6.3 Composite And Non Metalic Flashcards
What is the definition of composite
Two or more materials that are combined to form a much stronger structure than either material by itself
Most simple composites consist of what 2 simple elements
- A matrix which serves as a bonding substance (adhesives or resin)
- A reinforcing material (fabric)
What are the advantages of composite materials
- High strength to weight ratio
- Reduction of parts and fasteners
- Reduction of wear
- Corrosion resistance
What are the disadvantages of composites
- Generally expensive
- Not easy to repair, well trained staff, tools, equipment, facilities are required to repair composite components
What are the most common reinforcing fibres
Kevlar and graphite
What is nomex
- Very high thermal stability
- Temperature and flame resistant
- Radiation resistance is good
What is fibre glass
Made from stands of molten silica glass and spun together nod woven in to a cloth.
What are the most common forms of fibreglass used
E glass - Highly resistant to electricity
S glass - Very high tensile strength
What are the characteristics of glass fibre
- Low density
- High strength
- Height fracture toughness
- circular cross section and smooth surface
- Good wet out
- Does not absorb water or humidity
- Not ductile
Where does the name aramid come from
Aromatic polyamide
What are the characteristics of aramid
- Light weight and excellent tensile strength
- Heat resistance up to 260°c
- Non flammable and non fusible
- Good chemical resistance
- Very good resistance against abrasion
- High toughness
What is Kevlar
A man made fibre with properties such as high tensile strength and flexibility
Where would aramid be used
Rotor blades and bullet proof vests
Where would Kevlar be used
Bullet proof vests, radial tyres, cables, reinforced composites for aircraft panels
What are the disadvantages of Kevlar
- The fibres absorb moisture
- Compressive properties are poor
- Very difficult to cut (special scissors are required)
- UV light sensitive (turns brown when affected by sunlight)
What are the characteristics of carbon fibre
- High strength
- High E module
- Low density
- Low fracture resistance
- Thermal expansion in longitudinal direction of the fibre is close to 0
- High resistance against temperates
- Electrical conductivity
- Quickly wear out steel cutters
What must be used when bonding carbon fibre to aluminium
It has a problem of being corrosive when bonded to aluminium and so a layer of fibreglass is used to isolate the two materials
What is a matrix
The bonding material the completely surrounds he fibre to give strength no transfer the stress to the fibre
What are the characteristics of matrix materials
- Good stress distribution
- Heat resistance
- Chemical resistance
- Durable
Resins used as a matrix are two part systems consisting of what?
Resin and a hardener (catalyst) which acts as a curing agent
Resins are a type of plastic and are broken down in to what 2 categories
- Thermoplastics
- Thermoset
What are thermoplastics
Thermoplastic resins use heat to form the part in to a specified shape and this shape is not permanent.
If heat is added again it will flow in to another shape
Example - plexiglass/acrylic
Where can thermoplastics be used
Can only be used in areas where temperatures do not exceed 750°f
What are thermosets
Use heat to form and set the shape permanently
Even with addition on heat they cannot be reformed (most composites are made from these)
Example - Bakelite
What are epoxy resins
One type of thermosetting plastic resin
They are used to bond non porous and dissimilar materials like metal to composite
What is prepreg
Prepreg is the abbreviation of pre impregnated
They already have resin in them
How should prepreg materials be stored
In a freezer to prevent the resin from curing at room temperature
What is honeycomb
Type of material that has the shape of natural honeycomb
Has excellent strength to weight ratio
What are the 2 methods of honeycomb fabrication
Corrugation process - material which can be converted using this process include metals, plastics, plastic reinforced glass and paper
Expansion process
What is the corrugation process
Corrugation process - material which can be converted using this process include metals, plastics, plastic reinforced glass and paper
What is the expansion process
All bonds are made simultaneously ( the corrugation process is essentially one layer at a time operation)
What are other core materials used other than honeycomb
Styrofoam, urethane, wood and several other materials are also used as core materials
What are the different methods used by manufacturers to dissipate the electrical charge on composite components
- Flame spray
- Bonding jumpers
- Aluminium wires woven in to the top layer
- Thin aluminium foil bonded to the top layer
What is the MSDS
Material Safety Data Sheet
What type of details can be found on the MSDS
- Health precautions
- Flammability of the material
- Ventilation requirements
- Information for health professionals in the event of an accident
What is polymerisation
A chemical reaction, generally carried out in the presence of a catalyst, which combines small molecules (monomers), containing a double bond, into a long chain of molecules
What type of by products are produced during polymerisation
No byproducts are created
What type of monomer molecules are there
- Ethylene polymerising to polyethylene (PE)
- Styrene polymerising to Polystyrene (PS)
- Vinylchloride polymerising to polyvinyl-chloride (PVC)
Other than polymerisation what are the other types of reactions used to make polymers or giant molecules
- Polycondensation
- Polyaddition
What is polycondensation
A chemical reaction between two similar or dissimilar basic units which have at least two functional groups
What are the most important commercial polymers made by polycondensation
- Phenol-formaldehyde (Bakelite) resins (typical thermosets)
- Polyamides (nylon)
- Polyester
What is polyaddition
A reaction of two different types of molecules when reactive groups are brought together. No by products are produced
What two types of important plastics made by polyaddition
- Polyurethanes
- Epoxies
What is an adhesive
A substance used to bond two or more surfaces together
What are the advantages of adhesives
- Economical
- Distribute stress at the bonding point
- Eliminate need for fasteners
- Moisture and corrosion resistant
What factors does the effectiveness of adhesives depend on
- Resistance to slippage or shrinkage
- Malleability
- Cohesive strength
- Surface tension
What are thermosetting adhesives
Transformed in to tough, heat resistant solids by the addition of a catalyst or application of heat
What are thermoplastic resins
Can be softened by heating, used for bonding wood, glass, rubber, metal and paper products
Hat is aerosil
Very light white powder which is a product of combustion
Used as a thixotropic agent
Can find in a combination with resins, lacquers and even toothpaste and sauces
What are elastomeric adhesives
Synthetic or natural rubber cements
What are contact adhesives
Atmospheric pressure will press two solids together if there is no air between the two plane parallel surfaces
What are solvent adhesives
Contain plastics or resins dissolved in a solvent. The solvent has to evaporate to cure the adhesive
What are dispersion adhesives
Contain resins dissolved in water solution, such as wood glue or wood paste
What are resin adhesives
Cure by a chemical reaction. They contain two or more components
What are film adhesives
Similar to prepregs. Used for bonding metal to metal
What are foaming adhesive films
In adhesive in sheet form which expands during the curing cycle to fill gaps and adhere strongly to all parts of the structure which it will come in contact
What are thixotropic agents
A type of thickening agent
What are the two most commonly used thixotropic agents
- Micro balloons
- Aerosil
How should resins be mixed
- In a wax free container
- Process should take between 3-5 mins
- Always measured by weight
What are the 4 most commonly used resins in fibre reinforced composites
- Polyester resin
- Epoxy resin
- Vinyl ester resins
- Phenolic resins
What are the disadvantages of epoxy resins
- Toxicity
- Low pot and shelf life
- Moderate to high cost
What are the 3 levels of category for assessing damage
- Allowable
- Repairable
- Not repairable
For assessing damage on composite components what are the two main categories used
- Skin not perforated
- Skin perforated
What is BVID
Barely Visible Impact Damage
What is chemical degradation
Normally affects the resin in a structure and is generally due to accidental contact with aggressive chemical liquids or products
What does NDI stand for
Non Destructive Inspection
What equipment or methods are used for NDI
- Tap testing
- Visual or optical inspection
- Bond tester
- Thermography
- Holography
- Acoustic emission
- Ultrasonics
- Radiography
What is tap testing used for
Used for quick evaluation of any accessible aircraft surfaces to detect the presence of debonding or entrapped water
What is holography used for
The rapid assessment of flaws In surfaces of composite structures
What is the ultrasonic method of NDI
Quick and inexpensive evaluation of flaws in composite parts
What is the radiography method of NDI
Widely used for quick and low cost inspection of composite structures
Small parts to relatively large parts
What are the 3 repair types
- Temporary repairs
- Permanent cosmetic repairs
- Permanent structural repairs
What is scarfing
To remove damaged layers from a composite material it is necessary to remove one layer at a time
When cleaning and degreasing what must be remembered
Spray on rag first then apply, do not supply straight to structure
What is wet lay up
Consists of applying several layers of fabric material impregnated with resin on to the repair surface in order to restore the damaged skin
What things must be remembered when working with resins and catalysts
- Mix resin properly, inadequate mixture will result in poor strength
- Must be weight before mixing
- 3-5 mins for thorough mixing
- Do not mix too quickly as air bubbles will form and will be detrimental to strength
What are the two methods that can be used for the lay up application
- Prepreg
- Wet lay up
Why is vacuum bagging used
In advanced composite repairs
- Remove trapped air and gasses during the curing process
- To hold thermocouples and heater blankets in place
- Provide desired surface finish
What is peel ply
Also known as release fabric
A thin woven nylon or polyester fabric which has a non stick coating. It can be used instead of or underneath a perforated parting film
It’s main effect is on the surface finish of the repair
What are the 2 potential disadvantages of using a peel ply
- It absorbs some resin and may lead to a resin starved repair
- It may contaminate the repair surface with its non stick coating
What is a bleeder cloth
Also known as a surface bleeder or bleeder fabric
Used to provide an easy path for air to escape and to absorb any resin which comes through the holes in the perforated parting film
What is a caul plate
Also Known as a pressure plate or pressure intensifier
A thin metal plate which is placed between the breather fabric and the heater blanket
What is the purpose of the caul plate
- To help even out temperature over the entire repair
- To help even out the pressure over the whole repair
- To improve the flatness of the surface of the repair
What should caul plates not be used for
Complex shapes, they are designed for flat single curvature panels
What is a heater blanket
Also known as a heater mat
A silicone rubber pad which contains an electrical heating element
What is a heater mat used for
To control the temperature of the repair during the warm up, cure and cool down stages of a repair
How are thermocouples identified
K type, j type etc
What are the most commonly used thermocouples for hot bonding
J type
