CHAPTER II. 



STRESSES AND DEFORMATIONS BEYOND THE ELASTIC 



LIMIT. 



20. Non-elastic Stress and Deformation. For stresses 

 above the elastic limit, the deformation is chiefly permanent, and 

 deformations and stresses are not proportional the deformation 

 increases faster than the stress. Since working stresses are well 

 within the elastic limit, stresses and deformations beyond it are 

 of little importance in designing structures, but are important in 

 interpreting tests of materials. The character of the relation 

 which exists between stresses and deformations, may be seen best 

 if they are plotted on a stress-deformation diagram. 



21. Stress-deformation Diagrams. Fig. 15 shows a stress 

 deformation diagram of a piece of soft steel tested in tension. 



In accordance with Hooka's Law, 



T^fe^r^ %*%&*'* OA is a straight line, A A' being 



equal to the elastic limit, which, 

 in this case, is practically the 

 limit of proportionality between 

 stress and deformation. Above 

 -.1 the deformations increase 

 faster than the stresses, and at 

 B the material may be said to 

 flow, because no increase of 

 stress (or even a less stress) is 

 required to continue the deform- 

 ~ ation. The point B is called the 

 YIELD POINT because the metal 

 Fi S- 15 ' "breaks down" at this point; it 



is the "elastic limit" in commercial testing and is determined by 

 the drop of the scale beam of the testing machine ; it is, therefore, 

 much more conveniently gotten than the point A. 



For iron and structural steel, the points A and B are usually 

 close together, but for some ductile materials there is considerable 

 difference between the stress at the elastic limit and at the yield 



22 



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