§ 37. Effects of Progressively Augmented Strain

Ewing and Rosenhain1 found that if a metal is strained past its "yielding point "-elastic limit-the faces of the crystal grains (Fig. 54) show fine black lines, which increase in number as the strain increases. Lines appear on certain crystals nearly transverse to the pull; but as the strain increases, lines appear upon other grains. Intersecting lines then make their appearance on some of the grains. Such a strained surface is shown in Fig. 55.

What are these lines? Can they be cracks in the surface? Probably not, and for this reason. A piece of iron, strained beyond its elastic limits, will recover its original elasticity if it be allowed to rest for some time, or if it be heated to 100° C. But the dark lines do not disappear. Furthermore, the lines do disappear when the surface is lightly polished in the usual

manner.

The lines are not actual cracks in the surface, but rather slips along the cleavage planes of the crystal. They are called slip bands, or slip lines. Let AB (Fig. 56) represent a cross-section through a polished surface of metal. Let C be the junction between two contiguous grains, A and B. When the metal is pulled in the direction of the arrows a number of slips are developed along the cleavage planes a, b, c, d,

1 J. A. Ewing and W. Rosenhain, Phil. Trans., 193. 353, 1899; 195. 279, 1900; J. A. Ewing and J. C. W. Humfrey, ibid., 200. 241, 1902; W. Rosenhain, Journ. Iron and Steel Inst., 67. i. 335, 1904; F. Osmond and C. Frémont and G. Cartaud, Revue de Metallurgie, 1. i. 1904.

J. Muir, Phil. Trans., 193. 1, 1900; Proc. Roy. Soc., 67. 461, 1900.

FIG. 55.-Lead. (J. A. Ewing and W. Rosenhain.)

[To face p. 90.

and the surface now presents the appearance shown in Fig. 57. With still greater strains slip bands develop into actual cracks, and rupture takes place. Hence it follows that under a progressively augmented strain rupture takes place, not at the crystal boundaries, but through the crystals themselves.

Slip-bands have also been developed by compression, say pinching a button of polished silver or

copper in a vice, by twisting an iron bar, and by bending a strip of iron or copper backwards and forwards.

Plasticity is nothing but the yielding or slipping past each other of adjoining crystals; ice is only plastic when the strain is directed along the cleavage planes. In a large aggregate of crystals, only those crystals whose cleavage planes are in the right direction will suffer deformation.

You will remember that the elastic limit or yieldpoint of a body is the maximum distortion which the body can undergo, and yet return to its original form. If the strain is not very much greater than the elastic limit, the restoration of the original form will take place very slowly, as indicated above. The metal exhibits what is called fatigue. Steel bridges, for example, sometimes sag during the week's traffic, but recover during Sunday's rest.

It is possible to apply a much greater stress than

the elastic limit of the metal provided the stress be applied very quickly. Thus B. Hopkinson has recently shown that a piece of wire with an elastic limit of 17.8 tons per square inch withstood a stress of 35.5 tons per square inch applied for less than the thousandth part of a second.

§ 38. Effects of Repeated Alterations of Stress

Nearly fifty years ago Wöhler1 made the important discovery that the elastic limit is very much reduced by, and that metals will even break down undor, the repeated application of a stress very much less than would be required to produce rupture if the stress were applied gradually and continuously. The strength of a piece of metal when tested in the ordinary way is not a sure guide if the material is to be subjected to a series of constantly vibrating stresses-pushes and pulls-such as might occur in the piston of an engine, and the axles of a railway carriage. An iron bar, for instance, which is capable of bearing 20 tons per square inch of steady load can only withstand 9 tons per square inch for six hours if the stress vibrates about 144 times per minute. Bridges which might withstand the passage of a given number of trains per hour could not bear the same number passing in a minute.

The strength of a metal under the influence of alternations of stress depends on the number of alternations per minute; and the number of alternations which a metal can stand depends upon the intensity of the stress. The following table shows the number of

1 A. Wöhler, Engineering, 11. 199, 221, 243, 299, 326, 349, 1871; A. W. Kemp, Engineering Review, 11. 168, 1904.