ON STRESS DISTRISOTIONS IN ENGINEERING MATERIALS. 289 



desired value between the extreme cases of reversals of simple bending 

 and reversals of simple torsion. 



The fatigue-testing machine in which the experiments were made was 

 specially designed and constructed for the purpose of the research in the 

 engineering workshop of the National Physical Laboratory. The general 

 principle of the machine will be seen from fig. 1, which is a diagrammatic 

 representation of the manner in which the combination of bending and 



Fig. 1. 



twisting is applied to the specimen. In the position shown, the cross- 

 section of the specimen at S is subject to a twisting moment "WD, and 

 to a bending moment Wd. When the head has turned through 180" 

 the moments will be equal in amount but opposite in sign. When the 

 head has turned through 90° from the position shown the maximum 

 stress will be that due to a bending moment WD plus that due to the 

 direct loading, but as in all cases this stress is below the known fatigue 

 limit of the material under reversals of simple bending, its effect is supposed 

 to be negUgible, and the specimen is assumed to be subject to reversals 

 of the combination of bending and twisting moment alone. 



The form of specimen adopted is shown in fig. 2a,* which represents 

 a plan of the testing head with the specimen and hanger in position. By 

 varying the length of the collar c, and also, if necessary, the position of the 

 neck of the specimen, relative to the axis of rotation of the specimen, it 

 will be seen that the ratio of bending moment to twisting moment can be 

 varied within fairly wide limits. 



For the experiments in which the stresses were practically reversals 

 of simple shear, the arrangement described above was not suitable, and 

 the method of making the torsion tests is shown in fig. 2b. In this case 

 it will be seen that the fatigue of the specimen takes place simultaneously 



' It was found on trial that the variation ol sectional area in the neighbourhood 

 of the neck of the specimen, shown in fig. 2a, was a source of weakness and in the 

 tests, of whioh the results are given in Table V., the form of the specimen was slightly 

 modified. 



1916 u 



