178 < 'AST-IRON BEAMS. 



was used by Barlow for measuring the deflections of metal 

 beams, a fine steel wire being employed. 



105. Testing of Cast-iron Beams. The commercial test 

 for the quality of cast iron is most frequently carried out 

 upon a beam of rectangular section. This is more easily 

 performed than a tension test, and yields more uniform and 

 reliable results. The size of beam adopted for this pur- 

 pose is most generally either a 1 in. square section, tested 

 on a 1 ft. span ; or in some cases the span may be 3 ft. ; or 

 the section is rectangular, 2 in. deep, 1 in. broad, upon a 

 3 ft. span. The information required from the test is the 

 breaking load applied at the centre, the deflection at the 

 centre when fracture takes place, and the nature of the 

 surface of the fracture itself. 



The test may be carried out either in a testing machine, 

 or by the application of dead loads. Of these the former 

 is by far the more convenient, but deadweights may be 

 used when a machine is not available. Several of the 

 testing machine makers supply machines of small capacity, 

 specially adapted for the tests of cast-iron beams. If 

 the beam is of a 2 in. by 1 in. section on a 3 ft. span, the 

 load is to be gradually applied, and the deflection measured, 

 say, at every 500 Ib. increment of load until fracture takes 

 place. In connection with tests of this kind it is important, 

 as affecting the result, to place the beam in the proper 



FIG. 88. 



position in the machine. A beam is most often cast 

 edgewise, the vertical section being slightly taper. It 

 would at first sight appear that in order to obtain the 

 higher result from the test, that edge which has the 

 greater width should be placed so as to form the 

 tension surface. This would be so if the metal were 

 uniform, but it will most often be found that the better 

 and stronger metal is at the lower or narrower edge, 

 and the worst metal is to be found on the upper 

 edge, and therefore this must be put in compression. 

 Mr. Kirkaldy recommends that, in a commercial test of 

 cast iron, three test pieces should be cast in one, so that the 

 metal may be of a uniform character throughout. A com- 

 bined bar of this kind is shown on Fig. 88. The casting 



