6.3 Flashcards
What is the definition of a composite.
Two or more materials that are combined to form a much stronger structure than either material by itself.
Disadvantages of composites.
Inspection methods are difficult to conduct
Lack of long-term design database
High cost in production and repair
Very expensive processing equipment
General lack of repair knowledge and expertise
Products + materials are often toxic and hazardous
Lack of standardised methodology for construction and repairs.
Advantages of composites
Corrosion resistant
High strength to weight ratio
Longer life span
Fibre-to-fibre transfer of stress allowed by chemical bonding
Bonded constructions eliminates need for bolts and fasteners
Tensile strength 4-6 times more then steel or aluminium
Fibreglass
Most widely used in the industry
E-glass (electrically resistant)
S-glass (strong)
Does not conduct electricity
Aramid fibre (kevlar)
Two most common types in aviation:
- kevlar 49
- kevlar 29
Resistant to impact damage.
Sensitive to UV light turns it from yellow to brown.
Complex construction and maintenance.
Carbon fibre / black graphite
Used on primary aircraft structures (e.g. Ribs and floorboards)
Stronger compressive strength than kevlar, but more brittle.
Wears out tools
Corrosive when bonded with aluminium.
Boron fibres
Very hard wearing
More used on military aircraft
Repair of cracked aluminium
Apply boron vapour to tungsten
Corrosion resistant
Ceramic fibres
Used when high temperature applications are needed
Fabric orientation
Warp - runs along the length of the fabric
Weft - runs perpendicular to the warp
Selvedge edge
Tightly woven edge that runs parallel to the warp
Quasi-isotropic lay up
Orientation of fibres are balanced resulting in constant stress and stiffness
Mats
Chopped fibres, compressed together.
Not as strong.
Not common in aviation
Plain weave
Over under … etc
Lowest mechanical properties
Difficult to cover complex shapes
Twill weave
Superior wetout
The preferred weave
Superior mechanical properties (strongest)
Flatter
Satin weave
Often used for repairs.
Resin matrix
Transfers stress to the reinforcing fibres.
Consists of resin and hardener.
Two types
- thermosetting
- thermoplastics
Epoxy resins
Aviation use thermosetting
The MSDS sheets must always be referred to
Pre-impregnated fabrics
Fabrics already saturated with correct amount of resin.
Expensive
Stored in a freezer
Short shelf life / out of freezer time.
Laminated composites
Two or more layers of re-informing material bonded together and embedded in a resin matrix.
Sandwich composites
Core bonded between two fibreglass sheets.
Used in flying control surfaces.
Stronger then laminate
Core materials (sandwich)
Honeycomb
- aramid paper
- high strength to weight ratio
Foam
- found in smaller aircraft
- styrofoam
PEEK
Poly-ether-ether-ketone
- valve seats, pump gears
Thermosetting polyamide
A good insulator
Excellent mechanical properties, high chemical resistance
PCTFE (Teflon)
Fire + chemical resistant
Low moisture absorption
PTFE
Good insulator
Two types of transparent plastics
Monolithic (one layer)
Laminated (multiple layers)
- polyvinyl butyral (PVB) forms the inner layer.
Marking of thermoplastics
Heated dye
Sealants
Either one part or two part.
Used for fuel tanks, skin joins, aerodynamic smoother.
Defects in composites during manufacturing process
Micro-cracking, porosity, improper curing, improper drilling, impact damage, delamination.
Disadvantages of visual inspection of composites.
Damage is often not visible BVID
Visual inspections are often overlooked by engineers who are under pressure to dispatch the plane timely.
Effect of moisture on composites
Can worsen cracking as water freezes repeatedly.
Moisture ingress ion in the core can lead to skin to core bonding degradation, affecting structural integrity.
List the types of non-destructive testing/inspection of composites.
Tap testing, ultrasonic inspection, radiography inspection, thermography, dye penetrants, moisture detector.
Types of damage in composites.
Negligible, repairable, non-repairable.
List the 5 basic types of composite repair
- pre-cured patch
- potted
- resin injection
- laminate repair (one surface)
- laminate repair (through part)
Safety precautions when working with composites.
MSDS
PPE
Fire protection
Ventilated areas
Detection and determination of plastics.
Crazing
Cracking
Components of a truss type wing
Front spar
Drag and anti drag cables
Box structure and box spar
List the wood used for aircraft in order of most desirable to least desirable
Sitka spruce
Douglas fir
Noble fir
Western hemlock
Acceptable slope of grain for aviation wood
1 in 15
Factors assessed in aviation wood
Way it is cut
Shape of grain
No. Of growth rings
Slope of grain
How is aviation wood cut
It is quartersawn
Types of defects in wood
Knots
Pitch pocket
Compression
Dry rot
Stained wood
Grain deviation
Casein glue
No longer used, low grade and obsolete
Needs to be fully removed before applying modern adhesive
Bonding process for wood glue
Wood must be clean
Not sanded
Dry
Correct pressure in clamps
Inspection of wooden structures
Sniff test
Tap test
Remove screws and check for corrosion
Check for elongation in joints
Visual inspection
- use a light and a mirror
Repairing of wood
Reinforcing plates
- 1/4 as thick as the beam
- no more than 2 to a spar
- not permitted in some load bearing areas
- landing gear, wing root, engine mount
Plywood skin repair
Holes up to 1 inch
Fabric overlapping by 1 inch
In which direction can a fabric be stretched
Along the bias
Dopes
Nitrate
- flammable
Butyrate
- sensitive to UV light
- so added aluminium
Inspection methods of fabrics
Wood frames
Zip panels
Spring panels
Seyboth (punch tester)
Maule tester
How long is the lifespan of fabric and give the factors affecting this
5-10 years
Climate
Type of operation
Cyclic contraction + expansion
Repairing fabric
Herringbone stitch
Woods frame
Darning
Insertion
Boot stitch
Repairing fabric
Herringbone stitch
Woods frame
Darning
Insertion
Boot stitch
Components of a truss type wing
Front and rear spar which are the main spa wise members.
Compression struts made of alloy tubes.
Drag and anti drag wires
