Strain, per square inch, in pounds. CAST IRON. WROUGHT IRON. Extension, under tension. Contraction, under compression. Extension, under tension or contraction, under compression. 1000 0,08308 0,09155 0,0444 2000 0,17150 0,18404 0,0889 3000 0,26528 0,27747 0,1333 4000 0,36442 0,37185 0,1778 5000 0,46890 0,46715 0,2222 6000 0,57874 0,56341 0,2667 7000 0,69392 0,66061 0,3111 8000 0,81446 0,75875 0,3556 9000 0,94036 0,85782 0,4000 10000 1,07160 0,95784 0,4444 11000 1,20820 1,05880 0,4889 12000 1,35014 1,16070 0,5333 13000 1,49744 1,26354 0,5778 14000 1,65010 1,36733 0,6222 15000 1,80810 1,47205 0,6667 16000 ..... 1,57871 0,7111 17000 ..... 1,68432 0,7556 18000 ..... 1,79186 0,8000 19000 ..... 1,90035 0,84444 20000 ... 2,00978 0,8889 21000 ..... 2,11994 0,9333 22000 ..... 2,28145 0,9778 28000 ..... 2,84870 1,0222 24000 ..... 2,45690 1,0667 26000 ..... 2,57102 1,1111

Up to the elastic limit it is supposed that the amount of stretch (or shortening) is exactly proportional to the amount of strain, and that the material will recover its exact original length. Reference is here made to Formula (88). Neither of the above suppositions are exactly true, though in wrought-iron and mild steels it is very nearly so. Cast-iron, however, is very variable in its extensions or contractions under strains ; and all three show more or less "fatigue" and permanent set, under variable, or oft-repeated (on-and-off), or prolonged strains.

Box treats this subject very fully.

He finds the extension of cast-iron, subjected to tensional strains, to be :

Extension of Cast-iron.

e = L/10000000.(8,04. w + 0,00026 76.w2)

Where e = the total amount of extension, in inches, of a piece of cast-iron, subjected to tensional strains.

Where L= the original length, in feet, of the piece of cast-iron.

Where w= the tensional strain, in pounds, per square inch, of cross-section of the cast-iron piece.

It will be readily seen, that the increase in extension is in a higher ratio than the increase in strain, which is due, as already said, to the very defective elasticity of cast-iron.

For compressive strains in cast-iron, Box's researches give the following Formula:

Contraction of Cast-iron.

c =L/10000000.(9,108.w + 0,000047044.W2)

(105)

Where c = the total amount of shortening, in inches, of a piece of cast-iron, subjected to compression strains.

Where L = the original length, in feet, of the piece of cast-iron. Where w=the compression strain, in pounds, per square inch, of cross-section of the cast-iron piece.

Of course, in either case the extension or contraction from changes of temperature will be independent of the above.

Table XXXI gives the length of piece required to stretch or contract exactly one inch under different strains.

By comparing these formulae it will be seen that cast-iron yields more readily, that is, shortens more in proportion under very small compressive strains, than it extends under small tensional strains. But as the strains become greater the amount of shortening and