On speed effect and recovery in slow-speed alternating strees tests

Abstract

In a series of repeated torsion tests of steel the writer noticed that when the steel had become fatigued the angular strain became considerably greater when the frequency of the cycles of stress was decreased, and vice versâ although the maximum and minimum stresses of the cycle were unchanged The frequencies used were between 2 and 200 cycles per minute. This effect was absent when the cyclic strains were purely elastic, while it became increasingly apparent as non-elastic strain developed. Discussion of Results . The results may be stated thus:— If a piece of the steel used in the experiments has undergone a considerable number of cycles of torsional stress at 2 cycles per minute, then if the speed is changed to 200 per minute, the cyclic non-elastic strain immediately decreases about 50 per cent.; if the piece has endured a large number of cycles at 200 per minute, then on changing to 2 per minute the cyclic non-elastic strain is increased by 50 to 75 per cent. The reduction of range of non-elastic strain found for the above change of speed appears to be the counterpart, for slow speeds, of the comparatively small range of extra-elastic strain discovered by Prof. B. Hopkinson for very high frequencies. After a change of speed from 200 to 2 cycles per minute, or vice versâ , the condition of the steel is not a stable one. After the former change there is a shrinkage in the extra-elastic strain, and after the latter change the strain increases at a more rapid rate than normally would be the case. The shrinkage may be regarded as recovery—though not necessarily as recovery of elasticity—because, if the speed is put back to 200 per minute after an intermediate run at 2 per minute, the new range of the strain is found to be equal, approximately, to the original range at 200 less the shrinkage during the intermediate run. The development of cyclic strain appears to do definitely set back by the "recovery," though it is by no means certain that the progress of fatigue, whatever that may be, has received a corresponding check.