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The response of polycrystalline nitinol with solely austenite structure was studied in three series of planar impact tests characterized by loading of the nitinol samples of 0.5–10 mm thickness by 1 mm thick aluminum impactor accelerated up to velocities of about 387, 429, and 567 m/s. In all the tests, the velocities of the free surfaces of the samples were monitored by a laser velocity interferometer. It was found that in all three test series, the amplitude of elastic precursor wave, being initially greater than 4 GPa, rapidly decays with the propagation distance down to ∼2.5 GPa, below which the decay is hindered by atomic clusters of the nanometer size. Based on the part of the velocity histories indicating the shock-induced austenite–martensite transformation, the initial, of about 2.5 × 103 s−1, and the maximum, up to 1 × 105 s−1, rates of the transformation were determined. As well, the impact stress slightly greater than 4 GPa was determined as that required for the onset of the B2 → B19′ transformation under shock loading. The unloading parts of the same velocity histories allowed a rough estimate of the fraction of the shock-transformed martensite and the elucidation of the virtually complete reversibility of the transformation. [ABSTRACT FROM AUTHOR]

The response of plane-parallel 2 mm thick samples of 47.3Ni-52.7Ti alloy was studied in two series of planar impact tests at temperatures between 300 and 473 K and between 473 and 318 K (heating to 473 K followed by cooling). In two additional series, the samples of 0.4–4 mm thickness were tested at 300 and 338 K (after preheating up to 473 K). In all the tests, the samples were loaded by 1 mm thick copper impactors having velocities equal to 314 ± 2 m/s. The velocity of the rear sample surface was continuously monitored by a laser Doppler velocimeter. It was shown that substantial, by an order of magnitude, variation of Hugoniot elastic limit σ H E L and compressive strength Y of the nitinol with temperature are caused by the martensite–austenite transformation and its reversal. The variation of the dynamic tensile (spall) strength σ s p of the nitinol along the heating–cooling path was found similar to that of σ H E L although the difference between σ s p values of austenite and martensite, ∼20%, is much more modest than in the case of σ H E L . The test series performed at constant temperatures with samples of different thicknesses allows one to conclude that the plastic deformation in shocked austenite is presumably realized by dislocation motion and multiplication controlled by phonon viscosity. In the shocked martensite, the plastic deformation mechanism at a stress lower than ∼0.3 GPa is likely a thermally activated combination of deformation twinning and slip of kinking dislocations. [ABSTRACT FROM AUTHOR]

Impact response of <111> oriented germanium single crystals and polycrystalline samples obtained by high-pressure spark plasma sintering of pure germanium powder was studied in two series of planar impact tests performed at 300 and 1143 K with samples of different thicknesses and in a series of tests with 2 mm single crystals preheated up to the temperatures 300–1143 K. In all the tests, the samples were shock-loaded by tungsten impactors having velocity 980 ± 40 m/s, while the velocity of the interface between the germanium sample and the fused silica window was continuously monitored by velocity interferometer. Under compression, the cubic diamond (cd) germanium transforms into its high-pressure (β-Sn or liquid) modification. The stress corresponding to the upper bound of the existence of impact loaded cd germanium was found to depart upward from that obtained in the static experiments. At temperatures greater than 900 K, this departure increases due to the initiation of melting in the shock-loaded material. Part of the velocity histories recorded with either single or polycrystalline samples was characterized by a four-wave (instead of the expected three-wave) structure. This "surplus" wave seems to be caused by a short-term existence of an intermediate (nonequilibrium) germanium phase which, however, does not affect the principal germanium Hugoniot. [ABSTRACT FROM AUTHOR]

The effect of modest, 0.6% and 5.5%, pre-straining on the impact response of 2 mm thick samples of annealed polycrystalline vanadium of commercial purity was studied in a series of planar impact tests. The loading of the samples by 0.5 mm thick copper impactors having velocities varying between 300 and 610 m/s was accompanied by continuous laser Doppler velocimetry of their rear surface. Based on the recorded velocity histories, the dynamic compressive σ Y and tensile (spall) σ s p strengths and the strength σ Y s c of vanadium in the shock-compressed state were determined. Adjacent to the impact surface part of the cross sections of the softly recovered samples, the number of twins N t w per unit area was counted. It was found that the main parameter governing both the strength σ Y of pristine (in the shock sense) material and that in the shock-compressed state, σ Y s c , was the initial dislocation density η 0. Moreover, the dislocation surplus caused by pre-straining was responsible for complete suppressing of twinning in the 0.6% and 5.5% pre-strained samples. In undeformed vanadium, the twinning was partially suppressed by the presence of impurity atoms which, however, did not affect the twinning stress, which was equal to approximately 0.7 GPa. [ABSTRACT FROM AUTHOR]

The evolution of shock compressive pulses and dynamic tensile (spall) strength of pristine and pre-strained (0.6% and 5.4% compression) samples of pure [100]-oriented molybdenum single crystals were studied in a series of planar impact tests accompanied by continuous monitoring of the free surface velocity of the samples by an optic velocimeter. The impact loading of Mo samples of different thicknesses was produced by copper impactors accelerated in the smooth bore gun up to a velocity of about 350 m/s. Analyzing the recorded waveforms showed that pre-straining results in a substantial decrease of the molybdenum Hugoniot elastic limit while the dynamic tensile (spall) strength increases with pre-straining. The spall fracture of all tested (and spalled) samples was found to be brittle and characterized by a weak dependence of spall strength on the tensile strain rate. The obtained results are discussed in the terms of generally accepted theories of elastic precursor decay in ductile and spall fracture in brittle solids. [ABSTRACT FROM AUTHOR]

Impact response of a tungsten heavy alloy (WHA) prepared by liquid phase sintering of tungsten powder (∼80 vol. %) with an Ni–Co–Fe (3.50–1.25–1.0 weight ratio) binder was studied over a 23–1100 °C temperature range in a series of planar impact tests accompanied by continuous monitoring of the velocity of the WHA sample rear surface. The temperature dependence of the proof stress Y 0.1 (T) was found based on the 1D numerical simulations of the performed impact tests using a modified Steinberg–Cohran–Guinan constitutive model, and the temperature dependencies of the density ρ 0 (T) and longitudinal c l (T) and bulk c b (T) speeds of sound were found using rule of mixtures. The bulk speed of sound c b (T) was also used in determination of the temperature dependence of the spall strength σ s p (T) of the alloy based on the experimentally recorded velocity pull-backs Δ u p b . The strong decrease of both Y 0.1 (T) and σ s p (T) with temperature (Y 0.1 decreases almost sixfold between 23 and 1100 °C) allows one to assume that the tensile (spall) fracture of the alloy is controlled by the strength of its matrix. [ABSTRACT FROM AUTHOR]

A series of planar impact tests were performed aiming at measuring the compressibility of the paraffin–poly-methylmethacrylate (PMMA) mixture up to a pressure of 2.5 GPa. It was found that compressive waves in the mixture consist of the initial part, characterized by a virtually linear increase of parameters, followed by the part associated with their relaxation. The dependence of the maximum strain rate of compression of the mixture on the final shock pressure was found to be much weaker than observed in metals and liquids. This inconsistency was explained by the difference of the viscosity mechanisms in these media. Based on an assumption of additivity of volumes of the mixture constituents, the estimates of bulk PMMA compressibility were obtained. [ABSTRACT FROM AUTHOR]

The shock wave response of commercial poly(methyl methacrylate), PMMA, in the vicinity of its glass transition temperature (Tg = 110–120 °C) was studied in two series of impact experiments, the first of which was aimed at the variation of the PMMA spall strength with temperature. The purpose of the second series was the effect of temperature on the PMMA Hugoniot. It was found that the spall strength of PMMA experiences an abrupt twofold drop at ≈120 °C. The PMMA Hugoniot U s = C 0 + s u p was found to be that of a virtually elastic solid with C 0 close to the longitudinal speed of sound, c l , at room temperature and transformed above T g into a viscous material with no shear strength and C 0 close to the bulk speed of sound, c b. [ABSTRACT FROM AUTHOR]

The evolution of elastic-plastic wave in 70Cu-30Zn brass was measured so as to obtain information about its dynamic strength properties at normal and elevated temperatures. The different propagation velocities of plastic shock recorded in as-rolled and annealed brass can be explained in terms of a strong work-hardening. Measuring the decay of an elastic precursor wave revealed a transition from high-rate to low-rate mechanisms of plastic deformation. Unexpectedly, the flow stresses grow with increasing the temperature in both regimes. While at high strain rates this effect can be explained by phonon drag mechanisms, abnormal thermal hardening at a lower strain rate is apparently associated with different levels of short range order in the alloy at normal and elevated temperatures. [ABSTRACT FROM AUTHOR]

The evolution of elastic-plastic shock waves has been studied in pure polycrystalline tungsten and chromium at room and elevated temperatures over propagation distances ranging from 0.05 to 3mm (tungsten) and from 0.1 to 2mm (chromium). The use of fused silica windows in all but one experiment with chromium and in several high temperature experiments with tungsten led to the need for performing shock and optic characterization of these windows over the 300–1200K temperature interval. Experiments with tungsten and chromium samples showed that annealing of the metals transforms the initial ramping elastic wave into a jump-like wave, substantially increasing the Hugoniot elastic limits of the metals. With increased annealing time, the spall strength of the two metals slightly increases. Both at room and at high temperatures, the elastic precursor in the two metals decays in two distinct regimes. At propagation distances smaller than ~1mm (tungsten) or ~0.5mm (chromium), decay is fast, with the dislocation motion and multiplication being controlled by phonon viscous drag. At greater distances, the rate of decay becomes much lower, with control of the plastic deformation being passed to the thermally activated generation and motion of dislocation double-kinks. The stress at which this transition takes place virtually coincides with the Peierls stress τP of the active glide system. Analysis of the annealing effects in both presently and previously studied BCC metals (i.e., Ta, V, Nb, Mo, W, and Cr) and of the dependencies of their normalized Peierls stresses τP(θ) =τP(θ) on the normalized temperature θ= T/Tm allows one to conclude that the non-planar, split into several glide planes, structure of the dislocation core in these metals is mainly responsible for their plastic deformation features. [ABSTRACT FROM AUTHOR]

The evolution of elastic-plastic shock waves has been studied in pure molybdenum and niobium at normal and elevated temperatures over propagation distances ranging from 0.03 to 5mm. The experiments revealed that annealing of the metals substantially increases their Hugoniot elastic limits and, to a lesser degree, their spall strengths. Variations in the resistance of both the metals to fracture in tension with the test temperature can be described as modest. Measuring the decay of the elastic precursor waves with a propagation distance in the two metals has allowed a determining of the relationships between a flow stress τ and an initial plastic strain rate γp. It was found that, at the plastic strain rates greater than 3 ÷ 4 × 104s-1, the temperature sensitivity of the transient values of τ is much lower than that at the strain rates below this range. The τ(γp) data normalized on shear moduli of the metals have been approximated by simple functions that, despite substantial differences between the moduli and yield stresses, were found to be virtually identical for the two metals. [ABSTRACT FROM AUTHOR]

An experimental technique capable of determining the dynamic tensile (spall) strength of metals in the liquid state is described. Relying on this technique, spall data on samples of tin, lead, and zinc pre-heated to 20K above their melting points were obtained. It is found that the spall strength of the metals is low, 40-100MPa, but not zero and is, seemingly, affected by material purity and by the rate of tensile deformation preceding sample spallation. [ABSTRACT FROM AUTHOR]

The response of polycrystalline 99.5% pure iron was studied in a series of planar impact experiments, with samples of different thickness having an initial temperature that ranged between 300 and 1233 K. The free surface velocity histories of the shocked samples were recorded in the course of the experiments. Almost all recorded histories are characterized by a three-wave structure containing an elastic precursor Pel and two plastic, P1 and P2, waves. It was found that at 300, 900, and 1039 K (some 5 K away from iron's Curie point), the decay of the Pel wave with propagation distance is characterized by two different regimes; a fast one that corresponds to plastic strain rates above ~105 s−1 and a slower one at lower strain rates. Since the shear stress at which the change-over takes place is very close to the Peierls stress of iron, we assume that above this stress the decay is governed by the phonon-damped over-barrier dislocation motion, while below it, the thermally activated generation and motion of the dislocation kinks is the governing mechanism. Based on the parameters of the P1 and P2 waves, both the initial and the maximum rates of the α→ε transformation in iron have been estimated. The results indicate that the approach to the Curie point is associated with a substantial, by 2–3 times, increase of the transformation rate. [ABSTRACT FROM AUTHOR]

The evolution of the elastic precursor waves in pure tantalum and vanadium is presented at normal and elevated temperatures over propagation distances that ranged from 0.125 to 3mm. Measurements were performed in order to obtain experimental data about the temperature-rate dependence of the yield stress of the two metals. With increasing propagation distance, the rate of the decay of elastic precursor decreases, as the shear stress in the elastic precursor wave approaches the Peierls stresses. It has been found that the decay, with propagation distance, of the post-spike minimum of the spike-like elastic precursor wave in vanadium is essentially non-monotonous. The experiments also revealed that annealing of tantalum and vanadium increases their Hugoniot elastic limit. The anomalous increase of the high strain rate yield stress with temperature, as observed earlier for some FCC and HCP metals, has not been detected in these measurements. [ABSTRACT FROM AUTHOR]

The dynamic compressive strength (amplitude of the elastic precursor wave) of alumina of 99.8% purity has been studied in a series of planar impact experiments with the 0.25 to 3-mm thick alumina samples, preheated to 1300 K. The dependence of the dynamic tensile (spall) strength of alumina on the ultimate compressive strength was studied with 2-mm thick samples at 300 and 1300 K. The compressive strength of alumina was found to decline by some 30%-35% between 300 and 1300 K, while the absolute values of the spall strength and its dependence on the peak stress were found to stay almost unchanged between 300 and 1300 K. Over the studied temperature interval, the elastic precursor waves in alumina decayed with propagation distance with the decay parameters almost independent of temperature. The availability of decay parameters made it possible to determine the strain rate function [formula] of alumina and to derive some conclusions regarding the onset of inelastic deformation in this ceramic. [ABSTRACT FROM AUTHOR]

The evolution of elastic-plastic shock waves as a function of the propagation distance has been studied in 99.999% purity polycrystalline copper over the 300 to 1353 K temperature range. The free surface velocity histories of shock-loaded samples 0.1 to 2.0 mm in thickness have been recorded using the velocity interferometer. The measured decay of the elastic precursor waves has been converted into relationships between the shear stress at Hugoniot elastic limit and the initial plastic strain rate. Independently of the temperature, the initial densities of mobile dislocations in a range of 2.5×106 cm-2 to 5×108 cm-2 are required to provide observed initial strain rates varied from 2.3×103 s-1 to 2×106 s-1. Above 1100 K, the shape of the elastic precursor wave changes with the appearance of a sharp spike at its front part. This change is treated in terms of nucleation and multiplication of mobile dislocations. An analysis of the rise times of the plastic shock waves has shown that for the same level of shear stress, the plastic strain rates after a 2% compressive strain increase with respect to those just behind the elastic precursor front by a factor of 300 at 300 K and by a factor of 30 at 1353 K. [ABSTRACT FROM AUTHOR]

Solid solution treated aluminum alloy 6061 was tested in a series of planar impact experiments in which the initial sample temperature was varied between 296 and 902 K with special attention paid to the temperatures of the solid-liquid two-phase region. In the experiments, the free surface velocity histories were recorded using velocity interferometer system for any reflector (VISAR). It has been found that both the dynamic yield and the dynamic tensile strengths of the alloy decline monotonously through the two-phase domain and vanish completely as the initial sample temperature approaches 902 K and as the content of the liquid phase in the alloy approaches 20%. Through the same temperature interval the plastic wave rise-time and the time interval between elastic and plastic waves behave non-monotonously and display a sharp maximum at 876 K, some 20 K above the solidus temperature of the alloy. We presently assume that the non-monotonous behavior is caused by a compressive-induced solidification of a thin liquid layer between the grains of the solid phase. [ABSTRACT FROM AUTHOR]

The evolution of elastic-plastic shock waves in 99.9% purity silver samples of 0.127 to 2.0 mm thickness has been studied in a series of VISAR-instrumented planar impact experiments with initial sample temperature varied from 296 to 1233 K. The decay of elastic precursor wave at 933, 1173, and 1233 K temperatures is approximately inversely proportional to the square root of the propagation distance. The latter corresponds to the cubic dependence of initial plastic strain rate, ranged from 104 s-1 to 106 s-1, on the shear stress. At fixed strain rates, the flow stress grows linearly with the temperature but the dependence becomes stronger near the silver melting point, 1234 K. An analysis of the rise times of the plastic shock waves shows that for the same level of shear stress the plastic strain rate at the shock front is significantly higher than that at the top of the elastic precursor wave. [ABSTRACT FROM AUTHOR]

The impact response of potassium bromide single crystals with <100> and <110> orientation was studied in a series of planar impact experiments under continuous monitoring of the sample rear surface velocity. The samples whose initial temperature varied between 166 and 880 K were loaded with aluminum, copper or tungsten impactors having velocities ranging from 140 to 690 m/s. The velocity histories recorded in the experiments with maximum compressive stress, lower than about 2 GPa, include the elastic precursor wave followed by the plastic wave, while the waveforms, recorded in the experiments with stronger loading, are characterized by a three-wave structure caused by the shock-induced B1→B2 phase transformation in KBr. On the basis of the recorded velocity histories, the temperature dependences of the shear stress τI acting at the primary glide systems of KBr, of the transformation pressure ptr, and of the characteristic transformation time ttr were determined. [ABSTRACT FROM AUTHOR]

The shock response of polycrystalline iron of 99.995% purity was studied in a series of planar impact experiments using laser interferometer monitoring of the free surface of preheated/precooled samples. On the basis of the recorded waveforms, the dynamic yield and tensile (spall) strengths of iron were determined over the 143–1275 K temperature range. Part of the recorded waveforms had a three-wave (PEL, P1, and P2 waves) structure reflecting a stress-induced phase transformation in the impacted samples. By estimating the stress σtr and the temperature Ttr on the top of the P1 wave, it became possible to determine the upper borders of stability with respect to the shock loading for both the α- and the γ-iron phases. The analysis of the P2 waves of the recorded waveforms showed that irrespective of its initial, α or γ, state the high-pressure phase of iron is the [variant_greek_epsilon]-phase. The same analysis yields the width, 5.18 GPa, of the mixed phase region, above which the three-wave structures are substituted by two-wave ones (PEL and P2) with solely the [variant_greek_epsilon]-phase behind the front of the overdriven P2 wave. [ABSTRACT FROM AUTHOR]

Samples of polycrystalline nickel of high (99.9945%) and commercial (99.5%) purity were studied in a series of planar impact experiments with initial, preshot, sample temperature ranging from 150 to 1150 K. The free surface velocity of the impacted sample was monitored by velocity interferometer. On the base of the recorded waveforms, the temperature dependences of the dynamic tensile (spall) and the dynamic yield strength of the two kinds of nickel were determined. Both strength parameters of the two types of nickel decline gradually with temperature. The trend, however, is broken by strong anomalies of the yield strength near the Curie point of nickel, 630 K. While the yield strength of the high purity nickel shows a twofold increase over a narrow interval in close vicinity of the magnetic transformation, the strength of commercially pure material experiences, in the same temperature interval, a threefold drop approaching the strength level characteristic for high-purity nickel. The possible mechanisms of the observed phenomena are discussed. [ABSTRACT FROM AUTHOR]

The yield and spall strengths of polycrystalline high purity (99.99%) and commercial (grade 2) titanium were studied in a series of planar impact experiments with the initial sample temperature ranging from 20 to 1000 °C. In a separate series of impact experiments, the temperature dependence of the longitudinal speed of sound in pure Ti was measured making use of the reverberation of the stress pulse generated in the 3 mm Ti sample by an aluminum impactor. In all the experiments, the velocity of the sample free surface was continuously monitored by VISAR. The stress-strain diagrams of the two kinds of samples were derived from the free surface velocity profiles using the simple wave approximation. Analysis of these diagrams made allowed revealing the sequence of the dislocation-based processes governing the plastic flow in the two materials, from the ambient to some 100 °C above the hcp-bcc transformation temperature. On the basis of the spall data obtained from impact experiments with pure titanium, the limits of existence of nonequilibrium titanium phases were outlined. [ABSTRACT FROM AUTHOR]