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Chapter8_comp - Chapter 8 Composite Materials for Aero...

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Chapter 8 Composite Materials for Aero Applications
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Reading Material Callister p. 578 – 615 (607-610 will be covered Callister p. 578 615 (607 610 will be covered later)
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Composite Materials A composite material satisfies the following conditions: –It is manufactured; i.e., artificial rather than natural –It consists of 2 or more physically and/or chemically distinct, suitably arranged phases with an interface separating them –It has characteristics not displayed by the components in isolation
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Composites matrix + reinforcement Composites – matrix + reinforcement The matrix; e.g.,epoxy and the i f t l fib th t reinforcement; e.g., glass fibres, are the two essential components in any composite. Th t i i th ti h The matrix is the continuous phase. The reinforcement is the dispersed phase.
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Matrix The matrix is the material in which the reinforcement is embedded. It provides the bond between the reinforcement materials and protects them and impedes fracture. In aerospace applications, composites are based on polymer ceramic or metal matrices These based on polymer, ceramic or metal matrices. These are called • PMC – polymer matrix composites: PMC polymer matrix composites: CMC – ceramic matrix composites: • MMC – metal matrix composites: MMC metal matrix composites:
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Reinforcing materials can be in the form of particles, flakes, whiskers, discontinuous fibers, continuous fibers or sheets. It is these materials which carry up to 90% of the load (transferred from the matrix ) 90% of the load (transferred from the matrix.) Typically these are classified as: particle, fibre (US - fiber) or structural Orientations: • Particulate (uniform) - isotropic • Randomly oriented short fibres - isotropic • Unidirectional short fibres- anisotropic L i ( l l ) i i • Laminar (ply layers) - anisotropic • Nb: fibres can also be woven (cloth or tape), 3-D or braid
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Fig 16. 8 p 547, fibre reinforced composites The most effective strengthening and stiffening requires a critical fibre length lc. This is about 1 mm for many glass and carbon fibres. lc is dependent on the fibre diameter d, it t th * d th fib t i b d ( th h its strength σ f * and on the fibre-matrix bond (or the shear strength of the matrix, whichever is smaller) τ c . lc = σ f * d/ 2 τ c a. Continuous and aligned fibres, continuous fibres have l >> lc,
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Shorter, thicker fibre Longer thinner fibre σ (x) σ (x) Longer, thinner fibre Poorer fibre efficiency Better fibre efficiency Normally, many small d, long fibres are best. Smaller d fibres have a lower probability of defects (higher fraction of theoretical strength) Many smaller fibres provide a larger interfacial area strength). Many smaller fibres provide a larger interfacial area reducing shear stresses which develop. Note: for MMCs it's better to have maximum interfibre spacing so th t th k bl ti bilit f t l b l it d that the crack blunting ability of metals can be exploited.
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Typical Benefits PMCs have enhanced E, TS, creep performance CMCs have enhanced K c MMCs have enhanced σ y , TS, creep performance CMCs:
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