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National Advisory Committee for Aeronautics, Technical Notes - Extension of Pack Method for Compressive Tests

The pack method for determining compressive stress?"
strain.graphs described in EACA Report No. 649 has been
modified to extend its application to thinner gages and
stronger materials. The principal modifications consisted
in the provision of additional support against instabilitv
bv cementing the specimens of the pack torether with fused
shellac and the provision of special clamps that hold the _ ___
specimens in the pack together while the test is in prBFZ—r '
ress. The shellac was feund to increase the buckling load
of the pack without anv appreciaole effect on the compres— _ _
sive stress-strain graph of the material. The extended ..T7
pack method described.-in this note has made possible the '
application of stresses in excess of 220 kips per square
inch to sheet material having a thickness o.f only 0.02 inch.

The pack method described in reference 1 has permitted
the application of compressive stresses up.to 60 kips per_
square inch to packs conposed_of 13, 7; and 5 specimens of
0.032%, O.0647, and 0.081—inch aluminunéalloy sheet, re—
spectively, and of stresses up to 180 kips per.squafe”inch
to packs, composed of.5-specimens of 0-05—inch steel sEeet.
without failure- by instability. Extension of thi-s method
to still thinner gages and higher .stresses than heretofore L
obtained was urgently needed to test certain sheet materialfl
for aircraft submitted to the National Bureau of Standards
by the Bureau of Aeronautics,. Navy Department. “*‘

A number of modifications of the pack method were ac-
cordingly considered. Successful results were obtained
with the technique described in'this report; which was dc;
veloped with the support of the'Bureau of Aeronautics, Envy
Department, and the Rational Advisory committee tor Aero~
nautics. All details of the procedure are given because ___
time has not .cermitted an exploration of limitations of
the technique.

The testing procedure is an extension of the method
described in reference 1 for making pack compressive tests _
and involves the use of additional means of support which _w”_
increase the lateral stability of the pack so that it will
not fail before the yield strength of—the material is
reached.