A. V. Utkin

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...

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.

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.

Tensile strength of five metals and alloys in the nanosecond load duration range at normal and elevated temperatures

The paper presents results of measurements of the resistance to tensile fracture at spallation in nickel, cobalt, stainless steel, AlMg6% alloy, and Inconel IN 738 LC alloy. In the experiments carried out with a high-power ion beam as a shock-wave generator the load pulse duration was in the range of 50 ns. The measurements were performed at peak stresses varying by a factor up to 2 which had no influence on the dynamic tensile strength of the materials tested. For cobalt and Inconel measurements were also done at elevated temperatures. Whereas the response of cobalt was practically insensitive to temperature, IN 738 LC demonstrated a transition from viscous to relatively brittle fracture accompanied by a significant increase of the spall strength at higher temperatures.

Resistance of zinc crystals to shock deformation and fracture at elevated temperatures

Velocity profiles of the free surface of shock-loaded zinc crystals are measured in two different orientations. The test temperature is varied from room temperature to 410 °C. The results of the measurements show that the high-velocity deformation and fracture are athermal processes and that the fracture stresses are influenced by the preceding plastic deformation.

Spallations near the ultimate strength of solids

Spall strength measurements of aluminum, single crystals of molybdenum, niobium and aluminum oxide were performed over a wide range load durations. A very large dynamic tensile strength (20 GPa) has been registered for sapphire. Dynamic strength of metals at nanoseconds load durations reaches 35% of ultimate theoretical strength. Extrapolation of obtained results shows that ultimate strength can be reached at ∼10−10 s load duration for single crystals and ∼10−12 s for polycrystalline metals.

Response of high-purity titanium to high-pressure impulsive loading

Abstract Measurements of free surface velocity profiles of high-purity titanium samples under shock-wave loading were performed to study the dynamic strength and phase transition parameters. The peak pressure of the initial compression waves was within the range of 4 to 40 GPa, and the load duration was vaned between 10−8 and 10−6 s. An anomalous structure of shock waves was observed at pressures of ∼ 2.0 to 5.0 GPa due to the α-ω phase transition. The dynamic strength of pure titanium is lower than that of titanium alloys but exceeds the spall strength of commercial grade titanium.

Influence of the load conditions on the failure wave in glasses

Abstract Previously, a series of plate impact experiments have provided evidence for the appearance of failure waves in glasses under uniaxial compression. Presented herein is a continuation of research, performed on K9 crown glass and ZF1 dense flint glass at varied shock stresses with a goal to uncover the failure wave source and to make clear whether or not the failure wave can be formed at stresses above the Hugoniot elastic limit (HEL). It has been found that, at peak stresses below the HEL, the failure wave can be initiated on both the impact surface and the internal surface inside the sample, indicating that the glass surface is a direct source for the failure wave nucleation. However, when the peak stress exceeds the HEL, brittle glasses become ductile as a result of irreversible densification, and the Failure wave disappears. The high spall strength revealed in the stress range above the HEL indicates that ductility is also preserved under the subsequent tension.

Diagnostics of fast processes by charged particle beams at TWAC-ITEP accelerator-accumulator facility

A new setup for the experimental investigation of rapid dynamic processes using proton radiography techniques has been created at the TWAC-ITEP terawatt accelerator-accumulator facility. A set of equipment for conducting shock-wave experiments has been designed, constructed, and tested, and an instrumentation-software complex has been developed for the automation of experiments. The first series of experiments with dynamic targets representing high explosives have been carried out, in which the density distribution in detonation waves initiated in these explosives has been measured.

Hydrodynamic proton beam-target interaction experiments using an improved line-imaging velocimeter

The hydrodynamic response of 10- to 100-μm-thick target foils exposed to a high-power proton beam is investigated by laser Doppler velocimetry of the rear surface. For the first time, the ablative acceleration and the ablation pressure have been measured spatially resolved by use of an improved line imaging velocimeter. The instrument was operated in both the VISAR and the ORVIS mode, and takes advantage of the spatial localization of the interference fringes in the mirror planes for an intermediate imaging. This allows us to obtain optimum contrast, high spatial resolution of ⩽10 μm, and to vary the magnification in a wide range.