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National Advisory Committee for Aeronautics, Technical Notes - Effect of Fatigue Stressing Short of Failure on Some Typical Aircraft Metals

A number of tests were made to determineflthe.e£fecb cf
service stnesses on the impact resistance, the X—ray diffrac-
tion patterns, and the microstructure of 255 aluminum alloy.
The average impact resistance was found to be unimpaired even
in material cut from specimens previously broken bv repeated
stress. The X—ray diffraction patterns showed no structural
change resulting from the fatigue-stressing of the alloy.

Two structural conditions known as slip—plane precipitation
and veining were observed.-1$T§ining“'in the-structure could
be made to disappear and reappear by alternate solution heat
treatment and age—hardening} It was-concluded that the fa-
tigueqstressing was not responsible for these structural
features and the endurance limit was not reduced by it.

In attempts to detect and'evaluate damage resulting ‘
,from fatigue-stressing prior to the start of_cracks, the'ime
pact behavior pf-normalized-SAE.3£4130 steel was determined
after a.variety of repeated stress'treatments. The results
are valuabLe in showing- the effect of fatigue- -cracks on~'im-_
pact resistance, but they- do not give any indication of dam-
age occurring- beforegthe: cracks had formed.

Damage of- this kind was evaluated by determining the:
decrease i.n .enduranc_e due to stressing aboye- the fatigue
limit. In a large proportion of the tests made, a deflec-
ltion method was used toydetect crack formation, se.that the
damage- measurement could:be§:defdnitely limited to the pre-
crack.. stage._ The result.s showed that the apparent rate of
damage depends on- the stress- history. If the prestress is :
higher_ than: the test stresssthe.damage occurs rapidly at
first, then more slowly;. -The reverse is true if the damage'
is inflicted at a stress lower than that used to measure it.

Fatigue is ong;of the.most important factors contribut—
ing to mechanical failure in service of highly stressed
structural members of aircraft. It is apparent that the 4
metal is being damaged throughout the period of,fatigue_
stressing; but the damage cannot be detected until visible
cracks have formed, and when this occurs the life remaining
before failure is short; For several years a research proj~
act at the National Bureau of Standards has been directed
toward learning more about the damage occurring in the pre—
crack stage of fatigue of metals.