66 Mr. P. W. Bridgman on Breaking Jests under 



only, the ends being left unsupported. Fig. 1 illustrates 

 the manner of applying pressure. The rod A, the subject 



Fiff. 1. 



I <vt\^^v^/(r ^v^v%^> 



u 



Apparatus for producing the " pinching-off" effect, that is, separation of 

 the longitudinal iibres by the application of pressure to the curved 

 surface of a cylinder. The specimen is shown at A, the fluid 

 exerting the pressure by which rupture is produced is contained in 

 the annular space at B. 



of the test, passes completely through the cylinder B, 

 projecting at either end through the packing rings C The 

 cylinder is connected to the pressure-pump through the 

 indicated connexions and stress applied to the test specimen 

 by the pressure of the fluid in the annular space between the 

 specimen A and the interior wall of the cylinder. The 

 specimen fails by separation of the particles across some 

 plane perpendicular to the axis, the two disconnected ends 

 of the specimen being expelled with violence through the 

 packing rings. The fracture does not take place at the 

 packing rings as might be expected, but, whether for brittle 

 or ductile materials, takes place at some point well between 

 the rings. The nature of the process of rupture is evidently 

 merely that of squeezing the rod out sidewise. This type of 

 rupture may therefore be referred to as the " pinching-off- 

 effect." There is no longitudinal stress except that due to 

 the friction of the packing, and this a stress of compression 

 rather than one of tension, so that here we actually have the 

 fibres separating against tin; direction of stress. 



The nature of the fracture varies with the material. In 

 the case of a rod of mild steel, the rupture looks very much 

 like that of an ordinary tensile break, except that the 

 necking down is likely to be a little more abrupt. PI. II. 

 fig. 2 shows a photograph o\' one Buch specimen. Other 



soft, materials such as copper or brass -how the same manner 

 of rupture. Harder materials, such as hardened chrome- 

 nickel steel or vanadium steel, -how irregular fracture, a 



combination of necking down and of -lip <>n shear planes at 

 approximately 45° to the axis. Glass-hard tool-steel, on the 

 other hand, -how- a clean break ;it right angles to the axis 



without necking down. In the same wav it i- possible to 



