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National Advisory Committee for Aeronautics, Technical Notes - The Direct Stress Fatigue Strength of 17ST Aluminum Alloys Throughout the Range from 1-2 to 500,000,000 Cycles of Stress

Fatigue tests were conducted on six specimens mad-e
from 3/4—indh—diameter l7S—T rolled~and-drawn rod for the
purpose of obtaining additional data on the fatigue life
of the material at stresses up to t'he static strength.
The specimens were tested in direct tension using a stress
range from zero to a maximum in tension. A static testing "
machine Was used to apply repeated loads in the case of .
the first three specimens; the other three specimens were ". =~
tested in'a direct tensionécompression fatigue machine._ '

The direct—stress fatigue curve obtained’for the ma—
terial'indicates that, in the range of stresses above
about two—thirds the tensile strength the fatigue'strength
is higher than might be expected by simply extrapolating
the ordinary curve of stress plotted against number of
cycles determined at lower stresses. -

Fatigue—test results are ordinarily plotted with
stress as ordinates and number of cycles as abscissas.
The fatigue—strength curves for aluminum alloys usually
cover a range.of cycles starting at some value around" _ . ;
50,000 and extending to about 500,000,000. The larger of
these two numbers is an arbitrary limit used- to define '
endurance limit; and the smaller is a value repre_senting l
limitations imposed by capacity of testing _equipment, éx—_"‘ ”
cessive yielding of material in specimens, and various '
other practical considerations. For some applications of
aluminum alloys, useful engineering inform_ation would be
provided by extending the curves backward to smaller num—
bers of cycles and higher stresses. Such an extension of
a curve is difficult- however, because some of the stresses
involved exceed the yield strength of _the _material_with
the result that there is considerable yielding of the
specimen,and it is therefore difficult to maintain the
loading conditions constant dur'ing testing. In the case
of bending—fatigue. tests there is the additional diffi—
culty of accurately determining the extreme fiber stress
after yielding begins.