[Condensed from Nature.]
There has recently been a most interesting discussion at the Literary
and Philosophical Society, Manchester, on the above subject.
The paper which gave rise to the discussion was by Mr. Brockbank, who
detailed many experiments, and ended by stating his opinion that iron
does become much weaker, both in its cast and wrought states, under
the influence of low temperature; but Mr. Brockbank's paper was
immediately followed by others by Sir W. Fairbairn, Dr. Joule, and Mr.
Spence, which at once put an entirely new complexion on the matter.
Dr. Joule says:
"As is usual in a severe frost, we have recently heard of many severe
accidents consequent upon the fracture of the tires of the wheels of
railway carriages. The common-sense explanation of these accidents is,
that the ground being harder than usual, the metal with which it
is brought into contact is more severely tried than in ordinary
circumstances. In order apparently to excuse certain railway
companies, a pretence has been set up that iron and steel become
brittle at a low temperature. This pretence, although put forth in
defiance, not only of all we know, of the properties of materials, but
also of the experience of everyday life, has yet obtained the credence
of so many people that I thought it would be useful to make the
following simple experiments:
"1st. A freezing mixture of salt and snow was placed on a table. Wires
of steel and of iron were stretched, so that a part of them was in
contact with the freezing mixture and another part out of it. In every
case I tried the wire broke outside of the mixture, showing that it
was weaker at 50 deg. F., than at about 12 deg. F.
"2d. I took twelve darning needles of good quality, 3 in. long, 1/24
in. thick. The ends of these were placed against steel props, 2-1/8
in. asunder. In making an experiment, a wire was fastened to
the middle of a needle, the other end being attached to a spring
weighing-machine. This was then pulled until the needle gave way. Six
of the needles, taken at random, were tried at a temperature of 55 deg.
F., and the remaining six in a freezing mixture which brought down
their temperature to 12 deg. F. The results were as follow:--
Warm Needles. Cold Needles.
64 ounces broke 55 ounces broke
65 " " 64 " "
55 " " 72 " "
62 " " 60 " bent
44 " " 68 " broke
60 " bent 40 " "
--- ---
Average, 58-1/3 Average, 59-5/6
"I did not notice any perceptible difference in the perfection of
elasticity in the two sets of needles. The result, as far as it goes,
is in favor of the cold metal.
"3d. The above are doubtless decisive of the question at issue. But
as it might be alleged that the violence to which a railway wheel is
subjected is more akin to a blow than a steady pull; and as, moreover,
the pretended brittleness is attributed more to cast iron than any
other description of the metal, I have made yet another kind of
experiment. I got a quantity of cast iron garden nails, an inch and
a quarter long and 1/8 in. thick in the middle. These I weighed,
and selected such as were nearly of the same weight. I then arranged
matters so that by removing a prop I could cause the blunt edge of a
steel chisel weighted to 4lb. 2oz., to fall from a given height upon
the middle of the nail as it was supported from each end, 1-1/16 in.
asunder. In order to secure the absolute fairness of the trials, the
nails were taken at random, and an experiment with a cold nail was
always alternated with one at the ordinary temperature. The nails to
be cooled were placed in a mixture of salt and snow, from which they
were removed and struck with the hammer in less than 5"."
The collective result of the experiments, the details of which need
not be given, was that 21 cold nails broke and 20 warm ones.
Dr. Joule adds, "The experiments of Lavoisier and Laplace, of Smeaton,
of Dulong and Petit, and of Troughton, conspire in giving a less
expansion by heat to steel than iron, especially if the former be in
an untempered state; but this, would in certain limits have the effect
of strengthening rather than of weakening an iron wheel with a tire of
steel.
"The general conclusion is this: Frost does _not_ make either iron
(cast or wrought), or steel, brittle.
Mr. Spence, in his experiments, decided on having some lengths of
cast iron made of a uniform thickness of 1/2 in. square, from the same
metal and the same mould.
He writes:--"Two of the four castings I got seemed to be good ones,
and I got the surface taken off, and made them as regular a thickness
as was practicable.
"I then fixed two knife-edged wedges upon the surface of a plank, at
exactly nine inches distance from each other, with an opening in the
plank in the intervening space, the bar being laid across the wedges,
a knife-edged hook was hung in the middle of the suspended piece of
the bar, and to the hook was hung a large scale on which to place
weights.
"The bar was tried first at a temperature of 60 deg. F.; to find the
breaking weight I placed 56lb. weights one after another on the scale,
and when the ninth was put on the bar snapped. This was the only
unsatisfactory experiment, as 14 or 28lb. might have done it, but I
include it among others. I now adopted another precaution, by placing
the one end of the plank on a fixed point and the other end on to a
screw-jack, by raising which I could, without any vibration, bring the
weight to bear upon the bar. By this means, small weights up to 7lb.
could be put on while hanging, but when these had to be taken off and
a large weight put on, the scale was lowered to the rest, and again
raised after the change was made. I may here state that a curious
circumstance occurred twice, which seems to indicate that mere raising
of the weight, without the slightest apparent vibration, was equal in
effect to an additional weight. 33/4 cwts. were on the scale, a 14lb.
weight was added, then 7lb., then 4lb., 2lb., 1lb., and 1lb., making
4cwts. and 1lb. This was allowed to act for from one to two minutes,
and then lowered to take off the small weights, which were replaced by
a 56lb. with the intention of adding small weights when suspended; the
whole was then raised so imperceptibly by the screw, that the only way
of ascertaining that it was suspended, was by looking under the scale
to see that it was clear of the rest. As soon as it was half-an-inch
clear it snapped, thus breaking at once with one pound less than it
resisted for nearly two minutes.
"Six experiments were carefully conducted at 60 deg. F., the parts of the
bars being selected so as to give to each set of experiments similar
portions of both bars; the results are marked on the pieces. My
assistant now prepared a refrigerating mixture which stood at zero,
the bars were immersed for some time in this, and we prepared for the
breaking trials to be made as quickly as could be, consistently with
accuracy; and to secure the low temperature, each bar, on being placed
in the machine, had its surface at top covered with the freezing
mixture. The bars at zero broke with more regularity than at 60 deg., but
instead of the results confirming the general impression as to cold
rendering iron more brittle, they are calculated to substantiate
an exactly opposite idea, namely, that reduction of temperature,
_caeteris paribus_, increases the strength of cast iron. The only
doubtful experiment of the whole twelve is the first, and as it stands
much the highest, the probability is that it should be lower; yet,
even taking it as it stands, the average of the six experiments at
60 deg. F., gives 4cwt. 4lb. as the breaking weight of the bar at that
temperature, while the average of the six experiments at zero gives
4cwt 20lb. as the breaking weight of the bar at zero, being an
increase of strength, from the reduction of temperature, equal to 3.5
per cent."
Sir W. Fairbairn states: "It has been asserted, in evidence given at
the coroner's inquest, in a recent railway accident, that the breaking
of the steel tire was occasioned by the intensity of the frost, which
is supposed to have rendered the metal, of which this particular
tire was composed, brittle. This is the opinion of most persons, but
judging from my own experience such is not the fact. Some years since
I endeavored to settle this question by a long and careful series
of experiments on wrought iron, from which it was proved that the
resistance to a tensile chain was as great at the temperature of zero
as it was at 60 deg. or upwards, until it attained a scarcely visible red
heat."
The immense number of purposes to which both iron and steel are
applied, and the changes of temperature to which they are exposed,
renders the inquiry not only interesting in a scientific point of
view, but absolutely necessary to a knowledge of their security under
the various influences of those changes. It was for these reasons
that the experiments in question were undertaken, and the summary of
results is sufficiently conclusive to show that changes of temperature
are not always the cause of failure. Sir W. Fairbairn adds: "The
danger arising from broken tires does not, according to my opinion,
arise so much from changes of temperature as from the practice of
heating them to a dull red heat, and shrinking them on to the rim of
the wheels. This, I believe, is the general practice, and the unequal,
and in some cases, the severe strains to which they are subject, has a
direct tendency to break the tires."
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