It may be broken by direct slow tension, or by a falling weight, the breaking stress, elongation, contraction of area, and other particulars being noted. In the absence of facilities for breaking it, it may be subjected to certain rough tests which will be presently described.
Wrought iron is used in many structures in which it is liable to receive sudden and often-repeated shocks. This is the case, for example, in bridges, and to a certain extent in roofs. It must, therefore, be able not only to resist a great tensile stress, but also to withstand sudden concussion or continued vibration.
A very hard iron will withstand a very high tensile stress, but is brittle, and will snap under a sudden strain.
A good iron must, therefore, not only possess great tensile strength, but must be ductile, that is, able to stretch before it gives way. This ductility may be measured either by the proportion borne by the permanent elongation to the original length of the iron, or by the amount of contraction of area of section caused by the stretching.
A specimen of such iron when torn asunder by slow tension will not break off short as in Fig. 115,1 but will draw out as in Fig. 116,1 not only becoming longer, but also being reduced in diameter and sectional area at the centre. The dotted lines in Fig. 116 show the origmal size of the specimen.
In order that both strength and ductility may be secured, it is now usual for engineers to require that the iron for bridges and similar important work should fulfil at least two conditions : -
(1) That it shall not break with a tensile stress less than a certain specified amount.
(2) That before breaking it shall elongate not less than a named proportion of its origmal length; or
That before breaking its sectional area shall be reduced (as a consequence of its stretching) by not less than a certain named proportion of its origmal area.
Of these two forms of the test for ductility, the measurement of the elongation is generally simpler and more easily managed than the measurement of the reduction of area.
Iron can, however, be made which will possess both qualities in a very high degree.
In addition to ascertaining the strength and ductility of the iron, it is desirable to know how the iron will behave when reheated and worked.
This is ascertained by bending or otherwise distorting the iron when hot, as described at page 280, under the head of Forge Tests.
Such tests are especially valuable when the iron is to be forged into different shapes before use in the structures for which it is intended.
Mr. Kirkaldy's Experiments. - At one time it was thought that the tensile stress required to break a piece of iron was all that was necessary to be known in order to ascertain its quality.
The investigations of Mr. Kirkaldy founded upon an elaborate series of experiments made by him on iron of every description and quality, led him, however, to the following conclusions,1 among many others, some of which will be referred to presently : -
1 From Kirkaldy's Experiments on Iron and Steel.
"1. The breaking strain does not indicate the quality as hitherto assumed.
"2. 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.
"3. A low breaking strain may be due to looseness and coarseness in the texture, or to extreme softness, though very close and fine in quality.
"4. The contraction of area at fracture, previously overlooked, forms an essential element in estimating the quality of specimens.
"5. The respective merits of various specimens can be correctly ascertained by comparing the breaking strain jointly with the contraction of area.
"6. Inferior qualities show a much greater variation in the breaking strain than superior.
"7. Greater differences exist between small and large bars in coarse than in fine varieties.
"8. The prevailing opinion of a rough bar being stronger than a turned one is erroneous.
"9. Rolled bars are slightly hardened by being forged down.
"10. The breaking strain and contraction of area of iron plates are greater in the direction in which they are rolled than in a transverse direction." (The experiments show the difference to be about 10 per cent.)