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

Sandwich Panel (Composite) A panel consisting of two outer faces of wood, metal, or concrete bonded to a core of insulating foam.

Secant modulus The slope of a line drawn from the origin to any point on a nonlinear stress-strain curve. The ratio of nominal stress to corresponding strain at any specified point on the stress-strain curve. This measurement is usually employed in place of modulus of elasticity for materials whose stress-strain diagram does not demonstrate proportionality of stress to strain.

Sensor A device that responds to a physical stimulus (as heat, light, sound, pressure, magnetism, or motion) and transmits a resulting impulse for measurement or operating a control).

Shape The quality of a distinct object or body in having an external surface or outline of specific form or contour.

Shear A deformation in which planes of material slide with respect to one another.

Shear modulus of elasticity Tangent or secant modulus of elasticity of a material subjected to shear loading. Alternate terms are modulus of rigidity and modulus of elasticity in shear. Also, shear modulus of elasticity usually is equal to torsional modulus of elasticity. A method for determining shear modulus of elasticity of structural materials by means of a twisting test is given in ASTM E-143.

Shear strength Maximum shear stress that can be sustained by a material before rupture. It is the ultimate strength of a material subjected to shear loading. It can be determined in a torsion test where it is equal to torsional strength. The shear strength of a plastic is the maximum load required to shear a specimen in such a manner that the resulting pieces are completely clear of each other. It is reported in psi based on the area of the sheared edge (ASTM D-732).

Sinusoidal of, relating to, shaped like, or varying according to a sine curve or sine wave.

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

Sitecast Concrete poured and cured in its final position in a building.

Sizing The process of applying a material to a surface to fill pores and thus reduce the absorption of the subsequently applied adhesive or coating or to otherwise modify the surface. Also, the surface treatment applied to glass fibers used in reinforced plastics. The material used is sometimes called Size.

Skeleton A supporting internal framework.

Soap Bubble Gossamer skin, or film, of soap surrounding a globule of air.

Solid A three-dimensional shape or object, such as a sphere or a cube.

Space-covering Capacity to completely fill, or tile, all of the plane.

Space-filling Capacity to completely fill space. The combination of like or complementary figures in a three-dimensional packing continuously repeated in such a way that there is no unoccupied space.

Space frame, space truss A truss that spans with two-way action.

Span The distance between supports for a beam, girder, truss, vault, arch, or other horizontal structural device; to carry a load between supports.

Specific gravity The ratio of the density of a material as compared to the density of water at standard atmospheric pressure (1 ATM) and room temperature (73° F).

Sphere A closed surface consisting of the locus of points in space that are at a fixed distance, the radius r, from a fixed point, the center.

A sphere consists of all those points in space that are at some fixed, equal distance from a center point. The shortest connection between two points A and B on a sphere is an arc a of a great circle.

Sphericity Having the form of a sphere.

Spherical Array An ordered arrangement of items which has the form of a sphere.

Spin welding A process of fusing two objects together by forcing them together while one of the pair is spinning, until frictional heat melts the interface. Spinning is then stopped and pressure held until they are frozen together.

Spinneret A type of extrusion die, i.e., a metal plate with many tiny holes, through which a plastic melt is forced to make fine fibers and filaments. Filaments may be hardened by cooling in air, water, etc., or by chemical action.

Spinning Process of making fibers by forcing plastic melt through a spinneret.

Springback Degree to which a material returns to its original shape after deformation. In plastics and elastomers, it is also called recovery.

Stabilizer A brace, or bracing.

Stasis State of equilibrium or inactivity caused by opposing equal forces.

Steel Iron with a controlled amount of carbon, generally less than 1.7 percent.

Stiff, Stiffness Measure of resistance of plastics to bending. It includes both plastic and elastic behavior, so it is an apparent value of elastic modulus rather than a true value (ASTM D-747).

Equivalent Thickness

When a thermoplastic is specified as replacement for another material (a metal, for example) the new part often needs to have the same stiffness as the old one. Essentially, that means making sure that the new part, when subjected to the same load, will have the same deflection as the old part.

Deflection in bending is proportional 1/EI (E = modulus and I = moment of inertia), and I is proportional to t3 (t = thickness). Thus, the equivalent thickness of a plain, flat part to be made from a thermoplastic can be calculated by the following equation:

A thickness conversion factor (TCF) can be calculated on the basis of the cube root of the ratio of the moduli of the two materials. The Thickness Conversion Factors for Common Structural Materials Relative To Steel table lists the thickness conversion factors for several common structural materials relative to steel. These factors are based on the short-term, room temperature modulus values. Conversion factors based on the long-term and/or high temperature modulus (that is, the creep modulus) will be different from those shown here.

Thickness Conversion Factors for Common Structural Materials Relative To Steel

To find what thickness of a thermoplastic component is required for equal stiffness relative to steel, multiply the thickness of the steel component by the conversion factor, TCF, shown in the Thickness Conversion Factors for Common Structural Materials Relative To Steel table below:

To determine the thickness of material (Y) required for a thermoplastic part that will give the same stiffness as when the part is made with a material (Z) other than steel, multiply the thickness of the part in material (Z) by the TCF (from the Thickness Conversion Factors for Common Structural Materials Relative To Steel table) for the thermoplastic relative to steel, and then divide by the TCF for the material (Y) relative to steel.

The following calculations illustrate both methods of finding equivalent thickness when redesigning in polycarbonate.

To calculate the thickness of a part that, when made in polycarbonate, will have the same deflection as a 0.75 mm thick aluminum part at 73ºF (23ºC).

Using the moduli of the two materials:

Using the thickness conversion factors from the Thickness Conversion Factors for Common Structural Materials Relative To Steel table:

Remember that stiffness is proportional to thickness cubed (t3). This means an increase in thickness of only 26% will double part stiffness.

Stiffness-to-Weight Ratio Modulus of elasticity of the material of a body to the gravitational force acting on the body at the Earth's surface.

Strain Deformation under stress. The fractional change in dimension produced by a stress applied to a body. Change per unit length in a linear dimension of a part or specimen, usually expressed in % Strain, as used with most mechanical tests, is based on original length of the specimen. True or natural strain is based on instantaneous length, and is equal to: ln X l lo, where l is instantaneous length and lo is original length of the specimen. Shear strain is the change in angle between two lines originally at right angles. Criterion for use in designing parts. A measurable quantity from which many others are derived.

Strain energy Measure of energy absorption characteristics of a material under load up to fracture. It is equal to the area under the stress-strain curve, and is a measure of the toughness of a material. Splitting Resistance.

Strain hardening exponent Measure of increase in hardness and strength caused by plastic deformation. It is related to true stress and true strain by the equation: s = s0d h where s is true stress, s0 is true stress at unit strain, d is true strain and h is strain hardening exponent.

Strain Pattern Configuration of the deformation induced by the resolution of stress applied to a body.

Strain rate Time rate of elongation.

Strength The ability of a material to resist applied forces without yielding or fracturing. Strengths of materials may vary markedly with respect to the way they are deformed. A material that is strong and ductile under static load may appear weak and brittle under cyclic or impact stresses.

Strength of Materials - beams When a straight beam of uniform cross-sectional area is subjected to a perpendicular load, the beam bends. If shear is negligible, the vertical deflection is largely due to bending. Fibers on the convex side of the beam lengthen, and fibers on the concave side compress.

There is a neutral surface within any beam that contains the centroids of all sections and is perpendicular to the plane of the load for such deflections. In a uniform, symmetrical beam, the neutral axis of the beam is the horizontal, central axis. Tensile or compressive stress and strain on the neutral axis are essentially zero. At all other points within the beam, the stress is a tensile stress if the point lies between the neutral axis and convex surfaces of the beam, and is a compressive stress if the point lies between the neutral axis and concave surfaces of the beam, see the Bending of a Beam illustration.

The fiber stress s for any point (q) within the beam is calculated using the equation:

The maximum fiber stress in any section occurs at the points farthest from the neutral surface and at the section of greatest bending moment, i.e., when z = Max z, and M = Max M. Maximum fiber stress is given by the equation:

Such equations are valid if:

• The beam is of homogeneous material, so that it has the same modulus of elasticity in tension and compression.
• Plane sections remain planar.

If several loads are applied at the same time, the total stress and deflection at any point are found by superimposition. Compute the stress and deflection for each load acting on the point, and add them together.

Stress Force per unit area. When a system of opposing forces acts on a body the material is subject to some form of stress.

Stress amplitude One-half the range of fluctuating stress developed in a specimen in a fatigue test. Stress amplitude often is used to construct an S-N diagram.

Stress and Strain When a system of opposing forces acts on a body the material is subject to some form of stress. Normal stress is the ratio of applied load to the original cross sectional area: load divided by area. Strain is the change - due to force - in the linear size or shape of a body compared to its original size or shape. Normal strain or elongation is the measure of deformation of a material as a result of an applied load (stress). Strain is change in length divided by original length. Strain rate is the rate of change of strain with time. Stress-strain curve is a diagram in which corresponding values of stress and strain are plotted against each other: stress as ordinate and strain as abscissa. It provides information on tensile strength, elongation, and tensile modulus.

Stress Formulas - shear stress In addition to the normal stress calculated in the previous section, a plane at an angle to the force has a shear stress component. Here, unlike tensile and compressive stress, the force produces stress in the plane of the cross-section, i.e., the shear stresses are perpendicular to tensile or compressive stresses. The equations for calculating planar shear stress, based on the Representation of Shear Stress illustration are:

Stress Formulas - stress acting at an angle The standard stress equation is valid when the cross-section being considered is perpendicular to the force. However, when the cross-section is at an angle other than 90° to the force, as shown in the Diagram of Stress Acting at an Angle illustration, the equation must be adapted. These stresses are always less than the standard case, i.e., maximum normal stress occurs when = 0.

Stress Formulas - tensile or compressive stress Tensile or compressive stress “σ” is the force carried per unit of area and is expressed by the equation:

The force (P) produces stresses normal (i.e., perpendicular) to the cross section of the part. If the stress tends to lengthen the part, it is called tensile stress. If the stress tends to shorten the part, it is called compressive stress. (For compression loading, the part should be relatively short, or it must be constrained against lateral bucking.)

Stress Formulas - torsional stress When a stress acts to twist a component, it produces torsional stress. If a solid circular shaft, or shaft-like component, is subject to a twisting moment, or torsion, the resulting shear stress (q) is calculated by:

Stress ratio Ratio of minimum stress to maximum stress in one cycle of loading in a fatigue test. Tensile stresses are considered positive and compressive stresses negative.

Stress relaxation Decrease in stress in a material subjected to prolonged constant strain at a constant temperature. Stress relaxation behavior is determined in a creep test. Data often is presented in the form of a stress vs. time plot. Stress relaxation rate is the slope of the curve at any point.

Stress-Strain Diagram Graph of stress as a function of strain. It can be constructed from data obtained in any mechanical test where load is applied to a material, and continuous measurements of stress and strain are made simultaneously. It is constructed for compression, tension and torsion tests. An example is shown below.

Stress-Strain Ratio Stress divided by strain at any load or deflection. Below the elastic limit of a material, it is equal to tangent modulus of elasticity. An alternate term is the secant modulus of elasticity.

Stretch To distend or enlarge by tension.

Structural Of or pertaining to structure, structures, or construction.

Structural Element Component of a structure.

Structural Material Loadbearing materials of construction used in structural systems for framing; these include timber, wood, masonry, steel, and concrete.

Structural Foam A term originally used for cellular thermoplastic articles with integral solid skins having high strength-to-weight ratios, but now sometimes also used for high-density cellular materials that are strong enough for structural applications.

Structure Thing composed of elements which are organized or interrelated, for example a frame, network, building, etc.

Substrate (1) Structure which underlies or serves as the base or foundation. (2) Single crystal of semiconductor used as a basis for integrated circuit or transistor.

Supramolecular More complex than a molecule, e.g., composed of many molecules.

Surface Any locus of points extending in two dimensions. It is defined as an area. A surface may be flat (a plane surface) or curved and may be finite or infinite.

Surface Contact State in which two objects touch at a surface.

Surface Tension A property possessed by liquid surfaces whereby they appear to be covered by a thin elastic membrane in a state of tension, the surface tension being measured by the force acting normally across unit length in the surface. The phenomenon is due to unbalanced molecular cohesive forces near the surface.

Surface-to-Volume Ratio Surface area of an object divided by its volume.

Symmetry Correspondence in size, form, and arrangement of parts on opposite sides of a plane, line, or point. A symmetry of a figure is a transformation that leaves the figure invariant, in the sense that, taken as a whole, it looks the same after the transformation as it did before, although individual points of the figure may be moved by the transformation. Examples of transformations are rotations, translations, reflections, stretchings, or shrinkings of an object. Although there are two basic types of symmetry, mirror and rotational symmetry, probably the most familiar is rotational symmetry. This is because the shape of a ball is so familiar. A perfectly round ball is a sphere, and the sphere has perfect rotational symmetry. The rotational symmetry of any figure is determined by counting the number of times it repeats or reproduces itself in one rotation about an axis. The four kinds of rotational symmetry are twofold, threefold, fourfold, and sixfold. Balance.

Symmetrical Characterized by or exhibiting symmetry, or balance. Denotes, for example, any figure that can be divided into two parts that are mirror images of each other. The letter A, for example, is symmetrical, and does not change when viewed in a mirror, but the letter R is not. A symmetrical plane figure has at least one line that is an axis of symmetry, which divides it into two mirror images. (This is also known as bilateral symmetry.)

Symmetrical Array An arrangement, of figures or other items, which exhibit symmetry, or balance.

Syntactic Foams Composites made by mixing hollow microspheres of glass or plastic into fluid material to form a moldable, curable, lightweight, fluid mass, as opposed to foamed material, in which the cells are formed by gas bubbles released in the liquid plastic by either chemical or mechanical action.