TM 1-1500-204-23-11
(b) Braided iber architecture resembles a
coiled into a helix. The difference, how-
hybrid of ilament winding and weaving.
ever, is the mechanical interlocking. As
Like ilament winding, tubular braid fea-
a structure is exposed to high fatigue
tures seamless iber continuity from end
cycles, cracks will propagate through
to end of a part. Like woven materials,
the matrix of ilament wound or unidirec-
braided ibers are mechanically inter-
tional pre-preg laid up structures. While
locked with one another. The combi-
micro cracking will occur in a braided
nation, however, is quite extraordinary.
structure, the propagation is arrested
When functioning as a composite rein-
at the intersections of the reinforcing
forcement, braid exhibits remarkable
yarns. This is why braid is the reinforce-
properties because it is highly eficient
ment choice for aircraft propellers and
in distributing loads. Because all the
stator vanes in jet engines.
ibers within a braided structure are
continuous and mechanically locked,
(f)
Braid greatly improves interlaminar
braid has a natural mechanism that
shear properties when nested together
evenly distributes load throughout the
with other braids. While interlaminar
structure.
adhesion is no different from other rein-
forcement products, the layers move
together. As a result, it is very rare for
(c) With regard to strength and stiffness,
cracks to form and propagate between
properly molded biaxial braided com-
layers of braided reinforcement. Since
posites exhibit properties that are
braids are woven on the bias, they
essentially identical to unidirectional
provide very eficient reinforcement for
tape laminates of the same orienta-
parts that are subjected to torsional
tion and iber volume. Because of
loads. Braid is an ideal reinforcement
the load distributing feature inherent
for drive shafts and other torque trans-
in braided reinforcement, there is no
fer components such as langed hubs.
design strength reduction due to iber
crimp. This phenomenon is limited to
(g) Since braiding is typically done with dry
triaxial constructions that have signii-
iber, resin transfer molding is used to
cant iber volume in the axial direction.
apply resin to the preforms. The braided
The laid in axial yarns force added crimp
preform is placed in a closed die and
in the bias yarns, causing the biases to
liquid resin is injected. Resin transfer
lose in plane properties. The axial ibers
molding uses very low viscosity resins,
also tend to inhibit the distribution of
many times bismaleimide based, even
loads within the laminate, yielding lower
though the high temperature capability
breaking strength. However, triaxial
of the resin is not required.
constructions are very cost effective in
a myriad of applications because the
automated formation of complex net
c. Filament Winding. Filament winding involves
shape preforms frequently affords more
wetting out a continuous tow of iber as it is wound
savings than the additional cost asso-
around a mandrel. Each circuit of winding is at
ciated with the iber needed to meet
prescribed angles to achieve the desired structural
design loads.
properties. The mandrel rotates to insure coverage
around the part while the application head transverses
longitudinally. The iber is wet-out just prior to being
(d) Braid s eficient distribution of loads
wound. Cylindrical or spherical parts, such as pres-
also makes braided structures very
sure vessels, are commonly manufactured by ilament
impact resistant. Since all the ibers in
winding. This type of construction is typical in rocket
the structure are involved in a loading
motor casings and is usually found in aircraft appli-
event, braid absorbs a great deal of
cations on external fuel tanks and drive shafts.
energy as it fails. This is why braid is
used for energy absorbing crash struc-
tures, ballistic applications, and fan
d. Stitched Multiaxial Fabrics. In recent years,
blade containment in aircraft engine
multiaxial fabrics have begun to ind favor in the
applications.
construction of composite components. These fabrics
consist of one or more layers of long ibers held in
place by a secondary, nonstructural stitching thread.
(e) Braided structures are also excellent
The main ibers can be any of the structural ibers
with regard to fatigue. Like a ilament
available in any combination. The stitching thread is
wound structure, braided ibers are
usually polyester due to its combination of appropriate
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