466 REPORTS ON THE STATE OF SCIENCE,^ — 1919. 



course : the effect of surface scratches on the torsional strength of a shaft 

 is of special interest. In connection more essentially with airscrew con- 

 struction, the elastic constants of wood in the three principal directions 

 have been determined, and the complex stresses arising in an airscrew 

 due to thrust and centrifugal force have been studied by Berry, Griffith, 

 and Hague. 



The present report includes the following contributions : 



' The Strength of Tubular Struts.' By Andrew Robertson, Major R.A.F.. D.Sc. 

 (Mc). 



' Investigations of Stresses in Aeroplane Wing Frameworks.' By Professor L. N. G. 

 Filon, D.Sc, F.R.S. 



' The Soap Film Method of Stress Estimation.' By A. A. Griffiths. 



' Eccentric Loading in Tension and Compression Tests.' By W. A. Scoble. 



' Experiments on the Effect of Alternations of Tensile Stress at Low Frequencies 

 on the Elastic Properties of Mild Steel.' By Angus R. Fulton, M.B.E., B.Sc, A.M.Inst. 

 C.E., Temp. Major R.A.F. 



'The Strain-energy Function and the Elastic-limit.' By B. P. Haigh, M.B.E., 

 D.Sc. 



The Strength of Tubular Struts. 

 By Andrew Robertson, Major R.A.F., D.Sc. (Mc). 



The problem of the strength of a tubular strut differs from that of a solid 

 strut in that there may be a condition of clastic instability in the wall of 

 the tube under direct imiform compression. The theoretical analysis of 

 Southwell ' for this condition leads to the formula 



^ RV 3lm:'-l 



3(»f—l) 



where p = collapsing load per &q. in. 

 t = thickness of tube. 

 R = mean radius of tube. 



= Poisson's ratio. 



■in 



E = Young's modulus. 



This formula apj^lies only to cases in which the load per sq. in. is 

 less than the elastic limit of the material, so that it cannot be expected to 



t t) 



apply to any tube in which .fc is greater than j ^- — r — For a 



Ea / ^" 



V 3K-1)' 



mild-steel tube having an clastic limit of 20 tons per sq. in., ^ must 



be less than ^, i.e., if t = —" R must be greater than 4". Such tubes 



are never met with in practical work, so that the analysis suggests that 

 all practicable tubes should sustain an average stress equal to the elastic 

 limit before collapsing by wrinkling (the term used to denote the type 



' R. V. Southwell, Phil. Trans. Roi/. Soc. Series A. vol. 213. 



