TENSION TESTING. 



repeated once or several times. It is often found that the 

 first set of readings are not as uniform as could be wished, 

 and for this reason it is well to repeat the process. This, 

 is especially true in the case of observers who have not had 

 a great deal of practice. In some cases it is necessary to 

 take three, or even four, sets. Of course, it will be at once 

 apparent that, if the elastic readings are to be repeated, 

 the first loadings must not be up to the elastic limit. 

 Generally speaking, the limit occurs after the bar has been 

 stretched to about one-thousandth of its own length, so it 

 is safe to go to this point without affecting the initial state 

 of the bar. In the case we are considering the measured 

 length is 10 inches, and one-thousandth of this is one- 

 hundredth of an inch, so that the first set, or the two first 

 sets, may be carried up to this point in the mild steel bar 

 in question. This will be reached at a load of about twelve 

 or thirteen tons. Supposing a third set to have been taken 

 up to this point, and the readings are found to be 

 satisfactory and regular, then the test must be continued 

 by still applying increments of load and noting the 

 corresponding extensions. As the limit is approached, it 

 will be seen that the extensions per increment of load, 

 which up to this point have been uniform, will begin to 

 increase, and a point will be reached when these exten- 

 sions become too great for the capacity of the extenso- 

 meter. When this occurs the instrument must be removed 

 from the specimen, and the extensions of the bar beyond 

 this point measured by means of a pair of dividers and a 

 steel rule divided into inches and decimals of an inch, up 

 to hundredths. After each loading the dividers must be 

 set to the distance between the 10 in. marks by applying 

 them to the bar, and this distance measured by transferring 

 the dividers to the rule. It will be found more accurate 

 and convenient to apply the zero end of the dividers to 

 one of the inch divisions of the rule rather than to one 

 end. 



As the loads get well beyond the elastic limit, it may be 

 found convenient to make the increment greater; thus, 

 the increments may be changed from quarter-tons to half- 

 tons. Also, when the extensions become large, it will be 

 found that the extension in each case does not occur at 

 once, but that a little time must be allowed to elapse, 

 during which the bar goes on stretching, before the 

 measurement can be taken ; that is to say, the bar must, 

 as it were, be allowed to come to rest. 



