Bulletin of the American Physical Society
20th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 62, Number 9
Sunday–Friday, July 9–14, 2017; St. Louis, Missouri
Session Y4: Inelastic Deformations, Fracture and Spall XIII |
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Chair: Daniel Eakins, Imperial College London Room: Regency Ballroom A |
Friday, July 14, 2017 9:15AM - 9:30AM |
Y4.00001: The Taylor cylinder response of PC and PMMA A.D. Resnyansky, N.K. Bourne, E.N. Brown The well-known Taylor cylinder impact test, which follows the impact of a flat-ended cylindrical rod onto a rigid stationary anvil, is conducted over a range of impact speeds for two polymers, polymethylmethacrylate (PMMA) and polycarbonate (PC). Experiments and modelling were developed to capture the deformation behaviour of the cylinders after impact. The Taylor impact loading geometry enforces a strong influence of shear loading at impact velocities above a critical value. Introduction of shear stress at the rod-anvil interface was achieved in a new constitutive model by varying interface friction conditions. These works showed a region in which spatial and temporal variation of both longitudinal and radial deformation provided evidence of different failure modes. In a further series of experiments, we varied a range impact face conditions to control failure in the rod and resolve compressed regions within the recovered polymer cylinders. The combination of macroscopic high-speed photography and three-dimensional X-ray imaging with new constitutive modeling has identified and described the development of failure with these polymers. [Preview Abstract] |
Friday, July 14, 2017 9:30AM - 9:45AM |
Y4.00002: Simulation studies of shock wave propagation and reflection in semi-crystalline polyethylene Robert M. Elder, Tanya L. Chantawansri, Yelena R. Sliozberg, Timothy W. Sirk, In-Chul Yeh, Thomas C. O'Connor, Mark O. Robbins, Jan W. Andzelm Polyethylene (PE) fibers are used in many applications where high-strain-rate impacts occur, so understanding their response to such events is vital. Although PE fibers often have high crystallinity, they also contain defects such as amorphous domains. Using molecular dynamics simulations, we generate compressive shock waves of varying strength in crystalline, amorphous, and semi-crystalline PE models. The differing properties of amorphous and crystalline PE result in an impedance mismatch, which causes partial reflection/refraction of shock waves at interfaces between the phases. We quantify the properties (e.g., pressure) of these waves and the reflection/transmission of energy at interfaces, and we compare with a simple continuum-level theory. The theory and simulations agree that amorphous domains attenuate weak shocks more effectively than strong shocks. However, the simulations unexpectedly show that small amorphous domains reflect less energy than theoretically predicted. We identify nanoscale mechanisms that reduce the impedance mismatch, and thus reflection, at thin amorphous domains, including confinement-induced stiffness, chain ordering, and density. The significance of these results emerges as a design choice, in that processing techniques can be used to tune the size of amorphous inclusions for the requirements of a particular application. [Preview Abstract] |
Friday, July 14, 2017 9:45AM - 10:00AM |
Y4.00003: Spall Strength Measurements in Transparent Epoxy Polymers Jonathan Pepper, Meysam Rahmat, Oren Petel Polymer nanocomposites are seeing more frequent use in transparent armour applications. The role of the microstructure on the performance of these materials under dynamic tensile loading conditions is of particular interest. In the present study, a series of plate impact experiments was conducted in order to evaluate the dynamic response of an epoxy (EPON 828) cured with two differed hardeners. The purpose was to compare the role of these hardeners on the dynamic performance of the resulting transparent epoxy. The material response was resolved with a multi-channel photonic Doppler velocimeter. This system was used to determine the shock Hugoniot and dynamic tensile (spall) strength of the materials. The experimental results are presented in reference to spall theory and are evaluated against results predicted by an analytical model of the impacts. While varying the hardener did not change the shock Hugoniot of the epoxy, it did have an effect on the measured spall strengths. [Preview Abstract] |
Friday, July 14, 2017 10:00AM - 10:15AM |
Y4.00004: Jamming transitions and high pressure response of a block co-polymer Rodney Clifton Previous Pressure-Shear Plate Impact (PSPI) experiments on polyurea, an elastomeric block co-polymer, have shown the shearing resistance of polyurea to increase proportionately with increasing pressure, reaching a shearing resistance of 1 GPa at a pressure of 18 GPa. In an attempt to understand this remarkably high shearing resistance at high pressures, percolation theory has been introduced -- likening the hard regions of the co-polymer to grains in a granular medium. A second order phase transformation, called a jamming transition, is hypothesized to occur at a critical reciprocal density as in the modeling of the effect of pressure on the shearing resistance of granular media. From percolation theory, the pressure-volume relation near the percolation threshold must be a power law centered at the percolation threshold (identified in the context of a block co-polymer as at a critical compressive strain). Application of this theory to a quasi-isentrope of polyurea leads to a 3-parameter model that fits the experimental quasi-isentrope, not just in the vicinity of a critical compressive strain but over nearly the entire range of strains for which data are available. Possible application of this approach to other materials is examined to assess whether or not percolation theory may be useful in a broader range of shock wave compression studies. [Preview Abstract] |
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