Evening star., 1940-07-02
Of Stors, Men
And Atoms
Notebook Of Science Progress
In Field, Laboratory
And Study
By Thomas r. Henri.

By Thomas r. Henry.
Stones are made to talk and tell their
life histories by device which has Just
been perfected by physicists OF the
United States Bureau of Standards.
A block of stone-or bar of concrete
which is of more practical interest to
the experimenters at presents placed
against the vibrating membrane of a
loud speaker. The vibrations, all within
the sound range, become audible by
natural electrical effect roughly analog-
GUS to that of the old crystal radio set
The block, depending on its size and
shape, has a certain natural vibration
frequency. When@this frequency exactly
corresponds with the frequency Of the
vibrations against the side, the sound
coming from the stone Is most intense.
In this way the natural frequency of
the block is determined. This fire.
quency depends largely on one element
Of strength in building materials-ri-
gidity.
Then the block is subjected to all
sorts Of harsh treatment, such as it
might be expected to undergo in nature
1t is soaked, frozen, baked, pounded,
placed under heavy pressure. These
factors tend to change the rigidity, and
hence the strength, of the material. With
the new procedure the block being tested
can be removed at any time from the
tearing down process and its natural
frequency, or rigidity, remeasured in
few seconds. Thus the physicists can
tell what will happen to it, or to an es-
sentially identical specimen, when it is
used in the structure of a wall or dam
In the past, it was explained by John
Tocker, jr., and F. B. Horniprook, the
process of measuring the strength OF a
specimen has destroyed it. Thus 1t has
been practically impossible to measure
continuous effects of the various ele-
ments of nature. Now the stone can give
audibly continuous story OF its chang
ing state of health.
The method Is being used especially
to determine the changes of strength of
concrete. The actual work is done with
oAAAaiA chio VI cSScIllldllY 1Qen.
tical with that which will be used in a
building operation. The principal cor
dltlons OF temperature and pressure
to which block of similar size will be
subjected in the wall are known. In
this way the physicists are able to tell
the contractor more than ever before
on the prospective strength OF the
structure he builds, after certain lapses
of time
1t may also be possible by the same
method to determine the actual strength
OF standing wall. Within narrow limits
the natural vibration of concrete of
normal rigidity is known. The degree
10 which the slab taken from the wall
has departed from this normal can be
determined and the assumption made
that 1t is a representative specimen.
The method actually is applicable the
Bureau OF Standards physicists say. to
almost any building material for which
a natural vibration rate can be obtained.
The loud speaker will transmit to the
specimen vibrations ranging from to
10000 cycles a second, which would
practically cover the range from wooc to
fine steel. The technique is an adapta-
ton Of an English method for measuring
the strength Of structures. It Is ex-
tremely exact. The physicists do not
rely upon their ears to determine the
natural vibration period The sound
waves appear on screen as simmer
ing green lines, the length of which can
be measured within fraction OF a miln-
meter
In its cruder FORM the physicists saw.
the technique has been in practical Use
for centuries. 1t is the same in prjn-
ciple as that of the workman who taps
the wheels OF train when 1t makes
fairly long stop at station to deter
mine If any cracks, or weak spots, are
present. In this case the reliance is on
the trained ear alone and the work.
man Is satisfied If he gets a clear. bell
like tone. 1t is also common for per-
sons buying crockery to tap a plate or
cup with their fingernails.
The method now in use by the Bureau
OF Standards, however, is a thousand
fold more accurate. A similar principle.
but on much larger and cruder scale,
now is coming into use for the testing
OF whole buildings and, especially in
Germany, of railroad bridges. 1t is also
used in parts OF the earth subject to
earthquakes to make sure that the ma
tural frequency of building is not the
same as the fundamental frequency
OF a surface earthquake wave, about
three vibrations second
Tests have shown, the physicist say,
that specimen of concrete may lose
as much as 30 per cent Of its Rightly
not quite the same as strength but an
important element Of h-without any
change being apparent to the eye even
OF trained observer. Thus there Is
no easy way to follow the course of
deterioration.