Note. . The specimens tested were stool liars of different grades made from pure Swedish iron, and each bar was turned to a diameter of one inch for a length of 14 inches.

SPECIMENS.

Per rent, of carbon.

Breaking weight.

Elongation.

Resilience.

No. 1...........

0 33

68,100

0.098

4.450

No. 2...........

0.43

76.160

0.089

4,970

No. 3

0.48

84,000

0.089

5,040

No. 4...........

0.53

95,200

0.080

5,080

No, 5

0.53

92.960

0.058

8,600

No. 6...........

0.63

100,800

0.071

4,770

No. 7...........

0.74

101,920

0.050

3,400

No. 8...........

0 84

123,200

0.080

6.580

No. 9...........

[.00

134.400

0.071

6.360

No. 10...........

1.25

154,560

0.044

4,530

In the larger table, the ultimate resilience of metals is given as tested in the Stevens institute of technology, Hoboken, N. J. Phosphor bronze considerably exceeds ordinary bronze in ductility and resilience. 19. Heating wrought iron within certain limits, and then cooling under stress, increases its strength by relieving internal strain. Cold rolling and wire.drawing increase it, in some cases, 100 per cent. Mr. Dean of Boston and Uchatius of Vienna have similarly increased the strength and elasticity of bronze. Overheating, annealing, and cold hammering decrease its strength. Cast iron of open structure and low density is increased in strength by successive remelt. ings, sometimes to the amount of 100 per cent., over pig metal. Casting under a head, or under considerable pressure, similarly benefits both cast iron and cast steel. Sir Joseph Whitworth produced a steel of extraordinary strength and toughness by casting under heavy pressure. The internal strain consequent upon sudden cooling, or upon cooling awkwardly shaped castings, seriously reduces their strength and sometimes produces actual fracture. The character of cast iron is largely determined by its density, 7.2 to 7.3 representing the best limits for ordinary practice.

Cold wrought iron is more than twice as strong as red.hot. Strength, ductility, and resilience increase with diminishing temperatures, when the materials are of good quality. Cold.blast cast iron is usually stronger than hot-blast iron made from the same ores. Copper loses 25 per cent, .of its tenacity at 550° F., 50 per cent, at 810°, and G7 per cent, at 1,000°, the diminution of tenacity varying nearly as the square root of the third power of the temperature. Metals in large masses have usually less density and strength than when worked into sheets, bars, or wire. Wrought iron is particularly liable to loss of strength in large forgings. Bars two inches in diameter being made of the same metal as other bars one inch in diameter, the latter are sometimes found to have 20 per cent, more strength. Steel exhibits even greater differences. 20. Indentation is resisted by wrought iron nearly in proportion to its thickness. Fairbairn found the force necessary to push a blunt point or a ball 3 in. in diameter through boiler plate, one quarter of an inch thick, to be 17,000 1bs., and nearly equal to that required to drive the same instrument through a three-inch oak plank.

Resistance of armor plate to penetration by shot varies, if the plate be well backed, as the square of thickness, within the limit of moderate thickness. The material should be strong and ductile. 21. Generally, in designing machines or parts of machines, they should be so proportioned that all parts will havo factors of safety of nearly equal value. Economy of material is thus secured, and also the very important advantage, where exposed to severe shock or sudden strains, of utilizing the resilience of the whole machine in resisting them. Forms of uniform strength should therefore he used wherever possible. Suspension rods of uniform strength must have a greater section at the point of support than at the point of attachment of the load, as the upper portions carry not only the load but the weight of the lower part of the rod. Pump rods and wire ropes for deep mines are for this reason made tapering, with the largest section at the top. Care should always be taken that the pieces connected and their fastenings are, when possible, equally strong.

Tall columns are slightly swollen at the middle portion in order that they may be equally liable to break at all points, and the Hodgkinson form of cast-iron beams, and the Fairbairn (I) form of section of wrought. iron beams, are given their peculiar shapes in order that no surplus material may exist in either top or bottom flange. Beams of uniform strength, when fixed at one end and loaded at the other, if of uniform depth, are triangular in plan. If uniformly loaded, they represent in plan a pair of parabolas whose vertices touch at the outer end. When of uniform breadth, their vertical sections are parabolic in the first case, and triangular in the second. Beams of uniform depth, supported at the ends and loaded at the middle point, are in plan a pair of triangles with a common base at the load. If uniformly loaded, the plan is a pair of parabolas with their bases at the middle of the beam. When supported at the ends and uniform in breadth, they are in vertical section a pair of parabolas, in the first case with vertices at the ends and bases meeting at the load, and in the last case semi-ellipses extending between the points of support.

In building bridge girders, economy of material is secured by the use of isosceles bracing set at angles of 45°. In vertical and diagonal bracing, the proper angle for diagonals is 55° measured between the diagonal and the vertical. The amount of resistance of a cylinder to rupture by torsion is nearly double that to breaking across. Bolts exposed to shocks and sudden strains, as when used as armor-plate fastenings, are found to resist much more effectually where resilience is secured by turning down the shank to the diameter of the bolt at the bottom of the thread, or otherwise creating a uniform area of section between head and nut. Punching rivet holes weakens plates of hard iron and steel. The latter are injured so seriously that steel plates are never punched by careful engineers. (See Steel.) In hard iron the reduction of strength is often considerable (15 per cent, as shown by some experimenters); and in many cases, in boiler work, for this reason, the rivet holes are all drilled, notwithstanding the increased cost. Where the iron is very soft and ductile, punching produces less injury. 22. Elasticity is that quality by the possession of which the strain, or distortion of form, produced in any body by stress, is wholly or partially removed on the removal of the stress.

All bodies have more or less elasticity, and, when perfectly homogeneous and free from internal strain, are perfectly elastic within a certain limit, which is called the limit of elasticity. Within this limit, the displacement produced by any force is directly proportional to that force. Beyond the limit of elasticity, the strain produced by stress is not wholly removed on the cessation of the stress. The permanent change of form so produced is called the "set." This set takes place on the application of the slightest force where the material is not uniform in character and free from internal strain. Hodgkinson found that in iron, far within the elastic limit, the lightest loads produced slight set. Beyond the elastic limit the set becomes nearly proportional to the distortion, the resistance also increasing up to the point at which rupture begins, but in a far higher ratio. Repeatedly straining a piece beyond its elastic limit produces " fatigue " and ultimate fracture. This may occur by the application of force far less than that producing immediate rupture. 23. The modulus of elasticity, sometimes called the coefficient of elasticity, is the quotient obtained by dividing the measure of the force producing distortion by the measure of the distortion produced by it.

Its value varies with every material. The ordinary values of the modulus are given in the table. Those values, as is proved by autographic strain diagrams, are liable to variation, within very wide limits, by every circumstance which affects the physical character of the materials. It has no fixed relation to the ultimate strength. It will be seen that this quantity may be defined as the measure of that force which, supposing no limit to elasticity, would shorten or lengthen a bar, originally a unit in length, to the extent of one unit. Thus, a bar of ordinary forged iron, one foot long, would be altered in length 1/10,000 by a force equal to 25,000.000/10,000 = 2500 lbs. per square inch of section. 24. Testing Machines. The strength of materials is determined by means of testing machines. 25. Fig. 1 represents a machine for determining longitudinal resistance, as built by the Messrs. Riehle of Philadelphia. It consists of a weigh-beam, accurately made and nicely poised upon knife edges. At its outer end it sustains a scale pan upon which weights measuring 2,000 or 4,000 lbs. are placed. Intermediate weights are measured by a poise, not shown in the figure, which traverses the beam, the latter being divided into parts of 10 lbs. each, similarly to the steelyard balance.

The specimen is secured at the upper end by wedges or clamps, in a strong collar which is hung from two knife edges, one on each side the knife edge carrying the scale beam. These knife edges are placed at slightly different distances from the knife edge supporting the beam, thus making the latter a " differential lever," and permitting the measurement of a very great force without compelling the use either of large weights or of a series of levers. A similar collar below takes the lower end of the specimen to be tested. This second collar is secured to the head of a hydraulic press which is placed within the lower part of the frame of the machine. A small pump, worked by a hand lever, is used to force oil into the press. The breaking force is thus applied from below, and is measured upon the lever above. 26. With the autographic recording testing machine of Prof. E. II. Thurston, fig. 2, nearly all of the essential qualities as well as the strength of materials are determined by the automatic production of a strain diagram. This diagram is an exact graphical representation of all circumstances attending the distortion and fracture of the specimen. No system of personal observation yields results as trustworthy or with such precision as an autographic registry.

No other method gives simultaneously, and at every instant during the test, the intensity of the distorting force and the magnitude of the coincident distortion. In this machine two strong wrenches are carried by the A frames, and depend from axes which are both in the same line, but which are not connected with each other. The arm of one of these wrenches carries a weight at its lower end. The other arm is designed to be moved by hand in the smaller machines and by a worm gear in larger ones. The heads of the wrenches are fitted to take the head on the end of the test pieces, which are usually given the form shown in tig. 3. A guide curve of such form that its ordinates are precisely proportional to the torsional moments exerted by the weighted arm while moving up an arc to which the corresponding abscissas of the curve are proportional, is secured to the frame next the weighted arm. The pencil holder is carried on this arm, and as the latter is forced out of the vertical position, the pencil is pushed forward by the guide curve, its movement being thus made proportional to the force which, transmitted through the test piece, produces deflection of the weighted arm. The guide curve.is a curve of sines.

The other arm carries the cylinder upon which the paper receiving the record is clamped, and the pencil makes its mark on the table thus provided. This table having a motion, relatively to the pencil, which is precisely the angular relative motion of the two extremities of the test piece, the curve described upon the paper is always of such form that the abscissa of any point measures the amount of the distortion which the force produces. 27. The vertical scale of the diagrams produced is a scale of torsional moments, and the horizontal scale is one of total angles of torsion. Since the resistance to shearing, in a homogeneous material, varies with the resistance to longitudinal stress, the vertical scale is also for such materials a scale of direct resistance; and with approximately homogeneous substances this scale is approximately accurate, where, as here, all specimens compared are of the same dimensions. 28. By tig. 4 it will be seen that the first portion of the line rises at a slight inclination from the vertical, and very nearly straight. The amount of distortion here is seen to be approximately proportional to the distorting force, illustrating Hooke's law, Ut tensio sic vis.

After a degree of distortion which is determined by the specific character of each piece, the line becomes curved, the change of form having a rate of increase which varies more rapidly than the applied force. When this change begins, the molecules, which up to that point retain generally their original distribution, while varying their relative distances, begin to change their positions with respect to each other, moving upon each other in a manner similar to that action described by II. Tresca, and called the "How of solids." This point, at which the line begins to become concave toward the base, is considered as marking the torsional limit of elasticity. It is well defined in experiments upon woods; is less marked, but still well defined, in the fibrous iron3 and the less homogeneous specimens of other metals; and becomes quite indeterminable with the most homogeneous materials, as with the best qualities of well worked cast steel. This point does not indicate the first set, since a set occurs with every degree of distortion, however small. It is at this elastic limit that the sets begin to become proportional to the degree of distortion. The inclination of the straight portion of the line from the vertical measures the stiffness of the specimen.

This rigidity is very closely, if not precisely, proportional to the hardness, in homogeneous substances; and this quantity is taken, for practical purposes, as a measure of the hardness of the metals and of their elastic resistance to compression. After passing the elastic limit, the line becomes more and more nearly parallel to the base line, and then, with the woods invariably, and in some cases with the metals, begins to fall before fracture becomes evident in the specimen. With the more ductile substances, nearly all the particles are brought up to a maximum in resistance before fracture occurs, and this circumstance has an important influence in determining the resistance to rupture. The hardest and most brittle materials break with a snap before any flow is perceptible, before the line of the diagram begins to deviate from the direction taken at the commencement, and before the approach to the elastic limit is indicated. The elasticity of the material is determined by relaxing the distorting force, and allowing the specimen to relieve itself from distortion so far as its elasticity will permit.

In such cases, the pencil traces a line e, O, resembling, in its general form and position in respect to the coordinates, that forming the initial portion of the diagram, but almost absolutely straight, and more nearly vertical. The degree of inclination of this line indicates the elasticity, precisely as the initial straight line gives a measure of the original stiffness of the test piece. The homogeneity of the material tested is hardly less important than its strength. The degree of depression of the line immediately after passing the elastic limit exhibits the greater or lesshomogeneousness of the material. The resilience of the specimen is measured by the area included within the curve, this being the product of the mean force exerted into the distance through which it acts in producing rupture; i. e., it is proportional to the work done by the test piece in resisting fracture, and, taken up to the ordinate of the limit of elasticity, measures the capacity for resisting shock without serious distortion or injurious set.

The ductility of the specimen is deduced from the value of the total angle of torsion, and its measure is the elongation of a line of surface particles, originally parallel to the axis, which line assumes a helical form as the test piece yields, and finally parts at or near the point where the maximum resistance is observed. 29. The strain diagrams exhibit the characteristic differences of various materials. The-woods have a structure which differs in a distinguishing degree, both in the distribution of the substance and in the action of those molecular forces capable of resisting rupture, from that of the metals, the latter being far more homogeneous than the former. Wood consists of an aggregation of strong fibres, lying parallel, or approximately so, and held together often by a comparatively feeble force of lateral cohesion. The metals, on the other hand, are naturally homogeneous, both in structure and in the distribution and intensity of the molecular forces. Well worked and thoroughly annealed cast steel, as an example, is equally strong in all directions, is perfectly uniform in its structural character, and is almost absolutely homogeneous as to strain.

Wrought iron, as usually made, has a somewhat fibrous structure, which is produced by particles of cinder originally left in the mass by the imperfect work of the puddler while forming the ball of sponge in his furnace, which, not having been removed by the squeezers or by hammering the puddle ball, are, by the process of rolling, drawn out into long lines of non-cohering matter, and produce an effect upon the mass of metal which makes its behavior under stress somewhat similar to that of the stronger and more thready kinds of wood. In the low steels also, in which, in consequence of the deficiency of manganese accompanying almost of necessity their low proportion of carbon, this fibrous structure is produced by cells and bubble holes in the ingot, refusing to weld up in working, and drawing out into long microscopic, or less than microscopic, capillary openings. In consequence of this structure, a depression indicating this heterogeneousness of structure interrupts the regularity of their curves, immediately after passing the limit of elasticity. 30. The presence of internal strain constitutes an essential peculiarity of the metals which distinguishes them from organic materials.

The latter are built up by the action of molecular forces, and their particles assume naturally and invariably positions of equilibrium as to strain. The same is true of all naturally formed organic substances. The metals, however, are given form by external and artificially produced forces. Their molecules are compelled to assume certain relative positions, and these positions may be those of equilibrium, or they may be such as to strain the cohesive forces to their very limit. This peculiar condition is of serious importance where the metal is brittle, as is illustrated by the behavior of cast iron, and particularly in ordnance. Even in ductile metals, it produces a reduction in the power of the material to resist external forces. This condition of internal strain may be relieved by annealing hammered and rolled metals, and by cooling castings very slowly, so that the particles may naturally assume positions of equilibrium. In tough and ductile metals, internal strain may be removed by heating to a high temperature and then cooling under the action of a force approximately equal to the elastic resistance of the substance.

This process, called "thermo-tension," was first used by Prof. W. R. Johnson in 1836. The cause of this, which he terms an anomalous condition of the metal, was not then discovered. Ductile metals may be strengthened in a considerable degree by this relief of internal strain, and also by simply straining them while cold to the elastic limit, and thus dragging all their particles into extreme positions of tension, from which when released from strain they may all spring back into their natural and unstrained positions of equilibrium. This fact was noted by Prof. Thurston, and soon after independently by Commander Beardslee, U. S. N. It has an important bearing upon the resisting power of materials, and upon the character of all formulas in which it may be attempted to embody accurately the law of resistance of such materials to distorting or breaking strain. The initial portion of the diagram, when the material is free from internal strain, is a straight line up to the limit of elasticity. This line, with strained materials, becomes convex toward the base line. The initial portion of the diagram, therefore, determines whether the material tested has been subjected to internal strain, or whether it is homogeneous as to strain.

This is exhibited by the direction of this part of the line, as well as by its form. The existence of internal strain causes a loss of stiffness, which is shown by the deviation of this part of the line from the vertical to a degree which becomes observable by comparing its inclination with that of the line of elastic resistance. 31. In fig. 4, the strain diagram A is that of zinc. The concave form at the commencement indicates its inelastic nature, its slight altitude shows its weakness, and, breaking at 65°, it is shown to lack ductility. Tin, T, is vastly more ductile, but is still less tenacious. B and C are the diagrams given by cast and forged copper, the latter twisting 500°, and its fibres stretching to three times their original length. Cast copper is comparatively weak and brittle. Wrought iron gives the strain diagram D. It indicates the elasticity of the metal, its ductility, and its strength. The elastic limit is plainly indicated. The concavity of the initial portion of the line indicates some internal strain, and the horizontal portion immediately above the elastic limit shows that the metal was "seamy" and not perfectly homogeneous.

The lines e and O are " elasticity lines." They differ slightly in direction from the initial portion of the diagram, confirming the previously indicated presence of internal strain. E is the terminal portion of the diagram of a soft ductile iron. F is that given by a very strong and ductile and exceptionally homogeneous iron, a very smooth and symmetrical curve. G is a soft Bessemer steel. II is somewhat harder, the one containing 0.4 and the other 0.5 per cent, of carbon. I and J are tool steels containing 1 per cent, of carbon. K is medium, L spring, and M double shear steel. N and P are obtained from white and gray cast iron. One is stiff, hard, and brittle, the other weaker, soft, and comparatively tough. O is a malleableized cast iron made from N; it has lost no strength, and has gained considerable ductility. Strain diagrams may be produced by plotting data obtained by observation in the usual manner and similarly interpreted. 32. An examination of the fracture in each case assists in determining the character of the material, and in interpreting the strain diagram. The following figures exhibit the characteristics of various qualities of iron and steel.

Fig. 5 resembles that which gave the diagram marked D. The metal is good and tough, but seamy, and not thoroughly worked, as is shown by the cracks extending around the neck and by the irregularly distributed flaws on its end. Fig. 6 exhibits the appearance of the sample F. The surface of the neck was originally smoothly turned, polished, and fitted to gauge. Under test it became curiously altered and assumed a rough, striated appearance. The end has the peculiar appearance characteristic of tough and ductile metals, and the uniformly bright appearance of the fractured section shows that all held together up to the instant of rupture, and that fracture finally took place by shearing. Fig. 7 represents the appearance of low steels. The peculiarities of the finest tool steels are exhibited in fig. 8. In this the fracture is ragged and splintery, and the separated surfaces have a beautifully fine, even grain, which proves the excellence of the material. The surface, which was turned and polished in bringing the metal to size, remains as perfect as before the specimen was broken.

By an inspection of the broken test pieces in this manner, the grade of the steel, and by the practised eye the slightest strength of materials.

Richie's Longitudinal Testing Machine.

Fig. 1. - Richie's Longitudinal Testing Machine.

Thurston's Testing Machine.

Fig. 2. - Thurston's Testing Machine.

Test Piece.

Fig. 3. - Test Piece.

Strain Diagrams.

Fig. 4. - Strain Diagrams.

Before Breaking Ultimate Tensile Strength In Pound 1500270

Fig. 5.

Before Breaking Ultimate Tensile Strength In Pound 1500271

Fig. 6.

Before Breaking Ultimate Tensile Strength In Pound 1500272

Fig. 7.

TABLE OP COERFICIENTS OF RESISTANCE.

MATERIAL.

W.

Weight per cubic foot.

T.

Tension.

c.

Crushing.

K.

Transverse.

S. Torsion.

E.

| Coefficient of elasticity.

R.

Resilience.

!

METALS.

Antimony ..............

280

1.000

...

15

....

...

Bismuth ................

613

3,000

...

50

...

....

Brass: ............

Copper 10, zinc 1 .....................

585

22,000

50,000

...

200

8.000.000

6.000

6, " 1.........................

525

30.000

160,000

240

500

9,000,000

8,000

" 3, " 1.........................

525

28.000

...

8 000 000

4 000

Fine drawn ...................

535

80,000

165,000

...

1,000

14,000 000

15 000

Bronze:

Aluminum 10, copper 90..................

480

70.000

135,000

...

400

10,000.000

Copper 10, tin 1 ......................

535

36,000

...

...

500

11,000,000

8.000

8, " 1..........................

528

40.000

...

....

700

12,000,000

6.000

6, " 1..........................

540

40,000

...

...

...

...

2,000

Copper:

...

...

...

Cast....................................

540

20,000

...

...

350

3.000,000

200

Rolled...................................

550

30,000

100.000

...

400

...

Drawn ...............

555

60,000

100.000

...

750

...

Forged.............................. ...

552

40,000

100.000

220

600

3.750.000

40,340

Gold wire ......................

1,210

20,000

35,000

...

...

...

...

Iron:

Cast, pig ...................

440

20,000

100.000

500

400

13.000.000

15

" hard...............................

450

30,000

125,000

700

600

22,000,000

5

" tough .....................

450

25.000

120,000

600

500

15.000.000

25

" gun iron............................

455

30,000

125.000

700

700

25,000.000

50

Wrought bar ......................

486

60,000

50,000

900

750

22,000,000

20.000

'• sheet.....................

480

50,000

60,000

700

650

25.000.000

15.000

" tank...........................

480

45.000

65,000

500

600

25,000.000

10,000

" wire 1/8 inch ......................

485

80,000

60.000

900

1.000

28,000.000

40,000

" large forging.................

475

40,000

40,000

500

500

20.000,000

10.000

Lead, cast.................................

710

1,800

7,000

20

20

1,000,000

...

" rolled...............................

712

2,500

...

30

30

...

...

Platinum..................................

1,340

55.000

...

700

...

...

...

Silver.....................................

654

40,000

...

500

...

...

...

Steel:

...

...

Carbon 0.0038 .............................

488

65.000

80.000

800

900

25.000.000

35,000

" 0.0050............................

487

90.000

125.000

1.500

1.260

27.000.000

15.000

" 0.0075............................

486

100.000

150,000

2,000

1.350

29.090.000

10.000

" 0.0100............................

485

140.000

225.000

3,000

1.800

30,000.000

l0.000

" 0.0125............................

485

160.000

250.000

5,000

2.000

31 .000,000

5,000

Hardened in oil.........................

200.000

350.000

7,000

3 000

38.000,000

...

Rails...............................

488

70.000

100.000

900

900

...

30,000

Plate....................................

487

80.000

120,000

1.200

1.100

...

...

Blister..................................

488

100.000

150,000

2,000

1.500

...

...

Shear.......................,...........

486

120,000

180,000

2.500

2 000

...

5.000

Tin, block ....................................

455

4.000

15.590

50

60

4,500.000

2,500

" wire..................................

460

7.000

...

80

90

...

...

Zinc, cast ............................

437

2.500

...

30

30

13.000,000

500

" rolled................................

440

15,000

...

200

200

....

...

MINERALS.

Brick , red ..........................

130

150

1.000

5

...

...

" ..

135

300

2,000

10

14

...

...

Cement, 1 week............................

120

100

...

20

...

...

...

" 1 year ............................

120

400

2,000

5

69

....

...

Chalk....................................

117

334

...

...

...

....

Glass, plate,...............................

153

9,420

...

...

...

....

....

Granite___...............................

165

1,000

10.000

25

980

...

....

Limestone.................................

165

500

6,000

40

100

25.000.000

...

Marble ..................

165

9,000

40

...

25,000,000

...

Sandstone .................................

150

200

5,000

15

300

...

...

Mortar .....................

107

50

180

...

...

...

....

TIMBER.

....

Acacia ........................

47

16,000

...

140

...

1.152.000

...

Apple tree .......................

50

19,000

...

...

...

...

...

Ash........................................

45

16.000

9,000

150

120

1,500.000

...

Beech ........................

50

16,000

8.000

120

110

l,400.000

...

Birch ..........................

50

15,000

5.000

130

.....

1,500,000

....

. Box..........................

60

18,000

10.000

130

125

...

Cedar .............................

55

11.500

6.000

100

100

...

...

Elm .....................

37

13,000

10.000

75

...

700,000

...

Fir, N. E.........................

35

12,000

6,000

80

75

2,000,000

...

Larch .....................

35

9,000

10,000

100

80

1,000,000

...

Lancewood .. ,.....................

60

23.000

...

150

120

...

...

Lignumvitae .......................

75

12.000

10.000

160

150

...

...

Locust........................

60

20.000

...

250

220

...

...

Mahogany..................

50

16.000

8,000

120

180

...

...

Maple.......................

50

10.000

...

...

...

...

...

Oak..................

55

17.000

10.000

150

140

1.500.000

...

Pine .................

40

10,000

8.000

100

65

1,750,000

...

Spruce ...........

30

17.000

6.000

120

...

1,600.000

...

Teak................... ,

45

15.000

12,000

180

150

2,400,000

...

Walnut, white..............................

42

8.000

7.000

100

200

...

...

black.............................

40

8.000

8,000

150

130

...

...

Possible variations, are readily distinguished. Fig. 9 is white cast iron. Its surface, where fractured, has the general appearance of broken tool steel, but the color and texture of the metal are distinctive; it has none of the "steely grain." Fig. 10 represents dark foundery iron. Its color, its granular structure, and coarse grain are markedly characteristic. 33. Good iron plates should, in addition to the above tests, be subjected to the following: When red-hot, they should be capable of being bent sharply to a right angle without cracking, up to an inch in thickness. Ordinary boiler plate of good quality should bend double. When cold, they should bend along the grain without cracking, as follows : 1 in. thick, 15°; ¾ in. thick, 25°; ½ in. thick, 35°; ½ in. thick, 90°. Across the grain they should at least bend half as far. They should be bent on a slab rounded on the corners with a radius of ½ in. Steel plates should be 50 per cent, more ductile. 34. Testing within the Limit of Elasticity. In determining the value of materials of construction, it is usually more necessary to determine the position of the limit of elasticity, and the behavior of the metal within that limit, than to ascertain ultimate strength or resilience.

It should be possible to test every piece of material which goes into an important structure, and then to use it with confidence that it has been proved capable of carrying its load with a sufficient and known margin of safety. It is common to test bridge rods to a limit of strain determined by specification, and to compel their rejection when they are found to take a considerable permanent set under that strain. Specification now frequently (and it should invariably) makes the limit of elasticity the basis of calculation and test. - See Fairbairn, "Cast and Wrought Iron" (London, 1865); Haswell, "Engineers' and Mechanics' Pocket Book" (New York, 1868); Trautwine, "Civil Engineers' Pocket Book" (Philadelphia, 1872); Rankine, "Useful Rules and Tables" (London, 1872); Thurston, "Strength, Elasticity, Ductility, and Resilience of Materials of Construction" (Philadelphia, 1874); and Wood, "Resistance of Materials" (New York, 1875).

Before Breaking Ultimate Tensile Strength In Pound 1500273

Fig. 8.

Before Breaking Ultimate Tensile Strength In Pound 1500274

Fig.9.

Before Breaking Ultimate Tensile Strength In Pound 1500275

Fig. 10.