316 REPORTS ON THE STATE OF SCIENCE, ETC. 
cent., with a distribution curve as represented in fig. 3 (a), where it will be noted that | 
the maximum value is reached, not at the join of the straight with the curved part, 
bat at a point where the tangent to the contour makes an angle of about 15° with 
the axis of the specimen. With a radius of 4 in. the distribution (b) gives nearly 44 per 
cent. increase, while a further decrease to ;; in. radius gives (c), nearly 58 per cent. 
increase, and when the experiment was pushed to the extreme limit by making the 
curve as small as possible, a very small load was found to produce permanent stress at 
the re-entrant angle. 
These photo-elastic effects afford an explanation why materials like cast iron, hard 
brass, and even thin india-rubber specimens often break at sections which start from 
Stress distribution along the contours of test-bars near the enlarged ends. 
these points of maximum stress intensity, indicated in this reproduction of a natural 
colour photograph of a tension test-bar. 
The stress picture also raises another interesting matter, as it shows that the field 
of complex stress, represented by a variation in the colour field, invades the central 
parallel portion of the test-bar, so that not all of this can be in pure tension, and itis © 
at once seen to be necessary to so fix the gauge limits, inside which stress and strain are 
measured, that no complex stress is possible. 
An examination (1) of numerous models of test-bars given in standard specifications 
shows that very few specifications are free from criticism on this latter point, and 
especially is this so when the gauge length is short and the specimens are of circular 
cross-section. 
Taking the question of gauge length first, without the additional complication of a 
circular cross-section, we have, in a short member, a distribution of stress shown by 
the semi-perspective sketch, fig. 4, in which the greater of the two principal stresses 
Fia. 4. 
The stress distribution in a flat tension test-bar with enlarged ends. 
