Experimental study of effect of thermal activation on fatigue

This paper presents an experimental study of the effect of intermittent rest periods on the fatigue life of small rotating beam specimens of 75 S-T aluminum alloy. The number of specimens tested under like conditions in each of six different groups varied from 11 to 25; the test results were statistically interpreted to determine the mean life and standard deviation at several stress levels. The behavior of the aluminum alloy was similar to that observed in previous tests of copper; intermittent resting at 225°F. for 20 minutes or at 70°F. for 24 hrs., did not increase (and sometimes slightly decreased) the average fatigue life of specimens of the 75 S-T aluminum alloy. As in the previous tests of SAE 1045 and SAE 4340 steels, it was found that the use of log N (N=number of cycles of loading for failure) resulted in a better correlation than the number N in the presentation of the frequency distribution for fatigue life. Neither the S-log N curve nor the log S-log N curve were straight lines; both were concave upward. The scatter band for an average S-log N curve was not constant for different stress levels; the higher the stress level, the narrower the scatter band.Office of Naval Research, U. S. Navy, Contract N6-ori-71, Task Order IV; Project NR-031-00