Composite Materials

1
Composite Materials Krishan K. Chawla
Chapter 5. Polymer Matrix Composites
2
5.1 Fabrication of PMCs
Hand lay-up Glass fibers are laid onto a mold
by hand and the resin is sprayed on or brushed
on. Frequently, resin and
fibers are sprayed together onto the
mold. Filament winding another very versatile
technique in which continuous tow or roving is
passed through a resin
impregnation bath and wound over a rotating or
stationary mandrel. - a
roving consists of thousands of individual
filaments Pultrusion aligned fiber composites
in the form of section
Bag molding process - for making large parts.
Vacuum or pressure bags containing fibers in
predetermined
orientations in a partially cured matrix are used
in an autoclave
for densification and curing of resin.s
3
5.2 Structure and properties of PMCs
Essentially, it is a three-layer structure of
Different fiber orientations More fibers are
parallel to the mold fill direction In the two
surface layers, S , than in the transverse
Direction in the central layer, C.
A traverse section of continuous glass fiber in
an unsaturated polyester matrix
The layer structure of an injection -molded
composite consisting of short glass fibers in a
semicrystalline polyethylene terephthalate(PET)
- Laminates of polymer matrix composites made by
the stacking of appropriately oriented plies also
result In composites with highly anisotropic
characteristics.

• The epoxy resins commonly employed as matrices in
  the composites meant for use in the aerospace
• industry are fairly impervious to the range of
  fluids commonly encountered, for example, jet
  fuel,
• lubricants.
• - There are, however, two fundamental effects
  that must be taken into account when designing
• components made of PMCs, namely,
  temperature and humidity. The combined effect of
  these two,
• that is, hygrothermal effects, can result
  in a considerable degradation in the mechanical
  characteristics
• of the PMCs.

4

• Degradation owing to ultraviolet radiation is
  another important environmental effect.
• Ultraviolet radiation breaks the covalent bonds
  in organic polymers.

• Fracture in PMCs, as in other composites, is
  associated with the characteristics of the three
  entities
• fibers, matrix, and interface.
• fiber/matrix debonding, fiber pullout, fiber
  fracture, crazing and fracture of the matrix are
  the energy-
• absorbing phenomena that can contribute to the
  failure process of the composite.

5.3 Interface in PMCs

• Hull gives a good description of the glass
  fiber/polyester resin interface.
• The glass surface has randomly distributed
  groups of various oxides SiO2, Fe2O3, Al2O3,
  absorb water
• as hydroxyl groups, -M-OH where M can be
  Si, Fe, and Al and as molecular water attached to
  the
• (-M-OH) groups.
• - If the glass surface stays in contact with
  water for too long a time, hygroscopic elements
  can get
• dissolved and a porous surface consisting
  of nonhydrated oxides is left.
• The coupling agents create a chemical bridge
  between the glass surface and the resin matrix.

• The chemical bridge theory goes as follows
• R-SiX3 X hydrolyzable groups bonded to
  Si.(ex. OC2H5)
• R resin-compatible group

5
5.4 Applications
-Glass fiber reinforced polymers are used in a
wide variety of industries
- from sporting goods to civil
construction to aerospace.
- Tanks and vessels in the chemical
process industry.