Composites Flashcards
What are the elements that make up a composite material?
Matrix
Reinforcing material
What are the advantages of composites?
High strength to weight ratio
Reduces need for fasteners
Reduction of wear
Corrosion resistance
What are the disadvantages of composites?
Very expensive
Not easy to repair- needs specialists normally
What is Nomex?
Wholly aromatic polymer with high thermal stability, flame resistance, and radiation resistance
What is fibreglass?
Small strands of silica that is spun and woven into a cloth.
What are the two types of fibreglass that are commonly used?
E-glass, high resistance
S-glass, very high tensile strength
What makes fibreglass different to most other fibres?
Weighs more
Less strength
Why is fibreglass commonly used?
Wide range of applications
Low cost compared to other composites
Properties of fibreglass
Low density High strength High fracture toughness Smooth and circular cross section Good wet out Doesn’t absorb water
What is aramid short for?
Aromatic-polyamide
What is aramid?
Yellow fibres that are light weight and have a high tensile strength
What is Kevlar?
A man made organic fibre with properties such as high tensile strength and flexibility.
What company has developed Kevlar, Aramid, and Nomex?
DuPont
What are the main disadvantages of Kevlar?
Easily absorbs water Sensitive to the environment Poor compressive properties Difficult to cut- specialist tooling Sensitive to UV- turns brown as its effected
What is the positive characteristics of carbon fibre?
High strength High E modulus Low density High resistance to temperature Longitudinal thermal expansion almost 0
What are the negative effects of carbon fibre?
Electrically conductive
Will wear out high speed cutters quickly
Low fracture resistance
What is the difference between Kevlar and graphite?
Graphite has more compressive strength, but is more brittle
What is the downside to graphite?
Corrosive when in bonded with aluminium so a layer of fibreglass is needed
What is the warp of a fabric?
Length off the roll.
What is the weft/fill?
Dimension perpendicular to the warp.
What is the selvage edge?
Edge parallel to the warp that keeps the edge from unraveling
What is the bias?
Runs at 45 degrees to the warp, is what the fabric is manipulated on
What are unidirectional fibres?
Fibres run along the warps with occasional transverse threads to hold it together
What are bidirectional fibres?
Fibres run in two directions, typically more fibres on the warp for increased strength in that direction
What is are mats?
Chopped areas of fabric with little strength than unidirectional or bidirectional fibres
why are fabric weaves better?
More resistant to breakout,delamination, and damage compared to unidirectional.
What are the two types of resins?
Thermoset
Thermoplastic
What is a thermoset resin?
Heated to form then permanently retain that shape when cooled, this is mainly used in structural composites
What is a thermoplastic resin?
A resin that can be reformed by heating
What type of resin is an epoxy resin?
Thermoset
What are prepreg fabrics?
Fabrics that are pre impregnated with resin, they are kept in large rolls and need to be stored in a freezer.
What are the main types of core materials?
Foam
Honeycomb
Honeycomb core
What are the types of honeycomb fabrication?
Corrugation
Expansion
What are the main ways to dissipate charge?
Flame spray
Bonding jumpers
Aluminium wires woven into top layers
Aluminium foil bonded to top area
Why is paint used?
To seal against moisture
What is MSDS?
Material safety data sheets
What information is found on an MSDS?
Health precautions
Flammability
Ventilation requirements
Information for health professionals
What is polymerisation?
A chemical reaction that creates a long chain of double bonded molecules
Is there any by products made from polymerisation?
No
What is polycondensation?
A chemical reaction between 2 similar of dissimilar basic units with at least 2 functional groups.
What is polyaddition
Reaction of 2 different types of molecules when reactive groups are brought together.
No byproducts are produced
What are some key polycondensation polymers?
Polyester
Polyamides (nylon)
Phenol-formaldehyde (Bakelite)
What are some key polyaddition polymers?
Epoxies
Polyurethanes
What are the benefits of adhesives?
Moisture and corrosion resistant
Eliminate need for fasteners
Distribute stress at the bonding points
What are the main types of adhesives?
Synthetic Thermosetting Thermoplastic resins Elastomeric Contact Solvent Dispersion Film Foaming adhesive films
What are thixotropic agents?
Thickening agents to the resins
What are the 2 common types of thixotropic agents?
Aerosil
Micro-ballons
How should a resin be mixed?
In a wax free container with the accelerator added to the base
What will happen to a resin when a catalyst is added to it?
The pot life is reduced
Where are thermoplastic resins very unlikely to be used?
In composite structures
What are the two main thermosetting resins used for structures and why?
Epoxy- external components, good mechanical strength
Phenolic- cabin due to low toxicity and fire resistance
How can the mechanical properties of plastics be improved?
By bonding fibres to the plastic
What are the key features of a good polymer?
Easy to use
Coat every fibre and bond to them
Have the correct mechanical properties
What are the 4 most common resins in fibre reinforced polymers?
Polyester
Epoxy
Vinyl Ester
Phenolic
What must never be done with a catalyst for a resin?
Mix accelerator and catalyst to avoid an exothermic reaction
What are the disadvantages of epoxy resin?
Low pot and shelf life
Moderate to high costs
Toxicity
What are the positive characteristics of epoxy resins?
Low shrinkage when cured High adhesion Great cohesive strength when cure- can cause stress 100% solid- no byproducts Resistance to moisture and adhesives
What are the positive factors for surface wetting?
Removal of surface contamination
Viscosity reduction through heat or solvent
Time
Pressure
What factors have negative effects on surface wetting?
Leaving fingerprints Residues left on surface Incorrectly aligned glue lines Low temperatures Incorrect pretreatment
What are the important aspects for durable adhesives?
Mechanical hooking
Surface wettting
What are the pretreatment procedures for bonding?
Degrease only
Degrease, abrade, and remove loose particles
Degrease and chemically pretreat
What are the aim contamination causes for pretreatment?
Fingerprint Cloths not fully clean Poor degreasing Contaminated abrasives Chemical solutions Other work in bonding area
Where would repair procedures be for structure?
SRM (structure repair manual)
What are the 3 damage levels?
Allowable
Repairable
Not repairable
What would be classed as skin not perforated damage?
Abrasion Scratches Gouges Dents Nicks Debonding Delamination
What is classed as skin perforated damage?
Lightning strike
Holes
Impact by foreign object
What is BVID?
Barely visible impact damage
Is water absorption allowable?
No
What causes chemical degradation?
Accidental contact with aggressive chemical products
How are dents and depression inspected?
NDT
What are the different types of NDT?
Tap test Visual Radiography Thermography Holography Bond tester Acoustic emissions
What is holography?
Overlaying an image of what the surface should look like on top of the actual surface. Gets inspection done fast
What are the disadvantages of holography?
Tedious to set up
Difficult to get a good display of the overlay
What is an ultrasonic NDT?
Ultrasonic waves are transmitted onto the structure, which then bounces off and returns to the receiver
Why is ultrasonic used?
Quick
Not expensive
What waves are used for radiography?
X-rays
Gamma rays
How does radiography work?
By creating a photographic image from the em rays onto the film behind the area being tested
What are the 3 types of repairs?
Temporary
Permanent Cosmetic
Permanent structural
How should all surface protection be removed?
By mechanical means only
How can chemical strippers affect any composite surface?
It can cause the resin to deteriorate
What is scarfing?
The removal of layers one at a time with mechanical sanding tools
What should be done to damaged core material?
The damaged core must be removed fully and replaced with a suitable filler material to restore the full strength
What is wet lay up?
The application of multiple layers of hand pre-impregnated fabrics to restore the damaged skin
What is important to remember with resins and catalysts?
Mix properly for full strength Weigh each part of the resin first Advanced composites are very exacting Use a wax free container 3-5 minutes mixing usually Don’t mix too fast Do a drop test for the the viscosity
What is pot life?
Amount of time you have to work with the resin
What is shelf life?
The amount of time a resin can be stored in an unopened container
What is resin rich and how does it affect a composite?
There is too much resin
Increases the weight and makes it brittle
What is resin starved and how does it affect the composite?
Not enough resin
Can’t transfer stresses to the fibres
What should be done when working resins into fibres?
Ensure that the weave is not distorted
What are the two lay up methods?
Wet application
Pre-preg
What is vacuum bagging used for?
To hold parts such as thermo couple in lace during curing
To use atmospheric pressure to consolidate layers in a repair
To produce a desired surface finish
To remove trapped air or gasses during curing
