So important is the relation between cast iron, wrought iron, and steel that it may be well to illustrate this by the use of a diagram due to Professor Merriman. Assume that short rods of these materials, each of the same length and one square inch in cross section, are subjected to great stretching force by the use of a testing machine. As this force increases up to the elastic limit of six thousand pounds, the cast-iron rod becomes elongated proportionally. It breaks suddenly when the stress reaches twenty thousand pounds. At this limit of tenacity the rod has been increased in length less than one per cent, as shown in the diagram. The wrought iron becomes lengthened at a less rapid rate, reaching its elastic limit for a stress of about twenty-five thousand pounds. In each case, up to the elastic limit, if the stretching force be removed the rod will recover its former length and condition. On further increasing the stress, the wrought iron stretches at a more rapid rate, and bears a stress as great as fifty-eight thousand pounds. If now the force be withdrawn the iron remains in its deformed condition, the lengthening being about twenty-two per
Curves showing Tensile Strength of Timber, Cast Iron, Wrought Iron, and Steel. cent. On again applying the stress there is further rapid lengthening up to twenty-five per cent, this yielding causing a decrease of stress till the rod breaks at a limit below fifty-eight thousand pounds. The elastic limit and the breaking limit are thus widely different. In the case of steel the elastic limit is not reached until the stress becomes fifty thousand pounds. Its elastic limit is thus double that of wrought iron. Further increase of stress now causes the steel to increase its rate of stretching, and permanent strain results. Its breaking limit, one hundred thousand pounds, is nearly double that of the wrought iron, and is reached when the yielding attains fifteen per cent. This is not much more than half of the twenty-five per cent of yielding of the wrought iron.
The figures just given are only averages. Cast iron has been made with a tenacity in excess of forty thousand pounds, while that of steel may vary in different specimens from sixty thousand to three hundred thousand pounds. This wide range shows that for
