Harrisburg telegraph. [volume] (Harrisburg, Pa.) 1879-1948, 1919-05-07
How Metals Get Tired
BY Uverwork
BY Garrett T.. Seryiss.

BY Garrett p. Serviss.
Here, from popular viewpoint,
is an unpromiSing looking technical
question, which nevertheless opens
up glimpse of some of the most
| fascinating mysteries of the world
of matter, which affect us every day
| of our lives:
| Can tailshaft of an ocean-go-
| ing steamer get in a state where
you would call it tired from con-
slant work? Would rest of d
certain period increase it's strength
and better it's condition."
A tailshaft is the after section
OF propeller shaft, and one may
say that II any piece of machinery
| can get tired a tailshaft ought
to. Engineers use the term IA
tigue" to express an effect of over
strain on metals. Some say that
this is a misuse of the word, though
convenient; others affirm that it'll
| expresses a real and close resem-
blance to the effects of overwork on
living organism. In a broad, phil
osophical sense, the last assertion
may be substantially correct. The
road that leads from the inorganic
| into the organic world. appears
more continuous and less obStructed
the further the exploration of it is
pushed.
Certainly the manner in which a !
''fatigued" piece of structural iron
or steel RECEIVERS or Can be helped |
to recover its strength and, so to
speak, its working power, upon be- |
ing relieved from overstrain that has
injured or weakened it, is interest
ingly suggestive of what happens to
horse or man In similar cir
cumStances.
Speaking imaginatively, to be sure,
but still within the limits of reason
able analogy, one might say that
overworked metal "recuperates"
through rest much as an overworked |
muscle does. But the process is not
so rapid, and perhaps a stronger
tonic" needs to be applied from ex-
ternal sources.
The engineer in dealing with the |
question of the strength of his ma
/ terials uses certain terms which need
| to be understood by all who think |
of the subject even in the most gen.
eral way. When a force is eXerted
upon a body, that boJy Is said to
experience STRESS" and any
change of shape, however small, that
the stress produces in the body is
called a strain" Elasticity is that
to recover it's original shape when !
stress is removed. The elastic
limit" is the measure of greatest
stress that can be put upon a body
without straining it beyond it's power
to recover completely, or nearly
completely it's original shape. If it |
cannot recover it is said to have a |
permanent set" ultimate strength
is the measure of the stress re-
quired to produce rupture of the
body. working stress" is the |
amount of stress that is regarded |
in any particular case, as safe 10 |
apply, and the factor of safety" is |
the ratio of ultimate strength to the |
working strength. 1t should never
be less than 8, and often it exceeds |
5; that is to say, the stress is kept
down to from Ia to lis, or even |
less, of the ultimate strength. 1t VIII |
help to clearness of understanding |
if the meaning of these terms is
kept in mind.
One remarkable fact developed by
experiment is that time plays a not
able part in the results of the appl- |
cation of stress beyond the elastic |
limit. When the stress has been |
pushed beyond that limit a certain
time must elapse before the full
strain corresponding to that particu-
lar stress will be developed. And if |
by Il certain stress, constantly ap- |
piled, i. e., a certain load" a con- |
stant strain is maintained, after |
some time the stress required to |
maintain that degree of strain dimin-
Iishes. Still more remarkable is such |
an experiment as this:
If piece of iron or steel is SUBJ |
iected to a given load and the strain |
is pushed beyond the elastic limit, |
and then the load, or stress, is re-
moved and kept off for a consider
able interval of time, it will be found
when the load is again applied, that
the ultimate strength of the metal
has in the meantime increased. This
comes about through a hardening
process that has taken place during
the interval in the metal.
1t might be likened to the "hard- |
ening" of muscle by continued eX- |
ercise, although the analogy is not
precise Overstrain produces in the |
metal a change of properties which, |
it has been found, may continue to |
go on during weeks and months af-
ter the stress producing the strain
has been removed.
These effects are the result of |
something which occurs in the metal
affecting the arrangement of its con-
stituent crystalline grains Examina-
ton with the microscope shows that
overstrain alters the shape of the
metallic grains, elongating them in
the direction of the force. Alterna-
tions in the direction and also in the
amount of stress produce this ia
tigue" But rest, especially when
accom- panied by the application of |
heat, as in annealing, more or less. |