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Hybrid composites

Hybrid composites. John Summerscales. Hybrid: definition. incorporation of two or more fibres within a single matrix the resulting material is a hybrid composite , often abbreviated to just "hybrid". or it may be two resin systems e.g. an interpenetrating network (IPN).

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Hybrid composites

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  1. Hybrid composites John Summerscales

  2. Hybrid: definition incorporation of two or more fibres within a single matrix the resulting material is ahybrid composite, often abbreviated to just "hybrid". or it may be two resin systemse.g. an interpenetrating network (IPN)

  3. Hybrids: configuration • fibre-by-fibre mixtures also known as "intimate" hybrids • tow-by-tow mixtures a.k.a. "discrete" or "zebra" hybrids • layer-by-layer mixtures • skin-core-skin structures i.e. sandwich structures • internal ribs • external ribs

  4. Hybrids: configuration b a c d e f

  5. Hybrids: rule-of-mixtures For elastic modulus: Ec = ηlA ηoA VA EA + ηlB ηoB VB EB + Vm Em where subscripts A and B represent each of the fibre reinforcements

  6. Hybrids: strength (lower bound) • For unidirectional composites, assumethat the critical situation is fibre fracture and that contribution of resin matrix is negligible, then lower bound strength will be either:σ´c = either VA EA ε´Aor VB EB ε´B

  7. Hybrids: strength- constant strain (middle bound) • previous slide assumes that only one dominant fibre is carrying the load. • the low elongation fibre can be assumed to be the critical failure case and if the high elongation fibre is also carrying load at the same strain, then:σ´c = VA EA ε´A + VB EB ε´A • DoNOTuse this equationwithdifferentvalues of ε’

  8. Hybrids: strengthconstant strain graphical method • matrix contributionassumed negligible • plot strength (on y - axis)fromσ´c = VA EA ε´Aσ´c = VB EB ε´Bvspercent of each fibre composite (on x-axis) 100% A 100% B

  9. Hybrids: strengthconstant strain graphical method • as proportion of A decreases so doesits contribution to hybrid strength 100%A 100%B

  10. Hybrids: strengthconstant strain graphical method • as proportion of B decreases so doesits contribution to hybrid strength 100%A 100%B

  11. Hybrids: strengthconstant strain graphical method • read off thelower bound strengthfrom the graph(yellow dashed line) 100%A 100%B

  12. Hybrids: strengthconstant strain graphical method • A is low strain-to-failure fibre so carries no significant load when B fails • B is high strain-to-failure fibre so can carry load when A fails (up to ε’A) • assume ε’B = 3 ε’A 100%A 100%B

  13. Hybrids: strengthconstant strain graphical method • A is low strain-to-failure fibre so carries no significant load when B fails • B is high strain-to-failure fibre so can carry load when A fails (up to ε’A) • assume ε’B = 3 ε’A 100%A 100%B

  14. Hybrids: strengthconstant strain graphical method • by similar triangles,the orange line can be joined to S (σ’A) • i.e. the triangles PQR and SQR have the same vertical height S R P Q 100%A 100%B

  15. Hybrids: strengthconstant strain graphical method • hence, the composite middle-bound strength can now be read from a higher line 100%A 100%B

  16. The hybrid effect !(upper bound) hybrid strength exceeds the rule of mixtures ? … often referred to as “synergistic strengthening “

  17. The hybrid effect ! • for fibres with closely matched strains to failure, where the high-modulus fibre has the low strain to failure and vice versa • failure strain of the low elongation fibre might be increased to that for high elongation fibre by isolating the individual critical fibre failures such that broken fibres are uniformly distributed through the composite. • we might then predict a strength that exceeds the rule-of-mixtures prediction. 

  18. The hybrid effect • experimental data from Mark Gruber’sMMAE thesis, Delaware, 1981: Kevlar 49/E-glass. • theoretical model from JS’ PhD thesis, 1983 using the assumptions on the previous slide.

  19. However .... There is an alternative explanation: • carbon fibre contracts on heating • glass fibre expands on heating if fibre-matrix bond forms at cure temperature then, on cooling to ambient, • carbon tries to expand • glass tries to contract • but they are constrained by the matrix ...

  20. Thermal explanation of the hybrid effect • carbon fibre has residual compressive strain • glass fibre has residual tensile strain • when carbon is loaded in tension: σ constrained unconstrained ε

  21. Summary • six generic ways to combine the fibres • additional term in rule-of-mixtures • strength • lower bound (just the weaker fibre) • middle bound (all fibres at ε’LEC) • upper bound (hybrid effect)do NOT use this model for design purposes? • thermal effects with constraints

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