230 STRENGTH OF MATERIALS 



of the whole specimen, due to the twisting, is corrected in part by 

 the swinging head of the machine shown in Fig. 150. 



180. Shearing tests. To determine the shearing strength of timber 

 along the grain and the resistance of iron and steel to the pulling out 

 of rivets, many special tests are used. By means of a special piece of 

 apparatus, the force required to push off, along the grain, a projecting 

 piece from a test piece of timber is easily measured on tin 4 ordi- 

 nary tension-compression machine. The intensity of shearing stress is 

 computed by dividing the load by the area of the block pushed on". 



Tests are also made on wn.u^ht-iron plates to determine the force 

 required to pull out a rivet through the metal, both in the direct inn 

 of the fiber and perpendicular to it. A series of such tests may be 

 found in the Watertown Arsenal Report for 1882. Many tests have 

 also been made to determine t! i length of i: 



181. Impact tests. In actual service many materials are subject, d 

 to shock or impact (see Article 73). This is especially true of all 

 railway structural material, such as rail>. <>nj>lcrs, 

 bolsters, wheels, etc., which must be designed to withstand consid- 

 erahle shock. Two special machines have been designed to test 

 materials in impact. The first, called the drop testing mnchii 

 operated by allowing a given weight (hummer) t<> drop a gi\< -n dis- 

 tance upon a test piece mounted on an anvil under the hummer. 

 The other form of machine is operated by allowing a heavy pendu- 

 lum to strike the specimen when placed in the center of its swin^. 

 In either case the amount of the energy of the blow absorbed by the 

 specimen is desired. 



The results obtained from impact tests can only be comparative in 

 any case, since a part of the energy >f the blow must be abs< 

 by the parts of the machine itself. This is seen in the drop t- 

 machine in the absorption of energy by the anvil und hummer. 



Since the results of such tests cannot be absolute, it is highly 

 necessary that they should he standardized by making tests on the same 

 anvil with the same hammer. The M i Builders Association 



has taken a step toward such standardization by building an impact 

 testing machine for testing materials used by them. This machine 

 has been established at Purdue University. Its maximum Mow is 

 given by a hammer having a weight of 1640 lb., and dropping 50 ft. 



