What are the advantages of FRP composites?
- Good strength to weight ratio
- Complex shapes are easily accomplished e.g. double curvature
- Designer properties - can be optimised to meet stiffness, strength and manufacturing requirements
- Part consolidation to provide pre-fabricated/ pre-assembled product
- Texture and self-coloring
- Resistant to corrosion
- Resistant to fatigue damage with good damping characteristics.
What are the disadvantages of FRP composites?
- Properties depend on processes (poor mechanical properties transverse to the fibers)
- Limited design skills and standards/guides
- Poor heat and fire resistance
- Expensive (high production cost tools required)
- Difficult to join (design process is more complex than metals
- Low temperature applications
Why are fibers the most common method of reinforcing FRPs?
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.
Why are the properties of FRP highly dependent on the alignment of the fiber?
Because the introduction of fibers into the matrix induces directionality or anisotropy into the material
What are the variables that have a major influence on the properties of FRPs?
- Fiber type, cost and ease of processing into other fabrics
- Mechanical properties
- Alignment and distribution of the fiber
- fiber/matrix interface
- Size and shape of fiber
- loading direction
What are the 4 types of fiber materials?
Most common - glass. Other regularly used fiber materials: aramid, carbon and to a smaller extent, boron.
Do the fiber materials have a yield point/ what type of behaviour do they exhibit/ what is the strain to failure like?
All exhibit elastic behaviour up to the point of failure. There is no yield point and very low strain to failure.
Can you draw the tensile stress/strain graph for different fiber materials?
Yes or no.
What is glass fiber made up from?
A mixture of oxides making up silica with other materials such as aramid, carbon and a small amount of boron.
What process produces carbon fiber?
controlled pyrolysis.
How can a range of carbon fibers with different mechanical properties, strength and stiffnesses be achieved?
By the processes of oxidation, carbonization and graphitization.
What configuration should the composite have for max tenacity (break strength) and initial modulus?
Extended chain configuration and almost perfect crystalline configuration
Why are kevlar fibers highly crystalline?
Due to rigid back bone that arises from the para orientation on the benzene ring?
In general, what happens to the strength of a material which is drawn into fiber form?
The material becomes much stronger in the drawn direction than it was in its original form
What happens to glass when is is melt drawn?
The resulting glass fiber is isotropic
How are aramid fibers formed?
By wet spinning and drawing
How are carbon fibers formed?
By oxidation and carbonizaton
What are the resulting fibers for aramid and carbon once formed?
They are anisotropic - different properties in the longitudinal (parallel to fiber) and transverse (perpendicular to fiber) directions
How are glass fibers manufactured?
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.
What temp does silica melt at?
1720 degrees celcius
What is silica the basic element in?
Quartz, a naturally occurring rock. It is crystalline (rigid and highly ordered atomic structure).
How is silica made into quartz?
SiO2 is heated above 1200 deg. and then slowly cooled, it will crystalize and become quartz.
How is silica made into glass instead of quartz?
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.
What are the advantages of E-glass?
Good electrical insulator and high strength
What are the advantages of S-glass?
High silica content with high temp performance, high strength and stiffness.
What is the most widely used type of glass fiber?
E-glass, accounts for 95% of total glass fiber production.
What are the main advantages of glass fiber?
- low cost
- isotropic properties
What percent of composite materials are based on glass fiber?
over 90%
What are the different steps in making glass fibers?
Step 1: batching (raw materials) Step 2: melting Step 3: fiberization Step 4: Coating Step 5: Drying/packaging
What are the bonds like in aramid/kevlar fibers?
Strong covalent bonding in the fiber direction; relatively weaker hydrogen bonding in the fiber transverse direction.
What does the anisotropic nature of kevlar/aramid mean?
The fiber has poor compressive properties. Under compressive loads the fiber develops kink bands which eventually lead to ductile failure.
Name and describe 4 types of kevlar?
- Kevlar AP - a next-generation fiber that offers advanced performance, value, and increased design flexibility in many applications.
- 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.
- Kevlar 49 - high modulus type used in fiber optic cable, textile processing, plastic reinforcement, ropes, cables, and composites for marine sporting goods.
- Kevlar 100 - producer-colored kevlar yarns used in ropes and cables, tapes, strappings, gloves and other protective apparel, and sporting goods.
How is carbon fiber produced?
By the controlled pyrolysis of a precursor.
What are the most commonly used precursors for carbon fiber production?
pitch (petroleum based) and polyacrylonitrile (PAN).
What are the successive stages of PAN pyrolysis?
- Oxidation - oxidizing atmosphere at 200-250 deg.
2 - Carbonization - non-oxidizing atmosphere at 1000 deg for high strength fiber. - Graphitization - non-oxidizing atmosphere of 2500 to 3000 deg for high modulus fiber.
- surface treatment
- sizing
How can carbon fibers with different mechanical properties i.e strength and stiffness be produced?
By adjusting the process temperature.
How are carbon fibers grouped?
According to modulus and strength
What are the 5 different groups for carbon fiber and their corresponding modulus/strengths?
- Ultra-high modulus, UHM. (Modulus >450GPa)
- High modulus, HM. (Modulus between 350-450GPa).
- Intermediate-modulus, IM. (Modulus between 200-350 GPa).
- Low modulus and high-tensile, HT. (Modulus 3GPa).
- Super high-tensile, SHT. (tensile strength >4.5GPa).
What are the main characteristics for fiber selection?
- Drapability (conformity)
- Compressibility (change of thickness)
- Stackability (orientation of plies)
- Wetability by the resin
- Width and thickness of mat
- mass of mat
