Composites Terms

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Intermediate stage in the polymerization reaction of some thermosets in which the material softens with heat and is plastic and fusible but does not entirely dissolve or fuse. The resin of an uncured prepreg or premix is usually in this state. (See also A-stage, C-stage.)

Bag molding

Molding technique in which the composite structure is placed in a rigid mold and covered with a flexible impermeable layer of film whose edges are sealed, followed by consolidation and/or curing with pressure applied by vacuum, autoclave, press or inflation of the bag.

Balanced design

In filament winding, a winding pattern designed so that the stresses in all filaments are equal.

Balanced laminate

Any laminate that contains one ply of minus theta orientation, with respect to the principal axis of the laminate, for every identical ply with a plus theta orientation (e.g., a laminate with a principle axis of 0° combined with an equal number of plies having -45° and +45° orientations.

Barcol hardness

A surface hardness value obtained by measuring the penetration resistance of a given material to a sharp steel point under a spring load. The Barcol Impressor is an instrument that measures hardness on a 0-100 scale.

Basket weave

Woven reinforcement wherein two or more warp threads go over and under two or more filling threads in a repeating pattern; less stable than plain weave but produces a flatter, stronger, more pliable fabric (see plain weave).


Material made by the same process at the same time having identical characteristics throughout (same as lot).

Bias fabric

Fabric in which warp and fill fibers are at an angle to the length.

Biaxial fabric

Fabric with two non-interwoven layers - a unidirectional warp (0°) layer and a unidirectional weft (90°) layer - which are bonded together, usually by through-the-thickness stitching, to form a single sheet of fabric. (See also triaxial fabric, quadraxial fabric.)

Biaxial winding

Filament winding wherein helical bands are laid in sequence, side by side, with no fiber crossover.

Bidirectional laminate

Laminate with fibers oriented in more than one direction on the same plane.


The agent applied to glass mat or preforms to bond the fibers prior to laminating or molding.

Bismaleimide (BMI)

Type of thermoset polyimide that cures by an additional reaction, thus avoiding formation of volatiles. Exhibits temperature capabilities between those of epoxy and polyimide.

Bleeder cloth

Layer of woven or nonwoven material, not intended to become a part of the composite, that allows excess gas and resin to escape during cure.


Excess liquid resin appearing at the surface of the composite structure, particularly during filament winding.


See bulk molding compound.


See bismaleimide.

Bond ply

Ply or fabric patch that comes in contact with the honeycomb core during repair.

Bond strength

The adhesion between bonded surfaces. As measured by load/bond area, the stress required to separate a layer of material from another material to which it is bonded.

Boron fiber

Fiber produced by chemical vapor deposition of boron onto a core material, usually a tungsten filament. Because of the deposition process, a boron fiber is of a fairly large diameter, typically about 0.4 mils, and is thus often referred to as a wire.


Textile process that intertwines into a pattern three or more strands, yarns or tapes, typically into a tubular shape.


Separation or breakage of fibers when the edges of a composite part are drilled or cut.


Loosely woven material that does not come in contact with the resin but serves as a continuous vacuum path over a part in production.


Fabric plies over a curved edge that do not come in full contact with the core material. Also, excess resin that has formed on edges during cure.


General term for fibers woven into fabrics that may or may not be impregnated with resin; usually furnished in rolls.


A fire retardant (halogen) used to reduce or eliminate a resin's tendency to burn.


Failure mode usually characterized by unstable lateral deflection, rather than breakage, under compressive force.

Bulk molding compound (BMC)

A premixed blend of thermosetting resin, reinforcements, catalysts and fillers for use in compression-, transfer- or injection-molding processes.


General term for a collection of essentially parallel filaments.