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National Advisory Committee for Aeronautics, Technical Notes - Effects of Range of Stress and of Special Notches on Fatigue Properties of Aluminum Alloys Suitable for Airplane Propellers

Laboratory tests were made to obtain information on
the load~reSisting properties of X765—T aluminum aIIof”
when subjected to static, impact, and repeated_loads.
Results are presented from static—load tests 6T‘unnotched
specimens in tension and in tersion and of notched specié
mens in tension. Charpy impact Values obtained fess bend
tests on notched specimens and tension impactffialu95 £93.”
both notched and unnotched specimens tested at‘stttrsi_ '
different temperatures are included. Ehc endurancoflimé
its obtained from repeated bending fatigue tpéts made”_
on three different types of testing machino_are_gitbn fOr
unnotched polished specimens, and the endurance'limits of
notched specimens subjected to siX'different ranges of__'
bending stress are also reported. ” "‘”"

The results indicated that: (a) polished_rectangular
specimens had an endurance limit about 30 percent lessfthan
that obtained for round specimens; (b) a compariSon 5f anL'
durance limits obtained from tests on three different types
of machine indicated that there was no apparent effect of
speed of testing on the endurance limit for the'range of
speeds used (1,750 to 13,000 rpm); (c) the fatigue strength
(endurance limit) of the X7SS-T alloy was great17_decreased
by the presence Of a notch in the specimens; (d no com—
plete fractures of the entire specimens cesarean in notched
fatigue specimens when subdected to stress'affilés for which
the mean stress at the notch during the—efEIe Was"a EBmJ“
pressive stress; for this test condition a microscafiic- "
cracking occurred near the root of the notch and was used
as a criterion of failure of the specimen; (9) as the mean
stress at the notch was decreased from a tensile (+) stress
to‘a compressive (~) stress, it was found that the alter-
hating stress that could be'Supcrimposod on the mean stress
in the cycle without causing failure of the‘specimens was
increased.

Although the fatigue strengths of most of the common—
ly used wrought aluminum alloys as determined by labor—
atory tests of polished specimens are fairly well known,
little information seems to be aVailable on the relative
notch sensitivity of these alloys, especially for condi—
tions in which the stresses do not occur in completely re—
versed cycles. For applications of aluminum alloys in
service in a member such as a propeller blade, small
notches are often produced in the polished surface of the
blade by stones being thrown up in the backwash of the
propeller during a take-off or when taxying on the field.