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An Investigation of Strain Aging in Fatigue

Fatigue tests were performed on Low carbon steel at a rate Of 2, 600 
Load cyclee per minute and •t temperatures up to 7000 F. All teste Were 
conducted at a conetant stress amplitude of 35. 000 lb. per sq. inch. A num- 
ber of specimens, usually iO, were tested at each selected temperature, and 
the regult8 were anaiyced statistically. It was found that a penk occurred 
in fatigue life in the region Of 4500 F. There Wag a pronounced tendency 
for the acatter in life to increase as the mean life increased. 
Subsequent tests established that the magnitude of the observed peak 
depended on the amount of carbon and nitrogen in solid solution go that the 
effect could logically be classified ag one Of Strain aging which takea place 
during the course Of the test. When both carbon and nitrogen were present 
the peaking temperature Was somewhat higher than when nitrogen alone Tag 
present, which Beemed to indicate an interaction between the two types Of 
interstitial atoms. 
A recently deveioped theory Of strain aging hag been applied in 
which carbon and nitrogen atoms are preeumed to strengthen the metal by 
diffusing to dislocaiione in the crystal lattice. To appiy this theory tu 
fatigue conditions, the following a88umptionB were made: 
(Z) The time available for aging was of the same order as the 
time Of one loading cycle. 
(2) The distance to be travelied by the carbon and nitrogen atoms. 
in Order to re -anchor dislocations in fatigue affected zones, 
Wag 2 x 10—6 cm. (This corresponds to the distance they 
travel under tensile conditions and approximates to the closest 
approach Of dislocations in a heavily cold worked metal. ) 
Utilizing these assumptions, the calculated peaking temperature 
egreed with the observed value. A curve was constructed Of the pre— 
dieted temperature for peak life over a Wide range of cyclic rates.