CHAPTER XI. 



EXPEEIMENTS ON THE EELATION OF STEESS 



AND PEEMANENT STEAIN ; AND 



EEPEATED STEESSES. 



167. Relation between Stress and Permanent Strain. 



Just as in elastic deformation there is a definite relation 

 existing between the applied stress and the strain pro- 

 duced, so a law can be found for any given plastic 

 material, which formulates the relation existing between 

 a stress and its permanent strain. By making a series of 

 careful experiments on bars in a testing machine, noting 

 the stresses and corresponding strains, and plotting the 



10 



08 



\ -06 



K 

 at 



04 



02 



15 



16 



18 



20 



STRESS. TONS PER SQUARE INCH. 

 Fro. 161. STRESS-STRAIN DIAGRAM FOR WROUGHT-IRON BARS. 



stress-strain curve in the usual way, a graphic representa- 

 tion is obtained of the relation required in the form of a 

 curve, and an equation can be found to fit this curve. 



The method of obtaining the equation will be made 

 clear from a consideration of the following description of 

 a series of interesting experiments made by Dr. T. E. 

 Stanton,* now of University College, Bristol, with the 

 object of determining a definite relation between the 



Mem. and Proc. Manchester Lit. and Phil. Soc., 1893-4. 



