This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
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.