74G 



Prof. E. G. Coker on the 



The usual arrangement adopted is shown in fig. 2, in 

 which a symmetrically loaded tension specimen A and another 

 B with an eccentric load are both shown secured in the grips 



Fur. 2. 



of miniature testing machines. In this example the specimens 

 were cut from the same strip, and the interference effects 

 were projected on to a squared paper screen so that the 

 breadth of the eccentrically loaded specimen was 2 ins., cor- 

 responding to an actual breadth of 0*309 in. 



The stresses produced at different points in the cross- 

 sections were determined by loading the standard test-piece 

 until the uniform colour produced in it agreed with that 

 produced at a definite point in the cross-section of speci- 

 men B, and the stress was calculated from the load and 

 the dimensions of the levers. This method requires no cor- 

 rection for the tension side of the specimen as the diminution 

 of thickness due to the load is the same for both, but a small 

 correction is required for the compression side as the relative 

 retardation between the interfering rays is greater on account 

 of the increase in thickness due to the stress. 



Measuring in this way, the following values (Table IV.) of the 

 stress were obtained across the section of the specimen B, where 

 the abscissse refer to the projected dimensions measured from 

 the line of application of the load, which in this case coincided 

 with one edge of the specimen. 



These values are plotted in fig. 3, and the diagram shows 

 that the variation of stress in the specimen is approximately 

 a linear one, except for the highest load, when the specimen 

 failed on the tension side, thereby producing a change in the 



