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 K8: Soft Matter III |
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Chair: Trevor Cloete, University of Cape Town Room: Grand Ballroom C |
Tuesday, July 11, 2017 2:15PM - 2:30PM |
K8.00001: Framework for analyzing hyper-viscoelastic polymers Akash Trivedi, Clive Siviour Hyper-viscoelastic polymers have multiple areas of application including aerospace, biomedicine, and automotive. Their mechanical responses are therefore extremely important to understand, particularly because they exhibit strong rate and temperature dependence, including a low temperature brittle transition. Relationships between the response at various strain rates and temperatures are investigated and a framework developed to predict response at rates where experiments are unfeasible. A master curve of the storage modulus's rate dependence at a reference temperature is constructed using a DMA test of the polymer. A frequency sweep spanning two decades and a temperature range from pre-glass transition to pre-melt is used. A fractional derivative model is fitted to the experimental data, and this model's parameters are used to derive stress-strain relationships at a desired strain rate. Finite element simulations with this constitutive model are used for verification with experimental data. [Preview Abstract] |
Tuesday, July 11, 2017 2:30PM - 2:45PM |
K8.00002: A split Hopkinson pressure bar technique for measuring the stress relaxation behaviour of polymers on microsecond timescales David Williamson Stress relaxation experiments are commonly used to probe the time dependent properties of materials such as polymers: a constant strain is applied to the sample and the stress is observed to decay as a function of time. Such measurements are typically performed using quasi-static testing machines with corresponding timescales of minutes to hours. Here we describe a 'dynamic' stress relaxation experiment using a specially configured split Hopkinson pressure bar with a corresponding timescale of microseconds. Conventionally the lengths of the striker-, input- and output-bars of split Hopkinson pressure bar system are contrived such that the end of the loading pulse (strain ramp) denotes the end of the experiment and later time stress-strain information is overwritten by reflected waves within the input and output bars. Here we present a bar configuration which extends the measurement window to include not only the strain ramp but also a subsequent period of near-constant strain during which the stress in polymer samples can relax. The data so obtained can support the development and validation of models which aim to describe the dynamic deformation of polymers. [Preview Abstract] |
Tuesday, July 11, 2017 2:45PM - 3:00PM |
K8.00003: Low pressure shock response and dynamic failure of high density- and ultra-high molecular weight polyethylene Jennifer Jordan, Dana Dattelbaum, Benjamin Schilling, Cynthia Welch, Jamie Stull Polyethylene exhibits mechanical responses tailorable to a given application based on its network and chain structures (crystallinity) and molecular weight. Earlier reports have provided Hugoniot data for polyethylene over a broad range of conditions to very high shock stresses, while others focused on the discontinuous low pressure Hugoniot of crystalline forms of polyethylene. Surprisingly little is known about the influence of crystalline structure, and associated crystalline phase transitions including melt, on its dynamic compression response. Two different materials -- high density polyethylene and ultrahigh molecular weight polyethylene - were chosen for investigation of the influence of a high percentage of crystallinity (\textgreater 40{\%}) on the shock response and dynamic tensile failure (spall). We have applied in-situ electromagnetic gauges to measure the evolution of particle velocity wave profiles with propagation distance to elucidate the nature of the discontinuous Hugoniot at low pressures. The first evidence of a three-wave structure in highly crystalline polyethylene was measured above a shock stress of 0.5 GPa. Above this region of discontinuity in the principal Hugoniot, the transition is overdriven, and a single shock wave is observed to stresses exceeding 10 GPa. Details about the nature of the transition, including wave velocities and changes in density, will be presented. Further, a series of dynamic tensile (spall) experiments were performed on polyethylene and will be discussed. [Preview Abstract] |
Tuesday, July 11, 2017 3:00PM - 3:15PM |
K8.00004: Shock response and spall behavior of polycarbonate and polymethyl methacrylate Nobuaki Kawai, Takuya Seki, Tsutomu Mashimo Polycarbonate and polymethyl methacrylate (PMMA) are major transparent polymer materials used in a wide range of applications. It is well known that these polymers show the strain-rate-dependence of mechanical properties. However, the mechanical behavior under the deformation with very high rates such as shock compression is still not well understood. In this study, plate-impact experiments were conducted on polycarbonate and PMMA to investigate their shock response and dynamic tensile (spall) behavior. The stress wave profiles propagated into samples were measured using PVDF piezoelectric film stress gauges embedded in samples. The spall strengths were determined by measuring free-surface-velocity profiles by means of the VISAR. Both measurements were performed simultaneously. The obtained stress-wave and free-surface profiles show that structural relaxation occur under shock-compressed state in both materials, but the tendencies of the relaxation in stress-strain plane are different each other even though they are typical amorphous polymer. In contrast, the spall strengths of both materials show same behavior that they are substantially constant under the shock loading up to about 2 GPa but decrease in the case of the shock loading above it. [Preview Abstract] |
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