In order to apply these tests intelligently, it is necessary to know something of the peculiarities of the different descriptions of iron ordinarily met with, to see what their actual ultimate strength or resistance to rupture has been found to be by experiment, and to understand how that ultimate strength is modified by slight differences in their composition, form, treatment in working, and other surrounding circumstances.
Their resistance to torsion, though of importance in machines, does not come into play in buildings of any kind, and will, therefore, not be considered.
The breaking stresses, found by experiment and given in the following tables, were, in all cases, produced by a dead load, gradually applied. Very much smaller live loads, i.e. stresses suddenly applied, would cause rupture (see p. 318)
The average ultimate strength of the ordinary varieties of cast iron found in the market may be taken as follows; -
Tons per Square Inch.
. 13 1/2 B.
8 1/2 s.1
The above figures are intended to give a low average.
The following extracts from the most important experiments on the strength of cast iron show the wide differences that occur in different specimens.
The Table below is condensed from the records of Mr. Eaton Hodgkinson's experiments made for the Commission on the use of iron in railway structures.2
The experiments were made by crushing cylinders 3/4 in. diameter, some 3/4 in. and some 11/2 in. high. The figures given below show the resistance of the cylinders 11/2 inch high; the shorter cylinders offered a greater resistance.
Description of Iron.
In tons per square inch.
Lowmoor Iron, No. 1 ...
No. 2 ...
Clyde, No. 1 ...
No. 2 ...
No. 3 ,..
Blenavon, No. 1 ,..
No. 2 ...
Calder, No. 1 ...
Coltness, No. 3 ...
Brymbo, No. 1 ...
No. 3 ...
Bowling, No. 2 ......
Ystalyfera No. 2 (Anthracite) ....
Ynis-cedwyn No. 1 do. ....
No. 2 do. ...
Mean of irons testedby Mr. Hodgkinson in his experimental researches
Morris Stirling's iron tested by Mr. Hodgkinson - mean .
1 S. Stoney. B. Barlow.
2 Report of Commissioners appointed to inquire into the application of Iron to Mail-way Structures, 1849.
The mean of experiments made by the Ordnance authorities, as analysed by Professor Pole, give
Breaking weight in tons por square inch.
The specimens tried were generally samples received from the makers, of the second or third melting. The iron subsequently supplied in larger quantities was often inferior in strength to the samples.
The iron close to the surface of a casting has been found to be harder and stronger than that within. In a small bar the amount of this hard skin is greater in proportion to the section than in large castings, and hence the average strength is greater.
Again, the interior of large castings is more spongy and open than that of small castings.
Mr. Eaton Hodgkinson found the relative tensile strength - per square inch - of bars 1 in. 2 in. and 3 in. square to be 100, 80, 77.
Repeated remelting of cast iron increases its strength, probably in consequence of the carbon being burnt out of it, thus tending to assimilate it in composition to wrought iron. Sir William Fairbairn, experimenting upon Scotch iron, obtained the following results : -
Its resistance to cross breaking increased up to the twelfth remelting, and then fell off; at the twelfth remelting its strength was 7/5 of what it originally possessed. Its resistance to crushing was a maximum at the fourteenth remelting, i.e. nearly 21/4 times its original strength. Its resistance then fell off, until at the eighteenth remelting it possessed only twice its original strength.
In America the iron is kept in a state of fusion for two or three hours at each remelting.
Major Wade found the result to be as follows : -
Strength of pigs
5 to 61/2 tons per square inch.
First melting ....
9.3 ,, „
Fourth melting ....
The effect of remelting varied considerably, being greatest in No. 1 soft grey pig-iron.
This question can rarely be of any great importance to the engineer, though it might possibly have to be considered in using old iron.
Sir William Fairbairn's experiments led him to the following conclusion : - "Cast iron of average quality loses strength when heated beyond a mean temperature of 120°, and it becomes insecure at the freezing point, or under 32° Fahrenheit." 2
At a red heat its original strength is diminished by 1/3. A mass of cast iron raised to a red heat will crumble to pieces when struck. This property may be taken advantage of in breaking up large pieces of old cast iron, such as guns.
1 This is the value of the co-efficient C in the formula, W = Cbd2/l Where W=breaking weight in tons.
b = breadth d =depth I =span of beam in inches.
This subject will be explained in Part IV.
2 Application of Iron to Building Purposes, by Sir William Fairbairn, p. 73.
An increase or decrease of temperature amounting to 27° Fahr. causes such expansion or contraction that it would bring a stress of 1 ton per square inch upon the metal, if it was rigidly secured at the ends before the change of temperature took place.
The Effect of mixing Different Brands, when judiciously done, is doubtless to increase the strength of the iron beyond that of any single brand. The exact increase depends, of course, upon the mixtures used. As before mentioned, this is a question better left alone by the engineer.
Strength and Ductility of Wrought Iron.
The strength and ductility of wrought iron depend upon the quality of the material and the care with which it is manufactured.
A very small proportion of carbon is practically always present; if this is increased, the strength of the iron is considerably augmented, and its power of welding diminished, - in fact it approximates more to steel in its characteristics.
The presence of other impurities occasions the defects mentioned at pp. 262, 263.
The strength of different descriptions differs so greatly that an average is somewhat likely to be misleading in any particular case; but the following may be taken as a low average for the ultimate strength of wrought iron under different stresses.
Tons per sq. inch.
• • •
Tons per sq. inch.
. 16 to 20