Worcester morning daily spy., 1898-03-10
INAN1MArE THINGS GET TIRED.
From the Philadelphia Record,)

have been born tired" This capacity
for becoming fatigued by slight exertion
was always supposed to be Character
istic Of animate bodies; but the scien-
lists, 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 conditions, have discov-
cred that even such inanimate objects
grow tired, and positively lose much of
their strength and elasticity by long
continued subsection to quite moderate
stress or vibrations. When allowed to
rest for a time these metals recuperate
and recover their pristine strength.
| This observation : has been made so
often, and has been so thoroughly
proved by repeated laboratory tests, not
only on q small scale, conducted with
the greatest care and accuracy but also
upon large scale, with full-sized bridge
members and other commercial forms of
steel and iron, that 1t has become gen
orally accepted by engineers as a posi-
tive and well-understood law.
Inc old saying that there Is an ex-
| ceptlon to every law has again been YER
| ihed by report of the committee on
| science and arts of the Franklin Insti-
tute, upon whose recommendation the
city OF philadelphia has recently award
| ed the John Scott legacy premium and
medal for an original investigation
proving that cast iron is an exception
10 this law. Two years ago the an
| nouncement was made at a meeting of
| the American Institute of Mining En-
gineers that cast iron when subjected to
| long-continued shocks or vibrations in
creased in strength, the explanation be-
| ing given that the gain in strength is
due to the release of internal cooling
strains, owing to a Encouragement of
the molecules under the influence of
shock This is, in effect a process of
mechanical anneaIing analogous to an
\ healing by heat. Upon testing ma
chine over 1000 test bars We're broken.
one-half of which had been subjected
to shocks or vibrations in a revolving
iron barrel; the other bars, cast in the
same moulds and from the same metal,
We're not so treated. In all cases the
bars that had been subjected to shock
were found to be stronger than their
companions, 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 a sensation and ex-
cited widespread notice in this country
and in Europe. The tests were repeated
Il by expert ironmasters everywhere, and
the experiments subsequently published
by many observers not only corrobor-
ated the original statements, but the
records of these Independent investiga-
tions in most cases far exceeded the or
iginal claims
The Franklin Institute committee
made a great number of tests showing
large gains in most cases. and the maxi
mum gain in strength exceeded that OF
the original record by more than 100 pei
cent. The Investigations of the commit
tee extended over period of more thar
a. year, and no evidence was offered OF
| a mr. n nH N O
that such tests had ever been previous
ly made. On the contrary, these expert
ments explained for the first time GER
tain peculiar and heretofore mysterious
physical properties of cast iron.
1t has long been thought that iror
castings improved by age, and over 81
years ago Sir Frederick BramWell, theI
the director of the metallurgical work
Woolwich arsenal, found that some ole
! cast iron guns recently tested by hin
were immensely stronger than precisely
similar guns made and tested within 81
days after having been cast. TESTS tt
: destruction of both the old and the nev
guns were then ordered to be made, anC
it was found that gun which had beer
. kept for six years before being tested en-
dured 800 discharges before it burst; an
other gun, kept the same length Of time
did not burst after having been subject
2 ed to 25S2 discharges. Sir Frederiel
! Bramwell reported in 1860 that gums o
! the same description tried thirty day
after casting burst, one at the S4th ant
one at the 72d discharge"
This result" said he, showed that i
was not impossible that the superior
manner in which guns cast some years
ago but recently used, had stood theil
work as compared with those Of moderr
make, was not due, as was commonly
supposed, to the better quality Of the
4 41A&t,
having been cast a long time, and to the
strains that existed in them from un-
equal contraction when originally cast
, having ceased, while the strains in the
new guns were exerting a prejudicial
effect"
The recent discovery so fully corrobo-
rated by the committee on science and
arts of the Franklin Institute, proves
the correctness Of these assumptions.
But it does much more than this, The
tests not only measure quantitatively
the force and value of these internal
str"'- in cast iron, but the important
discovery has been made that such
strains can be quickly removed by sub
jecting castings to shocks or vibrations.
Steel and forged iron are inherently diff
ferent metals, and are differently treat
ed from cast iron in the process of man
ufacture.
1t is, therefore, not incomprehensible
that the effect of vibration upon steel
and iron is to cause fatigue and con-
sequent weakening of the metal. Lord
Kelvin thirty years ago directed atten-
ton to the fact that iron wires kept in
torsional oscillation during the week be-
haved quite differently after their Sun
days rest, and it has recently been
shown that owing to molecular rear
rangement which occurs during rest in
an overstrained bar the modulus of
elasticity" will rise 10 per cent after
three weeks holiday. This remarkable
statement was made by Sir Benjamin
Baker in his presidential address two
years ago at the meeting Of the English
institution of civil engineers.
1t is naturally to be expected that
many facts are still to be learned about
the physical qualities of steel, because
it's application to very large engineering
work is of comparatively recent origin;
but it was said at the meeting of the
mining engineers two years ago that ii
was long thought that practically all
properties of cast iron were known and
that the discovery of hitherto undeter-
mined law of the physics of cast iron at
this late day was, therefore, complete
surprise to the metallurgical world. The
Franklin Institute committee in its re-
port says:
''The principal result has been to
largely upset some of the generally held
ideas on the subject of cast iron, and
where formerly it was considered essen
tial that small or light castings must
be protected from shock, 1t Is now be-
llieved that they be generally strength
ened and relieved of their internal cool
ing strains by judicious treatment in this
way"
Recent specifications for certain cast
ings required to be made of iron of
guaranteed strength contain new
clause stipulating that the test bars