CHAPTER X. 







GENEEAL PHENOMENA EXHIBITED BY TEST 

 PIECES UNDEE VAEYING CONDITIONS. 



154. The strength properties of metals as gathered 

 from the results of mechanical tests can be divided into 

 two classes. Firstly, each specimen exhibits qualities in- 

 herent to the material itself, these qualities varying only 

 with the material, and not being dependent upon any 

 outside circumstances connected with the test. In the 

 second place, certain general results may be expected to 

 follow certain methods of treatment of the test piece, 

 either before or during the test. Among such external 

 influences may be mentioned the manner of holding the 

 specimen, the form of the specimen, the influence of 

 temperature both before and during the progress of 

 the test, and of the speed of loading. Some of these 

 general properties will now be described. 



*55- Forms of Test Pieces. In order to fully ap- 

 preciate how the results of a test are affected by the shape 

 and proportions of the test piece, consider the case already 

 referred to on page 143. Here is a bar of ductile metal- 

 mild steel broken in tension in the ordinary way. Pre- 

 vious to the test a number of lin. lengths were marked 

 off along the bar. After the test these were measured, 

 and all were found to have increased in length, but not 

 all to the same extent. Those inches near the fracture 

 were extended much more than those more remote, 

 owing to the local extension and the greater reduction in 

 the area of cross section at the point of fracture. The 

 percentage extension as reckoned on the full 10 inches is 

 30 '6 per cent. 



If, now, the first inch is omitted, and the extension 

 reckoned on the remaining nine (seep. 148), the extension 

 will be : 



2^87 

 9 



X 100 = 31-9 per cent. 



