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National Advisory Committee for Aeronautics, Technical Notes - The Compressive Yield Strength of Extruded Shapes of 24ST Aluminum Alloy

Tests were made by the Aluminum Company of America
on 26? extruded shapes of 24ST-aluminum alloy selected at
random from plant production to determine the relations
between the compressive yield strength and the tensile . _
properties of the material. The samples were divided in-
to three classes according to thickness: less than 0.250
inch, from O.l50 to 1.499 inches; and 1.500 inches and
over. Ratios were computed for the three classes by which
the compressive yield strength could be estimated from
either the tensile strength or’the tensile yield strength.
The'assumption that the compressive yield strength is
equal to the tensile yield strength was found to be fairly
accurate for the thicknesses 1.500 inches'and over' not
seriously in error for thicknesses from 0. 250 to 1. 49 9
inches, but unsatisfactory for the sections less than
0. 250 inch.

Navy Department specification-46A9c and Federal
specification QQdA-354 r-‘equire extruded Shapes o‘f 245T _n_
aluminum a'lloy to have tensile yield strengtns not less
than certain minimum values --tha-t are d-ependent upon the
thickness of the shape. Minimum compressive yield --__u. _:
strengths, although perhaps more i—mportant to the engineer
than minimum tensile- yield str-engths, are not specified
because they are too dif-ficult to determine to be included
in routine inspection tes.ts and ar_e not needed for the
control. of quality. In the -absence of specific informa—
tion concerning compressive yield strengths, it has been
common practice in the past to assume that the compres-
sive yield strength was equal to the tensile yield _
strength, even though it was generally understood that M

materials which are straightened by stretching usually
have compressive yield strengths lower than their tensile
yield strengths. Preliminary.tests disclosed that the
difference between compressive and_tensile,yield strengths
might be large enough to require attention in_designI and
it was decided to undertake a complete investigation-in—
volving a large number of samples selected at_random from
the general run of commercial production. The timely
development of the "pack" method for determining compres—
sive yield strengths of thin sections was an important
factor in making this'investigation possible. (See refer—
ence l.)