Composites And Non-metallic Structures Flashcards
What is a composite?
When two or more materials combine to form a stronger structure
What are some advantages of composites?
High strength to weight ratio
Reduction of parts and fasteners
Reduction of wear
Corrosion resistance
What are some disadvantages of composites?
Relatively expensive
Not easy to repair
Trained staff and equipment required
What does reinforcing materials do?
It gives major strength to the reinforcing component when combined with a matrix
Name a type of composite
Nomex
Name some properties of nomex?
It is manufactured only as a fibre
It has high thermal stability
It is flame resistant
How is fibre glass made?
It is manufactured from molten silica glass and spun together then woven into a cloth
Name one advantage and disadvantage of fibre glass
Advantage - low cost
Disadvantage - weighs more than other fibres
The simplest composite is made of two elements. What are they?
The matrix - adhesives or resins
The reinforcing material - fabric
What are the two types of fibreglass most commonly used in the aerospace sector?
E Glass (highly resistant to electricity) S Glass (very high tensile strength)
What are some characteristics of Aramid fibres?
Yellow in colour High strength to weight ratio High Tensile strength Heat resistant up to 260C Low density Abrasion resistant Chemical resistant Non flammable and non fusible High toughness
What is Kevlar?
A man made organic fibre with high tensile strength and flexibility
What is Kevlar used for?
Bullet proof vests, cables and reinforced composites for aircraft panels
How does Kevlar work?
NOT COMPLETED
What are some disadvantages of Kevlar
Difficult to cut
Absorbs up to 8% of moisture
What are some characteristics of Carbon Fibre?
High E modulus High strength Low density Low fracture resistance Thermal extension in longitudinal direction in the fibre close to 0 High resistance against temp Electrically conductive
What does CFRP stand for?
Carbon Fibre Reinforced Plastic
What are the different Fibre Orientations?
Warp
Weft/fill
Selvage edge
Bias
Uni directional Fibre
Bi directional Fibre
What is Warp?
The threads that run along the edge of the fabric
What is the weft/fill?
The fibres that run perpendicular to the warp fibres
What is the Selvage Edge?
The tightly woven edge that runs perpendicularly to the warp edges
Wha is the Selvage Edge used for?
Used to prevent unravelling edges
What is the Bias?
The thread that runs 45 degrees to the warp threads
What does the Bias do?
It allows manipulation of the material to create complex contours
What is Uni Directional?
When the threads are straight and go in one direction only
What are Bi directional fibres?
When the threads go in 2 or more different directions
What do Bi Directional Fibres provide more of?
Strength
What is the matrix?
Usually the wet form of the composite
What does a newer matrix material have?
Better stress distributions
What are some matrix characteristics?
Heat resistant
Chemical resistant
Durable
What can you use for a matrix?
Resins or adhesives
2 part systems include?
A matrix and a hardener
What do you need for a 2 part system between a resin and a hardener?
A catalyst acting agent
Resins are split into two categories, what are they?
Thermoplastics
Thermosets
What is the difference between a thermoplastic and a thermoset?
A thermoplastic uses heat to form a part into a specified shape and it is NOT permanent
A thermoset uses heat to form itself and then it is set permanently. It cannot be reformed
What temperature can a thermoplastic not exceed?
750F
Name an example of a thermoset resin
Epoxy resin
What are the characteristics of an epoxy resin?
Good adhesion
Good strength
Moisture and chemical resistant
Good for bonding NON-POROUS and DISSIMILAR metals
What is epoxy resin widely used in?
Resins for prepreg materials and structural adhesives
What do you refer to for composite safety?
MSDS - Material Safety Data Sheet
What does prepreg stand for and what does it mean?
Pre- impregnated
They are fabrics that have already been impregnated with resin
What type of construction is used when a core material is bonded between two thin face sheets ?
Sandwich construction
What is the honeycomb structure and what does it provide?
It takes the shape of a natural honeycomb and it provides strength
What are the 2 types of honeycomb Fabrication method?
Corrugation process
Expansion process
What materials can be converted in the corrugation process?
Metals
Plastic
Plastic reinforced glass
Paper
What is the Expansion process?
Where the bonds are made simultaneously
What are the different ways used to dissipate electrical charge on composite components?
Flame spray
Bonding jumpers
Aluminium wires
What safety precautions must always be taken?
Gloves at all times
Hands washed before and after
Wear masks and a gown
What is polymerisation?
A process carried out with the presence of a catalyst and it creates a chemical reaction where polymers are then created from monomers
What is an advantage for polymerisation?
There are no waste by-products
What are examples of polymers?
Polyethene, polystyrene
What is polycondensation?
A chemical reaction between two similar or dissimilar units which have at least two functional groups
Give examples of polymers formed by polycondensation
Bakelite
Polyester
What is polyaddition?
The reaction of 2 different types molecules when reactive groups are brought together
Name examples of polymers formed by polyaddition
Polyurethane
Epoxies
Give the 2 examples of bonding agents
Adhesives and resins
What is an adhesive?
A substance used to bond two or more surfaces together
What are some advantages of using adhesives?
Economical
Distribute stress at the bonding point
No need for fasteners
Depending on its factors, it is moisture and chemical resistant
What does the effectiveness of the adhesive depend on?
Resistance to shrinkage and slippage
Malleability
Cohesive strength
Surface tension - determines how far the adhesive penetrates the tiny depressions in the binding surfaces
What are the different types of Adhesives?
Synthetic Thermosetting Thermoplastic Elastrometric Contact Solvent Dispersion Resin Film Foaming Adhesive Film Plastic Foams
What are synthetic adhesives used for?
Used either alone or as modifiers of natural adhesives for better performance and better range of applications than the natural products.
What are thermosetting adhesives used for?
Used for structural functions as bonding metallic parts of aircraft and space vehicles
How is a thermoset transformed into a tough, heat resistant solid?
Through the addition of a catalyst or the application of heat
What use do thermoplastic resins have?
Used to bond wood, glass, rubber, metal and paper products
What are examples of elastomeric adhesives?
Synthetic or natural rubber cements
What do elastomeric adhesives do?
Join dissimilar metals without producing galvanic corrosion
Distributes stress more evenly throughout the body
What are contact adhesives?
The use of atmospheric pressure to press two solids together if there is no air between the two plane parallel surfaces
What do solvent adhesives contain?
Plastic or resins dissolved in a solvent
What must the solvent do to cure the adhesive?
It must evaporate
What do dispersion adhesives contain?
They contain resins dissolved in water solution
How are resin adhesives cured?
By chemical reaction
What do resin adhesives contain?
2 or more components
What are film adhesives?
Film adhesives are similar to prepegs.
They are used to bond metal to metal and sandwich the core to the skin
GFRP, AFRP, CFRP
What does GFRP stand for?
Glass fibre reinforced polymer
What does AFRP stand for?
Aramid Fibre Reinforced Plastic
What does CFRP stand for?
Carbon Fibre Reinforced Plastic
What do supported films contain?
A woven nylon carrier
At what temperature does a film adhesive have to be stored?
-18 degrees C
What are Foaming Adhesive Films?
An adhesive in sheet form which expands during the curing cycle to fill gaps and adhere strongly to all parts of the structure it comes into contact with
What are Foaming Adhesive Films mainly used for?
The repair of the honeycomb sandwich panels
What are plastic foams?
Plastic foams are either produced with closed or open pores and as either preformed rigid sheets or as fluids for injections into cavities
What are thixotropic agents?
Thickening agents
Why do we use thixotropic agents?
Some plastic resins are extremely sensitive to temperature and may run
How many thickening agents are used in aircraft maintenance and what are they?
2
Micro-balloons
Aerosil
What are microballoons?
Micro-balloons are made up of hollow phenolic balls with a range of diameters.
A paste of light consistency is made up and trowelled onto the surface where it cures into a hard, light-weight filler
What is aerosil?
Aerosil is a very light, white powder, which is a product of combustion
What are the two types of resin adhesives?
Thermoplastic
Thermosets
What do resins do?
Resins ensure the cohesion of the composite material
What are most of the mechanical performances of a composite given by?
The fibres and their orientation
Where should resins always be mixed?
In a wax free container
How do you get a wax free container?
By adding accelerator to the resin base
How long should the mixing process take?
3-5 minutes
What will the addition of too much accelerator do to the mixture?
Decrease the strength of the final mix
Exothermic reactions will take place and the resin may spontaneously combust
What are two main types of resin used for composite structures?
Epoxy
Phenolic
Why are epoxy resins used for external components?
Good mechanical properties
Why are Phenolic resins used for cabin furnishing?
They have good fire resistance and low toxicity
What happens to thermoplastic resins when it is heated?
It becomes a plastic
What are thermoplastics rarely used in?
Composite structures
What are the main features of a good polymer resin?
Must have the correct mechanical properties
It must coat every single and bond well to them
Must be fairly easy to use
What are the four most commonly used resins in fibre reinforced composites?
Polyester resin
Epoxy resin
Vinyl ester resin
Phenolic resin
When is polyester resin typically used?
Used with glass fibres to form a strong durable material
What do epoxy resins do?
Provide excellent adhesive strength and can be used for gluing metals together
What are characteristics of the vinyl ester resins?
Tough resins with high resistance to corrosion
What are characteristics of phenolic resins?
Good resistance to smoke generation in fires
What can the wrong amount of hardener or resin do?
Unreacted resin in the final cured resin
Unreacted harder in the final cured resin
How are polyesters cured?
By chain reactions, linking resin molecules to hardener molecules
What are some disadvantages of using epoxy hardeners?
Its toxicity can cause dermatitis
It has a moderate to high cost
They have a low pot and shelf life
What are epoxy resin characteristics?
Adhesion - sticks to most metals, glass and ceramics
Cohesion - material usually fades before resin
100% solution - no by products, cures without releasing water or other condensation products
Low shrinkage - cure with only a fraction of shrinkage of vinyl type adhesives
Resistance to moisture and solvents - effective barrier to heat and electric current
What is one of the most important factors of bonding?
Surface wetting
What are some positive effects of surface wetting?
No surface contamination
Viscosity reduction
Time
Pressure
What are some negative effects of surface wetting?
Fingerprints
Incorrect alignment of glue
Low temperature
Incorrect surface pre-treatment
What are the two important aspects of a durable adhesive joint?
Mechanical hooking
Surface wetting
What is mechanical hooking?
Takes into account that the adhesive must penetrate into cavities before curing, otherwise trapped air will decrease the strength of the joint
What are the pre-treatment procedures for bonding?
Degrease only
Degrease, abrade and wipe
Degrease and chemically pre-treat
How can contamination occur during pre treatment?
Fingerprints Dirty cloths Poor degreasing techniques Poor chemical solution Other tasks happening in the vicinity
How many levels of composite damage are there and what are they?
3 levels
Allowable
Repairable
Non-repairable
What does SRM stand for and what is it used for?
Structural Repair Manual
Used to determine whether a part is repairable or not and the applicable type of repair that needs to be done
In the SRM, damage is divided into 2 categories, what are they?
Skin not-perforated damage
Skin perforated damage
What sort of damage would go under skin not perforated damage?
Abrasions Scratches Dents Erosion Debonding/delamination Gouges
What types of damages would go into the skin perforated damages?
Lightening strikes
Holes
Impact from another object
What is delamination?
When reinforced materials separate from each other in the matrix
What is debonding?
When two materials stop adhering to one another
What is BVID?
Barely Visible Impact Damage
When the outer skin looks undamaged but there is a loss of skin bonding to core bonding which reduces the overall strength
When may galvanic corrosion occur?
When an aluminium alloy is in direct contact with a Carbon Fibre material in the presence of a corrosive environment
What material used on an aircraft is conductive?
Carbon fibre
If erosion erosion is undetected or unrepaired, what can it generate?
Composite deterioration
Where may erosion occur most on an aircraft?
The leading edges
Why must repairs be made when water absorption is present?
To avoid any material delamination during heat application
What must be repaired if chemical degradation is present?
The entire contaminated area
What is required if a dent or depression is found?
Further NDT to detect delamination or debonding.
If it is a sandwich structure, the honeycomb structure is usually damaged and requires repairs
What does NDT/NDI stand for?
Non-Destructive Testing
Non-Destructive Inspections
What are some examples of NDI tests?
Tap testing Visual/optical Holography Ultrasonic Radiography
Thermography
Acoustic emission
Bond tester/ resonator
What does tap testing do?
Detects debonding or trapped water quickly
How does tap testing work?
Tapping method using a coin/light hammer that creates a woodpecker effect. Places with debonding or trapped watter will give a flat/dead sound
What is visual/optical testing?
A visual / optical inspection to quickly locate SURFACE flaws/damages
What does visual/optical testing not locate?
It does not locate internal damages
What can you use to look for glass fibre component damages?
A strong light
How do you know delamination has occurred when doing a visual test with a strong light?
Delamination is indicated by a change in colour
What do holography testings do?
Locate small surface defects
What do ultrasounds do?
Locate internal defects
What are ultrasounds most commonly used for?
Laminates
What do ultrasounds rely on?
Surface cleanliness
What does radiography testing do?
Locates internal defects and surface defects
What are the 2 processes of radiography ?
X-ray
Gamma
What is a disadvantage and advantage of using radiography?
Disadvantage - highly radioactive
Advantage - produces a photographic record
How many types of repairs are there and what are they?
3 types of repair
Temporary
Permanent cosmetics
Permanent structural
Where are repair instructions found?
SRM
Before any repairs can be done, what must be done first?
Ensure the surface of the repair area has been prepared properly
When removing surface protection, what methods must be used?
Mechanical methods
Why cant chemical methods be used to remove the surface protection?
Can cause deterioration of the resin
What is a wet lay-up?
The application of several layers of fabric material impreganted with resin onto the repair surface in order to restore the damaged skin
What happens if the mixture is not mixed well?
It will not provide adequate strength
How long are you meant to mix components?
3-5 minutes
What are the two methods used for lay up operation?
Pre preg
Wet application
Why is the vacuum bag technique used in advanced composite repairs?
To use atmospheric pressure to consolidate the layers in the laminated repair
To remove trapped air and gases during curing
To hold heater blankets, thermocouples and other materials in place during the curing cycle
To produce the required surface finish on the repair
What are the two main principles that must be obeyed when doing a vacuum bagging repair?
The air and gases inside the vacuum bag must have an easy path to the vacuum pump over the entire repair surface
Amount of resin lost from the repair plies must be kept to a minimum
What is the term for removing one layer at a time?
Scarfing
What is one advantage of scarfing?
It is a safe way of cutting out damage of composite materials
Where should thermocouples be placed when using the vacuum bagging method?
Positioned under the heating mats in order to accurately monitor the repair temperature
What thermocouples are most commonly used for hot bonding?
J type
What are the most common factors for temperature variation with thermocouples?
Variation of thickness of the component
External draughts
Inaccuracy of thermocouples
