TRANSACTIONS OF SECTION J. 437 



commercial testing of materials ; what is really found is better 

 defined as the "yield-point." It is well known that the yield- 

 point can be raised by mechanical means, that the application 

 of a stress greater than the yield-point raises the yield-point, 

 and that it may be artificially raised almost to the breaking- 

 point. 



With very delicate instruments a permanent set is observ- 

 able with stresses well within the yield-point, and the stress 

 fixed upon as the elastic limit depends upon the delicacy of the 

 instruments used in determining it. The stretching of a bar 

 within the yield-point consists partly of an elastic extension 

 and partly of a permanent set ; and it is this permanent set 

 which makes it so extremely difficult to determine the true 

 elastic limit. 



Professor Bauschinger has shown that these artificially- 

 raised yield-points are entremely unstable, and may be lowered 

 considerably by hammering the test-bar on the end and reload- 

 ing it, and that, moreover, the yield-point cannot be raised in 

 tension without at the same time lowering the yield-point in 

 compression. When a ba,r is subjected to stresses alternating 

 between tension and compression the elastic limit cannot be 

 raised, and the yield-point settles down to the true limit of 

 elasticity. 



Professor Bauschinger further points out that ordinary 

 materials of construction have their yield-points artificially 

 raised in the process of manufacture, and proves by a most 

 elaborate series of experiments that the true elastic limit — which 

 he still defines as the stress beyond which the strains cease to 

 be proportional to the stresses producing them — can be cor- 

 rectly ascertained by first subjecting the bar to a series of 

 stresses alternating between tension and compression ; the 

 limit then decreases to a value not differing appreciably in 

 tension and compression, and below the initial elastic limit or 

 yield-point. The elastic limit found in this way is about 8 

 tons for wrought-iron and 9^ tons for mild steel. The stretch- 

 ing which occurs at the yield-point for hard and soft steels 

 does not differ materially, from which it is inferred that hard 

 and soft material may be relied upon to work together in a 

 built-up structure, under ordinary working-stresses. The same 

 has also been observed in the case of iron. 



It is very desirable in all important tests of materials to 

 have a record automatically registered by the machine itself : 

 such an apparatus is called an autographic stress-strain appa- 

 ratus, because it draws a diagi;am which shows the strain pro- 

 duced by stresses which vary from nothing to that required to 

 break the bar. The author possesses one designed by Professor 

 Kennedy, and made for the University of Sydney in liis labora- 

 tory. 



