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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