Cast Iron is very imperfectly elastic, that is, even a very small load will produce in it an appreciable permanent set. There is no clearly-defined elastic limit. The permanent sets are, however, very small at first, and may he practically ignored until the load applied is about 1/3 of that required to produce rupture.2 The sets then become partially appreciable.

Wrought Iron

The elastic limits for different descriptions of wrought iron vary according to the nature of the iron.

As an average, however, it may be said that the elastic limit, both in compression and tension, is as follows : -

Bars...........

•5 of ultimate strength.

Plates.................

.6 „ „

being about 13 tons per square inch for "Best Yorkshire" iron, and about 111/2 tons for Staffordshire crown iron3 (see p. 319).

Steel

The elastic limit of different kinds of steel varies considerably, according to the nature of the material and the degree of temper to which it has been subjected.

It ranges from about 123/4 tons in annealed Landore mild steel plates (see p.'323) to 26f tons in very hard cast steel (see p. 320), the proportion of the elastic limit to the ultimate strength varying from .45 to .8.