TESTING OF STRUTS. 



291 



151. Testing Columns. During the test of a strut, as 

 the load is increased the pillar is caused to deflect more and 

 more until the stress in the external 

 fibres, due to deflection and the simul- 

 taneous direct compressive load, carries 

 the metal beyond its elastic limit. When 

 this point is reached the deflection begins 

 to increase very rapidly and complete 

 collapse by buckling results. 



152. Fixing Struts in the Testing 

 Machine. The only really satisfactory 

 way of holding struts in the testing 

 machine is by having the ends hinged 

 or rounded, that is to say, quite free to 

 rotate. It is not usually convenient to 

 attempt to fix the ends, and this cannot 

 be done satisfactorily unless they are pro- 

 vided with very large feet, or firmly 

 bolted down. 



An easy and simple way of securing 

 the freedom of the ends is shown in 

 Fig. 150. 



Here the ends are cut square, and 

 two pieces of half- round steel are inserted 

 as shown. These allow the ends to rock 

 freely and effect the same purpose as 

 rounded or hinged-ends. Care should be 

 taken that the lines of contact coincide 

 precisely with the neutral plane produced, as other- 

 wise the load will be out of the centre. A better plan, 

 but rather more elaborate, is shown in Fig. 151. This 

 was first used by M. Considere, and also by Prof. 

 Bauschinger and Prof. Tejmajer. The latter provide 

 the ends of the struts to be tested with knife-edge or cone 

 bearings, the edge of the knife edges or the apices of the 

 cones being so placed so as to coincide with the geometric 

 axis of the pillar, as far as could be judged, without any 

 special means of adjustment. 



M. Considere adopted the plan shown in Fig. 151. 

 Here the ends of the strut are placed in sockets having 

 double knife-edge bearings on the compression plates of the 

 testing machine. The ends of the strut are to a small 

 extent adjustable in the sockets by means of screws. In 

 testing in this manner, before proceeding with the test 

 proper, moderate loads are applied and the direction of 



FIG. 150. 



