Ductile Materials under Combined Stress. 



Table II. 



Gauge Calibration : Comparison of Pressures in lbs. per 

 sq. inl, as determined by the different Grauges. 



87 



Commercial. 



Air. 



Special. 



Commercial. 

 1100 



Special. 



200 





196 



1030 



300 



290 



290 



1200 



1120 



400 



380 





1300 



1220 



500 



475 



475 



1400 



1320 



600 



570 



565 



1500 



1420 



700 



665 



662 



1600 



1520 



800 



750 



740 



1700 



1610 



900 



850 



851 



1800 



1705 



1000 





935 



1900 

 2000 



1795 

 1895 



32. Measurement of the Distortions. — In order to determine 

 the strains and to locate the poiiits in the various stress- 

 strain curves, I used instruments which I had devised with a 

 view to these tests. They were a combined axial extensometer 

 and twist-meter, and a diametral extensometer used to de- 

 termine the circumferential strain. 



As it seems to be essential that any extensometer to be 

 used on a part of the stress-strain curve likely to involve a 

 time-effect should be single-reading, and should not be 

 required to be manipulated in any way to obtain a reading, 

 optical methods were used throughout. 



33, A New Extensometer. — It will be convenient to de- 

 scribe the axitd portion of the apparatus first. It consists of a 

 lever-system and a mirror; the twist of the latter, produced 

 by an extension of the specimen in a manner similar to the 

 Unwin extensometer, being read by a telescope and scale. 

 The various parts are shown in fig. 4, which is a side- 

 elevation of the whole instrument in position on a specimen, 

 the " adjuster " being shown on the left-hand side. The 

 piece marked 1 is called the adjuster, and serves to adjust 

 the grips 2 and 3 correctly on the specimen, so that they are 

 at the correct distance apart, and so that certain points in 

 their configuration lie on the axis of the specimen. In fig. 5 

 is shown a view from below the grip 2 in fig. 4, and in the 

 broken lines is shown a view or the end of the adjuster. 

 This adjuster consists of surfaces A 1? A 2 , A 3 , A 4 , of which A x 

 and A 3 are parts of one plane, and A 2 , A 4 are parts of another 

 intersecting it. CD and EF are portions of kinematic slides 

 for the grips 2 and 3, the other elements of the slides being 

 formed by the spheres c l5 c 2 , d on the grip 2, and <?,, e 2 , f on 



