Pressure-shear Plate Impact Experiments Research Articles

Flow Strength Measurements of Wrought and AM SS304L via Pressure Shear Plate Impact Experiments

Flow Strength Measurements of Wrought and AM SS304L via Pressure Shear Plate Impact Experiments

Three-dimensional full-field velocity measurements in shock compression experiments using stereo digital image correlation.

Shock compression plate impact experiments conventionally rely on point-wise velocimetry measurements based on laser-based interferometric techniques. This study presents an experimental methodology to measure the free surface full-field particle velocity in shock compression experiments using high-speed imaging and three-dimensional (3D) digital image correlation (DIC). The experimental setup has a temporal resolution of 100ns with a spatial resolution varying from 90 to 200μm/pixel. Experiments were conducted under three different plate impact configurations to measure spatially resolved free surface velocity and validate the experimental technique. First, a normal impact experiment was conducted on polycarbonate to measure the macroscopic full-field normal free surface velocity. Second, an isentropic compression experiment on Y-cut quartz-tungsten carbide assembly is performed to measure the particle velocity for experiments involving ramp compression waves. To explore the capability of the technique in multiaxial loading conditions, a pressure shear plate impact experiment was conducted to measure both the normal and transverse free surface velocities under combined normal and shear loading. The velocities measured in the experiments using digital image correlation are validated against previous data obtained from laser interferometry. Numerical simulations were also performed using established material models to compare and validate the experimental velocity profiles for these different impact configurations. The novel ability of the employed experimental setup to measure full-field free surface velocities with high spatial resolutions in shock compression experiments is demonstrated for the first time in this work.

Inelastic behavior of tungsten carbide at high pressures

Tungsten carbide (WC) is commonly used in plate impact experiments for generating high pressures due to its combination of high impedance and high strength. In pressure-shear plate impact (PSPI) experiments, WC is used as anvil plates, which sandwich a thin sample of the material of interest. At pressures up to its HEL (∼5 GPa), an elastic response may be assumed for the anvil plate, enabling a straightforward analysis to extract the shear strength of the material. At higher pressures, exceeding the HEL, the inelastic behavior of WC must be taken into account to extract the shearing resistance of the material of interest. We report the results of pressure-shear plate impact experiments conducted in the pressure range of 9–37 GPa. Two modeling approaches are considered to account for the inelastic behavior of WC: (1) an elasto-plastic response, accounting for strain and strain-rate hardening, and (2) a brittle-like response, which incorporates pressure hardening and damage. The material models were calibrated using numerical simulations to reconstruct the measured free surface velocity components in the normal and transverse directions. Both models effectively simulate the measured velocity profiles. Above 20 GPa, the brittle model appears to better capture the experimental trends. The calibrated models for WC enable the reconstruction of the experimental data from the current work, as well as from results reported by others, for a broad range of pressures.

Dynamic shearing resistance of a polymer-bonded energetic simulant: Composite of sucrose and hydroxyl-terminated polybutadiene (HTPB)

The mechanical behavior of polymer-bonded explosives (PBXs) and polymer-bonded simulants has been widely studied under normal impact. However, their shearing response under large pressures and shear strain-rates has not been explored, to the best of our knowledge. Such measurements are crucial in informing constitutive models that aim to predict hot-spot formation and ignition behavior of PBXs subject to multi-axial loading. Pressure-shear plate impact experiments have been conducted on a composite of hydroxyl-terminated polybutadiene (HTPB) binder and sucrose (an energetic simulant) under normal stresses of 3–10 GPa nominally and shear strain-rates of the order of 105 s−1. The shear strength of the composite shows a dramatic drop after reaching a critical shear strain. Such a drop could be due to fracture or localization in either of the two phases, further compounded by other mechanisms such as friction between the resulting interfaces or collapse of voids. It is also observed that the shear strength of the composite is highly pressure-sensitive, increasing from 176 to 453MPa as the normal stress increases nominally from 3 to 9.5 GPa. Comparison of the shearing behavior of the composite with that of a granular aggregate of sucrose grains at low normal stresses (∼3 GPa) indicates that the binder plays the role of a lubricant between the sucrose grains at those normal stresses. However, at normal stresses of ∼9–10 GPa, the shear strength of HTPB becomes similar to sucrose, and beyond these normal stresses, a transition in the failure mode and its location is expected.

Dynamic Shearing Resistance of an Energetic Material Simulant: Sucrose

Sucrose is a commonly used energetic material simulant for β-HMX. However, its behavior under extreme loading conditions remains poorly understood. Pressure-Shear Plate Impact (PSPI) experiments have been conducted to provide an experimental foundation for developing a suitable constitutive model for sucrose. Experiments have been performed at two different nominal normal stress values, 3 and 9.5 GPa, with nominal shear strain-rates of the order of 106s−1. Sucrose exhibits a shear strength of ∼400-500 MPa in this pressure range, with a relatively small pressure sensitivity. However, pronounced strain softening is observed in sucrose at sufficiently large shear strains - even a dramatic drop in shearing resistance in some cases. A thermodynamically-consistent constitutive theory is presented to model the thermoelastic, thermo-viscoplastic response of sucrose. The use of the logarithmic strain and an appropriate set of invariants allows for separation of the Cauchy stress into pressure and deviatoric terms. A complete Mie-Gruneisen equation of state is presented to model the volumetric behavior while the deviatoric response is captured through the Johnson-Cook Model. The material model is able to capture experimental observations and predicts localization in the form of adiabatic shear bands, which explains the catastrophic drop in shearing resistance of sucrose observed in the experiments. Such localization events carry important implications for energetic materials in the context of hot-spot formation and underline the importance of dynamic shearing strength measurements.

Unraveling the implications of finite specimen size on the interpretation of dynamic experiments for polycrystalline aluminum through direct numerical simulations

Normal and Pressure-shear plate impact (NPI and PSPI) experiments are popular experimental techniques for studying the mean-field macroscopic behavior of polycrystalline metals under high-rate dynamic loading. However, since both configurations rely upon geometry for subjecting the specimen to high strain rates, these experiments often involve a limited specimen size. Moreover, because of the inherent heterogeneities present within polycrystalline metals, it is difficult to ascertain if the size of the specimen and/or regions where measurements are made are sufficiently large for making representative inferences about the mean-field macroscopic properties from single-point velocity measurements. In the present study, we quantify the expected measurement variability on observable point measurements in NPI and PSPI experiments by carrying out direct numerical simulations (DNS) of statistically representative polycrystalline microstructures subjected to dynamic compression and compression-shear loading. In particular, we consider the role of specific material heterogeneities (e.g. the grain-to-grain difference in size, crystallographic orientation) on dispersion in the normal and transverse particle velocity records and on local fluctuations in key state variables (e.g. velocity, accumulated plastic strain) by incorporating these effects directly into a representative synthetic microstructure geometry and crystalline description of pure polycrystalline aluminum. The form of the present study is a large parametric investigation, consisting of ten ensembles of one hundred simulations. Each of the thousand simulations reflects a randomly realized synthetic microstructure in one of five cases of decreasing average grain size for the two loading configurations. Our analysis of the DNS results demonstrates that for both of these experimental configurations, the grain size directly correlates with the coefficient of variation (CV) in simulated point measurements, showing a convergent decrease in CV to zero (i.e. particle velocity record approaches the mean-field value) with decreasing grain size. Remarkably, the magnitude of variations in the particle velocity record is shown to be largest where the deviatoric stresses are most significant. In the case of NPI, this occurs at the elastic and plastic wavefront, whereas, in the case of PSPI, the magnitude of fluctuations are approximately constant throughout the experimental window time. The reasoning for the scatter in particle velocity due to the heterogeneous microstructure is demonstrated to be dependent on the mechanisms for accommodating deformation and on the interaction of reflection waves generated at sites of heterogeneities occurring at the scale of grains. Lastly, we develop a power-law description for the magnitude of scattering versus characteristic length, which provides a statistical framework for assessing the required number of grains per characteristic specimen dimension for minimizing scatter within these two experimental configurations (NPI, PSPI).

Dynamic shearing resistance of hydroxyl-terminated polybutadiene (HTPB)

Hydroxyl-terminated polybutadiene (HTPB) is a widely used polymeric binder in polymer-bonded explosives (PBXs) and solid rocket propellants. Even though used in small fractions, the elastomeric binder absorbs much of the impact energy and therefore requires careful modeling of its mechanical behavior to accurately simulate the response of PBXs when they are subjected to large strains and strain rates. While the normal response of HTPB has been characterized under uniaxial stress and uniaxial strain loading, shear strength measurements under large pressures and large shear strain rates have not been made so far. Such measurements are critical for modeling localization and failure in PBXs and accurate prediction hotspot formation. In this study, pressure-shear plate impact (PSPI) experiments have been used to measure the shearing resistance of HTPB at different pressures. The shearing resistance of HTPB is found to be strongly pressure dependent. A quasi-linear viscoelastic model with pressure-dependent shear wave speeds and shearing resistance is used to describe the experimentally observed dynamic response of HTPB. The model consists of an instantaneous elastic response and viscoelastic relaxation of the elastic response.

Dynamic Strength of Copper at High Pressures Using Pressure Shear Plate Experiments

Measurement of strength at high pressures is critical to understanding the behavior of materials subjected to extreme loading conditions. Recent developments have extended the pressure shear plate impact (PSPI) technique to the high-pressure regime. Such experiments provide a unique opportunity to extract the complete stress–strain behavior of materials at relatively high pressures. The modified technique includes a new fiber-optic heterodyne transverse velocity interferometer system and new analysis methods to account for the inelastic response of the anvil plates. In this study, PSPI experiments are conducted on oxygen-free high conductivity (OFHC) copper at pressures ranging from 10 to 43 GPa and at strain rates of ~ 105 s−1. Complete stress–strain curves of copper are obtained using a hybrid methodology that relies on numerical simulations and particle velocity records gathered in these experiments. Experiments are conducted at similar pressures and strain rates using two types of anvils that undergo different levels of plastic strains: tool steel and tungsten carbide, to check the robustness of the hybrid method. The behavior of copper at these high pressures and strain rates is compared with previous literature data, and the effect of pressure and strain rate on the strength behavior of copper are discussed. It was observed that the rate of change in yield strength with pressure was 2.5 times the rate of change of shear modulus with pressure. The strength models that predict the response of copper at these extreme conditions are also examined and validated.

Strength of magnesium at high pressures and strain rates

The strength and deformation mechanisms in magnesium can be significantly affected by anisotropy, high strain rates, and pressure. In this study, pressure shear plate impact (PSPI) experiments are conducted to measure the strength of extruded polycrystalline magnesium at pressures varying from 5 to 10 GPa at a nominal strain rate of 105 s−1. The experimental technique enables to first shock load the material sample to the desired normal stress in one direction, and then shear the material in a perpendicular direction. A recently developed hybrid analysis method for PSPI experiments is used to extract the stress–strain curves of magnesium from the particle velocity records measured at the rear surface of the target. The PSPI experimental results reveal a slower twinning saturation at high pressures. To better understand the material behavior under the combined stress states in the PSPI experiments, the results were compared with that of a specimen deformed by a two-step process of quasi-static compression followed by dynamic shear loading at relatively low pressure. The two-stage loading at low pressure, and the calculation of temperature rise in the PSPI experiment revealed that the combined effect of the reorientation and the temperature rise lower the flow strength of magnesium at high pressures under multiaxial loading.

Dynamic flow stress of pure polycrystalline aluminum: Pressure-shear plate impact experiments and extension of dislocation-based modeling to large strains

The dynamic thermo-mechanical behavior of pure aluminum has attracted renewed interest lately due to experimental observations of an anomalous increase in Hugoniot Elastic Limit (HEL) at incipient plasticity and elevated temperatures in polycrystalline pure metals. In context of current dislocation-mediated plasticity models for metals, this increase in dynamic strength is indicative of a transition in the rate controlling mechanism for dislocation glide from being thermally assisted to phonon-drag restricted due to increase in phonon viscosity at elevated temperatures. Though these studies have helped to shed light on these important mechanisms operative in FCC metals at incipient plasticity, the extent to which they contribute to flow stress, particularly at larger strains, remains unclear. In the present study, we address these questions through a combined experimental and modeling effort focused on investigating the evolution of dynamic flow stress in polycrystalline aluminum using experimental data gathered from a series of combined pressure-and-shear plate impact (PSPI) experiments designed to reveal the flow stress of pure aluminum at strain rates ~ 105/s, plastic strains of up to 40% and temperatures ranging from room to 866 K. In all cases, the flow stress of aluminum, as inferred from the measured transverse particle velocity histories at the free surface of an elastic tungsten carbide target plate, reveals saturation with increasing plastic strains at stress levels that decrease with increasing test temperatures. Numerical simulations are performed to correlate the experimentally observed temperature and strain rate dependence of flow stress at small and large plastic strains using the Austin-McDowell dislocation-mediated plasticity model parametrized to normal plate impact experiments conducted in an earlier study by Zaretsky and Kanel (2012). Extensions to the model are made to better represent dynamic behavior of pure aluminum at larger plastic strains as observed in elevated temperature split Hopkinson Pressure bar (SPHB) experiments of Samanta (1971) and Lindholm and Yeakly (1965), and the combined pressure-and-shear plate-impact experiments conducted in the present study. The main theoretical extension to the Austin-McDowell model made in this paper is the introduction of a new rate- and temperature- dependent dynamic recovery function, which can potentially allow for an effective reduction in the rate of accumulation of dislocations at large plastic strains. The numerical predictions of the revised and re-calibrated plasticity model are brought into agreement with the experimental observations, correlating sufficiently well with dynamic yield stress at incipient plasticity and the flow stress levels at larger plastic strains, plastic strain rates in the range 103 – 106 /s, and elevated temperatures up to near melt. The model, in concert with the experimental measurements, suggests that at incipient plasticity the rate governing mechanism for plastic flow is phonon-drag restricted dislocation glide, whereas, at higher magnitudes of plastic strain, it transitions to stress-assisted thermally activated glide. The transition between these two rate governing mechanisms is controlled by the evolution of dislocations throughout the deformation process.

Pressure-Shear Plate Impact Experiments at High Pressures

The pressure shear plate impact (PSPI) experiment, developed over 40 years ago by R. J. Clifton at Brown University enables the study of the dynamic strength of materials at high pressures and strain rates. Traditional PSPI experiments were typically conducted at velocities and corresponding pressures, limiting the experimental conditions not to exceed the Hugoniot Elastic Limit (HEL) of the anvil materials, with typical values of 3–7 GPa. In this work, PSPI experiments are extended to higher pressures, significantly beyond the HEL, approaching 50 GPa, using a powder gun facility at Caltech. The high-pressure and high-velocity impact regimes introduce several experimental challenges which must be overcome: (1) the inelastic behavior of the anvils at high pressures precludes traditional elastic analysis to extract the material’s strength; (2) the potential for slip between impact faces as a result of elevated temperatures and shear forces on the impact surfaces due to higher velocities and pressures; and (3) accurate measurement of the transverse velocity at large normal displacements, resulting from the higher impact velocities. New experimental capabilities have been developed to overcome each of these technical challenges, which include a new all fiber-optic heterodyne transverse velocity interferometer system. New analysis methods that account for the inelastic response of the flyer and anvil plates have also been developed for the accurate extraction of material strength properties from PSPI experiments. Friction and slip were examined, pushing to the limits of their knowledge under these extreme conditions. The new PSPI capabilities have been demonstrated using tungsten carbide and D2 tool steel anvils for measuring the strength of soda–lime glass and magnesium.

Dynamic failure mechanisms of granular boron carbide under multi-axial high-strain-rate loading

Boron carbide is an excellent protection material but it is also very brittle, and granular flow, triggered by progressive fracture during impact, greatly influences its ballistic performance. Transmission electron microscopy observations of samples recovered from controlled pressure-shear plate impact experiments have elucidated the extent to which local failure mechanisms accompany dynamic granular flow. These observations point to the interplay of quasiplasticity, the formation of nanoscale amorphous bands, cleavage and intragranular cracking. Association of these mechanisms with granular flow and identification of a critical velocity for the onset of amorphization provides a foundation for modeling and predicting armor performance.

A Simple Dual-Beam Time-Multiplexed Photon Doppler Velocimeter for Pressure-Shear Plate Impact Experiments

Pressure-shear plate impact experiments generate normal and transverse particle velocities during high strain rate deformations. Traditionally, freespace lenscoupled tabletop laser interferometry techniques are used together with diffraction gratings to interrogate the evolving velocity vector at the back face of the target plate. Recently, fiberoptic velocimetry (photon Doppler velocimetry or PDV) has become commonplace for measuring normal particle velocities above 200m/sec. In this work, we demonstrate transverse velocity detection using a modified PDV system where we subtract the measured normal velocity history from a concurrent velocity history measured at a canted angle to the target surface to obtain the transverse velocity component. This modified system is time-multiplexed to reduce the number of components, and uses an erbium doped fiber amplifier (EDFA) to boost the angled signal intensity while maintaining low noise. The system operates as a heterodyne interferometer, but features a frequency upshifted reference leg to improve data analysis at the particle velocities expected in the experiment. We demonstrate by direct comparison that this inexpensive and simple approach is as effective as traditional grating methods.

Conducting Elevated Temperature Normal and Combined Pressure-Shear Plate Impact Experiments Via a Breech-end Sabot Heater System.

A novel approach for conducting normal and/or combined pressure-shear plate impact experiments at test temperatures up to 1000 °C is presented. The method enables elevated temperature plate-impact experiments aimed towards probing dynamic behavior of materials under thermomechanical extremes, while mitigating several special experimental challenges faced while performing similar experiments using the conventional plate impact approach. Custom adaptations are made to the breech-end of a single-stage gas-gun at Case Western Reserve University; these adaptations include a precision-machined extension piece made from SAE 4340 steel, which is strategically designed to mate the existing gun-barrel while providing a high tolerance match to the bore and keyway. The extension piece contains a vertical cylindrical heater-well, which houses a heater assembly. A resistive coil heater-head, capable of reaching temperatures of up 1200 °C, is attached to a vertical stem with axial/rotational degrees of freedoms; this enables thin metal specimens held at the front-end of a heat-resistant sabot to be heated uniformly across the diameter to the desired test temperatures. By heating the flyer plate (in this case, the sample) at the breech-end of the gun-barrel instead of at the target-end, several critical experimental challenges can be averted. These include: 1) severe changes in the alignment of the target plate during heating due to the thermal expansion of the several constituents of the target holder assembly; 2) challenges that arise due to the diagnostics elements, (i.e., polymer holographic gratings, and optical probes) being too close to the heated target assembly; 3) challenges that arise for target plates with an optical window, where crucial tolerances between the sample, bond layer, and window become increasingly difficult to maintain at high temperatures; 4) in the case of combined compression-shear plate impact experiments, the need for high-temperature resistant diffraction gratings for the measurement of transverse particle velocity at the free surface of the target; and 5) limitations imposed on the impact velocity necessary for unambiguous interpretation of the measured free surface velocity versus time profile due to thermal softening and possibly yielding of the bounding target plates.By utilizing the adaptations mentioned above, we present results from a series of reverse geometry normal plate impact experiments on commercial purity aluminum at a range of sample temperatures. These experiments show decreasing particle velocities in the impacted state, which are indicative of material softening (decrease in post-yield flow stress) with increasing sample temperatures.

Conducting Elevated Temperature Normal and Combined Pressure-Shear Plate Impact Experiments Via a Breech-end Sabot Heater System

Conducting Elevated Temperature Normal And Combined Pressure-shear Plate Impact Experiments Via A Breech-end Sabot Heater System - Video. JoVE publishes peer-reviewed scientific video protocols to accelerate biological, medical, chemical and physical research. Watch our scientific video articles. JoVE is the world-leading producer and provider of science videos with the mission to improve scientific research, scientific journals, and education. Millions of scientists, educators and students at thousands of universities, colleges, hospitals and biopharmaceutical companies worldwide use JoVE for their research, teaching and learning.

Heterodyne transverse velocimetry for pressure-shear plate impact experiments

Pressure-shear plate impact experiments have traditionally relied on free space beam interferometers to measure transverse and normal particle velocities at the rear surface of the target plate. Here, we present two different interferometry schemes that leverage heterodyne techniques, which enable the simultaneous measurement of normal and transverse velocities using short-time Fourier transforms. Both techniques rely on diffracted 1st order beams that are generated by a specular, metallic grating deposited on the rear surface of the target plate. The diffracted beam photonic Doppler velocimetry technique interferes each 1st order beam with a reference of slightly higher wavelength to create a constant carrier frequency at zero particle velocity. The second technique interferes the 1st order beams with each other and employs an acousto-optic frequency shifter on the +1st order beam to create a heterodyne transverse velocimeter. For both interferometer techniques, the 0th order beam is interfered in a heterodyne photonic Doppler velocimetry arrangement to obtain a measurement of the normal particle velocity. An overview of both configurations is presented along with a derivation of the interferometer sensitivities to transverse and normal particle velocities as well as design guidelines for the optical system. Results from normal impact experiments conducted on Y-cut quartz are presented as the experimental validation of the two proposed techniques.

Mechanism of shear attenuation near the interface under combined compression and shear impact loading

We investigate the compressional/shear coupling plastic wave propagation characteristics analytically for ideal elastic–plastic materials in both stress and particle velocity spaces, focusing on the shear wave attenuation near the interface occurring in pressure–shear plate impact experiments. The results show that the shear attenuation is strongly associated with the wave propagation characteristics of the coupling waves. In the stress space, as the shear stress increases, an adjustment of the stress components is observed and the final stress state along the wave path is a combined pure shear- and hydrostatic pressure-state. In the particle velocity space, the wave structures with different loading and maximal transverse particle velocity are obtained. The maximal transverse particle velocity varies with the longitudinal velocity and forms a boundary line. Once the loading transverse velocity exceeds this line, a transverse particle velocity discontinuity occurs at the impact interface. If the bonding strength is sufficiently high, there will be a shear band in the target in the extreme vicinity of the interface.

A physically-based, quasilinear viscoelasticity model for the dynamic response of polyurea

Polyurea, a promising material for damage mitigation in impact scenarios, has been investigated through plane-wave, pressure-shear plate impact (PSPI) experiments to obtain its mechanical response at high pressures and high strain rates. Based on these experimental results, a physically-based, quasi-linear, viscoelasticity model is introduced to capture the observed nonlinear pressure-volume behavior, the strong dependence of shearing resistance on pressure, and the strong relaxation of deviatoric stresses. This model has been implemented in finite element software ABAQUS to simulate the response of polyurea P1000 under the impact conditions of a variety of PSPI experiments. Simulation results agree reasonably well with those of the experiments.

Experimental and Computational Study of the Shearing Resistance of Polyurea at High Pressures and High Strain Rates

Mechanical response of polyurea, a nanophase segregated elastomeric co-polymer, is investigated using all-atom, equilibrium, molecular-dynamics methods and tools. Specifically, the effects of high pressure (1-30 GPa) and high strain rate (105-106 s−1) on the shearing resistance of polyurea are examined. Such loading conditions are encountered by polyurea coatings subjected to impact by high-velocity projectiles, shell shrapnel, and improvised explosive device fragments. Computed results are compared with their experimental counterparts obtained using the so-called pressure-shear plate impact experiments. Computed results have also been rationalized in terms of the nanosegregated polyurea microstructure consisting of rod-shaped, discrete, the so-called hard domains embedded in a highly compliant, the so-called soft matrix. By analyzing molecular-level microstructure and its evolution during high-rate deformation and under high imposed pressures, an attempt is made to identify and quantify main phenomena in viscous/inelastic deformation and microstructure-reorganization processes that are most likely responsible for the observed mechanical response of polyurea.

Measurement of the response of an elastomer at pressures up to 9 GPa and shear-rates of 105 – 106s−1

Pressure-shear plate impact (PSPI) experiments have been conducted to study the mechanical response of an elastomer at high pressures and high strain rates. The previously determined isentrope has been extended to 9 GPa. At this pressure, the high-strain-rate shearing resistance of polyurea is approximately 500 MPa – comparable to, or greater than, that of high strength steels and at much lower density. A new symmetric pressure-shear plate impact (SPSPI) configuration has been developed in order to enable the direct measurement of the thickness-averaged nominal strain rates of the sample – as well as the tractions on both of its interfaces with linear elastic plates. This enhancement is made possible by using a symmetric configuration for which the velocity of the mid-plane of the sample is known from symmetry to be one-half of the impact velocity. One dimensional elastic wave theory is used to obtain tractions and particle velocities at the sample/anvil interface from the measured rear-surface velocities. In this way, nominal strain-rate histories are obtained for both longitudinal and shear strains.