750 REPORT — 1 894. 



FRIDAY, AUGUST 10. 

 The following Papers were read : — 

 1. At a joint meeting with Section A : — 



(a) On Planimeters. By Professor O. Henrici, F.R.S. — See 



Reports, p. 496. 



(b) Note on the Behaviour of a Rotating Cylinder in a Steady 



Current. By Arnulph Mallock. 



(c) On the Resistance experienced by Solids moving through Fluids. 



By Lord Kelvin, P.R.S. 



(d) A Discussion on Flight. Opened by Mr. Hiram S. Maxim. 



2. On the Strength and Plastic Extensibility of Iron and Steel. 

 By Professor T. Claxton Fidler, M.Inst.C.E. 



1. For several reasons the stress-strain diagram, as autographically drown in 

 connection with the testing cf a ductile material, does not suffice to indicate any 

 definite relation between tensile stress and plastic strain. The stretching effect of 

 the load is commonly measured before it is fully developed ; the ordinates of the 

 diagram represent stress per unit of original area, but cannot represent the actual 

 unit stress, which varies in different parts of the bar ; and, lastly, the elongation 

 measured by the diagram is the elongation of the whole bar, and is the sum of the 

 component elements of elongation, which vary still more widely in different parts of 

 the bar. 



2. Analysing the total elongation, and plotting as co-ordinates the actual in- 

 tensity of stress in any short segment or element of the bar, and the actual elonga- 

 tion in the same element under a long-continued load, the curve takes a very 

 different form, and is probably already familiar to those who have taken a special 

 interest in the subject. 



3. So far as the author's direct experiments have gone, the curve so traced 

 appears to be almost identical for all parts of a fairly homogeneous bar, indi- 

 cating that the plastic extensibility, or the actual extension under any given 

 stress, is nearly the same in all segments of the bar's length, even when the ultimate 

 elongation varies, as it often does, between 12 per cent, and 120 per cent, in the 

 different segments. 



4. Volumetric measurements of the successive segments indicate that there is 

 no sensible telescopic shear, or internal flow of material from one segment to another 

 (except at one point, which need not be here noticed), and justify the general 

 application of the assumption of unchanging volume. 



5. Probably, therefore, the curve may be taken to represent the variable length 

 X assumed by each component internal element under the long-continued stress y, 

 as well as the variable length of any cylindrical segment under the same stress, the 

 original length of the element or segment being denoted by L. 



6. It may st first sight be supposed that a bar of uniform plastic extensibility 

 ought to draw out uniformly over its whole length ; but when the curve is con- 

 sidered along with the mechanical conditions affecting the question, they seem 

 sufficient to explain the observed behaviour of the bar. Beyond a certain critical 

 point a uniform extension is almost impossible, just as the uniform compression of 

 a long and slender strut is impossible. The formation of a short narrow neck in 

 some part of the tin-bar is inevitable — like the buckling of the strut. 



7. To ilhistr.te these points and some others the curve, as approximately 

 determined for a bar of mild steel, was shown in the diagrams exhibited, and its 

 regularity suggests the existence of a definite law of extension. As the length x 



