Materials under Combined Stress. 121 



The Apparatus for the Tests on Tubes, 



The tests on tubes were made in a machine which is part 

 of the equipment of the engineering laboratory at the City 

 and Guilds Technical College, Finsbury. The machine was 

 designed by Prof. E. G. Coker, and it has been recently 

 described *. A tube to be tested was sweated, and when it 

 was necessary pinned, on two steel holders, or mandrils, each 

 of which was provided with two keyways. One holder fitted 

 into a worm wheel so that this end of the tube could be 

 twisted, but the wormwheel casing was pivotted on roller 

 bearings so that there was no resistance to ben diner. A 

 lengthening piece 1 was fitted on the holder to carry an over- 

 hung load, which produced a bending moment on the spe- 

 cimen. The other holder was held in a special fitting which 

 was supported on a spindle of small diameter, so that the 

 resistance to torsion was negligible. The fitting was also 

 arranged to carry an overhanging load at this end, and an 

 arm projected at right angles to the specimen to support the 

 load which measured the torque. This latter arm was always 

 kept horizontal by turning the wormwheel as the twisting 

 load was increased. The loading in this machine was also 

 by dead weights. The torque was produced by the twisting 

 load and the extra supporting force which it required at it- 

 support. The torque was uniform along the tube. The 

 bending moment was caused by a couple at eacli end. com- 

 posed of the supporting force and the overhanging load : i; 

 was constant along the specimen. There was no shearing- 

 force on a section due to the bending loads. 



The twist of a tube was measured by Prof. Coker's torsion- 

 meter, and the bending deflexion by an -adaptation of the 

 Ayrton-Perry twisted strip. The latter apparatus was 

 designed for use during some bending tests in which the 

 deflexion was extremely small, and it was therefore very 

 sensitive for the purpose of the present tests. It was necessary 

 to measure the strains separately, because the writer has 

 shown that when a ductile material is under combined load- 

 ing, it does not always yield first in the way which is indicated 

 by the increasing load. The first yield is probably determined 

 by the loading which produces the greatest shearing stress. 



The Tests on Steel r /\rf>es. 

 Solid drawn steel tubes were tested, and the yield during 

 each test was kept very small so that several experiments 

 could be made with each tube. All these specimen- were 



* Proc. Phys. Soc. London, vol. xxi., and Phil. Mag. April 1909. 





