Imágenes de páginas
PDF
EPUB

STRENGTH OF MATERIALS.

CHAPTER I.

ON THE TENSILE STRAIN.

THERE are five principal strains to which materials may be subjected, namely, the Tensile, Shearing, Crushing, Transverse, and Torsional strains : essentially all strains are modifications or combinations of the Tensile and Crushing ones, but it will be convenient in practice to consider each of them as distinct and specific.

(1.) Central Strain.”—When the cross-section of a body is of a regular figure, and the tensile strain is in the centre, it is commonly admitted that the resistance is simply proportional to the

area, and that every part of the section is equally strained. This

may be practically true in many cases, but where the body is wide or large, the central part is more stretched than the edges, and the strain becomes very unequal. For example, Fig. 1 is a plate of very elastic material whose normal form unloaded is a, b, c, d, and when strained by the central load W it becomes e, f, g, h. Obviously the central part is more stretched and therefore more strained than the edges, and if the load be increased up to the point of rupture, the plate will break first at the centre.

(2.) “ Strain out of Centre."—When the strain coincides with one edge of a plate as in Fig. 2, the primitive form i, k, l, m, tends to take the form n, o, p, r, and we have this remarkable result, that the maximum extension and corresponding strain at n, o, is progressively reduced towards 8, t, where it becomes nil, and between 8, P, and t, r, the plate is compressed, not

B

stretched, and thus a crushing strain is created by a nominally tensile one.

Say, that we take a spiral spring whose normal length unloaded = 10 inches, and its elasticity such that it extends 1 inch per lb.; also let 4 lbs. be the breaking weight, the maximum length being then 14 inches. Let B, C, D, E in Fig. 3 be four such springs attached at equal distances to two rigid crossbars F, G: if now a tensile strain of 16 lbs. be applied at the centre-line H, J, obviously the whole of the springs will be extended to 14 inches, each yielding the 4 lbs. due to it.

In Fig. 4, K, L, M, N, are the centre lines of four springs similar to those in Fig. 3, but here the centre line of the strain coincides with L, Q, or the centre of the spring L. Now, it is essential that the forces on the two sides of the centre line should balance one another: they will arrange themselves as in the figure ; thus the strain on K being 4, and its distance from the centre 1.0, we have 4 x1 = 4, as the effect of the spring K. Then, on the other side, M = 2 x1 = 2.0, and N = 1 x 2 2.0 also : the sum of the two being 4, or the same as K. Then the weight at W, with which the spring K will break, becomes as in the figure, 4 + 3 + 2 + 1 = 10 lbs., whereas with a central strain as in Fig. 3 we obtained 16 lbs., or 60 per cent. more than in Fig. 4.

(3.) To show how a compressive strain may be generated by a nominally tensile load, let Fig. 5 be an arrangement similar to the preceding, but one where the tensile strain coincides with the centre line of the spring R, or the extreme edge of the combination. In this case the spring R bears the maximum load of 4 lbs., but $ = 2 lbs. only: the spring T is neither extended nor compressed, but retains its normal length of 10 inches; it is therefore useless. The spring U is compressed to the length of 8 inches, and bears a crushing strain of 2 lbs.

The tensile load at X from R 4 lbs., from S 2 x1=2

1.0; from T 0, and from U 2 x1=2 1.0; the total being 4 +1+0+1 = 6 lbs., whereas with a central load as in Fig. 3 we had 16 lbs.; hence the ratio = 6 + 16 = .375 to 1.0. Mr. Hodgkinson found by experiment that a cast-iron bar which broke with a central load of 7.65 tons, failed with

[ocr errors]
[ocr errors]

2.62 tons only when the force coincided with one side of the bar, the ratio being 2.62 = 7.65 = .342 to 1.0, or nearly as we found it by calculation.

These illustrations will serve to show the importance of arranging for the tensile strain to coincide with the axis of the body, or the centre line of the section, and that where this is impracticable, due allowance should be made for the fact.

(4.) “ Experimental Results."--Table 1 gives a general summary of the most important experiments on the tensile strength of materials, from which it appears than the mean breaking weight of:

Cast Iron Wrought Iron Steel Bar Copper Bolts may be taken at:7.142 25.7 47.84

16.0 tons per square inch, which is equivalent to :16,000 57,500

107,160

35,840 lbs. Table 2 gives the breaking weight of round bars from ( inch to 3 inches diameter, calculated from these data.

(5.) “ Effect of Re-melting Cast Iron.”—Ordinary cast iron is usually from the 2nd fusion, pig iron being the 1st: it has been found that with some kinds of iron at least, the tensile strength is very much increased by repeated re-melting; thus one set of experiments gave for iron of the 1 2 3

4th melting, the tensile strength per square inch 14,000 20,900 30,300

35,785 lbs. Another series gave 11,020 15,942 35,846

45,970 lbs. The mean of the two series in tons per square inch = 5.6

8.2
14.65

18.26

(6.) But Mr. Fairbairn obtained very different results, as given by Table 3, which shows that the transverse and tensile strengths were reduced by re-melting so far as the 3rd, then

TABLE 1.-Of the TENSILE STRENGTH OF MATERIALS.

[blocks in formation]

tons.

19

:::::

:::::

:::::

[ocr errors]

»

Wrought-iron, rolled bar, Yorkshire

Staffordsbire
Swedish
Russian

British
welded
screwed (new dies), or chased
rolled rivet-iron

shearing
single-shear
double-shear
mean of both

tensile strength, same iron
boiler-plate, lengthway, Yorkshire

crossway
lengthway, British
crossway
mean of both
solid unpunched plate
single-riveted joints
double-riveted joints
Lowmoor, solid, lengthway
Lowmoor, solid, crossway
Derbyshire, solid, lengthway
Derbyshire, solid, crossway
Shropshire, solid, lengthway

[ocr errors]

lbs.

Ibs.
62,886 60,075
62,231 56,715
48,232 47,885
56,805 49,564
68,848 44,584
57,342 33,288
47,846 34,344
66,848 47,095
55,277 50,127
58,464 53,536
51,296 48,384

.
56,005 52,000
50,515 46,221
62,544 37,474
60,756 | 32,450
61,650 34,962
61,579 43,805
45,743 | 37,161
58,286 52,352
58,915 | 49,852
61,588 53,570
51,191 46,047
44,291 41,391
53,349 | 48,912

lbs.
61,480 27:4 Napier and Sons.
59,473 26.5
48,043 21:4
53,185 23:7
57,555 25:7 Kirkalůy, 188. Exp.
47, 266 21:1
41,736 18.6
58,197 25.97
52,702 23.52 Fairbair (in joints).
54,073 24:14 E. Clark.
49,504 22:10
52,193 23.30
53,760 24.00
54,002 24 : 10 Napier and Sons.
48,368 21.7
50,737 22:6 Kirkaldy, 167 Exp.
46,171 20:6

160 Exp.
48,454 21.6
52,486 23:43 W. Fairbairn.
41,590 18:56
53, 635 23.94
55,029 24:56
54,993 24.55
48,619 21.70
42,841 19.12
51,130 22.82

[ocr errors]
[ocr errors]
[ocr errors]

..

Shropshire, solid, crossway .. Staffordshire, solid, lengthway Staffordshire, solid, crossway

mean of the four kinds, &c. Steel bar, rolled or

annealed drilled plate, annealed or not double-riveted joints, punched holes, annealed plate,

49,651 48,912

49,281 22.00 45,329 42,314 18,822 19:56 49,100

46,798 47,950 21.10
51,675 | 47,224 49,208 21.96
148,294 | 65,158 | 107,160 47.8

47.8 Kirkaldy, 66 Exp.
90,019 40.2
100,983 45.0
104,888 46.8
112,119 50.0
215,400 96.1

121,716 54:3
52,784 | 38,772 45,778 20.4

81,133 36.22 H. Sharp.
59,786 38,073 49,235 22:00
79,289 67,157 73,562 32.84
83,485 78,893 81,189 36.24
96,163 87,606 91,885 41.02
102,593 72,408 90,000 40:1 Kirkaldy.
97,150 67,686 81,960 36.6
99,870 70,047 85,977 38.4
58,128 42,000 53,215 23.77 H. Sharp.
48,272 36,557 42,380 18.92 Hawks and Cr ay.
48,765 33,376 38,443 17:43
56,089 45,002 48,160 21.5 Captain Brown.

14,270 6.37 E. Clark.
83,000 78,430 80,214 35.8 Telford.

16,000 7.142 Hodgkinson and Fair-
23,468 | 12,716

bairn.
32,077 | 23,461 27,916 12:46

Owen, &c.
96,300

73,185 32:67 Anderson, 1,273 0.568 Muschenbroek.

tilted hardened in water

tempered yellow spring temper

tempered blue. higlily heated and cooled in oil hardened and annealed

welded joints .. Steel-plate, solid or unpunched punched plate, not annealed

steel rivets
solid plate, length way

crossway
mean of the two
Steel rivets in double-riveted joints
Wrought-iron chain, short-linked

stud-linked, cable
drawn welded press-pipe

wire to inch diameter
Cast-iron, British

Stirling's toughened
Aluminium bronze
Antimony, cast

..

99

[ocr errors]
« AnteriorContinuar »