Mechanical Engineering Dictionary-Material Properties

Important Terms and their Meaning of Mechanical Engineering- Material Properties:

ACICULAR STRUCTURE – A microstructure characterized by needle shaped constituents.

ALLOTROPY – Ability of a material to exist in several crystalline forms.

AMORPHOUS – Non-crystalline, a random orientation of the atomic structure.

ANISTROPY – A material that has specific physical properties in different directions. Rolled steel is strongest in the direction of rolling.

API GRAVITY – Gravity expressed in units of standard American Petroleum Institute (hydrometer).

AUSTENITE – A solid solution of cementite or iron carbide, Fe₃C in iron.

BANDED STRUCTURE – A segregated structure of nearly parallel bands aligned in the direction of working.

BEL – A unit denoting the ratio of power levels of signals or sound. The number of bells may be given as the common logarithm of the ratio of powers.

BETA RAY – A ray of electrons emitted during the spontaneous disintegration of certain atomic nuclei.

BOILING POINT – The temperature at which a liquid begins to boil.

BOUND ELECTRONS – The inner orbit of electrons around the nucleus of the atom.

BREAKING POINT – The final rupture of a material which is being pulled in tension, after it has reached its ultimate strength.

BRINELL HARDNESS – The hardness of metal or alloy measured by pressing a hard ball (usually 10 mm diameter) with a standard load into the specimen. A number is derived by measuring the indentation with a special microscope.

BRITTLE METAL – A metal which exhibits only a very small change in dimensions before it fractures.

BRITTLENESS – The property of materials to not deform under load, but to break suddenly, for example, cast iron and glass are brittle. Brittleness is opposite to plasticity.

BULK MODULUS OF ELASTICITY – Ratio of a uniform, triaxial (equal in all directions) tensile or compressive stress to the change in volume it produces.

CAVITATION – The formation and instantaneous collapse of innumerable tiny voids or cavities within a liquid subjected to rapid and intense pressure changes.

CEMENTATION – Process of introducing elements into the outer layer of metal objects by means of high temperature diffusion.

CEMENTITE – Iron carbide, Fe3C, a hard brittle, crystalline compound observed in the microstructure of iron base alloys.

CHAFING FATIGUE – Fatigue initiated in a surface damaged by rubbing against another body.
CHLORINATION – A refining or degasification process, wherein dry chlorine gas is passed through molten aluminium base and magnesium base alloys to remove entrapped oxides and dissolved gases.

CLEAVAGE – Splitting (fracture) of a crystal in a crystallographic plane of low index.

CLEAVAGE FRACTURE – A fracture, usually of a polycrystalline metal, in which most of the grains have failed by cleavage, resulting in bright reflecting facets. It is one type of crystalline fracture.

CLEAVAGE PLANE – A characteristic crystallographic plane or set of planes on which cleavage fracture easily occurs.

COALESCENCE – The union of particles of a dispersed phase into larger units usually effected at temperatures below fusion point.

COHESIVE STRENGTH – (1) The hypothetical stress in an unnotched bar causing tensile fracture without plastic deformation. (2) The stress corresponding to the forces between atoms.

COLD SHORT – A condition of brittleness existing in some metals at temperatures below the recrystallisation temperature.

COLD SHUT – (1) A discontinuity that appears on the surface of cast metal as a result of two streams of liquid meeting and failing to unite. (2) A portion of the surface of a forging that is separated, in part, from the main body of metal by oxide.

COLUMNAR STRUCTURE – A coarse structure of parallel columns of grains having the long axis perpendicular to the casting surface.

COMPLETE FUSION – Fusion which has occurred over the entire base metal surfaces exposed for welding.

COMPOUND – A combination of two or more elements that are mixed together.

COMPRESSIBILITY – The property of a substance (e.g., air) by virtue of which its density increases with increase in pressure.

COMPRESSIVE STRENGTH (ultimate) – The maximum stress that can be applied to a brittle material in compression without fracture.

COMPRESSIVE STRENGTH (yield) – The maximum stress that can be applied to a metal in compression without permanent deformation.

COMPRESSIVE STRESS – Compressive stress is compression load per unit area perpendicular to the load.

CONDUCTIVITY – The quality or power of conducting or transmitting heat, electricity etc.

CONGRUENT TRANSFORMATION – An isothermal or isobaric phase change in which both of the phases concerned have the same composition throughout the process.

COOLING STRESSES – Residual stresses resulting from non-uniform distribution of temperature during cooling.

CORROSION – The destructive chemical or electro-chemical reaction of a material and its environment, usually associated only with metals in contact with liquids.

CORROSION EMBRITTLEMENT – The severe loss of ductility of a metal resulting from corrosive attack, usually inter-granular and often not visually apparent.

CORROSION FATIGUE – Effect of the application of repeated or fluctuating stresses in a corrosive environment characterized by shorter life than would be encountered as a result of either the repeated or fluctuating stresses alone or the corrosive environment alone.

COUPON – A piece of metal from which a test specimen is to be prepared, often an extra piece as on a casting or forging.

COVALENT BOND – A bond between two or more atoms resulting from the completion of shells by the sharing of electrons.

CRAZING – Minute surface cracks on the surface of materials often caused by thermal shock.

CREEP – Slow plastic deformation in steel and most structural metals caused by prolonged stress under the yield point at elevated temperatures.

CREEP LIMIT – (1) The maximum stress that will cause less than a specified quantity of creep in a given time. (2) The maximum nominal stress under which the creep strain rate decreases continuously with the time under constant load and at constant temperature. Sometimes called CREEP STRENGTH.

CRITICAL POINT – The temperature or pressure at which a change in crystal structure, phase, or physical properties occur.

CRYSTALLIZATION – Act or process of forming crystals or bodies formed by elements or compounds solidifying so that they are bounded by plane surfaces.

CRYSTAL UNIT STRUCTURE OR UNIT CELL – The simplest polyhedron that embodies all the structural characteristics of a crystal and makes up the lattice of a crystal by indefinite repetition.

CURIE TEMPERATURE – The temperature of magnetic transformation below which a metal or alloy is magnetic and above which it is paramagnetic.

DAMAGING STRESS – The minimum stress which, if exceeded in the material, would render the part unfit for service before the end of its normal expected life.

DAMPING CAPACITY – The ability to absorb vibration. More accurately defined as the amount of work dissipated into heat by a unit volume of material during a completely reverse cycle of unit stress.

DENDRITE – A crystal formed during solidification of a metal or alloy characterized by a structure like that of a fir tree.

DENSITY – The ratio of the mass of a body to its volume.

DUCTILE or MALLEABLE METAL – A metal that may be worked to a different size or shape without breaking or shattering.

DUCTILITY – The property of a material to deform permanently or to exhibit plasticity, elongation or bending or twisting without rupture (breaking or cracking) while under tension.

DYNAMIC CREEP – Creep that occurs under the conditions of fluctuating load or fluctuating temperature.

ELASTIC DEFORMATION – The movement or deflection of a material when an external load is applied that is less than the elastic limit.

ELASTICITY – The ability of a material to return to its original form after the load has been removed.

ELASTIC LIMIT – Maximum stress that can be applied to a metal without causing plastic deformation that will remain after the load is relaxed to zero.

ELASTIC RATIO – It is the ratio of yield point stress to tensile strength of a metal.

ELECTROLYSIS – Chemical change resulting from the passage of an electric current through an electrolyte.

ELONGATION – Elongation is the strain produced by uniaxial tension.

EMBRITTLEMENT – Reduction in the normal ductility of a metal due to a physical or chemical change.

ENDURANCE LIMIT – The limiting stress below which the metal will withstand without fracture an infinitely large number of cycles of stress.

ENDURANCE RATIO – It is the ratio between the endurance limit to tensile strength.

EUTECTIC – The alloy which has the lowest melting point possible for a given composition.

EUTECTOID – A solid solution of any series which cools without change to its temperature of final composition.

FACTOR OF SAFETY – The ratio of the damaging stress to working stress.

FATIGUE IN METALS – The tendency in a metal to fail, breaking or cracking under conditions of repeated cyclical stressing that take place well below the ultimate tensile strength.

FATIGUE STRENGTH – The amount of stress that can be applied to a metal without failure while it is subjected to ten million or more cycles of load reversals. In mild steel, the fatigue strength is about 50 per cent of the tensile strength.

FERRITE – Iron practically carbon free. It forms a body centered cube lattice and may hold in solution considerable amounts of silicon, nickel or phosphorous.

FLUIDITY – Ability of molten metal to flow readily, usually measured by the length of a standard spiral casting.

FRACTURE STRESS – The maximum principal true stress at fracture. Usually refers to unnotched tensile specimens.

FREE ELECTRONS – Electrons which are in the outer orbit of the atoms nucleus.

GRAIN BOUNDARY – The outer perimeter of a single grain where it is in contact with adjacent grains.

GRAIN GROWTH OR RECRYSTALLIZATION – Metal grains begin to reform to larger and more regular size and shape at certain temperatures, depending to some extent on the amount of prior cold working.

GRAIN REFINER – A material added to a molten metal to attain finer grains in the final structure.

GRAIN SIZE – For metals, a measure of the area or volume of grains in polycrystalline material, usually expressed as an average when the individual sizes are fairly uniform. Reported in terms of number of grains per unit area or volume, average diameter, or as a grain size number derived from area measurements.

GRANULAR PEARLITE – A structure formed from ordinary lamellar pearlite by long annealing at a temperature below but near to the critical point, causing the cementite to spherodize in a ferrite matrix.

GROWTH – With reference to cast iron, permanent increase in volume that results from continued or repeated cyclic heating and cooling at elevated temperatures.

HALF-LIFE – The characteristic time required for half of the nuclei of a radioactive species to disintegrate spontaneously.

HARDENABILITY – The property that determines the depth and distribution of hardness in a ferrous alloy induced by heating and quenching.

HARDNESS – The property of metal to resist being permanently deformed. This is divided into three categories, resistance to penetration, resistance to abrasion and elastic hardness.

HOT SHORT – Brittleness in hot metal. The presence of excess amounts of sulphur in steel causes hot shortness.

HYDROGEN EMBRITTLEMENT – A condition of low ductility in metals resulting from the absorption of hydrogen.

IMPACT TEST – A test in which small notched specimens are broken in Izod-Charpy machine. This test determines the notch toughness of a metal.

INCLUSIONS – Impurities, usually oxides, sulphides, silicates and similar compounds, retained from the ore or from processing.

INFRARED RADIATION – Electromagnetic energy with wave lengths from 770 to 1200 manometers.

ISOTROPY – Quality of having identical properties in all directions.

KISH – Graphite thrown out by liquid cast iron in cooling.

KNOOP HARDNESS – Micro-hardness determined from the resistance of metal to indentation by a pyramidal diamond indenter, having edge angles of 172° and 32 minutes and 130°, making a rhombohedral impression with one long and one short diagonal.

LATTICE, SPACE – The term that is used to denote a regular array of points in space. For example, the sites of atoms in a crystal. The points of the three dimensional space lattice are constructed by the repeated application of the basic translations that carry a unit cell into its neighbour.

LIQUIDUS – The temperature at which freezing begins during cooling and ends during heating under equilibrium conditions, represented by a line on a two phase diagram.

MACHINABILITY – The relative ease of machining that is related to the hardness of the material to be cut.

MACROSTRUCTURE – The structure of metals as revealed by examination of the etched surface of a polished specimen at a magnification not exceeding ten diameters.

MALLEABILITY – In metals, the property of being able to undergo mechanical deformation (flattened by rolling or hammering) without rupturing or developing a marked increase in resistance to change of shape.

MELTING POINT – The temperature at which a pure metal, compound or eutectic changes from solid to liquid, the temperature at which the liquid and the solid are in equilibrium.

MICROPOROSITY – Porosity visible only with the aid of microscope.

MICROSTRUCTURE – The structure of polished or etched metal specimens as seen enlarged through a microscope.

MODULUS OF ELASTICITY – A measure of the rigidity of metal. Ratio of stress, within proportional limit, to corresponding strain. Also called ELASTIC MODULUS and COEFFICIENT OF ELASTICITY and YOUNGS MODULUS.

MODULUS OF RIGIDITY – Ratio of shearing stress to shearing strain, within the proportional limit.

MODULUS OF RUPTURE – A fictitious stress calculated for either bending or torsion tests on the basis of the load causing failure, and the assumption that the elastic equation for stress applies up to the point of failure.

Ms TEMPERATURE – The temperature at which martensite begins to form in an alloy system on cooling.

MOTTLED – White iron structure interspersed with spots or flecks of gray.

NOTCH BRITTLENESS – Susceptibility of a material to brittle fracture at points of stress concentration.

NOTCH DUCTILITY – The percentage reduction in area after complete separation of the metal in a notch tensile test.

NOTCH RUPTURE STRENGTH – The ratio of applied load to original area of the minimum cross-section in a stress rupture test of a notched specimen.

NOTCH SENSITIVITY – A measure of the reduction in strength of a metal caused by the presence of stress concentration.

NOTCH TOUGHNESS – The resistance to fracture of a metal specimen having a notch or groove when subjected to a sudden load, usually tested on an Izod-Charpy testing machine.

OILINESS – The capacity of the lubricant to stick on to the surface under conditions of heavy load.

OXIDATION – The slow or rapid reaction of oxygen with other elements, burning. In metals, over oxidation during heating under oxidizing conditions often results in permanent damage to metals.

OXIDATION REDUCTION – A chemical reaction in which one or more electrons are transferred from one atom or molecule to another.

PEARLITE – A micro constituent of iron and steel consisting of alternative layers of ferrite and iron carbide or cementite.

PEEL LOAD – In metal, plastics or composites, the force that acts to peel apart joined pieces.

PH – The negative logarithm of the hydrogen ion activity. It denotes the degree of acidity or basicity of a solution.

PERMEABILITY – In casting of metals, the term is used to define the porosity of foundry sands in moulds and the ability of trapped gases to escape through the sand.

PERMANENT SET – When a metal remains deformed from its original dimensions after the forces applied to it have been reduced to zero, it is said to have undergone plastic deformation, and the amount of deformation is called the permanent set.

PHASE – A portion of an alloy, physically homogeneous throughout, that is separated from the rest of the alloy by distinct BOUNDARY surfaces. The following phases occur in the iron carbon alloy, molten alloy; austenite, ferrite, cementite, and graphite.

PHYSICAL CHANGE – A change which does not alter the composition of the molecules of a substance.

PIEZOELECTRIC EFFECT – The reversible interaction, exhibited by some crystalline materials between an elastic strain and an electric field. The direction of the strain is dependent upon the polarity of the field.

PLASTICITY – The quality of material such that it can be deformed without breaking. Clay is completely a plastic material. Metals exhibit plasticity in varying amounts.

PLASTIC DEFORMATION – Deformation that does or will remain permanent after the removal of load which caused it.

POISSON’S RATIO – When a rod of elastic material is elongated by stretching (strain), the lateral (crosswise) dimensions will contract. Poisson’s ratio is the ratio between the strain and the amount of lateral contraction.

PROPORTIONAL LIMIT – Proportional limit is the value of stress at which the stress strain curve first bends to the right. The proportional limit is high for steels and low for cast iron, copper, and aluminium.

PROOF LOAD – A predetermined load, generally some multiple of the service load, to which a specimen or structure is submitted before acceptance for use.

PROOF, RESILIENCE – Maximum amount of elastic energy that may be stored without permanent deformation in the material during its first loading cycle.

RADIOACTIVITY – The spontaneous nuclear disintegration with emission of corpuscular or electromagnetic radiation.

RESIDUAL STRESS – Stress induced within the structure of a material by cold working, machining, and heat treatments.

RESILIENCE – Ability of a material to store elastic energy without permanent deformation.

RETENTIVITY – The capacity of a material to retain a portion of the magnetic field set up in it after the magnetizing force is removed.

ROCKWELL HARDNESS – Hardness test that uses a penetrator and known weights. Several scales are used to cover very soft to very hard materials. The Rockwell C scale is used mostly for steel.

SACRIFICIAL ANODE – A metal slug, usually magnesium and zinc, designed to concentrate galvanic corrosion upon itself and thus save a more important structure on which the anode is attached, such as a ship’s hull or a buried pipe line.

SCRATCH HARDNESS – The hardness of a metal determined by the width of a scratch made by a cutting point drawn across the surface under a given pressure.

SEGREGATION – Non-uniform distribution of alloying elements, impurities or micro phases.

SHEARING – A concentration of forces in which the bending moment is virtually zero and the metal tends to tear or to be cut along a transversal axis at the point of applied pressure.

SHEAR LOAD – A load that tends to force materials apart by application of side slip action.

SHEAR STRESS – Shear stress is the shear load per unit area, exerted between the material on one side of an imaginary plane of separation in the part and the material on the other side.

SHORTNESS – A form of brittleness in metal. It is designated as cold, hot and red to indicate the temperature range in which the brittleness occurs.

SKELP – A name of the semi finished steel of which butt-welded pipe is made.

SLAG (dross) – A fused product that occurs in the melting of metals and is composed of oxidized impurities of a metal and a fluxing substance such as limestone. The slag protects the metal from oxidation by the atmosphere since it floats on the surface of the molten metal.

SLIP PLANES – Also called slip bands. These are lines that appear on the polished surface of a plastically deformed metal. The slip bands are the result of crystal displacement, defining planes in which shear has taken place.

SOLID SOLUTION – Found in metals at temperatures below the solidus. Some of the types of solid solutions are continuous, intermediate, interstitial, substitutional and terminal.

SOLIDUS – Seen as a line in a two phase diagram, it represents the temperatures at which freezing ends when cooling, or melting begins when heating under equilibrium conditions.

SOLUBILITY – The degree to which one substance will dissolve in another.

SPALLING – Breaking small pieces from a surface, often caused by thermal shock.

SPECIFIC GRAVITY – A numerical value that represents the weight of a given substance with the weight of an equal volume of water. The specific gravity for pure water is taken as 1.000.

SPHEROIDIZING – Consists of holding carbon steel for a period of time at just under the transformation temperature (heating and cooling). An aggregate of globular carbide is formed from other microstructures such as pearlite.

SPRING BACK – The tendency of a formed metal part to return to some extent to its former shape because of the elasticity of the metal.

SPUTTERING – To dislodge atoms from the surface of a material by collision with high energy particles for the purpose of depositing a metallic film on a part.

STIFFNESS – The ability of a metal or shape to resist elastic deformation. For identical shapes, the stiffness is proportional to the modulus of elasticity.

STRAIN – The measure of the change in the size or shape of a body, referred to its original size or shape. This is unit deformation of a metal when stress is applied.

STRENGTH – The ability of a material to resist external forces. It is called tensile, compressive, or shear strength depending on the load.

STRESS – The load per unit area on a stress strain diagram.

STRESS, TENSILE – Refers to an object loaded in tension, denoting the longitudinal force that causes the fibres of a material to elongate.

STRESS, COMPRESSIVE – Refers to a member loaded in compression, which either gives rise to a given reduction in volume or a transverse displacement of the material.

STRESS, SHEAR – Refers to a force that lies in a parallel plane. The force tends to cause the plane of the area involved to slide on the adjacent planes.

STRESS, TORSION – The shearing stress that occurs at any point in a body as the result of an applied torque or torsional load.

STRESS RAISER – Can be a notch, nick, weld under cut, sharp change in section, or machining grooves or hairline cracks that provide a concentration of stresses when the metal is under tensile stress. Stress raisers pose a particular problem and can cause early failure in members that are subjected to many cycles of stress reversals.

STRESS RELIEF ANNEAL – The reduction of residual stress in a metal part by heating it to a given temperature and holding it there for a suitable length of time. This treatment is used to relieve stresses caused by welding, cold working, machining, casting and quenching.

SUPER-CONDUCTIVITY – Property of zero electrical resistance (and zero magnetic induction) acquired by some metals at very low temperatures.

SURFACE TENSION – Interfacial tension between two phases one of which is a gas.

TARNISHING – A chemical reaction of a metal with its environment which results in formation of films which coat the metal.

TEMPER – (1) In ferrous metals, the stress relief of steels that are hardened by quenching for the purpose of toughening them and reducing their BRITTLENESS. (2) In non-ferrous metals, temper is a condition produced by mechanical treatment such as cold working. An alloy may be cold worked to the hard temper, fully softened to the annealed temper, or two intermediate tempers.

TENSION LOAD – A load applied to joined parts that attempt to separate them by a pulling or stretching action.

TENSILE STRESS – Tensile stress is the tension load per unit area perpendicular to the load.

THERMAL CONDUCTIVITY – The quantity of heat that is transmitted per unit time, per unit cross-section, per unit temperature gradient through a given substance. All materials are in some measure conductors of heat.

THERMAL EXPANSION – The increase of the dimension of a material that results from the increased movement of atoms caused by increased temperature.

THERMAL SHOCK – A stress induced on the surface of a material such as carbide tools or fire brick caused by a rapid rate of heating and surface expansion.

THERMAL STRESS – Shear stress that is induced in a material due to unequal heating or cooling rates. The difference of expansion and contraction between the interior and exterior surfaces of a metal that is being heated or cooled is an example.

THERMOPLASTIC – Material capable of softening or fusing when heated and of hardening again when cooled.

THERMOSETTING – Material capable of becoming permanently rigid when cured by heating will not soften by reheating.

TOUGHNESS – Ability of a metal to absorb energy and deform plastically before fracturing. Work per unit volume required to fracture a metal and is equal to the area under the stress strain curve.

TOXICITY – The degree of danger posed by a substance to animal or plant life.
TRANSFORMATION TEMPERATURE – The temperature at which one phase transforms into another phase, for example, where ferrite or alpha iron transforms into austenite or gamma iron.

TRANSITION TEMPERATURE – The temperature at which normally ductile metals become brittle.

ULTIMATE STRENGTH – Maximum conventional stress, tensile, compressive or shear that a material can withstand. The highest strength that a metal exhibits after it begins to deform plastically under load. Rupture of the material occurs either at the peak of its ultimate strength or at a point of further elongation and at a drop in stress load.

VALENCY – The capacity of an atom to combine with other atoms to form a molecule. The inert gases have zero valence, valence is determined by considering the positive and negative atoms as determined by the atoms gaining or losing of valence electrons.

VISCOSITY – The property of the fluids, either liquid or gaseous, which causes them to resist instantaneous change of shape or instantaneous rearrangement of their parts, due to internal friction.

VOID – A cavity or hole in a substance.

WEAR – A surface deterioration of contacting surfaces that destroys their operating relationship, or causes rupture if carried far enough.

WELDMENT – A unit formed by welding together an assembly of pieces.

WORKING STRESS – The actual stress that the part will be designed to handle.

YIELD POINT – The stress at which a marked increase in deformation occurs without an increase in load stress as seen in mild steel or medium carbon steel. This phenomenon is not seen in non-ferrous metals and other alloy steels.

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