December 5, 1912] 



NATURE 



38- 



THE APPLICATION OF OPTICAL METHODS 



TO TECHNICAL PROBLEMS OF STRESS 



DISTRIBUTION. 



IT is interesting, in the study of experimental 

 work on the properties of engineering 

 materials, to trace the general trend of the design 

 of apparatus for research, as the need of more 

 accurate knowledge has arisen. Much of our 

 fundamental knowledge of materials has been 

 gained from the study of strains in wires, bars 

 and beams, under uniform conditions of loading ; 

 and the experimental apparatus employed has 

 generally made these conditions necessary. The 

 l)ulk of the technical problems which still require 

 solution are, however, those in which the internal 

 stress varies enormously from point to point ; and 

 hence the strain-measuring apparatus now em- 

 ployed in researches has been so increased in/ 

 delicacy that it is possible to obtain average 

 measurements over \erv small distances which 

 approximate, if thev cannot reach, to the measure- 

 ment of the strain at a point. 



Fiii. t. — Transparent spur wtieels in circularly polarised ligtlt. 



Optical science has, however, provided a 

 very perfect method for investigating the stress 

 at a point, and the mathematical and physical 

 investigations of physicists, among them Neu- 

 mann, Clerk Maxwell, IMesnager and Filon, 

 on the temporary double refracting properties of 

 stressed glass have made it possible to enlist the 

 aid of a valuable experimental means of studying 

 internal stresses produced in models of structures 

 and machines. 



It is not necessary here to show that the stresses 

 in glass of good optical qualit}' agree very closely 

 with the calculated values of the theory of elas- 

 ticity. It Is worth while, however, to point out 

 that the apparent neglect of a valuable means of 

 technical research has been due to almost un- 

 avoidable causes, the chief of which have been 

 the great cost and fragility of glass specimens 

 when shaped to forms adapted for investigations, 

 and the necessity of employing verv small models 

 to suit the dimensions of the optical appliances 

 available. 



Some of these difficulties have been removed by 

 XO. 2249, VOL. go] 



the substitution for glass of one of the nitro-cellu- 

 lose compounds now available. These compounds 

 approach glass in the perfection of their optical 

 properties, and are considerably superior in ducti- 

 lity, and in the ease with .which the material 

 can be fashioned into complicated shapes at a 

 fraction of the cost of glass specimens. An 

 example of this is afforded by the accompanying 

 photograph, Fig. i, of a pair of toothed wheels 

 of transparent material shaped in a gear-cutting 

 machine in exactly the same way, and as accurately 

 as their metal counterparts. They are shown 

 here under somewhat heavy loads; and the con- 

 dition of internal stress is marked by colour fringes, 

 which appear as black bands in the photograph. 

 An important feature of this kind of material is 

 its ability to sustain stresses. of as much as several 

 thousands of pounds per square inch without injury, 

 so that the double refraction produced by the load- 

 ing can be made much more intense than in glass, 

 which usually fractures at verv small loads. 



briquette in plane polarised ligiit. 



The comparative rarity and great cost of large 

 Nicol prisms have also restricted optical investiga- 

 tions to very small objects, but, as will be shown, 

 this difficulty has been surmounted, and the size 

 of the specimen illuminated by plane or circularly 

 polarised light may be chosen at pleasure. 

 Although not an essential feature, it may be 

 mentioned that the brilliant colour effects of double 

 refraction may be permanently recorded in a very 

 convenient manner by any of the modern photo- 

 graphic plates now available. 



In describing in general outline a method of 

 obtaining the stress distribution in a loaded body, 

 it may be useful to recall that a glass or celluloid 

 body under stress causes an incident beam of 

 plane polarised light to divide into two rays, which 

 have different phases at exit, and also have their 

 planes of vibration in the directions of the principal 

 axes of stress in the body. A stressed object 

 between crossed Nicols, therefore, shows ilark 

 bands or brushes, and these mark the positions 



