,314 IJAISIXt: Till: LIMIT. 



The origin of the load strain curve is at A. As the 

 test proceeds, the diagram follows the straight line up to 

 B, where the yield point occurs. From this point the 

 curve follows the usual upward semiplastic curvature 

 towards C. If, now, at C, some point above the yield 

 point, the load is removed, an elastic contraction will take 

 place, and this is shown, either by an autographic apparatus 

 or by taking observations with an extensometer and 

 plotting, by the straight line C D parallel to A B. 



On the load being again increased, beginning from D, 

 the extensions will be such that the curve will be retraced 

 along D C ; when C is again reached, the point C being 

 found to be the yield point for this second loading, the 

 semiplastic extension will take place in the direction C K. 



And so on for any further loadings, a new limit being 

 established for each successive stage. It would appear 

 as if, after each overstrain, a new metal was produced, 

 having a higher yield point than before exposure to the 

 stress above its former limit. The curve A B C E G I, 

 and so on, would coincide approximately with the com- 

 plete load-strain curve drawn for the original bar without 

 any unloading having taken place. 



Owing to the fact that all the elastic portions of the 

 curves AB, C E, D C, F E, H G, I J, &c., are found to be 

 parallel, it is to be assumed that the modulus of elasticity 

 in each case is sensibly the same for the same bar. 



In order to prevent any mistakes as to these successive 

 limits, the first limit, as it is found for the unstrained 

 material, has been termed the primitive or original elastic 

 limit. 



Both the modulus of elasticity and the elastic limit 

 are affected by the temperature to which the bar is 

 exposed. The modulus in all metals decreases to a small 

 extent with a rise in temperature. Thus, if wrought 

 iron or steel is raised in temperature from 60 Fah. to 

 360 Fah., it will be found that the modulus has been 

 reduced about six per cent. This value is quoted by 

 Johnson* as deduced from published results of tests at 

 the Wartertown Arsenal. 



The elastic limit for steel also decreases continuously 

 as the temperature, increases, a rise in temperature of 



* Johnson's " Materials of Construction," page o62. 



