rxSYMMKTIiir.M. l.ii.MMXG. 



where y is half the depth of the section ; and k is the 

 " radius of gyration," whose value is 



In the case of the bar which has already been referred 

 to (C, Fig. 157), a nick is cut in one side of the 

 bar, thus throwing the remaining section out of centre, 

 the line of the load passing now to the right of the 

 centre of the section, so that, in addition to the bar 

 being weakened by the nicking itself, it is further 

 weakened by the load being out of the centre of the 

 section upon which the load is brought to bear. 



This question of loading a tension bar out of the 

 centre is fully discussed in the Proc. Inst. Civil Engineers, 

 vol. cxxxiv., by Professor "\V. K. Dalby. 



162. General Deductions. A few general principles 

 may be deduced from what has just been said as to the 

 influences which affect the results of tests, quite apart 

 from the inherent properties of the material tested. 



Perhaps the most important of these is in regard to 

 the form of the specimen itself. The bar should be fairly 

 long, if possible eight or ten times the diameter, so as by 

 this means to get a reliable extension. In some cases, of 

 "course, this is impossible, and one has to be content with 

 a very short specimen. If there are heads at the ends of 

 the bar, the parallel portion upon which the measurements 

 are taken should merge into the heads by gradual and 

 easy curves, and there should be no sharp corners. 

 Also, where possible, the marks denoting the ends of the 

 measured portion should be well clear of the heads, so 

 that the extension may not be affected by them. 



Unsymmetrical loading, caused by a faulty form of 

 the ends, should, of course, be avoided, and it is also 

 obvious that there should be no cracks or grooves on the 

 surface of the specimen. The extensions will not be 

 greatly affected by the speed of loading under the 

 ordinary conditions obtaining in most testing machines, 

 but the effect of very gradual and very sudden loading in 

 extreme cases, both as regards strength and deformation, 

 should be borne in mind. 



