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13,880 15,142

15,680

•00056

28,000,000

16,005

:::::

16,404
17,666

17,920

8

.00064

28,000,000

[blocks in formation]

•000725

27,806,900

[blocks in formation]

. 00081

27,654,000

22,713
23,975

.000495830 *000540000 000558628 -000571167 .000581660 000627500 000638774 000666083 000680830 *000724527 000728330 000766083 .000775830 000811457 .000826660 . 000880830 . 000873750 . 000908845 .000937500 001033300 ·001013580 001039175 ·001073330 ·001208300 001244605 .001288170 .001659200 . 002013840 ·002228670

27,993,000 28,041,000 28,069,000 28,022,000 28, 220,000 28,153,000 28,054,000 28,034,000 27,801,000 27,825,000 27,721,000 27,856,000 27,649,000 27,606,000 27,475,000 27,186,000 27,477,000 27,111,000 26,919,000 25,645,000 26,318,000 25,867,000 25,864,000 24,011,000 23,397,000 22,779,000 18,252,000 15,572,000 14,363,000

24,008

24,640

• 00091

::::::::::::::

27,077,000

25,237
26,499

26,675

26,880

[blocks in formation]

27,761
29,022

29,120

•00124

23,484,000

29,343

..

30,284

31,360

·00200

15,680,000

[blocks in formation]

(7)

(8)

[graphic]

(624.) The general conclusion from these experiments is, that for permanent structures the maximum working load on rolled bars of wrought iron should not exceed 8 tons per square inch, which is nearly one-third of the mean ultimate cohesive strength of British iron, namely, 25. 7 tons per square inch, as shown by Table 1. Within the limit of 8 or 9 tons, the extensions are practically simply proportional to the strains, and may be taken approximately at .00008 of the length per ton. With heavier strains, the extensions increase more rapidly than the load, and with all such strains are still further extended by time to an extent varying considerably with the ductility of the particular specimen of iron.

(625.) Another important practical lesson may be drawn from this investigation, namely, that where the length is fixed, whatever initial strain may be put on wrought iron, the permanent strain after a certain time will not exceed 8 or 9 tons per square inch. For instance, say that a wrought-iron hoop or wheel tire is shrunk hot on a cast-iron wheel, &c., as in Fig. 25 and (90), so that when cold it shrinks to such an extent as to yield at first say 20 tons per square inch; but that strain would not be permanent; the bar would immediately begin to stretch, and by so doing relieve the pressure upon it, at first very rapidly, afterwards, as the strain was reduced, more and more slowly, until after a long period it became 8 or 9 tons only, although the extent to which it was originally stretched was that due to 20 tons, and that strain was really obtained for a few seconds (90)

(626.) “ Compression of Wrought Iron.”—Experiments were made by Mr. Hodgkinson on two bars of wrought iron, each about 1 inch square and 10 feet long:—they were enclosed in a frame to prevent lateral flexure, in the same way as with the cast-iron bars in (607). The results are given in Table 97, but the experimental strains on the bars (which were rather more than one inch square) are reduced to equivalent strains per square inch, and the observed compressions are reduced to parts of the length of the bar.

The compressions for even tons were obtained by interpolating between the greater and lesser experimental numbers. The mean compression of the two bars is given by Table 98, and in cols. 4,5, reduced results are given, which may be taken as sufficiently correct for ordinary purposes.

(627.) “Effect of Time.”—The effect of time in increasing the amount of compression was observed with one of the bars only, and that not before a strain of 30,858 lbs., or 13.77 tons, was applied: the influence of time was then found to be very great, the compression being increased .002175 = .00158333

1.37, or 37 per cent. in } an hour; 48 per cent. in i hour; and 73 per cent. in 11 hour. In all probability, time would have been found influential with compressive strains of 8 or 10 tons per inch if observations had been made, and we may infer that for permanent structures the maximum working compressive strain should not exceed 8 tons per square inch, being the same as we found for the Tensile strain (625).

(628 Comparative Extension and Compression of Wrought Iron."—We found (616) that with small strains, when defect of elasticity was uninfluential, cast iron yielded more to compressive than to equivalent tensile strains. A similar comparison of the elasticity of wrought iron leads yet more clearly to the same result, as shown by Table 91; the elasticity of wrought iron is so nearly perfect that the ratio is nearly the same with all strains

up to 10 or 11 tons, the mean ratio in col. 7 from 1 to 11 tons is 1.236.

The length of rods and pillars of wrought iron that would be shortened 1 inch by different tensile and compressive strains is given by Table 93, and will suffice to give a general idea where it is desired to avoid the trouble of exact calculation,

(629.) “ Comparative Extension of Cast and Wrought Iron.”These two important materials are frequently combined in structures, and the differences in their elasticity cause unequal strains under circumstances where perfect equality might have been expected.

We will first consider the relative extensions of cast and wrought iron under the same tensile strain :-here the maximum strain must of course be limited by the strength of the weaker material of the two, namely, cast iron, which may be taken at

TABLE 97.–Of the COMPRESSION of WROUGHT IRON by CRUSHING

STRAINS : from Direct Experiments on two Bars 1 Inch Square.

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1 2

3 4

5

6

7

8

2,240 4,480 4,852 6,720 8,960 9,116 11,200 13,380 13,440 15,513 15,680 17,645 17,920 19,777 20,160 21,909 22,400 24,049 24,640 26,173 26,880 28,305 29,120 30,439 31,360 32,569

(2)

9

10

11

12

13

14

(1)

1 2

3

2,240 4,480 4,919 6,720 8,960 9,242 11,200 13,440 13,565

(2)

Shortening in Parts

of the Length.

Modulus of Elasticity E.

in Lbs. per Square Inch.

00010463 00020927 00023333 00032112 00042641 .00043333 .00051967 .00061000 .00061277 .00070833 *00071551 .00080000 •00081182 .00089167 .00090981 .00099267 ·00101445 ·00108333 00111115 ·00118333 00122117 00128333 .00134698 .00145000 ·00159414 00178333

(3)

21,409,000 21,409,000 20,795,000 20,926,000 21,013,000 21,037,000 21,552,000 21,934,000 21,934,000 21,898,000 21,914,000 22,062,000 22,074,000 22,183,000 22,158,000 22,071,000 22,081,000 (22,191,000) 22,168,000 22,115,000 22,012,000 22,056,000 21,619,000 20,993,000 19,666,000 18,263,000

(4)

SECOND BAR.

.00009814
00019629
00022500
•00029444
00038082
.00039167
.00046714
.00055352
.00055833

(3)

22,824,000 22,824,000 21,886,000 22,823,000 23,528,000 23,596,000 23,975,000 24,281,000 (24, 295,000)

5 6

(1)

TABLE 97.-0f the COMPRESSION of WROUGHT IRON by CRUSHING

STRAINS—continued.

[blocks in formation]
[blocks in formation]

00064813
•00074266
00074400
00083333
·00083884
00094167
.00095261
·00106667
00108216
00119167
·00121831
·00135834
·00140310
·00158333
·00217500
·00235000
·00273334

(3)

24,181,000
24,089,000
24,086,000
24,060,000
24,033,000
23,586,000
23,514,000
22,850,000
22,769,000
22,265,000
22,063,000
21,126,000
20,756,000
19,490,000

11

12

13

:::

[blocks in formation]

7 tons per square inch. In Table 99 the cols. 2 and 3 have been taken from cols. 4, 4, of Tables 88, 96, and col. 4 gives the ratio which increases regularly throughout, this fact being due to defect of elasticity in the cast iron as opposed to the almost perfect elasticity of wrought iron, the ratio rising from 2.066 with 1 ton to 3.008 with 7 tons per square inch. Thus, even with so low a strain as 1 ton, the extension of cast iron is more than double that of wrought iron.

(630.) But this statement does not give a clear idea of the effect of the unequal resistance to the same extension which happens in those numerous cases where the two materials are so combined that a given load must of necessity stretch them both to the same extent. Thus, in Fig. 123, let A be a rod of cast iron one square inch in area, and B a similar one of wrought iron stretched simultaneously and of necessity to the same extent by the weight W. Now, with say 4 tons per square inch on A the extension' by col. 4 of Table 88 would be :00077282,

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