G. I. Kanel

Dynamic yield and tensile strength of aluminum single crystals at temperatures up to the melting point

This article presents experimental results of the dynamic yield strength and dynamic tensile strength (“spall strength”) of aluminum single crystals at shock-wave loading as a function of temperature. The load duration was ∼40 and ∼200 ns. The temperature varied from 20 to 650 °C which is only by 10 °C below the melting temperature. A linear growth of the dynamic yield strength by more than a factor of 4 was observed within this temperature range. This is attributed to the phonon drag effect on the dislocation motion. High dynamic tensile strength was maintained over the whole temperature range, including the conditions at which melting should start in a material under tension. This could be an indication of the existence of superheated states in solid crystals.

Spall fracture properties of aluminum and magnesium at high temperatures

Measurements of the dynamic tensile strength of aluminum and magnesium have been carried out by investigations of the spall phenomena over a wide range of temperatures, shock‐wave intensities, and load durations. Free‐surface velocity profiles were recorded with VISAR and used to provide the spall strength measurements. The initial temperature of samples was varied from room temperature to near the melting point. The peak compressive pressure in the shock waves was varied from 5 to 50 GPa for aluminum and from 2 to 10 GPa for magnesium. The load duration was varied by more than one order of magnitude. The free‐surface velocity measurements showed a precipitous drop in the spall strength of preheated samples as temperatures approached the melting point. No significant influence of the peak pressure on the spall strength was observed. The strain‐rate dependencies of the spall strength could be represented as power functions with a power index of 0.060 for aluminum and 0.072 for magnesium. Unexpectedly large...

Behavior of aluminum near an ultimate theoretical strength in experiments with femtosecond laser pulses

The dynamics of the motion of the free surface of micron and submicron films under the action of a compression pulse excited in the process of femtosecond laser heating of the surface layer of a target has been investigated by femtosecond interferometric microscopy. The relation between the velocity of the shock wave and the particle velocity behind its front indicates the shock compression to 9–13 GPa is elastic in this duration range. This is also confirmed by the small (≤1 ps) time of an increase in the parameters in the shock wave. Shear stresses reached in this process are close to their estimated ultimate values for aluminum. The spall strength determined at a strain rate of 109 s−1 and a spall thickness of 250–300 nm is larger than half the ultimate strength of aluminum.

Spall strength of molybdenum single crystals

Spall strength measurements for commerical grade molybdenum and molybdenum single crystals were made in a wide range of load durations (∼10−9 s – 10−6 s) and intensities (∼5 – 100 GPa). Resistance to fracture of pure single crystals was found to exceed two times the spall strength of polycrystalline molybdenum and to increase with shorter load duration. The value of the shock wave amplitude does not influence the spall strength of single crystals. The largest spall strength obtained under nanosecond load duration amounts to 30% of the ultimate theoretical strength.

Transformation of shock compression pulses in glass due to the failure wave phenomena

A method of observing the compressive failure waves in glass is presented. Its advantages are good reproducibility of the recorded data and capabilities of measuring, the kinematic parameters of the failure wave and determining by one shot the failure threshold. The experiments presented herein confirm that the network of growing cracks in shock-compressed glass may indeed be considered as a failure wave with a small stress increment. Transformation was observed of the elastic compression wave followed by the failure wave in a thick glass plate into a typical two-wave configuration in a pad of thin glass plates.

Simulation of spall fracture of aluminum and magnesium over a wide range of load duration and temperature

Abstract Measurements of the dynamic strength of aluminum and magnesium have been carried out through investigations of spall phenomena. In experiments, free-surface velocity profiles were recorded with a VISAR. The initial temperature of samples was varied from room temperature to that close to the melting point. The peak pressure in shock waves was varied from 5 to 50 GPa for aluminum and from 2 to 10 GPa for magnesium. The load duration was varied by more than an order of magnitude. Measurements showed precipitous drop in the spall strength of preheated samples as temperatures approached the melting point. No significant influence of the peak pressure on the spall strength was observed until a residual temperature after unloading of shock-compressed matter approached the melting. The strain-rate dependencies of the spall strength can be represented as power functions with an exponent of 0.059 for aluminum and 0.072 for magnesium. An empirical constitutive relationship has been established to describe the fracture rate as a function of the tensile stress, ultimate tensile stress that has activated a damage in the point, the damage value, and the temperature. The constitutive relationshiop was constructed on a base of analysis of the wave dynamics at spalling. Computer simulations show reasonably good workability of the model over a wide range of the shock load parameters and the temperature of matter.

Resistance to dynamic deformation and fracture of tantalum with different grain and defect structures

This paper presents the results of measurements of the strength properties of technically pure tantalum under shock wave loading. It has been found that a decrease in the grain size under severe plastic deformation leads to an increase in the hardness of the material by approximately 25%, but the experimentally measured values of the dynamic yield stress for the fine-grained material prove to be less than those of the initial coarse-grained specimens. This effect has been explained by a higher rate of stress relaxation in the fine-grained material. The hardening of tantalum under shock wave loading at a pressure in the range 40–100 GPa leads to a further increase in the rate of stress relaxation, a decrease in the dynamic yield stress, and the disappearance of the difference between its values for the coarse-grained and fine-grained materials. The spall strength of tantalum increases by approximately 5% with a decrease in the grain size and remains unchanged after the shock wave loading. The maximum fracture stresses are observed in tantalum single crystals.

Response of seven crystallographic orientations of sapphire crystals to shock stresses of 16–86 GPa

Shock wave profiles of sapphire (single-crystal Al2O3) with seven crystallographic orientations (c, d, r, n, s, g, and m-cut) were measured with time-resolved VISAR (velocity interferometer for a surface of any reflector) interferometry at shock stresses in the range 16–86 GPa. Shock propagation was in the direction normal to the surface of each cut. The angle between the c-axis of the hexagonal representation of the sapphire crystal structure and the direction of shock propagation varied from 0 for c-cut up to 90° for m-cut in the basal plane. Based on published shock-induced transparencies for three directions of shock propagation, shock-induced optical transparency correlates with the smoothness of the mechanical shock wave profile. The ultimate goal was to find the direction of shock propagation for which shock-compressed sapphire is most transparent as a window material. In the experiments particle velocity histories were recorded at the interface between a sapphire crystal and a LiF window. In most ...

Shock-wave compression and tension of solids at elevated temperatures: superheated crystal states, pre-melting, and anomalous growth of the yield strength

Recent studies of the response of metals and alloys to shock-wave loading at elevated temperatures are summarized. Shock-wave tests have been carried out for metal single crystals, polycrystalline metals of different purity, and for alloys. High resistance to sub-microsecond tensile fracture of single crystals is maintained when melting should start. This is treated as evidence of a superheated solid state reached under dynamic tension. In polycrystalline metals, melting starts earlier at grain boundaries; this is known as the pre-melting phenomenon. As a result, their tensile strength drops to zero on approaching the melting curve. Anomalous growth of the dynamic yield stress was observed for low-strength metals, whereas the yield stress of high-strength alloys decreases with temperature. The different behaviour of metals and alloys is treated in terms of the relationship between the phonon drag of the motion of dislocations and the drag forces created by obstacles.

Thermal “softening” and “hardening” of titanium and its alloy at high strain rates of shock-wave deforming

The effect of temperature on the dynamic yield strength and ultimate tensile strength of high-purity and commercial-purity titanium and an α+β alloy Ti-6Al-2Sn-2Zr-2Cr-2Mo-Si upon submicrosecond-scale shock-wave loading was studied. An anomalous increase in the dynamic yield strength with temperature was detected in high-purity titanium, whereas the behavior of commercial-purity titanium and the titanium alloy was similar to that under regular conditions. It was found that the dynamic ultimate tensile strength is less sensitive to the composition and structure of the alloy and to the test temperature than is the yield strength. Our experiments corroborate the occurrence of polymorphic transformation during shock compression of high-purity titanium, but the transformation pressure and its temperature dependence are inconsistent with the data available in the literature.