24 



ULTIMATE STRENGTH. 



Art. 23. 



begins the piece "necks down" and the 

 elongation also becomes largely local. This is 

 shown in Fig. 16, rupture occuring at E. 

 This is the plastic state; the deformation is 

 all permanent or non-elastic. 



Ductility and plasticity are important 

 qualities in structural materials, especially 

 # in structures subject to impact - stresses. 

 Brittle materials break off short without giv- 

 ing any warning of failure. These qualities 

 are measured in a tensile test by the percent- 

 age of elongation and the percentage of re- 

 duction of area at the fracture. For struc- 

 Fig. 16. tural steel, the percentage of elongation in 



8 inches (including fracture) is 20% to 30% and the reduction 

 of area about 40% to 60% ; the larger figures apply to the softer 

 steel. 



23. Ultimate Strength. By ultimate strength is meant the 

 greatest unit stress which a test piece will show in a test, and is 

 obtained by dividing the maximum load on the piece by its origi- 

 nal area. The ultimate strength in tension, according to this 

 definition, is easily determined as explained in Art. 21, and has 

 been determined by many experiments on many kinds of mater- 

 ial; it is one of the results obtained in all commercial testing of 

 iron and steel. The ultimate strengths in pure compression (in 

 short blocks, not in columns), and in shear are of less importance, 

 and have been less definitely determined for various materials. 

 It is difficult to produce shearing stresses without bending 

 stresses. In compression, the character of failure depends very 

 much upon the kind of material; ductile and plastic materials, 

 like soft steel, copper, lead, asphalt, etc., will expand laterally 

 almost without limit, so that the intensity of stress reckoned on 

 the original area is much larger than the real stress; fibrous 

 materials, like wrought iron and wood, will split on account of 

 the lateral expansion, that is, they actually fail in tension ; brittle 

 materials, like cast iron, brick, stone, etc., fail with little lateral 

 expansion and usually by shearing on oblique planes, the friction 

 on the plane of failure, and on the faces to which the pressure 

 is applied, having considerable influence. In both compression 

 and tension, any interference with the lateral deformation, in- 



