Scott. — Resistance of Steel to Mechanical Shock. 519 



ranges of stress prescribed as safe soon raised a feeling amongst 

 engineers that the theory was incomplete, the more so that test 

 specimens cut from close to the plane of fracture usually gave 

 normal elongation, contraction of area, and elastic limit. (The 

 writer has himself tested with such results many samples from 

 fractured gun-mountings, drawhooks, chains, crank-pins, piston- 

 rods, &c). To account for such failures, the theory was ad- 

 vanced that the fatigue had been localised on planes of such 

 minute thickness that their influence on the physical constants 

 of the specimen was so small as to escape detection in ordinary 

 testing. It was pointed out that the presence on a stressed 

 piece of an initial, scratch, flaw, or tool-nick must lead to a 

 crowding of the stress-lines in its immediate neighbourhood 

 which might easily result in overstrain there ; with repeated 

 loading the portion so overstrained must eventually fail, and 

 a further crowding of the stress-lines and a further development 

 of the flaw take place, and in this way progressive fracture 

 would ensue. (See fig. 3 on next page.) 



Now, although this may happen, this theory in no way 

 accounts for the fact that material which has given most ex- 

 cellent results under tension test sometimes fails after having 

 been in service for a period far too short for fatigue fractures 

 to develop. In fact, the failure of a number of steel plates by 

 falling a few feet from a crane-sling, after strips cut from them 

 had given normal results in the testing-machine, led to the 

 important discovery by Mr. Seaton that these otherwise un- 

 accountable failures only occur when the piece is subjected 

 to " shock," and that the capacity of steel to resist shock — or % 

 in other words, its anti-brittleness — appears to be in no way 

 guaranteed by the passing of a satisfactory tensional test. 



Pure carbon steel may be considered to be a solid solution 

 of carbon in iron, and its principal properties can be illustrated 

 in no better manner than by the commonly used analogy of the 

 behaviour of solution of common salt in water. Guthrie has 

 shown that by the addition of salt to water its freezing-point 

 is lowered : the larger the percentage of salt up to about 23 - 5 

 per cent, the lower the freezing-point ; for water containing 

 23 "5 per cent, of salt it is 22° C. If there be less than 23-5 per 

 cent, of salt in solution, on the temperature being lowered ice- 

 crystals will first form ; a portion of the water thus crystalliz- 

 ing out, the solution becomes stronger, and a further lowering 

 of the temperature is required before further crystallization 

 can take place. In this way the percentage of salt in the resi- 

 dual solution is increased as the temperature is lowered, until 

 — 22° C. and 23*5 per cent, of salt is reached ; at this point 

 the water and salt crystallize out side by side, forming the cryo- 



