Planar Impact Experiments Research Articles

Compositional effects on the shock-compression response of alumina-filled epoxy

Alumina-filled epoxies are composites having constituents with highly dissimilar mechanical properties, resulting in complex behavior during shock compression and release. A previous study examined the shock properties of a particular composition in some detail. In the current study, the effects of compositional variations on shock properties were examined. Planar-impact experiments producing states of nearly equal strain were conducted to investigate the effects of changes in the size and shape of alumina particles, and in the total volume fraction of alumina. Laser interferometry and wave timing were used to obtain transmitted wave profiles, Hugoniot states, and release wave velocities. In addition, wave profiles and velocities were obtained in “thin-pulse” experiments that examined the combined effects of compression and release properties in different compositions. Changes in the size and shape of alumina particles were found to have little effect except in the viscous spreading of wave profiles during shock compression. Increasing the volume fraction of alumina resulted in steadily increasing Hugoniot states, wave rise times, and release wave velocities. An important observation was that differences between release wave and shock wave velocities increased significantly as the alumina loading was increased. Consequences of this effect were evident in the thin-pulse experiments, which showed that increased alumina loading resulted in stronger wave attenuation.

Shock wave compression of the ferroelectric ceramic Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3: Microstructural effects

Shock wave compression of poled Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 results in rapid depoling and release of bound charge. Different porous microstructures can be produced in the material by adding different types and amounts of organic pore formers prior to bisque firing and sintering. In previous studies, extensive planar-impact experiments on a baseline material having a fixed porous microstructure were conducted to determine Hugoniot states, to examine constitutive mechanical properties during shock propagation, and to investigate shock-induced depoling characteristics. Additional comparative experiments were conducted to investigate the effects of a different porous microstructure in a material having the same density, and also the effects of different initial densities. These comparisons indicated that differences in the porous microstructure of common-density materials have little effect on mechanical and electrical shock properties, in contrast to large effects observed when initial density is varied. To examine microstructural effects more extensively in the present study, additional common-density materials having distinctly different microstructures were prepared. Each material was made using spherical pore formers having diameters within a narrow range, with the mean diameter varying over a broad range between the different materials. Normally poled samples of each material were subjected to two particular experimental conditions that had proved useful for revealing important depoling and yielding properties in the baseline material. Results from materials made with larger pore formers again indicated that shock properties are insensitive to microstructural differences in common-density materials. Materials made with the smallest pore formers were an important exception, with the most noticeable difference being a significantly higher threshold for dynamic yielding.

Mechanisms of plastic deformation and shock-induced phase transformation in natural and synthetic single crystal scheelite

Dynamic response of natural, [0 0 1]-oriented, and synthetic, [2 0 1]-oriented, scheelite (CaWO4) was studied in planar impact experiments with shock up to 20 GPa. The velocity of the interface between the sample and the PMMA window was continuously monitored by VISAR. Although the waveforms recorded in the planar impact experiments revealed different dynamic responses of the two materials, the plastic deformation in both cases is governed by the dislocation glide in {1 1 2} planes with resolved shear stress of ∼0.6–0.7 GPa. The waveform obtained from [0 0 1]-oriented material contains the signature of the second-order scheelite–fergusonite transformation. The absence of the transformation signature in the waveforms obtained after strong impact of [2 0 1]-oriented crystals is probably due to faster transformation kinetics under loading in this direction.

Dynamic strength of uranium at high temperatures

Unalloyed uranium (PU) and uranium-0.78 wt% Ti alloy (UT) were studied in VISAR-monitored planar impact experiments with initial sample temperatures ranged from 27°C to 860°C. The recorded waveforms was used for obtaining the stress-strain σ(e) and deviator stress-strain s(e) diagrams, the conventional elastic limit Y 0.2 , and the spall strength of the alloys at different testing temperatures. The strengths Y 0.2 of both the materials stay almost constant up to the temperature of α - β transformation, increase strongly in the β-phase domain, and abruptly drop above the temperature of β - y transformation. The temperature dependences of the spall strength of alloys differ from those of the compressive strengths indicating the prevailing role of the void nucleation (over the void growth) in the spallation process. The most striking finding of the work is the existence of β-uranium at pressures some 3 GPa higher than that permitted thermodynamically. The life time and the borders of existence of this non-equilibrium phase are unknown and should be determined in future.

Progressive fracture modeling of the failure wave in impacted glass

The failure wave has been observed propagating in glass under impact loading since 1991. It is a continuous fracture zone which may be associated with the damage accumulation process during the propagation of shock waves. A progressive fracture model was proposed to describe the failure wave formation and propagation in shocked glass considering its heterogeneous meso-structures. The original and nucleated microcracks will expand along the pores and other defects with concomitant dilation when shock loading is below the Hugoniot Elastic Limit. The governing equation of the failure wave is characterized by inelastic bulk strain with material damage and fracture. And the inelastic bulk strain consists of dilatant strain from nucleation and expansion of microcracks and condensed strain from the collapse of the original pores. Numerical simulation of the free surface velocity was performed and found in good agreement with planar impact experiments on K9 glass at China Academy of Engineering Physics. And the longitudinal, lateral and shear stress histories upon the arrival of the failure wave were predicted, which present the diminished shear strength and lost spall strength in the failed layer.

Progressive fragment modeling of failure wave in ceramics under planar impact loading

Polycrystalline ceramics have heterogeneous meso-structures which result in high singularity in stress distribution. Based on this, a progressive fragment model was proposed which describes the failure wave formation and propagation in shocked ceramics. The governing equation of the failure wave was characterized by inelastic bulk strain with material damage and fracture. And the inelastic bulk strain consists of dilatant strain from nucleation and expansion of microcracks and condensed strain from collapse of original pores. Numerical simulation of the free surface velocity was performed in good agreement with planar impact experiments on 92.93% aluminas at China Academy of Engineering Physics. And the longitudinal, lateral and shear stress histories upon the arrival of the failure wave were predicted, which present the diminished shear strength and lost spall strength in the failed layer.

Hugoniot and strength behavior of silicon carbide

The shock behavior of two varieties of the ceramic silicon carbide was investigated through a series of time-resolved plate impact experiments reaching stresses of over 140 GPa. The Hugoniot data obtained are consistent for the two varieties tested as well as with most data from the literature. Through the use of reshock and release configurations, reloading and unloading responses for the material were found. Analysis of these responses provides a measure of the ceramic’s strength behavior as quantified by the shear stress and the strength in the Hugoniot state. While previous strength measurements were limited to stresses of 20–25 GPa, measurements were made to 105 GPa in the current study. The initial unloading response is found to be elastic to stresses as high as 105 GPa, the level at which a solid-to-solid phase transformation is observed. While the unloading response lies significantly below the Hugoniot, the reloading response essentially follows it. This differs significantly from previous results for B4C and Al2O3. The strength of the material increases by about 50% at stresses of 50–75 GPa before falling off somewhat as the phase transformation is approached. Thus, the strength behavior of SiC in planar impact experiments could be characterized as metal-like in character. The previously reported phase transformation at ∼105GPa was readily detected by the reshock technique, but it initially eluded detection with traditional shock experiments. This illustrates the utility of the reshock technique for identifying phase transformations. The transformation in SiC was found to occur at about 104 GPa with an associated volume change of about 9%.

Shock-compression response of an alumina-filled epoxy

Alumina-filled epoxies are composites having constituents with highly dissimilar mechanical properties. Complex behavior during shock compression and release can result, particularly at higher alumina loadings. In the current study, a particular material containing 43% alumina by volume was examined in planar-impact experiments. Laser interferometry was used to measure particle velocity histories in both reverse-impact and transmitted-wave configurations. Hugoniot states and release-wave velocities were obtained at shock stresses up to 10GPa, and represented smooth extensions of previous data at lower stresses. Surprisingly high release-wave velocities continued to be the most notable feature. Measured profiles of transmitted waves show a gradual transition from viscoelastic behavior at high shock stresses to a more complex behavior at lower stresses in which viscous mechanisms produce a broadened wave structure. This wave structure was examined in some detail for peak stress dependence, evolution towards steady-wave conditions, and initial temperature effects.

Shock wave compression of the ferroelectric ceramic Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3: Depoling currents

Shock wave compression of poled Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 (PZT 95/5-2Nb) results in rapid depoling and release of bound charge. In the current study, planar-impact experiments with this material were conducted on a gas-gun facility to determine Hugoniot states, to examine constitutive mechanical properties during shock propagation, and to investigate shock-induced depoling characteristics. A previous article summarized results from the first two of these areas, and this article summarizes the depoling studies. A baseline material, similar to materials used in previous studies, was examined in detail. More limited experiments were conducted with other materials to investigate the effects of different porous microstructures. Experiments were conducted over a wide range of conditions in order to examine the effects of varying shock strength, poling orientation, input wave shape, electric field strength, porous microstructure at a fixed density, and initial density. Depoling currents were recorded in an external circuit under either short-circuit or high-field conditions, and provide a convenient means of examining the kinetics associated with the ferroelectric–to–antiferroelectric phase transition. For sufficiently strong shock waves, the measured short-circuit currents indicate that the phase transition is very rapid and essentially complete. As shock strengths are reduced, short-circuit currents show increasing rise times and decreasing final levels at the end of shock transit. These features indicate that the transition kinetics can be characterized in terms of both a transition rate and a limiting degree of transition achieved in a given shock experiment. The presence of a strong electric field does not appear to have a significant effect on transition kinetics at high shock stresses, but has a strong effect at low stresses. As was found for constitutive mechanical properties, only small effects on measured currents resulted from differences in the porous microstructure of common-density materials, but large effects were observed when initial density was varied. To examine transition kinetics in more detail, short-circuit currents obtained with the baseline material and several approximate methods were used to estimate values for the rate and degree of transition as functions of shock properties. Differences between these currents and currents measured in high-field experiments using the same impact conditions were used to examine field effects on transition kinetics and corresponding dielectric properties.

Shock wave compression behavior of aluminum foam

The shock wave compression behavior of the open cell aluminum foam with relative density of 0.396 was studied through planar impact experiments. Using polyvinylidene fluoride (PVDF) piezoelectric gauge technique, the stress histories and propagation velocities of shock wave in the aluminum foam were measured and analyzed. The results show that the amplitude of shock wave attenuates rapidly with increasing the propagation distance in the aluminum foam, and an exponential equation of the normalized peak stress vs propagation distance of shock wave is established, the attenuation factor in the equation is 0.286. Furthermore, the Hugoniot relation, v 5=516.85 + 1.27v p, for the aluminum foam is determined by empirical fit to the experimental Hugoniot data.

1-D and 2-D modeling of U-Ti alloy response in impact experiments

Dynamic response of a U-0.75wt%Ti alloy has been studied in planar (disk-on-disk), reverse (disk-on-rod) and symmetric (rod-on-rod) ballistic impact experiments performed with a 25 mm light-gas gun. The impact velocities ranged between 100 and 500 m/sec and the samples were softly recovered for further examination, revealing different degrees of spall fracture (planar impact) and of adiabatic shear bands (ballistic experiments). The back (planar experiments) and the lateral (ballistic experiments) surface velocities were continuously monitored by VISAR. The velocity profiles and the damage maps were simulated using a 2-D AUTODYN Lagrangian finite differences code. Simulations of the planar experiments were performed with special attention to the compressive path of the loading cycle in order to calibrate a modified Steinberg-Cochran-Guinan (SCG) constitutive model. The Bauschinger effect and a single-parameter spall model were added to describe the unloading and tensile paths. The calibrated SCG model was then employed to simulate the ballistic experiments. An erosion AUTODYN built-in subroutine with a threshold value of plastic strain was chosen to describe the failure in the ballistic impact experiments. The results of the suggested experimental-numerical technique can be taken into account in estimating the different contributions to the shock-induced plastic deformation and failure.

Multipeak pulse x-ray diffraction study of shocked single crystals

The geometry of a pulse x-ray diffraction survey is suggested to obtain a multipeak x-ray diffraction pattern from a single crystal sample in planar impact experiments. This geometry allows simultaneous recording of x-ray reflections from different crystal planes. The corresponding mathematical expressions for screen (detector) positions of the reflections produced by both strained and unstrained crystals are developed. In the case of the reflections from (200) and (220) crystal planes, the expressions enable the strain tensor components to be extracted from the relative shifts of diffraction peaks. The expressions were applied to interpret the multipeak diffraction patterns obtained from NaCl single crystals both shock-compressed and unloaded after the shock compression. The lattice strain alterations observed may be explained within the framework of dislocation theory of plastic deformation.

Dynamic response of ceramic-metal composites: The TiC-Steel system

The dynamic response of a titanium carbide (TiC)–carbon steel, ceramic-metal composite, was studied in planar impact experiments, using a copper impactor with velocity in the 80–450 m/s range. The composites were prepared by pressureless infiltration of TiC ceramic preforms by molten steel. The metallic component had either a pearlitic or a martensitic microstructure, determined by an appropriate heat treatment. Fully dense composites, consisting of TiC and 1060 steel, in pearlitic and martensitic states, were used as reference samples. Values of the Hugoniot elastic limit and of the spall strength were derived from the velocity interferometer system for any refractor records of the free surface velocity profiles of the impacted samples. These properties are affected drastically by the confining stress that is induced in the TiC particles by the steel submatrix and is dependent on the microstructure of the latter. The results show unambiguously that the dynamic response of the cermets may be controlled by choosing an appropriate thermal treatment.

On the inelastic shock profile in alumina

The dynamic response of alumina specimens, above their elastic limits, was studied using planar impact experiments with different tile thickness. Stress-time measurements with manganin gauges show a steady spreading of the inelastic portion of the shock profile with increasing tile thickness. Such behavior is typical of elastic waves moving at a constant speed that depends on their amplitude. This finding supports recent interpretations of the failure ramp, by which the elastic response of these materials should be extended to higher stresses than the initial jump. However, further analysis of these profiles raises some questions regarding the exact determination of the Hugoniot elastic limit.

The role of thermal activation on dynamic stress-induced inelasticity and damage in Ti–6Al–4V

Planar impact experiments are performed on preheated Ti–6Al–4V specimens, at temperatures in the range 25–550 °C, to determine the role of thermal activation on dynamic stress-induced inelasticity and damage. Measurements in this high temperature and high strain rate regime are made possible by modification of the standard plate impact facility to include heating capabilities. This paper describes in detail needed hardware and experimental procedure. A symmetric planar impact configuration is employed to achieve high compressive and tensile stresses in the specimens. The targets are heated by a magnetic field generated by current flow on a coil surrounding the specimen. Interferometric techniques are employed to record the free surface velocity of the target plates. The experimental results show that thermal activation overcomes the role of rate dependence in the material constitutive behavior. The Hugoniot elastic limit (HEL) and spall strength of Ti–6Al–4V significantly decrease with temperature despite the high strain rate, about 10 5 s −1 , used in the tests. The damage mechanism remains the same at high and room temperatures, i.e., microvoid nucleation, growth and coalescence. Microscopy studies, performed on recovered samples, show that temperature substantially reduces the strain inhomogeneity leading to microvoid formation and that a change in void nucleation site occurs. A completely reversible shock-induced phase transformation α→ω appears to be present in the tested Ti–6Al–4V. Evidence of this phase transformation is observed in the velocity histories upon unloading of the first compressive pulse. The phase transformation is controlled by a combination of thermal and stress driven mechanisms.

Decay of elastic waves in alumina

The dynamic response of alumina under shock compression was studied using planar impact experiments with different tile thicknesses. Stress-time measurements were made with manganin gauges backed by different backing materials in order to optimize gauge response. The results show an apparent decay in the Hugoniot elastic limit with propagation distance. However, further analysis reveals that this phenomenon is probably a measurement artifact, resulting from the relatively slow response times of manganin gauges.

Experimental examination and numerical NAG model analysis of spall sensitivity to microstructure in copper

In order to achieve incipient and complete spallation, plane impact experiments were performed on three types of copper with different impurity levels. Dependence of the damage behavior on the material microstructure was found. The VISAR results of these experiments were used to examine the NAG model of Curran et al [Phys Rev. 147 (1987)], which includes a detailed description of microscopic void nucleation and growth dynamics. All measurements could be fitted by change of a single parameter in the model, the initial nucleation rate. The analysis demonstrates the competitive role of void nucleation and growth in the development of the spall.

A simple non-local spallation failure model

A new, simple theoretical model to characterise the average continuum behaviour of the spall process is presented. Unlike all previous models, it is based on a non-local view of spallation and is applicable to both ductile and brittle materials. The model was incorporated into one and two dimensional hydrodynamic code to predict the free surface velocity of ductile and brittle targets in planar plate impact experiments, and can be extended to three dimensional problems. The results compared well with experimental data. The model can be used with common equations of state and constitutive equations.

Material Behavior in Plane Polyurethane-Polyurethane Impact with Velocities from 10 to 400 m/sec

Shock response of the rubber-like polyurethane (PU) has been studied in symmetrical plane impact experiments in the impact velocity range 10 - 400 m/sec. The free surface velocity of the samples was measured by the laser velocity interferometer. It has been found that for a weak impact the step-like perturbation cannot penetrate into the polyurethane sample while the signal with the risetime of 1µsec and higher may propagate in the material with some steepening. The Poisson's ratio of PU estimated on the basis of the weak-impact data is close to 0.5. With the increase of the impact strength the PU shock response becomes more similar to that of a normal solid with Poisson's ratio 0.39.

Embryo-damage induced nucleation of microcracks in an aluminium alloy under impact loading

The nucleation of microdamage under dynamic loading was investigated through planar impact experiments accomplished with a light gas gun. The microscopic observation of recovered and sectioned specimens showed that microcracks were nucleated only by cracking of brittle particles inside material. However, for comparison the in situ static tensile tests on the same material conducted with a scanning electron microscope showed that the microcracks were nucleated by many forms those were fracture of ductile matrix, debonding particles from matrix and cracking of brittle particles. The quantitative metallographic observations of the specimens subjected to impact loading showed that most of the cracked particles were situated on grain boundaries of the aluminium matrix. These facts suggested the concept of critical size and incubation time of submicroscopic cavities in the dynamic case and the mechanism of embryo-damage induced nucleation by fracture of brittle particles in the aluminium alloy under impact loading was proposed.