COMPLEX STRESS DISTRIBUTION IN ENGINEERING MATERIALS. 3138 
On Certain of the More Complex Stress Distributions in 
Engineering Materials.—Report of Committee (Prof. E. G. Coker, 
Chairman ; Profs. L. N. G. Firon and A. Ropertson, Secretaries ; 
Profs. A. Barr, GitBert Cook, and W. E. Daxsy, Sir J. A. Ew1ne, 
Prof. A. R. Fuuron, Dr. A. A. Grirriras, Prof. J. J. Guzst, Dr. 
B. P. Haiau, Profs. Sir J. B. HenprErson, C. E. Inetis, F. C. Lza, 
A. E. H. Love, and W. Mason, Sir J. E. Peravet, Dr. F. Rogers, 
Dr. W. A. Scosir, Mr. R. V. SourHweu, Dr. T. E. Stanton, 
Mr. C. E. Stromeyer, Mr. G. I. Taytor, and Mr. J. 8. Witson). 
Introduction. 
Tue Committee submit as their Report the following contributions, which include 
an account of an investigation on the fatigue strength of a hard drawn steel, both 
with and without the stress concentrations arising from a circular hole, and several 
papers dealing with the scientific side of testing. 
I. Photo-Elastic Methods of Testing. Prof. E. G. Coker, F.R.S. 
II. A Standard Form of Test-Piece. Prof. W. E. Dalby, F.R.S. 
Ill. The Impressed Conditions of Fatigue Tests. A. A. Griffiths, D.Eng. 
IV. The Influence of Circular Holes on the Fatigue Strength of Hard Steel Plates. 
Prof. B. P. Haigh, D.Sc., and Mr. Albert Beale. 
VY. The Distribution of Stress in Fatigue Test-Specimens (Torsion and Bending). 
Prof. W. Mason, D.Sc. 
VI. The Effects of Inaccuracy of Axial Loading. Prof. Andrew Robertson, D.Sc. 
VII. The Drop of Stress at the Yield Point of Ductile Materials. Prof. Andrew 
Robertson, D.Sc. 
VIII. Note on Impact Experiments. Mr. R. V. Southwell, M.A. 
I. Photo-Elastic Methods of Testing. 
By Prof. E. G. Coxer, F.R.S. 
The apparent simplicity of ordinary mechanical tests of the strength of materials, as 
usually made by tension, compression, shearing, and like forces, is so far from being 
true, that photo-elastic experiments on transparent models of test bars form a useful 
qualitative guide to their real behaviour under load, and moreover provide, in many 
cases, means of measuring the stress distributions which actually occur, the study of 
which latter may eventually lead to a real simplification and fuller understanding of 
this branch of experimental enquiry. 
Although bending tests of materials come first historically, they have not quite the 
primary importance which simple tension tests have now assumed, when an estimate 
of the properties of an engineering material is required, and, since most of the interest- 
ing features in this latter kind of test lie in effects produced at the ends, whether en- 
larged or not for gripping purposes, it will be convenient to commence by considering 
the case of a very wide plate subjected to uniform stress at its ends, and to examine 
what effects are produced when semicircular notches are formed in the sides, with 
their centres at the ends of a transverse section. Members of this type with notches 
close together have occasionally been used for testing the strength of materials, although 
they give little information about its tensile properties, but, as is evident, this primary 
form is capable of development into forms now in use as standard types of test bars. 
