September io, 19 14] 



NATURE 



49 



experiment that made out this theory tried at 



. iiitehall, as also my spring watch." 

 Hooke had, in lact, discovered the fundamental 

 principle upon which a theory of the elasticity and 

 strength of materials could be based, and it would be 

 interesting to trace the great advances which were 

 rapidly made from this new vantage ground, whereby 

 the main facts of the distribution of stress in simple 

 members of structures became known, and a founda- 

 tion was laid for the great advances of the mathe- 

 matical theory. If I am silent upon the enormous 

 developments of the modern theory of the strength of 

 materials it is not from lack of appreciation, but 

 because I do not deem myself adequately fitted to 

 discuss the great work of the elasticians, which all 

 engineers admire, and so few are equipped to follow 

 with the full battery of mathematical tools which have 

 been pressed into service in the pursuit of this great 

 science. 



Among the greatest of the services rendered by early 

 pioneers was that of Young, who was the first to 

 notice that the elastic resistance of a body to shear 

 was different from its resistance to extension or con- 

 traction, and this led him to define a modulus of elas- 

 ticity for materials in compression. As Prof. Love 

 remarks, "This introduction of a definite physical con- 

 cept, which descends, as it were, from a clear sky on 

 the readers of mathematical memoirs, marks an epoch 

 in the history of the science.' 



From the viewpoint of the engineer, nothing is of 

 more practical importance than the great discoveries 

 of Hooke and Young, that bodies like metal, wood, 

 and stone are "springy" and have a simple linear 

 relation between stress and strain. It is probably 

 within the mark to say that nine-tenths of all the 

 experimental investigations on stress distributions in 

 structures have been entirely based on the funda- 

 mental principles which they enunciated, and new uses 

 are continually arising. The recent application of the 

 steam turbine to the propulsion of ships produced a 

 profound change in marine-engine practice, and inci- 

 dentally involved an entire reconstruction of methods 

 for obtaining the horse-power developed, which had 

 been gradually perfected from the time of Watt, but 

 were absolutely useless for the new system of pro- 

 pulsion. Hooke's discover}' of the essential springi- 

 ness of metals enabled engineers quickly to devise new 

 instruments capable of accurately measuring the in- 

 finitesimal angular distortions of propeller shafts, and 

 from these to determine the horse-power transmitted 

 by the aid of an appropriate modulus. 



The construction of tall buildings affords another 

 example where advantage has been taken to deter- 

 mine the loads upon columns bv measuring the 

 minute diminutions of length as the structure pro- 

 ceeds, thereby affording a valuable check upon the 

 calculations for these members, and a trustworthy 

 indication of the pressures supported by the founda- 

 tions. 



The distribution of stress in buildings constructed 

 of composite materials like concrete reinforced with 

 steel has also been examined by similar methods, and 

 much data for guidance in future constructional work 

 has been obtained, especiallv in the United States of 

 America. 



The still more difficult problems involved in the 

 determination of the stresses in joints and fastenings 

 of complicated structures have often been investigated 

 by purely mechanical measurements of strain, and the 

 experimental investigations of Profs. Barraclough and 

 Gibson and their pupils upon the distribution of stress 

 due to riveted joints and curved plates of boiler shells 

 afford a notable example of the successful application 

 of the measurement of small strains to a stress 

 problem of great complexity. 



NO. 2341, VOL. 94] 



That ■• science of measurement " is here sufficiently 

 obvious, and it seems only due to the memory of that 

 great engineer, Sir Joseph Whitworth, to refer to his 

 great mechanical achievements of a true plane and 

 well-nigh perfect screw, which enabled him to measure 

 changes of one-millionth of an inch, and thereby 

 gave experimental investigations of strains of a new 

 impetus, which is reflected in subsequent work on the 

 subject. Nor must we forget the no less important 

 exposition, by Kelvin and 'lait, of the scientific prin- 

 ciples of instrument construction which have done so 

 much for the design of instruments for the precise 

 measurement of strains. 



Mechanical measurements cannot, however, com- 

 pletely satisfy all our modern requirements, since 

 they are essentially average values, and fail to ac- 

 commodate themselves to many of the problems which 

 press for solution. 



In the quest for exact experimental knowledge, the 

 measurement of stress at a point becomes of paramount 

 importance, and we may, therefore, inquire what 

 further means the researches of pure science have 

 placed at our disposal for the determination of stress 

 distribution in materials. 



It is well known that many materials when tested to 

 destruction show a considerable rise of temperature 

 at the place of fracture, especially in very ductile 

 materials; but Weber was the first to discover that a 

 metal wire when stretched within the elastic limit is 

 cooled by the action of the load, and this result was 

 deduced later from the laws of thermo-elastic be- 

 haviour of materials by Lord Kelvin, who showed that 

 tension and compression loads produce opposite effects, 

 and that materials which have the property of con- 

 tracting with rise of temperature show thermal 

 effects of the reverse kind. Although the 

 changes of temperature produced by stress are 

 small within the elastic range— less than i° C. for 

 most materials — yet their effect upon a thermo-couple 

 is readily measurable if the equilibrating effects of sur- 

 rounding bodies are neutralisea or allowed for, so that 

 stress distribution can be determined by thermal 

 measurements at a point. The correction for such 

 disturbing causes is usually an important factor, and 

 is generally so large that experimental work is more 

 suitable for the laboratory than the workshop ; but if 

 all necessary- precautions are taken a linear relation of 

 stress to strain can be shown to hold up to the elastic 

 limit of the material, while above this point the break- 

 down of the structure causes a rise of temperature of 

 so marked a character that it has been utilised by 

 several investigators as an indication of the yield 

 point. 



Experiments upon members subjected to tension, 

 compression, and bending, show that thermal pheno- 

 mena afford trustworthy indications of the stress in 

 materials so diverse as a rolled-steel section, a block of 

 cement, and beams of stone and slate. Although no 

 attempt appears to have been made to investigate 

 stress distributions of any great complexitj-, it seems 

 not unlikely that thermal methods of investigation 

 will ultimately prove of considerable value. 



The transparency of metals to Rontgen rays is 

 another phenomenon which has often been suggested 

 as likely to be of service for work on stress distribu- 

 tion in materials, and Mr. Howgrave Graham and I 



j have examined a number of rolled metals under stress 

 up to the breaking point, without, however, discover- 

 ing any change in the appearance of the material as 

 seen on a fluorescent screen. Although our experi- 

 ments showed no perceptible change, it is, of course, 

 not impossible that an effect may have escaf>ed our 



j notice. 



I Another and still more fascinating field of research 



' on stress distribution is afforded bv the doublv refrac- 



