664 REPORT— 1882. 



3. On the increased Tenacity in perforated Test Bars of Steel and Iron. 

 By T. SVkightson, M.I.G.E. 



The author quotes the experiments of Professor Keuneilv,^ and of Mr. Edw.ard 

 Richards,^ as to the curious fact, that a drilled test har has a greater tenacity per 

 square inch than a plain bar, and proceeds to propound a theory as to the cause for 

 this apparent anomaly. 



If a plate or bar under tensile test be supposed to consist of a series of molecular 

 chains or filaments parallel one to another, but disconnected laterally except at the 

 ends, then, as the test proceeds, each filament being equally loaded would extend 

 without afiecting its neighbour. Again, if horizontal filaments be supposed to con- 

 nect the longitudinal filameut.s, these will descend as stretching takes place, but will 

 not affect the relative position of the longitudiual filaments. 



If, however, we suppose diagonal connections to exist between the upper end of 

 the extreme and the centres of the opposite filaments, it is evident that when, through 

 stretching, the centres of the longitudinal filaments descend, either an elongation of 

 the oblique filaments or a drawing in of the longitudinal filaments must occur. The 

 resistance of tbe latter in this direction being small, a lateral movement takes place. 

 The author contends that the well-kuowu ])henomenon of contraction in a test bar 

 arises from this cause, and is due to the oblique connection of the longitudinal 

 filaments. 



Assuming this view of the phenomenon of contraction to be correct, the author 

 supposes a hole drilled in the centre of a test bar, and shows that the two sections 

 left should be capable of resisting more than the same section in a single bar because 

 the contraction of area in the former case is resisted by the material immediately 

 above and below the hole, which must be stretched horizontally before the adjacent 

 iron can contract to its full extent. Hence the area being partially maintained by 

 the action of these cross strains the bar should be stronger. That this is so has 

 been abundantly proved, but the author goes further in describing an experiment 

 showing that in drilling several holes in a line across the bar, the tenacity increases 

 with an increased number of holes. Thus an iron bar two inches wide by half inch 

 thick broke at 22'68 tons per square inch of original area. When an inch hole was 

 drilled in the centre of a test piece cut from the same bar the tenacity increased to 

 22'93 tons. Two holes half an inch in diameter increased the tenacity to 2482 

 tons. Three holes one-third inch diameter increased the tenacity to 24-99 tons, or 

 about 1 per cent, total increase, comparing the plain bar with that perforated with 

 three holes cutting away half the section. 



These facts show that the form of a test bar may be such as to bring into action 

 the cross fibres of the material in such a way as considerably to afi'ect the results 

 obtained. 



FRIDAY, AUGUST 25. 

 The following Eeport and Papers were read : — 



1. Report on the Committee on Patent Legislation. — See Reports, p. 310. 



2. The Channel Tunnel. By J. Clarke Hawkshaw, M.A., F.G.S., M.I.C.E. 



See Reports, p. 404. 



3. A System of Excavating the Channel Tunnel by Hydraulic Machinery. 



By T. R. Crampton. 



The chalk from the cutters is received into a revolving drum, to which also the 

 waste water from the hydraulic motor employed to drive the machinery is led. 



' Proceedings of Mechanical Enr/ineers, April 1881. 

 2 Proceedings of Iron and Steel Institute, l\Iay 1 882. 



