6.3 Composite Materials✅(8) Flashcards
What is meant by ‘composite’
- Describe 2 or more materials that are combined to form a stronger structure
The simplest composite is composed of two elements which are
- Matrix (adhesives or resins)
- A reinforcing material (fabric)
What are the most commonly used reinforcing fibres in the aviation industry?
- Fibreglass
- Aramid
- Black graphite/carbon fibre
- Boron
- Ceramic
Fibreglass is now the most widely used composite material (T or F)
T
What are the advantages of fibreglass
- generally less expensive than other composites
- does not conduct electricity
2 common types of fibreglass?
- E glass
- S glass
What are the 2 types of Aramid (Kevlar)?
Kevlar 49 and Kevlar 29
How are Boron fibres made?
- applying small amount of vapourised boron onto the tungsten or carbon wire (core)
What are the disadvantages of boron fibres?
- very expensive
- hazardous for maintenance personnel
Is boron fibres mostly used for commercial aviation or military?
- military
Where are ceramic fibres used?
- Where a high temperature application is needed
What are aircraft firewalls sometimes manufactured from
-Ceramic fibre composites
explain warp (relating to fabric orientation)?
- threads run along the length of the fabric
Usually the fabric is stronger in the warp direction or weft direction?
- Warp
explain weft/fill?
- weft fibres run perpendicular to the warp fibres
explain selvedge edge?
- tightly woven edge that runs parallel to the warp threads
What is the function of the selvedge edge?
- prevents edges from unravelling
What is bias?
- fibre orientation that runs at a 45* angle to warp threads
what is unidirectional materials?
- major fibres run in warp direction
what is Bidirectional fibre?
- fibres run in 2 or more different directions
Chopped fibres that are compressed together are often called?
‘mats’
What are the most common weaves
- Plain weave
- Twill weave
- Satin weave
What are thermoplastics
- Use heat to form the part into the desired shape.
- Shape is not permanent (reversible)
Are epoxy resins classed as thermosetting plastic resins (T or F)?
T
What is pre-impregnated fabrics known as ?
Pre-pregs
What is pre-impregnated fabric?
- fabric that have the correct amount of resin already saturated on fabric
Pre-impregnated materials are made to eliminate what?
- any error during the mixing and application process
What would happen to resin systems in pre-preg materials if not kept in cold storage?
It will start to cure
What are laminated composites?
- consists of 2 or more layers of reinforcing material bonded together
What are sandwich composites?
- core bonded between 2 thin fibreglass sheets
Typical applications of sandwich composites?
- flying control surfaces
- wing to body fairings
- helicopter rotor blades
Sandwich composite is at least 10x stronger than laminate, and the weight is only increased by 6x (T or F)
T
What are the 2 most common types of core materials used in sandwich construction?
- honeycomb
- foam
What are core materials
- central members of an assembly
honeycomb refers to how a material is constructed to combine high stiffness with low weight (T or F)?
T
Honeycomb cores have an excellent strength to weight ratio (T or F)?
T
What are thermosetting plastics?
- When heated and cooled, it fully cures (cannot be reshaped)
What is the newest type of transparent plastics?
- stretched acrylic
Monolithic plastic sheets are made
- in single solid uniform sheets
Laminated plastic sheets are made
- from transparent plastic face sheets bonded by inner layer of clear resin material
What is a one part sealant ?
- prepared by manufacturer
- ready for application
What is a two part sealat?
- Require separate packaging to prevent curing prior to applications
What is a specialised form of adhesive known as
‘film adhesive’
What happens if you store the film adhesive in a freezer?
- Curing process slows down (increases shelf life)
What materials are prone to erosion
Composite materials
How are composite materials protected from ultraviolet (UV) light?
-special UV primers and paints
What is the primary inspection for all in service inspections of composites
visual inspection
What is audible testing (coin/automated tapping)?
- tapping defective area and listening for any change of sound
In what circumstance is the audible testing not reliable?
- structure with more than 4 piles
What is ultrasonic inspection?
-sound wave energy with a frequency above the audible range
what is the best method to detect internal delamination
ultrasonic inspection
What is radiography inspection
- often referred to as X-ray
- allows a view into the interior of the part/component
What is a thermograph inspection
- heat sensing devices used to measure temperature variations
What is dye penetrants used to detect?
- cracks and other defects in metallic surfaces
Should dye penetrants be used on composite structures?
- No, it should not be used unless manufacturer stated
What is a moisture detector?
- detects water in sandwich honeycomb structures
How does the moisture detector work?
- meter measures the RF power loss caused by the presence of water
What is a cosmetic defect
- defect on outer skin (does not involve damage to structure)
What are the 3 classifications of damage
- Negligible
- Repairable
- Non- repairable
What is impact damage
- when a foreign object strikes the part
What is delamination?
separation of fabric layers of material in a laminate
What does it mean by ‘disbonds’?
- separation of the composite materials from another material to which it has been bonded
The separation between the skin and core of composite sandwich structure is referred as?
core disbond
What are the 5 basic types of composite repairs
- Pre-cured patches
- Potted
- Resin injection
- Laminate repair (one surface)
- Laminate repair (through part)
A temporary repair can be performed to cover holes or deep scratches by using what?
pre-cured patch installed with blind fasteners/adhesive
Potted repairs are restricted to what structures and how much damage tolerance?
- honeycomb core sandwich structures
- damage up to 2.5cm (1in) in diameter
How can minor delamination be repaired?
-Resin injection (potting compound)
Honeycomb sandwich structures are prone to what damage
- Impact damage
- Puncture damage
What are the 2 more common honeycomb sandwich structure repair?
- puncture repair
- honeycomb core repairs
What are the 2 commonly used solvents in composite repair?
- Methyl-Ethyl-Ketone (MEK)
- Acetone
Methyl-Ethyl-Ketone (MEK) is commonly used for cleaning dust and grease from composite components (T or F)?
T
Acetone is used for general equipment and tool equipment (T or F)?
T
What are the 4 major types of plastic failure?
- Environmental failure
- Thermal failure
- Chemical failure
- Mechanical failure
What is crazing?
- when subjected to load, plastic materials develop small surface fissures
How is crazing caused
improper installation, machining, or cold forming
What is buffing?
- remove fine, hairline scratches
- final polishing process
When sanding, water/detergent must be used as a lubricant to prevent the paper from becoming clogged (T or F)?
T
How can cracks be limited?
stop drilling
An adaption of the truss-type wing is the ‘box spar’ (T or F)
T
What loads does a box spar accept
-torsional and bending loads
What is the most common wood used for aircraft structure
Sitka spruce
To meet the Sitka spruce grade specifications, the wood must be kiln-dried(T or F)
T
What is the maximum slope of grain that is acceptable when carrying out a wood assessment
- 25mm-380mm (1inch-15inches)
Aviation wood is usually quarter-sawn to reduce the amount of shrinkage over the lifetime of the component (T or F)?
T
Most wood damage is caused by what conditions?
- moisture
- temperature
- sunlight
How can wood be protected from the attack of decay fungi, harmful insects, or marine borers?
by applying chemical preservatives
How much strength does wood approximately lose at temperatures of 50*c
25%
End grain portions of wooden members are much more absorbent than side grain (T or F)
T
Excessive moisture at bolt holes promotes what
corrosion of the bolts
How can sealing of wood surfaces in bolt holes be accomplished
- varnish or other acceptable sealer
What is a knot
identifies a point where a branch grew from the trunk of the tree
Which knots are not acceptable for aircraft construction or repair
spike knots
What are pitch pockets
- small openings (weaken the wood)
When are pitch pockets acceptable
centre portion of beam
Can wood that has been compressed, be used in aircraft construction or repair?
No
What does it mean by ‘compression failure’
- tree falls and lands across a log or uneven ground
How is decay caused
fungi that grows on damp wood
Can stains or decay be used in aircraft construction or repair
No
What is dry rot?
-brown rot decay caused by fungi
How do you prevent dry rot
- ensure kiln-dried wood with a moisture content of 20% or less is used
- good surface finish (varnish, paint)
What is Casein glue
- powdered glue made from milk
-deteriorates over the years it is exposed to moisture and temperature
Casein glue is considered low grade and has become obsolete for all aircraft repairs (T or F)
T
What is plastic glue also known as
urea-formaldehyde adhesive
Plastic resin glue has been considered obsolete for all aircraft repairs (T or F)?
T
Does plastic glue deteriorate when exposed to moisture and warm humid environments?
yes
What is a Resorcinol glue
- 2 part synthetic resin glue which consists of a resin and a hardener
Which glue is the most water resistant
Resorcinol glue
Which glue is most commonly used on repairs of aircraft wooden structures
Resorcinol glue
What is epoxy resin
- 2 part synthetic glue (consists of a resin and a hardener)
Wood pieces that are joined, must be kept at a constant temperature for a minimum of how many hours
24 hours ( so moisture content can equalise)
When testing a joint sample, the fractured glue must show at least 75% of wood fibres, evenly distributed over the fractured glue surfaces (T or F)
T
What are the inspection methods for wooden structures
- Non destructive
- Destructive
what must always be the first reference for any repair scheme?
- AMM
What is the simplest method to repair a small hole in plywood
a fabric patch
What did nitrate dope help with
-tighten and stiffen the fabric
What was the drawback of nitrate dope
extremely flammable
To address the flammability issues, what dope was used
butyrate dope
Why did they add aluminium solids to butyrate coatings?
- as it reflected the suns rays, preventing UV damage
All materials used for fabric covering, have to be stored at temperatures above what
20c (68F)
What are the 2 types of fabric testers
- Maule punch tester
- Seyboth punch tester
Seyboth and Maule fabric strength testers are designed for cotton and linen covered aircraft (T or F)
T
How should fabric cracks be corrected?
- sealed
- replace
What is the most common honeycomb construction in the aviation industry
Aramid paper
Does a composite repair restore the full strength characteristics of the original structure
No
What are resin injection or potted repairs used to repair
Small holes
Resin injection or potted repairs are only performed on non-structural parts (T or F)
T
What is the preferred type of repair
Permanent
What procedures should a surface preparation involve
- Thorough solvent degreasing
- water break test
What fasteners are pre-cured patch installed with to cover holes or deep scratches
Blind fasteners
What are the 2 ways that a pre-cured patch can be secured
- pre-applied adhesive
- mechanical (metal) blind fasteners
What is the problem with using fasteners on a pre cured patch
- can cause damage to honeycomb core and lead to delamination
If a honeycomb sandwich structure was damaged more than 2.5cm (1 inch) what can be used
Honeycomb plug
How can you restore scratches
- polishing, buffing, or even sanding
Transparent plastics must never be sanded unless absolutely necessary (T or F)
T
What does solid wood refer to on aircraft construction
Beams or spars
What is plywood
Made of odd number of thin plies with each grain of each layer placed at 90*
What temperature must fabric coverings be stored at?
20*c
Seyboth/Maule punch tester are only designed for what covered aircraft
- cotton and linen
What is the problem with blind fasteners?
-can damage honeycomb core and lead to delamination
