TOOLED THICK COMPOSITES by ARVEN H. SAUNDERS III ...

to carry the load. The most commonly used fibers are graphite and fiberglass. Fibers can be
oriented in a single direction (unidirectional) or can be woven into a variety of fabrics onto which
the resin can be impregnated. Most of the plies in flight-critical thick composites are of
unidirectional prepreg material to provide the tensile strength along its length. The multiplicity of
fibers held in relative position to each other by the cured resin accounts for the tremendous
strength-to-weight advantage of composite material.
The design of a thick composite part specifies a target value or range for the final cured
part thickness and weight. It also defines a set of optional plies that can be added to the
nominal configuration in the case when the estimated per ply cured thickness is less than the
nominal ply thickness; it also specifies certain plies that can be omitted from the layup if the
estimated per ply thickness is above the nominal. Kitting specifies exactly which and how many
plies will be contained in the laminate.
Properties of the prepreg are tested and evaluated to provide input to the kitting
decision. These properties include resin content, fiber areal weight, gel temperature, and
estimated ply thickness. However, the actual ply thickness within the cured laminate is
stochastic in nature and depends on the cure and consolidation process.
Through the layup process the laminate is built ply by ply, much like each page can be
stacked up horizontally to form a book. Although automation of the layup process has been
envisioned for a long time, most thick laminates are still layed up by hand. The mechanic lays
each ply down, the prepreg resin “tack” or stickiness keeping it stuck to the previous plies in the
stack. The bond tool is the vessel that holds the laminate to its desired geometry during layup
and cure.
Computer-controlled cure control systems are commonly used to perform a cure cycle
recipe of predefined segments comprising temperature and pressure setpoints over time. The
combination of heat and pressure over time transforms the laminate into a durable and fatigue-
resistant component.
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