ULTRASONIC VELOCITY IN EPOXY-RESIN AT TEMPERATURES DOWN TO 4.2 K

Results are presented from measurements of the velocity of longitudina l and shear elastic waves (f = 5 MHz) in ED-20 epoxy resin, at tempera tures from 4.2 to 293 K. These data were used to calculate the moduli of elasticity E' and G'. Also, the moduli E' (f = 250 Hz) and G' (f = 2 Hz) of the ED-20 resin were measured directly. The calculated veloci ties of the low-frequency elastic waves c(l) = (E'/rho)(1/2) and c(t) = (G'/rho(1/2) (where rho is the density) were compared with the resul ts from the ultrasonic measurements. The frequency dependence of elast ic-wave velocity in the epoxy resin was evaluated. It was found that w hen the frequency was varied by four orders of magnitude, the dispersi on of longitudinal-wave velocity was 18% at 4.2 K and 39% at 293 K. A quantitative criterion is proposed for estimation of the dispersion of sonic velocity from the results of measurements at any two frequencie s. It was established that the sonic velocity at low temperatures vari es linearly with the temperature, and in the case of longitudinal wave s changes from c(t) = 3360 m/sec at 4.2 K to 2820 m/sec at 293 K. The shear-wave velocity changes from c(t) = 1630 m/sec at 5 K to 1340 m/se c at 293 K. The values of the dynamic Young's modulus E' and shear mod ulus G' at 4.2 K are 8.6 and 3.2 GPa, respectively. On a plot of c = f (T) for ED-20, a transition is observed at 180 K, due to reorientation al rotation of methyl groups. The activation energy of the relaxation process is 3.6 kcal/mole. Large values are obtained for the dispersion of the dynamic modulus E': 37.8% at 4.2 K and 93% at 293 K.