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National Advisory Committee for Aeronautics, Technical Notes - Biaxial Plastic Stress Strain Relations for 24S-T Aluminum Alloy

The object of this investigation was to determine the yield m
strength, ultimate strength, ductility, and plastic stress—strain
relations for ens-T aluminum alloy when subjected.to biaxial stresses. >é
Both biaxial stresses considered were tensile and the influence of various
biaxial stress ratios on the mechanical properties was determined.
Biaxial tensile stresses were produced in_a tubular specimen by a
specially_designed testing machine. This testing machine applies both
an axial tensile load and internal pressure to the tubular specimen,
thereby producing biaxial tensile stresses in the tube wall. Strains
were measured in the plastic range up to rupture by means of special
electric SR- 4 clip gages. Nominal stress- strain diagrams for the elastic-w
range and true stress- strain diagrams for the plastic range were plotted
for various biaxial stress ratios. ”—

The data were interpreted by a generalized St. venant theory in an
attempt to predict the biaxial stress— strain relations in terms of the
uniaxial tensile stress- strain relations. The stress- strain relations,
as predicted for combined stresses by this theory agree approximately
with the test results. The yield—strength values, as determined by
tests, agree Quite well with the distortion energy theory, and the
ultimate and fracture strengths agree well with the maximum stress theory._

Stress-strain data were obtained from flat control specimens cut
from the tubular specimens and compared with tension test data obtained
from tubular specimens. Except for ductility values, the results show
that the tension test results for these control specimens agree with the
values for the longitudinal tension tests on the tubular specimens.

Aircraft members may be subjected to stresses beyond the yield
strength of the material. In many cases the stresses are not simple ,,
stresses acting in one direction, but the stresses act in several
directions; that is, the biaxial stresses often occur in place of
uniaxial stresses. It is of importance, therefore, to determine the
plastic stress—strain relations and the mechanical properties in air—
craft materials subjected to coMbined stresses.