B42 .- REPORTS ON THE STATE OF SCIENCE, ETC. 
Many examples could be given, but perhaps the best is afforded by the ‘ Notched-bar 
Impact Test.’ of which the early history has been recounted by Stanton and Batson in 
a recent paper.! Originally devised by Barba as a method of detecting brittleness in 
steel, and subsequently developed by Charpy, Izod, and others, this has come more and 
more into favour as a means of determining the ‘ toughness’ of a material, by which 
is meant its ability to resist fracture by sudden shock. And, as its use has extended, 
the need of standardisation has become more imperative ; but against this standardisa- 
tion has always been the objection that very variable results are given by different 
individual tests of the same material. Moreover, it has been urged that ‘ the conditions 
of test are highly artificial in comparison with the conditions of practice, and that 
the results depend not only on the shape of the notch but also on the actual size of 
the specimen,’ so that ‘there is no certainty that the relative impact resistance of 
two materials as given by tests on a notched specimen of standard dimensions has any 
relation to the relative impact strength of the materials in a loaded structure.’ 2 
In this country the most common form is the Izod test, in which the specimen is 
held by a vice at one end and struck at the other by a falling pendulum ; but abroad 
the Charpy test is more generally employed, in which the specimen’ is supported 
horizontally at its ends on knife edges of given form, and struck in the centre, opposite 
the notch, by a pendulum. Stanton and Batson, holding the view ‘that, although 
standardisation of the method of test and of the dimensions of the specimen was very 
desirable, such standardisation was to be deprecated until a clearer interpretation of 
the test had been attained,’ beganin 1913 a careful study of the dimensional effect in 
such tests, the proposal being ‘ to carry out a large number of impact tests in which 
geometrical similarity as regards both form of specimen and testing appliances should 
be preserved.’ * This work, after frequent interruptions during the period of the war, 
was brought to a conclusion in 1920, and reported in the paper cited above. Notches 
of the standard Charpy form, and also V notches having an angle of 45°, were tested, 
and in each case an important dimensional effect was observed—the energy absorbed 
in fracture, per cubic centimetre of the specimen, falling steadily as the dimensions of 
the specimen were increased ; the serious implications of this result, in regard to the 
value of the test as an absolute indication of ‘ toughness,’ were pointed out, and the 
conclusion was drawn that ‘the value of the impact test lies, not in discriminating 
between the impact resistances of different materials, but as a means of ensuring that 
the impact strength of any given material is at its highest.’ 4 
In a note > communicated to the Aeronautical Research Committee, I endeavoured 
to examine these results in the light of dimensional theory. I showed, ‘on the 
assumption that the occurrences at fracture, for a given quality of material, are com-_ 
pletely determined by the velocity of striking, and by the dimensions of the specimen 
and apparatus (the material of the apparatus being kept the same throughout, so that 
the mass of the striker varies as the cube of its linear dimensions), that the energy 
of fracture per unit volume should be a function only of the striking velocity, provided 
that the stress » at which fracture begins is an absolute constant of the material ; 
but that the energy may vary with the dimensions of the specimen, if » depends in 
addition upon the rate of straining.” Thus it appeared that there is a possibility of 
scale effect in material characterised by what may be termed ‘ solid viscosity,’ but not 
in others; and I was informed by Dr. Stanton that this theoretical conclusion is in 
general agreement with his experimental results, since he found that the impact figure, 
for a constant velocity of striking, varied with the absolute dimensions of the specimen 
and apparatus in the case of ductile materials such as mild steel, but not in the case of 
brittle materials, where the distortion of the specimen before fracture was comparatively 
small, 
If this application of dimensional theory is legitimate, it will follow that the results 
of impact tests conducted on specimens of varying size but of similar proportions, and 
with apparatus in which geometrical similarity with the specimen is maintained, 
should enable us to classify materials under the two headings of materials which 
exhibit ‘viscosity’ and those which do not. And if confirmed by experiment these 
conclusions would not be devoid of practical significance since they indicate the 
importance, in the determination of ‘impact figures’ for materials which come under 
the first heading, of standardising not only the shape but also the dimensions of the 
specimen and apparatus; for materials of the second class, only the shape would 
appear to be important. 
But the conditions of test must be such that geometrical similarity is maintained 
in the apparatus as well as in the specimen, and I pointed out that it is doubtful whether 
