Dynamic Spall Strength Research Articles

Dynamic response ofCu46Zr54metallic glass to high-strain-rate shock loading: Plasticity, spall, and atomic-level structures

We investigate dynamic response of ${\text{Cu}}_{46}{\text{Zr}}_{54}$ metallic glass under adiabatic planar shock wave loading (one-dimensional strain) with molecular dynamics simulations, including Hugoniot (shock) states, shock-induced plasticity, and spallation. The Hugoniot states are obtained up to 60 GPa along with the von Mises shear flow strengths, and the dynamic spall strengths, at different strain rates and temperatures. The spall strengths likely represent the limiting values achievable in experiments such as laser ablation. For the steady shock states, a clear elastic-plastic transition is identified (e.g., in the shock velocity-particle velocity curve), and the shear strength shows strain softening. However, the elastic-plastic transition across the shock front displays transient stress overshoot (hardening) above the Hugoniot elastic limit followed by a relatively sluggish relaxation to the steady shock state, and the plastic shock front steepens with increasing shock strength. The local von Mises shear strain analysis is used to characterize local deformation, and the Voronoi tessellation analysis, the corresponding local structures at various stages of shock, release, tension and spallation. The plasticity in this glass, manifested as localized shear transformation zones, is of local structure rather than thermal origin, and void nucleation occurs preferentially at the highly shear-deformed regions. The Voronoi and shear strain analyses show that the atoms with different local structures are of different shear resistances that lead to shear localization (e.g., the atoms indexed with $⟨0,0,12,0⟩$ are most shear-resistant, and those with $⟨0,2,8,1⟩$ are highly prone to shear flow). The dynamic changes in local structures are consistent with the observed deformation dynamics.

Dynamic shear and spall strengths of preliminarily quasi-statically extruded fine-grained uranium and U-0.3% Mo alloy

It is shown that substantial changes in the average grain size (by two orders of magnitude) and a twofold increase in the quasi-static yield stress and strength for uranium and 1.3-fold increase for the U-0.3% Mo alloy did not lead to a change in their shear strength upon a shock-wave loading. There is no correlation between the change in the shear and spall strength and the decrease in the average grain size. A tendency toward a regular increase (other conditions, i.e., amplitudes and the durations of the loading pulse, being equal) in the spall strength of materials in the row “extruded U, extruded U-0.3% Mo alloy, cast U-1.5% Mo alloy” was noted. The increase in the spall strength is connected with alloying and the real content of molybdenum in the alloy rather than with the effect of extrusion.

Dynamic shear and spall strengths of quasi-statically pre-extruded fine-grain uranium and U-0.3% mo alloy

Dynamic shear and spall strengths of quasi-statically pre-extruded fine-grain uranium and U-0.3% mo alloy

Characterization of Incipient Spall Damage in Shocked Copper Multicrystals

Correlations between spall damage and local microstructure were investigated in multicrystalline copper samples via impact tests conducted with laser-driven plates at low pressures (2—6 GPa). The copper samples had a large grain size as compared to the thickness, which was either 200 or 1000 μm, to isolate the effects of microstructure on the local response. Velocity interferometry was used to measure the bulk response of the free-surface velocity of the samples to monitor traditional spall tensile failure and to examine heterogeneities on the shock response due to microstructure variability from sample to sample. The shock pressure, dynamic yield strength and spall strength were determined from the measured velocity history via standard hydrodynamic approximations, while the effect of strength was explored via 1D hydrocode calculations. Electron Backscattering Diffraction, both in-plane and through-thickness, was used to relate crystallography to the presence of porosity around microstructural features such as grain boundaries and triple points. It was found that the dynamic yield strength measured from velocity histories in different samples correlated well with the crystallographic dependence reported for the dynamic yield strength in single crystals. Transgranular damage dominated in thin specimens with 230 μm grain size, where porosity appeared close to, but not exactly at, grain boundaries. However, a transition to dominant intergranular damage was observed as the grain size was reduced to 150 μm. Thick specimens (450 μm grain size) showed both modes, with intergranular damage found mostly where grains were smaller than average and the sites for preferred damage nucleation in these samples included grain boundaries and triple points. In particular, twin boundaries, especially tips of terminated twins, showed a large mismatch in surface displacements on the diagnostic surface as compared to the surrounding grains as well as a tendency for damage localization on the through-thickness sections.

Temperature dependence of dynamic yield strength and spall strength for LY12 aluminum alloy under shock loading

A velocity interferometer system for use with an arbitrary reflector was used to measure the free surface velocity history of LY12 aluminum alloy in a wide range of pre-heating from the room temperature approaching to the melting point, and the dependences of dynamic yield strength and spall strength on the initial sample temperature have been investigated. Experimental results indicate that the dynamic yield strength decreases rapidly with the heating temperature, and only 15% of the initial strength at room temperature is retained when heated up to 847K (86K lower than the melting temperature). The measured yield strength has been compared with the Zerilli-Armstrong (ZA) model and Steinberg-Cochran-Guinan (SCG) model calculation, which points out that the ZA model could describe the dependence of the yield strength on temperature well and the SCG model has overestimated the dynamic yield strength at high temperature significantly. Decrease of the spall strength with the increase of heating temperature for LY12 aluminum alloy was observed also, and nearly 80% loss of spall strength was determined in the present experimental temperature range of 296—847 K. An empirical formula has been proposed to normalize the dependence of spall strength on the heating temperature, and it gives a fairly good fitting for a number of aluminum based materials, including pure aluminum, aluminum alloy and single crystal aluminum, measured in this work and by others in the literature.

Does the pressure‐induced a ‐ e phase transition occur for all low–alloy steels?

When designing many products, especially vehicles, it is important to take into consideration impact properties. This paper presents fundamental dynamic data for two examples of bainitic steels. Hugoniots and shear–strength variation with longitudinal stress is reported. These were similar for both materials and in agreement with values for mild steel. The ferrite in the lower–temperature bainite was found to undergo a shock–induced phase transition at 13 GPa, whereas no phase transition was observed in the upper bainite. Further research is being conducted to account for this difference by studying the microstructure and the effect of impact on it. The lower–temperature bainite was found to behave in a brittle fashion, fragmenting extremely, whereas upper bainitic samples were usually recovered intact indicating ductile behaviour. The dynamic tensile or spall strength was also measured in these materials over a range of impact conditions. For the upper bainite, the spall strength dropped slightly with increasing longitudinal stress. However, for the lower–temperature bainite, there was a significant drop in spall strength above the longitudinal stress at which the phase transition occurs. Microstructural studies were also undertaken.

Strength of Some Grades of Steel and Armco Iron under Shock Compression and Rarefaction at Pressures of 2 – 200 GPa

This paper reports results from a comparative experimental study of the dynamic strength of St. 20 and 09G2S steels (which are of interest as structural materials for the load‐bearing casings of explosion‐proof chambers) under shock compression and tension (spalling) at shock‐front pressures of 1–5 GPa and strain rates of 103 – 104 sec-1. A comparative analysis is performed of the obtained and available data on the dynamic yield point and spall strength of St. 3, St. 20, 09G2S, 12Kh18N10T, EI712, 30KhGSA, 36NKhTYu, KhVG, and 35Kh3NM steels and armco iron, whose strengths and ductilities under static loading conditions differ by a factor of up to five. Experiments are described in which spheres of St. 3, St. 20, 12Kh18N10T, and 30KhGSA steels with markedly different strengths and ductilities are loaded by a convergent quasispherical shock wave with a pressure at the center of 200 GPa and a strain rate of 105sec-1.

Impact response of titanium alloys at elevated temperatures

The paper presents results of investigations of temperature effect on dynamic yield stress and spall strength of high-purity titanium, commercial grade titanium, and α+β alloy Ti-6Al-2Sn-2Zr-2Cr-2Mo-Si at submicrosecond durations of shock-wave loading. The high-purity titanium exhibited anomalous growth of the yield stress whereas the behaviors of commercial titanium and titanium alloy are similar to that under normal conditions. In comparison with the yield stress the spall strength of the studied materials is much less sensitive to their composition and the test temperature The measurements confirmed the polymorphous transformation in high-purity titanium, although both the transformation pressure and it temperature dependence differ from known data.

Hydrogen effects on the spall strength and fracture characteristics of amorphous Fe-Si-B alloy at very high strain rates

A novel approach is suggested, using laser-induced shock wave measurements to estimate the effects of cathodic hydrogen charging on the mechanical properties and fracture characteristics of materials. This approach is applied to (1) determine the dominant mechanism of hydrogen embrittlement (HE) in an amorphous Fe80B11Si9 alloy; and (2) estimate the effects of the high pressures involved in cathodic charging. The dynamic spall strength of an amorphous Fe80B11Si9 alloy shocked before and after hydrogenation by a high-power laser to very high pressures (tens of giga Pascals) is measured. The dynamic spall strength of crystalline iron is measured as well for comparison. An optically recording velocity interferometer system (ORVIS) is used to measure the profile of the free surface velocity in time. The spall strength and the strain rate are calculated from the measurement of the free surface velocity as a function of time. Fracture characteristics are studied by scanning electron microscopy (SEM). The main conclusions are (1) the most reasonable mechanism of HE in the amorphous Fe-Si-B alloy is the high-pressure bubble formation; (2) the high pressures involved in cathodic hydrogen charging or laser-induced shock waves measurements may have similar effects on fracture characteristics; and (3) at very high strain rates, the spall strength is determined mainly by the interatomic bonds.

Spallation model for the high strain rates range

Measurements of the dynamic spall strength in aluminum and copper shocked by a high power laser to pressures of hundreds of kbars show a rapid increase in the spall strength with the strain rate at values of about 107 s−1. We suggest that this behavior is a result of a change in the spall mechanism. At low strain rates the spall is caused by the motion and coalescence of material’s initial flaws. At high strain rates there is not enough time for the flaws to move and the spall is produced by the formation and coalescence of additional cavities where the interatomic forces become dominant. Material under tensile stress is in a metastable condition and cavities of a critical radius are formed in it due to thermal fluctuations. These cavities grow due to the tension. The total volume of the voids grow until the material disintegrates at the spall plane. Simplified calculations based on this model, describing the metal as a viscous liquid, give results in fairly good agreement with the experimental data and predict the increase in spall strength at high strain rates.

An increase of the spall strength in aluminum, copper, and Metglas at strain rates larger than 107 s−1

Measurements of the dynamic spall strength in aluminum, copper, and Metglas shocked by a high-power laser to hundreds of kilobars pressure are reported. The strain rates in these experiments are of the order of 107 s−1, which cannot be reached in impact experiments. The free-surface velocity behavior associated with spallation is characterized by oscillations caused by the reverberations of the spall layer. An optically recording velocity interferometer system was developed to measure the free-surface velocity time history. This diagnostic method has the advantages of being a noninterfering system and produces a highly accurate continuous measurement in time. The spall strength was calculated from the free-surface velocity as a function of the strain rate. The results show a rapid increase in the spall strength, suggesting that a critical phenomenon occurs at strain rates ∼107 s−1, expressed by the sudden approach to the theoretical value of the spall strength.

Dynamic Yield Strength and Spall Strength of Alumina Short Fiber Reinforced ZL109 Cast Aluminium Alloy

This paper describes the results of plate impact experiments conducted on a alumina short fiber reinforced ZL109 cast aluminium alloy. The loading was produced by a 100mm bore light gas gun. The metal-matrix composite specimen was backed with a PMMA. Manganin gauges were used to measure the normal stress history at the interface between the specimen and the PMMA. The dynamic yield strength and spall strength of the metal-matrix composite were determined.

Development of an optically recording velocity interferometer system for laser induced shock waves measurements

An optically recording velocity interferometer system, ORVIS, was developed for measurements of the time evolution of the free surface and the particle velocity in laser induced shock waves experiments. This system produces interference fringe shifts which are proportional to the Doppler shift of a laser beam reflected from the moving surface. These fringe shifts are recorded with a high speed electronic streak camera, which has a 70 ps time resolution. Using this method, the free surface velocity was measured with an accuracy better than 5% and the pressure in laser shocked aluminum targets was calculated. Shock waves of order of hundreds of kilobars are produced by a Nd:YAG laser system with a wavelength of 1.06 μm, pulse width of 5 ns (FWHM) and energy in the range (30–50) J, focused to spots diameters in the range (200–1000) μm. The dynamic spall strength reported here for Al is (14.47±1.45) kbar for a strain rate of ε̇∼8×106s−1, consistent with studies performed with other methods.