ARTICLES 269 



such a value a material is tough, and below that value brittle. 

 For a steel with a maximum stress of 100 tons per square inch, 

 an impact value of 30 foot-pounds would be extraordinarily 

 good ; if the same figure were associated with a tenacity of 

 60 tons per square inch it would be moderate, while for a 

 20 or 30 ton steel it would be distinctly poor. 



In a crystalline aggregate, as all metallic test-pieces are, 

 both the crystalline portion and the intercrystalline surfaces 

 make their influence felt. As pointed out by Mr. H. Brearley, 

 brittle materials, which, like the hardened steel, yield low 

 impact figures, break between the adjacent crystals, which 

 are hardly affected at all. The fractured surface of a brittle 

 test-piece is strikingly crystalline. In the case, however, of a 

 tough sample in which the intercrystalline strength exceeds 

 that of the crystalline portion the reverse of the foregoing 

 conditions obtains. The crack must pass through the crystals 

 themselves ; in so doing, considerable work is done on the sur- 

 rounding parts, and a fibrous or silky fracture is shown. The 

 transition of a hardened steel from the brittle to the tough 

 condition on tempering is in part at least due to the softening 

 of the crystalline material, rather than to the increased strength 

 of the intercrystalline areas ; and in determining at what tem- 

 perature any given steel should be tempered to yield the 

 toughest product, the impact test gives the highest results when 

 tempering is carried out 50-1 00 ° C. below the carbon change- 

 point. To the impact test, therefore, a hard material composed 

 of large crystals is typically brittle, a more finely crystalline 

 and softer one tough. 



In the case of a forged material, such as steel, the resistance 

 to impact is closely bound up with the direction in which the 

 test-piece itself is cut A sample cut longitudinally will yield 

 far better results than one cut transversely to the direction of 

 forging. The following results (Journ. Iron and Steel Inst, 

 191 8, ii, p. 24) will illustrate this influence : 



Angle of Notch with Work absorbed on 



Direction of Rolling. Rupture (Kilogram-metres). 



0; 1 1*3 



20 rj 



45° 3-4 



90° 135 



1 The notch being perpendicular to the test-piece, this sample is cut trans- 

 versely to the direction of rolling. 



