The Working of Steel | Page 9

the action of metal in compression and in tension is closely allied, and the designer is usually satisfied with the latter.
IMPACT TESTS
Impact tests are of considerable importance as an indication of how a metal will perform under shock. Some engineers think that the tensile test, which is one made under slow loading, should therefore be supplemented by another showing what will happen if the load is applied almost instantaneously. This test, however, has not been standardized, and depends to a considerable extent upon the type of machine, but more especially the size of the specimen and the way it is "nicked." The machine is generally a swinging heavy pendulum. It falls a certain height, strikes the sample at the lowest point, and swings on past. The difference between the downward and upward swing is a measure of the energy it took to break the test piece.
FATIGUE TESTS
It has been known for fifty years that a beam or rod would fail at a relatively low stress if only repeated often enough. It has been found, however, that each material possesses a limiting stress, or endurance limit, within which it is safe, no matter how often the loading occurs. That limiting stress for all steels so far investigated causes fracture below 10 million reversals. In other words, a steel which will not break before 10,000,000 reversals can confidently be expected to endure 100,000,000, and doubtless into the billions.
About the only way to test one piece such a large number of times is to fashion it into a beam, load it, and then turn the beam in its supports. Thus the stress in the outer fibers of the bar varies from a maximum stretch through zero to a maximum compression, and back again. A simple machine of this sort is shown in Fig. 10, where B and E are bearings, A the test piece, turned slightly down in the center, C and D ball bearings supporting a load W. K is a pulley for driving the machine and N is a counter.
[Illustration: FIG. 10.--Sketch of rotating beam machine for measuring endurance of metal.]
HARDNESS TESTING
The word "hardness" is used to express various properties of metals, and is measured in as many different ways.
"Scratch hardness" is used by the geologist, who has constructed "Moh's scale" as follows:
Talc has a hardness of 1 Rock Salt has a hardness of 2 Calcite has a hardness of 3 Fluorite has a hardness of 4 Apatite has a hardness of 5 Feldspar has a hardness of 6 Quartz has a hardness of 7 Topaz has a hardness of 8 Corundum has a hardness of 9 Diamond has a hardness of 10
A mineral will scratch all those above it in the series, and will be scratched by those below. A weighted diamond cone drawn slowly over a surface will leave a path the width of which (measured by a microscope) varies inversely as the scratch hardness.
"Cutting hardness" is measured by a standardized drilling machine, and has a limited application in machine-shop practice.
"Rebounding hardness" is commonly measured by the Shore scleroscope, illustrated in Fig. 11. A small steel hammer, 1/4 in. in diameter, 3/4 in. in length, and weighing about 1/12 oz. is dropped a distance of 10 in. upon the test piece. The height of rebound in arbitrary units represents the hardness numeral.
[Illustration: FIG. 11.--Shore scleroscope.]
Should the hammer have a hard flat surface and drop on steel so hard that no impression were made, it would rebound about 90 per cent of the fall. The point, however, consists of a slightly spherical, blunt diamond nose 0.02 in. in diameter, which will indent the steel to a certain extent. The work required to make the indentation is taken from the energy of the falling body; the rebound will absorb the balance, and the hammer will now rise from the same steel a distance equal to about 75 per cent of the fall. A permanent impression is left upon the test piece because the impact will develop a force of several hundred thousand pounds per square inch under the tiny diamond-pointed hammer head, stressing the test piece at this point of contact much beyond its ultimate strength. The rebound is thus dependent upon the indentation hardness, for the reason that the less the indentation, the more energy will reappear in the rebound; also, the less the indentation, the harder the material. Consequently, the harder the material, the more the rebound.
"Indentation hardness" is a measure of a material's resistance to penetration and deformation. The standard testing machine is the Brinell, Fig. 12. A hardened steel ball, 10 mm. in diameter, is forced into the test piece with a pressure of 3,000 kg. (3-1/3 tons). The resulting indentation is then measured.
[Illustration: FIG. 12.--Hydraulic testing machine. (Brinell principle.)]
While under load, the steel ball in a