H. C. Chen

Controlled high‐rate localized shear in porous reactive media

It is demonstrated that controlled high‐strain‐rate plastic deformation of heterogeneous reactive porous materials (Nb+Si, Mo+Si+MoSi2) produces shear localization. Within the shear bands, having thicknesses 5–20 μm, the overall strains (γ≤100) and strain rates (γ≤107 s−1) result in changes in particle morphology, melting, and regions of partial reaction. The shear band thickness is smaller than the initial characteristic particle size of the porous mixture (≤44 μm). This ensures quenching of the deformed material structure in the same time scale as the deformation time (10−5 s). In the shear localization region, two types of patterning are observed: (a) a characteristic shear fracture which subdivides the Nb particles into thin parallel layers and (b) the formation of vortices.

SHEAR LOCALIZATION IN HIGH-STRAIN-RATE DEFORMATION OF GRANULAR ALUMINA

Abstract Dynamic deformation of densified granular alumina of two different particle sizes was investigated by the radial symmetric collapse of a thick-walled cylinder. The densified granular alumina was used to model the flow in ballistic impact and penetration of fragmented ceramic armor. Shear localization was a well developed deformation mode at an overall radial strain of ∼0.2–0.4 and strain rate of 10 4 s −1 . The following qualitative features of shear bands were established: • Shear bands have clear boundaries and their thickness does not depend on the initial particle size and has a typical value ∼10 μm. • The structure of the shear bands was dependent on initial particle size, suggesting differences in the mechanisms of flow. For the ∼4 μm alumina, comminution (break-up) and softening of particles were observed. For the ∼0.4 μm particles, a peculiar structure consisting of a central crack with two lateral cracks was formed. • Distributions of shear bands and displacement magnitudes were dependent on initial particle size. The observed differences in powder behavior are associated with different mechanisms of powder repacking. For large particles (∼4 μm), additional hardening resulting from microfracture and subsequent repacking of different size particles in the powder takes place. The small-sized (∼0.4 μm) ceramic does not go through the particle fracturing stage and the hardening is due to “classical” repacking.

SHEAR LOCALIZATION AND CHEMICAL REACTION IN HIGH-STRAIN, HIGH-STRAIN-RATE DEFORMATION OF Ti-Si POWDER MIXTURES

Ti-Si mixtures were subjected to high-strain-rate deformation at a pressure below the threshold for shock-wave initiation. Whereas the collapse of interparticle pores did not initiate reaction, regions of localized macro-deformation initiated reaction inside shear bands at suAciently high strains (g010), and propagation of the reaction through the entire specimen at higher strains (g020-40). This study demon- strates that temperature increases in shear localization regions can initiate chemical reaction inside a reac- tive powder mixture. The shear band spacing was 00.6-1 mm. Thermodynamic and kinetic calculations yield the reaction rate outside the shear bands, in the homogeneously deformed material, which has a

High temperature shock consolidation of hard ceramic powders

Abstract High-temperature shoch consolidation of hard ceramic powders was used as a means to improve bonding between powders and to decrease the number of cracks generated in the consolidated sample. A converging underwater shock-wave assembly was used for the compaction, and TiB 2 , c-BN and their mixed powders were consolidated at various conditions up to 850°C. The positive effects by heating the powders were confirmed by the experiments conducted.

The structure of controlled shear bands in dynamically deformed reactive mixtures

AbstractThe structure of controlled high-strain-rate shear bands generated in heterogeneous reactive porous materials (Nb + Si, Mo + Si + MoSi2) has been investigated using axially symmetric experimental configurations in which the source of energy is the detonation of low velocity explosives. The deformation was highly localized, with profuse formation of shear bands, which have thicknesses of 5 to 20 μm. The experimental method generated overall strains up to 100 and strain ratesn

Shear localization and chemical reaction in Ti–Si and Nb–Si powder mixtures: Thermochemical analysis

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Shear localization in granular and comminuted alumina

n of approximately 107 s-1. Changes in particle morphology, melting, and regions of partial reaction on three different length scales were observed. The shear band thickness is smaller than the initial characteristic particle size of the porous mixture (≤44 μm), ensuring a cooling time of the deformed material on the same order of magnitude as the deformation time (10-5 s). In the shear localization regions, two characteristic phenomena were observed: (a) a shear fracture subdividing the Nb particles into thin parallel layers and (b) the formation of vortices. A mechanism for the reaction inside the shear bands is proposed, and an expression for the largest size of chemical products as a function of shear deformation is obtained.

Shear-Induced Exothermic Chemical Reactions

Abstract Nbue5f8Si and Moue5f8Si elemental powder mixtures contained within cylindrical capsules were subjected to co-axial shock-wave loading at varying pressures (2.8–70 GPa). Shock-induced or shock-assisted chemical reactions were observed in these powder mixtures along the capsule axis. Three concentric regions with the capsules were observed: (1) fully reacted (Mach stem region); (2) partially reacted; and (3) unreacted. These results confirm the Krueger-Vreeland concept of threshold energy for shock-induced chemical reactions. Analysis of partially reacted regions enabled the identification of the reaction micromechanisms in accordance with the model proposed by Meyers, Yu and Vecchio (Acta Metall. Mater., 42 (1994) 715). Asymmetric shock-wave loading experiments on the above powder mixtures were also conducted. Significant macroscopic plastic deformation (i.e. ϵ ≌ 0.2–0.5 ) along with consolidation were achieved by modifying the explosive loading configuration. Because of the asymmetric loading, regions of shear localization were produced. These regions were also characterized by the onset of the chemical reaction resulting from the local thermal excursion due to both the frictional dissipation of kinetic energy and plastic deformation. The results obtained in this investigation confirm the earlier hypothesis that the shock energy dissipated by plastic deformation does play an important role in the initiation of the chemical reaction. It is proposed that the Krueger-Vreeland threshold energy be modified to take into account the plastic deformation energy.