| Safe working load (Unit N) |
The steady or unsteady load against which a component or structure is designed for normal operation. It is lower than that which would cause failure by buckling, fracture, or yielding, so as to accommodate uncertainty, possible fault or accident conditions. |
| Safety valve (safety-relief valve) |
A mechanical valve, fitted by law to all pressure vessels (e.g. steam boilers), which opens to prevent the internal pressure exceeding the maximum design value for that vessel. The valve closes again once the pressure reduces to a safe level. |
| Salt bath heat treatment |
Heat treatment carried out in a bath of molten salt. |
| Salt-velocity meter |
A volume flow meter based on detecting the transit time for a small quantity of salt or radioactive isotope in a flow by measuring electrical conductivity or radiation level. |
| Sample average |
The sum of all the observed values in a sample divided by the sample size. It is a point estimate of the population mean. Also known as arithmetic mean. |
| Sample median |
The middle value when all observed values in a sample are arranged in order of magnitude. If an even number of samples are tested, the average of the two middlemost values is used. It is a point estimate of the population median, or 50% point. |
| Sample percentage |
The percentage of observed values between two stated values of the variable under consideration. It is a point estimate of the percentage of the population between the same two stated values. |
| Sample standard deviation (s) |
The square root of the sample variance. It is a point estimate of the population standard deviation, a measure of the “spread”of the frequency distribution of a population. This value of sprovides a statistic that is used in computing interval estimates and several test statistics. For small sample sizes, s underestimates the population standard deviation. |
| Sample variance (s2) |
The sum of the squares of the differences between each observed value and the sample average divided by the sample size minus one. It is a point estimate of the population variance. |
| Sampling |
The process of measuring a series of values of a time-varying quantity. |
| Sampling frequency (Unit Hz) |
(sampling rate) The number of measurement samples per unit time taken from a continuous analogue signal to produce a discrete signal. It is the inverse of the time between successive measurements (sampling interval, sampling period, or sampling time). |
| Sawing |
Sawing is the process of cutting a workpiece with power band saws, hacksaws, and circular saws. Each of these methods is used in cutoff operations (cutting pieces to a required length), although band sawing also provides a method of cutting contours. |
| Scale effect |
The change in behaviour with size of a material, component, or structure. For example, cube-square scaling where one element changes with volume and another with area, as in the range of a projectile or ship, droplet formation, and fracture mechanics. |
| Scale height (Unit m) |
The altitude H at which the atmospheric pressure has decreased to a certain fraction of its value at sea level B. It depends on the model adopted for the atmosphere: for an isothermal atmosphere, the pressure decreases exponentially with altitude. If H is taken as the altitude at which the pressure has fallen to B/e, then H = RT/g where R is the gas constant for air, T is the absolute temperature T, and g is the acceleration due to gravity. |
| Scatter |
Data points or calculations are said to be scattered when they are not all the same. A ‘‘lot of scatter in preload’’ means wide variation in the preloads found in individual bolts. |
| Scleroscope hardness number (HSc or HSd) |
A number related to the height of rebound of a diamond-tipped hammer dropped on the material being tested. It is measured on a scale determined by dividing into 100 units the average rebound of the hammer from a quenched(to maximum hardness) and untempered AISI W5 tool steel test block. |
| Scleroscope hardness test |
A dynamic indentation-hardness test using a calibrated instrument that drops a diamond-tipped hammer from a fixed height onto the surface of the material being tested. The height of rebound of the hammer is a measure of the hardness of the material. Also known as Shore hardness test. |
| Scoring |
In tribology, a severe form of wear characterized by the formation of extensive grooves and scratches in the direction of sliding. |
| Scragging machine |
1. A machine to test springs by impulsive loading 2. A machine to increase the service life of coil springs by compressing them to their minimum solid length before use, thereby inducing favourable residual stresses. |
| Scratch hardness |
The resistance of a material, such as a metal, alloy, plastic, or mineral, to scratching by a much harder indenter, such as a diamond stylus, moved slowly across the surface. The scratch hardness number is computed from the loads and the dimensions of the residual scratch. |
| Scratch hardness test |
A form of hardness test in which a sharp-pointed stylus or corner of a mineral specimen is traversed along a surface so as to determine the resistance of that surface to cutting or abrasion. |
| Scratching |
In tribology, the mechanical removal or displacement, or both, of material from a surface by the action of abrasive particles or protuberances sliding across the surfaces. |
| Screw |
Threaded fastener designed to be used in a tapped or untapped (e.g., wood screw) hole, but not with a nut. |
| Screw |
A fastener with a screw thread cut or rolled into its cylindrical or conical shank, intended either to cut its own thread (as in a wood screw) or engage in a threaded hole. A self-tapping screw (sheet-metal screw, tapping screw) has a sufficiently hard thread that it cuts an internal thread in thin sheet or a component made of soft material when driven into a hole in the sheet or component. The screw head is the part of a screw used to apply torque to the screw. Common screw-head designs are circular in cross section with a diametral slot or cross, or hexagonal and recessed with a cross or hexagon. A screw thread is one or more continuous helical grooves of uniform section along either the exterior surface (male thread) or the interior surface (female thread) of a cylindrical or conical body. The three-dimensional shape that results when the thread cross section is rotated and axially advanced along an axis is called a helicoid, the angle that the thread makes when projected on to the axis being called the angle of inclination. Threads are employed in fasteners such as bolts, nuts, and screws; location and measuring instruments; in power drives; in some electrical fittings (Edison thread); and on the ends of crankshafts to suppress oil leakage (thrower thread). Parallel threads are formed on cylinders; tapered threads on cones, typically with a taper rate of 1:16. A screw with a righthanded thread appears to move away from the observer when turned clockwise. All standard screws, bolts and nuts have right-hand threads, but left-handed threads are sometimes employed. The axial distance between corresponding points on adjacent threads is called the screw pitch or screw rate and, for a single continuous helical groove (a single-start thread), is the same as the change in axial distance (the lead) between a nut and the head of a bolt during one revolution, the number of thread forms per mm then being the reciprocal of the pitch. For the same screw diameter, coarse threads have fewer threads per mm than fine threads. A multiple-start screw thread (usually coarse, see later) consists of two or more identical threads running simultaneously along its axis so as to provide greater bearing area and greater velocity ratio. The starts are separated by 180° (double start), 120° (triple start), 90° (quadruple start) etc., depending upon the number of threads. Thus, in double-start threads, the lead is twice the pitch; and so on. The axial distance between corresponding points on two adjacent threads in a multiple start thread is called the divided pitch. The position on a screw thread where there is equal distance between the flanks on the solid part of the thread and in the space between the threads is the pitch point, the associated diameter of which is termed the pitch diameter. The basic nomenclature for threads, some of which is common with that for toothed gearing, is shown in the diagrams. |
| Screw |
A helix formed or cut on a cylindrical surface which may advance along the axis to the right or left. The helix may be single or multiple. |
| Screw area (Unit m2) |
The area of a circle described by the tips of a propeller. |
| Screw compressor |
A positive-displacement rotary compressor in which gas is progressively compressed by two intermeshing, counter-rotating, helical screws. |
| Screw conveyor |
(auger, spiral conveyor, worm conveyor) A machine for bulk handling of semi-solid materials, consisting of a helical screw which rotates in a trough or casing. |
| Screw displacement |
A rotation of a rigid body about an axis accompanied by a translation of the body along the same axis. |
| Screw extractor |
A device for removing broken-off screws from threaded holes. It resembles a drill with, for broken right-hand threads, a fast (long-pitch) left-hand thread that is driven into a hole drilled in the broken screw, thus untwisting it. |
| Screw feeder |
A mechanism for handling bulk materials in which a rotating helicoidal screw moves the material axially forward. Similar to a screw conveyor, but required to discharge material at a controlled rate very accurately. It operates with the screw completely full. |
| Screw jack |
A lifting device (jack) consisting of a nut and square-threaded shaft at the top of which is a load-bearing pad. Rotation of the nut raises or lowers the pad. |
| Screw machine |
A machine tool for high-volume manufacture of small turned components from rod or bar. |
| Screw pair |
Two links connected together to form a kinematic pair, in which the contacting surfaces are screw threads, so that their relative motion consists of rotation and sliding. |
| Screw pump |
A positive-displacement pump that uses one or more helical rotors rotating within a casing to transfer liquids or slurries. |
| Screw thread |
A ridge of uniform section or shape in the form of a helix on the external or internal surface of a cylinder, or in the form of a conical spiral on the external or internal surface of a cone. |
| Scuffing |
A form of adhesive wear that produces superficial scratches or a high polish on the rubbing surfaces. It is observed most often on inadequately lubricated parts. |
| Seal |
A component which controls or prevents leakage of fluids into or out of parts of a machine. |
| Secant modulus |
The slope of the secant drawn from the origin to any specified point on a stress-strain curve. Compare with chord modulus. |
| Second (s) |
The SI base unit of time equal to 9 192 631 770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. |
| Secondary hardening |
The process whereby some low-alloy steels, quenched to produce martensite, produce fine precipitates when tempered above 550°C, which inhibit dislocation motion and reverse the trend towards lower strength at higher tempering temperatures. |
| Secondary stresses (Unit Pa) |
Stresses different from those induced by the major loading but nevertheless resulting from the major loads; for example, the hoop stresses that occur around the circumference of barrelled compression testpieces and forgings. |
| Selective heating |
Intentionally heating only certain portions of a workpiece. |
| Selective quenching |
Quenching only certain portions of an object. |
| Self-locking nut |
A nut with an inherent locking action which minimizes loosening due to vibration. A self-locking screw locks itself in place without the need for a separate selflocking nut or lock washer. |
| Self-loosening |
The process by which a supposedly tightened fastener becomes loose, as a result of vibration, thermal cycles, shock, or anything else which cause transverse slip between joint members and between male and female threads. Vibration loosening is a common, but special, case of self-loosening. |
| Semiguided bend |
The bend obtained by applying a force directly to the specimen in the portion that is to be bent. The specimen is either held at one end and forced around a pin or rounded edge, or is supported near the ends and bent by a force applied on the side of the specimen opposite the supports and midway between them. In some instances, the bend is started in this manner and finished in the manner of a free bend. |
| Semi-rotary pump |
A form of self-priming pump, often hand-operated, suitable for pumping water and light oils such as diesel oil and petrol. As shown in the diagram, liquid is sucked into one side of the pump through flap valves and simultaneously ejected from the other side on one stroke. The sequence is reversed on each successive stroke. |
| Serrations |
A row of notches or tooth-like projections on an edge or surface. On an engineering drawing, serrations on the surface of a circular component are shown over about 60° of arc. |
| Set pressure (Unit Pa) |
The pressure at which a relief valve or safety valve is set to open, corresponding to the relevant code or standard which applies to the pressure vessel being protected. |
| Set screw |
Usually a hardened steel screw having either no head or a square head and with various degrees of points or ends to lock or tighten adjustable machine parts in position on a shaft. |
| Set screw (grub screw, set bolt) |
A short, headless screw with a recess at one end to receive a screwdriver or key, the other end being pointed, square, or otherwise shaped. Typically used to secure a pulley, gear, or other component on a shaft. |
| Shank |
Portion of a bolt which lies under the head. |
| Shank |
1. The stem of a tool, such as a broach, drill bit, reamer, or tap, which fits into a holder such as a chuck. 2. The stem of a rivet or the unthreaded part of a screw or bolt. 3. The shaft of a tool connecting the tip and the handle. |
| Sharp-notch strength |
The notch tensile strength measured using specimens with very small notch root radii (approaching the limit for machining capability); values of sharp-notch strength usually depend on notch root radius. |
| Shear |
The type of force that causes or tends to cause two contiguous parts of the same body to slide relative to each other in a direction parallel to their plane of contact. |
| Shear fracture |
A ductile fracture in which a crystal (or a polycrystalline mass)has separated by sliding or tearing under the action of shear stresses. Contrast with cleavage fracture. |
| Shear joint |
A joint which is subjected primarily to loads acting more or less perpendicular to the axes of the bolts. |
| Shear lip |
A narrow, slanting ridge along the edge of a fracture surface. The term sometimes also denotes a narrow, often crescent-shaped, fibrous region at the edge of a fracture that is otherwise of the cleavage type, even though this fibrous region is in the same plane as the rest of the fracture surface. |
| Shear modulus (G) |
The ratio of shear stress to the corresponding shear strain for shear stresses below the proportional limit of the material. Values of shear modulus are usually determined by torsion testing. Also known as modulus of rigidity. |
| Shear strain |
The tangent of the angular change, as a response to force, between two lines originally perpendicular to each other through a point in a body. Also known as shearing strain. |
| Shear strain rate(Unit 1/s) |
In the deformation of solids, the rate at which shear strain is applied in a test or changes with time in a loaded component or structure. |
| 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 dimensions of the cross section of the specimen. |
| Shear strength (Unit Pa) |
The maximum shear stress that can be withstood by a material either (a) before plasticity occurs or (b) before rupture. |
| Shear stress |
(1) A stress that exists when parallel planes in metal crystals slide across each other. (2) The stress component tangential to the plane on which the forces act. Also known as tangential stress. |
| Shear stress (Unit Pa) |
(shearing stress, tangential stress, τ) 1. The stress which acts parallel to any plane within a solid material. It can arise due to a bending moment, a shear force, or torque applied to the body. 2. The stress corresponding to velocity gradients within a flowing viscous fluid. |
| Shear test |
Any of several tests to obtain shear strength of a metal. Common tests used on mill products include the double-shear test, single-shear test, the blanking shear test (also known as the punching shear test) and the torsion test. |
| Shell hardening |
A surface-hardening process in which a suitable steel workpiece, when heated through and quench hardened, develops a martensitic layer or shell that closely follows the contour of the piece and surrounds a core of essentially pearlitic transformation product. This result is accomplished by a proper balance among section size, steel hardenability, and severity of quench. |
| Shim |
A thin piece of material, such as metal of accurately-known thickness, placed between two surfaces to ensure they are the correct distance apart. |
| Shim |
A thin piece of material placed between two surfaces to obtain a proper fit, adjustment, or alignment. The piece can also be analyzed to measure furnace carbon potential (that is, because while in the furnace it will quickly carburize to a level equal to the furnace carbon potential). |
| Shock load |
The sudden application of an external force that results in a very rapid build-up of stress. |
| Shore hardness |
A measure of the hardness of materials such as elastomers, plastics, and rubbers determined using a Shore durometer which measures the penetration depth of a hardened steel rod (the indenter) 1.1–1.4 mm in diameter into the test material for a given force applied rapidly with a calibrated spring. There are two main indenter designs: type A (for softer materials) for which the end of the rod is a truncated 35° cone tapering to a diameter of 0.79 mm, and type D (harder materials) for which the end is a 30° cone with a 0.1 mm radius tip. The spring force is 8.05 N for type A and 44.45 N for type D. The hardness value is related to the penetration depth (limited to a maximum of 2.5 mm) such that for zero penetration the hardness is 100 and for maximum penetration the hardness is zero. The relevant standards are ISO 48-1, 48-4, and 48-7. Shore hardness values of selected materials are as follows: ABS 100, cellulose acetate 50–95, high-density polyethylene 60–70, low density polyethylene 40–50, polymethylmethacrylate 90–99, PTFE 50–65, and plasticized PVC 15–70. |
| Shoulder |
The portion of a shaft, stepped component, or flanged component, where a change in diameter or other dimension occurs. Even for a nominal step change, in practice there will be a radius where shoulder and shaft meet in order to avoid too severe a stress concentration. |
| Shoulder bolt |
A bolt for which the unthreaded cylindrical section between the thread and the head is of larger diameter than the threaded section, precisely machined to length and diameter and hardened |
| Shoulder joint |
The second joint on an articulated robot, which has a horizontal axis and is analogous to the human shoulder. |
| Shoulder screw |
A screw having two or more diameters or shoulders and commonly used for supporting levers and other machine parts that have to operate freely. |
| Shrink fit |
A tight interference fit between two components resulting from heating an outer part and/or cooling an inner part to allow easy assembly. The outer component contracts on cooling, while the inner part expands on warming to ambient temperature, thus gripping the two parts together. |
| Shrink ring |
A ring, which is expanded by heating, is placed around an assembly of parts, and then contracts upon cooling to hold the assembly in place. |
| SI system |
SI is an abbreviation of Système International d’Unités, i.e., the international unit measurement system – an internationally established system of measurement units built on older metric systems. |
| Sigma phase |
A hard, brittle, nonmagnetic intermediate phase with a tetragonal crystal structure, containing 30 atoms per unit cell, space group P42/mnm, occurring in many binary and ternary alloys of the transition elements. The composition of this phase in the various systems is not the same, and the phase usually exhibits a wide range in homogeneity. Alloying with a third transition element usually enlarges the field of homogeneity and extends it deep into the ternary section. |
| Sigma-phase embrittlement |
Embrittlement of iron-chromium alloys (most notably austenitic stainless steels) caused by precipitation at grain boundaries of the hard, brittle intermetallic sigma phase during long periods of exposure to temperatures between approximately 565 and 980 °C (1050 and 1800 °F). Sigmaphase embrittlement results in severe loss in toughness and ductility and can make the embrittled material structure susceptible to intergranular corrosion. |
| Significance level |
The stated probability (risk) that a given test of significance will reject the hypothesis that a specified effect is absent when the hypothesis is true. |
| Silky fracture |
A metal fracture in which the broken metal surface has a fine texture, usually dull in appearance. Characteristic of tough and strong metals. |
| Single thread |
A screw thread cut around a cylinder having a single start in which the lead is equal to the pitch. |
| Single-shear test |
(1) A shear test similar to the double-shear test used for round-bar specimens, but that uses only one stationary shear blade. (2) A shear test that uses a sheet or thin-plate specimen. |
| Sinter (sintering) |
A solid-state diffusion densification process for the production of objects, particularly porous objects, from raw material in powder, granular, mesh, perforatedsheet, or fibre form by heating to a temperature below the melting point until the constituents bond together. It is used for ceramic materials and metals. Applications of sintered materials include filtration, separation, flow control, fluidization, and noise reduction. |
| Sintering |
The bonding of adjacent surfaces in a mass of particles by molecular or atomic attraction on heating at high temperatures below the melting temperature of any constituent in the material. Sintering strengthens a powder mass and normally produces densification and, in powdered metals, recrystallization. |
| Size dimension (Unit m) |
The specified value of a diameter, length, width, etc. of a feature required to specify the finished form of a component or assembly. |
| Size effect |
Effect of the dimensions of a piece of metal on its mechanical and other properties and on manufacturing variables such as forging reduction and heat treatment. In general, the mechanical properties are lower for a larger size. |
| Sizing |
1. A finishing operation to ensure the specified dimensions and tolerances for a component are met. 2. (size classification) Separating an aggregate of mixed particles into groups according to size using a series of screens. |
| Slack quenching |
The incomplete hardening of steel due to quenching from the austenitizing temperature at a rate slower than the critical cooling rate for the particular steel, resulting in the formation of one or more transformation products in addition to martensite. |
| Slant fracture |
A type of fracture appearance, typical of plane-stress fractures, in which the plane of metal separation is inclined at an angle (usually about 45°) to the axis of the applied stress. |
| Slenderness ratio |
The effective unsupported length of a uniform column divided by the least radius of gyration of the cross-sectional area. |
| Sliding-vane compressor |
(rotary-vane compressor, vane compressor) A rotary compressor in which gas is compressed as the spaces between spring-loaded sliding vanes held in an offset rotor reduce as the rotor revolves within a cylindrical housing. |
| Slip |
Plastic deformation by the irreversible shear displacement (translation)of one part of a crystal relative to another in a definite crystallographic direction and usually on a specific crystallographic plane. Sometimes called glide. |
| Slip band |
A group of parallel slip lines so closely spaced as to appear as a single line when observed under an optical microscope. |
| Slip joint |
A mechanical connection which allows limited endwise relative movement of two components such as pipes, rods, and ducts. |
| Slip line |
The trace of the slip plane on a viewing surface; the trace is usually observable only if the surface has been polished before deformation. The usual observation on metal crystals (under the light microscope) is of a cluster of slip lines known as a slip band. |
| Slot |
A groove machined into a component, e.g. to allow for thermal expansion, or in which the tongue or tip of another component can fit or slide. |
| Slug |
1. A starting workpiece for forging and similar operations, such as a length of wire or rod to make a bolt blank on which a thread can be rolled or cut. 2. (geepound) An obsolete imperial (non-SI) unit of mass, being the mass which under an acceleration of one foot per second squared gives a force of one pound-force. The conversion to SI is 1 slug = 14.593 902 94 kg. 3. A large-scale flow structure which occurs in the transition from laminar to turbulent flow in a pipe. |
| Slug wrench |
A box wrench with an anvil on the end of the handle. Torque is produced by striking the anvil with a sledge hammer. |
| Snap temper |
A precautionary interim stress-relieving treatment applied to high-hardenability steels immediately after quenching to prevent cracking because of delay in tempering them at the prescribed higher temperature. |
| Socket head |
Screw head having a hexagonal or other form of recessed socket in the head so that the screw can be turned with a wrench or key, as a hexagon key. |
| Socket wrench |
A form of spanner with an internally ridged steel socket to fit a nut or the head of a bolt or screw. |
| Solenoid |
A coil of electrically-conducting wire wrapped around a metal core, typically iron, to produce a magnetic field and hence a force on the core when an electric current passes through the coil. Solenoids are widely used to produce linear movement to actuate valves (solenoid valves) and other devices. |
| Solution heat treatment |
Heating an alloy to a suitable temperature, holding at that temperature long enough to cause one or more constituents to enter into solid solution, and then cooling rapidly enough to hold these constituents in solution. |
| Sound pressure (Unit μPa) |
The instantaneous deviation of pressure from the ambient pressure caused by the passage of a sound wave, which can be measured by a microphone in air and a hydrophone in water. |
| Spalling |
The separation of macroscopic particles from a surface in the form of flakes or chips. |
| Span (Unit m) |
A dimension measured between the extremities of a body or structure, such as between the tips of a wing or the supports of a bridge. |
| Spanner (wrench) |
A tool for applying torque to tighten or loosen a nut, bolt, or screw. Grip is applied by a serrated ring or an open U-shape at the end of the tool. On imperial-size spanners, the BSW and BSF marking refers to the diameter of the screw thread; similarly for BA spanners. Spanners for unified and metric threads are marked with the distance across the flats (‘A/F’) of the nut or head of the bolt. |
| Specific heat capacity |
The heat necessary to raise the temperature of 1 kg of a substance by 1 K. It is measured in J/(kg K). |
| Specific weight (γ) (Unit N/m3) |
The weight per unit volume of a substance: if W is the weight of a volume of the substance, ρ is its density, and g is the acceleration due to gravity, γ = W/ = ρg. |
| Specimen |
A test object, often of standard dimensions or configuration, that is used for destructive or nondestructive testing. One or more specimens may be cut from each unit of a sample. |
| Speed of rotation (N) |
(Unit rps or rpm) For an object rotating about a fixed axis, the speed of rotation is the number of turns (revolutions) of the object per unit time. It is equal to the angular velocity Ω (or ω) (unit rad/s) divided by 2π. |
| Spherical washer |
A washer whose upper surface is semispherical. Used with a nut whose contact face is also semispherical. Reduces bending stress in a bolt or stud, by allowing some self-alignment and some compensation for nonparallel joint surfaces or Angularity. |
| Spinner |
A co-axial streamlined fairing enclosing the hub of a propeller and rotating with it. |
| Spray quenching |
A quenching process using spray nozzles to spray water or other liquids on a part. The quench rate is controlled by the velocity and volume of liquid per unit area per unit of time of impingement. |
| Spring |
An elastic component which stores mechanical energy and exerts a force when deformed. The slope of the curve of applied force F to the deflexion of a spring x, dF/dx, is termed the spring rate or spring modulus k with unit N/m. For a linear spring, the F(x) curve is proportional and k is called the spring constant. |
| Spring |
An elastic device which yields under stress or pressure but returns to its original state or position when the stress or pressure is removed. |
| Spring clip |
A clip made of a material such as spring steel which grips an inserted part. |
| Spring constant |
The ratio between the forces exerted on a spring (or a bolt) and the deflection thereof. Has the dimensions of force per unit change in length (e.g., lb=in.). Also called Stiffness. |
| Spring load (Unit N) |
A load exerted on a component by bending, compressing, extending, or twisting an attached spring. Mechanical energy stored in the spring can be released if any constraints on the component are removed. |
| Spring materials |
Mainly metals including plain-carbon and corrosion-resisting steels, phosphor bronze, spring brass, beryllium copper and nickel alloys, all of which in their workhardened states permit large reversible elastic strains without permanent deformation. |
| Spring temper |
A temper of nonferrous alloys and some ferrous alloys characterized by tensile strength and hardness about two-thirds of the way from full hard to extra spring temper. |
| Springback |
The extent to which metal tends to return to its original shape or contour after undergoing a forming operation. |
| Square |
1. A regular quadrilateral, i.e. a polygon with four equal sides and four right angles. 2. An instrument to check angles of internal and external surfaces and flatness. |
| Square thread |
A form of screw thread in which the cross–section of the thread forms a square, making the width of the thread equal to the space between the threads. |
| Stability |
1. A term relating to whether a body, structure, system, or flow is in a state of stable, unstable, or neutral equilibrium. 2. In numerical analysis, whether numerical errors, such as roundoff or input-data errors, are damped, propagate, or cause divergence of the solution as the numerical method proceeds. 3. A control system is often described as stable when a step change in input results in a steady output without continuing oscillation. |
| Stabilizing treatment |
(1) Before finishing to final dimensions, repeatedly heating a ferrous or nonferrous part to or slightly above its normal operating temperature and then cooling to room temperature to ensure dimensional stability in service. (2) Transforming retained austenite in quenched hardenable steels, usually by cold treatment. (3) Heating a solution-treated stabilized grade of austenitic stainless steel to 870 to 900 °C (1600 to 1650 °F) to precipitate all carbon as TiC, NbC, or TaC so that sensitization is avoided on subsequent exposure to elevated temperature. |
| Stainless steels |
Iron-base alloys that are highly resistant to corrosion in many environments. The predominant alloying element is chromium, which, like iron at room temperature, has BCC crystal structure. Corrosion resistance by passivation is achieved by the formation of chromium oxide on the steel surface. Nickel, which has FCC crystal structure, is another important addition for some steels. Other additions used to enhance properties include manganese, molybdenum, niobium, and titanium. There are several classes of stainless steel. |
| Standard |
(standard specification) A set of specifications for components, machines, materials, or processes intended to achieve uniformity, efficiency, and a specified quality. In the UK, standards issued by the British Standards Institution (BSI) are now generally those of the International Organization for Standardization (ISO). |
| Standard deviation |
A statistical term used to quantify the Scatter in a set of data points. If the standard deviation is small, most of the data points are ‘‘nearly equal.’’ A large deviation means less agreement. |
| Standard deviation |
The most usual measure of the dispersion of observed values or results expressed as the positive square root of the variance. |
| Standard deviation (σ) |
The square root of variance, a measure of the spread of data about the mean value. |
| Standard fit |
The fit of a component machined or otherwise manufactured to standardized clearances and tolerances. |
| Standard gauge |
A highly-accurate reference gauge against which to check working gauges. |
| Standard hole |
A hole in a workpiece bored to a specified tolerance where clearance with a shaft is accomplished by allowance on the shaft. A standard shaft is machined to a specified tolerance where clearance with a hole is accomplished by allowance on the hole. |
| Standardization |
1. National and international agreements for design, manufacture, materials, performance, practices, requirements, strength, etc. which ensure common results wherever an item is made and wherever used. 2. The manufacture of components so that interchangeability of parts during assembly of new, or repair of old, items is possible without ‘fitting’. |
| Starting friction |
(limiting friction, static friction, stiction) (Unit N) The force required to initiate movement between two bodies in contact. |
| Starting taper |
The taper on the end of a reamer or tap which aids in starting the cut. |
| Starting torque (Unit N.m) |
The torque developed by a motor at zero rotational speed in order to initiate rotation of the applied load. |
| Starting valve (regulator) |
A valve which admits steam from the boiler to the cylinder(s) of a steam engine. |
| Static equilibrium |
A solid body, structure, or physical system (or any subdivided part) will be in static equilibrium, either at rest or moving with constant velocity, if the resultants of all external forces and moments acting on it are zero. |
| Static fatigue |
A term referring to a time-dependent reduction in strength with a static (noncyclic) load. The term may refer to hydrogen-induced delayed cracking or the effect of creep on the strength of plastics. |
| static fatigue |
Delayed fracture caused by environmental effects under dead-weight loading of materials such as glass. The name is unfortunate, as cyclic loads are not involved and glass is not susceptible to fatigue. |
| Statistic |
A summary value calculated from the observed values in a sample. |
| Stator |
A ring of non-rotating blades or nozzles in a compressor, turbine, or other turbomachine, which directs fluid flow into an adjacent rotor. |
| Stead’s brittleness |
A condition of brittleness that causes transcrystalline fracture in the coarse grain structure that results from prolonged annealing of thin sheets of low-carbon steel previously rolled at a temperature below about 705 °C (1300 °F). The fracture usually occurs at about 45° to the direction of rolling. |
| Steady loads |
Loads that do not change in intensity or that change so slowly they may be regarded as steady. |
| Steady pin |
A dowel, key, or pin that prevents a pulley from turning on its shaft. |
| Steam gauge |
A pressure gauge used to measure gauge pressure in a line, boiler, cylinder, or other device operating with steam. |
| Steel-wire rope |
A rope or cable made up of many steel strands wrapped helically about an axis, each strand being made of metal wires twisted together like a helix. |
| Stiffness |
(1) The ability of a metal or shape to resist elastic deflection.(2) The rate of stress increase with respect to the rate of increase in strain induced in the metal or shape; the greater the stress required to produce a given strain, the stiffer the material is said to be. |
| Stove bolt |
This bolt has been so named because of its use in stove building. It is made in a number of different forms, either with a round button, or flat countersunk head, the head having a slot for a screwdriver and the threaded end being provided with a square or hexagon nut. |
| Strain |
The unit of change in the size or shape of a body due to force. Also known as nominal strain. |
| Strain aging |
The changes in ductility, hardness, yield point, and tensile strength that occur when a metal or alloy that has been cold worked is stored for some time. In steel, strain aging is characterized by a loss of ductility and a corresponding increase in hardness, yield point, and tensile strength. |
| Strain energy |
A measure of the energy absorption characteristics of a material determined by measuring the area under the stress-strain diagram. |
| Strain gage |
A device for measuring small amounts of strain produced during tensile and similar tests on metal. A coil of fine wire is mounted on a piece of paper, plastic, or similar carrier matrix (backing material), which is rectangular in shape and usually about 25 mm (1.0 in.) long. This is glued to a portion of metal under test. As the coil extends with the specimen, its electrical resistance increases in direct proportion. This is known as bonded resistance-strain gage. Other types of gages measure the actual deformation. Mechanical, optical, or electronic devices are sometimes used to magnify the strain for easier reading. |
| Strain hardening |
An increase in hardness and strengthcaused by plastic deformation at temperatures below the recrystallization range. Also known as work hardening. |
| Strain hardening |
(workhardening) The increase in stress above the initial yield stress required to deform a metal to greater strains in the plastic range at temperatures below the recrystallization range (cold working). Unloading after some strain or work input gives a harder and stronger material. |
| Strain rate |
The time rate of straining for the usual tensile test. Strain as measured directly on the specimen gage length is used for determining strain rate. Because strain is dimensionless, the units of strain rate are reciprocal time. |
| strain-age embrittlement |
A loss in ductility accompanied by an increase in hardness and strength that occurs when low-carbon steel (especially rimmed or capped steel) is aged following plastic deformation. The degree of embrittlement is a function of aging time and temperature, occurring in a matter of minutes at about 200°C (400 °F), but requiring a few hours to a year at room temperature. |
| Strain-hardening exponent (n value) |
The value n in the relationship σ = Kεn, where σ is the true stress, ε is the true strain, and K, the strength coefficient, is equal to the true stress at a true strain of 1.0. The strain hardening exponent is equal to the slope of the true stress/true strain curve up to maximum load, when plotted on log-log coordinates. The n-value relates to the ability of a sheet of material to be stretched in metalworking operations. The higher the n-value, the better the formability(stretchability). Also known as the strain-hardening coefficient. |
| Strength (Unit Pa) |
The maximum stress, in tension, compression, shear, or combinations thereof, that may be monotonically applied to a material, component, or structure before failure (defined as fracture, yielding, buckling, etc. as appropriate). |
| Strength of bolt |
An ambiguous term which can mean Ultimate strength or Proof load or Endurance limit or Yield strength. |
| Strength of materials |
A confusing name for what is really stress analysis, reflecting older engineering design based upon elasticity and ‘theories of strength’ for different materials, without regard to cracks. |
| Stress |
The intensity of the internally distributed forces or components of forces that resist a change in the volume or shape of a material that is or has been subjected to external forces. Stress is expressed in force per unit area and is calculated on the basis of the original dimensions of the cross section of the specimen. Stress can be either direct (tension or compression)or shear. |
| Stress (Unit Pa) |
1. Stress ‘at a point’ (i.e. over a volume of material that is very small compared with that of the component or structure) is the load per unit area for every face of an infinitesimal cube surrounding the point. 2. In fluid flow the stress tensor σij is the sum of an isotropic part −pδij, δij being the Kronecker delta, having the same form as the stress tensor for a fluid at rest, p being the static pressure, and the non-isotropic, deviatoric stress tensor dij which is due entirely to the fluid motion. |
| Stress amplitude |
One-half the algebraic difference between the maximum and minimum stress in one cycle of a repetitively varying stress. |
| Stress area |
The effective cross-sectional area of the threaded section of a fastener. Used to compute average stress levels in that section. Based on the mean of pitch and minor diameters. |
| Stress corrosion |
(stress-corrosion cracking) Corrosion failure accelerated by local stresses (particularly tensile) which enhance cracking in both monotonic and fatigue loading. |
| Stress corrosion cracking (SCC) |
A common form of Stress cracking in which an Electrolyte encourages the growth of a crack in a highly stressed bolt. Only a tiny quantity of electrolyte need be present, at the tip or face of the crack. |
| Stress cracking |
A family of failure modes, each of which involves high stress and chemical action. The family includes Hydrogen embrittlement, Stress corrosion cracking, stress embrittlement, and hydrogen-assisted stress corrosion. |
| Stress cycle |
The smallest segment of the stress-time function that is repeated periodically. |
| Stress cycles endured (N) |
The number of cycles of a specified character (that produce fluctuating stress and strain) that a specimen has endured at any time in its stress history. |
| Stress difference (Unit Pa) |
The algebraic difference between the largest and least principal stresses in a loaded body, equal to twice the greatest shear stress in the system. For σ1 > σ2 > σ3, it is given by (σ1 − σ3). |
| Stress distribution |
The manner in which tensile, compressive, and shear stresses are distributed within a loaded body, indicated by loci of constant stress. |
| Stress equalizing |
A low-temperature heat treatment used to balance stresses in cold-worked material without an appreciable decrease in the mechanical strength produced by cold working. |
| Stress factor |
A calibration constant used in ultrasonic measurement of bolt stress or strain. It is the ratio between the change in ultrasonic transit time caused by the change in length of the fastener, under load, to the total change in transit time. |
| Stress raisers |
Changes in contour or discontinuities in structure that cause local increases in stress. |
| Stress ratio (A or R) |
The algebraic ratio of two specified stress values in a stress cycle. Two commonly used stress ratios are the ratio of the alternating stress amplitude to the mean stress, A = Sa/Sm, and the ratio of the minimum stress to the maximum stress, R = Smin/Smax. |
| Stress relaxation |
The slow decrease in stress level within a part (e.g., a bolt) which is heavily loaded under constant deflection conditions. A ‘‘cousin’’ to creep, which is a slow change in geometry under constant stress conditions. |
| Stress relaxation |
The time-dependent decrease in stress in a solid under constant strain at constant temperature due to creep. The stress-relaxation behavior of a metal is usually shown in a stress-relaxation curve. |
| Stress relieving |
Heating to a suitable temperature, holding long enough to reduce residual stresses, and then cooling slowly enough to minimize the development of new residual stresses. |
| Stress wave |
An unbalanced force, such as an impact, applied at one location in a body sets the material there into vibration that is transmitted to adjacent elements and ultimately to all parts of the body in the form of stress waves or wave packets. |
| Stress-concentration factor (Kt) |
A multiplying factor for applied stress that allows for the presence of a structural discontinuity such as a notch or hole; Kt equals the ratio of the greatest stress in the region of the discontinuity to the nominal stress for the entire section. Also known as theoretical stress-concentration factor. |
| Stress-corrosion cracking (SCC) |
A time-dependent process in which a metallurgically susceptible material fractures prematurely under conditions of simultaneous corrosion and sustained loading at lower stress levels than would be required in the absence of a corrosive environment. Tensile stress is required at the metal surface and may be a residual stress resulting from heat treatment or fabrication of the metal or the result of external loading. Cracking may be intergranular or transgranular, depending on the combination of alloy and environment. |
| Stress-intensity calibration |
A mathematical expression, based on empirical or analytical results, that relates the stress-intensity factor to load and crack length for a specific specimen planar geometry. Also known as Kcalibration. |
| Stress-intensity factor |
A scaling factor, usually denoted by the symbol K, used in linear-elastic fracture mechanics to describe the intensification of applied stress at the tip of a crack of known size and shape. At the onset of rapid crack propagation in any structure containing a crack, the factor is called the critical stress-intensity factor, or the fracture toughness. |
| Stress-relaxation curve |
A plot of the remaining, or relaxed, stress in a stress-relaxation test as a function of time. The relaxed stress equals the initial stress minus the remaining stress. Also known as a stress-time curve. |
| Stress–strain curve |
(stress–strain diagram) A plot of stress vs strain for a solid material obtained from a test in which a specimen is loaded in tension, compression, shear, or combinations thereof. Diagrams of true stress vs true strain deviate from those of nominal stress vs nominal strain only at large strains where appreciable changes in cross section of testpieces occur. |
| Stress-strain diagram |
A graph in which corresponding values of stress and strain are plotted against each other. Values of stress are usually plotted vertically(ordinates or y-axis) and values of strain horizontally (abscissas or xaxis). Also known as deformation curve and stress-strain curve. |
| Stretch forming |
A manufacturing process in which sheet-metal components are physically stretched over a forming die using a press, punch, or edge clamps. Stretching combined with bending reduces springback. |
| Stretch ratio (extension ratio, λ) |
A measure, used for highly-extensible materials such as rubber, of the extensional or normal strain of a component subjected to tension and defined as the ratio of the final length l, for a given load, to the initial or gauge length l0, where the coordinate axes are those of principal strain. It is related to the engineering strain e by λ = 1 + e. |
| Stretch-bending test |
A simulative test for sheet metal formability in which a strip of sheet metal is clamped at its ends in lock beads and deformed in the center by a punch. Test conditions are chosen so that fracture occurs in the region of punch contact. |
| Striation |
A fatigue fracture feature, often observed in electron micrographs, that indicates the position of the crack front after each succeeding cycle of stress. The distance between striations indicates the advance of the crack front across that crystal during one stress cycle, and a line normal to the striation indicates the direction of local crack propagation. |
| Stroke |
1. (stroke length) (Unit m) The linear distance between top dead centre and bottom dead centre of a piston in a reciprocating engine or mechanism. 2. The movement of a piston or plunger in a reciprocating machine to execute a particular function; for example, the exhaust stroke of an engine in which the exhaust gases are expelled from a cylinder. |
| Structural analysis |
The determination of the forces, displacements, stresses, and strains in a given structure. |
| Structural engineering |
A branch of civil engineering concerned with the design of buildings, dams, bridges, and other large structures. |
| Structural frame |
The underlying load-bearing framework of beams, columns, bracing, etc., of steel, concrete, brick, stone, timber etc., required to support the static and dynamic forces a building or component is subjected to, including its weight and that of its contents. |
| Structural-integrity analysis |
Assessment of components and structures for likelihood of failure by buckling, fracture, yielding, etc. under conditions of normal and accident loading. |
| Structure |
An assembly, usually load bearing, of connected (pinned, riveted, screwed, bolted, welded, etc.) components, such as bars, tubes, beams, and plates. |
| Stud |
A headless threaded fastener, threaded on both ends, with an unthreaded body in the middle section, or threaded from end to end. Used with two nuts, or with one nut and a tapped hole. |
| Stud |
A rod having thread on both ends. |
| Stud |
A plain cylindrical piece having a threaded portion of suitable length at each end or a continuous thread over the entire length. One end is screwed into a machine or workpiece after which a second part is placed over the stud and held in place by a nut. |
| Subcritical annealing |
A process anneal performed on ferrous alloys at a temperature below Ac1. |
| Submerged-electrode furnace |
A furnace used for liquid carburizing of parts by heating molten salt baths with the use of electrodes submerged in the ceramic lining. |
| Supercharging |
The process of increasing the mass flow rate of air (or air/fuel mixture) into the cylinder(s) of a piston engine using a compressor driven from the crankshaft (the supercharger). The power output is increased compared with a naturally-aspirated engine of the same capacity. The process also increases the air pressure and density to greater than ambient. In the past, most superchargers were mechanically driven from the engine’s crankshaft, but these have been largely superseded by turbochargers. |
| Supercooling |
Cooling below the temperature at which an equilibrium phase transformation can take place, without actually obtaining the transformation. |
| Superheating |
Heating above the temperature at which an equilibrium phase transformation should occur without actually obtaining the transformation. |
| Surface hardening |
A generic term covering several processes applicable to a suitable ferrous alloy that produces, by quench hardening only, a surface layer that is harder or more wear resistant than the core. There is no significant alteration of the chemical composition of the surface layer. The processes commonly used are carbonitriding, carburizing, induction hardening, flame hardening, nitriding, and nitrocarburizing. Use of the applicable specific process name is preferred. |
| Surface hardness |
(superficial hardness) Indentation hardness on, and for some distance below, the surface of a component, as opposed to its bulk hardness. Depending on the resolution required, micro- or nano-hardness testing machines are employed. |
| Surface roughness (roughness) |
The small-scale, irregular peaks and troughs in a solid surface which are quantified using a surface analyser. Roughness may be a consequence of wear and corrosion, or of the manufacturing process. It leads to increased friction in solid– solid contact or fluid flow over a surface. Errors of form and waviness are excluded, but surface texture includes roughness and waviness. Surface topography incorporates the small-scale, three-dimensional geometry of a surface including surface roughness, machined, and etched features, typically at sub-mm scales. |
| Surface tension (σ, γ) (Unit N/m) |
At the interface between a liquid and a gas or two immiscible liquids, unbalanced cohesive forces acting on the liquid molecules at the interface lead to the property surface tension which causes tensile forces to develop as if it were a skin or membrane. |
| Surface treatment |
Any process, including chemical, electrochemical, mechanical, and thermal, designed to protect a surface against corrosion and wear or to alter its mechanical properties. |
| Swept volume (Unit m3) |
In the cylinder of a piston engine or other device, the volume between top and bottom dead centre positions of a piston; equal to πR2S where R is the piston radius and S is the stroke. |
| Swift cup test |
A simulative cupping test in which circular blanks of various diameters are clamped in a die ring and deep drawn into cups by a flat-bottomed cylindrical punch. |
| System engineering |
A methodology which integrates all disciplines and specialty groups into a team effort, forming a structured development process that proceeds from concept to production to operation. |
