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We present a series of experiments on the response of additive manufactured (AM) tantalum to dynamic loading, specifically the spall strength. Rectangular plates of AM tantalum were produced, with subsequent characterization revealing a highly anisotropic microstructure. Samples were taken from these plates to investigate the effect of anisotropy on the spall strength: the resistance to high strain-rate tensile damage. A conventional, wrought tantalum sample, possessing an equiaxed microstructure, was also tested to serve as a control. Shock loading was performed via light gas-gun flyer-plate impact experiments, with laser velocimetry on the rear of the samples to record the shock wave profiles and soft-recovery techniques to allow post-mortem analysis. In general, the AM samples were found to have a higher Hugoniot elastic limit, the dynamic yield strength under shock loading, while having a reduced spall strength, when compared to the wrought tantalum samples. [ABSTRACT FROM AUTHOR]

Materials subjected to dynamic environments experience a complex and wide range of stress, strain, and strain-rate conditions. To have confidence in material models, an accurate, predictive capability is required. In this study, we present a series of flyer-plate impact tests on well characterized, high purity tantalum. The shock-waves generated at impact release from the free-surfaces, reflect, and interact to produce incipient spall fracture. By varying the flyer-plate material and impact velocity, both the peak stress and the strain-rate in the samples were controlled independently. Velocimetry was used on the rear free-surface of the samples to measure the shock-response and the spall strength. While this measurement provided the same spall strength for all cases, at approximately 5.1 GPa, when the samples were sectioned during post-mortem, the quantity and distribution of internal damage was markedly different. For the high-strain rate cases, voids remained small and isolated, whereas in the lower strain-rate experiments, the spall damage was far more localized, with a well-defined continuous spall plane. With the use of hydrocode simulations, this was discovered to result from how the different release rates affect the interaction volume inside the sample. These results highlight the importance of careful sample recovery, and the risks of relying solely on free-surface velocity measurements. [ABSTRACT FROM AUTHOR]

We present a study on the spall strength of additive manufactured (AM) Ti-6Al-4V. Samples were obtained from two pieces of selective laser melted (SLM, a powder bed fusion technique) Ti-6Al-4V such that the response to dynamic tensile loading could be investigated as a function of the orientation between the build layers and the loading direction. A sample of wrought bar-stock Ti-6Al-4V was also tested to act as a baseline representing the traditionally manufactured material response. A single-stage light gas-gun was used to launch a thin flyer plate into the samples, generating a region of intense tensile stress on a plane normal to the impact direction. The rear free surface velocity time history of each sample was recorded with laser-based velocimetry to allow the spall strength to be calculated. The samples were also soft recovered to enable post-mortem characterization of the spall damage evolution. Results showed that when the tensile load was applied normal to the interfaces between the build layers caused by the SLM fabrication process the spall strength was drastically reduced, dropping to 60% of that of the wrought material. However, when loaded parallel to the AM build layer interfaces the spall strength was found to remain at 95% of the wrought control, suggesting that when loading normal to the AM layer interfaces, void nucleation is facilitated more readily due to weaknesses along these boundaries. Quasi-static testing of the same sample orientations revealed a much lower degree of anisotropy, demonstrating the importance of rate-dependent studies for damage evolution in AM materials. [ABSTRACT FROM AUTHOR]

For ductile metals, dynamic fracture occurs principally through void nucleation, growth, and coalescence at heterogeneities in the microstructure. Previous experimental research on high purity metals has shown that microstructural features, such as grain boundaries, inclusions, vacancies, and heterogeneities, can act as initial void nucleation sites. In addition, other research on two-phase materials has also highlighted the importance of the properties of a second phase itself in determining the dynamic response of the overall material. However, previous research has not investigated the effects of the distribution of a second phase on damage nucleation and evolution. To approach this problem in a systematic manner, two copper alloys with 1% lead materials, with the same Pb concentration but different Pb distributions, have been investigated. A new CuPb alloy was cast with a more homogeneous distribution of Pb as compared to a CuPb where the Pb congregated in large "stringer" type configurations. These materials were shock loaded at ~1.2 GPa and soft recovered. In-situ free surface velocity information, and post mortem metallography, reveals that even though the spall strength of both the materials were similar, the total extent and details of damage in the materials varied by 15%. This suggests that altering the distribution of Pb in the Cu matrix leads to the creation of more void nucleation sites and also changed the rate of void growth. [ABSTRACT FROM AUTHOR]

The response of polycrystalline metals, which possess adequate mechanisms for plastic deformation under extreme loading conditions, is often accompanied by the formation of pores within the structure of the material. This large deformation process is broadly identified as progressive with nucleation, growth, coalescence, and failure the physical path taken over very short periods of time. These are well known to be complex processes strongly influenced by microstructure, loading path, and the loading profile, which remains a significant challenge to represent and predict numerically. In the current study, the influence of loading path on the damage evolution in high-purity tantalum is presented. Tantalum samples were shock loaded to three different peak shock stresses using both symmetric impact, and two different composite flyer plate configurations such that upon unloading the three samples displayed nearly identical "pull-back" signals as measured via rear-surface velocimetry. While the "pull-back" signals observed were found to be similar in magnitude, the sample loaded to the highest peak stress nucleated a connected field of ductile fracture which resulted in complete separation, while the two lower peak stresses resulted in incipient damage. The damage evolution in the "soft" recovered tantalum samples was quantified using optical metallography, electron-back-scatter diffraction, and tomography. These experiments are examined numerically through the use of a model for shock-induced porosity evolution during damage. The model is shown to describe the response of the tantalum reasonably well under strongly loaded conditions but less well in the nucleation dominated regime. Numerical results are also presented as a function of computational mesh density and discussed in the context ofimproved representation of the influence of material structure upon macro-scale models of ductile damage. [ABSTRACT FROM AUTHOR]

Previous experimental research has shown that microstructural features such as interfaces, inclusions, vacancies, and heterogeneities can all act as void nucleation sites. However, it is not well understood how important these interfaces are to damage evolution and failure as a function of the surrounding parent materials. In this work, we present results on three different polycrystalline materials: (1) Cu, (2) Cu-24 wt. %Ag, and (3) Cu-15 wt.%Nb which were studied to probe the influence of bi-metal interfaces on void nucleation and growth. These materials were chosen due to the range of difference in structure and bulk properties between the two phases. The initial results suggest that when there are significant differences between the bulk properties (for example: stacking fault energy, melting temperature, etc.) the type of interface between the two parent materials does not principally control the damage nucleation and growth process. Rather, it is the "weaker" material that dictates the dynamic spall strength of the overall two-phase material. [ABSTRACT FROM AUTHOR]

A longstanding question in the field of multi-material behavior pertains to the treatment of interfaces possessing finite frictional strength under high dynamic pressures and shear. Here, we examine the effects of constrained interface sliding on local deformation near the boundary using new, highresolution measurements combined with simulations to infer friction strength. The experiments use laser driven plate impacts at the Los Alamos National Laboratory TRIDENT Laser Facility to launch a shock wave into a target consisting of a central cylindrical plate of Be and an outer ring of Cu oriented, such that the shock propagates at nearly a 90° angle to the interface normal producing a large velocity gradient across the material boundary. Impact experiments were performed on targets that underwent diffusion bonding of the two materials and on targets that were only press fit together. Friction-induced surface deformation was diagnosed using line-imaging velocity interferometry and surface Transient Imaging Displacement Interferometry in the immediate region of the interface. In these studies, we observed a significant behavioral change in both simulations and experiments between targets with diffusion bonded interfaces and those that were press fit. Bonded targets exhibited a mutual dragging between the Be and Cu parts throughout the entire experiment, whereas unbonded targets displayed a surface slope reversal on the Cu side of the interface, which simulations suggest arise due to altered wave interactions from a 3× lower frictional force compared to the bonded interface. [ABSTRACT FROM AUTHOR]

We provide a unique set of observations of the behavior of the α→ϵ phase transition under a complex axially symmetric loading path created by sweeping a detonation wave along the end surface of a cylindrical sample. The primary data sets are the measured mass density distributions acquired at 5 independent times during the sweep of the detonation along the surface. Shocked regions and boundaries are measured, as well as regions and boundaries of elevated density (presumed to be the ϵ-phase iron). The formation and dynamics of these regions were captured and are available for comparisons to material descriptions. We also applied 16 Photon Doppler Velocimetry probes to capture the free surface velocity along a discrete set of radially distributed points in order to compare and correlate the density measurements with previous shock wave studies. The velocimetry data are in nearly exact agreement with previous shock wave studies of the α→ϵ phase transition, the density distributions, while generally in agreement with expectations evolved from the shock wave studies, show that the epsilon phase is generated in regions of high shear stress but at hydrostatic stresses below the typically quoted 13 GPa value. The density field measurements are particularly useful for observing the effects of the forward and reverse transformation kinetics, as well as the reverse transformation hysteresis. [ABSTRACT FROM AUTHOR]

For ductile metals, dynamic fracture occurs through void nucleation, growth, and coalescence. Previous experimental works in high purity metals have shown that microstructural features such as grain boundaries, inclusions, vacancies, and heterogeneities can act as initial void nucleation sites. However, for materials of engineering significance, those with, second phase particles it is less clear what the role of a soft second phase will be on damage nucleation and evolution. To approach this problem in a systematic manner, two materials have been investigated: high purity copper and copper with 1% lead. These materials have been shock loaded at ∼1.5 GPa and soft recovered. In-situ free surface velocity information and post mortem metallography reveals the presence of a high number of small voids in CuPb in comparison to a lower number of large voids in Cu. This suggests that damage evolution is nucleation dominated in the CuPb and growth dominated in the pure Cu. [ABSTRACT FROM AUTHOR]

The shock response of the aluminum alloy 6061, and its variation according to heat treatment have been monitored via the placement of stress gauges in such orientations so as to be sensitive to the lateral component of stress, and hence the shear strength. To complement these measurements, the postshock microstructure and mechanical response have also been determined via full one-dimensional recovery techniques. Results have shown that the solution treated (T0) state, as a largely single phase material displays a fast rising shock pulse with a significant degree of hardening behind the shock front. This indicates that a high degree of dislocation generation is expected. Postshock analysis of recovered samples has confirmed this hypothesis, with dislocation cells being observed and a notable increase in the yield strength in comparison to the as-received material. In contrast, the aged (T6) experiments showed a much longer rise time with a lower degree of hardening behind the shock front. Microstructural analysis postshock shows a more randomized dislocation distribution, with little or no postshock hardening occurring once the shock induced strain has been accounted for. This has been attributed to the presence of fine Mg2Si precipitates inhibiting the motion and generation of dislocations. These measurements are in agreement with work previously carried out on this material. Comparison of the shear strengths of the two heat treatments also shows that although the T6 condition is a little higher than T0, the differences are somewhat lower than expected. [ABSTRACT FROM AUTHOR]

A magnetic loading technique was used to ramp load pure aluminum and 6061 aluminum alloy to peak stresses of approximately 29 GPa. The peak loading rate was approximately 106/s, followed by unloading from peak stress at a rate of about 105/s. The pure aluminum samples had impurity levels ranging from about 10 ppm to 0.5 wt % and average grain sizes in the range of 144–454 μm. The 6061 alloy was prepared in either the T6 condition with grain sizes of 5–50 μm, or in the T0 or T6 heat treatment condition with a grain size of about 40 μm. A wave profile technique was used to estimate the compressive strength during unloading. It was found that the compressive strength estimated during unloading increased with peak stress for all materials and that the change in strength was insensitive to initial material properties. This observation is in agreement with previous results obtained from shock loading of the same materials [H. Huang and J. R. Asay, J. Appl. Phys. 98, 033524 (2005)] and suggests that the deformation mechanisms, which govern strength properties at high strain rates, are similar for the two cases. It was also found that the quasielastic strain occurring during unloading saturated at about 2% in 6061-T6 alloy for peak stresses in the range of 10–50 GPa. The change in quasielastic strain was found to be similar for pure aluminum and 6061 alloys. [ABSTRACT FROM AUTHOR]

This effort investigates surface-preparation methods to enhance dynamic surface-property measurements of shocked metal surfaces. To assess the ability of making reliable and consistent dynamic surface-property measurements, the amount of material ejected from the free surface upon shock release to vacuum (ejecta) was monitored for shocked Al-1100 and Sn targets. Four surface-preparation methods were considered: Fly-cut machine finish, diamond-turned machine finish, polished finish, and ball rolled. The samples were shock loaded by in-contact detonation of HE PBX-9501 on the front side of the metal coupons. Ejecta production at the back side or free side of the metal coupons was monitored using piezoelectric pins, optical shadowgraphy, and x-ray attenuation radiography. [ABSTRACT FROM AUTHOR]

The shock response of the intermetallic compound Ni3Al (doped with boron to enhance ductility) has been investigated to determine equation of state and shock induced mechanical properties, and the response is compared to that of pure nickel. Examination of the Hugoniot data suggests that the Ni3Al has a higher compressibility than pure nickel. Elastic precursor decay has been observed, although due to the limited number of specimens available, we were not able to determine the point where the elastic amplitude became stable. Spall and shear strengths were observed to increase to a peak value (at a shock stress of between 6 and 7 GPa) before dropping again. We believe that this may be due to increasing amounts of damage behind the shock front as shocks of increased stress are applied. In a similar manner, spall strength was observed to decrease with pulse duration, again, most likely due to damage accumulating behind the shock front. Finally, it was observed that this material was brittle under all shock loading conditions studied in this investigation. We have proposed, due to the extremely high dislocation velocities imposed by the shock induced strain rates, that boron has effectively been removed from dislocations and hence cannot enhance ductility. [ABSTRACT FROM AUTHOR]

It is important to understand the mechanical behavior of polymeric materials that have a range of industrial and defence applications. In particular, several are used as the binder phase in plastic bonded explosives and propellants. Binder materials used in energetics need to be understood first to ensure their long-term, high-temperature stability when cycled during service, and second to ensure that the accidental loading of such materials leaves the explosives crystals within safe to handle. This work presents experimental data on equation-of-state and shock-induced damage evolution of the polymers teflon, estane™ and Kel-F-800™. The tensile (spall) strength at high rate was quantified using real-time laser interferometric techniques on each polymer as a function of impact stress. Measurements were made of the lateral stress under impact using manganin gauges, which, when combined with longitudinal data, allowed investigation of the shear strength of each polymer. An increase of shear strength, determined in this manner, was noted for two polymers and comparisons are drawn with others investigated using the same technique. A discussion of the mechanical response of these polymers is presented. [ABSTRACT FROM AUTHOR]

Presents information on a study which examined the effects of pulse duration and tensile strain rates on spall fracture of copper. Experimental design; Finite-difference simulations of copper spall fracture; Summary.

Grain boundaries (GBs) are often the preferred sites for void nucleation in ductile metals. However, it has been observed that all boundaries do not contribute equally to this process. We present a mechanistic rationale for the role of GBs in damage nucleation in copper, along with a quantitative map for predicting preferred void nucleation at GBs based on molecular dynamics simulations in copper. Simulations show a direct correlation between the void nucleation stress and the ability of a grain boundary to plastically deform by emitting dislocations, during shock compression. Plastic response of a GB, affects the development of stress concentrations believed to be responsible for void nucleation by acting as a dissipation mechanism for the applied stress. [ABSTRACT FROM AUTHOR]

Grain boundaries (GBs) can play an important role in governing the mechanical behavior and damage evolution of a material during both quasistatic and dynamic loading. However, the general consensus of the shock physics community has been that minute details about theGBstructure should not affect the response of a material to dynamic loading. In this paper, we present results of molecular-dynamics simulations investigating whether or not small changes in boundary structure are 'recognized' by the shock wave and can in turn affect the spall strength of a material. As a test case, we study a Σ11 asymmetric tilt GB in copper with an ordered and a disordered structure. The results are also compared with face-centered-cubic single crystals which correspond to the crystal orientations of the two grains in the bi-crystal. These results show that ordered and disordered boundaries undergo dissimilar amounts of plastic deformation during shock loading, which leads to spall strengths that vary by 12%, likely due to differences in the GB structures. [ABSTRACT FROM AUTHOR]

The response of pure nickel (Ni), a binary Ni-60 at. pct cobalt (Co) alloy exhibiting a low stacking fault energy (SFE), and the ordered face-centered-cubic (fcc) alloy Ni-24Al-0.01B to shock loading has been studied using the technique of plate impact. Changes in the variation of mechanical properties with shock amplitude and pulse duration have been explained in terms of a shift from dislocation dominated to twin dominated plasticity in the case of the Ni-Co alloy and the increasing effect of brittle failure in the case of Ni3Al. [ABSTRACT FROM AUTHOR]

A foreword to "Metallurgical and Materials Transactions A" is presented.