Characterization and modeling of materials used in improvised explosive devices
The mechanical response of energetic materials, especially those used in improvised explosive devices, is of great interest in the defense community. By understanding the mechanical behavior of the explosive material, it is believed that a remote acoustic or electromagnetic excitation may be tuned to produce signature that can be used to indicate the presence of explosives. The goals of the investigation were to identify macroscopic uni-axial mechanical material properties, and to develop robust models of uni-axial behavior of polymer energetic materials. Attention was restricted to uni-axial deformation of hydroxyl-terminated polybutadiene (HTPB) binder embedded with ammonium chloride crystals (NH4Cl). An elastic Ogden model was fitted to stress-strain data from uni-axial compression tests conducted on the HTPB binder. From the low-strain compression test data estimates of the Young's Modulus of the binder matrix where found to range from 2.67 MPa to 7.56 MPa. A series of swept sine-wave base-excitation tests were conducted on 0% and 50% crystal/binder volume fraction materials attached to a mass to examine the behavior and repeatability of the material-mass system dynamic responses. A continuous-time system identification approach is applied to develop models that predicted the harmonic base excitation responses. The estimated models were analyzed to produce estimates of the material properties. Six different models containing different combinations of linear and nonlinear expressions of stiffness, damping and viscoelastic terms were considered. While good agreement between the response measured in experiments and responses predicted from a linear model without viscoelasticity were often obtained, the inclusion of a hereditary viscoelastic term significantly improved the results. Some improvements were achieved when using a nonlinear viscoelastic model for the 50% material. Estimates of the damping ratio ranged from 0.10 to 0.13 for the 0% material and from 0.20 to 0.22 for the 50% material. Lastly, the Young's Modulus (E), was estimated from linear approximations to the stiffness terms in the equation of motion; for HTPB 0%, Young's Modulus ranges from 3.307 MPa to 7.251 MPa, and for HTPB 50%, Young's Modulus ranges from 29.63 MPa to 66.29 MPa. The Young's Modulus estimates of the HTPB binder matrix are of the same order and range as the estimated from the room-temperature compression tests.
Davies, Purdue University.
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