New Britain herald., 1928-10-05
DURMUMlN USED
IN AIRnANE WORK
Whining Bus Give Facts
@ Siisty


precisely measured bars of duralum-
In, Heating FREQ on films OF COM
pressed air and vibrating at the rate
Of about 100000 cycles an hour,
maintain 4 continuous loud hum in
one OF the laboratories of the bureau
of standards.
Driven pneumatically. each bar is
kept tn vibration until it cracks, or
has withstood two or three hundred
million cycles of this rapid slight
bending to and fro which subjects
the metallic fibers of each side to
alternate stresses of compression and
tension.
The purpose is to ascertain just
how much stress this aluminium al?
10y can be expected to Withstand
without fatigue" failure when used
in the construction of aircraft. where
ightneSs, dependability and strength
are Important considerations.
The investigation Is being con-
Quoted with the cooperation OF the
national advisory committee for
aeronautics under the personal
supervision of DIR. L. J. Briggs, as
sistant director in charge OF the
bureaus research and testing work.
with special time saving apparatus
devised by himself and DR.. l.. b..
Tuckerman OF the division of
mechanics and sound.
Results thus far obtained indi-
cate that. while a duralumin bar
subjected to steady pressure does
not begin to yield until about &c.
600 pounds have been applied to
the square inch, its fatigue limit
IL considerably less than half that
amount. As far as known, DIR.
Briggs explains. the strength of ali
metals is similarly decreased when
they are subjected to repeated load
ing and unloading.
The value of measuring the
fatigue limits" Of metals beyond
which they cannot be expected to
stand up Indefinitely. he points
out, is emphasized by their use in
high speed machinery and mechan-
ical devices in which they are sub
jected to repeated loading condi-
tons either directly or through vi
bration.
In the case OF steels, which have
been investigated widely, 1t has
been found that the maximum
stress they can be expected to
withstand Indefinitely under such
conditions may be ascertained by
subjecting specimens to about TO-
100000 cycles of vibration under
different loads. With the lighter
alloys, however. it has been dis
covered that tests comprising two
q three hundred million cycles are
necessary to obtain reliable meas
urements.
This raised the problem Of de.
vising means of loading and I'M
loading the test bars more rapidly
than was possible under the old
system. DR.. Tuckerman saw tn,
possibility Of employing discov,
cry made about 80 years ago by
Lord Rayleigh, noted English
physicist, who learned during eX-
periments with sound that, know
ing the dimensions of free bar
and the extent of its movement
from rest position at the center
when IN vibration, 1t was impossible
to compute the fiber stresses.
With experience he had gained
In the aerodynamics laboratory, Dr.
Briggs worked out method where
by the bars could be kept free
during the tests by supporting
them on films of compressed air at
the points where they remain sta
tionary during vibration and de-
veloping a pneumatic drive utiliz-
ing lets Of compressed air direct
ed against them on both sides at
each end.
The device has been able to de-
velop 200 vibration cycles a sec
ond. compared with 80 under the
old method. and to produce fiber
stresses In the free bars ranging
TIP to $4,000 pounds to the square
inch.