Art. 23 



ULTIMATE STRENGTH. 



25 



creases the ultimate strength, and decreases the longitudinal de- 

 formation. In compression, the ratio of height to lateral dimen- 

 sion has a marked influence on the ultimate strength. 



Ultimate strength in compression has no definite meaning 

 unless the conditions of the test are fully specified 1 . This is true 

 in a lesser degree for tension also ; a bar of steel of large cross 

 section, for example, will show a lower ultimate strength than 

 one of small cross section. The usual tensile tests of steel and 

 iron are not exactly comparable because the test pieces vary 

 somewhat in shape and size, for practical reasons. 



For the purposes of the engineer, the limit of strength is 

 determined by that load which produces the first sign of fracture 

 or lateral flow. This limit, in both tension and compression, is 

 the yield point for such materials as have a yield point, and in 

 steel and iron, is about the same for both kinds of stresses. 



Some materials (timber, wrought iron, stratified stone) do 

 not have the same strength in all directions. 



ULTIMATE STRENGTHS 2 . 



J For a discussion of this point see Kent's Mechanical Engineer's 

 Pocket-book, Johnson's Materials of Construction, and Burr's Elasticity 

 and Resistance of the Materials of Engineering. 



2 For detailed information with regard to ultimate strengths, see 

 Burr's Elasticity and Resistance of the Materials of Engineering, Jolin- 

 son's Materials of Construction, Ewing's Strength of Materials, Kent's Me- 

 chanical Engineer's Pocket-book, and the Cambria Steel Co.'s Hand-book. 



3 A single grade of steel, having an average ultimate strength of 

 about 60000 Ibs. per sq. in., is now being specified by a number of engineers. 

 Nickel-steel containing 39K of nickel, having an ultimate strength of 

 85000 Ibs. to 100000 Ibs. , and a minimum elastic limit of 50000 Ibs. , lias 

 been specified for the eye-bars of the Blackwell's Island Bridge, now being 

 built over the East River in New York. 



