V. F. Nesterenko

Dynamics of heterogeneous materials

1. Nonlinear Impulses in Particulate Materials. 2. Mesomechanics of Porous Materials Under Intense Dynamic Loading. 3. Transformation of Shocks in Laminated and Porous Materials. 4. Shear Localization and Shear Band Patterning in Heterogeneous Materials. 5. Non-Equilibrium Heating of Powders Under Shock Loading. 6. Advanced Materials Treatment by Shock Waves.

Propagation of nonlinear compression pulses in granular media

The study of mechanics of a granular medium is of substantial interest, both scientifically and for the solution of applied problems. Such materials are, for example, good buffers for shock loads. Their, study is important for the development of processes of the pulse deformation of several porous materials. A review of studies of small deformations and elastic wave propagation in these media was carried out in [i] on the basis of discrete models. The structure of a stationary shock wave was analyzed in [2] as a function of its amplitude. i. Statement of the Problem. The problem of nonstationary, nonlinear perturbations in one-dimensional granular media is stated in the present paper on the basis of the wellknown interaction between neighboring granules. As an interaction law we choose the Hertz law [3]

Shear localization in dynamic deformation of materials: microstructural evolution and self-organization

The plastic deformation of crystalline and non-crystalline solids incorporates microscopically localized deformation modes that can be precursors to shear localization. Shear localization has been found to be an important and sometimes dominant deformation and fracture mode in metals, fractured and granular ceramics, polymers, and metallic glasses at high strains and strain rates. Experiments involving the collapse of a thick walled cylinder enable controlled and reproducible application of plastic deformation at very high strain rates to specimens. These experiments were supplemented by hat-shaped specimens tested in a compression Hopkinson bar. The initiation and propagation of shear bands has been studied in metals (Ti, Ta, Ti–6Al–4V, and stainless steel), granular and prefractured ceramics (Al2O3 and SiC), a polymer (teflon) and a metallic glass (Co58Ni10Fe5Si11B16). The first aspect that was investigated is the microstructural evolution inside the shear bands. A fine recrystallized structure is observed in Ti, Cu, Al–Li, and Ta, and it is becoming clear that a recrystallization mechanism is operating. The fast deformation and short cooling times inhibit grain-boundary migration; it is shown, for the first time, that a rotational mechanism, presented in terms of dislocation energetics and grain-boundary reorientation, can operate within the time of the deformation process. In pre-fractured and granular ceramics, a process of comminution takes place when the particles are greater than a critical size ac. When they are smaller than ac, particle deformation takes place. For the granular SiC, a novel mechanism of shear-induced bonding was experimentally identified inside the shear bands. For all materials, shear bands exhibit a clear self-organization, with a characteristic spacing that is a function of a number of parameters. This self-organization is analyzed in terms of fundamental material parameters in the frame of Grady–Kipp (momentum diffusion), Wright–Ockendon, and Molinari (perturbation) models.

Self-organization of shear bands in titanium and Ti-6Al-4V alloy

The evolution of multiple adiabatic shear bands was investigated in commercially pure titanium and Ti–6Al–4V alloy through the radial collapse of a thick-walled cylinder under high-strain-rate deformation (10 4 s 1 ). The shearband initiation, propagation, as well as spatial distribution were examined under different global strains. The shear bands nucleate at the internal boundary of the specimens and construct a periodical distribution at an early stage. The shear bands are the preferred sites for nucleation, growth, and coalescence of voids and are, as such, precursors to failure. The evolution of shear-band pattern during the deformation process reveals a self-organization character. The differences of mechanical response between the two alloys are responsible for significant differences in the evolution of the shear band patterns. The number of shear bands initiated in Ti (spacing of 0.18 mm) is considerably larger than in Ti–6Al–4V (spacing of 0.53 mm); on the other hand, the propagation velocity of the bands in Ti–6Al–4V (556 m/s) is approximately three times higher than in Ti (153 m/s). The experimental shear-band spacings are compared with theoretical predictions that use the perturbation analysis and momentum diffusion; the shortcomings of the latter are discussed. A new model is proposed for the initiation and propagation that incorporates some of the earlier ideas and expands them to a two-dimensional configuration. The initiation is treated as a probabilistic process with a Weibull dependence on strain; superimposed on this, a shielding factor is introduced to deal with the deactivation of embryos. A discontinuous growth mode for shear localization under periodic perturbation is proposed. The propagating shear bands compete and periodically create a new spatial distribution.  2002 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.

Tunability of solitary wave properties in one-dimensional strongly nonlinear phononic crystals

One-dimensional strongly nonlinear phononic crystals were assembled from chains of PTFE (polytetrafluoroethylene) and stainless-steel spheres with gauges installed inside the beads. Trains of strongly nonlinear solitary waves were excited by impacts. A significant modification of the signal shape and an increase of solitary wave speed up to two times (at the same magnitude of dynamic contact force) were achieved through a noncontact magnetically induced precompression of the chains. The data for the PTFE based chains are presented for the first time and the data for the stainless-steel beads chains are extended into a range of maximum dynamic forces more than one order of magnitude lower than previously reported. Experimental results agreed reasonably well with the long-wave approximation and numerical calculations based on the Hertz interaction law for particles interactions.

Dynamic response of conventional and hot isostatically pressed Ti–6Al–4V alloys: experiments and modeling

Abstract This paper presents the results of a systematic comparative study of the dynamic thermomechanical response of Ti–6Al–4V alloys with three different microstructures. Two of the alloys are produced by the hot isostatically pressed technique using rapidly solidified granules, with one alloy milled prior to hot pressing. Experiments are performed over a broad range of strain rates, 10−3– 7000 s −1 , and initial temperatures, 77–1000 K. Depending on the test temperature, compressive strains of 10–60% are achieved. The microstructure of the undeformed and deformed specimens is investigated, using optical microscopy. The dependence of the flow stress on the temperature and the strain rate is examined for various strains and it is related to the corresponding material microstructure. The results show that adiabatic shearbands develop at high strain rates, as well as at low strain rates and high temperatures. Depending on the test temperature, shearbands initiate once a sample is deformed to suitably large strains. The flow stress is more sensitive to temperature than to the strain rate. Based on these results and other published work, the thermally activated mechanisms associated with the dislocation motion are identified. The physically based model proposed by Nemat-Nasser and Li (1997) for OFHC copper, is suitably modified and applied to this class of titanium alloys. In the absence of dynamic strain aging, the model predictions are in good accord with the experimental results. Comparing the results for the three considered Ti–6Al–4V alloys, with different microstructures, it is found that the initial microstructural features affect only the magnitude of the threshold stress and the athermal part of the flow stress, but not the functional dependence of the thermally activated part of the flow stress on the temperature and the strain rate.

Shear localization and recrystallization in high-strain, high-strain-rate deformation of tantalum

Tantalum was subjected to high plastic strains (global effective strains between 0 and 3) at high strain rates (>104 s−1) in an axisymmetric plane strain configuration. Tubular specimens, embedded in thick-walled cylinders made of copper, were collapsed quasi-uniformly by explosively-generated energy; this was performed by placing the explosive charge co-axially with the thick-walled cylinder. The high strains achieved generated temperatures which produced significant microstructural change in the material; these strains and temperatures were computed as a function of radial distance from the cylinder axis. The microstructural features observed were: (i) dislocations and elongated dislocation cell (eeff 2.5, T > 1000 K). Whereas the post-deformation (static) recrystallization takes place by a migrational mechanism, dynamic recrystallization is the result of the gradual rotation of subgrains coupled with dislocation annihilation. A simple analysis shows that the statically recrystallized grain sizes observed are consistent with predicted values using conventional grain-growth kinetics. The same analysis shows that the deformation time is not sufficient to generate grains of a size compatible with observation (0.1–0.3 μm). A mechanism describing the evolution of the microstructure leading from elongated dislocation cells, to subgrains, and to micrograins is proposed. Grain-scale localization produced by anisotropic plastic flow and localized recovery and recrystallization was observed at the higher plastic strains (eeff > 1). Residual tensile ‘hoop’ stresses are generated near the central hole region upon unloading; this resulted in ductile fracturing along shear localization bands.

Damage evolution in dynamic deformation of silicon carbide

Abstract Damage evolution was investigated in silicon carbide by subjecting it to dynamic deformation in (a) a compression Hopkinson–Kolsky bar (compressive stresses of 5 GPa), and (b) high-velocity impact under confinement (compressive stresses of 19–32 GPa) by a cylindrical (rod) tungsten alloy projectile. Considerable evidence of plastic deformation, as dislocations and stacking faults, was found in the fractured specimens. A polytype transformation was observed through a significant increase in the 6H–SiC phase at compressive stresses higher than 4.5 GPa (in the vicinity of the dynamic compressive failure strength). Profuse dislocation activity was evident in the frontal layer in the specimen recovered from the projectile impact. The formation of this frontal layer is proposed to be related to the high lateral confinement, imposed by the surrounding material. It is shown that plastic deformation is consistent with an analysis based on a ductility parameter ( Δ =K C /τ y πc ). The microstructural defects and their evolution were found to be dependent on the concentration of boron and aluminum, which were added as sintering aids. Several mechanisms are considered for the initiation of fracture: (a) dilatant cracks induced by mismatch in the effective elastic moduli between two adjacent grains, leading to internal tensile stresses and creating transgranular fracture. Finite element calculations show that high tensile stresses are generated due to elastic compatibility strains; (b) Zener–Stroh cracks nucleated by the piled up dislocations along grain boundaries, and resulting in intergranular fracture; (c) cracks due to existing flaws connected with grain-boundary phases, voids, etc.; and (d) stress concentrations due to twinning and stacking faults. The high dislocation density observed in the impacted specimen is consistent with existing models of microplasticity.

Hot isostatic pressing of powder in tube MgB2 wires

The critical current density (Jc) of hot isostatic pressed (HIPed) MgB2 wires, measured by dc transport and magnetization, is compared with that of similar wires annealed at ambient pressure. The HIPed wires have a higher Jc than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field (Hirr). The HIPed wires are promising for applications, with Jc>106 A/cm2 at 5 K and zero field and >104 A/cm2 at 1.5 T and 26.5 K, and Hirr∼17 T at 4 K. The improvement is attributed to a high density of structural defects, which are the likely source of vortex pinning. These defects, observed by transmission electron microscopy, include small angle twisting, tilting, and bending boundaries, resulting in the formation of subgrains within MgB2 crystallites.

Influence of microstructures and crystalline defects on the superconductivity of MgB2

This work studies the influence of microstructures and crystalline defects on the superconductivity of MgB2, with the objective to improve its flux pinning. A MgB2 sample pellet that was hot isostatic pressed (HIPed) was found to have significantly increased critical current density (Jc) at higher fields than its un-HIPed counterpart. The HIPed sample had a Jc of 10 000 A/cm2 in 50 000 Oe (5 T) at 5 K. This was 20 times higher than that of the un-HIPed sample, and the same as the best Jc reported by other research groups. Microstructures observed in scanning and transmission electron microscopy indicate that the HIP process eliminated porosity present in the MgB2 pellet resulting in an improved intergrain connectivity. Such improvement in intergrain connectivity was believed to prevent the steep Jc drop with magnetic field H that occurred in the un-HIPed MgB2 pellet at H>45 000 Oe(4.5 T) and T=5 K. The HIP process was also found to disperse the MgO that existed at the grain boundaries of the un-HIPed MgB2...