IKON AND STEEL 225 



fractured portion of the test piece. Two points are to be noted in 

 this connection : the geometrical form and the appearance of the fractured 

 material. Under the first we may have, as in tensile tests of hard 

 steel, a straight fracture where the material breaks squarely off in a 

 plane at right angles to the axis of the test piece ; or, as in tensile tests 

 of mild steel and high-grade wrought iron, a fracture which is cup- 

 shaped, half-cup, etc. The appearance of the material for the cup-shaped 

 fracture may be described as dull granular in the bottom of the cup 

 and silky around the edge ; or, in the case of wrought iron, as fibrous 

 in the bottom of the cup and silky around the edge. A cast-iron 

 fracture appears crystalline, the crystals being fine, coarse, or medium. 



In reporting a test the character and appearance of the fracture 

 should always be given. It should also be noted whether or not any 

 longitudinal seams occur, or whether the fracture shows the material 

 to be homogeneous and free from blowholes and foreign matter. If 

 the specimen has not been properly placed in the machine, so that 

 there is a bending moment, the fracture will indicate this. The axis 

 of the test piece should always coincide with the axis of the machine. 



176. Measurement of extension, compression, and deflection. The 

 extension in a tension specimen of iron or steel up to the elastic 

 limit is so slight that very accurate measurements must be made to 

 determine the elongations. Instruments for making such measure- 

 ments are known as extensometers, and are usually made to read to 

 .0001 of an inch. Fig. 148 shows a type of such instrument known 

 as the Yale-EiehlS extensometer. The method of using the instrument 

 is to mark off an 8-iii. gauge length on the test piece and fasten 

 the extensometer to it by inserting the screws in the extreme punch 

 marks of the gauge length. The backpiece is then removed and a 

 battery with a bell in circuit is attached ; the instrument is then 

 ready for use. As the piece elongates the elongations are measured 

 by turning the micrometer screw until it touches the armature, when 

 the circuit is closed and the bell rings. 



The instrument is used only a little past the elastic limit (the limit 

 of proportionality of stress to deformation), and about twenty elonga- 

 tions for corresponding loads are taken below the elastic limit. The 

 instrument is then removed and the test continued to failure, the 

 maximum load being noted. From the data obtained in making 



