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A B S T R A C T A viscoelastic fracture analysis has been developed for rocket grain fracture predictions. The fracture analysis uses a stress intensity factor technique to predict crack velocity histories under thermal and pressurization loading conditions. The theory is compared with two-dimensional pressurized tests of two typical rocket motor geometries using the viscoelastic material, Solithane 113.

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Samples approximately 2!/z x2!/2 in. square and 4 in. long were prepared from a single batch of TP H1011 propellant aged 29 months at 105°F. The well-aged propellant was used to minimize the chemical aging contribution and therefore emphasize the strain aging effects. Figure 1 illustrates the skmple hardware and instrumentation for measuring stress and strain as a function of time. Accurate sample displacement measurements were obtained with dual-dial gages (accurate to 0.0001 in. providing strains within ±0.13%). The force data were obtained with a stable force transducer (Transducer, Inc. accurate to ±0.4 psi and repeatable to ±0.12 psi) and were recorded with digital recorders (Monitor Lab accurate to ±0.003 mV, repeatable to ±0.001 mV) during and immediately after strain incrementing. Long-term data were acquired using a portable digital meter (Fluke accurate to ±0.002 mV, repeatable to ±0.001 mV). Time was recorded in seconds at the beginning of each increment and in minutes after two hours. Both excitation and output readings were recorded at all test times. The strain was induced by manually turning a turnbuckle connecting the sample holder to the test rack and thereby generating strain rates of approximately 0.2 in./in./min. Sample strain histories comprised two groupings: 1) samples were aged under gravity loadings for various times and then displacement incremented to 2% or greater tensile strain; 2) samples were tested and stored 2% tensile strain for various periods of time and were then displacement incremented to higher strain levels. Results and DiscussionsA reference 70° F test was conducted on a large sample strained to ~ 1 % and then incremented in ~ 1 % strain levels to -8% strain. The relaxation modulus for each strain increment is presented in Fig. 2.The viscoelastic modulus was essentially the same for each strain increment where all strains and stresses are referenced to the initial zero stress and strain. The viscoelastic modulus relaxed approximately 50% during each time increment of about 500 min. Figure 3 illustrates the relaxation modulus of a typical strain aged specimen history at 70°F. It was strained to 2.3% and monitored for 33 days. At this time the sample was incremented to 3.6% and then monitored another 36 days. The