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Composite materials

Learn the isostrain and isostress rules of mixtures for fiber-reinforced composites, volume fractions, CFRP and GFRP, critical fiber length, anisotropy of laminates, the role of the interface, and metal matrix and ceramic matrix composites with multiple-choice questions.

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Composite materials: key points

Composite materials combine materials with different properties to obtain properties that none achieves alone. In fiber-reinforced plastics, strong, stiff fibers carry most of the load and the resin matrix transfers load to them. The Young's modulus parallel to the fibers is estimated by the isostrain rule of mixtures and perpendicular to them by the isostress rule, and unidirectional material differs greatly in stiffness by direction. It is therefore used as laminates stacked from plies with different orientations.

The interface joining fiber and matrix strongly affects performance. For short fibers, the critical fiber length is the length over which stress is transferred by interfacial shear, and glass fibers use coupling agents to strengthen adhesion. In ceramic matrix composites, on the other hand, the interface is deliberately made weak so that fiber pull-out raises toughness. Metal matrix composites are used to raise stiffness and wear resistance and to lower thermal expansion.

All Composite materials questions (12)
  1. Which correctly describes the main role of the matrix in a fiber-reinforced composite?
  2. A composite of carbon fiber and epoxy has a carbon fiber mass fraction of 70%. Taking the density of carbon fiber as 1.80 g/cm³ and that of epoxy as 1.20 g/cm³, and assuming no voids, which value is closest to the fiber volume fraction?
  3. For a composite of aligned continuous fibers, which correctly describes when to use the isostrain and isostress forms of the rule of mixtures?
  4. A composite of glass fibers (Young's modulus 72 GPa) aligned in one direction at a volume fraction of 40% in a resin (Young's modulus 3.0 GPa) is loaded in tension parallel to the fibers. In the elastic range, which value is closest to the fraction of the load carried by the fibers?
  5. A composite contains glass fibers (Young's modulus 72 GPa) aligned in one direction at a volume fraction of 50% in a resin (Young's modulus 3.0 GPa). Using the isostress rule of mixtures, which value is closest to the Young's modulus perpendicular to the fibers?
  6. Which correctly compares carbon-fiber-reinforced plastic (CFRP) and glass-fiber-reinforced plastic (GFRP)?
  7. Which value is closest to the ratio CFRP/steel of the specific modulus (Young's modulus/density) in the fiber direction, for unidirectional CFRP (Young's modulus 140 GPa in the fiber direction, density 1.6 g/cm³) and steel (Young's modulus 210 GPa, density 7.9 g/cm³)?
  8. When a short fiber embedded in a matrix is loaded in tension along its length, which correctly describes the distribution of tensile stress within the fiber? Assume the interface does not slip and transfers load at a constant shear stress.
  9. Which statement about the in-plane stiffness of a laminate in which unidirectional plies are stacked crosswise and symmetrically, as in [0/90]ₛ, is correct?
  10. When glass fibers are used to reinforce a resin, what is the main purpose of treating the fiber surface with a silane coupling agent?
  11. Which correctly describes the properties of a metal matrix composite (MMC) of an aluminum alloy reinforced with SiC particles?
  12. In a ceramic matrix composite (CMC) reinforced with SiC fibers, what is the main purpose of applying a thin coating of boron nitride (BN) or pyrolytic carbon at the fiber–matrix interface?

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