Mr. Kirkaldy's experiments led him, however, to the following conclusions.2
"1. Whenever wrought iron breaks suddenly, a crystalline appearance is the invariable result; when gradually, invariably a fibrous appearance.
"2. Whether, on the one hand, it is finely or coarsely crystalline, or on the other, the fibre be fine and close, or coarse and open, depends upon the quality of the iron.
"3. When there is a combination in the same bar or plate of two kinds - the one harder or less ductile than the other - the appearance will be partly crystalline and partly fibrous, the latter produced by the gradual drawing asunder action previous to and at the time of rupture; whilst in the former the iron breaks suddenly, without elongating at time of rupture.
" 4. When the proportion of the harder is considerably less than the softer, the former snaps suddenly, whilst the latter continues stretching; but when nearly equal, or the less ductile predominates, both portions break together, or almost at the same moment;
1 Graham Smith in Proceedings Liverpool Engineering Society, from "Engineer." 2 Kirkaldy's Experiments on Wrought Iron and Steel.
the one part, gradually arriving at its limit of endurance, breaks with a fibrous appearance, whilst a greatly increased strain consequently coming on the remaining portion, it suddenly gives way producing a crystalline appearance.
"5. The relative qualities of various irons may be pretty accurately judged of by comparing their fractures, provided they have all been treated in precisely the same way, and all broken under the same sort of strains similarly applied.
"6. By varying either the shape, the treatment, the kind of strain, or its application, pieces cut off the same bar will be made to present vastly different appearances in some kinds of iron, whereas in others little or no difference will result."
It will be seen then that the appearance of the fractured surface of wrought iron is to a certain extent an indication of its quality, provided it be known how the stress was applied which produced the fracture.
Good iron may be either crystalline or fibrous, according as the stress which caused fracture was sudden or gradual, but it should be remarked that bad iron is never fibrous.
Coarse crystals, blotches of colour caused by scoriae or other impurities, loose and open fibres, are signs of bad iron, and flaws in the fracture surface are signs that the piling, welding, and rolling processes have been imperfectly carried out.
Fractures examined should be those of bars at least half-an-inch thick, or they will become distorted and will not exhibit the characteristic peculiarities to be seen in larger bars.
The fibres of wrought iron are readily exposed by immersing the specimen for a few days in very weak hydrochloric or nitric acid, which eats away the material between the fibres, leaving the latter exposed.
In testing iron for very important situations, where it will be subject to sudden shocks, it is well to subject it to the tension produced by a weight falling from a height, so as to imitate as nearly as possible the action of the force to which it will be subjected.
This is done in the case of bolts for fastening the thick iron plates of armour-plated forts.
These are tested by means of a ton weight falling through a distance of 30 feet. The testing apparatus is so arranged that the blow acts in the direction of the length of the bolt. This, it is found, will pull asunder a 3-inch bolt in six or seven blows. The fracture is required to be "silky fibrous, not crystalline in any degree,"and the contraction of area 40 per cent.
Iron rails are also sometimes tested by a falling weight.