratio: Derived from constituent interactions.

These constants are then used to characterize lamina behavior under small deformations.

3. Macromechanics – Theoretical Perspective

In macromechanics, the composite lamina is treated as a homogeneous orthotropic material, and the
internal structure (fiber and matrix) is not considered explicitly.

3.1 Purpose and Viewpoint

This approach focuses on how the lamina behaves as a whole under applied stress, regardless of the
microscopic configuration. It is rooted in continuum mechanics, which assumes the material has
continuous properties throughout.
This is most useful for:

• Engineering-level design and simulation.

• Laminated composite analysis (stacking multiple laminae).

3.2 Orthotropic Nature of Lamina

A unidirectional lamina exhibits orthotropic behavior, meaning it has three mutually perpendicular
planes of symmetry with different material properties.

• E1E_1: Young’s modulus in the fiber direction.

• E2E_2: Young’s modulus transverse to the fiber.

• G12G_{12}: Shear modulus in the plane.

• ν12,ν21\nu_{12}, \nu_{21}: Poisson’s ratios.

Theoretical insight: Unlike isotropic materials, orthotropic laminae require multiple constants to define
elastic response.

3.3 Stress-Strain Behavior

The macromechanics approach establishes a linear elastic stress-strain relationship using these
orthotropic constants. This is usually expressed in matrix form (stiffness matrix), which is used to relate:

• Applied stresses to resulting strains.

• Under plane stress or plane strain conditions.

This theory supports finite element modeling and failure prediction, making it indispensable in
practical applications.

3.4 Transformation of Properties

In many real-world scenarios, laminae are oriented at angles. Macromechanics provides the
mathematical framework (transformation laws) to:

• Rotate stress and strain tensors.

• Predict behavior of off-axis plies.

• Analyze laminates with multiple orientations.

This theoretical capability is vital in laminate stacking and design optimization.

 4. Comparative Summary

Aspect Micromechanics Macromechanics

Focus Constituents (fibers, matrix) Homogenized lamina

Key Use Predict material constants from Analyze lamina behavior under
microstructure loads

Assumptions Perfect bonding, uniform fiber distribution Continuum behavior, orthotropic

nature

Directional Analysis Derived from fiber-matrix interaction Defined via engineering constants

Practical Material design and optimization Structural design and laminate

Applications analysis

5. Concluding Theoretical Insights

• The micromechanical approach is rooted in materials science and provides insight into how the
internal architecture affects performance.

• The macromechanical approach is rooted in solid mechanics and is essential for structural
analysis and design.

• Both theories complement each other—micromechanics informs material selection, while

macromechanics guides engineering application.
 Chapter 3

1. Introduction to Macromechanics

Macromechanics is the study of composite materials by treating them as homogeneous continua.

Instead of analyzing individual fibers and matrices, we use averaged or effective properties. This is
suitable for most practical engineering applications.

Composite laminae often have direction-dependent behavior, meaning their mechanical properties
vary along different material axes.

2. Stress-Strain Relations in Composite Lamina

General Form (3D Hooke’s Law)

For a linearly elastic, general anisotropic material, the stress-strain relationship is:

Plane Stress Condition (Thin Lamina)

Most composite laminae are thin and experience in-plane loading. So, we consider a 2D plane stress
condition:
 3. Types of Material Symmetries

A. General Anisotropic Material

• No symmetry in any plane.

• Requires 21 independent elastic constants.

• Rare in practical composites.

B. Orthotropic Material

• Properties differ in three mutually perpendicular directions.

• Has three planes of symmetry.

• Requires 9 independent constants:

E1,E2,E3,G12,G13,G23,ν12,ν13,ν23E_1, E_2, E_3, G_{12}, G_{13}, G_{23}, \nu_{12}, \nu_{13},
\nu_{23}E1,E2,E3,G12,G13,G23,ν12,ν13,ν23

Orthotropic lamina (e.g., unidirectional fiber-reinforced composites) is the most common case in
practice.

C. Transversely Isotropic Material

• One axis of material symmetry (usually fiber direction).

• Isotropic in the plane perpendicular to the fiber.

• Requires 5 independent constants:

E1,E2,G12,ν12,ν23E_1, E_2, G_{12}, \nu_{12}, \nu_{23}E1,E2,G12,ν12,ν23

D. Isotropic Material

• Identical mechanical properties in all directions.

• Requires only 2 constants:

EEE: Young’s modulus, ν\nuν: Poisson’s ratio

4. Reduced Stiffness Matrix for Orthotropic Lamina

For a specially orthotropic lamina (with material axes aligned to loading axes), we simplify the stiffness
matrix:
 5. Thermal Expansion of Lamina

Composite laminae expand or contract due to temperature variations. These strains are independent of

6. Moisture Expansion of Lamina

Similar to thermal effects, moisture absorption leads to swelling in the composite, known as
hygroscopic strain.
 7. Total Strain in Composite Lamina

The total strain in a lamina is the superposition of mechanical, thermal, and moisture strains:
 Chapter 4

In-Depth Theory: Testing of Composites

1. Introduction

Composite materials, due to their heterogeneous and anisotropic nature, present unique challenges in
testing. The mechanical properties vary with direction, composition (fiber/matrix), layup sequence, and
environmental exposure. Testing aims to:

• Understand mechanical behavior under tensile, compressive, shear, and fracture loads

• Predict the structural integrity in real service conditions

• Determine hygrothermal degradation and expansion behavior

Conventional metal testing methods are not suitable for composites due to:

• Direction-dependent stiffness and strength

• Different failure modes (e.g., delamination, matrix cracking, fiber breakage)

• Influence of fiber orientation and layup

2. Mechanical Testing of Composites

2.1 Tensile Testing

Objective:

Evaluate axial stiffness, strength, and Poisson’s ratio in:

• Longitudinal direction (parallel to fibers, 0∘0^\circ)

• Transverse direction (perpendicular to fibers, 90∘90^\circ)

Specimen Geometry:

• Rectangular flat strip (with tabs to avoid stress concentrations at grips)

• Gauge length ~ 100–200 mm

Load Application:

• Uniaxial load applied through Universal Testing Machine (UTM)

• Strains are measured using strain gauges or extensometers

 Theory:

Observations:

• For UD lamina in 0∘ C, failure is abrupt: fiber fracture

• In 90∘ C, matrix-dominated failure: matrix cracking, interfacial debonding

2.2 Compression Testing

Objective:

Measure compressive modulus and strength. Important in aerospace and structural components
where compressive loads dominate.

Challenges:

• Global buckling and fiber micro-buckling distort results.

• Requires anti-buckling fixtures.

Common Test Methods:

• Shear loading compression (SLC) (ASTM D3410)

The Shear Loading Compression (SLC) method is a mechanical test used to measure the
compressive properties of composite materials, especially unidirectional (UD) fiber-
reinforced composites. This method is specifically designed to prevent global buckling, which
is a major challenge in testing thin composite laminates under compression.
 1. Purpose of SLC Method

• To determine compressive strength and compressive modulus of fiber-reinforced composites.

• To simulate actual loading conditions that composite structures face, particularly in aerospace
and automotive applications.

• To eliminate errors caused by instabilities like buckling during traditional compressive testing.

2. Basic Principle

Instead of applying direct axial load (which often leads to buckling), the compressive load is
introduced via shear through specially designed test fixtures. The specimen is clamped
between two wedge-shaped grips that apply load at the ends, transferring the compressive force
through pure shear loading.

3. Test Configuration and Procedure

Specimen:

• Short, flat rectangular specimen

• Unidirectional laminate

• Tabbed ends to ensure even load distribution

Fixture:

• Special SLC fixture designed to induce pure in-plane shear that results in compressive stress in
the specimen

• Typically involves V-notched grips or wedge-shaped clamps

Procedure:

1. The specimen is clamped between the gripping wedges of the SLC fixture.

2. The load is applied through the fixture such that shear forces transfer compressive load to the
specimen.

3. Strain is measured using strain gauges or extensometers aligned along the axial direction.

4. The test is continued until failure.

 4. Theoretical Background

In SLC, the compressive load is indirectly induced via shear load paths that transfer
compressive stress into the body of the laminate.

5. Advantages of SLC Method

• Reduces risk of global buckling—critical for thin laminates

• More realistic and stable than traditional axial compression methods

• Useful for high-performance composites like carbon/epoxy

6. Failure Modes Observed

• Fiber microbuckling (most common): fibers begin to kink due to matrix yielding

• Delamination: layers separate due to shear stresses

• Kinking: plastic deformation leads to localized bending

• Matrix crushing in weaker directions

7. Standard Reference

• ASTM D3410: "Standard Test Method for Compressive Properties of Polymer Matrix Composite
Materials with Unsupported Gage Section by Shear Loading"
• Combined loading compression (CLC) (ASTM D6641)

Combined Loading Compression (CLC) Method – Detailed Explanation

The Combined Loading Compression (CLC) test is a standardized and widely used method to
determine the compressive strength and compressive modulus of fiber-reinforced composite
materials. It is considered more practical and reliable than older methods like SLC and
addresses issues like buckling and improper load introduction.

1. Purpose of CLC Test

• To measure compressive mechanical properties of composite laminates, especially

unidirectional or multidirectional fiber composites.

• To provide accurate, repeatable, and stable results without complex fixturing.

• To reduce the influence of specimen buckling, misalignment, and grip-induced damage.

2. Principle of CLC Method

The compressive load is applied to the specimen through a combination of shear and end-
loading via a special fixture. This hybrid approach ensures uniform stress distribution and
minimizes buckling.

3. Test Standard

• ASTM D6641 / D6641M:

“Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials Using a
Combined Loading Compression (CLC) Test Fixture”

4. Specimen Details

• Flat, rectangular specimen

• Standard dimensions: ~140 mm (length) × 13 mm (width) × laminate thickness

• Tabs are optional (unlike tensile tests)

• Can be made from UD, cross-ply, or angle-ply laminates

 5. Fixture Description

The CLC fixture includes:

• Two compression platens or clamp assemblies

• A fixed back support and a movable front plate

• Side supports to prevent lateral movement and buckling

• Load is applied through both gripping surfaces and fixture ends

6. Loading Mechanism

The load is transferred to the specimen via:

• End loading: force applied axially to the specimen ends

• Shear loading: force applied through clamping friction and shear contact with the fixture

This combination provides more stable and accurate load transfer than either method alone.

7. Theory and Calculations

Strains are typically measured using strain gauges or non-contact extensometers.

8. Common Failure Modes

• Kinking or microbuckling of fibers

• Shear crimping of matrix

 • Delamination between plies

• Crushing in resin-rich zones

The failure pattern depends on the fiber orientation and layup type.

9. Advantages of CLC Method

Advantage Description

Buckling Control Side supports reduce lateral deformation

Standardized ASTM D6641 provides reliable protocol

Applicable to Many Layups Works for UD, woven, and multi-directional laminates

Easy Setup No tabs required, straightforward fixturing

Failure Mechanisms:

• Micro-buckling: local fiber instability

• Kinking: plastic matrix deformation leads to shear banding

• Delamination: layer separation under compressive shear

Interpretation:

• Non-linear stress-strain behavior due to plastic flow in matrix

2.3 In-Plane Shear Testing

Objective:

Evaluate shear modulus G12G_{12} and shear strength of the lamina.

Key Methods:

(a) ±45° Tensile Test:

• Laminate with fibers at ±45° to load axis

• Under axial load, induces shear in ply

• Simple setup, but sensitive to edge effects

(b) Iosipescu Shear Test:

• V-notched specimen loaded to create pure shear zone

 • Strain measured across notched section

• More accurate and uniform than ±45° test

(c) Short Beam Shear:

• Interlaminar shear strength (not in-plane)

• Uses 3-point bending on a short beam (ASTM D2344)

• Fails by interlaminar cracking

2.4 Fracture Toughness Testing

Fracture in composites occurs via matrix cracking, fiber pull-out, fiber breakage, and delamination.
Tests evaluate energy required to initiate and propagate a crack.

Fracture Modes:

Mode Description Crack Plane Orientation

Mode I Opening Normal to the crack surface

Mode II Sliding In-plane shear

Mode III Tearing Out-of-plane shear

Common Tests:

• Double Cantilever Beam (DCB) – Mode I

• End-Notched Flexure (ENF) – Mode II

• Mixed-Mode Bending (MMB) – Combined I & II

Key Concepts:

Critical Energy Release Rate (GcG_c):

Higher GcG_c → better damage tolerance and delamination resistance

 3. Characterization of Mechanical Constants

Orthotropic composite lamina is defined by 9 independent elastic constants:

• E1E_1: Longitudinal modulus

• E2E_2: Transverse modulus

• E3E_3: Out-of-plane modulus

• ν12,ν13,ν23\nu_{12}, \nu_{13}, \nu_{23}: Poisson’s ratios

• G12,G13,G23G_{12}, G_{13}, G_{23}: Shear moduli

Most important for thin lamina:

• E1,E2,G12,ν12E_1, E_2, G_{12}, \nu_{12}

These are extracted from:

• Tensile testing (0° & 90°)

• ±45° shear test

• Compression testing

Results feed into macromechanics models and laminate theory equations.

4. Hygrothermal Testing

Composites are often used in outdoor, aerospace, or marine environments where they are exposed to
temperature fluctuations and moisture.

4.1 Moisture Absorption Behavior

Follows Fickian diffusion in most cases.

 4.2 Thermal Expansion

Thermal expansion in composites is directionally dependent.

4.3 Hygrothermal Expansion

4.4 Hygrothermal Conditioning Protocol:

1. Dry specimen at 50–60°C to remove moisture.

2. Immerse in boiling water or store in humidity chamber (95% RH).

3. Periodically weigh specimen → measure % weight gain.

4. Test mechanical properties and compare with dry specimens.

 5. Summary Table

Property Test Type ASTM

E1,ν12E_1, \nu_{12} 0° Tensile Test D3039

E2E_2 90° Tensile Test D3039

G12G_{12} ±45° Tensile / Iosipescu Shear D3518 / D5379

Compressive Modulus Compression Testing D6641

Shear Strength Short Beam Shear D2344

Fracture Toughness (GcG_c) DCB / ENF / MMB Tests D5528 / D7905

Thermal Expansion (α\alpha) Free Expansion Test -

Moisture Expansion (β\beta) Humidity Conditioning + Testing -

 Chapter 5

Failure and Maintenance of Composites (In-depth Notes)

1. Failure Types in Laminates

Composite laminates can fail due to complex stress distributions, material heterogeneity, and
environmental influences. Unlike metals, composites exhibit multiple progressive failure mechanisms.

A. Matrix Cracking

• Occurs due to tensile or thermal stresses exceeding the matrix's strength.

• Typically the first damage to appear, especially in transverse plies.

• Often induced by fatigue, impact, or shrinkage.

• Allows moisture ingress and accelerates other failure modes like delamination.

B. Fiber Breakage

• Occurs when fibers are overstressed in tension.

• Most critical in unidirectional (UD) composites.

• Localized fiber failure can trigger stress redistribution → progressive failure.

• Dominates in the primary load-carrying direction.

C. Fiber-Matrix Debonding

• Interface failure due to incompatible bonding, thermal mismatch, or loading.

• Weakens the load-transfer mechanism.

• Increases crack propagation rate and reduces strength.

D. Delamination

• Separation between adjacent layers (plies).

• Triggered by impact, out-of-plane shear, thermal cycling, or manufacturing defects.

• Reduces stiffness and causes a sudden loss of load-carrying ability.

• Extremely dangerous in structural parts like aircraft skins or wind blades.

E. Buckling and Kinking

• Occurs in compression-dominated loading, especially in thin laminates.

• Fiber instability leads to misalignment or permanent deformation.

• Can initiate delamination or fiber breakage.

 F. In-plane Shear Failure

• Due to excessive shear forces along the laminate plane.

• Common in ±45° or multidirectional laminates.

• Results in slippage or matrix yielding.

2. Damage to Laminate Structures

Composite structures are susceptible to progressive, often invisible damage. Key sources:

Damage Type Mechanism

Manufacturing Defects Voids, fiber misalignment, dry spots during layup or curing

Impact Damage (BVID) Low-energy hits (e.g., tool drop) causing hidden delamination or cracks

Fatigue Damage Microscopic damage accumulation under cyclic loading

Thermal Stress Cracking Due to mismatch in coefficients of thermal expansion (CTE)

Moisture Ingress Leads to matrix plasticization, swelling, and interfacial degradation

These damage types interact and evolve, reducing performance over time without obvious external
signs.

3. Quality Control in Composites

Maintaining quality in composites requires both in-process and post-process evaluation.

A. Non-Destructive Testing (NDT) Techniques

• Ultrasonic Testing: Pulse-echo and through-transmission detect voids, delaminations, and

cracks.

• X-Ray / CT Scanning: Detailed visualization of internal structure, fiber distribution, and defects.

• Thermography (IR): Identifies anomalies in thermal conductivity—used for impact or

delamination.

• Acoustic Emission: Real-time failure monitoring under stress.

B. Process Monitoring

• Cure temperature, pressure, and vacuum control in autoclave/oven processes.

• In-situ fiber alignment monitoring via machine vision.

 • Use of smart materials (e.g., embedded sensors) for active damage detection.

C. Mechanical Testing for Quality Assurance

• Tensile, compressive, and flexural testing on sample coupons.

• Interlaminar shear strength (ILSS) testing for delamination resistance.

4. Maintenance and Repair of Composites

Due to their nature, composites require proactive and specialized maintenance.

Scheduled Inspection:

• Use of NDT techniques to track damage progression over time.

• Thermographic scanning or ultrasonic mapping.

Repair Techniques:

1. Scarf Repairs: Removing damaged plies in a tapered shape and bonding new layers.

2. Step-Lap Repairs: Similar to scarf but with stepped transitions.

3. Resin Injection: For microcracks and small voids.

4. Bolted/Adhesive Reinforcement: Strengthening damaged areas with bonded or bolted patches.

5. Sandwich Core Replacements: In honeycomb structures, core replacement and skin patching.

Preventive Measures:

• Use of coatings (e.g., UV-blockers, water-repellent paints).

• Storing parts in controlled environments (humidity, temperature).

• Designing for damage tolerance using toughened matrices or hybrid fibers.

5. Case Studies of Composite Failure & Repair

Aircraft Wing Panel

• Damage: Low-velocity impact during ground handling.

• Inspection: Barely Visible Impact Damage (BVID) found using ultrasonic C-scan.

• Action: Local delaminated region removed and scarf-repaired with prepreg layup.

• Outcome: >90% strength recovery, passed airworthiness criteria.

 Wind Turbine Blade

• Damage: Delamination due to moisture and fatigue near the root section.

• Inspection: Thermographic and tap testing.

• Action: Cut-out section, core replaced, surface repaired with epoxy-glass layup.

• Outcome: Blade put back in service with minor performance loss.

Formula One Car Monocoque

• Damage: Internal cracking post crash, undetectable by surface inspection.

• Inspection: High-resolution CT scan confirmed deep delaminations.

• Action: Monocoque section replaced with modular insert.

• Outcome: Regained structural integrity, tested for crashworthiness again.

6. Summary Table

Aspect Description

Failure Types Matrix cracking, fiber breakage, delamination, shear, buckling

Damage Sources Manufacturing flaws, impact, fatigue, thermal, moisture

Inspection Methods NDT (Ultrasonic, CT, Thermography), Process monitoring

Repair Methods Scarf/step repairs, resin injection, core replacement

Maintenance Goals Early damage detection, prolong life, ensure safety