6.3.1 Aircraft Materials

Question 1

Under what category of materials is composite placed?

Answer 1

Non-Metallic

Question 2

What is meant by the word composite?

Answer 2

A composite is a combination of two or more materials differing in form or composition on a macro scale.

Question 3

How is the strength of a plastic material increased?

Answer 3

By combining them with other stronger and stiffer materials to form a composite.

Example adding fibres to them.

Question 4

What three properties are important when using reinforced plastics?

Answer 4

Strength

Stiffness

Impact Resistance

Question 5

The threads which run the length of the fabric as it comes off the bolt are referred to as?

Answer 5

Warp

Question 6

The threads which run at 90° to the warp are called?

Answer 6

Fill or Weft

Question 7

Give two useful forms that glass fibre strands can be converted to?

Answer 7

Woven and Roven

Question 8

Under what type of load are carbon fibres strongest?

Answer 8

Carbon fibres are strongest in compression

Question 9

What colour are the following?

Carbon Fibres:

Kevlar Fibres:

Glass Fibres:

Answer 9

Carbon Fibres: Black

Kevlar Fibres = Yellow

Glass Fibre = Silver

Question 10

What is the aircraft structural grade of Kevlar know as?

Answer 10

The aircraft structural grade of Kevlar fibre is known as Kevlar 49

Question 11

Which part of a roll of fabric does not have good strength?

Answer 11

Selvage Edge

Question 12

What special attribute does Boron have that makes it useful as an aircraft material?

Answer 12

Excellent Compressive Strength, Stiffness and it is extremely hard.

Also prevents/doesn’t crack

Question 13

Describe Plain weave and list it’s properties.

Answer 13

Each warp and weft passes over one thread and underneath the next.

Good general purpose cloth

Question 14

Draw a roving based product and explain the advantage it has as a material?

Answer 14

Chopped Strand Mat

The rovings are chopped into short lengths and bonded together in random directions to form a mat

The mat gives equal properties in all directions and is easy to use

Question 15

Chopped strand mat has a weakness in one direction, explain?

Answer 15

The chopped strand mat gives equal properties in all directions and it easy to use.

Question 16

What type of polymer is most easily formed into a composite and how is it usually referred to?

Answer 16

The type of polymer which can be formed into a composite most easily is a liquid one, and this is usually referred to as a resin

Question 17

What is the main disadvantage of polyester?

Answer 17

The main disadvantage of polyester is its high rate of shrinkage

Question 18

What type of material is known as a thermoset?

Answer 18

Plastics which are not softened by heat are known as thermosets.

Question 19

What type of material is known as a thermoplastic?

Answer 19

Thermoplastics are polymers in which the molecules are held together by weak secondary bonding forces that soften when they are heated.

Question 20

Explain what is meant when it is said that a material is an elastomer?

Answer 20

If the molecules in a polymer are linked together by a small number of valency bonds so that a loose network is formed, then a rubber like metal is formed called an elastomer

Question 21

What is the purpose of a catalyst and how does it work?

Answer 21

A catalyst is a substance which produces the crosslinks which will finally set the resin.

(referred to as catalyst or hardener)

The curing process starts once the catalyst is added and it will set.

Question 22

Name the advantages that the hot curing system has over the cold curing system?

Answer 22

(Hot curing systems cure the resin at high temperatures 250 F +)

The catalyst only becomes active when it is heated, so it can be mixed with resin quite a long time before it is used.

The properties of the final composite depend on the curing temperature, and how quickly the resin is heated up. These can be controlled quite accurately by the curing equipment

Hot cured composites tend to behave better in service than cold cured composites.

Question 23

What is the pot life of a resin?

Answer 23

The amount of time that the resin remains usable after the catalyst is added is called the pot life.

Question 24

What is the shelf life of a resin?

Answer 24

The length of time that a resin can be stored before it is used is known as its shelf life.

(normally 2 years)

Question 25

What is a thixotropic agent used for?

Answer 25

Thixotropic is an agent used to thicken a resin system without adding weight

It adds volume but little weight and increases the viscosity of the resin

Question 26

Give some of the advantages of using an epoxy resin as opposed to a polyester resin?

(ratios and special note)

Answer 26

Epoxy is the most popular resin used for laminated structures.

Epoxy doesn’t use a catalyst to cure, as does a polyester resin

The hardener of curing agent combines with the epoxy to cure it whereas the polyester catalyst sets off a chemical reaction

Epoxies are cured with a ratio of about 2:1

Polyester uses a ratio of about 50:1

Note: As the hardener and curing agent are combined, it is vital that the correct ratio is used otherwise the mixture will not cure fully.

Question 27

Which resin does not need a catalyst to cure?

Answer 27

Epoxy Resin

Question 28

Which plastic can be softened and reshaped by heating?

Answer 28

Thermoplastic

(Thermoplastics are Polymers in which the molecules are held together by weak secondary bonding forces that soften when they are heated)

Question 29

What is the amount of time that resin remains usable after the catalyst is added?

Answer 29

(Pot life)

30 mins ????

Question 30

What is meant by the term GFRP?

CFRP

AFRP

Answer 30

Glass Fibre Reinforced Plastic

Carbon Fibre Reinforced Plastic

Aramid Fibre Reinforced Plastic (Kevlar)

Question 31

What does the following part number refer to 3K-70-PW?

Answer 31

3k = 3000 filaments per bundle

70 = thickness of each filament in microns

p/ PW = plain weave

Question 32

What is the purpose of an accelerator?

Answer 32

To speed up the cure (Hasten)

These are very reactive substances and must always be added to the resin before the catalyst.

Question 33

What are the words used to describe the (a) rate of increase in a cure cycle and the (b) time period the temperature is held at?

(a)

(b)

Answer 33

(a) Ramp up
(b) Soak Time

Question 34

Why should a catalyst and an accelerator not be mixed together?

Answer 34

It would result in a violent explosion.

Question 35

What potential hazard is associated with the use of peroxides?

Answer 35

They are oxidising agents and can cause other flammable substances to burst into flames if they come in contact.

Question 36

There are four main types of honeycomb used in aircraft manufacture, what are they?

Answer 36

Aluminium

Glass Fibre

Nomex

Kevlar

Question 37

Describe the honeycomb core material Flexibe core

Answer 37

The cells are often columnar and hexagonal in shape. A honeycomb shaped structure provides a material with minimal density and relative high out-of-plane compression properties and out-of plane shear properties.

Question 38

What is a pre-preg, and what are the advantages of them?

Answer 38

A pre-peg is a sheet of fibre reinforcement impregnated with the resin/hardener/catalyst mixture.

The resin/catalyst ration and the fibre/resin ratio are carefully controlled by the manufacturer

Each batch of pre-peg is identical. Also the resin is already spread evenly throughout the fibres. Of course pre-pegs can only be used for hot curing systems because they ust not cure during storage.

Pre-pegs must be stored under refrigeration at -18^C

Question 39

What is a coin tap test used to detect, and explain what indications are given by the test.

Answer 39

A coin tap test is used to detect internal flaws or areas of suspected of delamination.

A sharp solid sound indicates a good bond

A dull thud indicates bond separation

Question 40

Explain what radiographic (x-ray) inspection is used to detect?

Answer 40

Useful tool for the inspection of composite parts

Can examine the interior of honeycomb parts for conditions such as misalignment, missing parts and core damage.

Can also check for ply separation in a laminate.

Question 41

Why is a water break test necessary when carrying out a composite repair?

Answer 41

The water break test is a simple method of determining whether the surface to be bonded is clean.

Uniform wetting of the surface by distilled water indicates that it will probably be likewise wetter by adhesive.

Question 42

Why is vacuum bagging a recommended method for applying pressure to a repair?

Answer 42

(1 atmosphere/ 14.7 PSI)

A uniform pressure is applied throughout enabling large and complicated shapes to be accommodated

Question 43

Describe Twill weave and list it’s properties.

Answer 43

Varied Weave

Typical weave is 2 x 2:

This means each weft passes over 2 warps and under 2.

Good tensile strength

Question 44

Describe Satin weave and list it’s properties.

Answer 44

Each weft passes over all but one of the warps.

Smooth surface + very high tensile strength

Question 45

Describe Unidirectional weave and list it’s properties.

Answer 45

The warp threads are straight and parallel.

The weft threads are occasionaly added to them to keep them in place.

Very high strength in one direction but weak in the weft direction.

Question 46

Describe the honeycomb rectangular core

Answer 46

The cells are often columnar and hexagonal in shape. A honeycomb shaped structure provides a material with minimal density and relative high out-of-plane compression properties and out-of plane shear properties.

Question 47

Describe the Honeycomb Hexagonal core

Answer 47

The cells are often columnar and hexagonal in shape. A honeycomb shaped structure provides a material with minimal density and relative high out-of-plane compression properties and out-of plane shear properties.