TM 1-1500-204-23-11
COMPOSITE MATERIALS
4-1. COMPOSITE MATERIALS. To produce the
accomplished by reheating the part until the material
required strength characteristics, advanced compos-
softens and applying pressure to produce a newly
ites rely on a combination of matrix type, iber type,
molded shape. Thermoplastics are consolidated at
ply orientation, and processing method. All ibrous
temperatures up to 800 F and pressures up to 200
materials are characterized as having strength par-
psi. Continuous length iber/thermoplastics are not
allel to the iber direction and a lack of strength
common, but are seen at times. Refer to system
perpendicular to the iber direction. Plywood was the
speciic repair manuals for repairs to thermoplastic
irst practical laminated composite material, formed
systems.
by laying plies in two directions. Fiberglass brought
improved structural strength to this class of materials.
b. Advanced Composite Structure. A structure
The much stiffer ibers of boron and carbon have
constructed by combining ibers in speciic orientations
given composite materials structural properties supe-
with a resin to form a solid shape. Structures made
rior in several ways to the metal alloys they have
exclusively from chopped iber are not considered to
replaced. The term ADVANCED applies to these and
be advance composite structures; however, chopped
other composites, which have ibers of high stiffness
ibers could be part of an advanced material system.
and strength. The development of advanced compos-
ite materials has been accompanied by a substantial
c. Pre-preg. Pre-preg material is the basic build-
improvement in the theory required for an adequate
ing block of most advanced composites. It consists of
understanding of laminated composites. This theory
ibers preimpregnated with a partially cured (B-staged)
may be applied equally well to iberglass composites.
matrix material. It is supplied by a pre-preg manufac-
The same is true of research in composite repair
turer in thin sheets in two different forms, unidirec-
technology. In this manual, the adjective ADVANCED
tional pre-preg (ibers all in one direction) and woven
will be implied, although the adjective may not be
pre-preg (ibers woven into a speciied weave style).
used. This manual refers speciically to composite
The ibers in both of these forms are continuous.
materials with ibers oriented in selected directions
rather than those composites with ibers of random
d. Laminate. Considering its light weight, a lamina
orientation, though at times random ibrous materials
(ply) of iber reinforced composite is remarkably strong
would also be utilized in particular systems.
along the iber direction. However, the same ply is
considerably weaker in all off iber directions. To
a. Thermoset and Thermoplastic Materials. The-
address this issue and withstand loadings from multiple
re are both thermoset and thermoplastic matrix mate-
angles, aircraft structure is manufactured by laminating
rial systems being used in the construction of advanced
plies of composite, usually pre-preg. Plies are cut
composite parts. Thermoset materials are systems
to the required orientation and shape and stacked
that take a permanent set or shape when cured. Cure
together in speciied directions to obtain the required
temperatures range from room temperature to 350
stiffness and strength. To obtain repeatable laminate
F for polyesters, vinylesters, epoxies, and cyanate
properties, approved manufacturing operations must
esters, around 450 F for Bismaleimide (BMI) mate-
be followed to exacting detail. This stack up is then
rials, and close to 700 F for polyimides. Cure
cured using heat and pressure to form a solid laminate.
pressures are usually less than 100 pounds per square
Process control is essential for manufacturing basic
inch (psi). Thermoplastic materials are noncuring sys-
structural materials such as aluminum or composite
tems that can be reshaped or reformed after the part
laminates. See igure 4-1 for laminate details.
has been processed or consolidated. Reshaping is
