Uniformity

In choosing iron for railway bridges and similar structures it is not only important that the iron should be strong and tough, but also that it should be uniform in quality.

Iron structures should be so proportioned that an equal stress shall come upon every square inch of the section of every part. It is of no advantage that the iron in one part should be so good as to enable it to take more than this working stress, when at the same time another part would give way if the stress were applied.

Different Methods Of Testing

(1) Upon receiving a quantity of iron for any work, pieces may be taken at random and tested to breaking in the manner before described.

This is the best way of testing - all the particulars required to be known with regard to the iron may be ascertained - and though some bad pieces may escape detection, yet the general average of the whole, and the degree of uniformity which exists, is pretty well arrived at.

In order that the iron may be uniform in ductility as well as in tensile strength, it has been recommended that a maximum percentage of elongation or contraction of area should be specified as well as a minimum. This, however, is not done, the minimum only being referred to in most specifications.

There are, however, other ways in which engineers endeavour to ascertain the quality of a lot of iron by applying a tensile stress.

These may just be mentioned.

(2) Sometimes every piece to be used in the work is tested under a small stress, any bars which appear to elongate more than the others, and sooner to take a permanent set (see p. 329), being considered inferior. This test gives no information regarding the ultimate strength of the iron. Moreover, there is danger of testing each piece beyond its limit of elasticity (see p. 329), and thus doing it a permanent injury.

(3) In other cases it is specified that all bars shall be rejected the elongation of which exceeds a certain fixed proportion under a specified stress.

This is a bad test of the quality of the iron, for the large elongation may be due either to the iron being a good tough material, which stretches considerably long before breaking, or it may be due to the iron being of a weak description and on the point of breaking.

1 Unwin's Iron Bridges and Roofs.

Testing Machines

The machines for accurately testing iron and steel are too cumbrous and expensive for ordinary use. Engineers generally send their specimens to be tested by Mr. Kirkaldy of Southwark, to other testers of materials, or to one of the chain-testing establishments, such as those at Birkenhead and Sunderland.

A description of Mr. Kirkaldy's admirable machinery for testing is given in Spon's Dictionary of Engineering.

Tensile Tests For Wrought Iron

The Tables at page 318 give the tensile strength, the contraction of area, and other particulars with regard to several different descriptions of iron.

These particulars differ in nearly every case. It is not usual to make shades of difference in the tests applied, so that they do not vary with each minute difference in the description of iron that is to be used.

The following Tables, showing the tests that are applied to the various classes of iron by the different Government departments, will therefore be useful.

India Office

The following Table is extracted from one prepared for the India Office by Mr. Kirkaldy:1 -

Scale Of Tensile Tests For Iron Of Various Qualities

Description.

Class C.

Class D.

Class E.

Class F.

Class G.

Ultimate stress per square inch.

Contraction of area at fracture.

Ultimate stress per square inch.

Contraction of area at fracture.

Ultimate stress per square inch.

Contraction of area at fracture.

Ultimate stress per square inch.

Contraction of area at fracture.

Ultimate stress per square inch.

Contraction of area at fracture.

Bars, round or square ...

Tons.

Per cnt.

Tons.

Per cnt.

Tons.

Per cnt.

Tons.

Per cnt.

Tons.

Per cnt.

27

45

26

35

25

30

24

25

23

20

Bars, flat ...

26

40

25

30

24

25

23

20

22

16

Angle and Tee or T

25

30

24

22

23

18

22

15

21

12

Plates, grain lengthways..

24

23

20

16

23

21 1/2

15

12

22

201/2

12

91/2

21

191/2

10

71/2

20

181/2

8

51/2

Plates, grain crossways..

22

12

20

9

19

7.

18,

5

17.

3.

Swedish Bars.

Ultimate Stress per square inch.

22 tons.

Contraction of area at fracture.

60 per cent.

Comparing this Table with the Tables of strength given at page 318, it will be seen that the best Yorkshire iron might be expected to stand the tests under Class C.

1 Wray's Theory of Construction.

The Best Best irons of the market should stand the test under Class E. The ordinary Best iron of the market should stand those of Class G.

A and B are reserved for special qualities of iron which might he required at any future time, and the Classes D to F would be for qualities intermediate between the others.

Recent India Office specifications are summarised as in the following Table, which shows the ultimate tensile stress per square inch, and the percentage of elongation for each description of iron.

BB

Staffordshire.

B.B.B.

Staffordshire.

Yorkshire.

Miscellaneous.

For Iron Roofing.

Ultimate tensile stress per sq. inch.

Elongation per cent.

Ultimate tensile stress per sq. inch.

Elongation per cent.

Ultimate tensile stress per sq. inch.

Elongation per cent.

Ultimate tensile stress per sq. inch.

Elongation per cent.

Ultimate tensile stress per sq. inch.

Elongation per cent.

Tons.

Tons.

Tons.

Tons.

Tons.

Bars round and square .

23

30

24

40

23

50

24

40

24

20

Flat bars over 5" wide.

Do. flat .

22

25

23

35

22

45

...

22

15

Flat bars under 5" wide.

Angle Iron

22

25

23

35

22

45

22

20

22

15

T or H ,,

22

25

23

35

22

45

22

20

20

10

Plate

grain lengthways

20

10

22

12

21

20

21

10

18

5

grain crossways .

17

5

18

7

19

12

18

5

...

Sheet

grain lengthways

20

10

22

12

21

20

...

\ grain crossways .

19

5

18

7

19

12