G. — ENGINEERING. 131 



to the scratches being too small to see. The strength — under alternat- 

 ing stresses — appears to depend on the form of the scratches, and if the 

 ratio of radius at the bottom of the scratch to its depth is fairly large, 

 very little weakening occurs. It seems probable in the ordinary engineer- 

 ing finish produced by emery and oil that the scratches are broad and, 

 shallow. This subject is being investigated. A considerable amount of 

 evidence has been collected from practical experience pointing to the 

 important effect which a smooth finish has on the strength of heavily 

 stressed engine parts. 



Fatigue is probably the cause of failure of wires in wire ropes. A 

 good deal of valuable experimental work has been done on the life of 

 ropes, but so far as I am aware there is no satisfactory theory of their 

 strength. This subject also requires research, and it seems probable 

 that valuable practical results might follow if the true explanation of 

 the cause of the breakages of the wires was determined. 



These are only examples, but they may be sufficient to show how 

 much work both experimental and theoretical requires to be done to 

 give the engineer a really sound basis for the simplest strength calcula- 

 tions on any moving machinery. But there are more fundamental 

 questions still which must be tackled before the simplest questions of 

 all which meet the engineer can be answered scientifically. The two 

 most urgent and most important questions which I met with during the 

 war in connection with aircraft were always the same — Why did some 

 part break? and, What is the best material to use for that part? It was 

 most disconcerting to find how inadequate one's knowledge was to 

 answer these two simple questions. The common answers are : To the 

 first : ' It broke because it was too weak, make it stronger, ' and to the 

 second : ' General practice indicates such a material as the best — better 

 not try any other or you may have trouble. ' In aircraft weight is 

 paramount, and to make a part stronger— i.e., heavier — had to be the 

 last resort, and when used was almost a confession of failure. ' General 

 practice ' was no guide in aeroplane engines, which are built of the 

 strangest materials. The origins of fractures were traced to many 

 causes, often lying far away from the site of the breakage; but with 

 these I am not concerned to-day. I wish to confine our consideration to 

 the actual fracture and to ask, ' What stress caused the fracture? ' and 

 ' What property of the metal was absent which would have enabled 

 it to withstand that stress ? ' And again, ' What other material pos- 

 sesses suitable properties to withstand the stresses better? ' These are 

 the fundamental questions which I have referred to — and which urgently 

 need answers. 



As an example I will take a broken propeller shaft. It has broken 

 in a beautiful spiral fracture. What stress causes that? I have failed 

 to explain it by any of the facts I know about the steel it is made of. 

 It is, of course, a fatigue fracture — i.e., it spread gradually. The 

 questions to be answered are. Did it fail under tension, bending or 

 torsion? and, Why was a spiral direction followed by the failure as it 

 spread? j "i 



It may be objected that the question is unimportant. I think not. 

 For example, till we can determine the nature of the stress we cannot 



K a 



