M. A. Zocher

Twin structures in copper after shock and Shockless high-rate loading

The structure of copper formed after high-rate loading up to pressures of 20–80 GPa with a strain rate of 105–109 sec−1 is considered. In situations with pressures above 20 GPa and strain rates above 106 sec−1, the deformation twins are grouped into packets, which are seen in an optical microscope as parallel bands of localized strains inside individual grains. The number of bands in the structure increases with increasing grain size and strain rate, with decreasing sample temperature, and with increasing period of sample loading. The characteristic time of formation of twin bands in copper is estimated as 0.3 µsec.

Measurement of the sound velocities behind the shock wave front in tin

Results obtained by two methods for the measurement of the sound velocity in tim samples (initial density of 7.28 g/cm3 and impurities less than 0.085%) are presented. In the range of pressures from 30 to 150 GPa, the sound velocity is determined by the overtake method with the use of indicator liquids. The luminescence of the indicator liquids is detected by photodiode-based optical gauges. At shock compression pressures of 5–18 GPa, the sound velocity in tin is measured by the counter release method with the use of manganin-based gauges. The experimental data are compared with numerical predictions and results of other authors. The boundaries of the tin melting region on the shock adiabat are found.

Twinning and Dynamic Strength of Copper During High-Rate Strain

Results are presented of a study of the conditions under which microstructural changes involving the formation of complex bi‐periodic twin structures occurs in copper during shock wave and high strain rate (e>107 s−1) shock‐less loading. We have observed that the formation of these bi‐periodic twin structures results in an initial loss of shear strength that is significant over a time period of about 0.2 to 0.4 μs.

Study of cerium phase transitions in shock wave experiments

Cerium has a complex phase diagram that is explained by the presence of structural phase transitions. Experiments to measure the sound velocities in cerium by two methods were carried out to determine the onset of cerium melting on the Hugoniot. In the pressure range 4–37 GPa, the sound velocity in cerium samples was measured by the counter release method using manganin-based piezoresistive gauges. In the pressure range 35–140 GPa, the sound velocity in cerium was measured by the overtaking release method using carbogal and tetrachloromethane indicator liquids. The samples were loaded with plane shock wave generators using powerful explosive charges. The onset of cerium melting on the Hugoniot at a pressure of about 13 GPa has been ascertained from the measured elastic longitudinal and bulk sound velocities.

TEMPORAL SOFTENING AND ITS EFFECT UPON STRENGTH

Temporal softening that is associated with the passage of a shock wave is investigated along with its effect upon material strength and issues of macroscopic scaling. Specimens are loaded in such a way that a moderate tensile pulse is applied during the temporal window of softening. It is found that strength is reduced as a consequence of temporal softening. It appears that temporal softening contributes to a reversal of typically observed macroscopic scaling effects.

MEASUREMENT OF SOUND VELOCITIES AND SHEAR STRENGTH OF CERIUM UNDER SHOCK COMPRESSION

Sound velocity in shock‐compressed cerium was measured over the pressure range of 35–140 GPa using the rarefaction overtake technique. Indicator liquids carbogal and tetrachloromethane were used. The samples were loaded with planar shock wave generators using powerful high explosives (HE). Luminescence of the liquid indicators was recorded using photodiodes. For the pressure range of 13–35 GPa, sound velocity was measured in cerium samples using the counter release method with manganin‐based piezoresistive gauges. From the measured longitudinal and bulk sound velocities, Poissons ratio and shear strength of cerium were determined. The melting boundary on the shock Hugoniot was estimated. Experimental data is compared with calculation results.

Specific features of the damage nucleation stage under severe loading of copper

The nucleation and evolution of damage in annealed coarsely crystalline M1-type copper subjected to fast loading to a pressure P ∼ 32 GPa, followed by the action of tensile stresses σp with an intensity of ≈−2.0 GPa for a time t ≈ 0.3–1.5 μs, have been investigated numerically and experimentally. It has been shown that, at a specific combination of amplitude-time characteristics of the tensile stress pulse, damage localization in some cases at t < 1 μs has been observed in zones (∼10–14 mm in size) alternating with “dead” zones (∼3–5 mm in size) containing no visible damages. Pores are connected by “yield streamlets.” The existing multistage models of fracture kinetics have neither explained nor predicted the formation of a “band” damage structure or the presence of “yield streamlets” in specimens.

INFLUENCE OF DYNAMIC PROPERTIES ON PERTURBATION GROWTH IN TANTALUM

The perturbation method is employed in an experimental‐numerical study of the behavior of Ta subjected to both shock and shockless (approximately quasi‐isentropic) loading. The loading produces large deformation plastic flow, pressures on the order of 10–80 GPa, and strain rates on the order of 105–109 s−1. Metallographic analysis is used to assess microstructural changes. Perturbation growth is reasonably well predicted with the use of a finite element continuum code and the Steinberg‐Glushak model. Perturbation growth in Ta is compared to that of Al and Cu. Observations are made concerning fundamental differences in the behavior of fcc versus bcc materials subjected to the studied load environment. Ramifications of these differences on model development is discussed.

Measurement of sound velocities in shock-compressed tin under pressures up to 150~GPa

Sound velocity in shock‐compressed tin was measured over the pressure range of 31–138 GPa by the overtake method with using indicator liquids. Photodiode‐based optical gauges were used to record luminescence of the liquid indicators. For shock compressions of 5–18 GPa, the sound velocity in tin was measured with manganin gauges by determining the oncoming release wave in the tin. The experimental data were compared to calculated results and data obtained by other authors. According to data obtained in this work, tin melts on the hugoniot between ∼63–90 GPa.

TEMPORAL SOFTENING AND ITS EFFECT UPON SPALL STRENGTH

Metals subjected to compressive shock of sufficient intensity often undergo microstructural changes involving deformation localization and temporal softening. Temporal softening has a significant effect on the formation of damage (the material is much more susceptible to damage during the period of temporal softening). The present on polycrystalline M1 copper clearly exhibits this phenomenon.