The morning news. [volume] (Savannah, Ga.) 1887-1900, 1898-03-03
FATIGUE OF METALS.
Cast Iron Exempted from the other
wise General Law.


binary conversation, commonly said to
have been born tired This capacity for
becoming fadgued by slight exertion was
always supposed 10 be characteristic or
animate bodies; but the scientists, who
have been probing with their microscopes,
spectroscopes, and other scopes into the
molecular constitution Of steel or iron and
testing their strength under varying con-
ditions, have discovered that even such
inanimate objects grow tired, and posi-
lively lose much of their strength and
elasticity by long-continued subsection TO
quite moderate stress or vibrations. When
allowed TO rest for time these metals re-
cuperate and recover their pristine
strength. This observation has been made
so often, and has been so thoroughly
proved by repeated laboratory tests, no;
only on a small SCALE conducted with the
greatest care and accuracy, but also upon
large scale, with fuli-sized bridge mem
bers and other commercial forms of steer
and iron, that it has become generally ac-
cepted by engineers as positive and we'll
understood law
The old saying that there is an exception
TO every law has again been verified by a
report of the committee on science ard
arts of the Franklin Institute, upon whose
L
has recently awarded the John Scott leg
acy premium and medal for an original
investigation providing that cast iron is
an exception TO this law. Two years ego
the announcement was made at meet
Ing of the American Institute of Mining
Engineers that cast iron when subjected
to ong-continued shocks or vibrations in
creased in strength, the explanation being
given that the gain in strength Is due to
the release of internal cooling strains, CIV
ing to Encouragement of the molecules
under the influence of shock This is. in
ERECT process of mechanical annealing
analogous to annealing by heat. Upon a
testing machine over LOCO test bars were
broken, one-half of which had been sub
| Jected to shocks or vibrations In a revolv-
ing iron barrel; the other bars. cast in
the same molds and from the same metal
were not so treated. In sll cases the bcrs
that had been subjected to shock were
found to be stronger than their compan-
ions, the average being over 10 per cent,
and the maximum gain, as shown in the
published tables, being nearly 20 per cent.
This surprising statement created o SEN
satlon and excited wide notice in this coun-
try and in Europe. The tests were repeat-
ed by expert iron masters everywhere, and
the experiments subsequently published by
many observers not only corroborated the
original statements but the records of
these independent Investigations in most
cases far exceeded the original claims.
The Franklin Institute committee made
great number of tests showing large
gains in most cases, and the maximum
gain in strength exceeded that of the
original record by more than 100 per cent
The Investigations of the committee ex-
tended over period of more than year
and no evidence was offered. or found af-
ter prolonged search, indicating that such
tests had ever been previously made. On
the contrary these experiments explained
for the first time the certain peculiar and
heretofore mysterious physical properties
of east iron.
It-has long been thought that iron cast
ings improved by age, and over thirty
years ago Sir Frederick Bramweu. then
the director of the metallurgical work at
Woolwich arsenal. England, found that
some old cast iron guns recently tested
by him were immensely stronger than
precisely similar guns made and tested
Within thirty days after having been cast
Tests TO destruction of both the old
and the new guns Were then ordered to be
made, and it was found that a gun which
had been kept for six years before being
tested endured 800 discharges before II
burst: another gun, kept the same length
Oh time, did not burst after having been
subjected to 2,5S2 discharges Sir Frede-
rick Bramwen reported in 1860 that gums
or the same description tried thirty. days
after casting burst one at the eightv
fourth, and one a t the seventy-second dis
charge." This result said he, showed
that II was not impossible that the sumo
Ilor manner in which guns cast some
years ago but recently used had stood
thelr work as compared with those of
modern make was not due, as was com
monty supposed, to the better quality of
the metal used in those days but to their
having been cast long time, and to the
strains that existed in them from unenual
contraction when originally cast. having
ceased, while the strains in the new guns
We're eXerting a prejudiciaI effect
Inc recent discovery, so fully c6rrohor-
dteu Dy the committee on science and arts
Oh the FRANKLIN Institute, proves the cor
RICHNESS of these assumptions. But il does
much more than this. The tests not onI,
measure quantativeiy the force and val
ue of these internal strains in cast iron
but the important discovery has been
made that such strains can be Quickly re-
moved by subjecting castings to shocks
or vibrations. Steel and forged iron are
inherently different metals, and are DIFF
ferenuy treated from cast iron IN the
process of manufacture.
IL is, therefore, not incomprehensible
thal tAS Suecl Oh VIBRATION upon steel and
iron is to cause fatigue and conseonent
weakening, of the metal Lord Kelvin
thirty years ago directed attention 10 the
fact that iron wires kept in torsio1al os-.
Citation during the week behaved o,,ita
differently after their Sundays rest. and
1t has recently been shown that owing 10
molecular re-arrangement which occurs
during rest in an overstrained bar the
modulus of elasticity win rise 10 per
cent. after three weeks holiday. This re-
markable statement was made bv Sir
Benjamin Baker in his Presidential ad-
dress two years ago at the meeting of the
English Institution of civil engineers.
1t is naturally TO be expected that manv
facts are still 10 be learned about the nhv-
steal qualities of steel, because its applica-
ton TO very large engineering work is of
comparatively recent origin; but it was
said at the meeting of the mining ene,-
neers two years ago that it was lone
thought that practically all properties of
cast iron were known, and that the dis
covery of hitherto undetermined law of
the physics of cast iron at this late dav
was therefore, complete surprise 70 thu
metallurgical world The Franklin In
stitute committee, in its report, says:
THE principal result has been TO largo
ly upset some of the generally held ideas
on the subject of cast iron, and where for