The Salt Lake herald. [volume] (Salt Lake City [Utah]) 1870-1909, 1894-07-22
FATIGUE OF METALS.
ITS RESEMBLANCE TO THE FA-
TIGUE OF THE MUSCLES.
Some Interesting Experiments By1
Prof. Michael Foster-Reeupern-
tive Power of Metals Demonstrate
ed,
[The Literary Digest.I

Lie AllStdLy OlSCol-I
The term fatigue" applied to metals,
has a technical significance. 1t is the con-
dition which results in the breaking
down of parts of machinery after they
have worked satisfactorily for months or
years. The causes which give rise to this
condition are explained by D. s. Smart in
Knowledge" London, June l.. in an
article in which he argues that the term
fatigue" in the technical sense in which
it is applied to muscles, may with equal
propriety be applied to metals. He says;
Fatigue of metals is an expression
which has come into use only in recent
years, and it describes a condition of the
material not previously understood. The
expression stands for Straining of the
relationship to each other of the molecules
of which the metal is constituted.
I'm the metals. there is a point in their
resistance to pulling. bending, or crush
ing, known as the elastic limit. the point
at which permanent strain commences.
The elastic limit of iron or mild steel, for
example, in its normal condition, IS
reached, roughly speaking. when about<
half the breaking load is applied. If thel
stresses brought to bear upon piece ofF
these stresses without injury. however
long they may be applied. If the stresses
exceed the elastic limit, in however small
a degree, fatigue OF the metal Will result,
and, if they are continued breakage soon
er or later will inevitably take place. To
guard against accident from such 1nsldf-
GUS fatigue, it has now become usual, in
the best practice, to withdraw permanent
" from use parts of machinery, such as
railway cranks and carriage axles. upon
the soundness Of which the safety or
many lives may depend, after they have
performed a certain number of revolu-
tions, even although no flaw or sign OF
injury can be detected.
'If, however, metals are strained be-
yond the elastic limit, but not broken,
and if the stralning is not continued the
material will recover its elasticity by reSs1
alone. Some years ago Professor Is. W.
Kennedy demonstrated by many eXper1-i
ments the recuperative property of metals
. a q NT
strained in a testing beyond the
elastic limit, and so weakened thereby,
| that if they were tested again the follow
ing day they would take permanent set
at one-third or less of their former load.
vould, if allowed to rest for about two
years, be found not only to have recov-
cred theirs original elastic limit of strength
but to have exceeded it, and to have be-
come stronger than before in the direc-
ton in which they had been pulled II
the period of rest was materially short
ened, the restoration of strength was
found to be incomplete.
The writer then cites some experiments
of Professor Michael Foster, in his Cam
bridge University Rede lecture last year,
which went to show that this capacity 10
recover tone after exhaustion due to eN-
ceSSive strain Is a property of muscle as
well as of metal In one of his expert
ments, Professor Foster caused the mus
cles severed from the leg of frog to
stimulus until thoroughly wearied-the
expression he used-aud no longer able TO
respond TO the electrical eXClIemenJs ana
then showed that. with rest alone, they
recovered their elasticity and were able
to resume work as before. Professor FoS-
ter demonstrated that the WEAKNESS WaSN
in the muscle and not in the nerve, and
argued that since the muscles recovered
tone by rest alone, it was evident thar
the renewed vigor was due entirely TO
readjustment of molecules-to their recoV-
cry from a state of fatigue. 'It is there
fore clear saws Mr.. Smart. that the
fatigue which we experience in our own
bodies must be largely fatigue in the tech
meal sense, in addition to WEAKNESS pro-
per or wornness. Rest is. therefore. re-
qured, not only to enable wasted tissues
to be restored by fresh material from the
blood, and by the carrying away of waste
material, but also to afford opportunity
for the strained muscles to recover d
IF" continues Mr. Stewart. "a piece of
machinery is of ample strength for it,
work, it does not undergo fatigue and Is
safe for any length of time if not W0TRt
out by friction or by rust. In like mahnerj
it is only those parts of our bodies which
are too light for their work which be-
come really fatigued- If one is accus-
toned only to very light work, he be-
comes fatigued when he undertakes hea-
vier work. his muscles are untitled for
it. and are consequently strained. In fact
all heavy work is fatiguing because our
bodies are only fitted for the average
work. Of the race, and if heavy work were
{ to be done without excessive ia
tigue. special training through many
generations would be required. For
work only little above the
average, a course of training is neces-
sary. If, as in the case of machinery. the
bodies OF men and animals were unpro-
vided with a sensitive nervous system to
register fatigue and demand rest we
should continually be having sprained and
ruptured muscles and broken bones. .
The necessity for such rest, however. is
apparent. quite apart from any theory OF
fatigue in the technical sense. as the rest
is evidently required in which to repair
the waste of tissue and of unguent which
must continually be going on"