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National Advisory Committee for Aeronautics, Report - Cooperative Investigation of Relationship Between Static and Fatigue Properties of Wrought N-155 Alloy at Elevated Temperatures

SUMMARY

Extensive data were obtained relating properties of wrought
N~155 alloy under static, combined static and dynamic, and
completely reversed dynamic stress conditions. Time periods
for fracture ranaed from 50 to 500 hours at room temperature,
1,000°, 1,200°, 1,350°, and 1,500° F. Correlation of the
data showed:

1. Increasingly higher percentages of dynamic stress super—
imposed on the steady loads for rupture in 50, 150, and 500
hours were required to change rupture strength appreciably as
the temperature increased. At 1,500° F the dynamic stresses
approached the completely reversed fatigue strength before a
substantial reduction in fracture time occurred.

.9. A given amount of superimposed dynamic stress had
decreasingly less ezfect on the rupture time as the steady stress
was reduced to increase rupture time.

3. Completely reversed stress tests showed fatigue strengths
which were of the order of 40 to 60 percent of the static tensile
strength over the entire temperature range. As the temperature
and time periods considered were increased, the fatigue strengths
increased relative to static rupture strengths to values twice the
rupture strength for 500 hours at 1,500° F. This means that
the material can tolerate a cyclic load of larger magnitude than
a steady load for fracture in time periods longer than some limit-
ing value under conditions where creep occurs.

4. Limited data indicated that under combined stresses up to
67 percent of the steady load at 1,350° and 1,500° F the fatigue
stress did not appreciably alter the creep characteristic through
the second stage of creep. Apparently superimposed fatigue
stresses exerted theirmain influence during the third stage of creep,
at least for this particular test material and these test conditions.

5. Properties evaluated on the basis of number of cycles to
failure dizfered from those evaluated on a time basis only when
the properties were time dependent. Thus two fatigue machines
operating at different cyclic speeds gave differing values on a
cyclic basis when a fatigue limit was not attained. This was
particularly evident in combined stress tests at 1,350° and
1,500° F.

6‘. Increasing amounts of fatigue loading decreased elonga—
tion from the rupture test progressively to very low values in
completely reversed stress tests.

7. Bending tests gave slightly higher fatigue strengths than
completely reversed arial stress tests. The difi'erence decreased

with increasing test temperature so that the two types of tests
gave comparable results at 1,600° F. There seemed to be little
definite ezfect from variation in cyclic speed. Axial tests did
not show the “knee” typical of many S—N curves. The knee
also tended to disappear with increasing temperatures in. all
tests.