STRESSES IN OVERSTRAINED MATERIALS 331 
‘ The Effect of Fluid Pressure on the Permanent Deformation of Metals 
by Shear,’ by Prof. G. Cook, D.Sc. (Inst. of Civil Eng., Selected Engineer- 
ing Paper No. 170). 
‘The Lower Yield-Point in Mild Steel,’ by Prof. B. P. Haigh, D.Sc. 
(Section G, British Association, Aberdeen, September 1934). 
The Committee has considered these investigations together with other 
work bearing on the subject, particularly in relation to the requirements of 
the engineering profession and industry for present and future developments. 
The need for reconsideration arises in part from the increasing use of 
electric and other methods of welding in lieu of riveting in structural and 
general practice. While the practice of welding eliminates certain difficul- 
ties and dangers, it leaves the problem of plastic overstrain more pro- 
minently in the position of the limiting factor in design in many cases of 
economic importance. In such circumstances, it appears desirable that 
current methods of design should be adapted to recognise the danger of 
plastic strain in a more definite and formal manner, so that the margin 
of safety of a structure under its working loads may be judged by direct 
comparison with the overloads that would be required definitely to produce 
undue plastic overstrain in the ductile metal. 
The investigations carried out by the Committee confirm the view that 
has long been held by many engineers, that the elastic-limit in the case of 
mild or moderately high-tensile structural steel is unsuitable for use as a 
reference value for purposes of design. ‘The value depends unduly upon 
the method of testing adopted ; and when special precautions are observed 
to ensure that scientific accuracy is attained, the value appears to be much 
higher than is generally believed, and misleading for general application. 
The yield-point of the material is considered to afford a more reliable 
basis for design in cases where plastic overstrain is regarded as the limiting 
factor ; but the yield-point also—as now generally measured in accordance 
with current standard specifications—is open to the objection that the value 
also varies according to the method of testing and probably with the rate 
of application of load in the tensile test. 
The so-called ‘lower yield-point’ has been studied with particular 
attention and is considered to afford the most satisfactory and reliable basis 
for the comparison of structural steels and for the design of structures in 
which plastic overstrain must be limited ; and it is recommended that a 
specification of the lower yield-point should be adopted for the use of those 
who may desire to use it. At present, although the lower yield-point is 
already used by many who have experience of its advantages, no specifica- 
tion for its measurement appears among the British Standard Specifications 
for yield-point. 
The advantages of the lower yield-point, as a basis for the comparison 
of different structural steels and as a basis for design, may be summarised 
as follows : 
(1) In samples of mild or moderately high-tensile structural steel the 
value of the lower yield-point is readily determined in any ordinary 
tensile testing machine without the use of an extensometer or other 
sensitive or costly equipment. 
(2) Consistent values are obtained without any undue change in the 
current procedure for tensile testing, and are likewise obtained when 
the method of procedure is varied within limits to permit of changes 
to suit local conditions. 
(3) Although the value depends perceptibly upon the rate of straining 
adopted in the test, the variations that may be found when the rate 
