S-glass

The standard abbreviation for "structural glass," which is a magnesia/alumina/silicate glass fiber reinforcement designed to provide the very high tensile strength required in high-performance composites.


Sandwich structure

A composite component featuring a lightweight core material (usually honeycomb, foam or balsa wood) placed between (hence the term "sandwich") two relatively thin, dense, high-strength, functional and/or decorative skins. (Also see core.)


Scrim

Low-cost, woven reinforcing fabric in an open mesh construction.


Sealant

A paste or liquid that, when applied to a joint, hardens in place to form a seal.


Secondary bonding

The joining, by means of adhesive, of two or more already cured composite parts.


Secondary structure

Aerospace structure that is not critical to flight safety. (In contrast to primary structure.)


Separator

A permeable layer that separates and also acts as a release film (e.g., porous, Teflon-coated fiberglass). Often placed between lay up and bleeder to facilitate bleeder systems' removal from laminate after cure.


Shear

An action or stress resulting from force applied in a direction parallel to the plane of adhesion between the surfaces of two adjacent components or layers, causing or tending to cause one to slide relative to the other.


Shear strength

The maximum shear stress that a material is capable of sustaining.


Sheet molding compound (SMC)

A ready-to-mold, glass fiber-reinforced polyester material primarily used in compression molding.


Shelf life

Length of time a material can be stored and continue to meet specification requirements, remaining suitable for its intended use. (Also see storage life.)


Shot

One complete cycle on an injection-molding machine.


Shot weight

The measured amount of compound required to completely fill the mold in injection or transfer molding processes.


Silicon carbide fiber

Reinforcing fiber with high strength and modulus; density is equal to that of aluminum. May be formed as wires by chemical vapor deposition onto a carbon-filament core, or as filaments. Used in both organic and metal-matrix composites.


Sizing

A chemical solution used to coat fiber filaments, facilitating operations such as weaving or braiding. Sizing protects the filament from water absorption and abrasion (to minimize fiber wear) and also can be used to bind together and stiffen warp yarns during weaving. Sizing is usually removed and replaced with finish before matrix application. Also called size.


Skin

The relatively dense laminate adhered to the outer surfaces of the core material in a sandwich structure.


Soft tool

Tool made of composites or a similar "soft" material that is vulnerable to damage during use, storage or transportation. (In contrast to hard tool.)


Solvent

A liquid capable of dissolving another substance. Certain solvents find application as evaporative diluents in paints or coatings and/or as cleaning solutions in maintenance operations.


Spec

Colloquial abbreviation for "specification"; describes the required properties and characteristics a particular material or part must have in order to be acceptable to a potential user.


Specific gravity

The density (mass per unit of volume) of a material divided by the density of water at a standard temperature.


Sprayup

A technique in which continuous strand roving is fed into a chopper gun, which chops the roving into predetermined lengths and sprays the chopped fiber, along with a measured amount of resin and catalyst, onto an open mold.


Stacking sequence

Arrangement of ply orientations and material components in a laminate specified with respect to some reference direction (also see ply schedule).


Staple

Collection of short filaments of spinnable length.


Stiffness

Measure of the resistance of a material to deformation. The ratio of applied stress to resulting strain for a particular material.


Storage life

The length of time a material can be stored and retain specific properties. (Also see shelf life.)


Strain

Deformation resulting from applied stress. Measured as the change in length per unit of length in a given direction; expressed as a percentage or in inches per inch.


Strand

See tow.


Stress

Internal resistance to change in size or shape, expressed in units of force (load) per unit area.


Stress concentration

A magnification of applied stress in the region of a notch, void, hole or inclusion.


Stress corrosion

Preferential attack of areas under stress in a corrosive environment that alone would not have caused corrosion.


Stress crack

External or internal cracks in a composite caused by tensile stresses. Cracking may be present internally, externally or in combination.


Structural adhesive

An adhesive used to transfer loads between adhesively bonded surfaces.


Structural bond

A bond that joins load-bearing components in an assembly.


Structural Reaction Injection Molding (SRIM)

A closed molding process employing a fiber reinforced preform or mat that is injected with a reactive resin to impregnate the fibers and cure quickly.


Structural repair manual (SRM)

Document prepared by an OEM that designates original structural materials (both composite and metal) used for a specific aircraft. It usually includes schematics for all parts and listings of fastener types and adhesives. It also suggests general repair methodologies and curing parameters (e..g., autoclave requirements) that will maintain structural integrity. Updated periodically by OEMs based on input from repair technicians.


Substrate

Material that provides the surface on which an adhesive-containing substance is applied for any purpose, such as bonding or coating.


Surfacing veil

A reinforcing fabric specifically designed to block out the fiber patterns of underlying reinforcements. It often adds ultraviolet protection to the structure as well. (Also see veil.)


Symmetric laminate

Laminate in which the stacking sequence for the plies located on one side of the geometric midplane are the mirror image of the stacking sequence on the other side of the midplane.


Synthetic fiber

Fiber made of materials other than glass or carbon, such as polyester.