Composites- Intro and Natural Composites Flashcards
A definition of a composite
A material produced by the beneficial combination of two or more monolithic materials to produce a single material with properties that exceed those of the constituents
Key features of composites
Two or more constituent phases combined together. Has a continuous phase (matrix phase) and the dispersed phase (reinforcement phase). The constituent phases have distinctly different properties. Matrix often relatively compliant and tough and reinforcement phase often relatively stiff and brittle. Constituent phases remain separate in the composite (so microstructure nom-uniform and discontinuous). Properties of composite combine properties of constituent phases
Can phases be separate
Theoretically can and potentially be reused but in practice is almost impossible and not economically viable
What factors need to be considered for the combination of properties of constituent phases?
Reinforcement volume fraction, aspect ratio, orientation and distribution, interface between reinforcement and matrix, fibre shape and size
Typical roles of reinforcement and matrix
Reinforcement bears load in tension
Matrix keeps reinforcements together and maintains part shape, transfers stress onto reinforcement, protects reinforcement, improves toughness and supports in shear and compression
Example of wind turbine blades
Very high stresses at tips of blade so want them light and strong (composites)
Composites with thermosetting polymer matrices
Glass fibre reinforced polymers: E 10-40GPa, car bodies, baths and showers, boats.
Carbon fibre reinforced polymers: E 80- over 200GPa, brittle, aircraft, sports goods, expensive cars, satellites.
Kevlar reinforced polymers: E 50-140GPa, very tough, bullet proofing, abrasion protection
Ways of making composites with thermoplastic polymer matrices
For glass or carbon fibre reinforced thermoplastics.
Injection moulded: short fibres, better properties than infilled thermoplastic, for many engineering parts.
Compression moulded: long or short fibres, thermally shapeable and reshapeable, not as strong or stiff as thermosetting generally, more difficult to produce than thermosetting.
Ceramic matrix composites
Brittle matrix that’s less brittle than fibres/particles.
Armour, brakes (aircraft and high performance cars).
Example carbon-carbon composite brakes, carbon fibres in a graphite matrix
Metal matrix composites
Ductile matrix, brittle high modulus fibres/particles.
Carbide tools, armour, brake components.
Example cemented carbide tools which are tungsten carbide particles in a metallic cobalt matrix
Natural composite: Nacre
Sea shells. Platelets of crystalline calcium carbonate (calice or aragonite) held together in a complex protein-based matrix. Plates hard and brittle but matrix tough. Even the plates are a composite of proteins and chitin with 30nm grains (of calcium carbonate?). Holding plates together are mineral bridges, nano-asperities resist inelastic shearing, organic layer acts as viscoelastic glue, tablet interlocking where adjacent plates have sloes at interface
Natural composite: Bone
Is a complex hierarchical mixture of nano-crystalline hydroxyapatite and fibrous collagen in a variety of organic and inorganic matrices within a cellular framework. Grows in columns. Centre of column is spongy bone and rest compact bone. Contains holes to stop crack propagation
Natural composite: Bamboo
Polysaccharides in two forms provide highly directional strength. Cellulose fibrils held together in a lignin matrix within a cellular framework. Microfibrils of cellulose (cylinders) next to each other to form rings around other rings forming the cell wall layers
Why do tough biological materials perform better than expected?
Based on the combination of properties of their individual hard and soft phases they would perform worse than they actually do. Perform better due to their multiscale/hierarchical layered structures
Plant fibres used as reinforcement
Flax
Hemp
Nettle
Banana
Coir (coconut)
