Series Of Plate Impact Experiments Research Articles

Role of Crystal Orientation in the Dynamic Strength of Magnesium Alloy AZ31B

The effect of grain orientation distribution on the dynamic strength of highly textured magnesium alloy AZ31B has been studied in a series of plate-impact experiments. Specimens with thicknesses between 0.45 mm and 2 mm were cut parallel and perpendicular to the material extrusion direction and shock loaded to impact stresses between 1.4 GPa and 3.4 GPa. The dynamic strength is found to be highly dependent on the loading direction, with loading along the extrusion direction exhibiting significantly higher Hugoniot elastic limits than the transverse direction, including a much slower precursor decay rate. Application of an orientation-based analysis framework shows that the yield point of the polycrystalline material can be predicted reasonably well from its grain orientation distribution, predicated upon the use of dynamic critical resolved shear stress values from single-crystal data modified by a fitted strengthening factor. It is shown that the strong dependence on loading orientation in Mg AZ31 is caused by the relative differences in slip system activity and the slip anisotropies inherent to the hexagonal close packed crystal structure.

The mechanical and microstructural response of single crystal aluminium to one dimensional shock loading: The effects of orientation

A series of plate impact experiments have been performed on the three principal orientations of single crystal aluminium: [100], [110] and [111]. Experiments were designed to probe the Hugoniot Elastic Limit, spall strength and Equation of state as well as understand the microstructural response, via transmission electron microscopy, Transmission Kikuchi Diffraction and mechanical testing of shocked material. Results from this work, show that orientation affects the HEL and spall strength in a similar manner to the quasi-static uniaxial stress compressive response, with [100] and [110] being similar and [111] significantly stronger. This correlates with expectations from considerations using the Schmid analysis. However we have noted that similar studies in copper yield a different ordering of the HELs. We have reconciled those differences using an analysis based on second order elastic constants. However the shocked microstructures and post shock quasi-static mechanical response are strongly influenced by orientation. The [100] orientation shows a response typical of high stacking fault face centred cubic metals, consisting of equiaxed subgrains and a strong post shock hardening during quasi-static mechanical testing, similar to results in polycrystalline aluminium. In contrast, the [110] and [111] orientations have a microstructural response more similar to polycrystalline aluminium after repeated shock loading excursions, combined with much lower post shock hardening during mechanical testing. This suggests that these orientations require much greater dislocation activity to achieve the same plastic deformation as [100]. This would appear to agree with the much higher degree of work hardening observed in these orientations compared to [100].

Spall strength and equation of states for 2050-T84 Al–Li alloy under shock compression up to 120 GPa

Aluminum-lithium (Al–Li) is an attractive alloy for aerospace and military applications due to its low density, excellent mechanical performance, and corrosion resistance. In this study, a series of plate-impact experiments were conducted using a powder gun and a two-stage light-gas gun to investigate the dynamic response of 2050-T84 Al–Li alloy under shock compression. The Hugoniot elastic limit, spall strength, and Hugoniot state were determined up to 120 GPa according to the measured free-surface velocity profiles and shock wave velocities. The spall strength measured for 2050-T84 Al–Li alloy was about 0.6 GPa, which is lower than that for non-lithium-bearing Al alloys due to the large grain size and different alloying compositions. The high-pressure Hugoniot data agreed well with theoretical calculations based on first-principles molecular dynamics. The dynamic fracture and Hugoniot data obtained for 2050-T84 Al–Li alloy in the present study can guide the design of protective structures for spacecraft or artificial satellites, and lightweight aluminum alloy armor plates.

Effect of grain size on the shock-induced response and spall fracture behavior of a metastable β titanium alloy subjected to repeated impact

The dynamic failure behavior of titanium alloys has received increasing attention due to their promising application in high-strain-rate service environments. In this research, a series of plate impact experiments were carried out on a metastable β titanium alloy Ti–2Al-9.2Mo–2Fe (2A2F) to reveal the influences of multiple shock and β grain size on its spall fracture behavior. The results show that the spall behavior of 2A2F is significantly dependent on the number of impacts and β grain size. For the alloy with a small grain size, after first shocked at 6 and 12 GPa, the spall strength is increased by more than 60% during the second shock loading process. However, the pre-shock process has little effect on the spall strength of the alloy with the larger grain size. The microstructure analyses show that dislocation proliferation and β-to-α″/ω phase transformations occurred during the shock pulse. The α″ variants and fishbone-like α″ structures were also formed in 2A2F at the higher shock stress. These shock-induced substructures strengthen the β grains, leading to the shift in the nucleation site and propagation path of the spall micro-damages and the improvement of the spall strength.

Probing the properties and mechanisms of failure waves in soda-lime glass

Soda-lime glass (SLG) and other silica glasses exhibit the failure wave phenomenon under shock compression. The mechanism responsible for this peculiar behavior of glasses is still unresolved. In this study, a series of plate impact experiments was performed at three different impact stresses of 6.4, 8.3, and 10.8 GPa to better understand the mechanisms underlying the failure wave phenomenon. Specifically, spall experiments were conducted to probe the speed and existence of failure waves at different stresses in SLG. A layered glass target was used to probe the possibility of a “renucleation” of the wave at the SLG–SLG interface. When it existed, the failure wave was inferred to propagate at a speed of 1.3 km/s. However, it was observed that the failure wave phenomenon ceases to exist for impact stresses higher than 10 GPa. In experiments with a 6.4 GPa impact stress, the peak free surface velocity was significantly less than what is predicted by stress-Hugoniot calculations. This velocity deficit and other important features of the measured free surface velocity profiles were simulated using finite element analysis by incorporating an abrupt densification of SLG at a critical stress in the equation of state. This densification feature is similar to what would be expected of a phase transition. Although unable to unambiguously reveal the mechanism causing the failure wave phenomenon, the results of the present work clearly indicate that the failure wave causes a secondary compression and densification in SLG.

Mechanical response and deformation mechanisms of porous PZT95/5 ceramics under shock-wave compression

A series of plate impact experiments were performed to investigate the shock response of porous PZT95/5 ceramics. Based on the measured free surface velocity profiles, the influences of impact velocity and porosity on the shock response of porous PZT95/5 ceramics were explored. Then, the shock response of porous PZT95/5 ceramics was numerically simulated by using the discrete element method (DEM) based on flat-joint bonded contact model to analyze the microscopic deformation mechanisms. The simulation results of the free surface velocity profiles agreed well with experimental results. The macroscopic deformation process of porous PZT 95/5 ferroelectric ceramics under shock-wave compression can be divided into four stages: elastic deformation, cracks spread deformation, void collapse deformation and Hugoniot state. The microscopic deformation mechanisms of porous PZT95/5 ceramics under shock-wave compression could be mainly attributed to the nucleation and growth of large numbers of shear cracks and tension cracks around voids, as well as the collapse of voids in ceramics.

Shockwave compression and dissociation of ammonia gas.

We performed a series of plate impact experiments on NH3 gas initially at room temperature and at a pressure of ∼100 psi. Shocked states were determined by optical velocimetry and the temperatures by optical pyrometry, yielding compression ratios of ∼5-10 and second shock temperatures in excess of 7500 K. A first-principles statistical mechanical (thermochemical) approach that included chemical dissociation yielded reasonable agreement with experimental results on the principal Hugoniot, even with interparticle interactions neglected. Theoretical analysis of reshocked states, which predicts a significant degree of chemical dissociation, showed reasonable agreement with experimental data for higher temperature shots; however, reshock calculations required the use of interaction potentials. We rationalize the very different shock temperatures obtained, relative to previous results for argon, in terms of atomic versus molecular heat capacities.

Bayesian calibration of strength parameters using hydrocode simulations of symmetric impact shock experiments of Al-5083

Predictive modeling of materials requires accurately parameterized constitutive models. Parameterizing models that describe dynamic strength and plasticity require experimentally probing materials in a variety of strain rate regimes. Some experimental protocols (e.g., plate impact) probe the constitutive response of a material using indirect measures such as free surface velocimetry. Manual efforts to parameterize constitutive models using indirect experimental measures often lead to non-unique optimizations without quantification of parameter uncertainty. This study uses a Bayesian statistical approach to find model parameters and to quantify the uncertainty of the resulting parameters. The technique is demonstrated by parameterizing the Johnson-Cook strength model for aluminum alloy 5083 by coupling hydrocode simulations and velocimetry measurements of a series of plate impact experiments. Simulation inputs and outputs are used to calibrate an emulator that mimics the outputs of the computationally intensive simulations. Varying the amount of experimental data available for emulator calibration showed clear differences in the degree of uncertainty and uniqueness of the resulting optimized Johnson-Cook parameters for Al-5083. The results of the optimization provided a numerical evaluation of the degree of confidence in model parameters and model performance. Given an understanding of the physical effects of certain model parameters, individual parameter uncertainty can be leveraged to quickly identify gaps in the physical domains covered by completed experiments.

Damage Characteristics of Dynamically Induced Incipient Spall in Ultrapure Aluminum

A series of plate-impact experiments were performed to study the damage characteristics of incipient spall in ultrapure aluminum (99.999%). Surface profile measurement system based on the axial chromatic aberration was used to measure the cross section of “soft-recovered” samples and reconstruct the metallographic and 3D surface topography. Statistical analyses show that the size distribution, coalescence condition and connectivity factor of the voids are regularly affected by loading conditions. Based on this finding, the mechanisms that control the damage properties and evolution process of dynamic fracture are analyzed in detail. The results of this research offer an experimental quantitative microstructure description of the ductile fracture process.

Spall behaviour of single crystal aluminium at three principal orientations

A series of plate impact experiments have been conducted to study the spall strength of the three principal crystallographic orientations of single crystal aluminium ([100], [110] and, [111]) and ultra-pure polycrystalline aluminium. The samples have been shock loaded at two impact stresses (4 GPa and 10 GPa). Significant differences have been observed in the elastic behaviour, the pullback velocities, and the general shape of the wave profiles, which can be accounted for by considerations of the microscale homogeneity, the dislocation density, and the absence of grain boundaries in the single crystal materials. The data have shown that there is a consistent order of spall strength measured for the four sample materials. The [111] orientation has the largest spall strength and elastic limit, followed closely by [110], [100], and then the polycrystalline material. This order is consistent with both quasi-static data and geometrical consideration of Schmid factors.

Spall strength and fracture behavior of Ti–10V–2Fe–3Al alloy during one-dimensional shock loading

The spall fracture behavior of a typical metastable β titanium alloy, Ti–10V–2Fe–3Al, is systemically investigated during one-dimensional shock loading. Shock pressures over 14 GPa are achieved through a series of plate impact experiments. The results show that the shock-induced β-to-α″ martensite (SIM) phase transformation strongly influences the spall fracture behavior of this alloy. The spall strength of Ti–10V–2Fe–3Al ranges from 2.5 to 3.1 GPa, and drops by nearly 20% as the shock pressure exceeds 9 GPa. The acicular SIM phases provide additional nucleation sites and propagation paths for micro-damage, leading to the reduction of spall strength of Ti–10V–2Fe–3Al. Microstructural analyses also suggest that Ti–10V–2Fe–3Al is likely to spall in a mixed fracture manner containing both brittle and ductile failure features. The spall fracture surface of this alloy is very rough and consists of facets covered by ductile dimples.

Shock and spall behaviors of a high specific strength steel: Effects of impact stress and microstructure

A series of plate-impact experiments were conducted to investigate the influences of impact stress and microstructure on the shock and spall behaviors of a high specific strength steel (HSSS). The HSSS shows a strong positive strain rate sensitivity on the yield strength. With increasing impact stress up to about 6 GPa, the spall strength is found to decrease significantly and then levels off with further increasing impact stress. This trend is proposed to be attributed to the accumulation damage within the target as the initial shock-induced compression wave propagates through the target. The microcracks are clearly observed to nucleate from the interfaces between γ-austenite and B2 phase and propagate along the interfaces or cut through the B2 phase in the HSSS during the spalling process. The Hugoniot elastic limit and the spall strength were found to be highly dependent on the microstructure. The spall strength was found to be higher when the density of the void nucleation sites is lower, indicating that the spall strength should be a microstructure parameter of the HSSS under impact tensile conditions depending on the density of phase interfaces. It was also found that there is a tradeoff between the specific yield strength and the spall strength for this HSSS; thus, the current findings should provide insights for achieving an optimal combination of both mechanical properties for impact-resistant applications by tailoring the microstructure.

Experimental research on HEL and failure properties of alumina under impact loading

Abstract A series of plate impact experiments on alumina was conducted using a light gas gun in order to further investigate Hugoniot elastic limit (HEL) and failure properties of alumina under shock compression. The velocity interferometer system for any reflector (VISAR) was used to record the rear-free surface velocity histories of the alumina samples. According to the experimental results, the HELs of tested alumina samples with different thicknesses were measured, and the decay phenomenon of elastic wave in shocked alumina was studied. A phenomenological expression between HEL and thickness of sample was presented, and the causes of the decay phenomenon were discussed. The propagation of failure wave in shocked alumina was probed. The velocity and delayed time of failure wave propagation were obtained. The physical mechanism of the generation and propagation of failure was further discussed.

Behaviour of moist and saturated sand during shock and release

Relatively little is known about the changes that occur in the shock compaction and release of granular matter with varying levels of moisture. Here, we report a series of plate impact experiments giving shock Hugoniot and release data for a well characterized sand at dry, 10% moist, and saturated water contents. The results reveal that at low moisture content the shock impedance is slightly reduced, while the release remains predominantly inelastic. Close to saturation, much more substantial changes occur: the shock impedance stiffens substantially, the Hugoniot appears to split into two branches, and the release becomes almost completely elastic. We discuss mechanisms underpinning these changes in behavior.

Stress relaxation in vanadium under shock and shockless dynamic compression

Evolutions of elastic-plastic waves have been recorded in three series of plate impact experiments with annealed vanadium samples under conditions of shockless and combined ramp and shock dynamic compression. The shaping of incident wave profiles was realized using intermediate base plates made of different silicate glasses through which the compression waves were entered into the samples. Measurements of the free surface velocity histories revealed an apparent growth of the Hugoniot elastic limit with decreasing average rate of compression. The growth was explained by “freezing” of the elastic precursor decay in the area of interaction of the incident and reflected waves. A set of obtained data show that the current value of the Hugoniot elastic limit and plastic strain rate is rather associated with the rate of the elastic precursor decay than with the local rate of compression. The study has revealed the contributions of dislocation multiplications in elastic waves. It has been shown that independently of the compression history the material arrives at the minimum point between the elastic and plastic waves with the same density of mobile dislocations.

The Mechanical and Optical Response of Polychlorotrifluoroethylene to One-Dimensional Shock Loading

A series of plate impact experiments have been performed to probe the shock behavior of polychlorotrifluoroethylene (PCTFE), in terms of its optical and mechanical response. Interfacial velocity measurements using interferometric techniques have shown differences between measured and actual velocities, and been used to determine changes in refractive index due to shock-induced density increases. These have further been used to determine an optical correction factor, and allow the possibility of PCTFE being used as an optical window in future shock loading experiments. The shear strength of shock loaded PCTFE has also been shown to be near-constant behind the shock front, in common with other fluorinated polymers, although the strength variation with impact stress is greater than other similar materials. It has been suggested that the presence of a larger chlorine atom replacing a fluorine allows for a degree of tacticity between polymer chains, with local variations of charge density along the chain (due to the presence of the chlorine atom) also having an effect.

Elastic precursor decay in S-200F beryllium

We have performed a series of plate impact experiments on vacuum hot-pressed (VHP) S-200F Be at peak shock stresses between 2.1 and 23.0 GPa to gain insight into the dynamic strength (Hugoniot elastic limit (HEL)), equation-of-state, and damage behavior of this technologically important material. In this paper we focus on our VISAR observations of the evolution of elastic precursor amplitude with Be target thickness in a series of plate impact experiments conducted in both transmission and reverse geometry. We observe monotonic decay in precursor amplitude with run distance for sample thicknesses between 4 and 8 mm and present the HEL values obtained from these experiments. We will discuss the observed precursor decay with respect to the relative roles of twinning and dislocation-mediated slip in the overall dynamic material mechanical response.

Behaviour of sand during release from a shocked state

The dynamic response of granular materials to an applied shockwave is of wide ranging importance. While the shock Hugoniot has been studied, the shock-release of granular systems has never been experimentally characterised. Here, we present a simple approach to such measurements and present a series of plate impact experiments providing release data for a well characterised dry sand. We discuss the origin of the release behaviour, which we support with further measurements on a weakly bound sandstone.

An experimental investigation into the micro-mechanics of spall initiation and propagation in Ti–6Al–4V during shock loading

Titanium alloys have long been considered candidate materials for use in monolithic armor systems due to their excellent specific mechanical properties and relative ease of manufacture. However, high production costs combined with a propensity to fail due to adiabatic shear band formation and spallation have limited the adoption of titanium alloys in armor systems. In this paper, the spall behavior of the workhorse titanium alloy, Ti–6Al–4V, is investigated via a series of plate impact experiments. Back-scatter electron (BSE) microscopy and electron back-scatter diffraction (EBSD) are carried out on recovered specimens to investigate the nature of spall initiation and propagation. It is found that spall initiation occurs by the nucleation of voids at the grain boundaries between plastically hard/soft grains of the dominant hcp α phase. Further, it is shown that spall then propagates via the growth/coalescence of the nucleated voids into facets, followed by the coalescence of the facets through highly localized ductile bridging. A number of suggestions are made to improve the spall resistance of titanium alloys based on these observations.

Experimental investigation on delayed failure of alumina under shock compression

A series of plate impact experiments have been conducted on alumina specimens with different thicknesses through the light gas gun in order to further investigate the delayed failure phenomenon under shock loading. With the aid of the velocity interferometer system for any reflector, an evident recompression signal has been observed in the temporal curve of the rear free surface velocity of the specimen, which indicates the generation of the failure wave. According to the experimental results, the failure wave propagation was probed. The results show that the failure wave generates a delayed time of ∼0·105 μs behind the shock wave front and propagates into the stressed body with a velocity of ∼5051 m s−1 under the impact stress of 7·16 GPa. The physical mechanism of the failure wave was further discussed.