222 STRENGTH OF MATERIALS 



In case the beam is loaded at the third points, uniformly, eccen- 

 trically, or otherwise, the corresponding expressions are used for liber 

 stress and deflection (see Articles 52, 66). 



170. Method of holding tension specimens. To make a tension 

 test of a material a special test piece is usually provided. This test 

 piece has the same composition as the rest of the material, but has a 

 special form, being larger at the ends than in the central portion (see 

 Article 20). Fig. 146 illustrates a test piece made from a carbon 

 steel bar turned down in the central portion.* The machines are 



provided with serrated wedges 

 for holding the large ends of the 

 test piece, and as the load is ap- 

 plied these serrations sink into 

 the specimen, thus holding it 

 firmly. 



The behavior of the specimen 

 in tension is studied by noting 

 FlG - 147 the behavior of the re<lu ! 



portion, which should be far enough from the ends so that the local 

 stress caused by the wedges will have no effect upon it. 



Flat pieces, such as pieces of boiler plate, are left as they come 

 from the rolls on two sides, and the edges are machined to get the 

 reduced cross section, as shown in Fig. 147. The lower specimen, 

 of cast iron, is made with rounded corners t< eliminate sin-in' 

 stresses. Rolled material is often tested without bi-ing turned down. 

 Special holders and clamps are usually provided i. .r hnl.lin 

 specimens of timber. 



171. Behavior of iron and steel in tension. Wrought iron and mild 

 steel when tested in tension conform to Hooke's law up to the el 

 limit, a point which is usually well defined in these materials. They 

 then suffer a rapid yielding, with little increase of load reach: 

 point where the piece elongates very much for no increase of In: id. 

 This point is known as the yield point. It is indicated by the scaling 

 of the oxide from the specimen that has not been machined, and by 

 the dropping of the beam of the testing machine, if it has been kept 

 balanced up to this point. Beyond this point stress increases much 



* Dimensions for standard test specimens of different materials are given in Article 195. 



