220 TORSION TEST. 



The form of the fracture in the case of the brittle 

 materials, such as cast iron, is usually an almost perfect 

 spiral, making an angle of 45 deg. with the axis of the bar, 

 tne spiral portion of the fracture making a complete 

 revolution of the bar, that is to say, one whole pitch, and a 

 remaining part being roughly a straight line joining the two 

 ends of the spiral. This is the case in both solid and hollow 

 bars. 



In bars of ductile materials, especially in hollow bars of 

 fibrous metals such as wrought iron, the axis of the bar and 

 the whole bar itself often tends to assume a spiral or cork- 

 screw shape before fracture takes place. 



