1186 REPORT —1885. 
to fracture without any measurable elongation of the metal. In the case of crank- 
shafts, also, the fracture is invariably due to a tearing and not a crushing action. 
It appeared to me, therefore, eminently probably that repetition of stresses might 
be far more prejudicial to tension than to compression members, and, if so, the fact 
ought to be taken account of in proportioning a structure. 
This proved to be the case in my experiments. For example, the companion 
bars to those which had broken with 18,000 reversals of a stress less than half the 
original breaking weight behaved, when tested as columns thirty diameters in 
length, precisely the same as similar bars which had done no work at all, whereas 
when tested in tension the elongation was reduced from the original 25 per cent. to 
2:5 per cent., and the fracture appeared to indicate that the bars had been made of 
three different kinds of steel imperfectly welded together. With a stress reduced 
by one-fourth the number of bendings required to break the bars was increased to 
1,200,000. In this instance the calculated maximum working stress on the extreme 
fibres was 43 per cent. of the direct ultimate tensile resistance of the steel, and 
about 30 per cent. of the stress the bar was capable of sustaining as a beam under 
the single application of a load. Of course, the bars failed by tension, and the 
extreme fibres had thus deteriorated as regards tensile stresses to the extent 
indicated by the above percentages. Tested as a column, however, the injury the 
bar had received from the 1,200,000 bendings was inappreciable. The ductility 
was of course very largely reduced, but ductility is a quality of comparatively little 
importance when a material is in compression. There is no ductility in the slender 
Gothic stone columns of our cathedrals, which, though heavily stressed, have carried 
their loads for centuries. As I found repeated bendings raised the limit of elasti- 
city, I rather anticipated finding ar increased resistance from this cause in long 
columns. This did not prove to be the case, nor did I find any difference in short 
columns four diameters in length. 
In addition to the preceding experiments with rectangular bars, I have tested 
the endurance of many revolving shafts of cast iron, wrought iron, and steel, with 
similar results. About 5,000 reversals of a stress equal to one-half the static 
breaking weight sufficed generally to cause the snapping of a shaft of any of the 
above materials. When the stress was reduced and the number of applications 
increased, I found the relative endurance of solid beams to be more nearly pro- 
portional to the tensile strength of the metal than to the breaking weight of the 
beam, a distinction of great importance where axles, springs, and similar things 
are concerned. Many of my experiments were singularly suggestive. Thus, it 
was instructive to see a bar of cast iron loaded with a weight which, according to 
Fairbairn’s experiments, it should have carried for a long series of years, broken in 
two minutes when set gently rotating. Also to find a bar of the finest mild steel 
so changed in constitution by some months of rotation as to offer no advantages 
either in strength or toughness over a new cast-iron bar of the same section. 
Although, as already stated, many more experiments are required before uni- 
versally acceptable rules can be laid down, I have thoroughly convinced myself 
that, where stresses of varying intensity occur, tension and compression members 
should be treated on an entirely different basis. If, in the case of a tension 
member, the sectional area be increased 50 per cent. because the stress, instead of 
being constant, ranges from nz/ to the maximum, then I think 20 per cent. increase 
would be a liberal allowance in the case of a compression member. I have also 
satisfied myself that if a metallic railway bridge is to be built at a minimum first 
cost, and be free from all future charges for structural maintenance, it is essential 
to vary the working stress upon the metal within very wide limits, regard being 
had not merely to the effect of intermittent stresses, but also to the relative limits 
of elasticity in tension and compression members even under a steady load. 
Why an origivally strong and ductile metal should become weak and brittle 
under the frequent repetition of a moderate stress has not yet been explained. 
Lord Bacon touched upon the subject two or three centuries ago, but you may 
consider his explanation not wholly satisfactory. He said, ‘Of bodies, some are 
fragile, and some are tough and not fragile. Of fragility, the cause is an impotency 
to be extended, and the cause of this inaptness is the small quantity of spirits.’ I 
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