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Composites > Palm Cards > Flashcards

Palm Cards Flashcards

1
Q

Define a Laminate

A

Laminate is a product
made by bonding
together two or more thin
layers (plies or laminae)
of materials. Most
common composites
employed in aircraft
industry are polymer
matrix laminates.

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2
Q

Driving factors for composite selection

A

Weight, strength, resistance to environmental effects, performance under temperature extremes, resistance to damage and fatigue. Assembly costs

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3
Q

Role of both the fibers and the matrix in a complete composite.

A

Fibers: Oriented for principal stress/es in order to carry the major load/s

Matrix: Transmits load to/from fiber. Shapes and protects.

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4
Q

At what orientation relative to fiber direction is the composite strongest?

A

Along fiber direction

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5
Q

Polymer Matrix Composites (PMCs) vs Metals

A
  1. Properties are not uniform in all directions
  2. Strength and stiffness can be tailored to meet loads
  3. Possess a greater variety of mechanical properties
  4. Have poor through the thickness (short transverse) strength
  5. Are usually laid up in two dimensional form, while metals
    may be used in billets, bars, forgings, castings, etc.
  6. Have greater sensitivity to environmental heat and moisture
  7. Possess greater resistance to fatigue damage
  8. Damage propagates through delamination rather than
    through-the-thickness cracks
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6
Q

PMC Advantages Over Metals

A
  • Light weight
  • Resistance to corrosion
  • High resistance to fatigue damage

Reduced machining

Tapered sections and compound contours easily
accomplished

Strength and stiffness can be tailored

Reduced number of assemblies and reduced fastener count
when co-cured or co-consolidation is used

Absorb radar microwaves (stealth capability)

Near zero thermal expansion reduces thermal problems in
outer space applications

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7
Q

PMC Disadvantages Over Metals

A

Composite Materials are expensive

Less historical data and established design allowables

Corrosion problems can result from improper coupling with
metals, especially when graphite is used (sealing is
essential)

Degradation of structural properties under temperature
extremes and wet conditions

Poor energy absorption and impact damage resistance

May require lightning protection

Expensive and complicated inspection methods

Reliable detection of substandard bonds is difficult

Internal nature of defects makes damage assessment
difficult

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8
Q

Reasons for Lower-than-Expected Usage of Composites

A

High Cost of raw materials and manufacture

High Cost of Certification

Low Resistance to Impact

Damage and transverse cracking

Limited applicability at high temperatures

Advancement in aluminium alloys
(improved toughness,
fatigue resistance and corrosion resistance)

Development of new light weight alloys (Al-Li)

Low cost aerospace grade castings, mechanical alloying and
super-plastic forming, diffusion bonding

Advanced joining techniques for metals (laser and friction
welding, automated riveting

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9
Q

Different Types of Polymers

A
  • Polymer Matrix Composites
  • Metal Matrix Composites
  • Ceramic Matrix Composites
  • Carbon Carbon Composites

Hybrid Metal/PMC Composites

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10
Q

Types of Matrix Reinforcements

A
  • Continuous Fibers made from light elements (C, B, Si, etc)
  • Whiskers: Ultra strong, stiff, short fibers made up of single crystals
  • Particulates: Mainly ceramic, to improve toughness of brittle matrices
  • Nanotubes
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11
Q

Types of strengths provided by a matrix

A

Transverse, Shear, Compressive

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12
Q

Service temperatures of the different composite types

A

PMC: Less than 300 deg

MMC: Less than 650 deg

CMC: Less than 1400 deg

C/C: Greater than 1400 deg

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13
Q

Overview of PMC Benefits

A

Light weight, high performance
fibers in organic polymer matrices

Usually processed at low temperatures and pressures.

reasonably low cost.
Easier to machine, mould and fabricate

Limited high temperature applications

Susceptible to environmental degradation

Includes thermosets and thermoplastics.

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14
Q

MMCs compared to PMCs

A

Higher temperature resistance.

High melting point

High ductility and toughness

Higher dimensional stability (low CTEs)

Mostly heavier than PMCs

Costly, complex and limited fabrication techniques

Problems with thermal coefficient mismatch and poor
interfacial bonding between reinforcements and matrices

Susceptible to corrosion

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15
Q

Types of MMCs

A

Aluminium Metal

Magnesium and Copper

Titanium Alloys Intermetallics

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16
Q

Types of MMC Reinforcements

A

Continuous Fibers

Whiskers

Particulates

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17
Q

Manufacturing Methods for MMCs

A

Rapid liquid metal processes such as squeeze casting

Powder metallurgy processes based on heating

compacted metal powders to just below their melting
points to consolidate them (Hot Isostatic Press)

Some conventional metal working techniques for
particulate MMCs

Super plastic forming

Diffusion bonding (for Ti matrix systems)

Additive manufacturing.

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18
Q

Types of ceramic composites, and their operating temperatures

A
  1. Glass ceramics (up to 500deg)
  2. Oxides (?)
  3. Nitrides (up to 1400 deg)
  4. Carbides (Over 1400 deg)
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19
Q

Overview of ceramic composites

A

Ceramic matrices reinforced with continuous fibres,
whiskers or particulates

High temperature applications in gas turbine engines and
high temperature airframe/space structures

Ceramics are brittle and subject to microcracking at high temperatures

Hence reinforcing fibres with high strength and stiffness
that have chemical stability and resistance to oxidation at
high temperatures as well as matching CTEs are required

Hence it is common to use similar materials for both
reinforcement and matrix such as SiC/SiC and alumina
fibres in alumina matrix.

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20
Q

Features of C/C Composites

A

C/C has the best structural properties (specific
strength, specific stiffness, creep resistance, at the highest
operating temperatures (over 20,000 C) of all materials

Further it has no significant chemical or thermal expansion
compatibility problems

Its greatest disadvantage is susceptibility to oxidation at
high temperatures

Hence C/C is the main candidate for use in rocket nosecones, nozzles, and leading edges on hypersonic wings.

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21
Q

Advantages of Hybrid Metal/PMC Composites

A

Combines the advantages of metals (such as high
resistance to low velocity impact damage) with the advantages of
PMCs (such as high fatigue life).

They typically consist of thin sheets of metal (usually Aluminium)
bonded together with a fibre reinforced adhesive.

Much higher fatigue lives than
monolithic alloys and greater impact resistance than PMCs

They have lower densities than aluminium, higher post yield
strength and higher damping capacity

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22
Q

Disadvantages of Hybrid Metal/PMC Composites

A

Higher sensitivity to blunt notches,

lower elastic modulus than monolithic aluminium

Possibility of earlier
crack initiation, and

high cost.

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23
Q

Types of Fiber Metal Laminates

A

ARALL, GLARE

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24
Q

Explain how fiber form of a material relates to the probability of a defect being present

A

Elements of low atomic number have strong directional inter-atomic bonds.

The strong bonding inhibits plastic flow, so they are very
sensitive to sub-microscopic flaws.

These materials can achieve very high strengths when made
into fibers due to the probability of a flaw being present
being proportional to the volume of the materials for a given length.

However, for the same reason, they also become very sensitive to flaws, exhibiting a much higher variability in
strength compared to bulk materials.

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25
Q

Explain why fibers have high strength

A

The probability of a flaw being present is proportional to the
volume of the materials for a given length (and fibres have very
low cross sectional area).

Flaws can be minimized by proper manufacturing and coating procedures

Drawing and spinning impose high strains along the fibre axis
produces a more favourable orientation of crystal structure

High cooling rate or rapid molecular deposition produces ultrafine grained structures with much higher properties.

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26
Q

Explain the process used to manufacture glass fibers

A

Glass is first melted at temps between 12500 and 14000 C

Flows into an electrically heated alloy
bushing which contains a large number of holes at its base

The glass drops emerging from the holes are drawn into fibres at speeds of up to 50 m/s

They are cooled by a water spray and coated with a “size” by a rolling applicator

Finally the fibres are combined into a strand (of 52, 102 or
204 fibres) and wound on to a take-up spool

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27
Q

How do the properties of glass fiber differ from those of the bulk material?

A

The cooling rate experienced by the fibres is very high
(greater than 10,0000 C per sec)

Hence the fibre structure is different from that of bulk glass,

resulting in a higher tensile strength but lower
elastic modulus and chemical resistance

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28
Q

Outline how flaws affect glass fibres

A

Glass fibres, being essentially monolithic, linearly elastic brittle
materials, are very sensitive to flaws in the form of sub-microscopic inclusions and cracks

Commercial fibres are particularly prone to abrasion against
other fibres, resulting in a reduction in strength of up to 20% compared to pristine fibres

Humid environments reduce the strength of glass fibres under
sustained loading due to adsorption of moisture on to the surface causing “static fatigue”

Individual fibre strength is reduced by about 50% in a PMC, but
the bundling of the fibres overcomes this effect.

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29
Q

Define the two types of glass fiber

A
  1. E-Glass:
    Calcium Alumino-Borosilicate used for
    electrical applications due to its higher
    electrical resistivity.
  2. S Glass:
    Magnesium Alumino-Silicate, used for
    structural applications due to its higher
    strength and low cost
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30
Q

What is ‘Size’, and why is it used?

A

Consists of a lubricant, binders such as starch and
polyvinyl alcohol (to hold filaments together) and primers to
improve adhesion between fibres and matrices.

Reduces damage caused by friction upon fibres.

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31
Q

Carbon Fiber:
- Why is it used
- What Types
- What is it made from

A
  1. High specific strength and stiffness
  2. High Modulus, High
    Strength, Intermediate Modulus
  3. Organic precursor materials (PAN and Pitch) by a
    process of carbonization.
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32
Q

What is PAN

A

PolyAcryloNitrile is an acrylic textile fibre

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33
Q

How is PAN manufactured:

A

By wet or dry spinning of the polymer or co-polymer, and then stretching.

Stretching of fibres during spin reduces the diameter, aligns more molecular chains along the length, increasing stiffness

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34
Q

How is carbon fiber manufactured from PAN?

A

Stage 1 – Stabilisation: PAN is first stabilised in air at about 2500C
by oxidation to form a thermally stable ladder polymer, with a high
glass transition temperature (Tg).

Stage 2 – Carbonisation: Removal of N, O and H in an inert nitrogen atmosphere at 1200-16000C. Fibres develop full strength at 1500-16000C. Basal planes align along the fibre axis, but remain as extended 2D ribbons.

Stage 3 – Graphitisation: Final heat treatment at 1500-25000C in an
inert atmosphere (Argon), wherein basal layers grow and coalesce along the fibre direction, to provide higher E (up to 380 GPa) at the cost of lower strain capability (0.7%) and strength.

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35
Q

What is Pitch?

What are its advantages and drawbacks?

A

Precursor made of organic compounds

Cheap but poor mechanical properties

Low compression, shear and tensile properties.

High electrical and thermal conductivity.

No tension required to develop molecular orientation (unlike PAN).

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36
Q

How is Pitch manufactured into carbon fiber

A

Stage 1 – Isotropic to Mesophase Pitch: Isotropic pitch is
subjected to prolonged heating in an inert atmosphere at 400-
4500C to form a liquid crystal phase (mesophase) which is meltspun into fibre form

Stage 2 – Cross linking: To reduce relaxation, the pitch fibres are
cross linked by heating for a short time at 3000C in atmosphere containing O2.

Stage 3 – Pre Carbonisation

Stage 4 – Carbonisation

Stage 5 – Graphitisation

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37
Q

Benefits and drawbacks of Boron fibers.

How are they made

A

Very hard

Difficult to machine

Chemical vapor deposition

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38
Q

Properties of Aramid fibers

A

Superior Specific Properties: Higher specific properties compared to glass fibers.

Tensile Properties: Good tensile properties up to 400°C.

Compressive Strength: Poor compressive strength limits applications.

Energy Absorption: High energy absorption during fracture. High strain-to-failure.

Applications: Used in ballistic protection and engine containment rings

Anisotropic Properties: Strong covalent bonds within chains and weak hydrogen bonds between chains result in anisotropic properties.

Non-linear Compression: Highly non-linear under compression due to kink band formation from in-phase compressive buckling of fibrils.

Thermal Stability: Strength reduces by about 20% at 180°C, with rapid decline thereafter.

Hygroscopic Nature: Absorbs moisture (around 4% at 60% relative humidity for Kevlar 49), but tensile strength remains largely unaffected.

Creep Characteristics: Significant short-term creep, negligible long-term creep.

UV Degradation: Susceptible to UV degradation, but protected in PMCs by resin matrix.

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39
Q

Properties of polyethylene fibres

A

Tough

Operate best under 100 deg temps

Poor creep behavior

Low compressive strengths due to kink band formation

Specific strengths comparable to aramids.

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40
Q

List the types of dry fiber forms

A
  1. Mats
  2. Woven Fabrics
  3. Warns
  4. Tows
  5. Rovings
  6. Braided Fabrics
  7. Non-Crimp Fabrics
  8. Tapes
  9. 3D textile preforms
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41
Q

Function of the matrix

A

Provides shape

Holds the fibres together

Transfers load in and out of fibres

Separates the fibres to prevent failure from
adjacent fibres

Protects fibres from environmental damage

Determines “matrix dominated” properties:
- Longitudinal compressive strength
- Transverse tensile strength
- Intra and interlaminar shear strength

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42
Q

Service temps for different matrix types

A
  1. Low temp thermosets (epoxies, polyesters, vinyl esters, phenolic resins)
    = 100 - 150 deg
  2. Medium temp thermosets (Bismaleimides)
    = Up to 230 deg
  3. High temp thermosets (Polyimides)
    = Up to 300 deg
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43
Q

Properties of an epoxy resin

A
  1. Excellent chemical and mechanical properties at low temps
  2. Low viscosity during curing, enabling easier forming
  3. Low shrinkage
  4. Good fiber adhesion
  5. Tg increases with increasing cure temperature
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44
Q

Limitations of epoxy resins

A

Low toughness, and sensitive to impact damage

Limited temp range

Absorbs moisture

Sensitive to UV exposure

High cost compared to polyesters

Less convenient curing compared to polyesters

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45
Q

List the additives used in epoxy resins, and describe their effects

A
  1. Diluents
    Added to reduce viscosity
    before cure
  2. Flexibilizers
    Added to reduce elastic modulus
    and increase elongation to failure
  3. Toughening agents Increase fracture toughness
    and reduce crack propagation rates
  4. Inert fillers, such as hollow glass microspheres:
    Added to alter density, resin flow and
    effective modulus
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46
Q

List and describe the mechanisms through which an epoxy resin can be toughened.

A

Formation of a solid solution with a more ductile polymer

Precipitation of elastomeric second phase

Development of interpenetrating polymer networks

The inclusion of elastomeric second phase can be achieved by
adding an elastomer (rubber) to form a copolymer with the
base resin which then precipitates out upon curing to form a
dispersed second phase

Adding a very fine powder to form a dispersion

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47
Q

List the advantages of polyester resins

A

Initial low viscosity that permits good wetting of the fibres

Low cost (inexpensive and readily available raw
materials, easy long term storage)

Cure conditions can be modified easily without too much expertise

Easy manufacture of modifications for a variety of specific applications
Excellent environmental durability

48
Q

List the disadvantages of polyester resins

A

High exotherm and high shrinkage on cure (this
leads to residual stresses and poor interfacial
bonding with fibres)

Lower values for mechanical properties than epoxies

Systems with adequate shear strength tend to be
brittle (low toughness)

Toughening mechanisms are relatively ineffective

Poor chemical resistance

49
Q

List the general properties of vinyl-ester resins

A

Intermediate class of materials between epoxies and polyesters

The major ester ingredient is produced by reaction of a standard epoxy and methacrylic acid

The resins are cured by free radical reaction process like polyesters

Tougher and more chemically resistant than polyesters due to less cross linking

Large range with different mechanical properties,
including rubber toughened resins

50
Q

Outline the advantages and disadvantages of vinyl-ester resins

A

Advantages:

Combines chemical resistance of epoxies with easy processing of polyesters

Lower cross linking, hence better mechanical properties than polyesters

Improved interfacial bond strength with fibres

Disadvantages:

High cost compared to polyesters

Higher shrinkage levels than epoxies

51
Q

Outline a general description of phenolic resins

A

Condensation polymerisation of phenol with formaldehyde under alkaline or strong acid conditions is used to produce a pre-polymer called “resol”.

This is polymerised under heating or with acidic or basic catalysts to form densely cross-linked structure forming
the matrix

Water and volatile byproducts are formed in the reaction,
hence high pressures are required for curing

High void content

52
Q

State the advantages and disadvantages of phenolic resins

A

Advantages

Excellent resistance to high temperatures, especially under oxidizing conditions

Phenolics have good ablation properties, forming char
readily, yielding a superficial layer of carbon, which burns away protecting the underlying composite

Good as fire retardant materials

Disadvantages

High pressures needed for polymerisation, hence
difficult to fabricate

Significantly lower mechanical properties due to high
void content

53
Q

What are the two types of Polyimide Resins?

What is special about them?

A
  1. Condensation polyimides
  • Thermoplastics used as
    matrices or toughening mechanisms in high temperature epoxy
    formulations
  1. Addition polyimides thermoSETs.

In general they release large quantities of volatiles unless high temperatures and pressures
are employed, hence fabrication is complex and expensive

  1. Very high Tg
54
Q

Advantages and disadvantages of Polyimide Resins?

A

Advantage:

Stability at high temperatures (up to 3000C)

Resistance to most chemicals

Can be formulated to give better mechanical properties

Disadvantages

High Cost

Difficulty in processing

55
Q

List the requirements of the component manufacturing process

A

To orient the fibres in the appropriate directions and
proportions to obtaine the desired 2D or 3D
properties

Produce the required shape of the component

Verify properties

To ensure that the fibres state and orientation

Ensure free of defects

56
Q

Procedure, Pros, Cons:
Wet Lay-Up

A

Fibres laid up in open mould, with resin added by brushing or spraying

Labour intensive, with challenging quality control

Cost effective

57
Q

Procedure, Pros, Cons:

A

“Pre-pregs” are sheets of fibre reinforcement pre-impregnated with partially cured resin.

Safer, cleaner, more consistent, less labor intensive

Costly with limited shelf life

58
Q

List all composite manufacturing processes

A
  1. Wet Lay Up
  2. Pre-Preg
  3. Wrapping
  4. Compression Die Moulding
  5. Open Die Moulding
59
Q

How must pre-preg materials be stored

A

Refridgerated

60
Q

Draw a pre-preg manufacturing set-up

A

Picture

61
Q

Draw an autoclave process graph (Temp/Time)

A

Picture

62
Q

List the resin moulding techniques, and draw each one

A

Resin Transfer Moulding

Resin Film Infusion

Vacuum Assisted RTM

63
Q

Advantages/Disadvantages of Liquid Resin Moulding

A

Advantages:

Can be used for manufacturing complex shapes and parts

Materials can be selected to obtain the desired properties

Process can be tailored to each individual type of
component to maximise efficiency

Disadvantages:

Tooling is very costly

Initial set up time is time
consuming and expensive

Process is time consuming and laborious

High operator skill and experience required

Very low through put

64
Q

Explain Filament Winding

A

Process that enables
continuous reinforcement to be laid down at high speed and precision in predefined paths

The process involves impregnating
continuous fibres with resin and winding them on to a stationary or rotating mandrel.

65
Q

Pros/Cons of filament winding

A

Advantages:

High speed and high precision process

Fully automated

High fibre volume ratios can be achieved

Can be employed to make very large components

Disadvantages:

Tooling is very costly

Initial set up time is time consuming and expensive

66
Q

Explain Pultrusion

A

Highly automated, continuous linear process for manufacturing
of components with constant cross-sectional profiles with fibre
reinforced PMCs

The process essentially involves impregnating continuous fibres
with a thermosetting resin and pulling them through a heated die to shape and cure

67
Q

Pultrusion pros/cons

A

Advantages:

Continuous process
Fully automated

High throughput

Can use inexpensive forms of reinforcement

Can be employed to make very long components

No lamination or autoclave curing is required

No requirement to refrigerate raw materials

Disadvantages:

Tooling can be costly

Initial set up time is time consuming and expensive

Process can only be applied to parts with constant cross
section

68
Q

Outline the manufacturing process for thermoplastics

A
  • Very viscous, so liquid resin moulding techniques don’t work
  • Requires only heat and pressure over a short period of time
  • High temp and High Pressure
69
Q

Define:
1. Isotropic
2. Anisotropic
3. Orthotropic
4. Transversely Isotropic

A
  1. Has the same material properties in all directions
    (infinite number of planes of symmetry)
  2. Has different material properties in different
    directions with no planes of symmetry
  3. Has three planes of symmetry for its material
    properties
  4. Material with one plane (at every point) in
    which the properties are the same in all directions
70
Q

Define the longitudinal and transverse modulus

A

Longitudinal: Stiffness in fiber direction
= E1

Transverse: Stiffness across fiber direction
= E2

71
Q

Define the Major and Minor Poisson’s Ratio

A

Major = v12 = -E2/E1

Minor = v21 = -E1/E2

72
Q

Relation between E and v

A

(V12/E1) = (v21/E2)

73
Q

List the methods used to calculate the mechanical properties of a single lamina from the known properties of the fiber and matrix

A
  1. Mechanics of materials approach
  2. Theory of elasticity approach
  3. Finite Element Modelling
74
Q

What assumptions are involved in determining elastic constants using
Micromechanics:

A

The fibres are assumed to be homogeneous, linearly
elastic, isotropic, regularly spaced, perfectly aligned and of uniform strength

The matrix is assumed to be homogeneous, linearly
elastic and isotropic

The fibre matrix interface is assumed to be perfect with no voids or dis-bonds

The lamina is orthotropic along its principal directions

The composite is considered to be homogeneous

75
Q

State the Rule of Mixtures Mechanics of Materials Approach Equation

A

E1 = Ef*Vf + Em * Vm

For:
- E1 = Longitudinal Modulus

  • Ef = E of fibres
  • Vf = Volume fraction of the fibers
  • Em = E of Matrix
  • Vm = Volume fraction of matrix (1-Vf)
76
Q

What is G12

A

In Plane Shear Modulus

77
Q

“Sigma 1 t” notation meaning

A

Tensile strength in fiber direction (1)

78
Q

Types of glass fibers and their mechanical properties

A
  1. E-Glass: moderate strength and stiffness, low cost, non-structural and semi-structural applications
  2. S-Glass: higher strength and higher stiffness than
    E-Glass, cost comparable to that of Carbon/epoxy,
    employed for some structural components
  3. D-Glass: better dielectric properties than E-glass,
    protection against lightning strikes
79
Q

Compare the general properties of glass fibres with those of other types of PMCs

A

Cheaper for non or less load bearing structures

Density comparable to that of Boron/epoxy (B),
higher than aramid/epoxy (A) and carbon/epoxy (C)

Lower specific stiffness than all three (A,B and C)

Not employed for weight critical load bearing
structures

Better dielectric properties than other PMCs

80
Q

Describe the fatigue properties of Glass fiber composites

A

Poor comparative fatigue performance

Particularly vulnerable in hot and wet conditions due to low conductivity and moisture ingress tendencies

81
Q

Outline the properties of GFRP relating to impact strength and environmental degradation

A

Superior ability to absorb impact energy

Degrades significantly in extreme environments

82
Q

Pros and Cons of GFRP

A

Advantages of GFRP

Low cost

Superior Impact Resistance

Better dielectric Properties

Disadvantages of GFRP

Lower specific stiffness than all other PMCs

Poorer fatigue
performance

Susceptible to Stress rupture

Degradation of Properties due to moisture and
other aggressive environments

83
Q

Advantages and Disadvantages of Aramid composites

A

Advantages:

  • Low density
  • Mid-range specific strength and stiffness
  • Low creep rate
  • Good tensile properties
  • High impact resistance

Disadvantages:

  • Poor compressive strength
  • Poor fiber adhesion
  • Difficult to machine
  • High absorption
84
Q

4 Different classifications of CFRP fibers

A
  1. High Strength
  2. High Modulus
  3. Intermediate Modulus
  4. Ultra High Modulus
85
Q

2 different TYPES of CFRP fibres. Pros/Cons to each

A
  1. PAN: Low Cost, higher strain to failure. Less stiff
  2. Pitch: More conductive, More stiff. More expensive
86
Q

4 different CFRP Matrix types in order of low-to-high temp ranges

A
  1. Thermoplastics (*?)
  2. Epoxy Resins
  3. Bismaleimides
  4. Polyimides
87
Q

What properties in a CFRP matrix are dependent on the matrix?

A
  1. Strain-to-Failure
  2. Compressive Strength
  3. Interlaminar Shear
88
Q

Explain what is meant by ‘adequate’ interfacial bonding.

A

Carbon fibres are normally surface treated to develop
adequate interfacial bonding. Too strong bonding is not
desirable as the fibre should be capable of disbonding to
alleviate local stress concentrations such as at
microcracks

89
Q

Rank composite impact resistance (most to least)

A
  1. GFRP
  2. Aramid
  3. CFRP (least)
90
Q

What is BVID and what does it affect?

A
  • Barely Visible Impact Damage

-Can significantly affect the residual compressive
strength and buckling resistance by causing significant
reductions in the bending stiffness of the laminate

91
Q

Outline the desirable trend for fibre to matrix stiffness ratio, and why:

A

Maintain high fibre to matrix stiffness ratio
so that the matrix strains are kept low to avoid matrix
cracking

92
Q

Discuss the fatigue resistance properties of CFRP:

A
  • Generally, fatigue resistant
  • At moderate strain levels fatigue resistance of CFRP is strain dependent with fibre bridged matrix cracking and disbonding
  • Fatigue of CFRP at low strain levels causes only a small amount of matrix cracking which can lead to environmental damage.
93
Q

Fatigue Resistance of GFRP

A

Glass fibres exhibit degradation of strength under fatigue

Low modulus of glass fibres causes high matrix strains leading to matrix cracking, which
exacerbates fatigue sensitivity by causing strain concentration and exposing fibres to environment

94
Q

Fatigue Resistance of Aramids

A

Between GFRP and CFRP

95
Q

Why is combined high matrix toughness and high adhesion bad?

A

Allows damaged fibres to be isolated from the good fibres by
matrix cracking, disbonding and formation of delaminations.

96
Q
A
97
Q

Three phases of tension fatigue in cross-ply laminates

A
  1. Cracking in the off
    axis plys accumulate leading to saturation of matrix cracks
  2. Crack coupling and delaminations develop
    which slowly increase the stresses carried by the on-axis ply
  3. Zero degree fibres begin to fail under the
    tensile load leading to final fracture
98
Q

Effects of stress concentrations on composites

A

Minimal due to formation of microcracking
and delamination

99
Q

Effects of loading frequency on GFRP and CFRP

A

GFRP: Frequencies over 5 Hz can reduce fatigue
performance due to accumulation of heat damage due to
the poor thermal conductivity of glass fibres

CFRP: Minimal

100
Q

Outline the factors involved in compressive strength of composites

A

Depends on the ability of the matrix to support the fibres to avoid micro buckling which depends on interfacial bond strength and the presence of matrix cracks

101
Q

What negatively effects the compressive strength of a composite

A

Hot/wet conditions, due to lowering of the Tg and
softening of the matrix

102
Q

What type of composite is greatly affected by BVID

A

CFRP

  • The BVID delaminations grow marginally under fatigue loading, which leads to a small drop in fatigue strength or
    fatigue life.
  • Greater effect on static residual strength
103
Q

What is the mechanism for water absorption into a matrix

A

hydrogen bonding with polar sites in
the polymer molecule

104
Q

What types of matrices are most/least polar

A
  1. Epoxies most
  2. Phenolics least
105
Q

What law governs water ingress

A

Fick’s Law

106
Q

Effects of temperature in moisture absorption

A
  1. High Temp: vaporizes leading to matrix cracking
  2. Low temp: freezing = expansion
  3. Room temp: plasticiser of the resin system and
    degrades the mechanical properties at room temperature
107
Q

Water affect on Tg - and result

A

Reduces.

Lower high temp performance

108
Q

Water ingress differing effects on matrix and fiber dominated composites

A
  1. Fiber: Compression and buckling
  2. Matrix: Tensile strength and stiffnes
109
Q

4 stages of quality assurance

A
  1. Control of raw materials
  2. Monitoring of manufacturing
  3. QA of finished product
  4. In service damage monitoring
110
Q

Mechanical properties tested

A

longitudinal
tensile strength, elastic modulus, elongation, yield, density,
twist, creep and sizing content

111
Q

Types of QA tests

A
  1. Mechanical
  2. Chemical
  3. Xray
  4. Pressure
  5. Optical
  6. Ultrasonic
112
Q

Method of cure monitoring

A

Measurement of temperature variations and electrical conductivity

KNOW GRAPH (slide 13)

113
Q

Types of composite damages

A
  1. Fiber breaks
  2. Matrix cracks
  3. Delaminations
114
Q

Methods of ultrasonic testing (common in aviation)

A
  1. Pulse-Echo
  2. Through transmission
115
Q

Techniques of US testing

A

A and C scans

116
Q
A

Composites (2 decks)

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