76 



SCIENCE 



[N. S. Vol. XLVII. No. 1204 



miliarity has dulled our sense of incon- 

 gruity that we do not rebel against its irra- 

 tionality. 



Machines for determining quantitatively 

 the resistance of steel to impact of any de- 

 sired degree of violence have been on the 

 market for about twenty years, and they 

 have come into use to a considerable extent 

 in Europe for reception tests, though even 

 there they have not received a fraction of 

 the attention which they deserve, so fully 

 has habit blinded us to the irrationality of 

 our present practise. These machines meas- 

 ure quantitatively the energy consumed in 

 breaking a test piece under impact of 

 given violence, and also the capacity of the 

 material for plastic deformation under 

 these same conditions. 



It is pure empiricism to measure fitness 

 for these kinetic conditions by applying a 

 factor of safety to the results of quiescent 

 tests. Not only must such a factor be a 

 matter of guesswork, but that which suf- 

 fices for certain materials is wholly insuffi- 

 cient for others. We know that certain 

 steels, which we call ' ' fragile, ' ' behave well 

 under quiescent tests, but are very brittle 

 under impact. A factor suitable for them 

 would involve a great waste if applied to 

 infragile steel, while one suited to infragile 

 steel might lead to death and disaster if ap- 

 plied to that which is fragile. Unfortu- 

 nately the fragile and infragile can not be 

 distinguished by our present quiescent 

 tests, though certain causes of fragility can 

 be detected with the microscope. Under 

 these conditions, unhesitatingly preferring 

 waste to disaster, we adopt a factor suited 

 for fragile steels. This in turn means that 

 we load the community with the cost of a 

 superfluity of material in that large major- 

 ity of cases in which we use infragile steel, 

 lest we invite certain disaster by our pres- 

 ent neglect to take the simple precautions 

 of using the impact test so as to detect fra- 



gility. To secure infragility by regulating 

 the composition of the product and the 

 process of manufacture is far better than 

 nothing; but these precautions, which the 

 buyer can enforce with difficulty if at all, 

 should in his interest be supplemented by 

 the direct and positive exclusion of fragile 

 material by means of the impact test. 



A like overloading occurs through our 

 trying to provide for the shocks incident to 

 these kinetic services by requiring great 

 plastic deformability, which we currently 

 call "ductility," in the shape of permanent 

 elongation and contraction of area under 

 quiescent tests. This provision at first 

 seems wholly irrational, because these serv- 

 ices imply no appreciable plastic deforma- 

 tion, so that we demand properties which 

 will never be utilized'. Neither guns, nor 

 rails, nor tires deform plasticallj' in use to 

 an appreciable extent. The excuse is that 

 to increase the ductility, even that deter- 

 mined quiescently, is to increase the power 

 to endure shock. Unfortunately the shock 

 resistance thus implied is far less in fragile 

 steels than in infragile ones, so that in order 

 to secure enough of it for the fragile steels 

 which we may use unwittingly we specify 

 a degree of ductility wholly superfluous in 

 infragile ones. This superfluous ductility 

 is very costly, whether it is got without 

 sacrifice of strength by means of special 

 heat treatment or composition, or, sacrific- 

 ing unit strength, by using much more of 

 a much weaker steel, following the prin- 

 ciple that for given grade of steel, every 

 gain of ductility carries with it a corre- 

 sponding loss of strength. We could save 

 this needless expense by using the impact 

 test and thus detecting fragility, for then 

 we could lessen the ductility which we ex- 

 act to that needed for infragile steels. 



It is beside the mark that for many uses, 

 including some kinetic ones, more mate- 

 rial is needed for rigidity than for strength, 



