286 



DR E. G. COKER ON 



assume its own position of equilibrium without affecting the readings of the measuring 

 instrument. 



Attention has been directed to the influence of bending on the torsional properties 

 of a bar. 



1. Bending within the Elastic Limit, and Torsion within the Elastic Limit. 



In a previous section it has been shown that the effect of a tension produced little 

 or no effect upon the torsional properties of a bar while in the elastic state. It might 

 be expected, therefore, that a bending action which results in a varying tension and 

 compression upon the longitudinal fibres would have but little effect upon' the strain. 

 As an example we may take the case quoted in Section II. of a rivet steel bar, in which 

 an increase in the bending moment caused a slight diminution of the strain per unit 

 torque. 



Similar decrements were found in every case of the same type ; as, for example, in 

 the case of a semi-mild steel bar of diameter 0"869 inch, and of length 8'00 inches 

 under test. 



The unit reading corresponds to -^ minute of arc. 



Table X. 





No Bending Moment. 



640 inch lbs. 



Torque in 

 lbs. 



















Reading. 



Difference. 



Reading. 



Difference. 











166 







163 



75 



166 



166 



163 



164 



150 



332 



166 



327 



166 



225 



498 



165 



493 



166 



300 



663 



167 



659 



165 



375 



830 





824 





2. Bending beyond the Elastic Limit. 



A more interesting case was that of a steel bar in which the bending moment was 

 gradually increased until a permanent set w r as given to the bar in the plane of bending. 

 The readings obtained are shown in Table XL, and a summary of the results in Table 

 XII. It will be noticed that when the bar is bent the true value of the torque is given 

 by its apparent value multiplied by cos 6, where = angle of bending of the weigh- 

 lever about its axis. 



The results show that the slight diminution in the readings within the elastic limit 

 is followed by a much greater rise when the yield-point is reached. 



