MI:. .1. .\iriu ON THI: I;KO>\T.I;N <>|- n;<>N FI;<IM <>VKI;STI;.\I\ 



when recovery from overstrain had once more l)een effected fracture took place at 

 45 tons per square inch. It is however probable since this material is the same ;i* in 

 Diagram VIII., that had the primary loading in the present case been carried only 

 to 29 tons per square inch, then a yield-point would have been obtained at a stress 

 of 44 tons, and fracture would not have taken place until a load of over 49 tons per 

 square inch had been applied. A yield-point obtained at a high stress is thus a 

 crisis in the history of the specimen under test ; the material is in danger of giving 

 way, but if it does not, then, after recovery it will stand, before fracture occurs, 

 a stress 5 or 6 tons higher than that at the critical yield-point. 



It should, perhaps, be pointed out that in Diagram No. VII. no uniformity exists 

 in the position of the yield-points. In this case the specimen cannot, perhaps, be 

 taken as illustrating the behaviour of a certain material, for it will be remembered 

 that a small flaw ran through the centre of the bar from which this specimen was 

 taken, and probably this flaw had a considerable influence in determining the position 

 of the yield-points.* Chemically this material differed only slightly from that of the 

 other steel rods used, as is shown by the analyses given on page 4. 



Before concluding this section of the paper, attention should perhaps be directly 

 called to Diagram No. XL, which has already been incidentally referred to. It 

 gives the history of a specimen of common wrought iron, the diameter of the 

 specimen being 1 inch. Curve No. 1 illustrates the primary loading and shows 

 that the yield-point has occurred at a stress of 15^ tons per square inch. After the 

 large stretching had ceased, and the load had been removed, the 8-inch length of the 

 specimen was found to have been stretched about 0'20 of an inch. On re-loading, 

 the material exhibited comparatively little semi-plasticity, as is shown by Curve No. 2. 

 The load was, therefore, increased until a stress of 20 tons per square inch was 

 attained, the specimen being thereby stretched further by about a quarter of an inch 

 on the 8-inch length. On re-testing, the curve obtained was still found to agree 

 closely with Curve No. 2 up to the stress of 15 tons, but as the loading was now 

 continued to 20 tons the semi-plasticity was more clearly shown. Curve No. 5 shows 

 that a night's rest at the ordinary temperature has been sufficient to produce complete 

 recovery of elasticity ; so common iron recovers much more quickly than the semi- 

 mild steel employed for the most part in the course of these experiments. It may 

 be of interest here to recall that the half-inch specimens of comparatively mild steel, 

 employed for Diagram No. V., recovered at a very much slower rate than the harder 

 steel usually employed in these experiments. 



After Curve No. 5 was obtained the specimen was put in boiling water for a few 

 minutes to ensure perfect recovery. On testing, Curve No. 5 was repeated, and on 

 increasing the load a yield-point was got at 23 tons per square inch, as shown by 

 Curve 5'. Curve No. 8 shows that a few minutes in boiling water has effected perfect 

 recovery from this second overstrain. The maximum load of 23 tons was kept on in 

 this test for 45 hours, and only the slight creeping shown in the diagram occurml. 



* Sec p. 24. 



