ARTICLES 267 



The Charpy test differs from the foregoing chiefly in that 

 the test-piece is loosely supported at both ends, the distance 

 between the supports being 40 mm., and that the notch is placed 

 on the opposite side of the test-piece to that on which the 

 blow is received. Although, of course, a notch identical with 

 that used in the Izod test can be and often is used, the specified 

 form consists of a cut 5 mm. deep, terminating in a cylindrical 

 hole 1$ mm. in diameter. The knife edge which comes in 

 contact with the test-piece is here placed vertically in the 

 pendulum ; in the Izod machine it is horizontal. In the machine 

 of the usual size the weight of the tup is 22-5 kilograms, the 

 striking energy being 30 kilogram-metres. 



In the Fremont test the blow is received from a tup which 

 has fallen vertically through a height of several metres, the 

 striking velocity being higher than in either the Izod or Charpy 

 forms. The test-piece, 8 mm. by 10 mm., rests horizontally on 

 supports 21 mm. apart, with a square notch 1 mm. deep and 

 1 mm. wide on the longer lower side. 



The standardisation of the impact test is, as a result of 

 the work of Charpy and Cornu-Thenard, now fairly complete. 

 Despite the differences in the types of apparatus described, the 

 results obtained from similar samples with the same form of 

 notch on all of the machines are practically the same. Not 

 only is the type of machine without influence, but so also is 

 the energy of the blow, provided that this is ample to produce 

 fracture. The work done in breaking the test-piece is, therefore, 

 a definite characteristic of the material. 



The results of the test are commonly expressed in terms of 

 the difference in energy of the tup before and after fracturing 

 the test-piece. As Mr. H. Brearley has pointed out, the work 

 done upon the material is expended in the first place in starting 

 the crack at the root of the notch, secondly in extending the 

 crack across the bar, and thirdly in deforming the material in 

 the neighbourhood of the crack. In view of this complexity, 

 it is not surprising that no satisfactory general connection has 

 so far been discovered between the dimensions of the bar and 

 the work done in fracturing it in impact. All that it is at 

 present possible to do is to record the work expended in breaking 

 a clearly specified but quite arbitrary type of test-piece. 



The unfortunately fairly common practice of expressing these 

 results in such terms as kilogram-metres per square centimetre 



