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WALTER H. BUCHER 



strips of soft steel. The second set of fractures, formed after 

 shearing was well under way along Liiders' lines, consisted of 

 tension fractures, one started from the lower, the other from the 

 upper surface. 



Glass, on the other hand, being a highly brittle substance, 

 in contrast to soft steel, will fail along tension fractures rather 

 than along planes of shearing. The fractures marked t in Figure 5 

 are the only ones that form when the glass plate used in the experi- 

 ment is very thin. They must, therefore, be tension cracks, one 

 set produced on the under side, the other, symmetrical to it, on 



,j — 



Fig. 6. — Diagram illustrating the position of the planes of shearing in a brittle 

 body subjected simultaneously to compression and tension, both in a horizontal 

 direction. 



the upper surface, and both finally extended to both surfaces, 

 owing to the thinness of the plate. The gentle curving of these 

 cracks is quite in harmony with this interpretation. 



The other set of fractures, marked s in Figure 5, intersects 

 with the tension cracks at angles varying from 15° to 25°. This, 

 however, is one-half of the angle of shearing characteristic of glass.^ 



The same angle for soft steel is approximately 45°. It is 

 evident, therefore, that these fractures represent shearing planes 

 produced by the compressive stress acting in the direction normal 

 to the tensile stress. In the experiments made with glass, however, 

 in contrast to those with mild steel, only one set of the shearing 

 planes forms in connection with a tension crack, that only which 



* To verify this, it is sufficient to compress small pieces of thick plate glass in 

 a strong vise. The resulting angle of shearing can be measured conveniently by 

 means of Penfield's contact goniometer. 



