Tangent Modulus The instantaneous slope at any point on the stress-strain curve, while the secant modulus is the slope of a line drawn from the origin to any point on a nonlinear stress-strain curve.
Technology The practice, description and terminology of any or all of the applied sciences which have practical value and/or industrial use.
Tenacity The tensile stress expressed as force per unit linear density of an unstrained specimen.
Tendon A steel strand used for prestressing a concrete member.
Tensile Modulus An indicator of the stiffness of a part. It is basically the applied tensile stress, based on the force and cross-sectional area, divided by the observed strain at that stress level. It is generally constant before the material approaches the point at which permanent deformation will begin to occur. It is most easily observed as the slope of the stress-strain curve prior to the yield point. Because plastics are viscoelastic materials, stress-strain relationships are nonlinear and curved (usually convex upward). If the curvature is pronounced, the stress-strain ratio must be either a tangent modulus or a secant modulus. The tangent modulus is the instantaneous slope at any point on the stress-strain curve, while the secant modulus is the slope of a line drawn from the origin to any point on a nonlinear stress-strain curve. According to GE Plastics, tensile modulus is a much more accurate measure than flexural modulus and should be used in all cases to predict part deflection or strain level.
Tensile Modulus of Elasticity Tangent or secant modulus of elasticity of a material subjected to tensile loading. Alternate terms are Young's modulus and modulus of elasticity in tension. It can be measured in a tension test or in a dynamic test where it is related to resonant frequency on a cylindrical rod by the equation:
where E is modulus of elasticity; 1, length of the rod; p, density; f, resonant frequency; k, radius of gyration of the rod about an axis normal to the rod axis and plane of motion (d/4 for cylindrical rods) and j, a constant dependent on the mode of vibration. Tensile modulus of elasticity is approximately equal to compressive modulus of elasticity within the proportional limit.
Tensile Strength The ability of a structural material to withstand bending and stretching forces.
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.
Tensile Strain Strain caused by bending and/or stretching a material
Tensile Structure Structure which relies primarily on the tensile strength of its components for its strength and stability. Tensile structures have a primary advantage of not being subject to buckling failure and therefore not depending on the stiffness of materials but on their tensile strength. High-tensile-strength wires of cold drawn steel, made into cables, are the material for a variety of tent-like roofs that can span almost unlimited distances. With anticlastic (saddle-shaped) curvature, cable stays, or other ways of restraining the cable net, hanging roofs can be made rigid against wind uplift and flutter.
Tensile Yield Strength The maximum stress that can be applied to a part being pulled apart without suffering permanent deformation. According to GE Plastics, it is a much more accurate measure than flexural strength and should be used in all cases to determine the acceptability of a material or design for an application. The ability of a structural material to withstand bending and stretching forces.
Tension A bending or stretching force; to stretch.
Tension Impact Test Method for determining energy required to fracture a specimen under shock tensile loading (ASTM D-1822).
Tension Test Method for determining behavior of materials under axial stretch loading. Data from test are used to determine elastic limit, elongation, modulus of elasticity, proportional limit, reduction in area, tensile strength, yield point, yield strength and other tensile properties. Tension tests at elevated temperatures provide creep data. Procedures for tension tests of metals are given in ASTM E-8. Methods for tension tests of plastics are outlined in ASTM D-638, ASTM D-2289 (high strain rates), and ASTM D-882 (thin sheets).
Tetrahedron (triangular pyramid) A solid figure bounded by four triangular faces. A regular tetrahedron has four congruent equilateral triangles as its faces.
Thermal Degradation Deterioration of the material by heat, characterized by molecular scission.
Thermal Expansion Increase of dimensions or volume of a specimen due to an increase in its temperature.
Thermal Stress, Designing for Thermal expansion and contraction are important considerations in plastics design, and are often overlooked. Expansion-contraction problems often arise when two or more parts made of materials having different coefficients of thermal expansion are assembled at a temperature other than that of the end-use environment. When the assembled parts go into service in the end-use environment, the two materials react differently, and the resultant thermal stresses can cause unexpected part failure.
So, you must consider the effects of thermal expansion and/or contraction early in the design of parts that involve close fits, molded-in inserts, and mechanical fastenings. Coefficients of thermal expansion for some common materials are given in the Coefficients of Thermal Expansion of Various Structural Materials table below.
Thermal stress can be calculated by using the following equation:
The following calculations illustrate the use of thermal stress equations:
Calculate the strain on a part made of polycarbonate and close fitting onto a steel bracket. The parts are assembled at a room temperature of 73ºF (23ºC) and operated at an environmental temperature of 180ºF (82ºC). Because the steel bracket restrains the expansion of the polycarbonate part, a strain of 0.33% is induced in the part.
Coefficients of Thermal Expansion of Various Structural Materials
|
Material S.I.
|
mm/mm/°C English
|
in/in/°F Metric
|
mm/mm/°C
|
| ABS |
9.5 x 10-5
|
5.3 x 10-5
|
9.5 x 10-5
|
| Aluminum |
2.2 x 10-5
|
1.2 x 10-5
|
2.2 x 10-5
|
| Brass |
1.8 x 10-5
|
1.0 x 10-5
|
1.8 x 10-5
|
| Nylon |
8.1 x 10-5
|
4.5 x 10-5
|
8.1 x 10-5
|
| PBT |
7.4 x 10-5
|
4.1 x 10-5
|
7.4 x 10-5
|
| PC |
6.8 x 10-5
|
3.8 x 10-5
|
6.8 x 10-5
|
| PE |
12.0 x 10-5
|
6.7 x 10-5
|
12.0 x 10-5
|
| PP |
5.8 x 10-5
|
3.2 x 10-5
|
5.8 x 10-5
|
| PS |
8.1 x 10-5
|
4.5 x 10-5
|
8.1 x 10-5
|
| SAN |
6.7 x 10-5
|
3.7 x 10-5
|
6.7 x 10-5
|
| Steel |
1.1 x 10-5
|
0.6 x 10-5
|
1.1 x 10-5
|
Thermoforming The process of forming a thermoplastic sheet into a three-dimensional shape by clamping the sheet in a frame, heating it to render it soft and flowable. Then applying differential pressure to make the sheet conform to the shape of a mold or die positioned below the frame.
Thermoplastics (1) Capable of being repeatedly softened by heat and hardened by cooling. (2) [n.] - A material that will repeatedly soften when heated and harden when cooled. Typical of the thermoplastics family are the styrene polymers and copolymers, acrylics, cellulosics, polyethylenes, vinyls, nylons, and the various fluorocarbons materials.
Thermoplastics vs. Thermosets A thermoplastic is a polymeric material or plastic which becomes soft and formable when heated and rigid when cooled. This process may be repeated a number of times without chemically altering the material. A thermoset is a polymeric material that undergoes irreversible chemical changes when cured through heat, catalysts or ultraviolet light: cross-linking prevents movement of molecular chains after curing. Once cured, the structure cannot be changed.
Thermosets A material that will undergo or has undergone a chemical reaction by the action of heat, catalysts, ultra-violet light, etc., leading to a relatively infusible state. Once the structural framework is set, these plastics cannot be reformed. Typical of the plastics in the thermosetting family are the aminos (melamine and urea), most polyesters, alkyds, epoxies, and phenolics.
Thrust A lateral or inclined force resulting from the structural action of an arch; a pushing force.
Tile -v.t. to cover with or as with tiles. -n. Thin slab of baked clay, terra-cotta, glass, cement, or asbestos-cement, used for roofing or for covering walls or floors.
Torque Twisting action; moment.
Torsional Stress Stress resulting from the twisting of a structural member.
Toughness A measure of the ability of a material to absorb work, or the actual work per unit volume or unit mass of material that is required to rupture it. Toughness is proportional to the area under the load-elongation curve from the origin to the breaking point.
Transverse Perpendicular to the long axis, such as an axis of rotational symmetry.
Triangle A three-sided rectilinear plane figure. The triangle is the only polygon that is stable (rigid) by virtue of its geometry.
Triangulated Composed of or having the form of triangles.
Triangulated System Assemblage or set of correlated members organized as triangles.
Triangulation Technique for organizing a structure or system in triangular form (see Triangulated System).
True Strain Instantaneous % of change in length of specimen in mechanical test. It is equal to the natural logarithm of the ratio of length at any instant to original length.
True Stress Applied load divided by actual area of the cross section through which load operates. It takes into account the change in cross section that occurs with changing load.
Truncated Describing a solid generated from a given solid by two non-parallel planes cutting the given solid.
Truncated Octahedron An octahedron with its vertices sliced off along a plane.
Truncated Rhombic Dodecahedron A rhombic dodecahedron with either or both its vertices and edges sliced off along a plane. In Reflexive Material Technology, a rhombic dodecahedron with its tetra-edge vertices removed.
Truncation Operation of slicing off either the vertices or edges of a polyhedron along a plane; replacing a vertex or edge with a plane.
Truss A triangular arrangement of structural members that reduces nonaxial forces on the truss to a set of axial forces in the member. See also "Space frame, Space truss."
Two-way Action Bending in which bending stresses are approximately equal in the two principal directions of the structure.