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National Advisory Committee for Aeronautics, Technical Notes - The Influence of Plastic Deformation and the Heat Treatment on Poisson's Ratio for 18;8 Chromium Nickel Steel

An investigation of the elastic properties bf high
strength aircraft metals has been conducted at the
National Bureau of Standards during the past 8 years
under the sponsorship of the Rational Advisory Committee
for Aeronautics. Reports have been presented upon the
tensile elastic (references 1, 2, and 3) and the tor—
sional elastic (reference 4) properties of metals. The
elastic properties studied were: (a) the proof stresses
which produce various proof sets and (b) the elastic
modulus and its variation with applied stress. The role»
tionship of these indices to cold work and to annealing
treatment also was investigated. The method of obtaining
correlated stresswstrain and stress—set curves and of
deriving-the elastic properties from these curves is
given in the afore—mentioned publications (references 1.
2, 3, and 49.

The present paper correlates the tensile and shear
elastic secant moduli of 18:8 Gr—Ni steel, by deriving
from them an effective value of Poisson's ratio. This
effective value becomes identical with the classical
value (ratio of unit lateral contraction to unit axial
extension under axial loading in the elastic range) only
for isotropic material obeying Hookes law. The effective
value of Poisson‘s ratio as defined in this paper cannot
be used for computing stresses under conditions of multi—
axial strain beyond the range of validity of Hooke's law,
or for anisotrOpic material. It may be used, however,
as an index of anisotropy — that is, an index for a change
in properties with direction. The significance of vari—
ations in the effective value of Poisson‘s ratio with the
degree of cold work or with thermal treatment is discussed
in the present paper.

Bar stock 18:8 Cr—Ni steel was_used in preparing
specimens for.the tensile tests (references 1, 2, and 3 1
whereas tubular stock material was selected for the tor—
sion tests (reference 4). In order to obtain reliable
values of Poisson's ratio from tensile and torsional
modulus data, the materials tested in tension and torsion
should be of nearly the same chemical composition,

structureziand'hardness.""A'"half—hard" rod material.and
the "-hard" grade tubular' material were' selected for this
"evaluation ; in' an earlier’ report (reference 4) it was
shown that of the- cold—worked- rbd and tubular materials,
these two Were in- the most nearly equivalent cold—worked
condition. Rockwell hardness readings for both metalsgave'
valuesranging from 039 -to C42.’ Moreover. microscqpic
examination orf the 'cross sections of the tubular and rod
material in the "as— received“'condition indicated similar
microstructurefl Table 1 shows that the compo_sitions of.
these_' two materials do not differ greatly.