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Key Terms

*The definitions below have been adapted from the glossaries compiled by Composites One ® and NetComposites. For a more complete list please visit their websites.


  • Activator/Accelerator: An oxidizing agent used to accelerate the chemical reaction (curing) between a catalyst and resin.
  • Addition Polymerization: Chemical reaction in which monomers (chemical subunits) chemically bond to each other to form long-chain molecules (polymers) without forming chemical byproducts.
  • Additive: A material incorporated into polymer resins to modify their physical properties.
  • Amine Resin: A synthetic resin derived from the reaction of urea, thiourea, melamine or allied compounds with aldehydes, particularly formaldehyde.
  • ASTM: American Society of Test Methods.
  • Autoclave: A vessel capable of simultaneous application of heat and pressure, usually 50-200 psi, in order to compact the composite during curing.


  • Bleeder: A cloth material (commonly non-woven) that provides a pathway for gas, resin, and volatiles to escape from a composite during vacuum bagging.
  • Braid/Braider: A narrow tubular fabric produced by intertweaving yarns according to a controlled angle and pattern.


  • Carbon Fiber: Fibers (~7-10 microns in diameter) produced by the pyrolysis of organic precursor fibers (commonly polyacrylonitrile (PAN) and pitch) under an inert atmosphere.
  • Catalyst (Hardener): A chemical compound that facilitates the reaction of thermoset matrices via the production of free radicals.
  • Composite: A material composed of two or more distinct phases. A composite contains a matrix and a reinforcement that is chemically or mechanically bonded to via an interface.
  • Compressive Strength: The compressive load a material can sustain before buckling or crushing occurs.
  • Condensation Polymerization: A chemical reaction between two or more monomers and which produces a molecular byproduct (commonly water).
  • Continuous Reinforcement: Filaments or fibers that span a substantial fraction of the length of a composite.
  • Copolymer: A polymer which is composed of two or more distinct monomer chemical subunits.
  • Creep: The slow deformation of a material under sustained load over time.
  • Cross-linking: The chemical links that form between polymers to create a 3D network in thermoset matrices.
  • Cure Cycle: The combination of time, temperature, and pressure used to cure thermoset matrix composite.
  • Curing Agent: A chemical molecule that reacts with a resin to form a cross-linked matrix.


  • Deep-Draw Mold: A mold whose depth is large relative to the part thickness. Fabrics can wrinkle, tear, or exhibit other defects during deep-draw processes.
  • Delamination: The debonding of neighboring composite layers.
  • Density: The mass per unit volume of a material.


  • E Glass: Electrical glass used in general-purpose composites. It is a borosilicate-based glass.
  • Elasticity: The ability of a material to recover the original size and shape after the application of a load.
  • Ejection (Demolding): Removing a composite part from its mold.
  • Epoxy: An important composite matrix resin containing epoxide groups.
  • Extruder: A machine that pushes molten plastic through a die.


  • Fiber: Used as a reinforcement material within composites.
  • Filler: An additive that is used to reduce plastics usage, improve properties, and lower cost.
  • Flexural Strength: The maximum stress of a material in bending.


  • Gelation: The point during resin cure when the material transitions from a liquid (flows) to a solid (no flow).
  • Glass Transition Temperature (Tg): The temperature range over which polymers transition from partially crystalline to amorphous rubbery materials.


  • Honeycomb: A cellular material manufactured from metal or resin-impregnated sheets and used as a lightweight core material in sandwich composites.


  • Initiator: A chemical used to create free radicals.
  • Instron: A mechanical testing machine to determine the mechanical properties of materials.
  • Isotropic: Having uniform properties in all directions.


  • Laminate: A composite material composed of layers, commonly fiber-based textiles infiltrated with a polymer matrix.
  • Lay-up: The placement of layers of matrix and reinforcement at determined orientations prior to curing.


  • Matrix: The continuous phase surrounding the reinforcement in a composite.
  • Micron: One micron is 0.001 millimeter.
  • Mold: The shaped tooling into or onto which the composite material is placed prior to curing.
  • Monomer: A molecular unit capable of chemical reactions to form a polymer chain.


  • Polymer: Chain molecule composed of many monomers.
  • Polymerization: Chemical bonding of monomers during the curing reaction.
  • Ply: A layer within a laminate.
  • Prepreg: A fabric with the matrix material already present prior to lay-up.


  • Release Agent: Chemical added to the mold surface to reduce adhesion of the composite part to the mold.


  • Sandwich Composite: A composite composed of face materials that are attached to a lightweight core, commonly a honeycomb or plastic foam.
  • Stiffness: The relationship of load to deformation.
  • Stress-Strain Curve: The plot of the load per unit area on a sample versus the material strain.
  • Surfactant: A chemical additive that modifies the surface properties of a material.


  • Tensile Strength: The maximum stress a material can sustain prior to failure.
  • Thermoplastic: A polymer matrix capable of being repeatedly heated to soften and cooled to harden the material.
  • Thermoset: A polymer matrix that chemically crosslinks during curing and cannot be re-formed by heating.


  • Ultimate Tensile Strength: The final stress sustained by a specimen under tensile loading.


  • Vacuum Bag Molding: A fabrication method which pulls a vacuum on composite lay-up under an impermeable bag. Atmospheric pressure compresses the composite-lay-up to reduce voids and increase part quality.
  • Viscosity: The resistance of a liquid to flow.


  • Weave: The interlaced pattern of a fabric .