Composites Terms

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See polyacrylonitrile.

Part consolidation

A design-and-fabrication process in which a number of previously discrete parts are combined in a single component to reduce or eliminate assembly operations and associated costs.

Parting film

A layer of thin plastic that prevents bagging materials from sticking to a part. It may be perforated to vent excess resin. It is removed after cure.


See poly p-phenylene-2,6-bensobisoxazale.

Peel ply

A layer of material that, when applied to a layup surface, can be removed from the cured laminate prior to bonding operations, leaving a clean, resin-rich surface suitable for bonding.

Peel strength

Strength of an adhesive bond between sheet materials; determined by applying parting stress at a right angle (perpendicular) to the plane of the adhesive interface.

Phenolic resin

A thermosetting resin produced by a condensation reaction of an aromatic alcohol with an aldehyde (usually phenol with formaldehyde).

Pin holes

Small voids open to and visible on the surface of a cured composite part.


Residual petroleum product used as a precursor in the manufacture of certain carbon fibers.

Planar winding

Filament winding method in which the filament path lays on a plane that intersects the winding surface.


General term for a range of high-molecular-weight thermoplastic or thermosetting polymers that have characteristics and properties that make them suitable for use in molding, casting, extruding or laminating processes.

Plied yarn

Two or more yarns collected together, with or without twist.


A single layer (or lamina) used to fabricate a laminate. Also, the number of single yarns twisted together to form a plied yarn.

Ply orientation

Acute angle (theta) - including 90° - between a reference direction and the ply principal axis. the ply orientation is positive if measured counterclockwise from the reference direction and negative if measured clockwise.

Ply schedule

A prescribed sequence for laying up individual plies or layers to form a laminate, indicating the arrangement of plies by material type and other characteristics, such as fiber orientation.

Poisson's ratio

When a material is stretched, its cross-sectional area changes as well as its length. Poisson's ratio is the constant relating these changes in dimensions, and is defined as the ratio of the change in width per unit width to the change in length per unit length.

Polar winding

Filament winding in which the filament path passes tangent to the polar opening at one end of the chamber and tangent to the opposite side of the polar opening at the other end of the chamber.

Poly p-phenylene-2,6-bensobisoxazale (PBO)

A relatively new polymer fiber, with a modulus and tensile strength almost double that of aramid fiber and a decomposition temperature almost 100°C/212°F higher.

Polyacrylonitrile (PAN)

Polymer base material that is spun into a fiber form and used as a precursor in the manufacture of certain carbon fibers.


Thermosetting resins produced by dissolving unsaturated, generally linear, alkyd resins in a vinyl-type active monomer, such as styrene. The resins are usually furnished in solution form, but powdered solids are also available.

Polyetherimide (PEI)

A high-performance thermoplastic resin with repeating aromatic imide and ether molecular units. Characterized by high strength and rigidity over a wide range of temperatures, as well as long-term heat resistance, highly stable dimensional properties and broad chemical resistance.

Polyimide (PI)

Highly heat-resistant thermoplastic polymer resin.


Large organic molecule formed by combining many smaller molecules (monomers) in a regular pattern.

Polymer alloy (or polymer blend)

A blend of polymers, copolymers or elastomers.


Chemical reaction that links monomers to form polymers.


The presence of voids open to the surface of a solid material into which air or liquids may pass.


Exposure of a molded component to elevated temperature after initial in-mold curing, performed for the purpose of improving the component's mechanical properties. Postcure may occur after demolding and is often done without the use of pressure.

Pot life

Length of time in which a catalyzed thermosetting resin retains sufficiently low viscosity for processing.


Full or partial hardening of a resin or adhesive before pressure is applied.


Material from which carbon fibers are made by pyrolysis. Common precursors are polyacrylonitrile (PAN), rayon and pitch.


Pre-shaped fibrous reinforcement, supplied without matrix, but often containing a binder to facilitate manufacture and maintain shape. A preform's fiber components are distributed or arranged, typically on a mandrel or mock-up, to approximate the contours and thickness of the finished part, saving time and labor during the molding process.


Fibrous reinforcement (sheet, tape, tow, fabric or mat) preimpregnated with resin and capable of storage for later use. For thermosetting matrices the resin is usually partially cured or otherwise brought to a controlled viscosity, called B-stage. Additives (e.g., catalysts, inhibitors and flame retardants) are used to obtain specific end-use properties and/or improve processing, storage and handling characteristics.

Primary structure

An aerospace critical load-bearing structure; if damaged the aircraft or space vehicle cannot operate safely.

Prime contractors

Referred to as "primes"; companies that are awarded government contracts and usually work with subcontractors (or "subs") who provide individual and specific components or systems relevant to the contract. Primes often team on contracts, sharing portions of the contract funding.

Principal axis

Laminate coordinate axis that coincides with the direction of maximum inplane Young's modulus. Within a ply, for a balanced weave fabric either warp or fill direction may be chosen. (See also laminate coordinate axes and x-axis.)


A chemical which hastens the reaction between a catalyst and a resin (also known as an accelerator).


A test part not intended for commercial release, which establishes design, material and fabrication parameters for a new product. Also, to fabricate such a test part (a process that can entail multiple iterations to arrive at final/commercial part design).


Local areas on prepreg where material has blistered and pulled away from the separator film or release paper.


Continuous process for manufacturing composite rods, tubes and other linear structures that have constant cross-sections. The process involves drawing continuous reinforcement through a resin-impregnation bath (or an alternative resin-impregnation method is used), then pulling the wetout material through a heated shaping die, where cure takes place, securing the desired cross-section before the laminate departs from the die.


A break in the composite skin of a sandwich structure that may or may not go through to the core material or completely through the part thickness.


The decomposition or other transformation of a compound caused by exposure to heat.