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In this paper, we present results of plate impact simulations of shock compressed woven glass fiber-reinforced plastic (GRP) performed using the Arbitrary Lagrangian–Eulerian three-dimensional finite element code. A hyperelastic large-strain-based empirical Continuum Damage Mechanics (CDM) formulation is employed to describe damage initiation and growth in the shock-compressed GRP. The model parameters calibration scheme utilizes the Velocity Interferometer System for Any Reflector normal particle velocity measurements at the free surface of the GRP target plates. The impact velocity in the experiments ranged from 8.5 to 418 m/s. The finite element model considered planar 0°/90° bidirectional plies with an individual ply thickness of 0.68 mm, stacked to reach a total laminate thickness of 6.8 mm. The anisotropic elastic strains were estimated from the experimentally determined tetragonal symmetry stiffness matrix for the GRP. The strain-based damage model captures several salient features observed in the measured free surface particle wave profiles, including the shock rise time, onset of Elastic—Elastic Cracking, and the shape of the nonlinear portion of the experimental particle velocity profiles. The CDM model predicts the dominant damage mode to be matrix microcracking due to shear and the associated bulk expansion (bulking) under the global compressive loading in the plate impact configuration. [ABSTRACT FROM AUTHOR]

This article presents results from laser-driven shock compression experiments performed on pre-heated pure aluminum films at temperatures ranging from 23 to 400 °C. The samples were vapor deposited on the surface of a 500 μm thick sapphire substrate and mounted onto a custom holder with an integrated ring-heater to enable variable initial temperature conditions. A chirped pulse amplified laser was used to generate a pulse for both shocking the films and for probing the free surface velocity using Ultrafast Dynamic Ellipsometry. The particle velocity traces measured at the free surface clearly show elastic and plastic wave separation, which was used to estimate the decay of the elastic precursor amplitude over propagation distances ranging from 0.278 to 4.595 μm. Elastic precursors (which also correspond to dynamic material strength under uniaxial strain) of increasing amplitudes were observed with increasing initial sample temperatures for all propagation distances, which is consistent with expectations for aluminum in a deformation regime where phonon drag limits the mobility of dislocations. The experimental results show peak elastic amplitudes corresponding to axial stresses of over 7.5 GPa; estimates for plastic strain-rates in the samples are of the order 109/s. The measured elastic amplitudes at the micron length scales are compared with those at the millimeter length-scales using a two-parameter model and used to correlate the rate sensitivity of the dynamic strength at strain-rates ranging from 103 to 109/s and elevated temperature conditions. The overall trend, as inferred from the experimental data, indicates that the temperature-strengthening effect decreases with increasing plastic strain-rates. [ABSTRACT FROM AUTHOR]

We present results of a reverse non-equilibrium molecular dynamics study of thermal transport in single-walled carbon nanotube (SWCNT)-graphene junctions comprised of carbon-carbon (C-C) bonds with either sp2 or mixed sp2/sp3 hybridization. In both cases, a finite interfacial thermal resistance is observed at the SWCNT-graphene junctions for thermal transport in the out-of-plane direction. The interfacial thermal resistance at the junctions is attributed to the combined effects of scattering of the phonons at the SWCNT-graphene junctions due to the presence of distorted sp2 bonds in the junction region and the change in dimensionality of the medium along the phonon transport path as the phonons propagate from SWCNT pillars (quasi-1D) to graphene sheet (2D) and then again to SWCNTs. Moreover, the thermal resistance is found to depend on the C-C bond hybridization at the intramolecular junctions with mixed sp2/sp3 hybridization showing a higher interfacial resistance when compared to pure sp2 bonding. Thermal conductivity of typical SWCNT-graphene unit cells was observed to increase nearly linearly with an increase in cell dimensions, and then reaches a plateau as the pillar height and the inter-pillar distance approach the critical length for ballistic thermal transport in SWCNT and single layer graphene. These results indicate that the thermal transport characteristics of the three-dimensional SWCNT-graphene (hybrid) structures can be tuned by controlling the unit cell size. [ABSTRACT FROM AUTHOR]

Recently, tin has been identified as an attractive electrode material for energy storage/conversion technologies. Tin thin films have also been utilized as an important constituent of thermal interface materials in thermal management applications. In this regards, in the present paper, we investigate thermal conductivity of two nanoscale tin films, (i) with thickness 500 ± 50 nm and 0.45% porosity and (ii) with thickness 100 ± 20 nm and 12.21% porosity. Thermal transport in these films is characterized over the temperature range from 40K-310K, using a three-omega method for multilayer configurations. The experimental results are compared with analytical predictions obtained by considering both phonon and electron contributions to heat conduction as described by existing frequency-dependent phenomenological models and BvK dispersion for phonons. The thermal conductivity of the thicker tin film (500 nm) is measured to be 46.2W/m-K at 300K and is observed to increase with reduced temperatures; the mechanisms for thermal transport are understood to be governed by strong phonon-electron interactions in addition to the normal phonon-phonon interactions within the temperature range 160K-300K. In the case of the tin thin film with 100 nm thickness, porosity and electron-boundary scattering supersede carrier interactions, and a reversal in the thermal conductivity trend with reduced temperatures is observed; the thermal conductivity falls to 1.83W/m-K at 40K from its room temperature value of 36.1W/m-K. In order to interpret the experimental results, we utilize the existing analytical models that account for contributions of electron-boundary scattering using the Mayadas-Shatzkes and Fuchs-Sondheimer models for the thin and thick films, respectively. Moreover, the effects of porosity on carrier transport are included using a previous treatment based on phonon radiative transport involving frequency-dependent mean free paths and the morphology of the nanoporous channels. The systematic modeling approach presented in here can, in general, also be utilized to understand thermal transport in semi-metals and semiconductor nano-porous thin films and/or phononic nanocrystals.V 2014 AIP Publishing LLC. [ABSTRACT FROM AUTHOR]

Thermal conductivity measurements in commercially available, chemical vapor deposition-grown, heat-treated and non-heat-treated multiwalled carbon nanotubes (MWCNTs) are reported. The thermal conductivity of individual samples is measured using a suspended platinum wire as a thermal resistance probe in a 'T-type' configuration. Changes in third harmonic voltage are measured across the heated suspended platinum wire as a specimen is attached to the platinum wire's midpoint. An analytic model is used to correlate the reduction in the average temperature of the probe wire to the thermal resistance (and thermal conductivity) of the attached sample. Experiments are implemented inside a scanning electron microscope equipped with nanomanipulators for sample selection, and a gas injection system for platinum based electron beam-induced deposition to improve thermal contact resistances. The results indicate a nearly 5-fold increase in the average thermal conductivity of MWCNT samples annealed with a 20-h 3000 °C annealing heat treatment compared to the as-grown samples. However, specimen-specific morphological defects, such as kinking, Y-branches, etc., are found to negate, to a large degree, the advantage of the heat treatment process. The thermal contact resistance between the MWCNT and the electron beam-induced deposition contacts is estimated using an anisotropic diffusive mismatch model that includes the effect of fin resistance. Adjusting the thermal conductivity to include the effect of thermal contact resistance is found to increase the thermal conductivity by approximately 5%. Once adjusted for thermal contact resistance, the average thermal conductivity of the heat-treated MWCNT specimens is 228 W/m-K, with the highest measured thermal conductivity being 765 ± 150 W/m-K. The results highlight the importance of MWCNT quality in thermal management applications. [ABSTRACT FROM AUTHOR]

A multilayer elasticity model is developed to investigate the effects of acoustically stiffened surfaces (increased surface moduli) on the specific heat and thermal conductivity of typical nanowire and nanotubes as a function of temperature. Changes in phonon dispersion are analyzed using approximated phonon dispersion relations that result from the solutions to the frequency equation of a vibrating elastic tube or rod. The results of the investigation indicate a 10% reduction in specific heat and a 2% decrease in lattice thermal conductivity at 50 K for a 10 nm outer diameter crystalline nanotube with an inner diameter of 5 nm when the average Young's modulus of the first three atomic layers on both the inner and outer free surfaces are increased by a factor of 1.87. In contrast, a 10 nm outer diameter nanowire composed of the same material and with an acoustically stiffened outer shell shows an approximate 30% increase in thermal conductivity and specific heat near 50 K. Our simplified model can potentially be extended to investigate the acoustic tuning of nanowires and nanotubes by inducing surface stiffening or softening via appropriate surface chemical functionalization protocols or coatings. [ABSTRACT FROM AUTHOR]

In the present study plate-impact pressure-shear friction experiments are conducted to study dynamics of high-speed slip at metal-on-metal interfaces. By using a CH tool-steel/Ti–6Al–4V tribo-pair and appropriate selection of the flyer and target plate thicknesses, the experiment has been used to investigate the frictional resistance of sliding interfaces subjected to step changes in slip rates. In order to describe the frictional memory effects a model based on Ruina–Dieterich slip law has been utilized. It is noted that the model parameter a in the range of 0.01–0.25 with characteristic lengths of L=5–7 μm provides a reasonably good fit to the experimental data. [ABSTRACT FROM AUTHOR]

Synthetic heterogeneous material systems, e.g., layered composite materials with organic matrices reinforced by glass fibers (GRP), are attractive materials for a variety of lightweight armor applications. However, while the dynamic response of homogeneous materials, such as, metals and ceramics, has been well documented, the ballistic response of heterogeneous material systems is poorly understood. In the present study, in an attempt to better understand the shock-induced compression response of GRPs, a series of plate impact experiments were conducted on a S2-glas fiber reinforced polymer composite comprising S2-glass woven roving in a Cycom 4102 polyester resin matrix. The plate-impact experiments were conducted using an 82.5 mm bore single-stage gas-gun at the Case Western Reserve University. The history of the shock-induced free-surface particle velocity at the rear surface of the target plate was monitored using the multibeam VALYN™ VISAR system. The results of the experiments indicate the absence of an elastic front in the shock-induced free-surface particle velocity profile in the GRP. Moreover, in the low impact velocity range, relatively weak late-time oscillations are observed in the particle velocity profiles. Increasing the amplitude of the shock-induced compression resulted in a decrease in the rise-time of the shock wave front. The critical shock stress amplitude at which a clear shock-front is seen to develop during the shock loading was determined to be between 1.5 and 2.0 GPa. The results of the experiments are used to obtain the equation of state of the GRP in the stress range 0.04–20 GPa. Moreover, the Hugoniot curve (Hugoniot stress versus Hugoniot strain) was calculated using the Rankine–Hugoniot relationships; the departure of the Hugoniot stress versus the particle velocity curve from linearity allowed the estimation of the Hugoniot elastic limit of the GRP to be about 1.6 GPa. [ABSTRACT FROM AUTHOR]

The mechanical behavior of Zr41.25Ti13.75Cu12.5Ni10Be22.5 (LM-1) has been extensively characterized under quasistatic loading conditions; however, its mechanical behavior under dynamic loading conditions is currently not well understood. A Split–Hopkinson pressure bar (SHPB) and a single-stage gas gun are employed to characterize the mechanical behavior of LM-1 in the strain-rate regime of 102–105/s. The SHPB experiments are conducted with a tapered insert design to mitigate the effects of stress concentrations and preferential failure at the specimen-insert interface. The higher strain-rate plate-impact compression-and-shear experiments are conducted by impacting a thick tungsten carbide (WC) flyer plate with a sandwich sample comprising a thin bulk metallic glass specimen between two thicker WC target plates. Specimens employed in the SHPB experiments failed in the gage-section at a peak stress of approximately 1.8 GPa. Specimens in the high strain-rate plate-impact experiments exhibited a flow stress in shear of approximately 0.9 GPa, regardless of the shear strain-rate. The flow stress under the plate-impact conditions was converted to an equivalent flow stress under uniaxial compression by assuming a von Mises–like material behavior and accounting for the plane strain conditions. The results of these experiments, when compared to the previous work conducted at quasistatic loading rates, indicate that the peak stress of LM-1 is essentially strain rate independent over the strain-rate range up to 105/s. [ABSTRACT FROM AUTHOR]

Potential use of γ-TiAl alloys in aerospace and other structural applications require knowledge of their impact behavior for better evaluation and modeling. In the present study plate impact experiments are conducted using a single-stage gas gun to better understand the shock behavior of the recently developed class of gamma titanium aluminide alloys—the Gamma-Met PX. The Gamma-Met PX showed superior shock properties when compared to the conventional titanium aluminide alloys. The spall strength of Gamma-Met PX is 1.8±0.09 GPa, which is four to six times higher than those reported for other gamma titanium aluminide alloys. Moreover, it has a Hugoniot elastic limit of 1.88 GPa at a target thickness of 3.86 mm, which drops to 1.15 GPa at target thickness of 15.8 mm. The decay in the elastic precursor is continuous without showing an asymptote to a constant level within the range of target thicknesses studied. [ABSTRACT FROM AUTHOR]

In the present paper, a series of plate impact shock-reshock and shock-release experiments were conducted to study the critical shear strength of a S2 glass fiber reinforced polymer (GRP) composite under shock compression levels ranging from 0.8 to 1.8 GPa. The GRP was fabricated at ARL, Aberdeen, using S2 glass woven roving in a Cycom 4102 polyester resin matrix. The experiments were conducted by using an 82.5 mm bore single-stage gas gun at Case Western Reserve University. In order to conduct shock-reshock and shock-release experiments a dual flyer plate assembly was utilized. The shock-reshock experiments were conducted by using a projectile faced with GRP and backed with a relatively high shock impedance Al 6061-T6 plate; while for the shock-release experiments the GRP was backed by a relatively lower impedance polymethyl methacrylate backup flyer plate. A multibeam velocity interferometer was used to measure the particle velocity profile at the rear surface of the target plate. By using self-consistent technique procedure described by Asay and Chabbildas [Shock Waves and High-Strain-Rate Phenomena, in Metals, edited by M. M. Myers and L. E. Murr (Plenum, New York, 1981), pp. 417–431], the critical shear strength of the GRP (2τc) was determined for impact stresses in the range of 0.8 to 1.8 GPa. The results show that the critical shear strength of the GRP is increased from 0.108 GPa to 0.682 GPa when the impact stress is increased from 0.8 to 1.8 GPa. The increase in critical shear strength may be attributed to rate-dependence and/or pressure dependent yield behavior of the GRP. [ABSTRACT FROM AUTHOR]

The shock-induced compression response of organic matrices reinforced by glass fibers (GRPs) is understood by conducting a series of plate impact experiments on a S2-glas fiber reinforced polymer composite comprising S2-glass woven roving in a Cycom 4102 polyester resin matrix. The results have shown the absence of an elastic front in the shock-induced free-surface particle velocity profile in the GRP and the Hugoniot curve is calculated by using the Rankine-Hugoniot relationships.

A series of plate impact shock-reshock and shock-release experiments were performed to study the critical shear strength of a S2 glass fiber reinforced polymer (GRP) composite under shock compression levels ranging from 0.8 to 1.8 GPa. The results showed that the critical shear strength of the GRP is increased when the impact stress is increased.