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Loading from sweeping detonation waves possess enhanced shear relative to loading from ordinary one-dimensional plane detonation waves. Our experiments use cylindrical detonation waves that are driven into flat samples of high-purity titanium. The waves driven into the sample by the explosive interact obliquely from the free surface as the wave sweeps along the sample, develop toward quasi-steady motion from effectively zero obliquity, and therefore induce a variation of shear along the sample. The type and thickness of the explosive is also varied, from experiment to experiment, in order to access conditions that bridge the α → ω phase transition in pure titanium. The primary dynamic diagnostic is Photon-Doppler-Velocimetry (PDV) using a crossed pair of probes directed at each spot with sufficient view direction (vectorial) independence such that the normal and in-surface tangential velocity are found. Various quantities that characterize the dynamic response of the material are derived from the PDV data and reported, such as spall strength. The samples are recovered and metallurgical analysis performed to characterize the deformation mechanics, such as twinning, and likelihood of the presence of the α → ω phase transition via retained ω −phase associated with various loading conditions. [ABSTRACT FROM AUTHOR]

Certification and product qualification of an engineering component generally involves meeting engineering and physics requirements tied to its functional requirements. . In this paper, the results of a study quantifying the microstructure and the dynamic damage evolution of Tantalum (Ta) fabricated using an EOS laser-powder-bed machine are presented. The microstructure of the AM-Ta is detailed and compared / contrast to wrought Ta. The dynamic damage evolution and failure response of the AM-Ta material, as well as wrought Ta, was probed using flyer-plate impact driven spallation experiments. The differences in the spallation response between the AM and wrought Ta were measured using in-situ velocimetry as well as postmortem quantification of damage in "soft-recovered" samples. The damage evolution of the AM and wrought Ta were characterized using optical metallography. [ABSTRACT FROM AUTHOR]

With PBX9501 we explosively loaded fully annealed OFHC-Cu and an OFHC-CuPb (extruded with 1% Pb that aggregates at the Cu grain boundaries) to study the effects of the 1% Pb on the elastic-plastic yield Y of Cu. The yield-stress Y was studied through observation of surface velocimetry and total ejected mass ρA from periodic surface perturbations machined onto the sample surfaces. The perturbation's wavelengths were λ ≈ 65 μm, and their amplitudes h were varied to determine the wavenumber (2π=λ) amplitude product kh at which ejecta production for the Cu and CuPb begins, which relates to Y. The Y of the two materials is apparently different. [ABSTRACT FROM AUTHOR]

Widespread research since 1950 has provided a wealth of experimental data concerning shock hardening and the spallation response of materials subjected to square-topped shock-wave loading profiles. Sweeping-wave loading is a significantly different loading history than that achieved by a square-topped impulse or 1-D HE-driven plane-wave shock in terms of the evolving spherical and shear stresses applied. Sweeping-wave loading in a flat-plate geometry was previously observed to: a) yield a lower spall strength than previously documented for 1-D supported-shock-wave loading, b) exhibit increased shock hardening as a function of increasing obliquity, and c) lead to an increased incidence of deformation twin formation with increasing shock obliquity (1). The current sweeping-wave loading of a 10 cm radius curved Ta plate is observed to: a) lead to an increase in the shear stress as a function of increasing obliquity, and b) display a more developed level of damage evolution, extensive voids and coalescence, and lower spall strength with obliquity in the curved plate than in the flat-plate sweeping-detonation wave loading for an equivalent HE loading. [ABSTRACT FROM AUTHOR]

There is a range of thresholds for the response of condensed matter under loading in compression, from the yield point to that at which the bond strength is overcome and warm dense matter is formed. Yield stress shows a correlation between the length scale swept by the rise of the pulse and the defect distribution within the target for a range of materials. Strain rate is also a useful term that reflects the evolution of the stress state within a target but must also be defined for a volume element containing a particular defect distribution to reflect continuum conditions acting within; it thus applies to a defined length scale within a target. Examples are shown using shock pulses that spall metal targets. Different stacking shows differing behaviour yet in each case momentum is conserved. This overview suggests simple observations must take account of mechanisms operating at different timescales and lengthscales in the development of damage in materials and structures. [ABSTRACT FROM AUTHOR]

Widespread research over the past five decades has provided a wealth of experimental data and insight concerning the shock hardening, damage evolution, and the spallation response of materials subjected to square-topped shock-wave loading profiles. However, fewer quantitative studies have been conducted on the effect of direct, in-contact, high explosive (HE)-driven Taylor wave (unsupported shocks) loading on the shock hardening, damage evolution, or spallation response of materials. Systematic studies quantifying the effect of sweeping-detonation wave loading are yet sparser. In this study, the shock hardening and spallation response of Ta is shown to be critically dependent on the peak shock stress and the shock obliquity during sweeping-detonation-wave shock loading. Sweeping-wave loading is observed to: a) yield a lower spall strength than previously documented for 1-D supported-shock-wave loading, b) exhibit increased shock hardening as a function of increasing obliquity, and c) lead to an increased incidence of deformation twin formation with increasing shock obliquity. [ABSTRACT FROM AUTHOR]

HT-9 is a 12Cr - 1Mo ferritic/martensitic steel with significant experience as cladding material in fast reactor applications. Recent investigations into precipitation of α in HT-9 steel after irradiation at elevated temperatures suggests that it nucleates at dislocation loops. It is recognized that steel shocked below the α to ε high-pressure phase transformation results in a material with a high density of stored defects. These defects have profound effect on subsequent mechanical properties and with additional elevated temperature exposure could serve as nucleation sites for the α' precipitate. To investigate the possibility of precipitating α' at dislocations, HT-9 steel was shocked at a peak pressure of 11 GPa and subsequently annealed at 475 °C for up to 16 weeks. The mechanical response of shock prestrained HT-9 steel was investigated and compared to the mechanical response of the shocked and annealed material. Substructure and texture evolution due to shock loading was examined and mechanical response of shock prestrained HT-9 is rationalized in terms of these observations. [ABSTRACT FROM AUTHOR]

We present an experimental study of crystalline structure evolution of polytetrafluoroethylene (PTFE) due to pressure-induced phase transitions in a semi-crystalline polymer using soft-recovery, shock-loading techniques coupled with mechanical and chemical post-shock analysis. Gas-launched, plate impact experiments have been performed on pedigreed PTFE 7C, mounted in momentum-trapped, shock assemblies, with impact pressures above and below the phase II to phase III crystalline transition. Below the phase transition only subtle changes were observed in the crystallinity, microstructure, and mechanical response of PTFE. Shock loading of PTFE 7C above the phase II–III transition was seen to cause both an increase in crystallinity from 38% to ∼53% and a finer crystalline microstructure, and changed the yield and flow stress behavior. We particularly focus on the effect of pulse duration on the microstructure evolution. [ABSTRACT FROM AUTHOR]

The mechanical behavior of, and damage evolution in high-purity Cu is influenced by strain rate, temperature, stress state, grain size, and shock prestraining. The effects of grain size on the tensile mechanical response of high-purity Cu have been probed and are correlated with the evolution of the substructure. The dynamic extrusion response of shock prestrained Cu demonstrates the significant influence of grain size on the large-strain dynamic tensile ductility of high-purity copper. Eulerian hydrocode simulations utilizing the Mechanical Threshold Stress constitutive model were performed to provide insight into the dynamic extrusion process. Quantitative comparisons between the predicted and measured deformation topologies and extrusion rates are presented. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

When uranium-niobium (6 wt.%) alloy is shock loaded, the expected elastic precursor is absent. A prior model attributed this absence to shear-induced twinning and the concomitant shear stress reduction that prevented the shocked material from reaching the plastic yield point. In the present study, carefully prepared U6Nb was subjected to shock loading to verify the adequacy of the prior model. Other samples were loaded with a ramp pressure pulse with strain rate large enough so that significant twinning would not occur during the experiment. Backward integration analyses of these latter experiments’ back surface motion give stress-strain loading paths in U6Nb that suggest ordinary elastic-plastic flow. Some of the U6Nb was pre-strained by cold rolling in an effort to further ensure that twinning did not affect wave propagation. Shock and ramp loadings yielded similar results to the baseline material except, as expected, they are consistent with a higher yield stress and twinning shear stress threshold. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

While much has been learned over the past five decades concerning shock hardening and the spallation response of materials shock-loaded using “square-topped” shock profiles, achieved via flyer plate loading, considerably less quantitative information is known concerning direct in-contact HE-driven or triangular-wave loading profile shock prestraining on metals and alloys. In this paper the influence of shock-wave profile, using both “square-topped” and triangular-wave pulses, on the shock hardening and spallation response of 316L stainless steel is presented. The shock hardening in 316L SS, using a triangular-shaped pulse and square-topped pulse (pulse duration of 0.75 μsec) to a peak shock pressure of 6.6 GPa was found to be reasonably similar. Square-wave loading at 6.6 GPa is observed to result in incipient spallation in 316L SS while triangular-wave loading to an equivalent peak stress is quantified to exhibit no wave-profile “pull-back” nor damage evolution. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The post shock mechanical behavior and substructure evolution of zirconium (Zr) under shock prestrained at 5.8 and 8 GPa, above and below the pressure induced α-ω phase transition, has been quantified. The reload yield stress of Zr shock prestrained to 8 GPa was found to exhibit enhanced shock hardening when compared to the flow stress measured quasi-statically at an equivalent strain. In contrast, the reload yield behavior of Zr specimens shocked to 5.8 GPa did not exhibit enhanced shock hardening. The microstructure of the as-annealed and shock prestrained materials were examined. The presence of a reduced available glide distance due to a relatively more well developed dislocation substructure and increased twinning over quasi-static specimens deformed to comparable strains correlates with the increased yield stresses after shock prestraining at 8 GPa. Additionally, the retention of ∼ 40% by volume metastable high-pressure ω-phase in specimens shocked to 8 GPa and its absence in the 5.8 GPa specimen, is thought to contribute to the increased yield stress in the 8 GPa specimens. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

U-Nb(6-wt%) exhibits plastic yield strength of a few-tenths of a GPa that can vary depending upon the starting microstructure and heat-treatment. However, when a several-mm thick specimen of U-Nb(6-wt%) is shock loaded in the range between 1.5 and 10 GPa, no elastic precursor is observed in interferometer measurements at the rear free surface. The absence of the elastic precursor and other features of the compression and release measurements are explained by assuming shear stress-relaxation rate is dependent upon the shear stress. The resulting stress waves are unsteady and broaden so that shear stress relaxation can occur in the front preventing the plastic yield point from being reached. U-Nb(6-wt%) is known to twin in quasi-static compression and shear-induced, rate-dependent twinning is likely the underlying cause for shear stress relaxation in our experiments. Recent experiments in which U-Nb(6-wt%) was heavily cold-rolled ( work-hardening to ∼25% strain) display no evidence of a precursor, admitting the possibility of a pressure or temperature induced stress-relaxation process. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The shock response of poyether ether ketone (PEEK) has been investigated using manganin stress gauges mounted in longitudinal and lateral orientation to the impact axis. Measurements of the longitudinal stress with gauges at different positions within the shock assembly have determined the Hugoniot in terms of shock stress, shock velocity and particle velocity. It has been shown that the shock velocity has a simple linear response to particle velocity, in common with many but not all polymers. Measurements of lateral stress show a decrease behind the shock front, implying an increase in shear strength, possibly due to the viscoplastic nature of PEEK. Shear strength was also observed to increase with shock stress. A break in slope was observed at ca. 1.0 GPa, indicating a divergence between elastic and inelastic behaviour. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Jim Williams has made seminal contributions to the field of structure / property relations and its controlling effects on the mechanical behavior of metals and alloys. This talk will discuss experimental results illustrating the role of interstitial content, grain size, texture, temperature, and strain rate on the operative deformation mechanisms, mechanical behavior, and substructure evolution in titanium, zirconium, hafnium, and rhenium. Increasing grain size is shown to significantly decrease the dynamic flow strength of Ti and Zr while increasing work-hardening rates due to an increased incidence of deformation twinning. Increasing oxygen interstitial content is shown to significantly alter both the constitutive response and α-ω shock-induced phase transition in Zr. The influence of crystallographic texture on the mechanical behavior in Ti, Zr, and Hf is discussed in terms of slip system and deformation twinning activity. An example of the utility of incorporation of operative deformation mechanisms into a polycrystalline plasticity constitutive model and validation using Taylor cylinder impact testing is presented. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Post-shock-recovered metallurgical analysis of solid metal spheres shock loaded via spherical energetic(HE) loading provides a unique opportunity to quantify the substructure evolution in a material subjected to converging Taylor-wave (triangular-shock pulse) loading. In this paper detailed quantitative metallographic, orientation-imaging microscopy (OIM), and texture analysis is presented characterizing the gradient in substructure generated in Cu subjected to a spherical HE shock loading pulse at VNIIEF. The substructure in the recovered sphere is seen to include: 1) a spherical cavity generated in the center of the sphere due to shock-wave convergence and release, displaying ductile dimpled failure and no evidence of melting, 2) a gradient in deformation (slip and deformation twins) from the center outward to the surface, and 3) numerous shear cracks and/or spall planes. The substructure evolution is discussed relative to that previously observed in Cu shock prestrained via either 1-D triangular-shaped shockwave loading or 1-D square-topped pulse shock loading. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The response of simple polymers to shock loading is governed by a number of factors such as the complexity of the polymer chains and nature of the atoms attached to the main carbon-carbon backbone. In the case of polyethylene based materials such as polyethylene, polypropylene and polytetrafluoroethylene the competing effects of inter chain tangling (tacticity) and electrostatic repulsion between adjacent polymer chains have been shown to have a profound effect on shock velocity, release velocity and shear strength development. In this work, we apply these considerations to a commercially available fluoro-tripolymer, Viton-B, where all these molecular features are present. [ABSTRACT FROM AUTHOR]

The tensile response of ductile, polycrystalline metals is often accompanied by the formation of pores within the material, which coalesce and fail a plane within a metal. This large deformation process is broadly progressive, with a physical path consisting of nucleation, growth, coalescence, and failure, which occur individually over short periods of time. Thus distinct micro-mechanisms operate, each influenced by microstructure, loading path, and loading profile, which remains a significant challenge to represent and predict numerically at the macroscale. In a previous study, the influence of loading path on damage evolution in high-purity tantalum has been presented; in this paper, complimentary measurements are made on a pure, FCC copper. Samples were shock loaded to three different peak stresses using both symmetric impact, and two different composite flyer plate configurations, such that upon unloading, the three samples were subject to nearly identical tensile loading. The damage evolution in the soft-recovered copper samples was then quantified using optical metallography and electron-back-scatter diffraction. We shall compare metallurgical observations, velocimetry histories and simulations to discuss the dynamic failure mechanics observed. [ABSTRACT FROM AUTHOR]

For ductile metals, the process of dynamic fracture occurs through nucleation, growth and coalescence of voids. For high purity single-phase metals, it has been observed by numerous investigators that voids tend to heterogeneously nucleate at grain boundaries and all grain boundaries are not equally susceptible to void nucleation. However, for materials of engineering significance, especially those with second phase particles, it is less clear if the type of bi-metal interface between the two phases will affect void nucleation and growth. To approach this problem in a systematic manner two bi-metal interfaces between Cu and Pb have been investigated: {111} and {100}. Qualitative and quantitative analysis of the collected data from molecular dynamics shock and spall simulations suggests that Pb becomes disordered during shock compression and is the preferred location for void nucleation under tension. Despite the interfaces being aligned with the spall plane (by design), they are not the preferred location for void nucleation irrespective of interface type. [ABSTRACT FROM AUTHOR]

At high pressures zirconium is known to undergo a phase transformation from the hexagonal close packed (HCP) alpha phase to the simple hexagonal omega phase. Under conditions of shock loading, the high-pressure omega phase is retained upon release. However, the hysteresis in this transformation is not well represented by equilibrium phase diagrams and currently models that accurately represent such a solid-solid phase transformation coupled with the multi-phase plasticity likely under shock conditions do not exist. For this reason, the influence of peak shock stress on the retention of omega phase in Zr is explored in this study. In-situ VISAR measurements along with post-mortem metallographic and neutron diffraction characterization of soft recovered specimens have been utilized to quantify the volume fraction of retained omega phase, morphology of the shocked alpha and omega phases, and qualitatively understand the kinetics of this transformation. This understanding of the role of peak shock stress will be utilized to address physics to be encoded in our present macro-scale models. [ABSTRACT FROM AUTHOR]

The role of shock wave loading profile is investigated for the failure processes in a brittle material. The dynamic damage response of ductile metals has been demonstrated to be critically dependent on the shockwave profile and the stress- state of the shock. Changing from a square to triangular (Taylor) profile with an identical peak compressive stress has been reported to increase the "spall strength" by over a factor of two and suppress damage mechanisms. The spall strength of tungsten heavy alloy (WHA) based on plate impact square-wave loading has been extensively reported in the literature. Here a triangular wave loading profile is achieved with a composite flyer plate of graded density in contrast to the square-wave loading. Counter to the strong dependence in wave profile in ductile metals, for WHA, both square and triangle wave profiles the failure is by brittle cleavage fracture with additional energy dissipation through crack branching in the more brittle tungsten particles, largely indistinguishable between wave profiles. The time for crack nucleation is negligible compared to the duration of the experiment and the crack propagation rate is limited to the sound speed as defined by the shock velocity. [ABSTRACT FROM AUTHOR]

Time-temperature equivalence is a widely recognized property of many time-dependent material systems, where there is a clear predictive link relating the deformation response at a nominal temperature and a high strain-rate to an equivalent response at a depressed temperature and nominal strain-rate. It has been found that high-density polyethylene (HDPE) obeys a linear empirical formulation relating test temperature and strain-rate. This observation was extended to continuous stress-strain curves, such that material response measured in a load frame at large strains and low strain-rates (at depressed temperatures) could be translated into a temperature-dependent response at high strain-rates and validated against Taylor impact results. Time-temperature equivalence was used in conjuction with jump-rate compression tests to investigate isothermal response at high strain-rate while exluding adiabatic heating. The validated constitutive response was then applied to the analysis of Dynamic-Tensile-Extrusion of HDPE, a tensile analog to Taylor impact developed at LANL. The Dyn-Ten-Ext test results and FEA found that HDPE deformed smoothly after exiting the die, and after substantial drawing appeared to undergo a pressure-dependent shear damage mechanism at intermediate velocities, while it fragmented at high velocities. Dynamic-Tensile-Extrusion, properly coupled with a validated constitutive model, can successfully probe extreme tensile deformation and damage of polymers. [ABSTRACT FROM AUTHOR]

While numerous studies have investigated the low-strain-rate constitutive response of beryllium, the combined influence of high strain rate and temperature on the mechanical behavior and microstructure of beryllium has received limited attention over the last 40 years. In the current work, high strain rate tests were conducted using both explosive drive and a gas gun to accelerate the material. Prior studies have focused on tensile loading behavior, or limited conditions of dynamic strain rate and/or temperature. Two constitutive strength (plasticity) models, the Preston-Tonks-Wallace (PTW) and Mechanical Threshold Stress (MTS) models, were calibrated using common quasi-static and Hopkinson bar data. However, simulations with the two models give noticeably different results when compared with the measured experimental wave profiles. The experimental results indicate that, even if fractured by the initial shock loading, the Be remains sufficiently intact to support a shear stress following partial release and subsequent shock re-loading. Additional "arrested" drive shots were designed and tested to minimize the reflected tensile pulse in the sample. These tests were done to both validate the model and to put large shock induced compressive loads into the beryllium sample. [ABSTRACT FROM AUTHOR]

For ductile metals, the process of dynamic fracture during shock loading is thought to occur through nucleation of voids, void growth, and then coalescence that leads to material failure. Particularly for high purity metals, it has been observed by numerous investigators that voids appear to heterogeneously nucleate at grain boundaries. However, for materials of engineering significance, those with inclusions, second phase particles, or chemical banding it is less clear what the role of grain boundaries versus other types of interfaces in the metal will be on nucleation of damage. To approach this problem in a step-wise fashion two materials have been investigated: high purity copper, and copper with 1% lead. These materials have been shock loaded at 1.4 GPa and soft recovered. In-situ VISAR and post mortem metallography reveals significantly less damage in the metals with no lead. The role of lead at grain boundary triple points and its behavior during shock loading will be discussed. [ABSTRACT FROM AUTHOR]

In order to understand the role of interface structure during shock loading, and specifically the role of interfaces in damage evolution due to shock, four copper bi-crystal grain boundaries (GBs) were studied under shock loading and incipient spall conditions. These boundaries, two [100]/[111] boundaries and two [100]/[100] boundaries, were characterized prior to deformation using optical microscopy (OM), electron back scattered diffraction (EBSD), and transmission electron microscopy (TEM) to determine axis/angle pair relationships and interface plane. Samples containing these boundaries were then subjected to incipient spall at 2.1 GPa and shock loading at 10 GPa, respectively, in an 80 mm gas gun. Samples were soft recovered and characterized post-mortem via EBSD and TEM. Preliminary results indicate that typical GBs readily form damage during shock loading but that special boundaries, such as twin boundaries, are resistant to failure. Differences in slip and defect transmissibility across these types of boundaries likely play a role in the failure modes. [ABSTRACT FROM AUTHOR]

Polyurea was investigated under Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) loading where spherical projectiles were propelled at 440 to 509 ms-1 through a conical extrusion die with an area reduction of 87%. Momentum of the leading edge imposes a rapid tensile deformation on the extruded jet of material. Polyurea is an elastomer with outstanding high-rate tensile performance of interest in the shock regime. Previous Dyn-Ten-Ext work on semi-crystalline fluoropolymers (PTFE, PCTFE) elucidated irregular deformation and profuse stochastic-based damage and failure mechanisms, but with limited insight into damage inception or progression in those polymers. The polyurea behaved very differently; the polymer first extruded a jet of apparently intact material, which then broke down via void coalescence, followed by fibrillation and tearing of the material. Most of the material in the jet elastically retracted back into the die, and only a few unique fragments were formed. The surface texture of all failed surfaces was found to be tortuous and covered with drawn hair-like filaments, implying a considerable amount of energy was absorbed during damage progression. [ABSTRACT FROM AUTHOR]

Over the past few years, a research programme has been in place to examine the shock response of body centred cubic metals such as tantalum and tungsten. Examination of the development of shear strength behind the shock front has shown common behaviour in that a marked decrease has been noted, both in the pure metals and their simple alloys. This has been ascribed to the low generation of new dislocation line length due to the high Peierls stresses found in these metals. However more recent work in niobium and molybdenum has shown a more constant response in shear strength due to either a much lower Peierls stress (niobium) or the possibility of twin formation (molybdenum). Examination of the rise times in rear surface velocity traces in these materials has also shown a degree of agreement with changes in lateral stress behind the shock front. [ABSTRACT FROM AUTHOR]

To examine the high strain and high strain rate response of structural metals, a dynamic extrusion technique has been developed at Los Alamos National Laboratory. In this study, several structural metals (copper, tantalum, and most recently zirconium) have been accelerated up to velocities of 600 m/s and extruded through a high strength steel die. A novel use of Photonic Doppler Velocimetry (PDV) has been employed to track the time and distance of the evolved deformation through the die. This integrated small-scale experiment is used to study and provide in-situ data, assisting in the modeling and understanding the dynamic response of materials. Time and distance data as well as the influence of crystallography and texture on the dynamic extrusion response of high purity zirconium will be presented. [ABSTRACT FROM AUTHOR]

Both Ti and Zr exhibit phase transitions from the α (hcp) to the ω phase at pressures of a few GPa. In addition, the Hugoniot of Zr shows a second phase transition at 23 GPa. We have developed multi-phase equations of state for these metals based on ultrasonic, static compression, and shock data. The second transition in Zr is consistent with a phase diagram in which the high-temperature and high-pressure bcc phases are a single continuous phase. Time-resolved experiments using plate impact and continuous magnetic loading are compared to simulations to investigate the kinetics of these phase transitions. Strong kinetic effects are observed in the α - ω transition in both metals, with the dynamic phase transition observed at pressures well above the equilibrium phase boundary. Data on Zr samples of varied purity are consistent with a strong reduction of the transformation rate by impurities. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]