CROSS-BREAKING TESTS. 171 



ing tackle, the two most important conditions for accurate 

 testing are, first, that the load be applied in a direction 

 truly at right angles to the centre line of the test bar, and, 

 secondly, that there should be no possibility of a bending 

 stress being introduced. The latter condition can only be 

 attained by the bar being rigidly held in its place and 

 being allowed no play or freedom. In order that this may 

 be so the bar must either be very accurately turned and 

 fitted into the holes, or it must be firmly bolted in its place, 

 as in the Johnson gear, which has been described. The 

 true direction of the load is more easily preserved in a gear 

 in which the load is applied in a tension machine than 

 when applied in a compression machine. Probably a form 

 of tackle in which the holding arrangements of Professor 

 Johnson were combined with an attachment for holding 

 and applying the load in a tension machine, instead of as 

 at present, between the compression plates, would be as 

 reliable as any. 



CROSS-BREAKING TESTS. 



102. Systematic tests of materials under cross-breaking 

 stresses often require more preparation and more elaborate 

 appliances than tests in tension ; but for some purposes 

 and for some materials they provide valuable informa- 

 tion, and serve their purpose better. For instance, 

 bending tests of cast iron are more easily carried out, 

 and, as a rule, are more reliable than those in ten- 

 sion. Timber, again, is most frequently subjected to 

 bending stresses when in actual use, and bending tests 

 provide the most reliable information as to its qualities 

 for the purposes for which it is to be utilised. Wrought- 

 iron and steel, though not usually tested under bending 

 stresses in the form of small test pieces, are frequently 

 tested as rolled joists and girders, and in some cases in the 

 form of actual built-up beams ; this also applies to rails of 

 various sections. 



103. Testing Appliances used in Cross -breaking. As 



we have already seen, most testing machines intended for 

 general work are provided with special parts for the carry- 

 ing out of bending tests. In addition to these, there are 

 many small machines designed and used almost exclusively 

 for work of this kind. The plan adopted is very much the 

 same in all cases. To one ot the shackles of the machine 



