224 STRENGTH OF MATERIALS 



173. Relative strength of large and small test pieces. It has been 

 found by Tetmajer * and others that the values obtained in testing 

 small test pieces taken from different parts of a steel girder or I-beam 

 are higher than those obtained in testing the girder itself. The aver- 

 age of a series of tests of small test pieces gave an elastic limit of 

 49,000 lb./in. 2 and a maximum strength of 62,000 lb./in. 2 . Tests on 

 the complete girders themselves gave an elastic limit of 33,500 lb./in. 2 

 and a maximum strength of 54,500 lb./in. 2 . The same has been found 

 true for the elastic limit of wrought-iron girders, but in this case the 

 maximum strength is greater in the girder than in the small test piece. 



174. Strength of iron and steel at high temperatures. From a 

 series of tests made at Cornell University,! it was found that wrought 

 iron having a tensile strength of 30,000 lb./in. 2 at ordinary tem- 

 peratures increased in strength with increase of tempera lure up to 

 475 F., and then decreased as the temperature* was further raised. 

 Machinery steel of 60,000 lb./in. 2 maximum strength ^ave at 47". I . 

 a maximum strength of 111,500 lb./in. 2 . Tool steel having a strength 

 of 114,000 lb./in. 2 at ordinary temperatures a\v 145,000 lb./in.* 

 maximum strength at 350 F. 



Professor C. Bach also reports an elaborate series <>i testa <>n the 

 strength of steel at high temperatures.^ At ordinary temperatures 

 one bar had a maximum strength of 54,000 lb./in. 2 , an elongation in 

 8 in. of 26.3 per cent, and a contraction of area of 46.9 per cent. 

 Up to a temperature of 572 F. the strength increased by about 

 7000 lb./in. 2 , and from this point fell, approximately in proportion to 

 the temperature, to 26,200 lb./in. 2 at 1022 F. The ultimate el 

 tion decreased to 7.7 per cent at 392 F., and then increased to 39.5 

 per cent at 1022 F. The contraction of area fell until 392 F. wafl 

 reached, and did not rise until about 572 F. 



While the tensile strength is increased for a moderately hi^li tem- 

 perature, the elastic limit is lowered in proportion to the in< re;, 

 temperature, being diminished about 4 per cent for each increase of 

 100 F. 



175. Character and appearance of the fracture. The kind and 

 quality of the metal are usually indicated by the character of the 



* Communications, Vol. IV. t Journal Western Society of Engineers, Vol. I. 



J Journal Franklin Institute, December, 1904. 



