STRENGTH OF MATERIALS  , that See also:

• PART

part of the theory of See also:

• ENGINEERING

engineering which deals with the nature and effects of stresses in the parts of engineering structures . Its See also:

• PRINCIPAL

principal See also:

• OBJECT

object is to determine the proper See also:

• SIZE

size and See also:

• FORM (Lat. forma)

form of pieces which have to See also:

• BEAR
• BEAR, BLACK
• BEAR, BROWN
• BEAR, GRIZZLY
• BEAR, ISABELLINE
• BEAR, WHITE

bear given loads, or, conversely, to determine the loads which can be safely applied to pieces whose dimensions and arrangement are already given . It also treats of the relation between the applied loads and the changes of form which they cause . The subject comprises experimental investigation of the properties of materials as to strength and See also:

• ELASTICITY

elasticity, and mathematical discussion of the stresses in ties, struts, beams, shafts and other elements of structures and See also:

• MACHINES

machines . Stress is the mutual See also:

• ACTION

action between two bodies, or between two parts of a See also:

• BODY

body, whereby each of the two exerts a force upon the other . Thus, when a See also:

• STONE
• STONE (0. Eng. shin; the word is common to Teutonic languages, cf. Ger. Stein, Du. steen, Dan. and Swed. sten; the root is also seen in Gr. aria, pebble)
• STONE, CHARLES POMEROY (1824-1887)
• STONE, EDWARD JAMES (1831-1897)
• STONE, FRANK (1800-1859)
• STONE, GEORGE (1708—1764)
• STONE, LUCY [BLACKWELL] (1818-1893)
• STONE, MARCUS (184o— )
• STONE, NICHOLAS (1586-1647)

stone lies on the ground there is at the See also:

• SURFACE

surface of contact a stress, one aspect of which is the force directed downwards with which the stone pushes the ground, and the other aspect is the equal force directed upwards with which the ground pushes the stone . A body is said to be in a See also:

• STATE
• STATE, GREAT OFFICERS OF

state of stress when there is a stress between the two parts which See also:

• LIE, JONAS LAURITZ EDEMIL (1833—1908)
• LIE, MARIUS SOPHUS (1842–1899)

lie on opposite sides of an imaginary surface of See also:

• SECTION
• SECTION (Lat. sectio, cutting, secare, to cut)

section . A See also:

• PILLAR (0. Fr. piler, Mod. pilier, Late Lat. pilare, from pila, column)

pillar or See also:

• BLOCK
• BLOCK (from the Fr. bloc, and possibly connected with an Old Ger. Block, obstruction, cf. " baulk ")
• BLOCK, MAURICE (1816–19or)

block supporting a See also:

• WEIGHT

weight is in a state of stress because at any See also:

• CROSS

cross section the part above the section pushes down against the part below, and the part below pushes up against the part above . A stretched rope is in a state of stress, because at any cross section the part oneachside is pulling the part on the other See also:

• SIDE
• SIDE (mod. Eski Adalia)

side with a force in the direction of the rope's length . A See also:

• PLATE

plate of See also:

• METAL
• METAL (through Fr. from Lat. metallum, mine, quarry, adapted from Gr. µATaXAov, in the same sense, probably connected with ,ueraAAdv, to search after, explore, µeTa, after, aAAos, other)

metal that is being cut in a shearing See also:

• MACHINE
• MACHINE (through Fr. from Lat. form machina of Gr. µnxavil)

machine is in a state of stress, because at the See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place where it is about to give way the portion of metal on either side of the See also:

• PLANE

plane of shear is tending to See also:

• DRAG (from the Old Eng. dragan, to draw; the word preserves the g which phonetically developed into w)

drag the portion on the other side with a force in that plane . Normal and Tangential Stress.—In a solid body which is in a state of stress the direction of stress at an imaginary surface of See also:

• DIVISION (from Lat. dividere, to break up into parts, separate)

division may be normal, oblique or tangential to the surface . When oblique it is conveniently treated as consisting of a normal and a tangential component .

Normal stress may be either push (compressive stress) or pull (tensile stress) . Stress which is tangential to the surface is called shearing stress . Oblique stress may be regarded as so much push or pull along with so much shearing stress . The amount of stress per unit of surface is called the intensity of stress . Stress is said to be uniformly distributed over a surface when each fraction of the See also:

• AREA

area of surface bears a corresponding fraction of the whole stress . If a stress P is uniformly distributed over a plane surface of area S, the intensity is P/S . If the stress is not uniformly distributed, the intensity at any point is SP/SS, where SP is the amount of stress on an indefinitely small area SS at the point considered . For See also:

• PRACTICAL

practical purposes intensity of stress is usually expressed in tons weight per square See also:

• INCH (O. Eng. ynce from Lat. uncia, a twelfthpart; cf. " ounce," and see As)

inch, pounds weight per square inch, or kilogrammes weight per square millimetre or per square centimetre . See also:

• SIMPLE

Simple See also:

• LONGITUDINAL

Longitudinal Stress.—The simplest possible state of stress is that of a See also:

• SHORT, FRANCIS JOB (1857– )

short pillar or block compressed by opposite forces applied at its ends, or that of a stretched rope or other tie . In these cases the stress is wholly in one direction, that of the length . These states may be distinguished as simple longitudinal push and simple longitudinal pull . In them there is no stress on planes parallel to the direction of the applied forces .

See also:

• COMPOUND
• COMPOUND (from Lat. componere, to combine or put together)

Compound Stress.—A more complex state of stress occurs if the block is compressed or extended by forces applied to a pair of opposite sides, as well as by forces applied to its ends—that is to say, if two simple longitudinal stresses in different directions See also:

• ACT (Lat. actus, actum)

act together . A still more complex state occurs if a third stress be applied to the remaining pair of sides . It may be shown (see ELASTICITY) that any state of stress which can possibly exist at any point of a body may be produced by the See also:

• JOINT (through Fr. from Lat. junctum, jungere, to join)

joint action of three simple pull or push stresses in three suitably chosen directions at right angles to each other . These three are called principal stresses,‘ and their directions are called the axes of principal stress . These axes have the important See also:

• PROPERTY

property that the intensity of stress along one of them is greater, and along another it is less, than in any other direction . These are called respectively the axes of greatest and least principal stress . See also:

• RESOLUTION

Resolution of Stress.—Returning now to the See also:

• CASE
• CASE, JOHN (d. 1600)

case of a single simple longitudinal stress, let AB (fig . I) be a portion of a tie or a strut A which is being pulled or pushed in the direction of the See also:

• AXIS (Lat. for " axle ")

axis AB with a See also:

• TOTAL

total stress P . On any plane CD taken at right angles to the axis we have a normal pull or push of intensity p=P/S, S being the area of the normal cross-section . On a plane EF whose normal is inclined to the axis at an See also:

• ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle 0 we have a stress still in the direction of the axis, and therefore oblique to the plane EF, of intensity P/S', where S' is the area of the surface EF, or S/See also:

• COS
• COS, or STANKO (Ital. Stanchio, Turk. Islan-keui, by corruption from Etc rav KW)

cos 0 . The whole stress P on EF may be resolved into two components, one normal to EF, and the other a shearing stress tangential to EF . The normal component (P., fig .

2) is P cos 0; the tangential component (Pe) is P See also:

• SIN
• SIN (O. Eng. syn: a common Teutonic word, cf. Dutch zonde, Ger. Siinde)

sin 0 . Hence the intensity of normal pull or push on EF, or p,,, is p cos' 0, and the B intensity of shearing stress EF, or p,, See also:

• COMBINATION (Lat. combinare, to combine)

Combination of Two Simple Pull or Push Stresses at Right Angles to One Another.--Suppose next that there are two principal stresses: in other words that in addition to the simple pull or push stress of fig . I there is a second pull or push stress acting at right angles to it as in fig . 33 . See also:

• CALL (from Anglo-Saxon ceallian, a common Teutonic word, cf. Dutch kallen, to talk or chatter)

Call these Ps and Py respectively . On any inclined surface EF there will be an in-tensity of stress whose normal component p° and tangential component pi are found by summing up the effects due to P, and P„ g separately . Let ps and py be the intensities of stress produced by P. and Py respectively on planes perpendicular to their own direc- tions . Then p = (P. cos' 0+p,, sine 0, Pt = (pz—Py) sin 0 cos 0, B being the angle which the normal to the surface makes with the direction of P . The tangential stress Pt becomes a maximum when 0 is 45°, and its value then is Max. p, = i (Pr pv) • If in addition there is a third principal stress it will not produce any tangential component on planes perpendicular to the plane of the figure . Hence the above expression for the maximum tangential stress will still apply, and it is easy to extend this result so as to reach the important See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

general proposition that in any See also:

• CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition of stress whatever the maximum intensity of shearing stress is equal to one-See also:

• HALF

half the difference between the greatest and least principal stresses and occurs on surfaces inclined at 45° to them . Q State of Simple Shear.—A See also:

• SPECIAL

special case of See also:

• GREAT

great importance occurs when there are two principal stresses only, equal in >Q magnitude and opposite in sign; in other words, when one is a simple push and the other a simple pull . Then on surfaces inclined at 45° to the axes of pull and push there is nothing but tangential stress, for p„ = 0; and this intensity of tangential stress is numerically equal to P. or to N .

This condition of stress is called a state of simple shear . The state of simple shear may See also:

• FIGS

Figs . 3, 4, 5, 6, 15, 16, 17, 18, 19, 20, 23, 24 and 25 are from See also:

• EWING
• EWING, ALEXANDER (1814-1873)
• EWING, THOMAS (1789-1871)

Ewing's Strength of Materials, by permission of the See also:

• CAMBRIDGE
• CAMBRIDGE, EARLS AND DUKES OF
• CAMBRIDGE, RICHARD OWEN (1717-1802)

Cambridge University See also:

• PRESS (through Fr. presse from Lat. pressare, frequentative of premere, to crush, squeeze, press)

Press.as in the figure . The effect is to set up a state of simple shear . On all planes parallel to A and B there is nothing but tangential stress, and the same is true of all planes parallel to C and D . The intensity of the stress on both systems of planes is equal throughout to the intensity of the stress which was applied to the See also:

• FACE (from Lat. fades, derived either from facere, to make, or from a root fa-, meaning " appear "; cf. Gr. cbatvstv)

face of the block . To see the connexion between these two ways of specifying a state of simple shear consider the See also:

• EQUILIBRIUM (from the Lat. aequus, equal, and libra, a balance)

equilibrium of the parts into which the block may be divided by ideal See also:

• DIAGONAL (Gr. &a, through, ywvia, a corner)

diagonal planes of section . To See also:

• BALANCE (derived through the Fr. from the Late Lat. bilantia, an apparatus for weighing, from bi, two, and lanx, a dish or scale)

balance the forces QQ (fig . 5), there must be normal pull on the diagonal plane, the amount of which is P=Qsl2 . But the area of the surface over which P acts is greater than that of the surface over which Q acts in the proportion which P bears to Q, and hence the intensity of P is the same as the intensity of Q . Again, taking the other diagonal plane (fig . 6), the same See also:

• ARGUMENT

argument applies except that here the normal force P required for equilibrium is a push instead of a pull .

Thus the state of stress represented in fig . 4 admits of See also:

• ANALYSIS (Gr. avci and Meta, to break up into parts)

analysis into two equal principal stresses, one of push and one of pull, acting in directions at right angles to one another and inclined at 45° to the planes of shear stress . Equality of Shearing Stress in Two Directions.—No tangential stress can exist in one direction without an equal intensity of tangential stress existing in another direction at right angles to the first . To prove this it is sufficient to consider the equilibrium of the ele- Q mentary See also:

• CUBE (Gr. K46os, a cube)

cube of fig . 4 . The tangential FIG . 6 . forces acting on two sides A and B produce a couple which tends to rotate the cube . • No arrangement of normal stresses on any of the three pairs of sides of the cube can balance this couple; that can be done only by equal tangential forces on C and D . Fluid Stress.—Another important case occurs when there are three principal stresses all of the same sign and of equal intensity p . The tangential components on any planes See also:

• CANCEL (from the Lat. cancelli, a plural diminutive of cancer, a grating or lattice, from which are also derived " chancel " and " chancellor ")

cancel each other: the stress on every plane is wholly normal and its intensity is p . This is the only state of stress that can exist in a fluid at See also:

• REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

rest because a fluid exerts no statical resistance to shear .

For this See also:

• REASON (Lat. ratio, through French raison)

reason the state is often spoken of as a fluid stress . See also:

• STRAIN (through O. Fr. straindre, estraindre, mod. i treindre, from Lat. stringere, to draw tight, related to stress, stretch, string, &c.)

Strain is the See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change of shape produced by stress . If the stress is a simple longitudinal pull, the strain consists of lengthening in the direction of the pull, accompanied by contraction in both directions at right angles to the pull . If the stress is a simple push, the strain consists of shortening in the direction of the push and expansion in both directions at right angles to that; the stress and the strain are then exactly the See also:

• REVERSE

reverse of what they are in the case of simple pull . If the stress is one of simple shearing, the strain consists of a distortion such as would be produced by the sliding of layers in the direction of the shearing stresses . A material is elastic with regard to any applied stress if the strain disappears when the stress is removed . Strain which persists after the stress that produced it is removed is called permanent set . For brevity, it is convenient to speak of strain which disappears when the stress is removed as elastic strain . Limits of Elasticity.—Actual materials are generally very perfectly elastic with regard to small stresses, and very imperfectly elastic with regard to great stresses . If the applied stress is less than a certain limit, the strain is in general small ,n amount, and disappears wholly, or almost whollyy when the stress is removed . If the applied stress exceeds this limit, the strain is, in general, much greater than before, and most of it is found, when the stress is removed, to consist of permanent set . The Pr .

E Pj ._gyp P C A Q Q Q limits of stress within which strain is wholly or almost wholly elastic are called limits of elasticity . For any particular mode of stress the limit of elasticity is much more sharply defined in some materials than in others . When well defined it may readily be recognized in the testing of a See also:

• SAMPLE (through the O. Fr. essemple, from Lat. exemplum; a doublet of " example ")

sample from the fact that after the stress exceeds the limit of elasticity the strain begins to increase in a much more rapid ratio to the stress than before . This characteristic goes along with the one already mentioned, that up to the limit the strain is wholly or almost wholly elastic . See also:

• HOOKE, ROBERT (1635–1703)

Hooke's See also:

• LAW
• LAW (0. Eng. lagu, M. Eng. lawe; from an old Teutonic root lag, " lie," what lies fixed or evenly; cf. Lat. lex, Fr. loi)
• LAW, JOHN (1671—1729)
• LAW, WILLIAM (1686-1761)

Law.—Within the limits of elasticity the strain produced by a stress of any one See also:

• KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind is proportional to the stress producing it . This is Hooke's law, enunciated by him in 1676 . In applying Hooke's law to the case of simple longitudinal stress —such as the case of a See also:

• BAR
• BAR (0. Fr. barre, Late Lat. barra, origin unknown)
• BAR, CONFEDERATION OF
• BAR, FRANCOIS DE (1538-1606)
• BAR, THE

bar stretched by simple longitudinal pull—we may measure the state of strain by the change of length per unit of See also:

• ORIGINAL

original length which the bar undergoes when stressed . Let the original length be 1, and let the whole change of length be Sl when a stress is applied whose intensity p is within the elastic limit . Then the strain is measured by Sl/l, and this by Hooke's law is proportional to p . This may be written I1/1=p/E, where E is a See also:

• CONSTANT, BENJAMIN (1845-1902)

constant for the particular material considered . The same value of E applies to push and to pull, these modes of stress being essentially continuous, and differing only in sign . See also:

• YOUNG
• YOUNG, A
• YOUNG, ARTHUR (1741-1820)
• YOUNG, BRIGHAM (1801-1877)
• YOUNG, CHARLES MAYNE (1777–1856)
• YOUNG, EDWARD (1683–1765)
• YOUNG, JAMES (1811-1883)
• YOUNG, THOMAS (1773-1829)

Young's Modulus.—This constant E is called the modulus of longitudinal extensibility, or Young's modulus .

Its value, which is expressed in the same See also:

• UNITS, DIMENSIONS OF
• UNITS, PHYSICAL

units as are used to See also:

• EXPRESS (through the French from the past participle of the Lat. exprimere, to press out, by transference used of representing objects in painting or sculpture, or of thoughts, &c. in words)

express intensity of stress, may be measured directly by exposing a sample of the material to longitudinal pull and noting the See also:

• EXTENSION (Lat. ex, out ; tendere, to stretch)

extension, or indirectly by measuring the flexure of a loaded See also:

• BEAM
• BEAM (from the O. Eng. beam, cf. Ger. Baum, a tree, to which sense may be referred the use of " beam " as meaning the rood or crucifix, and the survival in certain names of trees, as horn-beam)

beam of the material, or by experiments on the frequency of vibrations . It is frequently spoken of by See also:

• ENGINEERS, MILITARY

engineers simply as the modulus of elasticity, but this name is too general, as there are other moduli applicable to other modes of stress . Since E = pl/Sl, the modulus may be defined as the ratio of the intensity of stress p to the longitudinal strain Il/i . Modulus of Rigidity.—In the case of simple shearing stress, the strain may be measured by the angle by which each of the four originally right angles in the square See also:

• PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)

prism of fig . 3 is altered by the distortion of the prism . Let this angle be ¢ in radians; then by Hooke's law p/t = C, where p is the intensity of shearing stress and C is a constant which See also:

• MEASURES

measures the rigidity of the material . C is called the modulus of rigidity, and is usually determined by experiments on torsion . Modulus of Cubic Compressibility.—When three simple stresses of equal intensity p and of the same sign (all pulls or all pushes) are applied in three directions, the material (provided it be isotropic, that is to say, provided its properties are the same in all directions) suffers change of See also:

• VOLUME

volume only, without distortion of form . If the volume is V and the change of volume IV, the ratio of the stress p to the strain SV/V is called the modulus of cubic compressibility, and will be denoted by K . Of these three moduli the one of most importance in engineering applications is Young's modulus E . When a simple longitudinal pull or push of intensity is is applied to a piece, the longitudinal strain of extension or See also:

• COMPRESSION

compression is p/E . This is accompanied by a lateral contraction or expansion, in each trans-See also:

• VERSE (from Lat. versus, literally a line or furrow drawn by turning the plough, from vertere, and afterwards signifying an arrangement of syllables into feet)

verse direction, whose amount may be written p/uE, where a is the ratio of longitudinal to lateral strain .

It is shown in the See also:

• ARTICLE (from Lat. articulus, a joint)

article ELASTICITY, that for an isotropic material E= 901( and o=2(3K+C) 3K+C 3K—2C Plastic Strain.—Beyond the limits of elasticity the relation of strain to stress becomes very indefinite . Materials then exhibit, to a greater or less degree, the property of plasticity . The strain is much affected by the length of See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time during which the stress has been in operation, and reaches its maximum, for any assigned stress, only after a See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

long (perhaps an indefinitely long) time . Finally, when the stress is sufficiently increased, the ratio of the increment of strain to the increment of stress becomes indefinitely great if time is given for the stress to take effect . In other words, the substance then assumes what may be called a completely plastic state; it flows under the applied stress like a viscous liquid . Ultimate Strength.—The ultimate strength of a material with regard to any stated mode of stress is the stress required to produce rupture . In reckoning ultimate strength, however, engineers take, not the actual intensity of stress at which rupture occurs, but the value which this intensity would have reached had rupture ensued without previous alteration of shape . Thus, if a bar whose original cross-section is 2 sq. in. breaks under a uniformly distributed pull of 6o tons, the ultimate tensile strength of the material is reckoned to be 30 tons per square inch, although the actual intensity of stress which produced rupture may have been much greater than this, owing to the contraction of the section previous to fracture . The convenienceof this usage will be obvious from an example . Suppose that a piece of material of the same quality be used in a structure under conditions which cause it to bear a simple pull of 6 tons per square inch; we conclude at once that the actual load is one-fifth of that which would cause rupture, irrespective of the extent to which the material might See also:

• CONTRACT
• CONTRACT (Lat. contract us, from contrahere, to draw together, to bind)

contract in section if overstrained . The stresses which occur in engineering practice are, or ought to be, in all cases within the limits of elasticity, and within these limits the change of cross-section caused by longitudinal pull or push is so small that it may be neglected in reckoning the intensity of stress . Ultimate tensile strength and ultimate shearing strength are well defined, since these modes of stress (simple pull and simple shearing stress) See also:

• LEAD
• LEAD (pronounced iced)

lead to distinct fracture if the stress is sufficiently increased .

Under compression some materials yield so continuously that their ultimate strength to resist compression can scarcely be specified ; others show so distinct a fracture by crushing that their compressive strength may be determined with some precision . Some of the materials used in engineering, notably See also:

• TIMBER

timber and wrought See also:

• IRON [symbol Fe, atomic weight 55.85 (0=16)]

iron, are so far from being isotropic that their strength is widely different for stresses in different directions . In the case of wrought iron the See also:

• PROCESS

process of See also:

• ROLLING

rolling develops a fibrous structure on See also:

• ACCOUNT
• ACCOUNT (through O. Fr. acont, Late Lat. comptum, cornputare, to calculate)

account of the presence of streaks of slag which become interspersed with the metal in puddling; and the tensile strength of a rolled plate is found to be considerably greater in the direction of rolling than across the plate . See also:

• STEEL, FLORA ANNIE (1847– )

Steel plates, being rolled from a nearly homogeneous See also:

• INGOT

ingot, have nearly the same strength in both directions, provided the process of rolling is completed at a temperature high enough to allow recrystallization to take place in cooling . See also:

• COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

Cold-rolled or cold-See also:

• DRAWN

drawn metal is not isotropic because the crystals of which it is made up have been elongated in one direction by the process: but isotropy may be restored by See also:

• HEATING

heating the piece sufficiently to allow the crystals to re-form . Permissible Working Stress.—In applying a knowledge of the strength of materials to determine the proper sizes of parts in an engineering structure we have to estimate a permissible working stress . This is based partly on special tests and partly on experience of the behaviour of the material when used in similar structures . The working stress is rarely so much as one-third of the ultimate strength; it is more commonly one-See also:

• FOURTH

fourth or one-fifth and in some cases, especially where the loads to- be See also:

• BORNE, KARL LUDWIG (1786–1837)

borne are liable to reversal or to much change, it may be prudent to make the working stress even less than this . See also:

• FACTOR (from Lat. facere, to make or do)

Factor of Safety.—The ratio of the ultimate strength to the working stress is called the factor of safety . The factor should in general be such as to bring the working stress within the limit of elasticity and even to leave within that limit a margin which will be ample enough to See also:

• COVER (from the Fr. convert, from couvrir, to cover, Lat. cooperire)

cover such contingencies as imperfection in the theory on which the calculation of the working stress is founded, lack of uniformity in the material itself, uncertainty in the estimation of loads, imperfections of workmanship which may cause the actual dimensions to fall short of those that have been specified, alterations arising from See also:

• WEAR

wear, See also:

• RUST (O.E. rust, a word which appears in -many Teutonic languages, cf. Du. roest, Ger. rost); in origin it is allied with " ruddy " and " red," the reddish-brown powdery substance which forms on the surface of iron or steel exposed to atmospheric corrosio

rust and so forth . An important distinction has to be drawn in this connexion between steady or " dead " loads and loads which are subject to variation and especially to reversal . With the former the working stress may reach or pass the elastic limit without destroying the structure; but in a piece subject to reversals a stress of the same magnitude would lead inevitably to rupture, and hence a larger margin should be See also:

• LEFT

left to ensure that in the latter case the elastic limit shall not even be approached .

It is in fact the elastic limit rather than the ultimate strength of the material on which the question mainly depends of how high the working stress may safely be allowed to rise in any particular conditions as to mode of loading, and accordingly it becomes a See also:

• MATTER

matter of much practical importance to determine by tests the amount of stress which can be borne without permanent strain . From an engineering point of view the structural merit of a material, especially when variable loads and possible shocks have to be sustained, depends not only on the strength but also on the extent to which the material will bear deformation without rupture . This characteristic is shown in tests made to determine tensile strength by the amount of ultimate See also:

• ELONGATION

elongation, and also by the contraction of the cross-section which occurs through the flow of the metal before rupture . It is often, tested in other ways, such as by bending and unbending bars in a circle of specified See also:

• RADIUS

radius, or by examining the effect of repeated blows . Tests by impact are generally made by causing a weight to fall through a regulated distance on a piece of the material supported as a beam . Tests of Strength.—See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

Ordinary tests of strength are made by submitting the piece to See also:

• DIRECT

direct pull, direct compression, bending or torsion . Testing machines are frequently arranged so that they may apply any of these four modes of stress; tests by direct tension are the most See also:

• COMMON

common, and next to them come tests by bending . When the samples to be tested for tensile strength are See also:

• MERE
• MERE, ADALBERT (1838-1909)

mere wires, the stress may be applied directly by weights; for pieces of larger section some See also:

• MECHANICAL

mechanical multiplication of force becomes necessary . Owing to the plasticity of the materials to be tested, the applied loads must be able to follow considerable change of form in the test-piece: thus in testing the tensile strength of wrought iron or steel See also:

• PROVISION (Lat. provisio)

provision must be made for taking up the large extension of length which occurs before fracture . In most See also:

• MODERN

modern forms of large testing machines the loads are applied by means of See also:

• HYDRAULIC

hydraulic pressure acting on a See also:

• PISTON (through Fr. from Ital. pistone or pestone, a great pestle, from Late Lat. pistare, to pound, a frequentative form of classical Lat. pinsere)

piston or plunger to which one end of the specimen is secured, and the stress is measured by connectinglever . The See also:

• LOWER

lower holder is jointed to a cross-See also:

• HEAD (in 0. Eng. heafad; the word is common to Teutonic languages; cf. Dutch hoofd, Ger. Haupt, generally taken to be in origin connected with Lat. caput, Gr. KerbOvi7)
• HEAD, SIR EDMUND WALKER, BART
• HEAD, SIR FRANCIS BOND, BART

head C, which is connected by two See also:

• VERTICAL (from Lat. vertex, highest point)

vertical screws to a lower cross-head B, upon which the hydraulic plunger shown in section in fig . 7 exerts its thrust .

G is a counterpoise which pushes up the plunger when the See also:

• WATER

water is allowed to See also:

• ESCAPE (in mid. Eng. eschape or escape, from the O. Fr. eschapper, modern echapper, and escaper, low Lat. escapium, from ex, out of, and cappa, cape, cloak; cf. for the sense development the Gr. iichueoOat, literally to put off one's clothes, hence to sli

escape . Hydraulic pressure may be applied to the plunger by pumps or by an See also:

• ACCUMULATOR

accumulator . In the See also:

• PRESENT

present instance it is applied by means of an See also:

• AUXILIARY (from Lat. auxilium, help)

auxiliary plunger Q, which is pressed by See also:

• SCREW (O.E. scrue, from O. Fr. escroue, mod. ecrou; ultimate origin uncertain; the word, or a similar one, appears in Teutonic languages, cf. Ger. Schraube, Dan. skrue, but Skeat, following Diaz, finds the origin in Lat. scrobs, a ditch, hole, particularl

screw gearing into an auxiliary See also:

• CYLINDER (Gr. KvAwSpos, from KvXivaety, to roll)

cylinder . Q is driven by a See also:

• BELT (a word common to Teutonic languages, the Old Ger. form being bale, from which the Lat. balteus probably derived)
• BELT, THOMAS (1832-1878)

belt on the See also:

• PULLEY

pulley D . This puts stress on the specimen, and the weight W is then run out along the See also:

• LEVER (through O. Fr. leveour, levere, mod. levier, from Lat. levare, to lift, raise)
• LEVER, CHARLES JAMES (1806-1872)

lever so that the lever is just kept floating between the stops E, E . Before the test-piece is put in the distance between the holders is regulated by means of the screws connecting the upper and lower cross-heads C and B, these screws being turned by a handle applied at F . The See also:

• KNIFE (0. E. cuff, a word appearing in different forms in many Teutonic languages, cf. Du. knijf, Ger. Kneif, a shoe-maker's knife, Swed. knif; the ultimate origin is unknown; Skeat finds the origin in the root of " nip," formerly " knip "; Fr. canif is a

knife edges are made long enough to prevent the load on them from ever exceeding 5 tons to the linear inch . To adapt a machine of this class for tests in compression, a small See also:

• PLATFORM (Fr. plateforme, i.e. ground plan)

platform is suspended like a See also:

• STIRRUP (O. Eng. slirap, stigrap, M. Eng. stirop,styrope,&c.,i.e. a mounting or climbing-rope; O. Eng. stigan, to mount, climb, and rap, rope, cf. Du. stijbeugel, literally mounting bow or loop, Ger. Steigbugel)

stirrup by four rods from the weigh-beam, and hangs below the cross-head, which is pulled R—=eElOO the other end to a lever or See also:

• SYSTEM

system of levers provided with adjust-able weights . In small machines, and also in some large ones, the stress is applied by screw gearing instead of by hydraulic pressure . Springs are sometimes used instead of weights to measure the stress, and another See also:

• PLAN
• PLAN (from Lat. planes, flat)

plan is to make one end of the specimen act on a See also:

• DIAPHRAGM (Gr. Scat/pay,aa, a partition)

diaphragm forming part of a hydrostatic pressure See also:

• GAUGE, or GAGE (Med. Lat. gauja, jaugia, Fr. jauge, perhaps connected with Fr. jale, a bowl, galon, gallon)

gauge . Single-lever Testing Machine.—Figs . 7 and 8 show an excellent form of single-lever testing machine designed by J .

H . See also:

• WICK

Wick-steed (Prot . Inst . Mech . Eng., See also:

• AUGUST (originally Sextilis)

August 1882) in which the stress is applied by an hydraulic plunger and is measured by a lever or See also:

• STEELYARD, MERCHANTS OF THE

steelyard and a movable weight . The See also:

• ILLUSTRATION

illustration shows a 3o-ton machine, but machines of similar See also:

• DESIGN (Fr. desiin, drawing; Lat. designare, to mark out)

design are in common use which exert a force of too tons or more . AA is the lever, on which there is a graduated See also:

• SCALE
• SCALE (1) A

scale . The stress on the test-piece T is measured by a weight W oft ton (with an attached See also:

• VERNIER, PIERRE (c. 1580–1637)

vernier scale), which is moved along, the lever by a screw-See also:

• SHAFT
• SHAFT (O. Eng., sceaft, from scafan, to shave; the word is common to Teutonic languages)

shaft S; this screw-shaft is driven by a belt from a parallel shaft R, which takes its See also:

• MOTION (Lat. motio, from movere, to move)
• MOTION, LAWS OF

motion, through See also:

• BEVEL (from an O. Fr. word, cf. mod. biveau, a joiner's instrument)

bevel-wheels and a Hooke's joint in the axis of the fulcrum, from the See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

hand-See also:

• WHEEL (0. Eng. hweol, hweohl, &c., cognate with Icel. hjol, Dan. hiul, the Indo-European root is seen in Sanskrit chakra, Gr. r in Aos, circle, whence " cycle ")
• WHEEL, BREAKING ON THE

wheel H . (The Hooke's joint in the shaft R is shown in a See also:

• SEPARATE

separate See also:

• SKETCH (directly adapted from Dutch schets, which was taken from Ital. schizzo, a rough draft, Lat. schedium, something hastily made, Gr. oxE&os, sudden, off-hand, axeb6v, near by; Ger. Skizze and Fr. esquisse are from the same source)

sketch above the lever in fig . 8.) The holder for the upper end of the sample hangs from a knife-edge 3 in. from the fulcrum of thedown when the hydraulic cylinder is put in action . The arrangement is that of two stirrups linked with one another, so that when the two pull against each other a block of material placed between them becomes compressed . For tests in bending one of the stirrups, namely, the platform which hangs from the weigh-beam, is made some 4 or 5 ft. long, and carries two knife-edge supports at its ends on which the ends of the piece that is to be See also:

• BENT
• BENT (E1. BENI)
• BENT, JAMES THEODORE (1852–1897)

bent rest, while the cross-head presses down upon the See also:

• MIDDLE

middle of the piece .

In both cases the force which is exerted is measured by means of the weigh-beam and travelling weight, just as in the tension tests . To apply the force continuously, without See also:

• SHOCK, or COLLAPSE

shock, and either quickly or slowly at will, a very convenient plan is to use an hydraulic intensifier, consisting of a large hydraulic piston operating a much smaller one . By gradually admitting water to the large piston from any convenient source under moderate pressure, such as the See also:

• TOWN

town water mains, a progressively increased pressure is produced in the fluid on which the small piston acts, and this fluid is admitted to the straining cylinder of the. machine . One of the advantages of the vertical type of machine, with its single See also:

• HORIZONTAL

horizontal lever, is the facility with which the correctness of its readings may be verified . The two things to be tested are (r) the distance between the knife-edges, one of Which forms the fulcrum of the weigh-beam and from the other of which the shackle holding the upper end of the specimen is hung, and (2) the weight of the travelling poise . The weight of the poise is readily ascertained by using a supplementary known weight to apply a known moment to the beam, and measuring how far the poise, has to be moved to restore equilibrium . The distance between the knife-edges is then found by See also:

• HANGING

hanging a known heavy weight from the shackle, and again observing how far the poise has to be moved . Another example of the single-lever type is the See also:

• WERDER, KARL WILHELM FRIEDRICH AUGUST LEOPOLD, COUNT VON (1808—1887)

Werder testing machine, much used on the See also:

• CONTINENT (from Lat. continere, "to hold together "; hence " connected," " continuous ")

continent of See also:

• EUROPE

Europe . In it the specimen is horizontal; one end is fixed, the other is attached to the short vertical See also:

• ARM (a common Teutonic word; the Indo-European root is ar, to join or fit; cf. the Lat. armus, shoulder, and the plural word arma, weapons, Gr. apphs, joint, and the reduplicated apapilKSV, to join)

arm of a See also:

• BELL
• BELL, ALEXANDER MELVILLE (1819—1905)
• BELL, ANDREW (1753—1832)
• BELL, GEORGE JOSEPH (1770-1843)
• BELL, HENRY (1767-1830)
• BELL, HENRY GLASSFORD (1803-1874)
• BELL, JACOB (1810-1859)
• BELL, JOHN (1691-178o)
• BELL, JOHN (1763-1820)
• BELL, JOHN (1797-1869)
• BELL, ROBERT (1800-1867)
• BELL, SIR CHARLES (1774—1842)

bell-See also:

• CRANK

crank lever, whose fulcrum is pushed out horizontally by an hydraulic See also:

• RAM
• RAM, PIERRE FRANCOIS XAVIER DE (18o4-1865)

ram.' Multiple-lever Testing Machines.— In many other testing machines a system of two, three or more levers is employed to reduce the force between.the specimen and the measuring weight . In most cases the fulcrums are fixed, and the stress is applied to one end of the specimen by hydraulic See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power or by screw gearing, which takes up the stretch, as in the single-lever machines already described . See also:

• DAVID
• DAVID (a Hebrew name meaning probably beloved 1)
• DAVID, FELICIEN (1810—1876)
• DAVID, GERARD [GHEERAERT DAVIT], (?=1523)
• DAVID, PIERRE JEAN (1789–1856)
• DAVID, ST (Dewi, Sant)

David Kirkaldy, who was one of the earliest as well as one of the most assiduous workers in this See also:

• FIELD (a word common to many West German languages, cf. Ger. Feld, Dutch veld, possibly cognate with O.E. f olde, the earth, and ultimately with root of the Gr. irAaror, broad)
• FIELD, CYRUS WEST (1819-1892)
• FIELD, DAVID DUDLEY (18o5-1894)
• FIELD, EUGENE (1850-1895)
• FIELD, FREDERICK (18o1—1885)
• FIELD, HENRY MARTYN (1822-1907)
• FIELD, JOHN (1782—1837)
• FIELD, MARSHALL (183 1906)
• FIELD, NATHAN (1587—1633)
• FIELD, STEPHEN JOHNSON (1816-1899)
• FIELD, WILLIAM VENTRIS FIELD, BARON (1813-1907)

field, applied in his r,000,000 lb machine a horizontal hydraulic press directly to one end of the horizontal test-piece . The other end of the piece was connected to the short vertical arm of a bell-crank lever; the long arm of this lever was horizontal, and was connected to a second lever to which weights were applied .

Machines have been employed in which one end of the specimen is held in a fixed support; an hydraulic press acts on the other end, and the stress is calculated from the pressure of fluid in the press, this being observed by a pressure-gauge . Machines of this class are open to the obvious objection that the See also:

• FRICTION (from Lat. fricare, to rub)

friction of the hydraulic plunger causes a large and very uncertain difference between the force exerted by the fluid on the plunger and the force exerted by the plunger on the specimen . - It appears, however, that in the ordinary conditions of packing the friction is, very nearly proportional to the fluid pressure, and its effect may therefore be allowed for with some exactness . The method is not to be recommended for woik requiring precision, unless the plunger be kept in constant rotation on its own axis during the test, in which case the effects of friction are almost entirely eliminated . Diaphragm Testing Machines.—In another class of testing machines the stress (applied as before to one end of the piece, by gearing or by hydraulic pressure) is measured by connecting the other end to a flexible diaphragm, on which a liquid acts whose pressure is determined by a gauge . Fig . 9 shows Thomasset's testing machine, in which one end of the specimen is pulled by an hydraulic press A . The other end acts through a bell-crank lever B on a horizontal diaphragm C, consisting of a metallic plate and a flexible See also:

• RING (O.E. hring; a word common to Teutonic languages; and probably cognate with the Lat. circus, Gr. KtpKOS or KpLKOS, Skt'. chakra, wheel, circle, cf. also " harangue "); in art, a band of circular shape of varying sizes, made of any material and used f

ring of See also:

• INDIA
• INDIA, FRENCH

india-See also:

• RUBBER, INDIARUBBER

rubber . The pressure on the diaphragm causes a See also:

• COLUMN (Lat. columna)

column of See also:

• MERCURY
• MERCURY (MERCU1uus)
• MERCURY (symbol Hg, atomic weight = 2oo)

mercury to rise in the gauge-See also:

• TUBE (Lat. tuba)

tube D . The same principle is applied in the remarkable testing machine of See also:

• WATERTOWN

Watertown. See also:

• ARSENAL

arsenal, built in 1879 by the U.S. See also:

• GOVERNMENT
• GOVERNMENT (0. Fr. governement, mod. gouvernement, O. Fr. governer, mod. gouverner, from Lat. gubernare, to steer a ship, guide, rule; cf. Gr. xv(3epvav)

government to the designs of A . H . See also:

• EMERY (Ger. Smirgel)

Emery .

This is a horizontal machine, taking specimens of any length up Maschine zum Prufen d . Festigkeit d . Materialen, •&e . (See also:

• MUNICH (Ger. Munchen)

Munich, 1882).to 3o ft., and exerting a pull of 36o tons or a push of 48o tons by an hydraulic press at one end . The stress is taken at the other end by a See also:

• GROUP

group of four large vertical diaphragm presses, which communicate by small tubes with four similar small diaphragm presses in the scale case . The pressure of these acts on a system of levers which terminates in the scale beam . The See also:

• JOINTS

joints and See also:

• BEARINGS

bearings of all the levers are made frictionless by using flexible steel connecting-plates instead of knife-edges . The total multiplication at the end of the scale beam is 420,000.2 Stress-strain Diagrams.—The results of tests are very commonly exhibited by means of stress-strain diagrams, or diagrams showing the relation of strain to stress . A few typical diagrams for wrought iron and steel in tension are given in fig. to, the data for which are taken from tests of long rods by Kirkaldy a Up to the elastic limit these diagrams show sensibly the same See also:

• RATE

rate of extension for all the materials to which they refer . Soon after the limit of elasticity is passed, a point, which has been called by See also:

• SIR

Sir A . B . W .

See also:

• KENNEDY
• KENNEDY, BENJAMIN HALL (1804-1889)
• KENNEDY, THOMAS FRANCIS (1788-1879)
• KENNEDY, WALTER (c. 1460–c. 1508)

Kennedy the yield-point, is reached, Jv 3J 30 ~Id Beaseme Steel 28 A 20 n( For in' Pa t IS 15 pips 10 s 0 2 4 8 8 10 12 1a to 1R et Extension. per cent FIG . I0 . which is marked by a very sudden extension of the specimen . After this the extension becomes less rapid; then it continues at a fairly See also:

• REGULAR

regular and gradually increasing rate; neat the point of rupture the metal again begins to draw out rapidly . When this See also:

• STAGE (Fr. 6/age; from Lat. stare, to stand)

stage is reached rupture will occur through the flow of the metal, even if the load be somewhat decreased . The See also:

• DIAGRAM

diagram may in this way be made to come back towards the See also:

• LINE

line of no load, by with-See also:

• DRAWING

drawing a part of the load as the end of the test is approached . Fig . I1 is a stress-strain diagram for See also:

• CAST (from the verb meaning " to throw "; the word is Scand. in origin, cf. Dan. kaste, and Swed. kasta; " cast " in Middle Eng. took the place of the A.S. weor pan, cf. Ger. werf en)

cast iron in ex-tension and compression, taken from See also:

• EATON, DORMAN BRIDGMAN (1823-1899)
• EATON, MARGARET
• EATON, THEOPHILUS (c. 15go-1658)
• EATON, WILLIAM (1764-1811)
• EATON, WYATT (1849-1896)

Eaton See also:

• HODGKINSON, EATON (1759-1861)

Hodgkinson's experiments.4 The extension was measured on a See also:

• ROD (O.E. rodd, probably related to Norw. rudda, stick, rodda, stake)
• ROD, EDOUARD (1857-1910)

rod 50 ft. long; the compression was also measured on a long rod, which 2 See See also:

• REPORT (O.Fr. report or raport, modern rapport, from O.Fr. reporter, mod. rap porter, Lat. reportare, to bring back, in poetical use only, of bringing back an account, news, &c.)

Report of the U.S . See also:

• BOARD (O. Eng. bord)

Board appointed to test Iron, Steel and See also:

• ETHER, (C2H5)2O

ether Metals (2 vols., 1881) . For full details of the Emery machine, see Report of the U.S . See also:

• CHIEF (from Fr. chef, head, Lat. caput)

Chief of See also:

• ORDNANCE (a syncopated form of " ordinance " or " ordonnance," so spelt in this sense since the 17th century)

Ordnance (1883), app . 24 .

a Experiments on the Mechanical Properties of Steel by a See also:

• COMMITTEE (from commute, an Anglo-Fr. past participle of commettre, Lat. committere, to entrust; the modern Fr. equivalent comite is derived from the Eng.)

Committee of See also:

• CIVIL

Civil Engineers (See also:

• LONDON

London, 1868 and 1870) . Report of the Commissioners on the Application of Iron to Railway Structures (1849) . 'See also:

• SAN

San, 4 co: , reO O ••1 O m E,te -aioa . per eat 4 1 II O was prevented from buckling by being supported in a trough with partitions . The full line gives the strain produced by loading; it is continuous through the origin, showing that Young's modulus is the same for pull and push . (Similar experiments on wrought iron and steel in extension and compression have given the same result.) The broken line shows the set produced by each load . Hodgkinson found that some set could be detected after even the smallest loads had been applied . This is probably due to the existence of initial See also:

• INTERNAL

internal stress in the metal, produced by unequally rapid cooling in different portions of the cast bar . A second loading of the same piece showed a much closer approach to perfect elasticity . The elastic limit is, at the best, See also:

• ILL

ill defined; but by the time the ultimate load is reached the set has become a more considerable part of the whole strain . The pull curves in the diagram ex-tend to the point of rupture; the compression curves are drawn only up to a stage at which the bar buckled (between the partitions) so much as to affect the results . Autographic Recorders.—Testing machines are sometimes fitted with autographic appliances for drawing strain diagrams .

When the load is measured by a weight travelling on a steelyard, the diagram may be drawn by connecting the weight with a See also:

• DRUM (early forms drome or dromme, a word common to many Teut. languages, cf. Dan. tromme, Ger. Trommel: the word is ultimately the same as " trumpet," and is probably onomatopoeic in origin; it appears late in Eng. about the middle of the 16th century)

drum by means of a See also:

• WIRE (A.S. wir, a wire; cf. Swed. vire, to twist, M.H.G. wiere, a gold ornament, Lat. viriae, armlets, ultimately from the root wi, to twist, bind)

wire or See also:

• CORD (derived through the Fr. corde, from the Lat. chorda, Gr. xop(5rt, the string of a musical instrument)

cord, so that the drum is made to revolve through angles proportional to the travel of the weight . At the same time another wire, fastened to a clip near one end of the specimen, and passing over a pulley near the other end, draws a See also:

• PENCIL (Lat. penicillus, brush, literally little tail)

pencil through distances proportional to the strain, and so traces a diagram of stress and strain on a See also:

• SHEET

sheet of See also:

• PAPER
• PAPER (Fr. papier, from Lat. papyrus)

paper stretched See also:

• ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round the drum.' In Wicksteed's autographic See also:

• RECORDER
• RECORDER, FIPPLE FLUTE

recorder the stress is determined by reference, not to the load on the lever, but to the pressure in the hydraulic cylinder by which stress is applied . The See also:

• MAIN (from the Aryan root which appears in " may " and " might," and Lat. magnus, great)
• MAIN (Lat. Moenus)

main cylinder is in communication with a small auxiliary hydraulic cylinder, the plunger of which is kept rotating to avoid friction at its packing . This plunger abuts against a See also:

• SPRING (from " to spring," " to leap or jump up," " burst out," O. Eng., springau, a common Teut. word, cf. Ger. springer, possibly allied to Gr. QnrFpxecOac, to move rapidly)

spring, so that the distance through which it is pushed out varies with the pressure in the main cylinder . A drum covered with paper moves with the plunger under a fixed pencil, and is also caused to rotate by a wire from the specimen through distances proportional to the strain . The scale of loads is calibrated by occasional reference to the weighted lever ? In Kennedy's apparatus autographic diagrams are drawn by applying the stress to the test-piece through an elastic See also:

• MASTER (Lat. magister, related to tnagis, more, as the corresponding minister is to minus, less; the English form is due partly to the O. Eng. maegister, and partly to O. Fr. maistre, mod. maitre; cf. Du. meester, Ger. Meister, Ital. maestro)

master-bar of larger section . The master-bar is never strained beyond its elastic limit, and within that limit its extension furnishes an accurage measure of the stress; this gives motion to a pencil, which writes on a paper moved by the extension of the test-piece' In R . H . Thurston's pendulum machine for torsion tests, a See also:

• CAM (CAO), DIOGO (fl. 1480-1486)

cam attached to the pendulum moves a pencil through distances proportional to the stress, while a paper drum attached to the other end of the test-piece turns under the pencil through distances proportional to the angle of twist.' Strain beyond the Elastic Limit: See also:

• INFLUENCE (Late Lat. influentia, from influere, to flow in)

Influence of Time.—In testing a plastic material such as wrought-iron or mild steel it is found that the behaviour of the metal depends very materially on the time rate at which stress is applied . When once the elastic limit is passed the full strain corresponding to a given load is reached only after a perceptible time, sometimes even a long ' For descriptions of these and other types of autographic recorder, see a paper by See also:

• PROFESSOR (the Latin noun formed from the verb profiteri, to declare publicly, to acknowledge, profess)

Professor W . C .

Unwin, " On the Employment of Autographic Records in Testing Materials," Journ . See also:

• SOC

Soc . Arts (Feb., 1886) ; also Sir A . B . W . Kennedy's paper, " On the Use and Equipment of Engineering Laboratories," Proc . Inst . Civ . Eng . (1886), which contains much valuable See also:

• INFORMATION (from Lat. informare, to give shape or form to, to represent, describe)

information on the whole subject of testing and testing machines . On the general subject of tests see also Adolf See also:

• MARTENS
• MARTENS, GEORG FRIEDRICH VON (1756-1821)

Martens's Handbook of Testing Materials, trans. by G . C .

Henning . s Proc . Inst . Mech . Eng . (1886) . An interesting feature of this apparatus is a See also:

• DEVICE

device for preventing See also:

• ERROR (Lat. error, from errare, to wander, to err)

error in the diagram through motion of the test-piece as a whole . 'Prot . Inst . Mech . Eng . (1886); also Proc .

Inst . Civ . Eng. vol. lxxxviii. pl. i (1886) . • Thurston's Materials of Engineering, pt. ii . For accounts of See also:

• WORK

work done with this machine, see Trans . Amer . Soc . Civ . Eng . (from 1876) ; also, Report of the See also:

• AMERICAN

American Board, cited above.time . If the load be increased to a value exceeding the elastic limit, and then kept constant, the metal will be seen to draw out (if the stress be one of pull), at first rapidly and then more slowly . When the applied load is considerably less than the ultimate strength of the piece (as tested in the ordinary way by steady increment of load) it appears that this process of slow extension comes at last to an end .

On the other hand, when the applied load is nearly equal to the ultimate strength, the flow of the metal continues until rupture occurs . Then, as in the former case, extension goes on at first quickly, then slowly, but finally, instead of approaching an asymptotic limit, it quickens again as the piece approaches rupture . The same phenomena are observed in the bending of timber and other materials when in the form of beams . If, instead of being subjected to a constant load, a test-piece is set in a constant condition of strain, it is found that the stress required to maintain this constant strain gradually decreases . The See also:

• GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

gradual flow which goes on under constant stress—approaching a limit if the stress is moderate in amount, and continuing without limit if the stress is sufficiently great—will still go on at a diminished rate if the amount of stress be reduced . Thus, in the testing of soft iron or mild steel by a machine in which the stress is applied by hydraulic power, a stage is reached soon after the limit of elasticity is passed at which the metal begins to flow with great rapidity . The pumps often do not keep See also:

• PACE (through O. Fr. pas, from Lat. passus, step, properly the stretch of the leg in walking, from pandere, to stretch)
• PACE, RICHARD (c. 1482-1536)

pace with this, and the result is that, if the lever is to be kept floating, the weight on it must be run back . Under this reduced stress the flow continues, more slowly than be-t fore, until presently the pumps recover their lost ground and the increase of stress is resumed . Again, near the point of rupture, the flow again becomes specially rapid; the weight on the lever has again to be run back, and the specimen finally breaks under a diminished load . These features are well shown by fig . 12, which is copied from the autographic diagram of a test of mild steel . Hardening Effect of Permanent Set.—But it is not only through what we may call the viscosity of materials that the time rate of loading affects their behaviour under test .

In iron and steel, and probably in some other metals, time has another effect of a very remarkable kind . Let the test be carried to any point a (fig . 13) past the original limit of elasticity . Let the load then be removed; during the first stages of this removal the material continues to stretch slightly, as has been explained above . Let the load then be at once replaced and loading continued . It will then be found that there is a new yield-point b at or near the value of the load formerly reached . The full line be in fig . 13 shows the subsequent behaviour of the piece . But now let the experiment be repeated on another sample, with this difference, that an See also:

• INTERVAL

interval of time, of a few See also:

• HOURS, CANONICAL

hours or more, is allowed to elapse after the load is removed and before it is replaced . It will then be found that a process of hardening has been going on during this interval of rest; for when the loading is continued the new yield-point appears, not at b as formerly, but at a higher load d . Other See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

evidence that a change has taken place is afforded by the fact that the ultimate extension is reduced and the ultimate strength is increased (e, fig . 13) .

A similar and even more marked hardening occurs when a load (exceeding the original elastic limit), instead of being removed and replaced, is kept on for a sufficient length of time without change . When loading is resumed a new yield-point Extension is found only after a considerable addition has been made to the load . The result is, as in the former case, to give greater ultimate strength and less ultimate elongation . Fig . 14 exhibits two experiments of this kind, made with annealed iron wire . A 30 s to 15 0 5 10 15 Extension, per cent Extension.per cent load of 231 tons per square inch was reached in both cases; ab shows the result of continuing to load after an interval of five minutes, and acd after an interval of 451 hours, the stress of 231 tons being maintained during the interval in both cases.' It may be concluded that, when a piece of metal has in any way been overstrained by stress exceeding its limits of elasticity, it is hardened, and (in some cases at least) its See also:

• PHYSICAL

physical properties go on slowly changing for days or even months . Instances of the hardening effect of permanent set occur when plates or bars are rolled cold, hammered cold, or bent cold, or when wire is drawn . When a hole is punched in a plate the material contiguous to the hole is severely distorted by shear, and is so much hardened in consequence that when a See also:

• STRIP

strip containing the punched hole is broken by tensile stress the hardened portion, being unable to extend so much as the rest, receives an undue proportion of the stress, and the strip breaks with a smaller load than it would haveborne had the stress been uniformly distributed . This See also:

• BAD, BCD

bad effect of punching is especially noticeable in thick plates of mild steel . It disappears when a narrow ring of material surrounding the hole is removed by means of a rimer, so that the material that is left is homogeneous . Another remarkable instance of the same kind of action is seen when a mild-steel plate which is to be tested by bending has a piece cut from its edge by a shearing machine . The result of the shear is that the metal See also:

• CLOSE
• CLOSE (from Lat. clausum, shut)
• CLOSE, MAXWELL HENRY (1822-1903)

close to the edge is hardened, and, when the plate is bent, this part, being unable to stretch like the rest, starts a crack or See also:

• TEAR

tear which quickly spreads across the plate on account of the fact that in the metal at the end of the crack there is an enormously high See also:

• LOCAL

local intensity of stress .

By the simple expedient of planing off the hardened edge before bending the plate homogeneity is restored, and the plate will then See also:

• BEND

bend without damage . .4nnealing.—The hardening effect of overstrain is removed by the process of See also:

• ANNEALING, HARDENING AND

annealing, that is, by heating to redness and cooling slowly . In the ordinary process of rolling plates or bars of iron or mild steel the metal leaves the rolls at so high a temperature that it is virtually annealed, or See also:

• PRETTY

pretty nearly so . The case is different with plates and bars that are rolled cold: they, like wire supplied in the hard-drawn state (that is, without being annealed after it leaves the draw-plate), exhibit the higher strength and greatly reduced plasticity which result from permanent set . Extensometers.—Much See also:

• ATTENTION (from Lat. ad-tendo, await, expect; the condition of being " stretched " or " tense ")

attention has been paid to the design of extensometers, or apparatus for observing the small deformation which a test-piece in tension or compression undergoes before its limit of elasticity is reached . Such observations afford the most direct means of measuring the modulus of longitudinal elasticity of the material, and they serve also to determine the limits within which the material is elastic . In such a material ' J . A . Ewing, Proc . See also:

• ROY, WILLIAM (1726-1790)

Roy . Soc . (See also:

• JUNE

June, 188o) .

Oas wrought iron the elastic extension is only about Tyi-u- of the length for each ton per square inch of load, and the whole amount up to the elastic limit is perhaps ro-'o -o of the length; with a length of 8 in., which is usual in tensile tests, it is desirable to read the extension to, say, -i -oa-g in. if the modulus of elasticity is to be found with See also:

• FAIR

fair accuracy, or if the limits of proportionality between strain to stress are under examination . Measurements taken between marks on one side of the bar only are liable to be affected by bending of the piece, and it is essential either to make See also:

• INDEPENDENT

independent measurements on both sides or to measure the displacement between two pieces which are attached to the bar in such a manner as to See also:

• SHARE (O. Eng. scearu, chiefly in compounds, e.g. land-scearu, a share of land, from sceran to cut; cf. " shear" )

share equally the strain on both sides . In experiments carried out by Bauschinger, independent measurements of the strains on both sides of the bar were made by using See also:

• MIRROR (through O. Fr. mirour, mod. miroir, from a sup-posed Late Lat. miratorium, from mirari, to admire)

mirror micrometers of the type illustrated diagrammatically in fig . 15 . Two clips a and b clasp the test-piece at the place between which the extension is to be measured . The clip b carries two small rollers d, d2 which are See also:

• FREE

free to rotate on centres fixed in the clip . These rollers press on two plane strips c, c2 attached to the other clip . When the specimen is stretched the rollers consequently turn through angles proportional to the strain, and the amount of turning is read by means of small mirrors g1 and g2, fixed to the rollers, which reflect the division of a fixed scale f into the See also:

• READING

reading telescopes el e2 . In Martens's extensometer each of the rollers is replaced by a rhombic piece of steel with See also:

• SHARP, GRANVILLE (1735-1813)
• SHARP, JAMES (1618-1679)
• SHARP, JOHN (1645-1714)
• SHARP, RICHARD (1759-1835)
• SHARP, WILLIAM (1749-1824)
• SHARP, WILLIAM (1856-1905)

sharp edges, one of which bears against the test-piece, while the other rests in a groove formed