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National Advisory Committee for Aeronautics, Technical Notes - Effects of Cyclic Loading on Mechanical Behavior of 24S-T4 and 75S-T6 Aluminum Alloys and SAE 4130 Steel

An investigation was conducted to determine the effects of cyclic
loading on the mechanical behavior of EMS—Th and 75S-T6 aluminum alloys
and SAE #130 steel. Specimens of the three materials were subjected to
various numbers of prior fatigue cycles both below and above the fatigue
limits. Special slow-bend tests at constant deflection rates and temp
peratures were employed to show the effects of prior cycles of fatigue
stressing on the transition temperature to brittle fracture for SAE #130
steel and on the energy-absorption capacity of the aluminum alloys.
Micrographic studies were made to observe and measure crack formation
and propagation and additional Special tests were conducted to supple-
ment the results of the slow—bend tests. These included Charpy impact
tests, microhardness surveys, tension tests, and fretting-corrosion
studies.

Previous tests (reference 1) conducted by the authors have shown
that prior cycles of fatigue markedly raised the brittle transition
temperature of SAE 1020 steel. This has the practical effect of seri—
ously reducing the energy—absorption capacity of the metal, placing it
in a vulnerable condition where shocks or transient overloads may pro—
duce a brittle fracture in a normally ductile steel. The special testing
technique to be described showed its extreme sensitivity in detecting
early damage.

It is the purpose of the present investigation to utilize this
method in detecting any change in properties of EMS-Th and 75S-T6 alum—
inum alloys and SAE 4130 steel due to prior fatigue. In testing the
aluminum alloys it was realized that these metals might well not show
an actual transition temperature, at least within the range of strain
rates and temperatures available with the present equipment. Neverthe-
less, the primary purpose of studying these metals by this method was
to determine whether the special slow—bend test would distinguish in any
way between these two alloys. With the SAE 4130 steel it was expected
that actual transition temperatures would be determined.