# Texture Analysis and Materials Testing Terminology

A glossary of terms to help understand the vocabulary used in this field of physical testing.

Texture Analysis and Materials Testing have many technical terms that may need to be explained for a thorough understanding of these fields. Whilst terms may be specific to texture analysis or to materials testing (which usually takes a more fundamental approach) there are many terms that are common to both. Terms can be broken into the following categories:

**Sample Characteristics** – terms related to the structure and properties of the sample.

**Apparatus Terms** – terms related to the equipment used to carry out a test.

**Test Types** – an explanation of the methods that might be used for texture analysis or materials testing.

**Data Terms** – how to understand the words pertaining to the data that you have captured.

**Measured Parameters** – an explanation of vocabulary used to describe results that have been obtained from your graphs.

## Sample Characteristics

**Abrasion.** Roughening or scratching of a surface due to **abrasive wear.**

**Abrasive wear.** The removal or displacement of material from a surface when hard particles slide or roll across the surface under pressure. The particles may be loose or may be part of another surface in contact with the surface being worn. Contrast with **adhesive wear.**

**Anisotropy.** A variation in the mechanical properties of a material relative to direction.

**Asperity.** A protuberance in the small-scale topographical irregularities of a solid surface.

**Brittle crack propagation.** A very sudden propagation of a crack with the absorption of no energy except that stored elastically in the body. Microscopic examination may reveal some deformation not noticeable to the unaided eye.

**Brittle fracture.** Separation of a solid accompanied by little or no macroscopic plastic deformation. Typically, brittle fracture occurs by rapid crack propagation with less expenditure of energy than for ductile fracture.

**Buckling.** A compression phenomenon that occurs when, after some critical level of load, a bulge, bend, bow, kink or other wavy condition is produced in a beam, column, plate, bar or sheet product form.

**Cleavage.** The tendency to cleave or split along definite crystallographic planes or other planar structural feature.

**Creep.** Time dependent strain that occurs under stress. The creep strain occurring at a diminishing rate is called primary or transient creep; that occurring at a minimum and almost constant rate, secondary or steady state creep; that occurring at an accelerating rate, tertiary creep.

**Distortion. **Any deviation from an original size, shape or contour that occurs because of the application of stress or the release of residual stress.

**Ductile crack propagation.** Slow crack propagation that is accompanied by noticeable plastic deformation and required energy to be supplied from outside the body.

**Ductile fracture.** Fracture characterised by tearing of a material 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 or cupping in a cupping test, or by the radius or angle of bend in a bend test.

Elasticity. Ability of a material to return to its original shape when load is removed.

**Endurance.** The capacity of a material to withstand repeated application of stress.

**Fatigue.** The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the ultimate tensile strength of the material. Fatigue failure generally occurs at loads that applied statically would produce little perceptible effect. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.

**Flexibility.** The quality or state of a material that allows it to be flexed or bent repeatedly without undergoing rupture.

**Flow.** When essentially parallel planes within an element of a material move (slip or slide) in parallel directions; occurs under the action of shearing stress. Continuous action in this manner, at constant volume and without disintegration of the material, is termed yield, creep or plastic deformation.

**Fracture.** (1) The cracking or breaking of a hard material. (2) The irregular surface produced when a specimen is broken.

**Fragmentation.** A sample breaking into small pieces under load.

**Hysteresis.** The phenomenon of permanently absorbed or lost energy that occurs during any cycle of loading or unloading when a material is subjected to repeat loading.

**Isotropy.** A term indicating equal physical or mechanical properties in all directions within a material.

**Malleability.** A characteristic of materials that permits plastic deformation in compression without fracture.

**Meltability. **(1) The extent to which a product flows and spreads upon heating. (2) The ability of the particles of a product to flow together and form a continuous melted mass upon heating.

**Necking.** Localised reduction of cross-sectional area of a specimen under tensile load. It is disregarded in calculating engineering stress.

**Physical properties.** Properties of a material, the determination of which does not involve the deformation or destruction of the specimen. For example, density, electrical conductivity, coefficient of thermal expansion, magnetic permeability and lattice parameter. Does not include chemical reactivity or properties more appropriately regarded as mechanical properties.

**Plasticity.** The property that enables a material to undergo permanent deformation without rupture.

**Pliability.** The distance that a supported sample drops when extended horizontally (in a cantilever hold).

**Poisson’s ratio.** The absolute value of the ratio of transverse (lateral) strain to the corresponding axial strain resulting from uniformly distributed axial stress below the proportional limit of the material.

**Residual stress.** Stresses that remain within a body as a result of plastic deformation.

**Shear fracture.** A ductile fracture in which a crystal (or polycrystalline mass) has separated by sliding or tearing under the action of shear stresses.

**Springback.** Degree to which a material returns to its original shape after deformation. It is also called recovery.

**Strain hardening.** An increase in hardness and strength caused by plastic deformation at temperatures below the recrystallization range. Also known as work hardening.

**Strain relaxation.** Alternate term for creep.

**Stress raisers.** Changes in contour or discontinuities in structure that cause local increases in stress.

**Tear resistance.** Measure of the ability of materials to resist tearing.

**Thermal fatigue.** Fracture resulting from the presence of temperature gradients that vary with time in such a manner as to produce cyclic stresses in a structure.

**Viscoelasticity.** The property of materials that exhibit both viscous and elastic characteristics when undergoing deformation.

**Wear.** Damage to a solid surface, generally involving progressive loss of material, due to relative motion between that surface and a contacting surface or substance.

**Yield.** Evidence of plastic deformation in structural materials. Also known as plastic flow or creep.

## Apparatus Terms

**Calibrate.** To determine, by measurement or comparison with a standard, the correct value of each scale reading on a measuring (test) instrument.

Calibration. Determination of the values of the significant parameters by comparison with values indicated by a reference instrument or by a set of reference standards.

**Capacity. **In testing machines, the maximum load and / or displacement for which a machine is designed. Some testing machines have more than one load capacity. These are equipped with accessories that allow the capacity to be modified as desired.

**Constraint.** Any restriction to the deformation of a body.

**Extensometer.** An instrument for measuring changes in length over a given gauge length caused by application or removal of a force. Commonly used in tensile testing of specimens.

**Frame deflection.** Movement of a Texture Analyser setup during a test, primarily due to load cell bend. It can be accounted for using a frame stiffness calibration.

**Gauge length.** The original length of that portion of a specimen over which strain or change of length is determined.

**Load Cell.** Electro-mechanical device that records the mechanical response of a sample.

**Load.** In the case of testing machines, a force applied to a test piece that is measured in units such as Newtons.

**Lubricant.** Any substance interposed between two surfaces for the purpose of reducing the friction or wear between them.

**Notch depth.** The distance from the surface of a test specimen to the bottom of the notch. In a cylindrical test specimen, the percentage of the original cross-sectional area removed by machining an annular groove.

**Specimen.** A test object often of standard dimensions or configuration that is used for destructive or non-destructive testing. One or more specimens may be cut from each unit of a sample.

**Strain gauge.** A device for measuring small amounts of strain produced during tensile and similar tests on materials. A coil of fine wire is mounted on a piece of paper, plastic or similar carrier matrix (backing material), which is rectangular in shape. This is glued to a portion of a test specimen. As the coil extends with the specimen, its electrical resistance increases in direct proportion. This is known as a bonded resistance-strain gauge. Other types of gauges measure the actual deformation. Mechanical, optical or electronic devices are sometimes used to magnify the strain for easier reading.

**Target condition. **The force, distance or strain the Texture Analyser travels to once it has passed its trigger.

**Thermocouple.** A device for measuring temperature, consisting of lengths of two dissimilar metals or alloys that are electrically joined at one end and connected to a voltage-measuring instrument at the other end. When one junction is hotter than the other, a thermal electromotive force is produced that is roughly proportional to the difference in temperature between the hot and cold junctions.

**Tolerance.** The amount by which a quantity, such as a dimension, property, or composition, is allowed to vary; the tolerance is the difference between the maximum allowable limits.

**Trigger force.** The instrument will move the probe until it detects a force greater than the trigger force setting. Measurement starts once this point is reached. This ensure that products of different sizes are accounted for. This facility also allows the instrument to measure the product height.

**Verification.** Checking or testing the instrument to ensure conformance with the specification.

**Verified loading range.** In the case of testing machines, the range of indicated loads for which the testing machine gives results within the permissible variation specified.

**Vernier.** A short auxiliary scale that slides along the main instrument scale to permit more accurate fractional reading of the least main division of the main scale.

## Test Types

**Bend test.** See **flexure test.**

**Brinell hardness test.** A test for determining the hardness of a material by forcing a hard steel or carbide ball of specified diameter into it under a specified load. The result is express as the Brinell hardness number.

**Capillary rheometry.** Measurement and analysis of a liquid flowing into and within a capillary. The pressure difference along the capillary is measured as a function of the liquid flow rate and some rheological information is inferred from this data.

**Climbing drum peel test. **Method for determining peel resistance of adhesive bond between a relatively flexible and rigid material.

**Compressibility and recovery test.** Method for measuring the behaviour of materials in a compression / de-compression cycle. This test is not designed to indicate long term (creep) behaviour.

**Compression test. **Method for determining the behaviour of materials under compressive (crushing) loads.

**Creep rupture test.** A test in which progressive specimen deformation and the time for rupture are both measured. In general, deformation is much greater than that developed during a creep test.

**Creep test.** Method for determining the extension of a material under a given load at a given temperature. The determination usually involves the plotting of time-elongation curves under constant load; a single test may extend over many months. The results are often expressed as the elongation per hour on a given gauge length.

**Extrusion testing.** Compression of a sample until flow occurs through an outlet. Texture analysis uses both forward extrusion (forced through an aperture) and backward extrusion (forced back around the compressing probe).

**Fatigue test.** A method for determining the behaviour of materials under fluctuating loads.

**Flexure test.** Method for measuring behaviour of materials subjected to simple 3-point bend loading.

**Flexure.** A term used in the study of strength of materials to indicate the property of a body, usually a rod or beam, to bend without fracture.

**Fracture test.** Breaking a specimen and examining the fractured surface with the unaided eye or with a low-power microscope to determine such things as composition, grain size, case depth or internal defects.

**Lastometer.** Method of measuring the response of a circular leather sample due to uni-axial strain from a spherical penetrometer. Also known as a ball burst test when applied to materials other than leather.

**Penetration Testing.** A sample is penetrated with a probe of smaller cross-sectional area than itself. This results in shear and compressive forces within the sample as well as tensile stresses at its surface.

**Physical testing.** Methods used to determine the entire range of a material’s physical properties. In addition to density and thermal, electrical and magnetic properties, physical testing methods may be used to assess simple fundamental physical properties such as colour, crystalline form and melting point.

**Rockwell hardness test.** An indentation hardness test using a calibrated machine that utilises the depth of indentation, under a constant load, as a measure of hardness.

**Tensile stress.** A stress that causes two parts of an elastic body, on either side of a typical stress plane, to pull apart.

**Tribology.** The science and technology concerned with interacting surfaces in relative motion.

**Vickers hardness test.** An indentation hardness test employing a diamond pyramidal indenter and variable loads, enabling the use of one hardness scale for all ranges of hardness. Also known as diamond pyramid hardness test.

## Data Terms

**Accuracy.** (1) The agreement or correspondence between an experimentally determined value and an accepted reference value for a material undergoing testing. The reference value may be established by an accepted standard (such as those established by ASTM), or in cases the average value, obtained by applying the test method to all the sampling units in a lot or batch of the material may be used. (2) The extent to which the result of a calculation or the reading of an instrument approaches the true value of the calculated or measured quantity.

**Batch.** A definite quantity of some product or material produced under conditions that are considered uniform.

**Bias.** A systematic error that contributes to the difference between a population mean of the measurements or test results and an accepted reference or true value.

**Characteristic.** A property of items in a sample or population that when measured, counted, or otherwise observed helps to distinguish between the items.

**Error.** Deviation from the correct value. In the case of a testing machine, the difference obtained by subtracting the load indicated by the calibration device from the load indicated by the testing machine.

**Estimate. **The particular value, or values, of a parameter computed by an estimation procedure for a given sample.

**Estimation.** A procedure for making a statistical inference about the numerical values of one or more unknown population parameters from the observed values in a sample.

**Observed value.** The particular value of a characteristic determined as a result of a test or measurement.

**Percent error.** In the case of a testing machine, the ratio, expressed as a percentage, of the error to the correct value of the applied load.

**Population.** The hypothetical collection of all possible test specimens that could be prepared in the specified way from the material under consideration. Also known as universe.

**Precision.** The closeness of agreement between randomly selected individual measurements or test results. The standard deviation of the error of measurement may be used as a measure of “imprecision”.

**Repeatability.** A term used to refer to the test result variability associated with a limited set of specifically designed sources of variability within a single laboratory.

**Reproducibility.** A term used to describe test result variability associated with specifically defined components of variance obtained both from within a single laboratory and between laboratories.

**Sample.** One or more units of a product (or a relatively small quantity of a bulk material) that are withdrawn from a lot or process stream, and that are tested or inspected to provide information about properties, dimensions or other quality characteristics of the lot or process stream. Not to be confused with specimen.

**Standard deviation.** The most usual measure of the dispersion of observed values or results expressed as the positive square root of the variance.

**Statistic.** A summary value calculated from the observed values in a sample.

**Uncertainty.** An indication of the variability associated with a measured value that takes into account two major components of error. (1) bias, and (2) the random error attributed to the imprecision of the measurement process.

**Variance.** A measure of the squared dispersion of observed values or measurements expressed as a function of the sum of the squared deviations from the population mean or sample average.

## Measured Parameters

**Apparent deformability modulus.** Engineering stress divided by engineering strain at 20% deformation. From Chu and Peleg 1985.

**Apparent elastic limit.** Arbitrary approximation of the elastic limit of materials that do not have a significant straight line portion on a stress/strain diagram.

**Asymptotic (relaxation / residual) modulus.** In a stress relaxation test, the engineering stress horizontal asymptote divided by the engineering strain. This is an indicator of the degree of solidity of the sample.

**Average linear strain.** See engineering strain.

**Bend modulus.** This relates to the maximum force measured and is defined as 3WL/2bhh, where W= max. force, L= Length on the beam, b= Breadth of the beam, h= Height of the beam.

** Bond strength. **Stress (tensile load divided by area of bond) required to rupture a bond formed by an adhesive between two substrates. Also known as Peel strength.

**Breaking load.** The maximum load (or force) applied to a test specimen or structural member loaded to rupture. See also breaking strength.

**Breaking strength. **The stress at fracture. Also known as rupture strength or fracture strength.

**Brinell hardness number, HB. **A number related to the applied load and to the surface area of the permanent impression made by a ball indenter computed from the Brinell hardness equation.

**Bulk modulus of elasticity, K.** The measure of resistance to change in volume; the ratio of hydrostatic stress to the corresponding unit change in volume. Also known as bulk modulus, compression modulus, hydrostatic modulus and volumetric modulus of elasticity.

**Chord modulus.** The slope of the chord drawn between any two specific points on a stress-strain curve.

**Coefficient of compression.** A measure of the relative volume change of a fluid or solid as a response to a pressure change.

**Coefficient of friction, µ.** The ratio of the force resisting tangential motion between two bodies to the normal force pressing these bodies together.

**Cohesive strength.** Theoretical stress that causes fracture in a tensile test if the material exhibits no plastic deformation.

**Compliance.** The reciprocal of material stiffness.

**Compressive strength.** Maximum compressive stress a material is capable of developing. With a brittle material that fails in compression by fracturing, the compressive strength has a definite value. In the case of ductile, malleable or semiviscous materials (which do not fail by in compression by a shattering fracture), the value obtained for compressive strength is an arbitrary value dependent on the degree of distortion that is regarded as effective failure of the material.

**Compressive stress.** A stress that causes a body to deform (shorten) in the direction of the applied load.

**Compressive yield strength.** Stress which causes a material to exhibit a specified deformation.

**Creep rate.** Time rate of deformation of a material subject to stress at a constant temperature. It is found from the slope of the creep strain-time curve at a given time determined from a Cartesian plot.

**Creep rupture strength.** Stress required to cause fracture in a creep test within a specified time in a specified constant environment.

**Creep strain.** The time-dependent total strain (extension plus initial gauge length) produced by applied stress during a creep test.

**Creep strength.** The stress required to cause a specified amount of creep strain in a specified time in a specified constant environment.

**Creep stress.** The constant load divided by the original cross-sectional area of the specimen.

**Crush resistance.** Compressive load required to produce fracture.

**Deformation (under load).** A change in the form of a body due to stress, thermal change, change in moisture or other causes. Measured in units of length.

**Deformation energy.** Energy required to deform a material a specified amount.

**Density (absolute).** The mass per unit volume of a solid material.

**Elastic deformation.** A change in dimensions directly proportional to and in phase with an increase or decrease in applied force. Linear elastic fracture mechanics. A method of fracture analysis that can determine the stress (or load) required to induce fracture instability in a structure containing a crack-like flaw of known size and shape. Hooke's law. Stress is directly proportional to strain. Hooke's law assumes perfectly elastic behaviour. It does not take into account plastic or dynamic loss properties.

**Elastic energy. **The amount of energy required to deform a material within the elastic range of behaviour, neglecting small heat losses due to internal friction. The energy absorbed by a specimen per unit volume of a material contained within the gauge length being tested. It is determined by measuring the area under the stress-strain curve up to a specified elastic strain.

**Elastic limit. **Greatest stress than can be applied to a material without causing permanent deformation that remains after complete release of the stress. For materials that have a significant straight line portion in their stress/strain diagram, elastic limit is approximately equal to proportional limit. For materials that do not exhibit a significant proportional limit, elastic limit is an approximation (the apparent elastic limit).

**Elastic recovery.** In hardness testing, the shortening of the original dimensions of the indentation upon release of the load.

Elastic resilience. The amount of energy absorbed in stressing a material up to the elastic limit; or the amount of energy that can be recovered when stress is released from the elastic limit.

**Elastic strain energy.** The energy expended by the action of external forces in deforming a body elastically. Essentially all the work performed during elastic deformation is stored as elastic energy, and this energy is recovered upon release of the applied force.

**Elongation.** A term used in mechanical testing to describe the amount of extension of a test piece when stressed. This is often expressed as a per cent of the initial gauge length (this refers to a specimen of constant cross section).

**Endurance limit.** The maximum stress below which a material can presumably endure an infinite number of stress cycles. The value of the maximum stress and the stress ratio should be stated.

**Engineering strain. ** A term sometimes used for average linear strain or conventional strain in order to differentiate it from true strain. In tensile testing it is calculated by dividing the change in the gauge length by the original gauge length.

**Engineering stress. **A term sometimes used for conventional stress in order to differentiate it from true stress. In tensile testing, it is calculated by dividing the breaking load applied to the specimen by the original cross-sectional area of the specimen.

**Failure viscosity. **In tablet diametral compression tests, tensile strength divided by strain rate. Strain is calculated as the diametral deformation undergone during the test divided by the diameter. Strain rate is strain to failure divided by time taken.

**Fatigue life. **Number of cycles of fluctuating stress and strain of a specified nature that a material will sustain before failure occurs.

Flexural modulus of elasticity. Alternate term for modulus in bending.

**Flow stress. **See **yield stress.**

**Fracture strength. **The true normal stress on the minimum cross-sectional area at the beginning of fracture. This term usually applies to tensile tests of un-notched specimens. However, there are many experimental configurations that can be used to investigate fracture.

**Fracture toughness.** A generic term for measures of resistance to the extension of a crack. The term is sometimes restricted to results of fracture mechanics tests, which are directly applicable in fracture control. However, the term commonly includes results from simple tests of notched or precracked specimens not based on fracture mechanics analysis. Results from tests of the latter type are often useful for fracture control, based on either service experience or empirical correlations with fracture mechanics tests.

**Hardness.** A measure of the resistance of a material to surface indentation (or sometimes surface abrasion). It may be thought of as a function of the stress required to produce some specified type of surface deformation. There is no absolute scale for hardness; therefore, to express hardness quantitatively, each type of test has its own scale of arbitrarily defined hardness. Indentation hardness can be measured by Brinell, Rockwell, Vickers, Knoop and Scleroscope hardness tests.

**Indentation hardness.** The resistance of a material to indentation as determined by hardness testing. The indenter, which may be spherical or diamond shaped, is pressed into the surface of a material under specified load for a given time.

**Initial strain.** The strain in a solid immediately upon achieving the given loading conditions in a creep test. Sometimes referred to as instantaneous strain.

**Initial stress.** The stress in a specimen immediately upon achieving the given constraint conditions in a stress relaxation test. Sometimes referred to as instantaneous stress.

**Initial tangent modulus.** The slope of the stress-strain curve at the beginning of loading.

**Linear (tensile or compressive) strain. **The change per unit length due to force in an original linear dimension. An increase in length is considered positive.

**Maximum load. **(1) The load with the highest value in the load cycle. (2) Used to determine the strength of a structural member: the load that can be borne before failure is apparent.

**Maximum stress.** The stress with the highest value in the stress cycle, tensile stress being considered positive and compressive stress negative.

**Mechanical hysteresis.** Energy absorbed in a complete cycle of loading and unloading within the elastic limit and represented by the closed loop of the stress-strain curves for loading and unloading.

**Mechanical properties.** The properties of a material that reveal its elastic and inelastic behaviour when force is applied or that involve the relationship between the intensity of the applied stress and the strain produced. The properties included under this heading are those that can be recorded by mechanical testing – for example, modulus of elasticity, tensile strength, elongation, hardness and fatigue limit.

**Modulus of elasticity.** The measure of rigidity or stiffness of a material. The ratio of stress, below the proportional limit, to the corresponding strain. In terms of the stress-strain diagram, the modulus of elasticity is the slope of the stress-strain curve in the range of linear proportionality of stress to strain. Also known as Young’s modulus. For materials that do not conform to Hooke’s law throughout the elastic range, the slope of either the tangent to the stress-strain curve at the origin or at low stress, the secant drawn from the origin to any specified point on the stress-strain curve, or the chord connecting any two specific points on the stress-strain curve is usually taken to be the modulus of elasticity. In these cases, the modulus is referred to as the tangent modulus, secant modulus or chord modulus, respectively.

**Modulus of resilience.** The amount of energy stored in a material when loaded to its elastic limit. It is determined by measuring the area under the stress-strain curve up to the elastic limit.

**Modulus of rupture in bending.** The value of maximum tensile or compressive stress (whichever causes failure) in the extreme fibre of a beam loaded to failure in bending,

**Modulus of rupture.** Nominal stress at fracture in a bend or torsion test. In bending, modulus of rupture is the bending moment at fracture divided by the section modulus.

**Modulus of toughness.** The amount of work per unit volume of a material required to carry that material to failure under static loading.

**Normal stress.** The stress component perpendicular to a plane on which forces act. Normal stress may either be tensile or compressive.

**Offset yield strength.** Arbitrary approximation of elastic limit. It is the stress that corresponds to the point of intersection of a stress-strain diagram and a line parallel to the straight line portion of the diagram. Offset refers to the distance between the origin of the stress-strain diagram, and the point of intersection of the parallel line and the stress axis. Offset is usually expressed in terms of % strain.

**Offset.** The distance along the strain coordinate between the initial portion of a stress-strain curve and a parallel line that intersects the stress-strain curve at a value of stress (commonly 0.2%) that is used for a value of the yield strength. Used for materials that have no obvious yield point.

**Peel strength.** Measure of the strength of an adhesive bond. It is the average load per unit width of the bond.

**Plastic deformation.** Deformation that remains after the load causing it is removed. It is the permanent part of the deformation beyond the elastic limit of a material.

**Plastic instability.** The stage of deformation in a tensile test where the plastic flow becomes non-uniform and necking begins.

**Plastic strain.** Dimensional change that does not disappear when the initiating stress is removed. Usually accompanied by some elastic deformation.

**Principal stress (normal). **The maximum or minimum value of the normal stress at a point in a plane considered with respect to all possible orientations of the considered plane. On such principal planes the shear stress is zero. There are three principal stresses on three mutually perpendicular planes. The state of stress at a point may be (1) uniaxial, a state of stress in which two of the three principal stresses are zero, (2) biaxial, a state of stress in which only one of the three principal stresses is zero, or (3) triaxial, a state of stress in which none of the principal stresses is zero. Multiaxial stress refers to either biaxial or triaxial stress.

**Proof stress.** (1) Stress that will cause a specified small permanent deformation in a material. (2) A specified stress to be applied to a member or structure to indicate its ability to withstand service loads.

**Proportional limit.** Highest stress at which stress is directly proportional to strain. It is the highest stress at which the curve in a stress-strain diagram is a straight line. Proportional limit is equal to elastic limit for many materials.

**Recovery.** A material's ability to recover from deformation after a compressibility and recovery test. In the compressibility and recovery test, it is usually expressed as %.

**Relaxation curve.** A plot of either the remaining or relaxed stress as a function of time.

**Relaxation rate.** The absolute value of the slope of a relaxation curve at a given time.

**Relaxation.** Reduction of stress in a material due to creep. Also known as stress relaxation.

**Relaxed stress.** The initial stress minus the remaining stress at a given time during a stress-relaxation test.

**Remaining stress.** The stress remaining at a given time during a stress relaxation test.

**Resilience.** The ability of a material to absorb energy when deformed elastically and return to its original shape upon release of load.

**Rockwell hardness number, HR.** A number derived from the net increase in the depth of impression as the load on an indenter is increased from a fixed minor load to a major load and then returned to the minor load. Rockwell hardness numbers are always quoted with a scale symbol representing the penetrator, load and dial used.

**Rupture strength.** The stress at failure. Also known as the breaking stress or fracture stress.

**Secant modulus of elasticity.** Ratio of stress to strain at any point on curve in a stress-strain diagram. It is the slope of a line from the origin to any point on a stress-strain curve.

**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 the modulus of rigidity.

**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 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.

**Stiffness. **(1) The ability of a material to resist elastic deflection. (2) The rate of stress with respect to strain; the greater the stress required to produce a given strain, the stiffer the material is said to be.

**Strain energy. **Measure of energy absorption characteristics of a material under load up to fracture. It is equal to the area under the stress-strain diagram, and is a measure of the toughness of a material.

**Strain hardening exponent.** The value of the exponent in the true stress-true strain exponential relationship. Also called ‘n value’, it relates to the ability of a material to be stretched in working operations.

**Strain rate sensitivity.** The increase in stress needed to cause a certain increase in plastic strain rate at a given level of plastic strain and a given temperature.

**Strain rate.** The time rate of straining. Because strain is dimensionless, the units of strain rate are reciprocal time.

**Strain.** Change per unit length in a linear dimension of a part or specimen, usually expressed in %. Strain, as used with most mechanical tests, is based on original length of the specimen.

**Strength. **The maximum nominal stress a material can sustain. Always qualified by the type of stress (tensile, compressive or shear).

**Stress relaxation curve.** A plot of the remaining or relaxed stress as a function of time. The relaxed stress equals the initial stress minus the remaining stress. Also known as a stress-time curve.

**Stress relaxation.** Decrease in stress in a material subjected to prolonged constant strain at a constant temperature. Stress relaxation behaviour is determined in a creep test. The stress relaxation behaviour of a material is usually shown in a stress relaxation curve.

**Stress. **(1) Load on a specimen divided by the area through which it acts. As used with most mechanical tests, stress is based on original cross-sectional area without taking into account changes in area due to applied load. This sometimes is called engineering stress. True stress is equal to the load divided by the instantaneous cross-sectional area through which it acts. (2) 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-strain graph.** A graph in which corresponding values of stress and strain are plotted against each other. Values of stress are usually plotted vertically on the y axis and strain horizontally on the x axis.

**Stress-strain ratio.** Stress divided by strain at any load or deflection. Below the elastic limit of a material, it is equal to modulus of elasticity.

**Tangent modulus. **The slope of the stress strain curve at any specified stress or strain.

**Tensile test.** Method for determining the behaviour of materials subjected to uniaxial loading, which tends to stretch the test specimen. A longitudinal specimen of known length and cross-section is gripped at both ends and stretched at a slow, controlled rate until rupture occurs.

**Total elongation.** A total amount of permanent extension of a test piece broken in a tensile test.

**Toughness.** The ability of a material to absorb energy and deform plastically before fracturing.

**Transition temperature. **(1) An arbitrarily defined temperature that lies within the temperature range in which material fracture characteristics (as usually determined by tests of notched specimens) change rapidly, such as from primarily fibrous (shear) to primary crystalline (cleavage) fracture. (2) Sometimes used to denote an arbitrarily defined temperature within a range in which the ductility changes rapidly with temperature.

**True strain.** (1) The ratio of the change in dimension, resulting from a given load increment, to the magnitude of the dimension immediately prior to applying the load increment. (2) In a body subjected to axial force, the natural logarithm of the ratio of the gauge length at the moment of observation to the original gauge length. Also known as natural strain.

**True stress.** Applied load divided by actual area of the cross section through which load operates. It takes into account the change in cross section that occurs with changing load.

**Ultimate elongation.** Alternate term for elongation of material at rupture under tensile loading.

**Ultimate strength.** The maximum stress (tensile, compressive or shear) a material can sustain without fracture, determined by dividing maximum load by the original cross-sectional area of the specimen. Also known as nominal strength or maximum stress.

**Uniform elongation. **The elongation at maximum load and immediately preceding the onset of necking in a tensile test.

**Uniform strain. **The strain occurring prior to the beginning of localisation of strain (necking); the strain to maximum load in the tensile test.

**Vickers hardness number, HV. **A number related to the applied load and the surface area of the permanent impression made by a square-based pyramidal diamond indenter of specific geometry.

**Yield point elongation. **The amount of strain that is required to complete the yielding process. It is measured from the onset of yielding to the beginning of strain hardening.

**Yield point.** The first stress in a material, usually less than the maximum attainable stress, at which an increase in strain occurs without an increase in stress. Only certain materials – those that exhibit a localised, heterogeneous type of transition from elastic to plastic deformation – produce a yield point. If there is a decrease in stress after yielding, a distinction may be made between upper and lower yield points. The load at which a sudden drop in the flow curve occurs is called the upper yield point. The constant load shown on the flow curve is the lower yield point.

**Yield strength.** The stress at which a material exhibits a specified deviation from proportionality of stress and strain. An offset of 0.2% is often used.

**Young's modulus.** Alternate term for modulus of elasticity, specifically the ratio of stress to strain in the linear elastic region of a tensile or compressive graph.