292 REPORTS. ON THE STATE OF SCIENCE, ETC. 
In the present report a brief account will be attempted of some experimental 
investigations which have been made on discontinuities by photo-elastic methods 
described in a previous report (1), whereby stress distributions have been deter- 
mined sufficiently completely to allow of a fairly accurate value being assigned 
to the maximum stresses experienced under given loads. In all these cases the 
member, a flat bar, has been subjected to plane stress, and the results are, 
therefore, only applicable strictly to this kind of stress except in some cases 
which will be noted. 
An interesting case of a discontinuity of a simple kind arises where it is 
necessary to alter the cross section of a member in a definite manner, as for 
example in a flat tension member or a cylindrical rod. ‘This case occurs in a 
variety of instances in practice, and the salient fact in connection with this 
type of discontinuity is the increase of stress which occurs near the join of the 
smaller to the larger section when the contours are formed by circular arcs, 
which are tangential to the boundaries of the smaller section and intersect the 
larger at an angle. 
The maximum stresses reached are shown experimentally not to occur exactly 
at the joins (2), but slightly beyond, at places where a tangent to the contour 
makes a small angle to the axis of the specimen. Thus, for example, in a case 
ppt (6976.A) 
where the smaller breadth of a flat bar is .4488 in. and the larger 0.855 in. 
connected by arcs of 0.3 in. radius, the maximum stress is found at a point 
of the curved contour where a tangent line makes an angle of about 8° to the 
line of pull, and the stress intensity is nearly 20 per cent. greater than the 
uniform stress in the smaller section. 
The distribution in this member at the contour is shown in the accompany- 
ing fig. 1, from which it will be observed that a maximum stress of 1,470 lb. 
per sq. in. is reached when the uniform stress in the smaller section is 
1,230 Ib. per sq. in. As might be expected, this maximum stress increases with 
decrease of radius at the re-entrant angle, and in a very nearly similar contour, 
but with a re-entrant angle of j!; in. radius, an increase of stress of 57.5 per cent. 
was observed. 
The type of contour shown in fig. 1 is probably more frequently met with 
in British practice than any other. It is, for example, adopted by the 
Engineering Standards Committee as a standard form of end (with a radius of 
one inch for the connecting arc) for tension test members of boiler-plates and 
like material. 
As will be observed from the contour stresses, the distribution in the region 
of the discontinuity is extremely variable and in the interior of the plate is of 
a complex nature. 
