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Composites Terminology Database

This glossary is not an exhaustive list, however it does contain the majority of terms used in the composites industry and as part of the management of our website, it is updated on a continual basis.

The terms are accessed by clicking on the appropriate letter below, all the terms beginning with the letter S can be found below.

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Terms S

Click on a term and it will take you to the coresponding explanation.

The S-basis property allowable is the minimum value specified by the appropriate federal, military, Society of Automotive Engineers, American Society for Testing and Materials, or other recognised and approved specifications for the material.
"A-Basis" "B-Basis"

A magnesium aluminosilicate composition that is especially designed to provide very high tensile strength glass filaments. S-glass and S-2 glass fibres have the same glass composition but different finishes (coatings). S-glass is made to more demanding specifications, and S-2 is considered the commercial grade.
"glass" "C-glass" "D-glass""E-glass" "M-glass"

S-N diagram
A plot of stress (S) against the number of cycles to failure (N) in fatigue testing. A log scale is normally used for N. For S, a linear scale is often used, but sometimes a log scale is used here, too. Also, a representation of the number of alternating stress cycles a material can sustain without failure at various maximum stresses.

Ultimate strength in shear, measured in kpsi.

sandwich constructions
Panels composed of a lightweight core material, such as honeycomb, foamed plastic, and so forth, to which two relatively thin, dense, high-strength or high-stiffness faces or skins are adhered.

satin weave
See harness satin
"harness satin"

A type of finish having a satin or velvety appearance, specified for plastics or composites.

See short beam shear
"short beam shear"

scarf joint
A joint made by cutting away similar angular segments on two adherents and bonding the adherents with the cut areas fitted together. See also lap joint.
"lap joint"

A low-cost reinforcing fabric made from continuous filament yarn in an open-mesh construction. Used in the processing of tape or other B-stage material to facilitate handling. Also used as a carrier of adhesive, to be used in secondary bonding.

A material applied to a joint in paste or liquid form that hardens or cures in place, forming a seal against gas or liquid entry.

secant modulus
Idealised Young's modulus derived from a secant drawn between the origin and any point on a non-linear stress-strain curve. On materials whose modulus changes with stress, the secant modulus is the average of the zero applied stress point and the maximum stress point being considered. See also tangent modulus.
"tangent modulus"

secondary bonding
The joining together, by the process of adhesive bonding, of two or more already cured composite parts, during which the only chemical or thermal reaction occurring is the curing of the adhesive itself.

secondary structure
In aircraft and aerospace applications, a structure that is not critical to flight safety.

self-extinguishing resin
A resin formulation that will burn in the presence of a flame but will extinguish itself within a specified time after the flame is removed.

self-skinning foam
A urethane foam that produces a tough outer surface over a foam core upon curing.

The woven-edge portion of a fabric parallel to the warp.

In plastics, materials that exhibit localised crystallinity. See also crystalline plastic.
"crystalline plastic"

A permeable layer that also acts as a release film. Porous Teflon-coated fibreglass is an example. Often placed between lay-up and bleeder to facilitate bleeder system removal from laminate after cure.

service temperature
Continuous use temperature in degrees F.

set up
To harden, as in curing of a polymer resin.

The irrecoverable or permanent deformation or creep after complete release of the force producing the deformation.

shear edge
The cut-off edge of the mould.

shear modulus
The ratio of shearing stress to shearing strain within the proportional limit of the material.

shear strain
The tangent of the angular change, caused by a force between two lines originally perpendicular to each other through a point in a body. Also called angular strain.

shear strength
The maximum shear stress that a material is capable of sustaining. Shear strength is calculated from the maximum load during a shear or torsion test and is based on the original cross-sectional area of the specimen.

shear stress
The component of stress tangent to the plane on which the forces act.

An action or stress resulting from applied forces that causes or tends to cause two contiguous parts of a body to slide relative to each other in a direction parallel to their plane of contact. In interlaminar shear, the plane of contact is composed primarily of resin. See also shear strength and shear stress above.
"shear strength" "shear stress"

sheet moulding compound (SMC)
A composite of fibres, usually a polyester resin, and pigments, fillers, and other additives that have been compounded and processed into sheet form to facilitate handling in the moulding operation.

shelf life
The length of time a material, substance, product, or reagent can be stored under specified environmental conditions and continue to meet all applicable specification requirements and/or remain suitable for its intended function.

shell tooling
A mould or bonding fixture consisting of a contoured surface shell supported by a substructure to provide dimensional stability.

A device for gathering filaments into a strand, in glass fibre forming.

Shore hardness
A measure of the resistance of material to indentation by 3 spring-loaded indenter. The higher the number, the greater the resistance. Normally used for rubber materials.

short beam shear (SBS)
A flexural test of a specimen having a low test span-to-thickness ratio (for example, 4:1), such that failure is primarily in shear.

short shot
Injection of insufficient material to fill the mould.

shot capacity
The maximum weight of material an injection machine can provide from one forward motion of the ram, screw, or plunger.

The relative change in dimension from the length measured on the mould when it is cold to the length of the moulded object 24 hours after it has been taken out of the mould.

silicon carbide
Reinforcement, in whisker, particulate, and fine or large fibre, that has application as metal matrix reinforcement because of its high strength and modulus, density equal to that of aluminium, and comparatively low cost. As a whisker or particulate, it gives the composite isotropic properties and is easily machined.

silicone plastics
Plastics based on resins in which the main polymer chain consists of alternating silicon and oxygen atoms, with carbon-containing side groups. Derived from silica (sand) and methyl chlorides and furnished in different molecular weights, including liquids and solid resins and elastomers.

single-circuit winding
A winding in which the filament path makes a complete traverse of the chamber, after which the following traverse lies immediately adjacent to the previous one.

single-curvature panel
Panel with slightly curved shaped, frequently a sandwich panel.

sink mark
A shallow depression or dimple on the surface of an injection-molded part due to collapsing of the surface following local internal shrinkage after the gate seals. An incipient short shot.

The bonding of powders by solidstate diffusion, resulting in the absence of a separate bonding phase. The process is generally accompanied by an increase in strength, ductility, and, occasionally, density.

Any treatment consisting of starch, gelatin, oil, wax, or other suitable ingredient applied to yarn or fibres at the time of formation to protect the surface and aid the process of handling and fabrication or to control the fibre characteristics.

The treatment contains ingredients that provide surface lubricity, and binding action but, unlike a finish, contains no coupling agent. Before final fabrication into a composite, the size is usually removed by heat cleaning, and a finish is applied.

sizing content
The percent of the total strand weight made up by the sizing; usually determined by burning off or dissolving the organic sizing; known as loss on ignition.

A continuous filament, strand, yarn, or roving, wound up to some measurable length and usually used to measure various physical properties.

The relatively dense material that may form the surface of a cellular plastic or of a sandwich.

The extension of a motorcase from the tangency plane, used for interstage connections, usually wound or laid up as an integral part of the case.

slenderness ratio
The unsupported effective length of a uniform column divided by the least radius of gyration of the cross-sectional area.

slip angle
The angle at which a tensioned fibre will slide off a filament-wound dome. If the difference between the wind angle and the geodesic angle is less than the slip angle, fibre will not slide off the dome. Slip angles for different fibre-resin systems vary and must be determined experimentally.

A number of staple or continuous filament fibres aligned in a continuous strand without twist. Pronounced ''slyver.'' See also strand.

slurry preforming
Method of preparing reinforced plastic preforms by wet processing techniques similar to those used in the pulp moulding industry. For example, glass fibres suspended in water are passed through a screen that passes the water but retains the fibres in the form of a mat.

See sheet moulding compound
"sheet moulding compound"

softening range
The range of temperatures in which a plastic changes from a rigid to a soft state. Actual values will depend on the test method. Sometimes erroneously referred to as softening point.

The process of swelling, gelling, or dissolving a resin by a solvent or plasticiser.

specific gravity
The density (mass per unit volume) of any material divided by that of water at a standard temperature.

specific heat
The quantity of heat required to raise the temperature of a unit mass of a substance 1¡ under specified conditions.

specific properties
Material properties divided by the material density.

See superplastic forming
"superplastic forming"

A fan-like surface defect near the gate on a part.

The joining of two ends of glass fibre yarn or strand, usually by means of an air-drying adhesive.

Technique in which a spray gun is used as an applicator tool. In reinforced plastics, for example, fibrous glass and resin can be simultaneously deposited in a mould. In essence, the roving is fed through a chopper and ejected into a resin stream that is directed at the mould by either of two spray systems.

In foamed plastics, fast-reacting urethane foams or epoxy foams are fed in liquid streams to the gun and sprayed on the surface. On contact, the liquid starts to foam. spray-up

sprayed metal moulds
Moulds made by spraying molten metal onto a master until a shell of predetermined thickness is achieved. The shell is then removed and backed up with plaster, cement, casting resin, or other suitable material. Used primarily as a mould in the sheet forming process.

spring constant
The number of pounds required to compress a spring or specimen 25 mm (1 in.) in a prescribed test procedure.

A single hole through which thermoset moulding compounds are injected directly into the mould cavity.

spun roving
A heavy low-cost glass or aramid fibre strand consisting of filaments that are continuous but doubled back on themselves.

In carbon fibre forming, the process used to render the carbon fibre precursor infusible prior to carbonisation.

stacking sequence
A description of a laminate that details the ply orientations and their sequence in the laminate.

Heating a premixed resin system, such as in a prepreg, until the chemical reaction (curing) starts, but stopping the reaction before the gel point is reached. Staging is often used to reduce resin flow in subsequent press moulding operations.

standard deviation
A measure of dispersion of data from the average. The root mean square of the individual deviation from the average.

staple fibres
Fibres of spinnable length manufactured directly or by cutting continuous filaments to short lengths (usually 12.7 to 50 mm, or 1/2 to 2 in. long; 1 to 5 denier).

starved area
An area in a plastic part that has an insufficient amount of resin to wet out the reinforcement completely. This condition may be due to improper wetting, impregnation, or resin flow; excessive moulding pressure; or improper bleeder cloth thickness.

starved joint
An adhesive joint that has been deprived of the proper film thickness of adhesive due to insufficient adhesive spreading or to the application of excessive pressure during the lamination process.

static fatigue
Failure of a part under continued static load. Analogous to creep rupture failure in metals testing, but often the result of ageing accelerated by stress.

static modulus
The ratio of stress to strain under static conditions. It is calculated from static stress-strain tests, in shear, compression, or tension. Expressed in force per unit area.

static stress
A stress in which the force is constant or slowly increasing with time, for example, test of failure without shock.

A measure of modulus. The relationship of load and deformation. The ratio between the applied stress and resulting strain. A term often used when the relationship of stress to strain does not conform to the definition of Young's modulus. See also stress-strain.

Metal pieces inserted between die halves. Used to control the thickness of a press-moulded part. Not a recommended practice, because the resin will receive less pressure, which can result in voids.

storage life
The period of time during which a liquid resin, packaged adhesive, or prepreg can be stored under specified temperature conditions and remain suitable for use. Also called shelf life.

strain gage
Device to measure strain in a stressed material based on the change in electrical resistance.

strain relaxation
Reduction in internal strain over time. Similar molecular processes occur as in creep, except that the body is constrained.

strain, axial
See axial strain
"axial strain"

strain, initial
See initial strain
"initial strain"

strain, residual
See residual strain
"residual strain"

strain, shear
See shear strain
"shear strain"

strain, transverse
See transverse strain
"transverse strain"

strain, true
See true strain
"true strain"

Elastic deformation due to stress. Measured as the change in length per unit of length in a given direction, and expressed in percentage or mm/mm (in./in.).

strand count
The number of strands in a plied yarn. The number of strands in a roving.

strand integrity
The degree to which the individual filaments making up the strand or end are held together by the applied sizing.

strand tensile test
A tensile test of a single resin-impregnated strand of any fibre.

Normally an untwisted bundle or assembly of continuous filaments used as a unit, including slivers, tows, ends, yarn, and so forth. Sometimes a single fibre or filament is called a strand.

strength, compressive
See compressive strength
"compressive strength"

strength, flexural
See flexural strength
"flexural strength"

strength, shear
See shear strength
"shear strength"

strength, tensile
See tensile strength
"tensile strength"

strength, wet
See wet strength
"wet strength"

strength, yield
See yield strength
"yield strength"

stress concentration
On a macromechanical level, the magnification of the level of an applied stress in the region of a notch, void, hole, or inclusion.

stress corrosion
Preferential attack of areas under stress in a corrosive environment, where such an environment alone would not have caused corrosion.

stress crack
External or internal cracks in a plastic caused by tensile stresses less than that of its short-time mechanical strength, frequently accelerated by the environment to which the plastic is exposed. The stresses that cause cracking may be present internally or externally or may be combinations of these stresses. See also crazing.

stress cracking
The failure of a material by cracking or crazing some time after it has been placed under load. Time-to-failure may range from minutes to years. Causes include moulded-in stresses, post fabrication shrinkage or warpage, and hostile environment.

stress relaxation
The decrease in stress under sustained, constant strain. Also called stress decay.

stress, fracture
See fracture stress
"fracture stress"

stress, initial (instantaneous)
See initial (instantaneous) stress
"initial (instantaneous) stress"

stress, nominal
See nominal stress
"nominal stress"

stress, normal
See normal stress
"normal stress"

stress, relaxed
See relaxed stress
"relaxed stress"

stress, residual
See residual stress
"residual stress"

stress, shear
See shear stress
"shear stress"

stress, tensile
See tensile stress
"tensile stress"

stress, torsional
See torsional stress
"torsional stress"

stress, true
See true stress
"true stress"

stress-concentration factor
The ratio of the maximum stress in the region of a stress concentrator, such as a hole, to the stress in a similar strained area without a stress concentrator.

stress-strain curve
Simultaneous readings of load and deformation, converted to stress ;and strain, plotted as ordinates and abscissa, respectively, to obtain a stress-strain diagram.

Stiffness at a given strain.

The internal force per unit area that resists a change in size or shape of a body. Expressed in force per unit area.

structural adhesive
Adhesive used for transferring required loads between adherents exposed to service environments typical for the structure involved.

structural bond
A bond that joins basic load bearing parts of an assembly. The load may be either static or dynamic.

structural reaction injection moulding (S-RIM)
Evolution of two other plastic moulding processes - RIM and RTM. S-RIM uses the fast polymerisation reactions of RIM-type polymers, its intensive resin mixing procedures, and its rapid resin injection rates. S-RIM also employs preforms like RTM to obtain composite mechanical properties.
"reaction injection moulding"

superplastic forming (SPF)
A strain rate sensitive metal forming process that uses characteristics of materials exhibiting high elongation-to-failure.

Surface of Revolution
Part shape such as a rocket motor case; includes cylindrical, round or any shape that does not have re-entrant curvature.

surface preparation
Physical and/or chemical preparation of an adherend to make it suitable for adhesive bonding.

surface resistivity (electrical)
The surface resistivity of a material is the ratio of the potential gradient parallel to the current along its surface to the current per unit width of surface. Surface resistivity is numerically equal to the surface resistance between opposite sides of a square of any size when the current flow is uniform.

surface treatment
A material (size or finish) applied to fibrous material during the forming operation or in subsequent processes. For carbon fibre surface treatment, the process used to enhance bonding capability of fibre to resin.

surfacing mat
A very thin mat, usually 180 to 510 mm (7 to 20 mil) thick, of highly filamentized fibreglass, used primarily to produce a smooth surface on a reinforced plastic laminate, or for precise machining or grinding.

symmetrical laminate
A composite laminate in which the sequence of plies below the laminate midplane is a mirror image of the stacking sequence above the midplane.

syntactic foams
Composites made by mixing hollow microspheres of glass, epoxy, phenolic, and so forth, into fluid resins (with additives and curing agents) to form a mouldable, curable, lightweight, fluid mass; as opposed to foamed plastic, in which the cells are formed by gas bubbles released in the liquid plastic by either chemical or mechanical action.

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