Matthew Serge

The mechanism of instability and localized reaction in the explosively driven collapse of thick walled Ni-Al laminate cylinders

Thick-walled cylinders constructed from alternating concentric layers of Ni and Al foils were explosively collapsed. The prevalent mode of the high strain, high strain rate plastic deformation was the cooperative buckling of the foils originating in the interior layers. This phenomenon was reproduced in numerical simulations. Its mechanism is qualitatively different than that of shear localization seen in all previously investigated homogeneous solid and granular materials and from the independent buckling of single thin-walled cylinders. Localized chemical reactions were observed in the apex areas of the Ni foils, consistent with the localization of temperature due to high strain plastic deformation.

The mechanisms of plastic strain accommodation during the high strain rate collapse of corrugated Ni–Al laminate cylinders

The Thick-Walled Cylinder method was used on corrugated Ni–Al reactive laminates to examine how their mesostructures accommodate large strain, high strain rate plastic deformation and to examine the potential for intermetallic reaction initiation due to mechanical stimuli. Three main mesoscale mechanisms of large plastic strain accommodation were observed in addition to the bulk distributed uniform plastic flow: (a) the extrusion of wedge-shaped regions into the interior of the cylinder along planes of easy slip provided by angled layers, (b) the development of trans-layer shear bands in the layers with orientation close to radial and (c) the cooperative buckling of neighbouring layers perpendicular to the radius. These mesoscale mechanisms acted to block the development of periodic patterns of multiple, uniformly distributed, shear bands that have been observed in all previously examined solid homogeneous materials and granular materials. The high-strain plastic flow within the shear bands resulted in the dramatic elongation and fragmentation of Ni and Al layers. The quenched reaction between Al and Ni was observed inside these trans-layer shear bands and in a number of the interfacial extruded wedge-shaped regions. The reaction initiated in these spots did not ignite the bulk of the material, demonstrating that these mesostructured Ni-Al laminates are able to withstand high-strain, high-strain rate deformation without reaction. Numerical simulations of the explosively collapsed samples were performed using the digitized geometry of corrugated laminates and predictions of the final, deformed mesostructures agree with the observed deformation patterns.

Influence of mesoscale properties on the mechanisms of plastic strain accommodation in plane strain dynamic deformation of concentric Ni-Al laminates

The paper presents results on the mechanisms of plastic strain accommodation of Ni-Al laminates composed of concentrically aligned thin foils processed at different conditions undergoing a high strain radial collapse in thick walled cylinder experiments. Numerical simulations were conducted to examine the influence of mesoscale parameters (layer size, defects in mesostructure, and ductility) on the mechanisms of large plastic strain accommodation (high amplitude cooperative buckling; high frequency, low amplitude buckling; and kinking) at high strain rates in pure shear (plane strain) conditions. These mechanisms are dramatically different than observed in solid ductile and brittle homogeneous materials where a pattern of shear bands is the major mode of strain accommodation. It was observed that the layer thickness and ductility greatly influenced the dominant mode of plastic strain accommodation. The number of apices was related to the layer thickness. The presence of defects mainly had a localized area of influence. Numerical simulations showed good qualitative agreement with the experiments and provided the ability to simulate additional mesoscale and material dependencies: the role of friction/bonding, relative layer sizes, and sample thickness.

Mechanisms of large strain, high strain rate plastic flow in the explosively driven collapse of Ni-Al laminate cylinders

Ni-Al laminates have shown promise as reactive materials due to their high energy release through intermetallic reaction. In addition to the traditional ignition methods, the reaction may be initiated in hot spots that can be created during mechanical loading. The explosively driven thick walled cylinder (TWC) technique was performed on two Ni-Al laminates composed of thin foil layers with different mesostructues: concentric and corrugated. These experiments were conducted to examine how these materials accommodate large plastic strain under high strain rates. Finite element simulations of these specimens with mesostuctures digitized from the experimental samples were conducted to provide insight into the mesoscale mechanisms of plastic flow. The dependence of dynamic behaviour on mesostructure may be used to tailor the hot spot formation and therefore the reactivity of the material system.

Phase velocity enhancement of linear explosive shock tubes

Strong, high-density shocks in a gas can be generated by end-initiating a hollow cylinder of explosive surrounding a pressurized thin-walled tube. Implosion of the tube results in a pinch that travels at the detonation velocity of the explosive, thereby driving a strong shock into the gas ahead of it. In the present study, the pinch velocity was increased beyond the detonation velocity of known explosives by dragging an oblique detonation wave along the surface of the tube. The gas shock and detonation trajectories were measured for a variety of phase velocities and tube fill pressures. Strong shocks with an average velocity of 13 km/s were observed for fill pressures as high as 6.9 MPa in helium while transient velocities as high as 19 km/s were observed. Shock trajectory performance degraded strongly with increasing phase velocity and for a velocity of 16 km/s the gas shock barely advanced ahead of the detonation.

Reactivity of Ti-B, Cr-S, and Mn-S powder systems during explosively-driven collapse

Metal-metal and metal-sulfur reactive powder mixtures have been previously tested for initiation of reaction via planar, normal-shock loading. In addition to reacting under shock, such powder mixtures may undergo exothermic reaction from other types of mechanical loading. The thick-walled cylinder technique was performed on samples of Ti-B (1:2 molar ratio), Cr-S (1.15:1 molar ratio), and Mn-S (1:1 molar ratio). These experiments were aimed to determine the effect of large shear strains exerted on reactive metal powder mixtures and to establish the relative effectiveness of shear loading in comparison to shock loading for initiating reaction. Recovered samples were analyzed via SEM and XRD to determine the degree of reaction.

Strength effects in an imploding cylinder with constant mass-to-explosive loading

High explosives were used to implode thin-walled metal cylinders of different strengths (Al 6061-O, Al 6061-T6, mild steel, and stainless steel) at a constant mass-to-explosive (M/C) ratio. The velocity history of the inner surface of the imploding cylinder was measured via photonic Doppler velocimetry. These histories and maximum velocities were compared to an imploding Gurney model that used a detonation pressure-based time constant, giving good agreement with the experiments. The deceleration caused by strength effects was modeled via a simplified stress-strain curve, which was then used to predict the entire velocity history.

Implosion-driven technique to create fast shockwaves in high-density gas

Pressurized tubes surrounded by either one or two layers (separated by a secondary tube) of sensitized nitromethane and encased in a thick-walled tube (the tamper) were imploded. The distance between the detonation wave in the explosive and shock wave in the innermost tube were measured (the standoff). A simple model based on hoop stress and acoustic interactions between the tubing was developed and used to predict the standoff distance. At low initial pressures (on the order of 7MPa), results indicate that the secondary tube and two layers of explosive did not prove to significantly increase the standoff. However, at higher pressures (on the order of 10 MPa), standoff was noticeably greater when the secondary tube was inserted between the pressurized tube and the tamper. The measured values are in reasonable agreement with the predictions of the model.