206 THE CIVIL ENGINEERS OF THE BODY 



disappears, then the stress has not been beyond the elastic limit of 

 the material. If a stress beyond an elastic limit has been applied, 

 part of the resulting strain remains after the removal of the stress 

 and the material has become permanently distorted. In other 

 words, the residual strain has become a permanent set. The 

 determination of an elastic limit comes then to be the detection of 

 the minimum permanent set. Hooke's Law (1660) states that, 

 within the elastic liinits, the strain produced is proportional to 

 the stress producing it. This law holds for all kinds of stresses, 

 but is not exactly true for all materials. The deviations, however, 

 are few and slight. Hooke's law may be written as 



Stress intensity = strain X constant. 



The constant in this equation is of the same kind as the stress 

 intensity, and is measured in units of force per unit area. It 

 varies (a) with the material stressed, and (b) with the type of stress 

 developed. That is, for every material there will be a constant 

 or modulus for tensile, for compressive and for shear stress. The 

 modulus of elasticity might be defined as the intensity of stress 

 which would cause unit strain. 



Young's Modulus is the direct or stretch modulus. It is always 

 denoted by the letter E. From Hooke's law — 



Stress intensity = Strain X E, 



tensile stress intensity 



oyE 



tensile strain 

 load on specimen -i- area of cross-section 



alteration in length -i- original length 



Wertheim sives the moduli of the following substances in grams weight 



The above values are given in order of increasing " perfection " and decreas- 

 ing ■■ strength " of elasticity. The figure last given, that of arterial walls, 

 may be taken as substantially that of elastic fibrous tissue. 



The same values are obtained for most materials for a pure 

 compressive strain. 



Shear Modulus (or modulus of rigidity, or of transverse elasticity) 

 is the modulus expressing the relation l)etween the intensity shear 



