Composites- Mechanics and Processing Considerations Flashcards
Tensile moduli directions
1 is parallel to uniaxial fibres. 2 is perpendicular to fibres but in plane of lamina. 3 is vertically out of lamina (don’t normally consider)
Rule of mixtures for tensile modulus in 1 and 2 directions
E1=ffEf+(1-ff)Em
E2=EfEm/((1-ff)Em+ffEm)
Where f subscript f is fibre volume fraction
Ef is the fibre modulus
Em is matrix modulus
E2 equation isn’t accurate so often use Halpin-Tsai equation
When do fibre and matrix work in series of parallel?
In the 1 direction (along fibre length) they work in parallel.
In the 2 direction (perpendicular in plane) they work in series and the matrix is the weakest (most compliant) link
Why are unidirectional composites anisotropic?
It is stiffer axially as the fibres and matrix work in parallel and more compliant in transverse as they work in series. Small contribution is whether the fibres themselves are anisotropic (but glass isn’t)
Major poisson’s ratio formula
ν12=ffνf+(1-ff)νm
f and m mean fibre and matrix
This is for force applied in axial direction (length of fibres)
How are major and minor poisson’s ratio linked?
ν12/E1=ν21/E2
21 means stress in transverse direction
In which direction do composites contract more in?
More in the transverse direction when loaded axially because this direction is matrix (more compliant) dominated. Axial direction is stiffer as is fibre dominated. Therefore ν12 is greater
In-plane shear modulus formula
G12=GfGm/((1-ff)Gf+ffGm)
What is needed for successful manufacture of parts from fibre reinforced composites?
Fibres and matrix must be maintained at the correct shape, temperature, pressure. The conditions must be maintained throughout the process and the full part
Processing considerations for the reinforcement
Type (fibres, particulates). Continuous or discontinuous. Random or oriented. Sizing, coupling agent, surface treatment. Fibre volume fraction. Safety (nanomaterials in lungs).
Processing considerations for the matrix
Thermoset or thermoplastic. Temperature tolerance (shrinkage). Environmental tolerance (moisture). Compatibility with reinforcement. Additives, fillers. Void fraction (vacuum/gas won’t support load). Reaction chemistry- cure schedule (T, P, t), degree of cure, post-curing. Viscosity gelatinous time (pot life). Safety
Processing considerations for the component
Size, thickness, cross-section, curvature. Surface finish. Dimensional tolerance. Holes, fixings, strong points, inserts
Processing considerations for the cost
Equipment. Moulds. Materials. Labour. Energy
Processing considerations for the process type
Automation, rate. Machining, trimming, static electricity. Heat transfer- curing T (can overheat/overture and burn part), exothermic reaction (excess heat output). Mould fabrication. Reinforcement draping (getting fibres to follow shape of mould, e.g around corners)
