154 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1952 



O 1.85 



O 1.55 



1 2 3 4 5 6 7 8 10 20 30 40 50 70 100 



FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 23 — Relation between measured longitudinal velocity for polyisobutylene 

 of molecular weight 5590 and that calculated from shear stiffness measurements 

 assuming the Lam6 X elastic constant is independent of frequenc}-. 



constant as determined by the shear measurements. The points are 

 velocities measured for longitudinal waves and as can be seen, the meas- 

 urements agree closely with the calculated values. A slightly better 

 agreement would be obtained if X increased by a small amount as the 

 frequency increased. As discussed in the next section there is some 

 experimental evidence for an increase in X in nylon 6-6 and in poly- 

 ethylene. 



The question also arises as to how much of the attenuation is due to 

 shear mechanisms and how much due to pure compressional effects. 

 From longitudinal velocity and attenuation measurements at 30°C for 

 the polymers E, F and G of Table I, the values of X + 2ju and x + 2r? 

 can be determined and are shown by Table IV. The values of m and t] 

 can be obtained from Table I by interpolation and are given in columns 



Table IV 



Polymer 



X + 2m dynes/cm2 



X +2, 

 poise 



M dynes/cm^ 



\ dynes/cm^ 



5 megacycles 



8 megacycles 



