‘BEHAVIOUR OF MATERIALS IN TESTING MACHINE 175 
" . . Al/a,\?_ aif? 
ee, : the second bar is aE (4 vy ) SRE 
laine resilience of first bar _ A 
resilience of second bar a’ 
The unnotched bar has therefore a greater resilience than the notched 
_ bar when the minimum or effective cross section is the same in both. 
166. Fracture by Shearing in Tension and Compression Tests.— 
In Art. 140, p. 138, it was shown that a piece subjected to direct 
tension or direct compression is also placed under shear, the shear stress 
being a maximum in planes inclined at 45° to the axis of the specimen. 
Tt was also shown that the intensity of the maximum shear stress is half 
the intensity of the tensile or compressive stress on planes perpendicular 
to the axis of the specimen. 
__ Experimental evidence of the existence of this oblique shear stress in 
tension and compression tests is found with various materials. For 
_ example, if a test piece of mild steel be highly polished previous to 
testing in tension, a series of lines inclined at about 60° to the axis of 
‘the piece are clearly seen after the yield point is reached. If the piece 
‘is cylindrical, the lines referred to form helices on the polished surface, as 
‘shown roughly in Fig. 247. These lines, sometimes called Luders’ lines, 
show that the molecular slip is taking place in the direction of shear 
Stress, and in the case of mild steel and other materials it is no doubt 
the resistance to pure shear which determines the yield point. 
The actual fracture of a piece of mild steel in tension also shows that 
it really gives way by shear- 
‘ing, as is shown in Fig. 248, 
where the specimens are cylin- 
drical, and the fractures partly 
conical. Fig. 249 shows the 
common form of fracture of 
a cylindrical piece of cast-iron CR 
tested in compression. The All 
of the fracture to the axis of il 
_ the specimen being about 35°. 
_ Shearing of a piece under a crushing load is found when testing a hard 
Wide 
| 
| 
———— 
=————— 
zx 
ing obliquely, the inclination 
‘cast-iron gives way by shear- 
eel ball in compression between two hard flat plates, or between two 
_ other balls. Where contact takes place small circular flats are formed 
on the ball, and these become the bases of two cones, which form in the 
all by shearing, and these cones are forced into the ball and cause it to 
