| by Michael G. Busch., Associate Editor Materials Engineering Engineering materials are commonly defined and specified by their properties. And of all the properties a material may possess, mechanical properties often are the most important because virtually all fabrication processes and most service conditions involve some type of mechanical loading. Thus, in selecting materials you must know the mechanical properties of a material and what they stand for. As this glossary shows, there are literally hundreds of different mechanical properties and terms. And behind each property there generally is a test that defines the property and tells how to measure it. Thus, a large part of our "picture" of a material is based on the sum of its mechanical properties and tests for measuring these properties. Here is a summary of the properties, tests and terms you should know in order to make an intelligent choice of materials. Adherence. The extent to which a coating bonds to a substrate. Adherence index. Measure of the adherence of porcelain enamel and ceramic coatings to sheet metal. (ASTM C-313). |
| Alpha Rockwell hardness. Index of the resistance of a plastic to surface penetration by a specified indentor under specified load applied with a Rockwell hardness tester. Higher values indicate higher indentation hardness. (ASTM D-786). Annealing point. Temperature at which internal stress in glass is substantially relieved in 15 min. (ASTM C-336). ASTM hardness number. Depth (in thousandths of an inch) of penetration of an indentor into a rubber specimen under loads and conditions specified in ASTM D-314. While suitable for most common grades of rubber, ASTM hardness number is not applicable to extremely hard or soft rubbers. Bearing area. Area through which a bearing load is transmitted. Diameter of bearing hole multiplied by thickness of specimen is area used to compute bearing stress. Bearing load. Compressive load transmitted to a structural member through area of contact (bearing area). Bearing stiffness. Slope of the tangent at any point on the stress-strain diagram plotted from data obtained in test for bearing strength. (ASTM D-953, plastics and ASTM E-238, metals). Gives an indication of the behavior of materials subjected to edge-wise loads such as applied through mechanical fasteners. Strain used to determine bearing stiffness is defined as % deformation of bearing hole through which the load is applied. Bearing strength. Measure of maximum usable bearing stress that can be developed in a material. Equal to the stress that corresponds to the point on the bearing stress-strain diagram where the slope of the curve equals the stress divided by a strain of 4%. A standard procedure for determining bearing strength is given in ASTM D-953 (plastics) and ASTM E-238 (metals). While it is known that materials with higher compressive and tensile strengths have higher bearing strengths, there is no widely accepted method for estimating bearing strength from compression or tensile properties. Bearing stress. Bearing load applied to a material divided by original bearing area. In the bearing strength test (ASTM D-593 for plastics, ASTM E-238 for metals) a rectangular specimen is loaded in tension or compression by a pin or rod passing through a bearing hole. Bearing stress and bearing strain are recorded as bearing load is increased and are plotted to form a stress strain diagram. Maximum bearing stress is equal to load at rupture divided by original bearing area. Bending strength. Alternate term for flexural strength. It is most commonly used to describe flexure properties of cast iron and wood products. Bond strength. Stress (tensile load divided by area of bond) required to rupture a bond formed by an adhesive between two metal blocks. (ASTM D-952). Breaking load. Load which causes fracture in a tension, compression, flexure or torsion test. In tension tests of textiles and yarns, breaking load also is called breaking strength. In tensile tests of thin sheet materials or materials in form of small diameter wire it is difficult to distinguish between breaking load and the maximum load developed so the latter is considered the breaking load. Breaking strength. Tensile load or force required to rupture textiles (e.g., fibers, yarn) or leather. It is analogous to breaking load in a tension test. Ordinarily, breaking strength is reported as lb or lb/in. of width for sheet specimens. Brinell hardness number (BHN). Measure of the indentation hardness of metals, calculated from the diameter of the permanent impression made by a ball indentor of a specified size pressed into the material by a specified force. BHN increases with increasing indentation hardness. A standard test for determining BHN is given in ASTM E-10. Conversion tables that relate BHN to diamond pyramid hardness, Rockwell hardness and Rockwell superficial hardness are published in ASTM E-140. BHN test is particularly useful where deep penetration is required to avoid surface effects or where a large impression is required to avoid errors due to in homogeneity of material. It is not used for thin sections or very hard materials. Brittle fracture. Failure or rupture of a material with little or no plastic flow or deformation of a metal's crystal lattice. Usually this type of failure is associated with impact loads. However, many materials at low temperatures also show brittle fracture failures under static loads. Two common methods for determining resistance to brittle fracture are the Izod and Charpy impact tests. (ASTM E-23). Brittleness temperature. Temperature at which plastics and elastomers exhibit brittle failure under impact conditions specified in ASTM D 746. Brittleness temperature often is used as part of specifications for plastics or elastomers, but is not considered an accurate measure of materials lowest use temperature. A test for determining brittleness temperature of plastic film is given in ASTM D-1790. Bulk modulus of elasticity. Ratio of stress to change in volume of a material subjected to axial loading. Related to modulus of elasticity (E) and Poisson's ration (r) by the following equation: K=Er/3(1-2r). Bursting strength. Measure of ability of materials in various forms to withstand hydrostatic pressure. For round rigid plastic tubing it usually is reported as internal fluid pressure required to produce rupture (ASTM D-1180). For coated fabrics it is reported as force required to rupture a diaphragm divided by its area. (ASTM D-751). Charpy impact test. Method for determining behavior of materials under conditions favorable to brittle fracture. Used where results of a tension impact test would not be significant. It is the most popular impact test. Test is performed by striking a notched specimen supported as simple beam with a falling weight. Results are reported as energy absorbed in fracture and a description of the fracture. ASTM E-23 describes test for metals, ASTM A 327 for cast iron, ASTM D-256 for plastics and ASTM D-758 for plastics at subnormal and elevated temperatures. Clash-Berg test. Method for determining stiffness of plastics as a function of temperature by a torsion test (ASTM D-1043). Test consists of direct measurements of apparent modulus of rigidity over wide temperature range. Cleavage strength. Tensile load (lb/in. of width) required to cause separation of a 1-in. long metal-to-metal adhesive bond under the conditions set in ASTM D-1062. Climbing drum peel test. Method for determining peel resistance of adhesive bond between a relatively flexible and rigid material. (ASTM D1781). Coefficient of elasticity. Alternate term for modulus of elasticity. Cohesive strength. Theoretical stress that causes fracture in tension test if material exhibits no plastic deformation. Cold crushing strength. Load required to produce fracture in refractory bricks and shapes divided by average cross section area of specimen. (ASTM C-133). Cold flow. Permanent deformation of plastics remaining after load applied at temperature below distortion temperature is removed. It is an alternate term for creep in plastics (ASTM D-674) and rubber (ASTM D-530). Complex modulus. Measure of dynamic mechanical properties of material taking into account energy dissipated as heat during deformation and recovery. It is equal to the sum of static modulus of a material and its loss modulus. In the case of shear loading it is called dynamic modulus. See also damping capacity. Compressibility. Extent to which material is compressed in test for compressibility and recovery of gasket materials (ASTM F-36). It is usually reported with recovery. Compressibility and recovery test. Method for measuring behavior of gasket materials under short time compressive loading at room temperature. ASTM F-36 outlines a standard procedure. Test is not designed to indicate long term (creep) behavior and should not be confused with the plastometer test. Compression-deflection test. Nondestructive method for determining relationship between compressive load and deflection under load for vulcanized rubber. (ASTM D-575). Compression fatigue. Ability of rubber to sustain repeated fluctuating compressive loads. (ASTM D-623). Compression set. The extent to which rubber is permanently deformed by a prolonged compressive Ioad. (ASTM D-395). Should not be confused with low temperature compression set. Compression test. Method for determining behavior of materials under crushing loads. Specimen is compressed, and deformation at various loads is recorded. Compressive stress and strain are calculated and plotted as a stress-strain diagram which is used to determine elastic limit, proportional limit, yield point, yield strength and (for some materials ) compressive strength. Standard compression tests are given in ASTM C-528 (high strength ceramics), ASTM E-9 (metals), ASTM E-209 (metals at elevated temperatures), ASTM D-695 (plastics) and ASTM D-759 (plastics at high and low temperatures). Compressive deformation. Extent to which a material deforms under a crushing load. Compressive strength. Maximum stress a material can sustain under crush loading. The compressive strength of a material that fails by shattering fracture can be defined within fairly narrow limits as an independent property. However, the compressive strength of materials that do not shatter in compression must be defined as the amount of stress required to distort the material an arbitrary amount. Compressive strength is calculated by dividing the maximum load by the original cross section area of a specimen in a compression test. Compressive Yield strength. Stress which causes a material to exhibit a specified deformation. Usually determined from the stress-strain diagram obtained in a compression test. See also yield strength. Corrosion fatigue. Failure under fluctuating loads in a corrosive environment. Crack growth resistance. Degree to which rubber resists enlargement of cracks when subjected to repeated bend flexing. [ASTM D-813 and ASTM D-1052 (Ross flexing machine test)]. Cracking resistance. Ability of vulcanized rubber to withstand repeated stretching or flexing without developing cracks. (ASTM D-430). Creep. Deformation that occurs over period of time when a material is subjected to constant stress at constant temperature. In metals, creep usually occurs only at elevated temperatures. Creep at room temperature is more common in plastic materials and is called cold flow or deformation under load. Data obtained in a creep test usually is presented as a plot of creep vs time with stress and temperature constant. Slope of curve is creep rate and end point of curve is time for rupture. As indicated in accompanying diagram, creep of a material can be divided into three stages. First stage or primary creep starts at rapid rate and slows with time. Second stage (secondary) creep has a relatively uniform rate. Third stage (tertiary) creep has an accelerating creep rate and terminates by failure of material at time for rupture. See also stress-relaxation. Creep limit. Alternate term for creep strength. Creep rate. Time rate of deformation of a material subject to stress at a constant temperature. It is the slope of the creep vs time diagram obtained in a creep test. Units usually are in./in./hr or % elongation/hr. Minimum creep rate is the slope of the portion of the creep vs time diagram corresponding to secondary creep. Creep recovery. Rate of decrease in deformation that occurs when load is removed after prolonged application in a creep test. Constant temperature is maintained to eliminate effects of thermal expansion and measurements are taken from time load is zero to eliminate elastic effects. Creep rupture strength. Stress required to cause fracture in a creep test within a specified time. Alternate term is stress rupture strength. Creep strength. Maximum stress required to cause a specified amount of creep in a specified time. Also used to describe maximum stress that can be generated in a material at constant temperature under which creep rate decreases with time. Alternate term is creep limit. Creep test. Method for determining creep or stress relaxation behavior. To determine creep properties, material is subjected to prolonged constant tension or compression loading at constant temperature. Deformation is recorded at specified time intervals and a creep vs time diagram is plotted. Slope of curve at any point is creep rate. If failure occurs, it terminates test and time for rupture is recorded. If specimen does not fracture within test period, creep recovery may be measured. To determine stress relaxation of material, specimen is deformed a given amount and decrease in stress over prolonged period of exposure at constant temperature is recorded. Standard creep testing procedures are detailed in ASTM E-139, ASTM E-150 (metals under rapid heating), ASTM D-674 (plastics) and ASTM D-2294 (adhesives). Creepocity. Increase in creep from one specified time to another during creep test. Since it often is independent of stress a plot of creepocity vs temperature provides a basis for comparing creep behavior of some materials. Cross cut adhesion test. Method for determining adherence of paint to a metal substrate. Series of lines are scribed through paint to base metal at specified intervals 1/16 or 1/8 in.). Then perpendicular lines are scribed to form a grid. Lack of flaking indicates good adhesion. Crush resistance. Load required to produce fracture in a glass sphere subjected to crush loading. (ASTM D313). Crushing load. Maximum compressive force applied during a compression or crushing test. For materials that do not shatter, crushing load is defined as the force required to produce a specified type of failure. Crushing strength. Compressive load required to cause a crack to form in a sintered metal powder bearing (ASTM B-202). C o I d crushing strength of refractory bricks and shapes is the gross compressive stress required to cause fracture. (ASTM C-133). Cup blank diameter, maximum. Comparative index of the formability or ductility of a material in sheet or plate form. It is the diameter of the largest circular blank that can be drawn to a cup of a specified diameter without failure. Cup drawing test. Method for measuring ductility of sheet metal. Term covers several different tests and there is little standardization of procedure. However, ASTM A-344 gives a standard cup drawing test for magnetic materials and ASTM B-69 gives one for rolled zinc. In general, results are given as maximum cup blank diameter or cup height or depth. Cup height (or depth). Index of ductility or formability of a material in sheet or plate form. It is the height or depth of a punched dome at the time fracture or other specified failure occurs. Damping capacity. Measure of the ability of a material to absorb vibration by converting mechanical energy in to heat. It is equal to the area of the elastic hysteresis loop divided by the deformation energy of a vibrating material. It can be calculated by measuring the rate of decay of vibrations induced in a material. For details see "Mechanical Properties of Polymers," L. E. Niel son, Reinhold Publishing Corp.; "Nondestructive Testing," W. y. Mc Gonnagle, McGraw-Hill Book Co. and "Mechanical Testing of Materials," A. J. Fenner, Philosophical Library Inc. Deflection temperature. Temperature at which a plastic specimen deforms a specified amount under a specified load. It is not a direct guide to the high temperature limit of a plastic for a specified application, but rather a means for comparing the relative heat resistance of plastics. (ASTM D-648). Deformation energy. Energy required to deform a material a specified amount. It is the area under the stress-strain diagram up to a specified strain. Deformation under load. Measure of the ability of rigid plastics to with stand permanent deformation and the ability of nonrigid plastics to return to original shape after deformation. Standard test methods for determining both types of deformation under load are given in ASTM D-621. For rigid plastics deformation (which can be flow or flow and shrinkage) is reported as % change in height of specimen after 24 hr under a specified load. For nonrigid plastics results are reported as % change in height after 3 hr under load and recovery in the 11/2 hr period following removal of the load. Recovery is % increase in height calculated on basis of original height. Delamination strength. Measure of the node-to-node bond strength of honeycomb core materials. It is equal to the tensile load applied to a honeycomb panel at fracture divided by its width times thickness. (ASTM C-363). De Mattia flexing machine test. Method for measuring the cracking resistance (ASTM D-430) and crack growth resistance of rubber (ASTM D-813). Diamond pyramid hardness number (DPHN). Measure of the indentation hardness of a material. It is the amount of plastic deformation caused by a 136 deg pyramidal diamond indentor under a specified load. (ASTM E-92). Also known as Vickers hardness. Dissipation factor. Ratio of the loss modulus to static modulus of a material under dynamic loading. It is proportional to damping capacity. An alternate term is loss tangent. Drop ball impact test. Method for determining the energy absorption characteristics of a material subjected to shock loading. Metal ball of known weight is dropped on specimen from regularly increasing heights and height of drop, producing failure is reported. Test is used for hard metals, ceramics and plastics. Drop weight test. Method for determining the nil-ductility transition temperature of steel. Results are reported as temperature above which specimens no longer show brittle fracture after specified shock loadings. (ASTM E-208). Dry strength. Strength of an adhesive joint determined immediately after drying or after a period of conditioning in a specified atmosphere. (ASTM D-1144). Ductile-to-brittle transition temperature. Indication of temperature range in which metals undergo transition from ductile to brittle behavior. It is an indication of the minimum temperature at which metals have sufficient ductility for forming. For some refractory metals ductile-to-brittle transition temperatures are well above room temperature. Ductility. Extent to which a material can sustain plastic deformation without rupture. Elongation and reduction of area are common indices of ductility. Du Pont flexing machine test. Method for determining the cracking resistance of rubber (ASTM D-430). Rubber specimens are mounted to a fabric base and subjected to tensile and compressive flexing until failure occurs. Results are reported as a comparison of the severity of cracking in various samples, and number of cycles required to produce specified severity of cracking in the material. Durometer hardness. Measure of the indentation hardness of plastics and rubber. It is tze extent to which a spring loaded steel indentor protrudes beyond a pressure foot into the material. Standard procedures are given in ASTM D-1706 (plastics) and ASTM D-2240 (plastics and rubber). Dynamic creep. Creep that occurs under fluctuating load or temperature. Dynamic ductility test. Method for determining ductility of zinc strip or sheet. A series of cups are formed by a plunger and depth of deepest nonruptured cup is reported. (ASTM B-69). Dynamic modulus. Complex modulus of material under dynamic shear loading. It is equal to the sum of static shear modulus and loss modulus. Dynamic modulus takes into account energy dissipated as heat when material is deformed. Dynamic modulus, effective. Indication of the vibration absorption characteristics of elastomers. It is determined in the Yerzley mechanical oscillograph test (ASTM D-945). It is not an actual physical modulus, but an extension of dynamic modulus beyond the straight line portion of the load-deformation curve. Eccentricity of loading. Distance between the actual line of action of compressive or tensile loads and the line of action that would produce a uniform stress over the cross section of the specimen. Edge distance ratio. Distance from the edge of a bearing strength test specimen to the center of the bearing hole, divided by the diameter of the hole. Edge distance ratio is generally reported with results of a bearing strength test. Edge tearing strength. Measure of the resistance of paper to tearing when folded over a V-notch beam and loaded in a tensile testing machine (ASTM D-827). Results are reported in lb or kg. See also tear resistance. Elastic hysteresis. Difference between strain energy required to generate a given stress in a material and elastic energy at that stress. It is the energy dissipated as heat in a material in one cycle of dynamic testing. Elastic hysteresis divided by elastic deformation energy is equal to damping capacity. Elastic limit. Greatest stress that can be applied to a material without causing permanent deformation. For metals and other 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 arbitrary approximation (apparent elastic limit). Elastic limit, apparent. Arbitrary approximation of the elastic limit of materials that do not have a significant straight line portion on a stress-strain diagram. It is equal to the stress at which the rate of strain is 50% greater than at zero stress. It is the stress at the point of tangency between the stress-strain curve and a line having a slope with respect to the stress axis 50% greater than the slope of the curve at the origin. Elasticity. Ability of a material to return to its original shape when load causing deformation is removed. Elongation. Measure of the ductility of a material determined in a tension test. It is the increase in gage length (measured after rupture) divided by original gage length. Higher elongation indicates higher ductility. Elongation cannot be used to predict behavior of materials subjected to sudden or repeated loading. Embrittlement. Reduction in ductility due to physical or chemical changes. Endurance limit. Alternate term for fatigue limit. Energy absorption. Energy required to fracture a specimen in an impact test. It is a measure of toughness or impact strength. Nil ductility transition temperature is derived from a series of energy absorption measurements at various temperatures. Engineering stress. Load applied to a specimen in a tension or compression test divided by a cross section area of the specimen. The change in cross section area that occurs with increases and decreases in applied load is disregarded in computing engineering stress. It is also called conventional stress. Erichsen test. Cupping test in which a sheet metal blank restrained at its edges is deformed at its center by a cone-shaped, spherical-end plunger until fracture occurs. Height of the cup (in mm) at fracture is an indication of ductility. (ASTM A-344). Expansion test. Control and acceptance test for determining ductility of nonferrous metal tubing. A tapered pin is forced into the end of tubing to produce a specified increase in tube diameter. The tube is then examined for failure. (ASTM B-153). An alternate term is pin test. Extensometer. Instrument for measuring changes in linear dimensions. Also called a strain gage. Fatigue. Permanent structural change that occurs in a material subjected to fluctuating stress and strain. However, in the case of glass, fatigue is determined by long-term static testing and is analogous to stress rupture in other materials. In general, fatigue failure can occur with stress levels below the elastic limit. Fatigue life. Number of cycles of fluctuating stress and strain of a specified nature that a material will sustain before failure occurs. Fatigue life is a function of the magnitude of the fluctuating stress, geometry of the specimen and test conditions. An S-N diagram is a plot of the fatigue life at various levels of fluctuating stress. Fatigue limit. Maximum fluctuating stress a material can endure for an infinite number of cycles. It is usually determined from an S-N diagram and is equal to the stress corresponding to the asymptote of the locus of points corresponding to the fatigue life of a number of fatigue test specimens. An alternate term is endurance limit. Fatigue notch factor. Ratio of fatigue strength of a specimen with no stress concentration to fatigue strength of a specimen with a notch or other stress raisers. Fatigue notch factor is usually lower than the theoretical stress concentration factor because of stress relief due to plastic deformation. An alternate term is strength reduction ratio. Fatigue ratio. Ratio of fatigue strength or fatigue limit to tensile strength. For many materials fatigue ratio may be used to estimate fatigue properties from data obtained in tension tests. Fatigue strength. Magnitude of fluctuating stress required to cause failure in a fatigue test specimen after a specified number of cycles of loading. Usually determined directly from the S-N diagram. Fatigue strength reduction factor. An alternate term for fatigue notch factor. Fatigue test. A method for determining the behavior of materials under fluctuating loads. A specified mean load (which may be zero) and an alternating load are applied to a specimen and the number of cycles required to produce failure (fatigue life) is recorded. Generally, the test is repeated with identical specimens and various fluctuating loads. Loads may be applied axially, in torsion or in flexure. Depending on amplitude of the mean and cyclic load, net stress in the specimen may be in one direction through the loading cycle or may reverse direction. Data from fatigue testing often are presented in an S-N diagram which is a plot of the number of cycles required to cause failure in a specimen against the amplitude of the cyclical stress developed. The cyclical stress represented may be stress amplitude, maximum stress or minimum stress. Each curve in the diagram represents a constant mean stress. Most fatigue tests are conducted in flexure, rotating beam or vibratory type machines. Fatigue testing is generally discussed in "Manual on Fatigue Testing," ASTM STP 91-A and "Mechanical Testing of Materials," A. J. Fenner, Philosophical Library Inc. ASTM D-671 details a standard procedure for fatigue testing of plastics in flexure. Fiber stress. Stress through a point in a part in which stress distribution is not uniform. For example, the stress in a beam under bending load varies from compression to tension across the beam. It is more meaningful in determining the properties of the beam material to consider the maximum stress generated in the outer fibers of the beam. Similarly, stress in a beam under twist loading is a maximum in the material furthest from the axis of twist. File hardness. Simple determination of the comparative hardness of a metal. It is a statement as to whether a file does or does not bite into a material. Firestone flexometer test. Method for determining compression fatigue characteristics of rubber. A pyramidal rubber specimen is subjected to an oscillating compressive load and the number of load cycles required to produce a specified deflection is reported. (ASTM D-623). Flare test. Method for determining ductility of tubing material. It is similar to an expansion test and a pin test. Flattening test. Measure of the ductility of metal pipe. A short section of pipe is crushed diametrically between parallel plates to a specified extent and examined for failure. Flex resistance. Ability of foam rubber to sustain repeated compressive loads without damage to cell structure. (ASTM D-1055). Flexural modulus of elasticity. Alternate term for modulus in bending. Flexural strength. Maximum fiber stress developed in a specimen just before it cracks or breaks in a flexure test. Flexural yield strength is reported instead of flexural strength for materials that do not crack in the flexure test. An alternate term is modulus of rupture. Flexure test. Method for measuring behavior of materials subjected to simple beam loading. It is also called a transverse beam test with some materials. Specimen is supported on two knife edges as a simple beam and load is applied at its midpoint. Maximum fiber stress and maximum strain are calculated for increments of load. Results are plotted in a stress-strain diagram, and maximum fiber stress at failure is flexural strength. Flexural yield strength is reported for materials that do not crack. Standard test procedures are given in ASTM D 790 (plastics), ASTM C-328 and ASTM C-369 (fired whiteware), ASTM D-797 (elastomers), ASTM A-438 (cast iron) and ASTM C-158 (glass). Flow stress. Stress required to cause plastic deformation. Fluting diameter. Smallest diameter about which sheet metal can be bent to form a smooth curve rather than a series of planes with a fluted appearance. Fracture stress. True stress generated in a material at fracture. Fracture test. Visual test wherein a specimen is fractured and examined for grain size, case depth, etc. Fracture toughness. Ability of a material to resist crack propagation when subjected to shock load as in an impact test. Gehman torsional test. Method for measuring low temperature stiffening of rubber by a calibrated torsion wire. (ASTM D-1053). Results often are reported as apparent modulus of rigidity, which is equal to three times Young's modulus. Goodrich flexometer test. Method for determining compression fatigue characteristics of rubber having a durometer hardness less than 8.5. (ASTM D-623). Results may be reported as temperature rise during continued cyclic loading, permanent set in a specified time, duration of heat buildup, maximum temperature rise or time to failure. Hardness. Measure of a material's resistance to localized plastic deformation. Most hardness tests involve indentation, but hardness may be reported as resistance to scratching (file test), or rebound of a projectile bounced off the material (scleroscope hardness). Some common measures of indentation hardness are Brinell hardness number, Rock well hardness number, ASTM hardness number, diamond pyramid hardness number, durometer hardness, Knoop harness and Pfund hardness number. A table relating various type of hardness values of metals is given in ASTM E-140. Hardness often is a good indication of tensile and wear properties of a material. Heat distortion point. Temperature at which a standard plastic test bar deflects 0.010 in. under a maximum fiber stress of 66 or 264 psi. (ASTM D-648). An alternate term is deflection temperature. Heat distortion temperature. An alternate term for deflection temperature. Hooke's law. Stress is directly proportional to strain. Hooke's law assumes perfectly elastic behavior. It does not take into account plastic or dynamic loss properties. Hoop stress. Circumferential stress in a cylinder subjected to internal hydrostatic pressure. For thin wall cylinders it can be calculated by Barlow's formula: S = PD/2t where S is hoop stress in psi; P, applied pressure in psi; D, cylinder o.d. in in.; and t, wall thickness in in. Barlow's formula does not hold for thick-wall cylinders where stress varies across wall thickness. Hot hardness. Measure of hardness at elevated temperature. Often it is determined by heating a specimen, removing it from the oven and testing it with standard hardness testers. However, this is not a true indication of hardness at temperature because the surface cools quickly after removal from the oven and surface properties are critical in hardness testing. Several methods and apparatus for hot hardness testing are described in "Property Measurements at High Temperatures," W. D. Kingrey, John Wiley & Sons, Inc. Impact energy. Energy required to fracture a part subjected to shock loading as in an impact test. Alternate terms are impact value, impact strength, impact resistance and energy absorption. Impact resilience. Measure of the resilience of rubber obtained by dropping a pendulum hammer against a specimen and measuring rebound. (ASTM D-1054). It is the ratio of 1 minus the cosine angle of rebound, to 1 minus the cosine of the original angle of the pendulum, and is expressed as a %. It is sometimes called % rebound. Impact resilience of metal is an indication of hardness obtained in a rebound test such as the test for scleroscope hardness. Impact strength. Energy required to fracture a specimen subjected to shock loading, as in an impact test. Alternate terms are impact energy, impact value, impact resistance and energy absorption. It is an indication of the toughness of a material. Impact test. A method for determining behavior of material subjected to shock loading in bending, tension or torsion. The quantity usually measured is the energy absorbed in breaking the specimen in a single blow, as in the Charpy impact test, Izod impact test and tension impact test. Impact tests also are performed by subjecting specimens to multiple blows of increasing intensity, as in the drop ball impact test and repeated blow impact test. Impact resilience and scleroscope hardness are determined in nondestructive impact tests. Indentation hardness. Resistance of material to surface penetration by an indentor. See hardness. International rubber hardness degrees (IRHD). Measure of indentation hardness of rubber. For substantially elastic isotropic rubbers IRHD is related to Young's modulus by the equation: F/M = 0.00017 R0.65 Pl.35 where F is indenting force in kg; M, Young's modulus in kg/sq cm; R, radius of indentor in cm, and P, penetration in hundredths of mm. IRHD of a rubber is approximately equal to its durometer hardness. A standard test method for determining IRHD is given in ASTM D-1415. Izod impact test. Method for determining behavior of materials subjected to shock loading. Specimen supported as a cantilever beam is struck by a weight at the end of a pendulum. Impact strength is determined from the amount of energy required to fracture specimen. ASTM E-23 describes a standard test procedure, specimens (including notch shapes) and apparatus for Izod impact testing of metals. See also ASTM A-327 (cast iron), ASTM D-256 (plastics) and ASTM D-758 (plastics at sub normal and elevated temperatures). See also Impact test. Kink test. Method for determining ductility of metal wire. A short section of wire is looped and drawn in tension to produce a kink. Relative ductility is indicated by the occurrence or non-occurrence of failure and extent to which kink may be opened up without failure. Knoop hardness number. Measure of indentation hardness of a material (especially an organic coating) measured with a pyramidical diamond indentor of prescribed dimensions. (ASTM D-1474). Knot strength. Tenacity of a fiber in which an overhand knot is tied. Knot strength is a measure of a fiber's sensitivity to compressive and shear stresses. Load-deflection diagram. Plot of load vs corresponding deflection. Loss factor. Ratio of the real and imaginary components of complex modulus. Term is used in connection with dynamic tests to determine damping capacity of materials and combinations of materials. An alternate term is loss tangent. Loss modulus. Imaginary component of complex modulus. It takes into account mechanical energy dissipated as heat during deformation of a material under dynamic loading. Loss tangent. An alternate term for loss factor. Low temperature brittleness test. A method for measuring the resistance of rubbers and rubber-like materials to brittle cracking at low temperatures. Specimens are exposed to a specified temperature for 4 hr and flexed. Results are reported as failure or non-failure at specified temperatures. (ASTM D-736). Low temperature compression set. Measure of the ability of vulcanized rubber loaded at room temperature and subsequently subjected to prolonged exposure at low temperatures to recover from deformation at the low temperature. It is the % compressive deformation retained by a specimen subjected to the standard test set out in ASTM D-1229. The test simulates conditions encountered by seal and gasket materials used in aircraft and submarines. Low temperature stiffening. Measure of the effect of temperature change on the stiffness of rubber and rubber like materials. It is usually reported as the temperature at which a material has a specified relative modulus. (ASTM D-1063). See also Gehman torsional test. Maximum fiber stress. Maximum tensile or compressive stress in a homogeneous flexure or torsion test specimen. For a specimen loaded as a simple beam at its midpoint, maximum fiber stress occurs at mid-span and may be calculated by the formula (for rectangular specimens): S=3PL/2bd2 where S is maximum fiber stress; P, load; L, span; b, width of the beam and d, depth of the beam. For a circular cross section member loaded in torsion, maximum fiber stress may be calculated by the following formula: S=Tr/J where T is twisting moment; r, original outer radius and J, polar moment of inertia of original cross section. Mean stress. Algebraic difference between maximum and minimum stress in one cycle of fluctuating loading as in a fatigue test. Tensile stress is considered positive and compressive stress negative. Mechanical hysteresis. Alternate term for elastic hysteresis. Microhardness. Hardness of microscopic areas. Microhardness values differentiate hardness of constituents in a material. Minimum bend radius. Minimum radius to which a sheet or wire can be bent to specified angle without failure. Modulus. Alternate term for modulus of elasticity, often used in connection with rubber. Modulus in bending. Ratio of maximum fiber stress to maximum strain with in elastic limit of stress-strain diagram obtained in flexure test. Alternate term is flexural modulus of elasticity. Modulus of elasticity. Rate of change of strain as a function of stress. The slope of the straight line portion of a stress-strain diagram. Tangent modulus of elasticity is the slope of the stress-strain diagram at any point. Secant modulus of elasticity is stress divided by strain at any given value of stress or strain. It also is called stress strain ratio. Tangent and secant modulus of elasticity are equal up to the proportional limit of a material. Depending on the type of loading represented by the stress-strain diagram, modulus of elasticity may be reported as compressive modulus of elasticity (or modulus of elasticity in compression), flexural modulus of elasticity (or modulus of elasticity in flexure), shear modulus of elasticity (or modulus of elasticity in shear), tensile modulus of elasticity (or modulus of elasticity in tension) or torsional modulus of elasticity (or modulus of elasticity in torsion). Modulus of elasticity may be determined by dynamic mechanical testing where it can be derived from complex modulus. Modulus used alone generally refers to tensile modulus of elasticity. Shear modulus is almost always equal to torsional modulus and both are called modulus of rigidity. Moduli of elasticity in tension and compression are approximately equal and are known as Young's modulus. Modulus of rigidity is related to Young's modulus by the equation: E = 2G (1 + r) where E is Young's modulus (psi), G is modulus of rigidity (psi) and r is Poisson's ratio. Modulus of elasticity also is called elastic modulus and coefficient of elasticity. Modulus of rigidity. Rate of change of strain as a function of stress in a specimen subjected to shear or torsion loading. It is the modulus of elasticity determined in a torsion test. Alternate terms are modulus of elasticity in torsion and modulus of elasticity in shear. Apparent modulus of rigidity is a measure of the stiffness of plastics measured in a torsion test (ASTM D-1043). It is "apparent" because the specimen may be deflected past its proportional limit and the value calculated may not represent the true modulus of elasticity within the elastic limit of the material. Modulus of rupture. Ultimate strength determined in a flexure or torsion test. In a flexure test, modulus of rupture in bending is the maximum fiber stress at failure. In a torsion test, modulus of rupture in torsion is the maximum shear stress in the extreme fiber of a circular member at failure. Alternate terms are flexural strength and torsional strength. Modulus of strain hardening. Alternate term for rate of strain hardening. Monotron hardness. Measure of indentation hardness. It is the load (kg) required to press a specified ball indentor to a specified depth. Indentors consist of 1 mm diamond (M-2), 1/16 in. tungsten carbide (M-3) and 2.5 mm tungsten carbide (M-4). Standard depth of indentation is 0.045 mm, but for hard materials depth of indentation may be limited is multiplied by 3. Necking. Localized reduction of cross section area of a specimen under tensile load. It is disregarded in calculating engineering stress but is taken into account in determining true stress. Nil ductility transition temperature. Temperature above which a specimen no longer shows brittle fracture in a drop weight test. (ASTM E-208). Nominal stress. Stress calculated on the basis of the net cross section of a specimen without taking into account the effect of geometric discontinuities such as holes, grooves, fillets, etc. Notch brittleness. Phenomena by which brittle fracture occurs more readily in notched specimens than in notch free specimens. Notch ductility. Reduction in area of a notched specimen at fracture in a tension test. Notch sensitivity. Measure of reduction in load-carrying ability caused by stress concentration in a specimen. 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 0 stress axis. Offset is expressed in terms of strain (often 0.2%). Olsen cup test. Method for measuring metal ductility by determining cup height. A sheet metal blank, restrained on all edges, is deformed by pressing a specified steel ball at its center. Height (or depth) of the cup when rupture occurs is reported in thousandths of an inch. Operating stress. Stress imposed on a part in service. Overstressing. Application of high fluctuating loads at the beginning of a fatigue test and lower loads toward the end. It is a means for speeding up a fatigue test. Peel resistance. Torque required to separate an adhesive and adherent in the climbing drum peel test. (ASTM D-1781). It is a measure of bond strength. Peel strength. Measure of the strength of an adhesive bond. It is the average load per unit width of bond line required to part bonded materials where the angle of separation is 180 deg and separation rate is 6 in./min. (ASTM D-903). Penetration. Depth to which the striker of a rebound pendulum penetrates a rubber specimen under conditions set in ASTM D-1054. It is an index of the dynamic stiffness or complex modulus of rubber specimens. Permanent set. Extent to which a material is permanently deformed by a specified load. Usually expressed as % and calculated by dividing the difference in dimensions in the direction of loading before loading and after the load is removed by the original dimension and multiplying by 100. Pfund hardness number (PHN). Measure of the indentation hardness of coatings. Hemispherical quartz or sapphire indentor is used. (ASTM D1474). Photoelasticity. Method for observing stress distribution in a part through the use of a transparent model. Pin test. Alternate term for expansion test. 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. It also is called plastic strain and plastic flow. Plasticity. Tendency of a material to remain deformed after reduction of the deforming stress to a value equal to or less than its yield strength. Plasticity number. Index of the compressibility of rubber at elevated temperatures. Equal to 100 times the height of a standard specimen after 3 to 10 min compression by a 5 kg load. (ASTM D-926). Plastometer test. Method for determining ability of rubber to be compressed at an elevated temperature and to recover at room temperature. Results are reported as plasticity number and recovery. Test differs from compressibility and recovery test in that the latter measures behavior of material subjected to short-time loading at room temperature. Poisson's ratio. Ratio of lateral strain to axial strain in axial loaded specimen. It is the constant that relates modulus of rigidity to Young's modules in the equation: E = 2G (r + 1) where E is Young's modulus; G, modulus or rigidity and r, Poisson's ratio. The formula is valid only within the elastic limit of a material. A method for determining Poisson's ratio is given in ASTM E-1321. Proof stress. Stress that will cause a specified permanent deformation. 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 metals. Pusey and Jones indentation. Measure of indentation hardness of rubber. It is the depth of penetration of a ball indentor under a load applied by a plastometer. (ASTM D-531). Rate of strain hardening. Rate of change of true stress as a function of true strain in a material undergoing plastic deformation. An alternate term is modulus of strain hardening. Recovery. Index of a material's ability to recover from deformation in the compressibility and recovery test (ASTM F-36), the deformation under load test (ASTM D-621) and the plastometer test (ASTM D-926). In the compressibility and recovery test it usually is reported with compressibility and given as %. It is calculated by dividing the difference between recovered thickness and thickness under load by the difference between original thickness and thickness under load. In the deformation under load test it indicates the extent to which a nonrigid plastic recovers from prolonged compressive deformation at elevated temperature. It is given as % and is calculated by dividing the difference between height recovered 11/2 hr after load is removed and height after three hr of loading by the change in height under load. In the plastometer test it indicates the extent to which an elastomer recovers from compressive loading at elevated temperature. It is equal to plasticity number minus recovered height. See also Rockwell recovery. Recovery test. Method for measuring compressibility and recovery of gasket and seal materials. (ASTM F 36). Reduction of area. Measure of the ductility of metals obtained in a tension test. It is the difference between original cross section area of a specimen and the area of its smallest cross section after testing. It is usually expressed as % decrease in original cross section. The smallest cross section can be measured at or after fracture. For metals it usually is measured after fracture and for plastics and elastomers it is measured at fracture. Relative modulus. Ratio of the modulus of a rubber at a given temperature to its modulus at 73 F. It is determined in the Gehman torsional test. Relaxation. Rate of reduction of stress in a material due to creep. An alternate term is stress relaxation. Repeated bent test. Method for determining ductility of relatively ductile metals such as silicon steel sheet and strip. Specimens about 1 in. wide and 6 in. long are held in jaws and bent 90 deg. Then they are bent back and forth through 180 deg. Results are reported as number of bends (including the original 90 deg bend) required to cause failure. (ASTM A-344). Repeated blow impact test. Method for judging impact properties of cast iron. Hammer is dropped on specimen from increasing heights until fracture occurs. Results are reported as height of the last drop prior to fracture (repeated blow impact value). (ASTM A-327). Residual elongation. Measure of ductility of plastics. It is the elongation of a plastic specimen measured 1 min after rupture in a tension test. Resilience. Measure of recoverable elastic energy in a deformed material. It is the amount of energy released when a load is removed from a specimen. It is equal to deformation energy minus electric hysteresis. Rockwell hardness number (RHN). Index of indentation hardness measured by a steel ball or diamond cone indentor. RHN is given in various scales (B, C, R, etc.) depending on indentor and scales used. ASTM E-18 details a standard method for determining RHN for metals and gives a table of scale symbols. ASTM D-785 gives standard method for measuring RHN of plastics; ASTM B-294 covers cemented carbides and ASTM A-370, steel products. Rockwell penetration. Measure of indentation hardness of rubber. It is the resistance to penetration by a specified indentor under specified load applied with a Rockwell hardness tester. (ASTM D-530). It usually is reported with Rockwell recovery. Rockwell recovery. Extent to which hard rubber recovers from indentation. It is obtained in same test as Rockwell penetration (ASTM D-530) and is reported with it. Rockwell superficial hardness. Measure of surface hardness of thin strip or finished parts on which large test marks cannot be tolerated or shapes that would collapse under normal Rockwell hardness test loads. (ASTM E-18). Ross flexing machine test. Method for measuring crack growth resistance of rubber by repeatedly bending pierced specimen and measuring growth of crack that develops. (ASTM D-1052). Rupture resistance. Indication of ability of rubber to withstand tensile loading. It is the load required to rupture a rubber specimen under conditions set out in ASTM D-530. Rupture strength. Nominal stress developed in a material at rupture. It is not necessarily equal to ultimate strength. And, since necking is not taken into account in determining rupture strength, it seldom indicates true stress at rupture. S-N diagram. Plot of stress (S) against the number of cycles (N) required to cause failure of similar specimens in a fatigue test. Data for each curve on an S-N diagram are obtained by determining fatigue life of a number of specimens subjected to various amounts of fluctuating stress. The stress axis can represent stress amplitude, maximum stress or minimum stress. A log scale is almost always used for the N scale and sometimes for the S scale. St. Joe flexometer test. Method for measuring compression fatigue characteristics of rubber. Results are reported as time and flexing load required to fail specimen. (ASTM D-623). Scleroscope hardness. Measure of hardness or impact resilience of metals. A diamond-tipped hammer falls freely against specimen from a fixed height and rebound height is measured. Scleroscope hardness is read on an empirical scale where 100 rep resents average rebound from a quenched high carbon steel specimen. ASTM A-427 contains a table that relates scleroscope hardness to diamond pyramid hardness. Scratch hardness. Method for determining comparative hardness of materials by measuring width of a scratch made by a scriber drawn across the surface under specified pressure. Often performed on coatings. Secant modulus of elasticity. Ratio of stress to strain at any point on curve in stress-strain diagram. It is the slope of a line from the origin to any point on stress-strain curve. Shear modulus of elasticity. Tangent or secant modulus of elasticity of a material subjected to shear loading. Alternate terms are modulus of rigidity and modulus of elasticity in shear. Also, shear modulus of elasticity usually is equal to torsional modulus of elasticity. A method for determining shear modulus of elasticity of structural materials by means of a twisting test is given in ASTM E-143. A method for deter mining shear modulus of structural adhesives is given in ASTM E-229. Shear strength. Maximum shear stress that can be sustained by a material before rupture. It is the ultimate strength of a material subjected to shear loading. It can be determined in a torsion test where it is equal to torsional strength. The shear strength of a plastic is the maximum load required to shear a specimen in such a manner that the resulting pieces are completely clear of each other. It is reported in psi based on the area of the sheared edge. (ASTM D-732). The shear strength of a structural adhesive is the maximum shear stress in the adhesive prior to failure under torsional loading. (ASTM E-229). Methods for deter mining shear strength of timber are given in ASTM D-143 and ASTM D-198. Softening point. Temperature at which a uniform fiber of glass elongates under its own weight at a specified rate. (ASTM C-338). The Vicat softening point of plastics is the temperature at which a flat ended needle of 1 sq mm circular or square cross section penetrates a thermoplastic specimen to a depth of 1 mm under load and conditions specified in ASTM D-1525. Splitting resistance. Measure of the ability of felt to withstand tearing. It is the load required to rupture a slit felt specimen by gripping lips of the cut in jaws and pulling them apart. (ASTM D-461). An alternate term is tear resistance. Springback. Degree to which a material returns to its original shape after deformation. In plastics and elastomers it is also called recovery. Stiffness. Measure of resistance of plastics to bending. It includes both plastic and elastic behavior, so it is an apparent value of elastic modulus rather than a true value. (ASTM D-747). 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. True or natural strain is based on instantaneous length and is equal to In l/lo where l is instantaneous length and lo is original length of the specimen. Shear strain is the change in angle between two lines originally at right angles. 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. Measure of increase in hardness and strength caused by plastic deformation. It is related to true stress and true strain by the equation: O= oOn where O is true stress, Oo is true stress at unit strain, O is true strain and n is strain hardening exponent. Strain point. Temperature at which internal stress in glass is substantially relieved in about 1 hr. (ASTM C 336). Strain rate. Time rate of elongation. Strain relaxation. Alternate term for creep of rubber. Strength reduction ratio. Alternate term for fatigue notch factor. Stress. Load on a specimen divided by the area through which it acts. As used with most mechanical tests, stress is based on original cross section area without taking into account changes in area due to applied load. This sometimes is called conventional or engineering stress. True stress is equal to the load divided by the instantaneous cross section area through which it acts. Stress amplitude. One-half the range of fluctuating stress developed in a specimen in a fatigue test. Stress amplitude often is used to construct an S-N diagram. Stress concentration factor. Ratio of the greatest stress in the area of a notch or other stress raiser to the corresponding nominal stress. It is a theoretical indication of the effect of stress concentrators on mechanical behavior. Stress concentration factor usually is higher than the empirical fatigue notch factor or strength reduction ratio because it does not take into account stress relief due to local plastic deformation. Stress corrosion cracking. Failure of a material due to combined effects of corrosion and stress. Generally, stress corrosion cracking refers to the phenomenon by which stress in creases corrosion rate. Stress ratio. Ratio of minimum stress to maximum stress in one cycle of loading in a fatigue test. Tensile stresses are considered positive and compressive stresses negative. Stress relaxation. Decrease in stress in a material subjected to prolonged constant strain at a constant temperature. Stress relaxation behavior is determined in a creep test. Data often is presented in the form of a stress vs time plot. Stress relaxation rate is slope of the curve at any point. Stress rupture strength. Alternate term for creep strength. Stress-strain diagram. Graph of stress as a function of strain. It can be constructed from data obtained in any mechanical test where a load is applied to a material and continuous measurements of stress and strain are made simultaneously. It is constructed for compression, tension and torsion tests. Stress-strain ratio. Stress divided by strain at any load or deflection. Below the elastic limit of a material it is equal to tangent modulus of elasticity. An alternate term is secant modulus of elasticity. Stripping strength. Alternate term for peel strength. Tangent modulus of elasticity. Instantaneous rate of change of stress as a function of strain. It is the slope at any point on a stress-strain diagram. Tear length. Measure of the drawability of sheet metal. Two small parallel slots are cut in the edge of the sheet to form a tab which is gripped and torn from the sheet. The variation in length of tabs torn in different directions is an indication of crystal orientation in the sheet. (Tabs torn in the direction of orientation are longer.) The degree of orientation is an indication of difficulty to be expected in drawing the sheet to uniform shapes. Tear resistance. Measure of the ability of sheet or film materials to resist tearing. For paper it is the force required to tear a single ply of paper after the tear has been started. (ASTM D-689). Three standard methods are available for determining tear resistance of plastic films: ASTM D-1004 details a method for determining tear resistance at low rates of loading. A test in ASTM D-1922 measures the force required to propagate a pre-cut slit across the sheet specimen. ASTM D-1038 gives a method for determining tear propagation resistance that is recommended for specification acceptance testing only. Tear resistance of rubber is the force required to tear a 1 in. thick specimen under the conditions outlined in ASTM D-624. Tear resistance of textiles is the force required to propagate a single-rip tongue-type tear (starting from a cut) by means of a falling pendulum apparatus. (ASTM D-1424). Tearing strength. Tensile force required to rupture a pre-slit woven fabric specimen under the conditions out-lined in ASTM D-2261 and ASTM D-2262. Edge tearing strength of paper is the force required to tear a specimen folded over a V-notch and loaded in a tensile test machine. (ASTM D-827). Temper test. Method for measuring the ability of strip or sheet metal to recover its original shape after bending. Temper of strip and sheet metals for electronic devices is measured by clamping a rectangular specimen in a rotatable clamp, bending it and recording the angle of spring-back. (ASTM F-155). To determine temper of zinc strip, a rectangular specimen is clamped in a rotating mandrel, pressed against the mandrel and released. Spring back is reported in a scale based on the apparatus used. (ASTM B-69). Tenacity. Force required to break a yarn or filament expressed in grams per denier. It is equal to breaking strength divided by denier. Tensile impact energy. Energy required to break a plastic specimen in tension by a single swing of a calibrated pendulum. (ASTM D-1822). It is a measure of impact strength obtained in a tension impact test. Tensile modulus of elasticity. Tangent or secant modulus of elasticity of a material subjected to tensile loading. Alternate terms are Young's modulus and modulus of elasticity in tension. It can be measured in a tension test or in a dynamic test where it is related to resonant frequency of a cylindrical rod by the equation: E = 412 12 p f2/k2 j4 where E is modulus of elasticity; 1, length of the rod; p, density; f, resonant frequency; k, radius of gyration of the rod about an axis normal to the rod axis and plane of motion (d/4 for cylindrical rods) and j, a constant dependent on the mode of vibration. Tensile modulus of elasticity is approximately equal to compressive modulus of elasticity within the proportional limit. An alternate term is Young's modulus. Tensile strength. Ultimate strength of a material subjected to tensile loading. It is the maximum stress developed in a material in a tension test. Tension impact test. Method for determining energy required to fracture a specimen under shock tensile loading. (ASTM D-1822). Tension set. Extent to which vulcanized rubber is permanently deformed after being stretched a specified amount for a short time. It is expressed as % of original length or distance between gage marks (ASTM D-412). Tension test. Method for determining behavior of materials under axial stretch loading. Data from test are used to determine elastic limit, elongation, modulus of elasticity, proportional limit, reduction in area, tensile strength, yield point, yield strength and other tensile properties. Tension tests at elevated temperatures provide creep data. Procedures for tension tests of metals are given in ASTM E-8, ASTM E-150 (creep tension tests) and ASTM E-151 (elevated temperatures). Methods for tension tests of plastics are outlined in ASTM D 638, ASTM D-2289 (high strain rates), ASTM D-882 and ASTM D 1923 (thin sheets) and ASTM D 795 (high and low temperatures). ASTM D-2343 outlines a method for tension testing of glass fibers; ASTM D-897, adhesives; ASTM D-987, paper products; ASTM D 412, vulcanized rubber. Thermal stress. Internal stress in part caused by uneven heating. Time for rupture. Time required to rupture specimen under constant stress and temperature in a creep test. Torque twist diagram. Graph of torque vs torsional deformation plotted from data obtained in a torsion test. Torsion resistance. Measure of the resistance to cracking of porcelain enamel coatings on an iron or steel substrate when the base metal is twisted. (ASTM D-409). Torsion test. Method for determining behavior of materials subjected to twisting loads. Data from torsion test is used to construct a stress strain diagram and to determine elastic limit, torsional modulus of elasticity, modulus of rupture in torsion and torsional strength. Shear properties are often determined in a torsion test. (ASTM E-143). Torsional deformation. Angular displacement of specimen caused by a specified torque in torsion test. It is equal to the angular twist (radians) divided by gage length (in.). Torsional modulus of elasticity. Modulus of elasticity of material subjected to twist loading. It is approximately equal to shear modulus and also is called modulus of rigidity. Torsional strain. Strain corresponding to a specified torque in the torsion test. It is equal to torsional deformation multiplied by the radius of the specimen. Torsional strength. Measure of the ability of a material to withstand a twisting load. It is the ultimate strength of a material subjected to torsional loading, and is the maximum torsional stress that a material sustains before rupture. Alternate terms are modulus of rupture and shear strength. Torsional stress. Shear stress developed in a material subjected to a specified torque in torsion test. It is calculated by the equation: s = Tr/J where T is torque, r is the distance from the axis of twist to the outer most fiber of the specimen and J is the polar moment of inertia Torsional Yield strength. Yield strength of material under twist loading. Toughness. Extent to which a material absorbs energy without fracture. It is usually expressed as energy absorbed in an impact test. The area under a stress-strain diagram also is a measure of toughness of a material. (ASTM D-256, plastics and ASTM E-23, metals). Transition temperature. Alternate term for ductile-to-brittle transition temperature. Transverse bend test. Alternate term for flexure test, often used in connection with cast iron. True strain. Instantaneous % change in length of specimen in mechanical test. It is equal to the natural logarithm of the ratio of length at any instant to original length. 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. Twisting test. Method for measuring shear properties of structural materials. (ASTM E-143). Ultimate elongation. Alternate term for elongation of material at rupture under tensile loading. Ultimate strength. Highest engineering stress developed in material before rupture. Normally, changes in area due to changing load and necking are disregarded in determining ultimate strength. Vicat softening point. Temperature at which a flat-ended needle of 1 sq mm cross section penetrates a thermoplastic material to a depth of 1 mm under conditions and loads set out in ASTM D-1525. Vickers hardness. Alternate term for diamond pyramid hardness. Wet strength. Breaking strength of paper saturated with water. Also, the strength of an adhesive bond after immersion in water. Wrapping diameter. Measure of wire ductility. It is the minimum diameter to which wire can be wound without causing failure. Yerzley mechanical oscillograph test. Method for determining the dynamic mechanical properties of rubber. (ASTM D-945). Results of test are reported as deformation energy, effective dynamic modulus and Yerzley resilience. Yerzley resilience. Measure of resilience of rubber subjected to dynamic loading. Yield point. Stress at which strain increases without accompanying increase in stress. Only a few materials (notably steel) have a yield point and generally only under tension loading. Yield point elongation. Strain at yield point of a material. It is an indication of ductility. Yield strength. Indication of maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of elastic limit. Offset yield strength is determined from a stress-strain diagram. It is the stress corresponding to the intersection of the stress-strain curve and a line parallel to its straight line portion offset by a specified strain. Offset is usually specified as 0.2 %, i.e., the intersection of the offset line and the 0-stress axis is at 0.2 % strain. Yield strength elongation. Strain corresponding to yield strength of material. It is an indication of ductility. Yield value. Stress in an adhesive joint at which a marked increase in deformation occurs without an increase in load. Young's modulus. Alternate term for modulus of elasticity in tension or compression. Reprinted from MATERIALS ENGINEERING, June 1967 issue, Reinhold Publishing Corporation, |
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