Polyhedron A solid figure bounded by plane polygonal faces. The point at which three or more faces intersect on a polyhedron is called a vertex, and a line along which two faces intersect is called an edge. In a regular polyhedron, all the faces are congruent regular polygons. There are only five regular polyhedra: tetrahedron, octahedron, cube, icosahedron, and dodecahedron (see "Platonic Polyhedra" above).
Polymers These include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements; they have very large molecular structures. These materials typically have low densities and may be extremely flexible.
Polymer chains A linear polymer is a polymer in which the monomers are bound to each other in a straight chain without any branches. Branched polymers have branched connections of molecules.
Such polymers are called block-copolymers, characterized by both monomers A and B forming the backbone chain of the polymer. They have repeating monomers in linearly connected blocks. Another possibility is the formation of a graft-copolymer, which is essentially a branched-chain structure. It has side chains composed of one type of monomer unit attached to the backbone or main chain from another monomer unit.
Polymerization A chemical reaction in which the molecules of a monomer are linked together to form large molecules whose molecular weight is a multiple of that of the original substance. When two or more monomers are involved, the process is called copolymerization or heteropolymerization.
Polypropylene A tough, lightweight rigid plastic made by the polymerization of high-purity propylene gas in the presence of an organometallic catalyst at relatively low pressures and temperatures.
Porosity A condition of trapped pockets of air, gas, or vacuum within a solid material. Usually expressed as a percentage of the total non-solid volume to the total volume (solid plus non-solid) of a unit quantity of material.
Portland Cement The gray powder used as the binder in concrete, mortar, and stucco.
Post-and-Beam Construction Building construction system in which two upright members, the posts, hold up a third member, the beam (or lintel), laid horizontally across their top surfaces. The beam must bear loads that rest on it as well as its own load without deforming or breaking. Brick, or stone, weak in tensile strength (inelastic and brittle) can provide only a short beam; steel can be used for long beams. All structural openings have evolved from this system, which is seen in pure form only in colonnades and in framed structures.
Posttensioning The compressing of the concrete in a structural member by means of tensioning high-strength steel tendons against it after the concrete has cured.
Powder Finely divided solid; particles are smaller than one millimeter in maximum dimension. It is metallic, ceramic or a polymer; these can be blended in infinite combinations. An important characteristic is the relatively high surface-to-volume ratio of the particles which exhibit behavior that is intermediate between that of a solid and a liquid. Powders will flow under gravity to fill containers or die cavities, so in this sense behave like liquids. They are compressible like gas. Powders are easily shaped, with the desirable behavior of a solid after processing.
Powder Metallurgy The practice, description, study and terminology of processing metal powders, including the fabrication, characterization, and conversion of these powders into useful engineering components. The processing sequence involves the application of basic laws of heat, work, and deformation to the powder. Processing changes the shape, properties, and structure of a powder into a final product.
Precast Concrete Concrete cast and cured in a position other than its final position in the structure.
Precision The degree of refinement with which an operation is performed or a design replicated.
Precursor A substance, cell, or cellular component from which another substance, cell, or cellular component is formed.
Preform Form or shape beforehand.
Pressure The pressure on a surface due to forces from another surface or from a fluid is the force acting at right angles to unit area of the surface: pressure = force/area.
Prestressed Concrete Concrete that has been pretensioned or posttensioned.
Prestressing Applying an initial compressive stress to a concrete structural member, either by pretensioning or posttensioning.
Pretensioning The compressing of the concrete in a structural member by pouring the concrete for the member around stretched high-strength steel strands, curing the concrete, and releasing the external tensioning force on the strands.
Prism Polyhedron with two parallel opposite faces, called bases, that are congruent polygons. All the other faces, called lateral faces, are parallelograms formed by the straight parallel lines between corresponding vertices of the bases.
Product-centered engineering Concurrent engineering of materials properties and product architecture based on product specifications.
Proof stress Stress that will cause a specified permanent deformation.
Proportional limit Highest stress at which stress is directly proportional to strain. It is the highest stress at which the curve in a stress-strain diagram is a straight line. Proportional limit is equal to elastic limit for many metals.
Prototype A model suitable for use in complete evaluation of form, design, performance, and material processing.
Prototyping (rapid) Also known as solid freeform fabrication, automated fabrication, layered manufacturing, and other terms; consists of a range of technologies that are capable of taking computer-aided design (CAD) models and converting these to a physical form or part. This process is automatic, generally independent of the model geometry, and does not require special tooling or fixtures. Complex three-dimensional contours are quantized in the form of stacks of two-dimensional, finite thickness layers or cross sections.
Progenitor Thing that originates something or serves as a model.
Property Essential or distinctive attribute or quality.
Pultrusion A continuous process for manufacturing composites that have a constant cross-sectional shape. The process consists of pulling a fiber-reinforced material through a resin impregnation bath and through a shaping die, where the resin is subsequently cured.