2(58 SCIENCE PROGRESS 



is most unsatisfactory. The energy used up in breaking, say, 

 an Izod test-piece o*8 sq. cm. in cross-section at the notch, when 

 multiplied by 1*25, does not give the energy required to fracture 

 a test-piece whose dimensions are 1 cm. x 1 cm. at the base of 

 the V. It has been suggested by Mimey (Revue d 1 Artillerie , 

 July 191 1 ) that if T x and T 2 are the energies absorbed in 

 fracturing two geometrically similar bars of the same material, 



T 



then ~^ will be proportional to the cube of the homologous 



dimension, provided that 



Mi = fPL ! = D t _ #T X 

 M, ' ' v/H* " D 2 ' #% 



where M x and M 2 are the masses of the weights, H^ and H 2 the 

 distance fallen, and D x and D 2 the distances apart of the sup- 

 ports for the test-piece. The result is not, however, in agreement 

 with experiments, except where deformation does not proceed 

 as far as fracture and in the case of unnotched bars. For 

 purposes of reducing the results obtained in ordinary impact 

 tests on notched bars, it is useless. In the case of a bar of side 

 x and notched with a standard groove, the total work done in 

 producing fracture would require a formula of three terms 

 of the form ax-\- fix*+ yx\ The first term corresponds to the 

 energy expended in starting the crack along the line of the 

 notch, the second to that done in carrying it across the section 

 of the test-piece, and the third to that used up in deforming 

 adjacent material. Since it is often comparatively easy to 

 continue the crack when once it is formed, the importance 

 of the first time — neglected by Charpy and Cornu-Thenard — 

 is evident. The relative part which each of these factors 

 plays will vary considerably in different materials. In, for 

 instance, a hardened steel the resistance to deformation is very 

 great. A crack, therefore, once started extends with ease and 

 without appreciably deforming the surrounding material. A 

 low impact value will thus be recorded. In the case, however, 

 of a metal or alloy with a fairly low yield-point, it may not be 

 possible either to start or to propagate the crack without con- 

 siderable distortion of the surrounding material. In this 

 distortion considerable energy is used up, with the result that 

 a high impact figure will be given. It is not, therefore, possible 

 to draw any hard-and-fast line, and say that above such and 



