128 SECTIONAL ADDRESSES. 



it grows straight up the trunk; further out again it grows in a left- 

 hand spiral, and this is repeated again and again, with a pitch of about 

 two inches. The timber is strong and probably well adapted for use 

 in large pieces — it someAvhat resembles plywood — ^but it is doubtful 

 whether it is safe in small pieces. No theory is yet available for esti- 

 mating its strength, and very elaborate tests would he needed to 

 determine its reliability in all positions. I had to reject it for aero- 

 planes during the war for want of accurate knowledge of its 

 properties. 



These examples show how necessary it is to have a theory for the 

 strength of anisotropic materials before we can either understand the 

 causes of their failure or make full use of their properties or even test 

 them rationally. 



The second material we shall consider is steeT, and in dealing with 

 it I do not wish to enter into any of the dozen or so burning questions 

 which are so familiar to all metallm'gists and engineers, but to call 

 your attention to a few more fundamental questions. Steel is not 

 strictly isotropic — but we may consider it to be so to-day. The first 

 obvious question the engineer has to answer is, ' "What is its strength? ' 

 The usual tests give the Ultimate Strength, Yield Point, Elastic Limit, 

 the Elongation, the E-eduction of Area, and perhaps the Brinell and 

 Izod figiu'es. On which of these figures is the dimension of an engine 

 part, which is being designed, to be based? If we choose the Ultimate 

 Strength we must divide it by a large factor of safety — a factor of 

 ignorance. If we choose the Yield Point we must remember that none 

 of the higher-grade steels have any Yield Point, and the nominal Yield 

 Point depends on the fancy of the tester. This entirely imaginary 

 point cannot be used for accurate calculation except in a very few 

 special cases. Can we base our calculation on the Elongation — the 

 Reduction of Area — ^the Izod test ? If we face the question honestly we 

 realise that there is no known connection between the test results and 

 the stress we can safely call on the steel to bear. The only connecting 

 link is that cloak for our ignorance — the factor of safety. 



I feel confident that the only reliable property on which to base 

 the strength of any engine part is the suitable Fatigtie Liinit. We 

 Have not yet reached the position of being able to specify this figure, 

 but a considerable number of tests show that in a wide range of steels 

 (though there are some unexplained exceptions) the Fatigue Limit for 

 equal ± stresses is a little under half the Ultimate Strength, and is 

 independent of the Elastic Limit and nominal Yield Point, so that the 

 Ultimate Strength may be replaced as the most reliable guide to true 

 strength, with a factor — no longer of ignorance, but to give the fatigue 

 Hmit — of a little over 2. 



If the Fatigue Limit is accepted as the only sound basis for strength 

 calculation for engine parts, and it is difficult to find any valid objection 

 to it, then it is obvious that there is urgent need for extensive researches 

 in fatigue, for the available data are most meagre. The work is 

 laborious, for there is not one Fatigue Limit, but a continuous series, 

 as the signs and magnitudes of the stresses change. Many problems in 

 connection with fatigue are of great importance and need much fuller 



