Composites part1 - b

Question 1

What are the advantages of FRP composites?

Answer 1

1. Good strength to weight ratio
2. Complex shapes are easily accomplished e.g. double curvature
3. Designer properties - can be optimised to meet stiffness, strength and manufacturing requirements
4. Part consolidation to provide pre-fabricated/ pre-assembled product
5. Texture and self-coloring
6. Resistant to corrosion
7. Resistant to fatigue damage with good damping characteristics.

Question 2

What are the disadvantages of FRP composites?

Answer 2

1. Properties depend on processes (poor mechanical properties transverse to the fibers)
2. Limited design skills and standards/guides
3. Poor heat and fire resistance
4. Expensive (high production cost tools required)
5. Difficult to join (design process is more complex than metals
6. Low temperature applications

Question 3

Why are fibers the most common method of reinforcing FRPs?

Answer 3

Fibers, especially long and continuous forms, provide the stiffest and strongest materials and it is for this reason that they are also the most common method of reinforcing FRPs.

Question 4

Why are the properties of FRP highly dependent on the alignment of the fiber?

Answer 4

Because the introduction of fibers into the matrix induces directionality or anisotropy into the material

Question 5

What are the variables that have a major influence on the properties of FRPs?

Answer 5

1. Fiber type, cost and ease of processing into other fabrics
2. Mechanical properties
3. Alignment and distribution of the fiber
4. fiber/matrix interface
5. Size and shape of fiber
6. loading direction

Question 6

What are the 4 types of fiber materials?

Answer 6

Most common - glass. Other regularly used fiber materials: aramid, carbon and to a smaller extent, boron.

Question 7

Do the fiber materials have a yield point/ what type of behaviour do they exhibit/ what is the strain to failure like?

Answer 7

All exhibit elastic behaviour up to the point of failure. There is no yield point and very low strain to failure.

Question 8

Can you draw the tensile stress/strain graph for different fiber materials?

Answer 8

Yes or no.

Question 9

What is glass fiber made up from?

Answer 9

A mixture of oxides making up silica with other materials such as aramid, carbon and a small amount of boron.

Question 10

What process produces carbon fiber?

Answer 10

controlled pyrolysis.

Question 11

How can a range of carbon fibers with different mechanical properties, strength and stiffnesses be achieved?

Answer 11

By the processes of oxidation, carbonization and graphitization.

Question 12

What configuration should the composite have for max tenacity (break strength) and initial modulus?

Answer 12

Extended chain configuration and almost perfect crystalline configuration

Question 13

Why are kevlar fibers highly crystalline?

Answer 13

Due to rigid back bone that arises from the para orientation on the benzene ring?

Question 14

In general, what happens to the strength of a material which is drawn into fiber form?

Answer 14

The material becomes much stronger in the drawn direction than it was in its original form

Question 15

What happens to glass when is is melt drawn?

Answer 15

The resulting glass fiber is isotropic

Question 16

How are aramid fibers formed?

Answer 16

By wet spinning and drawing

Question 17

How are carbon fibers formed?

Answer 17

By oxidation and carbonizaton

Question 18

What are the resulting fibers for aramid and carbon once formed?

Answer 18

They are anisotropic - different properties in the longitudinal (parallel to fiber) and transverse (perpendicular to fiber) directions

Question 19

How are glass fibers manufactured?

Answer 19

By drawing molten glass into very fine threads and then immediately protecting them from contact with the atmosphere or with hard surfaces in order to preserve the defect free structure that is created by the drawing process.

Question 20

What temp does silica melt at?

Answer 20

1720 degrees celcius

Question 21

What is silica the basic element in?

Answer 21

Quartz, a naturally occurring rock. It is crystalline (rigid and highly ordered atomic structure).

Question 22

How is silica made into quartz?

Answer 22

SiO2 is heated above 1200 deg. and then slowly cooled, it will crystalize and become quartz.

Question 23

How is silica made into glass instead of quartz?

Answer 23

By altering the temperature and cool down rates. If pure SiO2 is heated to 1720 then cooled quicklym crystallization can be prevented and the process yields amorphous or randomly ordered atomic structure such as glass.

Question 24

What are the advantages of E-glass?

Answer 24

Good electrical insulator and high strength

Question 25

What are the advantages of S-glass?

Answer 25

High silica content with high temp performance, high strength and stiffness.

Question 26

What is the most widely used type of glass fiber?

Answer 26

E-glass, accounts for 95% of total glass fiber production.

Question 27

What are the main advantages of glass fiber?

Answer 27

• low cost

- isotropic properties

Question 28

What percent of composite materials are based on glass fiber?

Answer 28

over 90%

Question 29

What are the different steps in making glass fibers?

Answer 29

Step 1: batching (raw materials)
Step 2: melting
Step 3: fiberization
Step 4: Coating
Step 5: Drying/packaging

Question 30

What are the bonds like in aramid/kevlar fibers?

Answer 30

Strong covalent bonding in the fiber direction; relatively weaker hydrogen bonding in the fiber transverse direction.

Question 31

What does the anisotropic nature of kevlar/aramid mean?

Answer 31

The fiber has poor compressive properties. Under compressive loads the fiber develops kink bands which eventually lead to ductile failure.

Question 32

Name and describe 4 types of kevlar?

Answer 32

1. Kevlar AP - a next-generation fiber that offers advanced performance, value, and increased design flexibility in many applications.
2. Kevlar 29 - the original family of product types of kevlar. These yarns are used in ballistic applications, ropes and cables, protective apparel such as cut resistant gloves etc.
3. Kevlar 49 - high modulus type used in fiber optic cable, textile processing, plastic reinforcement, ropes, cables, and composites for marine sporting goods.
4. Kevlar 100 - producer-colored kevlar yarns used in ropes and cables, tapes, strappings, gloves and other protective apparel, and sporting goods.

Question 33

How is carbon fiber produced?

Answer 33

By the controlled pyrolysis of a precursor.

Question 34

What are the most commonly used precursors for carbon fiber production?

Answer 34

pitch (petroleum based) and polyacrylonitrile (PAN).

Question 35

What are the successive stages of PAN pyrolysis?

Answer 35

1. Oxidation - oxidizing atmosphere at 200-250 deg.
   2 - Carbonization - non-oxidizing atmosphere at 1000 deg for high strength fiber.
2. Graphitization - non-oxidizing atmosphere of 2500 to 3000 deg for high modulus fiber.
3. surface treatment
4. sizing

Question 36

How can carbon fibers with different mechanical properties i.e strength and stiffness be produced?

Answer 36

By adjusting the process temperature.

Question 37

How are carbon fibers grouped?

Answer 37

According to modulus and strength

Question 38

What are the 5 different groups for carbon fiber and their corresponding modulus/strengths?

Answer 38

1. Ultra-high modulus, UHM. (Modulus >450GPa)
2. High modulus, HM. (Modulus between 350-450GPa).
3. Intermediate-modulus, IM. (Modulus between 200-350 GPa).
4. Low modulus and high-tensile, HT. (Modulus 3GPa).
5. Super high-tensile, SHT. (tensile strength >4.5GPa).

Question 39

What are the main characteristics for fiber selection?

Answer 39

1. Drapability (conformity)
2. Compressibility (change of thickness)
3. Stackability (orientation of plies)
4. Wetability by the resin
5. Width and thickness of mat
6. mass of mat