THE STRENGTH AND DUCTILITY OF FLAT STEEL BARS. 209 
inset the origin is shifted slightly so as to correspond with the edge of the broken 
surface. In both cases the cross dimensions have been set up from a straight base, so 
that the figures do not represent the true shape of the bars. The profiles of the 1}-, 23-, 
3-, and 34-inch bars have been omitted for the sake of clearness. 
TABLE VI. 
Extension oF Bars or Dirrerent Wiprus, Mrasurep upon Lenctus PRoporTIONAL 
WiptH 
TO THE Ratio = 
THICKNESS 
Extension per cent. on a Length equal to 
Nominal Width. _ Width — 
aa eee) AG | Wedge Width Fae 
Thickness’ 2 Thickness * Thickness é 
a 
) $ 1:76 37:0 46°5 56°8 
P 1 3°81 31°3 40°5 50°6 
i 1} 5°64 316 39°9 49°9 
| 2 UT 30:2 38:4 49-0 
% 24 9°57 28°1 35°8 AT 5 
‘ 3 11:14 26°1 33°6 43-7 
34 13°32 24°9 32°5 42°8 
, 4 15°39 25°6 32°9 42°4 
5 In fig. 4 the actual outline of one of the bars is shown, and the variations in thick- 
ness are indicated by the contour lines, drawn at intervals of ‘02 inch (the contour for 
b a thickness of 0°21 inch is given also). There are evidently two nearly straight 
| 4 depressions crossing the bar, and intersecting each other at about the middle of its 
| width. This peculiar phenomenon, which is a characteristic of flat metal bars broken 
| ; by tension, is known as the Contractile Cross ; its essential features have been described 
elsewhere (5). Fracture takes place frequently, but not invariably, along one of the 
| i grooves ; in the figure the line of fracture has followed one groove, and the bottom of 
the other groove is indicated by the oblique dotted line. 
__ The extensions, as measured on various lengths proportional to the syuare root of 
the cross-sectional area of each bar, are given in Table VII., from which fig. 11 has 
been drawn. ‘he extension is much more nearly constant than in the case of a fixed 
‘gauge-leneth, but there are still variations. The change in the form of the constricted 
region has here a greater effect than in the case of the fixed length; in other words, 
the factor which formerly was sufficient to neutralise the effect of relative decrease of 
length and to keep the extension nearly constant over a certain range, is now able to 
give rise to an actual depression in the value of the extension within that range. The 
curves of extension of fig. 11 are therefore of wave form, the extension at first increasing, 
then decreasing, and then increasing again as the ratio width/thickness is increased 
continuously. There is no great difference in the form of the curves for lengths between 
2,/area and 15 ,/area. 
