Composites- Types of Composite Flashcards
Three types of composite
Particulate-reinforced composites
Fibre-reinforced composites
Structural composites
Subdivisions of particulate-reinforced
Large particles (micron scale) and nano particles
Subdivisions of fibre-reinforced
Discontinuous (short) fibres which can be aligned or random.
Continuous (long) fibres which are aligned
Subdivisions of structural composites
Laminates and sandwich panels
What do the particles do?
Often stiffer than the matrix and act to restrict movement of matrix in vicinity of particle. Strong particle-matrix interface critical. Particles usually provide stiffness and hardness with matrix providing ductility and toughness.
Aspects of particulates that affect properties of composite
Size, shape (aspect ratio), orientation, volume fraction, distribution and dispersion, surface treatment (for interface between reinforcement and matrix
Particulate size types
Large (micron scale), treated by continuum mechanics (e.g shear lag theory).
Dispersion strengthened: strengthening metals with metal oxides (like precipitation hardening), treated at atomic or molecular level.
Nano-particles: at least one dimension is on the nano scale, nano-composites
Particulate shape types
Spherical (3 similar dimensions)
Plate-like (2 long, 1 smaller)
Fibrous (1 long, 2 smaller)
What is the reinforcement effect of particulates controlled by?
Aspect ratio. Can be length over diameter
Aspect ratios of different particles and their effect
Spherical is about 1 so poorer reinforcement but uniform properties in all directions.
Plate- like or long and thin have greater or much greater than 1, better reinforcement in-plane or along long dimension, poorer reinforcement out-of-plane or transverse to long dimension
Three types of particulate orientation
Aligned
Aligned in-plane
Random
What does higher volume fraction of particulates lead to?
Higher composite stiffness
What dispersion and distribution do you want from particulates?
Good distribution so are present in whole of material and good dispersion so particulates not clumped together
Exfoliation example of layered silicate and polymer
Blocks of layered silicates surrounded by polymer to form a tactoid. Polymer gets between layers in intercalation. The layers then separated from each other in exfoliation with polymer in between
Surface treatment and example
In most cases reinforcement and matrix phases incompatible. Surface of reinforcement often modified to improve reinforcement-matrix interface.
For nano-clay reinforced epoxy, reinforcement Clay is inorganic, matrix polymer is organic, modify clay surface with amphiphilic alkyl ammonium ions, curing reaction exfoliates clay
Example of cemented carbide
Large tungsten carbide (or TiC) particles in cobalt matrix. Particles 10-100μm and approximately 90% by volume, provide stiffness and hardness. Cobalt matrix about 10% by volume and provides toughness and ductility. Gaps between particles (matrix) stop cracks in particles from traversing between particles so can’t keep propagating. Tougher matrix phase relives stress concentrations
Example of rubber automobile tyres
Large carbon black particles (20-50μm) in vulcanised rubber matrix. Carbon black particles essentially spherical and 15-30% by volume. Vulcanised rubber is natural rubber crosslinked with sulphur and is 70-85% by volume. Composite has improved tensile strength, toughness, wear and abrasion resistance. Many other filled polymers which are essentially particulate composites.
Why are air pockets bad in composites?
They are voids in the material and make properties unpredictable
Difference between nano-scale and large particulates
Nano-scale particulates have very high surface area.
Nano-scale fibres can have very high aspect ratio like 150.
Property improvement therefore potentially significantly increased for nano-composites over more traditional particulate composites.
Types of nano-scale reinforcement and examples of each
Spherical- ballotini and powdered silica
Plate-like- nanoclays, graphème and flaked silica
Fibrous- nanotubes/nanoropes and whiskers
Example of tennis balls
10-50μm thick nano-composite layer significantly reduces loss of air pressure. This has vermiculite nano-clay platelets (1nm thick aspect ratio 10,000) in a butyl rubber matrix. The platelets are exfoliated so the layers are separated from each other. Layers are also aligned to provide very effective barrier against pressurised gases leaking out
Example of flame retardant nano-composites
Addition of nano-clay to thermoplastic polymers significantly reduces their flammability.
Mechanism include creation of char layer on surface (barrier), recombination of radicals within clay layers (trap), reduced dripping (retention)
Critical fibre length for effective reinforcement
L_c. At lengths below this the stress in fibre never reaches maximum possible. At lengths above this fibres can efficiently be used. Is about 1mm for most carbon and glass fibres
Aspect ratios of short and long fibres
<15 are short fibres
>15 are long fibres
What does shorter fibre length mean?
Lower composite modulus and strength
Aligned or random alignment of fibres
Aligned gives good properties along fibre direction
Random gives equal properties in all directions
Why are short fibres cheaper than long?
Created by chopping or cutting to length long fibres. Lower length or grade long fibres can be chopped. Short fibres can be processed using conventional polymer processing techniques (like injection moulding) or composite processing techniques (like compression moulding).
Ideal length of fibres
Ideally they would continue from one end of the part to the other without breaks. Allows most efficient reinforcement as fermer fibre ends (where no stress can be maintained).
Long forms of fibres
Tows (continuous aligned fibre bundles)
Woven, knitted, braided, stitched fabrics (bending of fibres induces crimp)
Maximum and minimum modulus of forms of long fibre
Maximum modulus along fibres in tows
Minimum modulus across fibres in tows
All other between extremes
What is fibre pull-out?
Where the fibre-matrix interface has failed and ends of fibres can stick out of matrix
Structural composites: laminar structures
Stacked and bonded fibre-reinforced sheets (piles)
Fibres orientated in different directions to give relatively high strength and stiffness in number of directions in the plane
Structural composites: sandwich panels
Face sheet, adhesive, honeycomb, adhesive, face sheet.
Outer face sheets are relatively strong and stiff (e.g carbon fibre composite, aluminium, titanium). Low density core material in honeycomb (e.g foams, honeycombs, lightweight woods). Gives lightweight composite structure with enhanced bending stiffness. Structural applications are like aircraft fuselage and construction materials
