E. Zaretsky

Plastic flow in shock-loaded silver at strain rates from 104 s−1 to 107 s−1 and temperatures from 296 K to 1233 K

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.

Tantalum and vanadium response to shock-wave loading at normal and elevated temperatures. Non-monotonous decay of the elastic wave in vanadium

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 3 mm. 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.

Shock response of iron between 143 and 1275 K

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 e-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-w...

Impact response of titanium from the ambient temperature to 1000 °C

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 ...

Dislocation multiplication behind the shock front

A new model of fast operated dislocation source based on multiplication and motion of partial dislocations bounding the stacking fault is suggested. Stress‐activated stretching of lateral branches of the partial dislocation bowed‐out segment results in collapse of these branches with subsequent restoration of the ‘‘initial’’ dislocation half‐loop and generation of a ‘‘fresh’’ partial dislocation loop, both capable to produce the next multiplication act. The multiplication results in the exponential increase of both dislocations and stacking faults concentration and is accompanied with the plastic deformation having a strain rate de/dt∼2t/ΔT. The characteristic time of the process, ΔT, ranges from 10−9 to 10−10 s. The model explains the variations of x‐ray diffraction pattern for the material undergoing shock compression, the shock‐induced formation of twins, and shear bands and pre‐fracture voids nucleation in the rarefaction wave.

Time-Resolved Measurements of Impulse Generation in Pulsed Laser-Ablative Propulsion

Impulse generation mechanisms in pulsed laser ablation were experimentally studied using the velocity interferometer system for any reflector and framing Schlieren visualization. The impulse was estimated from the rear surface velocity at the center of the laser irradiated spot. The fluence was from 13 to 24 J/cm 2 . In most cases, the propulsive force generated even after the primary laser power peak significantly contributed to the total impulse. With the combination of transversely excited atmospheric CO 2 laser and aluminum target, only air breakdown was induced on the target surface without ablation and the impulse level was low. With decreasing ambient pressure P 0 , the impulse also decreased, and eventually vanished. With the combination of Nd:YAG laser and aluminum target, the ablation jet contributed to impulse generation and the impulse did not vanish even at vanishing P 0 . When a transversely excited atmospheric CO 2 laser pulse was directed onto the polyacetal target, the impulse increased by a factor of 10 in comparison with the aluminum target, yielding a momentum coupling coefficient exceeding 400 μN ·s/J. When P 0 was at atmospheric pressure, the laser plasma shielded the target surface against the proceeding laser power transmission and the impulse saturated at a lower value than at P 0 = 10 -2 Pa.

Yield stress, polymorphic transformation, and spall fracture of shock-loaded iron in various structural states and at various temperatures

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 irons 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 ...

Impact response of cobalt over the 300–1400 K temperature range

The yield and tensile (spall) strengths and the dynamic viscosity of 99.95% pure cobalt were studied in a series of interferometer-instrumented planar impact experiments over the 300 to 1423 K temperature range. The spall strength of cobalt declines monotonously from 3.2 GPa at 300 K to 0.8 GPa at 1400 K. The dynamic yield strength grows linearly with temperature in both the hcp and fcc cobalt phases. The hcp to fcc transformation at 690 K is accompanied by an abrupt twofold drop of the yield strength. The similarity between the temperature dependence of the cobalt’s dynamic viscosity and of the yield strength suggests that both properties are controlled by the interaction of the dislocations with the oncoming phonon flow (phonon drag).

Evidence of ductile response of alumina ceramic under shock wave compression

An experimental technique for controlled variation of the transversal stress in specimens subjected to planar impact has been developed to study the inelastic response mode of materials under one-dimensional shock compression. That technique was employed in order to test alumina ceramic at zero and 0.3 GPa confining pressure. The results unambiguously exhibit the ductile response of the ceramic under conditions of one-dimensional shock compression.

Multipeak pulse x-ray diffraction study of shocked single crystals

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