The tensile strength of iron is properly determined by ascertaining the load under which permanent set takes lace, and the amount of stretch under the proof load, rather than from the ultimate load that causes the fracture of the bar. In other words, the elastic limit rather than the breaking strain should be regarded as the measure of quality in a bar, and working loads should be proportioned with reference to the elastic limit instead of to the so-called ultimate strength.
Tough, sinewy iron is what is required in a tension bar, and although a hard, unyielding iron may show greater ultimate strength under a gradually applied strain, yet it is not suitable for use under tension for the reason that a sudden shock may cause it to snap under a weight that is ought to carry with entire safety.
Good bar iron should be of uniform character and possess a limit of elasticity of not less than 25,000 pounds per square inch. The ultimate resistance of prepared test-bars having a sectional area of about one square inch for a length of 10 inches should be not less than 50,000 pounds per square inch when the test-bars have been prepared from full-size bars having not more than 4 square inches of sectional area. For each additional square inch of full-sized bar area above 4 square inches a reduction of 500 pounds per square inch may be allowed down to a minimum ultimate resistance of 46,000 pounds. The amount of stretch under the breaking load should be not less than 15 per cent in 10 inches of the test-bar.
Bars that are to be used in tension should stand, without cracking, a cold bending test to 90 degrees to a curvature the radius of which is about the thickness of the bar under test, and at least one-third of the lot should stand bending to 180 degrees under the same conditions.
A round bar, one inch in diameter, should bend double, cold, without signs of fracture. A square bar of the same quality may show cracks on the edges under such a test.
Under a breaking pull the reduction of area should be not less than 25 per cent of the original section.
The shape of a bar has much influence in determining the breaking-strain. The ultimate strength of round bars is, for this reason, considerably greater than that of flat bars, but in either case the elastic limit will be found to occur at about the same point for equally good qualities of iron.
Within the elastic limit the extension of iron may, for all practical purposes, be stated as follows:
Wrought iron, 1/10000 of its length per ton per square inch.
Cast iron, 1/5000 of its length per ton per square inch.
The compression of wrought iron within the limits of elasticity follows the same law, and the amount of shortening under pressure will be in direct proportion to the weight applied. But with cast iron the amount of compression does not follow a constant ratio, the compression per ton becoming greater with the increase of the weight. Thus, a cast iron bar, one square inch in section was compressed 1/5900 of its length by a load of one ton; but under a load of 17 tons, instead of being compressed 17/5900, it was compressed 20/5900.
The modulus of elasticity is term used to designate such a weight as would extend a bar through a space equal to its original length, supposing the elasticity of the bar to be perfect. Or, the modulus of elasticity of any given material in feet is the height in feet of a column of this material, the weight of which would extend a bar of any determinate length through a space equal to this length. Thus, if one ton extends an inch bar of wrought iron one ten-thousandth of its length, it is evident that, upon the supposition that the bar is perfectly elastic, 10,000 tons would extend it to twice its original length. Hence, on this assumption, 10,000 tons, or 22,400,000 pounds, will be the modulus of elasticity of the wrought iron stated in weight. But an inch bar of wrought iron to weigh 22,400,000 pounds, at 3 1/3 pounds per foot, would be 6,720,000 feet long, and this would express the modulus of elasticity in feet.
The modulus of elasticity will, of course, vary according to the character of the material tested, being much higher in the better than it is in the lower grades of iron, but it forms a very useful and convenient standard of comparison in determining quality.
Mr. Kirkaldy sums up the results of his experimental inquiry in the following concluding observations, which the student should study carefully:
- The breaking-strain does not indicate the quality, as hitherto assumed.
- A high breaking-strain may be due to the iron being of superior quality, dense, fine, and moderately soft, or simply to its being very hard and unyielding.
- A low break-strain may be due to looseness and coarseness in the texture, or to extreme softness, although very close and fine in quality.
- The contraction of area at fracture, previously overlooked, forms an essential element in estimating the quality of specimens.
- The respective merits of various specimens can be correctly ascertained by comparing the breaking-strain jointly with the contraction of area.
- Inferior qualities show a much greater variation in the breaking-strain than superior.
- Greater differences exist between small and large bars in coarse than in fine varieties.