| Damage |
1. The deterioration of a component or structure in fault or accident conditions, reducing or preventing its ability to perform its intended function. 2. The accumulation of defects or microcracks in the microstructure of a body loaded monotonically or in fatigue, which weakens the body and can lead to crack propagation and failure. |
| Damage mechanics |
The theory of degradation in bodies, particularly fracture by accumulated microstructural damage. Analyses take two approaches: (a) the use of some critical integrated function of stress and strain at which cracking is initiated and propagated; (b) incorporation of damage in the stress–strain curves to reflect weakened material. |
| Damage tolerance |
(defect tolerance) A design philosophy that takes into account initial imperfections, crack-growth rates and conditions at final fracture, and uses fracture mechanics to demonstrate that cracks should not grow to their critical length within the design life (or at least should be capable of ready detection). |
| Dead centre |
The point at which, in a crank mechanism, the piston connecting rod and crank are all in line so that there can be no driving moment. |
| Dead load (Unit N) |
A load on a component or structure that is steady with time, e.g. the self-weight of a bridge. |
| Dead space |
(dead volume) (Unit m3) That volume of a gas-filled thermometer in which the gas is not at the same temperature as that being measured. |
| Decalescence |
A phenomenon, associated with the transformation of alpha iron to gamma iron on the heating (superheating) of iron or steel, revealed by the darkening of the metal surface owing to the sudden decrease in temperature caused by the fast absorption of the latent heat of transformation. |
| Decarburization |
Loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface. |
| Decarburization |
Loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface. |
| Decompression chamber |
A chamber in which ambient-air pressure can be increased to levels found in deep-sea diving. It is used to gradually acclimatize divers back to normal conditions and avoid ‘the bends’. |
| Deflection temperature under load (DTUL) |
The temperature at which a simple cantilever beam deflects a given amount under load. Formerly called heat distortion temperature. |
| Deformation |
A change in the form of a body due to stress, thermal change, change in moisture, or other causes. |
| Deformation (Unit 1/s) |
In solid mechanics, any change, reversible (elastic) or permanent (plastic), in the shape or size of parts of a body, or the whole body, caused by external or internal loading. It includes extension, compression, bending, and twisting. The same state of deformation in a body can appear as different combinations of elongation and shear. To state how much of the total strain is shear, it is split into dilatation and deviatoric components, the former changing volume but not shape, the latter shape but not volume. |
| Degradation |
The reduction with time of the physical properties of a material. |
| Degradation failure |
Failure of a system, component, or structure owing to material degradation. |
| Degradation of energy |
Conversion of energy into forms of lower usefulness due to irreversibilities in energy transfer and conversion processes. The increase in entropy can be regarded as a measure of the degradation of energy. |
| Degree (°) |
A measure of plane angle such that 1° is 1/360 of a complete revolution and equal to π/180 rad. |
| Degrees of freedom |
The number of independent variables (such as temperature, pressure, or concentration within the phases present) that may be altered at will without causing a phase change in an alloy system at equilibrium; or the number of such variables that must be fixed arbitrarily to define the system completely. |
| Delamination |
A mode of failure of composite materials, including radial-ply tyres, in which the layers separate due to repeated cyclic loading, impact, or weak bonding. |
| Densimeter |
An instrument used to determine the density or relative density of a solid or liquid. |
| Density |
The mass per unit volume of a solid material. |
| Density |
(mass density, ρ) (Unit kg/m3) The mass per unit volume of a substance that satisfies the continuum assumption. The reciprocal of specific volume. |
| Deposit gauge |
(deposition gauge) An instrument employed in air pollution studies for measuring the amount of pollutant deposited on a given area in a given time under given conditions. |
| Depth (Unit m) |
1. The vertical distance below a datum surface, especially the sea surface. 2. The distance between the top and bottom of a hole, step in a surface, or a container. |
| Depth gauge |
1. A precision instrument, typically consisting of a machine-divided steel rule passing through a hardened-steel cross head, used to measure the depths of slots, holes, shoulders, projections, etc. 2. A device used by divers to indicate the water depth. |
| Design code |
(design standard) A standard or specification for any aspect of engineering design, issued by national organizations such as ANSI, ASME, BSI, DIN, and ISO. |
| Design heating load (Unit kW) |
The heating requirements based on a specified number of heating degree days, or required to maintain a building or other enclosed space at a specified temperature for a given outside temperature. |
| Design load (Unit N) |
The greatest load that a component or structure is expected to experience under normal operating conditions. |
| Design pressure (Unit Pa) |
The greatest pressure that a closed container is expected to experience under normal operating conditions. |
| Design stress (Unit Pa) |
The greatest allowable stress in a component or structure that will not result in failure under normal operating conditions. |
| Destructive testing |
Measurement of the mechanical properties of a material, component, or structure, by increased loading until the sample fails by fracture, collapse, or buckling. |
| Deviation |
The difference between the actual value and the desired value of a controlled variable. |
| Dew point |
The temperature and pressure at which a gas begins to condense to a liquid. |
| Dew point analyzer |
An atmosphere monitoring device that measures the partial pressure of water vapor in an atmosphere. |
| DIN |
Deutsches Institut für Normung, the German Institute for Standardization. |
| Diagonal pitch (Unit m) |
The distance, when components in an assembly are staggered, between the position of a component in one row or column and the position of the corresponding component in the next row or column. The term is applied to rivets, turbine or compressor blades in a cascade, vortex generators on the surface of a wing, etc. |
| Diaphragm compressor |
A machine in which compression is achieved by the reciprocating motion of a flexible membrane of metal, plastic, or elastomeric material. This arrangement is ideally suited to pumping high-purity, toxic, or explosive gases. A similar machine incorporating check valves is a self-priming positive-displacement pump (diaphragm pump, membrane pump). |
| Diaphragm meter |
A dry flow meter in which there are two or more interconnected chambers, each having a diaphragm in the wall. The chambers empty and fill alternately and the flow rate of gas is determined from the movement of diaphragms. Diaphragm meters are commonly used to monitor domestic and commercial gas supply. |
| Die |
1. A tool having an appropriately-shaped hole through which material may be extruded or drawn. 2. A tool employed in forging. 3. A block having the male or female shape employed in stamping operations. 4. A thick circular disc, driven by a die wrench, with internally-threaded cutting edges for producing a screw thread. A die wrench (die stock) holds a screw-cutting die and has two projecting arms for applying the torque necessary to cut the thread. A die having a hexagonal or square shape (die nut), driven by a spanner, is used in confined spaces where a die wrench cannot be rotated through a full circle. |
| Die casting |
A process in which molten metal, particularly alloys of aluminium, magnesium, copper, and zinc, is forced under pressure (10 to 200 MPa) into a reusable hardened-steel mould machined into a die. |
| Die screw |
Tool used when threading cylindrical pieces beyond the capacity of a die plate. |
| Die threading |
Die threading is a machining process for cutting external threads in cylindrical or tapered surfaces by the use of solid or self-opening dies. Die threading is a slower method of producing external threads than thread rolling, but it is faster than single-point threading in a lathe. |
| Diesel engine |
(compression–ignition engine) A piston engine operating on the Diesel cycle in which the air is compressed to a temperature above the autoignition temperature of the fuel and combustion is initiated as the fuel is injected into the hot air. Diesel engines operate at higher compression ratios (typically in the range 12 to 24) than petrol engines. Although named after Rudolf Diesel, the inventor, Diesel engine is often spelled with a lower case d, contrary to normal practice. |
| Differential heating |
Heating that intentionally produces a temperature gradient within an object such that, after cooling, a desired stress distribution or variation in properties is present within the object. |
| Diffusion |
(1) Spreading of a constituent in a gas, liquid, or solid, tending to make the composition of all parts uniform. (2) The spontaneous movement of atoms or molecules to new sites within a material. |
| Diffusion coating |
Coating on metal for corrosion- and/or oxidation-resistance. Coatings are produced by heating a metal in contact with an alloy in powder, liquid, or gaseous form at elevated temperature such that atoms of the coating diffuse into the substrate. Examples include chromium, aluminium, or silicon on substrates such as nickel-based superalloys, steels, and refractory metals. |
| Diffusion coefficient |
A factor of proportionality representing the amount of substance diffusing across a unit area through a unit concentration gradient in unit time. |
| Dilatation (dilation) |
A change of volume caused by external load, compression, temperature change, chemical action, etc. |
| Dilatometer |
An instrument for measuring length or volume changes in a solid during heating and subsequent cooling or isothermal holding. |
| Dilatometer |
· An instrument for measuring the linear expansion or contraction in a metal resulting from changes in such factors as temperature and allotropy. |
| Dimension line |
A line on an engineering drawing with a numeral above it that shows the length of a feature, usually in millimetres. |
| Dimensional stability |
Ability of a plastic part to retain the precise shape in which it was molded, fabricated, or cast. |
| Dimensioning |
The specification on an engineering drawing of the size (e.g. length, radius, angle, or spacing) and the relative location (e.g. angular position) of each feature of a component. The numerical values often include the tolerances. There should be no more dimensions than are necessary to manufacture the component. |
| Direct quenching |
(1) Quenching carburized parts directly from the carburizing operation. (2) Also used for quenching pearlitic malleable parts directly from the malleablizing operation. |
| Dislocation density |
The total length of dislocation lines per unit volume, or the number of dislocation lines that cut through a unit cross-sectional area. |
| Displacement |
The distance that a chosen measurement point on a cracked specimen displaces normal to the crack plane as the crack grows. |
| Distortion |
Any deviation from an original size, shape, or contour that occurs because of the application of stress or the release of residual stress. |
| Double aging |
Employment of two different aging treatments to control the type of precipitate formed from a supersaturated matrix in order to obtain the desired properties. The first aging treatment, sometimes referred to as intermediate or stabilizing, is usually carried out at higher temperature than the second. |
| Double tempering |
A treatment in which a quench-hardened ferrous metal is subjected to two complete tempering cycles, usually at substantially the same temperature, for the purpose of ensuring completion of the tempering reaction and promoting stability of the resulting microstructure. |
| Double-shear test |
A shear test having two stationary, shear blades and one moving one and that uses solid round bars as test specimens. |
| Dowel |
A headless cylindrical pin that fits into corresponding holes in mating components, thus ensuring relative location. |
| Dowel |
A round metal or wooden pin. |
| Dowel |
A pin, usually of circular shape like a cylinder, used to pin or fasten something in position temporarily or permanently. |
| Drift pin |
A round tapered steel pin used to align rivet holes so that the rivet will pass through the holes easily. |
| DTI |
Direct tension indicator. A fastener used primarily in the structural steel industry, designed to indicate that a certain minimum amount of tension has been developed in the fastener during assembly. |
| Ductile |
(ductile material) Describes a material that can be deformed permanently to large strains without fracture. |
| Ductile |
If a bolt can be stretched well past its yield point before breaking, it is said to be ductile. |
| Ductile cast iron |
A cast iron that has been treated while molten with an element such as magnesium or cerium to induce the formation of free graphite as nodules or spherulites, which imparts a measurable degree of ductility to the cast metal. Also known as nodular cast iron, spherulitic graphite cast iron and SG iron. |
| Ductile crack propagation |
Slow crack propagation that is accompanied by noticeable plastic deformation and requires energy to be supplied from outside the body. |
| Ductile fracture |
Fracture characterized by tearing of metal accompanied by appreciable gross plastic deformation and expenditure of considerable energy. |
| Ductile fracture |
A fracture of a component or structure which is preceded by extensive ductile deformation so that the broken pieces cannot be re-fitted to regain the original size and shape of the component or structure. |
| Ductile fracture |
Fracture characterized by tearing of metal accompanied by appreciable gross plastic deformation and expenditure of considerable energy. |
| Ductility |
The ability of a material to deform plastically before fracturing. Measured by elongation or reduction in area in a tensile test, by height of cupping in a cupping test, or by the radius or angle of bend in a bend test. |
| Ductility |
The ability of a material to deform plastically without fracturing, measured by elongation or reduction of area in a tensile test, by height of cupping in an Erichsen test, or by other means. |
| Durometer hardness |
Measure of the indentation hardness of plastics. It is the extent to which a spring-loaded steel indenter protrudes beyond the pressure foot into the material. |
| Dynamic compressor |
A machine, such as a centrifugal or axial compressor, that compresses a gas by rotational rather than reciprocating motion. |
| Dynamic coupling |
In vibrations, the existence of inertial terms depending on mass in the governing equations, so that there is only a force if there is a corresponding acceleration. |
| Dynamic hardness |
(rebound hardness) The resistance of a material to local indentation by a rapidly-moving rigid indenter. In most practical methods the indenter is allowed to fall under gravity on to the surface of the material when the rebound height is a measure of the dynamic hardness. |
| Dynamic load (Unit N) |
1. Loading of a component or structure by a moving object whose point of application changes with time, e.g. the live load of a train passing over a bridge. 2. A load applied to a particular part of a component or structure in a short time interval. |
| Dynamic load rating (Unit N) |
The allowable load on a component or structure when the loading is not static. |
| Dynamic mechanical measurement |
A technique in which either the modulus and/or damping of a substance under oscillatory load or displacement is measured as a function of temperature, frequency, or time, or a combination thereof. |
| Dynamic modulus |
The ratio of stress to strain under cyclic conditions (calculated from data obtained from either free or forced vibration tests, in shear, compression, or tension). |
| Dynamic recovery |
A process occurring in hot working of metals in which a fine subgrain structure forms within the elongated grains due to annihilation of dislocations due to easy cross slip and climb. It results in a lowering of the flow stress. Dynamic recovery, as opposed to dynamic recrystallization that occurs in hot working, occurs in metals of high stacking fault energy such as aluminum, αiron, and most bcc metals. |
| Dynamic strain aging |
A behavior in metals in which solute atoms are sufficiently mobile to move toward and interact with dislocations. This results in strengthening over a specific range of elevated temperature and strain rate. |
| Dynamics |
The branch of mechanics that deals with objects in motion. Its two main branches are kinematics, which studies motion without regards to its cause, and kinetics, which also takes into account forces that cause motion. |
