Plastic Crystals of A 



mmonium 



Nitrate. 



11 



the crystal AA, if we consider the forces acting on the right 

 hand side, we see that the unknown fractional force T does 

 not take any appreciable part in the bending. Further, the 

 moment of the force N is almost directly proportional to the 

 distance measured along the crystal, since with even con- 

 siderable bending each half of the crystal is almost straight. 

 (See fig. 12-6.)! 



For the elastic bending of a uniform beam we have : — 



- = ^p, where o= radius of curvature of beam. 

 p HiL 



M = applied bending moment at the point, 

 E = Young's modulus for the beam, 

 I — moment of inertia of a transverse sec- 

 tion (constant). 



For simple viscous flow, the equation for calculating 

 bending may be written 



d /1\ M 



dt\p) ~ \T 



Kff. 11. 



Value oj Stress, S. 



Thus for moderate deflexions, the curved shapes for 

 elastic and simple viscous bending are of approximately the 

 same form, being identical for very small amounts of bending. 



It should be noticed that, -as the crystal bends, its ends 

 slide over the pegs inwards. Thus the parts now forming 

 the ends of the curve have not been subject to bending for 

 the full time. They should thus be less bent than the corre- 

 sponding elastic curve. 



On comparing these (see figs. 12a and 12 M, we find that 

 in either case the part concerned is sensibly straight, so that 

 no appreciable alteration is caused by this sliding over. 



