90 Degree
0 Degree
90 Degree
Figure 4. Schematic representation of a fiber
composite lamina.
could be either a flat arrangement of unidirec-
tional fibers, which are called a lamina, or struc-
tural shapes like rods, bars, or beams, which are
usually produced by continuous manufacturing
processes using roving of the fibers. Figure 4 gives
an example of three laminas, in each one of which
the fibers are oriented in a specific direction. The
glass-fiber-reinforced plastic bar which we have
considered for creep tests under this study is also
Figure 3. Photomicrograph of a polymer composite.
a unidirectional composite manufactured in the
form of a rod from glass fiber roving. The poly-
theoretical volume fraction of fibers under the
mer for the matrix of these composites is usually
square array is 78.5% and under the hexagonal
polyester or vinyl ester (the latter has slightly
array is 90.7%. In reality, however, the packing
higher fracture toughness, flexural strength, and
condition is quite arbitrary, and large spaces in
chemical resistance, but a somewhat higher cost).
between fibers are common, as shown by the
micrograph of a composite in Figure 3. In most
Laminated composites
composites the fiber volume varies from 55 to
The manufacturing of laminated composites
65%.
starts with the incorporation of a large number of
Figure 1 shows that the orientation of the
fibers into a thin layer of matrix called a ply. The
fibers controls the overall behavior of the compos-
thickness of the ply usually ranges from 0.1 to 1
ite, and it is necessary to refer to these orientations
mm (0.0040.04 in.). As said before, a laminate
and other directional properties with some des-
containing only unidirectional fibers is called a
ignated axes, say 1, 2, and 3. For example, in Fig-
lamina. For a laminate containing unidirectional
ure 1, we can designate x = 1, y = 2, and z = 3. Thus,
fibers, the material has the highest strength and
if we have to refer to a tensile or compressive
modulus in the longitudinal direction of the
stress σ in the x direction, we refer to it as σ11; simi-
fibers. However, in the transverse direction, its
larly, for the y direction we have σ22, and the z
strength and modulus are very low. Therefore, a
direction is σ33. Other elastic parameters, such as
varied amount of fibers in different directions
strains, ε, modulii (E for Young's modulus, and
(Fig. 5), as well as the use of different types of
G for the shear modulus), or Poisson's ratios, ν,
fibers, can be used to control the properties in dif-
as well as the thermal and moisture absorption co-
ferent directions. Generally, continuous fibers or
efficients (α and β, respectively), are also referred
mats of the reinforcing fibers in bidirectional ori-
to their directionality with appropriate subscripts.
entations are used to form the laminate. Sophis-
ticated laminate theories and computer-aided
Unidirectional composites
design methods are available to produce opti-
The bundle of rods shown in Figure 1 can be
mum properties of the desired laminates. The FRP
assumed to represent a model of the unidirec-
W-beam design developed under this program
tional composites. The unidirectional composites
was a laminate design, as will be discussed later.
4
Previous Page
