In very hard materials it is more than half the strain, for such materials not being elastic, will stretch (or shorten) but very little and will not show appreciable variation until a high strain is reached, when they show a set quickly, and break soon after. In soft, elastic, ductile materials they begin showing permanent stretching (or shortening) very early and continue to do so for a long time before breaking. The following will be quite safe to follow as general guides.

For cast-iron the elastic limit is about 1/3 of the ultimate tensional or compressive stress.

For wrougt-iron the elastic limit is about 1/2 of the ultimate tensional or compressive stress in bars, and about 3/5 in plates.

For mild steel the elastic limit is about 9/20 and from this it varies to about 4/5 for

Hard steel-both, of course, of either the ultimate tensional or compressive stress.

Table XXX gives the amount of extension and contraction, in inches, for different strains, of cast-iron and wrought-iron pieces, if each piece were one hundred feet long.

Permanent Set.

Elastic Limit.