More accurate analyses of adhesive EA946 can now be accomplished using new mathematical models. Tests involving this adhesive (used in the space shuttle's reusable solid-rocket-motor nozzle and other rocket-motor nozzles) indicated that the adhesive is nonlinearly viscoelastic at short equivalent times and linearly viscoelastic at long equivalent times. These tests also showed that the material properties of EA946 change with aging time after cure.

This mathematical model shows that Stress Relaxation Curves were divergent for short equivalent times and convergent for long equivalent times at 70 °F (21 °C).

The viscoelastic nature of EA946 was modeled using a strain-dependent time-shift factor. Aging effect with only a time-shift factor was also modeled. An assumption (from earlier test data) that the bulk modulus of this adhesive remains constant for all times and temperatures was used for these modeling investigations.

During previous testing of this adhesive, master stiffness curves at various strain levels were recorded. The curves were generated from stress-relaxation data at a given strain level, using several different temperatures.

The figure illustrates how stress-relaxation curves of 1, 3, 5, 7, and 10 percent were divergent for short equivalent times and convergent for long equivalent times. Aging time after the cure caused the trend differences between the 1-percent and the 3-, 5-, 7-, and 10-percent stress-relaxation data.

Strain-shift and aging factors were added to the testing to account for the nonlinearities and aging effects. The temperature-shift factor was also introduced into the model.

After many attempts to model the material response of EA946 using a linear viscoelastic model, nonlinearities were introduced into the model. Shift factors that were a function of strain-level and time were used to evaluate the nonlinearities. This approach for evaluating the nonlinearity of adhesive EA946 accounted for the strain-level dependence of uniaxial tests (as noted in the illustration).

Tests showed the material properties of EA946 to be sensitive to the effects of age-time after cure. A series of additional aging tests conducted at 0 day, 1 day, and 3, 5, 10, 60, 90, and 365 days indicated that the elastic modulus increased 110 percent with 90-day aging and 370 percent with one-year aging. The ultimate strain decreased by approximately 75 percent, while the strength of the bulk adhesive did not appear to increase. Additionally, the bondline strength increased by approximately 25 percent during one year of aging.

This work was done by David E. Richardson and Russell A. Crook of the Thiokol Corporation for Marshall Space Flight Center. MFS-31161