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