THROW-TESTING MACHINE FOR REVERSALS OF MEAN STRESS. 267 
Sir B. Baker in 1886 gave the results of a series of experiments on iron and steel 
(Am. Soc. Mechanical Engineers ), which were obtained with machines similar to 
Wohler’s, the bars being rotated in one set of tests and subjected to bending in the 
other sets, the repetitions taking place 50 to 60 times per minute. 
About the same time Bauschinger^ published his important memoir on the 
“Variation of the Elastic Limits,” in which it is shown that when the elastic limit 
in tension is raised, the elastic limit in compression is lowered, and that by subjecting 
a material to a few alternations of equal stresses, the elastic limits tend towards fixed 
positions, in which positions he called them the natural elastic limits. Bauschinger 
then proceeded to explain the results obtained by subjecting material to repeated 
stresses, by showing that the limiting range of stress coincided with the difference of 
the two elastic limits. 
Objects of the Research. 
The present research, which was carried out in the Whitworth Engineering 
Laboratory of the Owens College, was undertaken at the suggestion of Professor 
Osborne B-eynolds, who proposed an investigation of “ repeated stress ” on the 
following lines:—(1) The stress should be direct tension and compression; and 
(2) of approximately equal amounts, such tension and compression being obtained by 
means of the inertia force of an oscillatory weight ; (8) the rapidity of repetitions 
should be much higher than in the experiments of Wohler, Spangenberg, 
Bauschinger, and Baker, in fact, ranging as high as 2000 reversals per minute. 
The importance of these points will be seen from the following considerations :— 
(1) By far the greater number of experiments on “repeated stress” have been 
carried out on bars subjected to bending, the ordinary formula for stress in a bent 
bar being used to calculate the stress at breaking, that is, in such experiments it has 
been assumed that the distribution of stress at the breaking-down point is the same 
as for an elastic bar. Calculations on this assumption are not expected to give the 
tensile strength of a material for an ordinary cross-breaking experiment. This difficulty 
is completely overcome, and no such assumptions are necessary, when the stresses are 
direct as in the present work. The (direct) stress in a bar of metal could easily be 
obtained by having one extremity rigidly connected to a part of a machine having a 
known periodic motion, the other extremity being attached to a known weight. 
(2) The tensile stress being in all experiments nearly equal to the compressive 
stress, the elastic limits would, as shown by Bauschinger, soon be changed to their 
natural positions, and the range of stress for unlimited reversals would be this 
natural elastic range. Tf then, the limiting range coincides with the natural range 
it will be constant whatever the rate of reversals. The author considered this point 
an interesting one, and it will be found that most of the tests recorded in this paper 
* ‘ Mittheilungen aus clem Mech. Techn. Laboratoiium in Munchen,’ 1886. 
