| In order to absorb the explosive stress wave, a new kind of composite material was developed. Polyurea elastomer, which has a good application in the field of impact- and blast-tolerant composite structure, was selected as the matrix. Fly ash hollow sphere, which has low-density and low-cost, was selected as the filler. The viscoelastic properties of polyurea and fly ash/polyurea (FA/PU) composites, including tensile properties at various strain rates, low frequency, high-frequency dynamic mechanical properties and relaxation behavior, were studied systematically by using tensile testing machine, dynamic mechanical analyzer (DMA) and ultrasonic technique. Appropriate mechanical models were introduced to characterize and predict the properties of polyurea and the FA/PU composites. Furthermore, the microstructure of the composites was researched by using scanning electron microscopy (SEM), modulated differential scanning calorimetry (MDSC).Results show that the hollow spheres are distributed homogeneously in the matrix and have good interfacial adhesion with the matrix. Polyurea shows microphase separation and the glass transition temperature of its soft segment increases due to adding hollow spheres.FA/PU composites is much more strain rate sensitive than polyurea in the strain rate range of 10-2～10-4s-1, which may be due to the inertia of polyurea "cell wall".The Young's modulus of FA/PU composites is in the range of 51～96MPa, 4%～97% higher than that of polyurea. The tensile strength and elongation at break of FA/PU composites are in the range of 34.8 ~ 11.3MPa and 410%～270%, respectively, both lower than those of polyurea. The large strain tensile behaviors of polyurea and FA/PU composites are characterized by using 4-order Odgen model, and hence, the parameters in the hyperelastic constitutive model are obtained. Dynamic mechanical properties of polyurea and FA/PU composites were studied in the temperature range -80°C to 70°C and the frequency range 1～20Hz.Results show that the temperature spectra of polyurea and FA/PU composites appear only one relaxation peak, corresponding to the glass transition of the soft segment. The Young's storage moduli of polyurea and FA/PU composites increase and the peak values of loss moduli decrease with an increase of frequency. It is also found that the glass transition temperatures of polyurea and FA/PU composites increase linearly with logf as frequency increases. The Young's storage and loss moduli of FA/PU composites increase with increasing of hollow sphere volume fraction. The composites filled with large-sized hollow spheres indicate higher dynamic moduli. When T>Tg, the effect of hollow spheres is more significant and the relative storage and loss moduli are both higher than 3.0. Frequency master curves of polyurea and FA/PU composites have been constructed through application of the time-temperature superposition. Then it can be seen that the storage and loss moduli of the composites both increase with increasing hollow sphere volume fraction over a wide frequency range, and the effect of the hollow spheres is more noteable at low frequencies than at high frequencies. The continuous relaxation time spectra of polyurea and FA/PU composites were calculated by the second approximation method. They show that segment movement is the main relaxation mode. The hollow spheres have little effect on the main relaxation time and main relaxation modes. However, contribution of the motor units with long relaxation time and the apparent activation energy (T> Tg) increase with the increase of hollow sphere volume fraction. These results indicate that movement of some segments of the polyurea matrix was restricted. As a result, the relaxation time of some segments becomes longer.The dynamic longitudinal, shear and bulk moduli of polyurea and FA/PU composites decrease and the Poisson's ratio of polyurea and FA/PU composite increases with increasing the temperature at 1MHz. As the hollow sphere volume fraction increases, the dynamic longitudinal and shear moduli of the FA/PU composites increase, and the Poisson's ratio decreases, however, the dynamic bulk modulus has little change. The experimental values of Young's storage moduli of polyurea and FA/PU composites at high frequencies are in good agreement with the predictions based on the frequency master curves, however, the experimental values of Young's loss moduli are much larger than those predicted ones. Hashin composite sphere assemblage model can be used to predict the high-frequency Young's storage moduli of the composites, whereas the prediction errors for Young's loss moduli are significant.Quasi-static confined compression tests were conducted. The results show that the FA/PU composites is able to absorb large amounts of energy while keeping the stress at a low level, as a result of the collapse of the hollow spheres. Quasi-static bulk moduli and dynamic moduli (1MHz) of polyurea and FA /PU composites increase linearly with increasing the stress. Effect of frequency on the bulk modulus of the composites is significant, however, it becomes unconspicuous when pressure increases.The results of the quasi-static relaxation tests (ε=1%) show that stress relaxation rates of polyurea and FA/PU composites increase and the stress retention ratios decrease with decreasing temperature. Both the initial stress and the stress relaxation rate of the composites increase with an increase of the hollow sphere volume fraction, resulting in a maximum stress retention ratio at Vf=10%. The stress relaxation rate varies slightly and the initial stress decrease as the hollow sphere size increases. As a result, the stress retention ratio of the composites decreases with increasing the size of the hollow spheres. The relaxation master curves of polyurea and FA/PU composites were obtained through application of the time-temperature superposition. Their time spans decrease when the volume fraction of hollow sphere increases. However, the relaxation modulus shows the opposite trend. The fractional derivative model was used to characterize the relaxation behavior of polyurea. Combined with Eintein model and Guth model, relation between the relaxation modulus of FA/PU composites and the hollow sphere volume fraction Vf and the parameters E0,E1 of polyurea was obtained, i.e., E (t )= A+B/(Γ(1-0.2367)t0.2367), A = (1 +2.786Vf)E0, B = (1 +2.5Vf +14.1Vf2)E1.
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