| Partial annealing |
An imprecise term used to denote a treatment given cold-worked material to reduce the strength to a controlled level or to effect stress relief. To be meaningful, the type of material, the degree of cold work, and the time-temperature schedule must be stated. |
| Pascal (Pa) |
The SI unit of pressure, 1 Pa = 1 N/m2. |
| Pascal’s law |
When there is a change in pressure at any point in a confined fluid at rest, there is an equal change at every other point in the fluid volume. |
| Peel strength (Unit N) |
(peel test) The strength of an adhesive bond between two materials as measured by the force required in a given direction to peel apart adhered strips. This force depends on the dimensions and thickness of the adhered strips and their yield stress, together with the fracture toughness of the bond. |
| Percent error |
For testing machines, the ratio, expressed as a percentage, of the error to the correct value of the applied load. |
| Permanent set |
The deformation or strain remaining in a previously stressed body after release of load. |
| Permissible variation |
For testing machines, the maximum allowable error in the value of the quantity indicated. It is convenient to express permissible variation in terms of the percent error. |
| Physical crack size (ap) |
The distance from a reference plane to the observed crack front. This distance may represent an average of several measurements along the crack front. The reference plane depends on the specimen form, and it is normally taken to be either the boundary or a plane containing either the load line or the centerline of a specimen or plate. |
| Physical properties |
Properties of a material the determination of which does not involve the deformation or destruction of the specimen—for example, density, electrical conductivity, coefficient of thermal expansion, magnetic permeability, and lattice parameter. Does not include chemical reactivity or the properties more appropriately regarded as mechanical properties. |
| Physical testing |
Methods used to determine the entire range of physical properties of a material. In addition to density and thermal, electrical, and magnetic properties, physical testing methods can be used to assess simple fundamental physical properties such as color, crystalline form, and melting point. |
| Pin expansion test |
A test for determining the ability of a tube to be expanded or for revealing the presence of cracks or other longitudinal weaknesses in it, made by forcing a tapered pin into the open end of the tube, similar to flare test. |
| Pin or mandrel |
In bend testing, the plunger or tool used in making semiguided, guided, or wrap-around tests to apply the bending force to the inside surface of the bend. In free bends or semiguided bends to an angle of180°, a shim or block of the proper thickness may be placed between the legs of the specimen as bending is completed. This shim or block is also referred to as a pin or mandrel. |
| Piston pump |
A positive-displacement pump in the basic configuration of which a piston reciprocates in a cylinder. On the suction stroke, fluid is drawn into a chamber through the inlet valve, which is then open while the outlet valve is closed. On the delivery stroke the fluid is forced out of the chamber through the outlet valve, which then opens while the inlet valve is closed. Both axial and radial configurations are common. |
| Pitch |
The nominal distance between two adjacent thread roots or crests. |
| Pitch |
In screw threads, the distance from a point on one thread to a corresponding point on the next thread measured parallel to the axis. In the case of spur gears, indicates the size of the gear teeth and is correctly called diametral pitch. |
| Pitch (Unit m) |
1. In a cascade of identical turbine or compressor blades, the distance between successive blades measured parallel to the leading edges. 2. For a screw thread, the distance between adjacent thread forms measured parallel to the thread axis. 3. For an aircraft, the vertical relationship between the nose and the horizon. The pitch axis is an axis in the plane of the wings of an aircraft, perpendicular to the centreline, about which the aircraft rotates. Pitch motion is the corresponding up or down movement of the aircraft nose. Pitch attitude is the angle between the centreline of an aircraft and the horizontal. The angle is positive when the nose is above its position when the centreline is horizontal. |
| Pitch angle (Unit °) |
For a bevel gear, the angle between the axis and the pitch-cone generator. |
| Pitch diameter |
For screw threads, the diameter of an imaginary cylinder, the surface of which would pass through the threads at such points that would make the width of the groove and width of the land equal to one half the pitch. |
| Pitch-circle diameter (Unit m) |
The diameter of the circle (the pitch circle) centred on a component’s axis, around which holes or bolts are equally spaced. |
| Pitting |
In tribology, a type of wear characterized by the presence of small, sharp surface cavities formed by processes such as fatigue, local adhesion, wear, corrosion, or cavitation. |
| Planar anisotropy |
A variation in physical and/or mechanical properties with respect to direction within the plane of material in sheet form. |
| Plane strain |
The stress condition in linear-elastic fracture mechanics in which there is zero strain in a direction normal to both the axis of applied tensile stress and the direction of crack growth (i.e., parallel to the crack front); most nearly achieved in loading thick plates along a direction parallel to the plate surface. Under plane-strain conditions, the plane of fracture instability is normal to the axis of the principal tensile stress. |
| Plane stress |
The stress condition in linear-elastic fracture mechanics in which the stress in the thickness direction is zero; most nearly achieved in loading very thin sheet along a direction parallel to the surface of the sheet. Under plane-stress conditions, the plane of fracture instability is inclined 45° to the axis of the principal tensile stress. |
| Plane-strain fracture toughness (KIc) |
The crack extension resistance under conditions of crack-tip plane strain. |
| Plane-stress fracture toughness (Kc) |
In linear-elastic fracture mechanics, the value of the crack-extension resistance at the instability condition determined from the tangency between the R-curve and the critical crack-extension force curve of the specimen. |
| Plastic deformation |
The permanent (inelastic) distortion of a material under applied stress that strains the material beyond its elastic limit. |
| Plowing |
In tribology, the formation of grooves by plastic deformation of the softer of two surfaces in relative motion. |
| Plug |
1. An object, usually tapered, used to block a hole to prevent fluid from escaping or flowing. 2. The central moveable part of a valve. 3. In the flow of a yield-stress fluid through a pipe or duct, the central region is often assumed to take the form of a plug with uniform axial velocity. 4. In thermoforming, a shaping tool that forces a heated plastic sheet into a female mould. |
| Plug gauge |
A steel gauge used to determine the dimension or dimensions of a hole. It may be straight or tapered, and is typically circular, but in principle can have any cross section. |
| Pneumatic control valve |
(pneumatic valve) A valve in which the position of the valve stem, which varies the open area, is determined by the net force generated by compressed air acting on a diaphragm operating against the force of a compression spring. Depending upon the arrangement of the spring, in the event of air-supply failure, the valve may open or close. |
| Poisson’s ratio (ν) |
The absolute value of the ratio of transverse (lateral) strain to the corresponding axial strain resulting from uniformly distributed axial stress below the proportional limit of the material. |
| Pop rivet |
A hollow rivet that enables a connexion to be made from one side only of an assembly. |
| Population |
The hypothetical collection of all possible test specimens that could be prepared in the specified way from the material under consideration. Also known as universe. |
| Positive-displacement compressor |
Any type of compressor, including piston and rotary-screw types, that delivers a fixed volume of gas at high pressure per unit time. A positive-displacement pump delivers a fixed volume of fluid, usually a liquid, per unit time. |
| Positive-displacement flow meter |
An instrument of high accuracy that determines volumetric flow rate by dividing the flowing fluid into successive fixed volumes, and measuring their times of passage through the meter. |
| Positive-displacement machine |
A machine that incorporates pistons, valves, etc. to ensure positive admission and delivery of a working fluid and prevent undesired reversal of flow. This class of machine incorporates all reciprocating compressors and expanders and some types of rotary compressor, the Roots blower, agear pump, a lobe pump, and a vane pump. |
| Power |
The rate of doing work or of producing or consuming energy. The unit of power is the watt, W, where 1 W = 1 N m/s. |
| Precipitation hardening |
Hardening caused by the precipitation of a constituent from a supersaturated solid solution. |
| Precipitation heat treatment |
Artificial aging in which a constituent precipitates from a supersaturated solid solution. |
| Precipitation-hardenable stainless steels |
Precipitation-hardenable stainless steels contain typically 11–18% chromium, 3–10% nickel, 0.05–0.2% carbon, and small additions of manganese, silicon, aluminium, molybdenum, niobium and titanium. These steels can be supplied in a solution-treated condition, which is readily fabricated or machined, and then hardened by an ageing treatment that produces a fine dispersion of second-phase precipitates. They are available in austenitic, semi-austenitic, and martensitic grades. Uses include springs, knives, and pressure vessels. |
| Precision |
The closeness of agreement between the results of individual replicated measurements or tests. The standard deviation of the error of measurement may be used as a measure of “imprecision.” |
| Preheating |
Heating before some further thermal or mechanical treatment. For tool steel, heating to an intermediate temperature immediately before final austenitizing. For some nonferrous alloys, heating to a high temperature for a long time, to homogenize the structure before working. In welding and related processes, heating to an intermediate temperature for a short time immediately before welding, brazing, soldering, cutting, or thermal spraying. |
| Preload |
The tension created in a threaded fastener when the nut is first tightened. Often used interchangeably, but incorrectly, with Working load or bolt force or bolt tension |
| Preload accuracy |
A measure of the precision with which a given tool or procedure creates preload in a bolt when the bolt is tightened. A common torque wrench, for example, is said to produce preload with an accuracy of +30%. The mean preload, however, may not be that which the designer intended, or may not be what he should have intended. Accuracy as used here, in other words, is synonymous with Scatter. |
| Preload adjustments |
Preloads may be applied directly by axial loading or indirectly by turning of the nut or bolt. When preload is applied by turning of nuts or bolts, a torsion load component is added to the desired axial bolt load. |
| Preload relaxation |
Local yielding, due to excess bearing stress under nuts and bolt heads (caused by high local spots, rough surface finish, and lack of perfect squareness of bolt and nut bearing surfaces), may result in preload relaxation after preloads are first applied to a bolt. |
| Press quenching |
A quench in which hot dies are pressed and aligned with a part before the quenching process begins. Then the part is placed in contact with a quenching medium in a controlled manner. This process avoids part distortion. |
| Pressure (p) (Unit Pa) |
In thermodynamics and fluid mechanics, the compressive force exerted by the fluid per unit area. The pressure exerted by a fluid on a surface acts normal to the surface. |
| Pressure chamber |
A chamber in which components or devices can be subjected to high or low fluid (liquid or gas) pressure. |
| Pressure control valve |
A valve used to set the pressure level in a pressure vessel or piping system. |
| Pressure difference (Unit Pa) |
(pressure differential, Δp) The difference between two pressures, one of which may be a reference pressure such as barometric pressure. In many flow processes, the pressure difference is more important than the absolute pressure level. |
| Pressure gauge |
An instrument used to measure absolute or gauge pressure. The sensing element may be a tube which deflects when pressurized, as in a Bourdon gauge, a bellows as in an aneroid barometer, a piezoelectric crystal, a piezoresistive element, etc. |
| Pressure loss (Unit Pa) |
The loss in stagnation pressure in internal flow due to wall friction and minor losses in fittings. |
| Pressure rating (Unit Pa or bar) |
The internal pressure at which a pressure vessel, boiler, tank, piping, etc. is designed to operate safely. |
| Pressure ratio |
One pressure divided by another which may be a reference pressure. In many flow and thermodynamic processes, the pressure ratio is more important than the absolute pressure levels. For example, in compressible gas flow the Mach number is determined by the ratio of the stagnation pressure to the static pressure. |
| Pressure recovery (Unit Pa) |
The progressive increase in static pressure for unseparated flow through a diverging nozzle or diffuser. |
| Pressure regulator |
(pressure-regulating valve) A device installed in a pneumatic or gas system to maintain the downstream pressure at the required level. |
| Pressure snubber |
(pressure-pulse snubber) A device used for filtering, dampening, and the protection of transducers, pressure gauges, and switches, from pressure surges, pressure spikes, and water hammer. A typical design uses a fixed mesh or porous metal disc installed in an inline body. In a piston-type pressure-gauge snubber, a piston is forced against an orifice leading to the gauge. |
| Pressure storage tank |
(pressure vessel) A closed container for storing gases or volatile liquids, such as liquefied gases, at pressures significantly above atmospheric pressure. Such tanks are commonly cylindrical with domed ends, spherical, spheroidal, torispherical or hemispherical. |
| Pressure surface |
The high-pressure surface of an aerofoil, turbine, or compressor blade. In normal aircraft applications, this is the lower surface of a wing. For applications, such as to high-performance motor vehicles, where the aerofoil is inverted to generate downforce, it is the upper surface. |
| Pressure tap |
A small hole in the wall of a pipe or pressure vessel to which is attached a tube, the other end of which is connected to one side of a pressure transducer. |
| Pressure-relief valve |
A valve that limits the maximum pressure in a pressure vessel or fluid-power system to a specified level. |
| Pre-stress |
To induce stresses into a component or structure before it is subjected to operating loads. |
| Prevailing torque |
Torque required to run a nut down against the joint when some obstruction, such as a plastic insert in the threads, or a noncircular thread, or other, has been introduced to help the fastener resist vibration loosening. Prevailing torque, unlike normal torque on a nut or bolt, is not proportional to the Preload in the fastener. |
| Principal stress (normal) |
The maximum or minimum value of the normal stressat a point in a plane considered with respect to all possible orientations of the considered plane. On such principal planes the shear stress is zero. There are three principal stresses on three mutually perpendicular planes. The state of stress at a point may be: (1) uniaxial, a state of stress in which two of the three principal stresses are zero; (2) biaxial, a state of stress in which only one of the three principal stresses is zero; or (3) triaxial, a state of stress in which none of the principal stresses is zero. Multiaxial stress refers to either biaxial or triaxial stress. |
| Process annealing |
An imprecise term denoting various treatments used to improve workability. For the term to be meaningful, the condition of the material and the time-temperature cycle used must be stated. |
| Profile |
The shape of a cross section of an object, such as an aerofoil or cam. |
| Profiling machine |
(profile milling machine) A milling machine in which the cutter is guided by the contour of a pattern having the required shape. |
| Profilometer |
An instrument used to quantify the roughness of a surface. For a contact profilometer, a diamond stylus sweeps across the surface along a series of parallel lines. Noncontact profilometers use optical techniques to map the surface irregularities. |
| Progressive aging |
Aging by increasing the temperature in steps or continuously during the aging cycle. |
| Proof load |
The maximum, safe, static, tensile load which can be placed on a fastener without yielding it. Sometimes given as a force (in lb or N) sometimes as a stress (in psi or MPa). |
| Proof load (Unit N) |
The test load in quality control to which different components made of different materials must be subjected without failing in order to perform properly. |
| Proof strength (Unit Pa) |
(proof stress, Rp, Rp,) The yield strength (offset yield strength) at some fixed value of the permanent strain given by the intersection of a line offset from, but parallel to, the elastic loading line and the engineering stress-engineering strain curve. The offset is arbitrary but is usually 0.1, 0.2, or 0.5% permanent strain (the proof strain). Used for materials which do not exhibit a sharply defined yield point. The notation Rp0.2 or Rp0.2 is employed for the 0.2% proof stress. |
| Proof stress |
(1) The stress that will cause a specified small permanent set in a material. (2) A specified stress to be applied to a member or structure to indicate its ability to withstand service loads. |
| Proportional limit |
The greatest stress a material is capable of developing without a deviation from straight-line proportionality between stress and strain. |
| Prying |
The magnification of an External load by a pseudolever action when that load is an Eccentric tensile load. |
| PSI (pounds per square inch) |
Pressure measurement. Psia (pounds per square inch absolute) measures actual pressure with no allowance for atmospheric pressure. Psig (pounds per square inch gauge) measures pressure with the gauge set to zero (0) at atmospheric pressure (14.7 psia). In other words psig = psia – 14.7. |
| Pulley |
A free or driven wheel on a shaft with an appropriately shaped rim to carry a flat belt, vee belt, notched belt, rope, or chain. Used to transmit power or motion. |
| Pulse |
An increase or decrease in the magnitude of a physical quantity, such as pressure, voltage, or force, with a time scale short compared with other time scales in a process, after which there is a return to the original level. |
| Pump |
A machine designed to cause a liquid, gas, vapour, or slurry to flow due to the reciprocating motion of pistons, rotation of vanes, or rotation of an impeller. |
| Punch |
1. A hand tool with a sharp point, used to mark the position of a hole centre in a workpiece prior to drilling. 2. A flat-ended tool with sharp edges, used to shear out a hole in a plate that is supported underneath on a die having a slightly larger same-shaped orifice. |
| Push rod |
A rod which opens and closes a valve via a valve rocker in an overhead-valve piston engine. The rod is actuated by a camshaft located in the crankcase. |
