Adam Stover

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.

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.

LASER SHOCK COMPRESSION AND SPALLING OF REACTIVE NI‐AL LAMINATE COMPOSITES

Reactive laminates produced by successive rolling and consisting of alternate layers of Ni and Al (with bi‐layer thicknesses of 5 and 30 μm) were investigated by subjecting them to laser shock‐wave loading. The laser intensity was varied between ∼2.68×1011 W/cm2 (providing an initial estimated pressure P∼25 GPa) and ∼1.28×1013 W/cm2 (P∼333 GPa) with two distinct initial pulse durations: 3 ns and 8 ns. Hydrodynamic calculations (using commercial code HYADES) were conducted to simulate the behavior of shock‐wave propagation in the laminate structures. SEM, and XRD were carried out on the samples to study the reaction initiation, and the intermetallic compounds. It was found that the thinner bilayer thickness (5 μm) laminate exhibited the most intensive localized interfacial reaction at the higher laser intensity (1.28×1013 W/cm2); the reaction products were identified as NiAl and other Al‐rich intermetallic compounds. The reaction front and the formation of intermetallic compounds extend into the sample wit...

MESO‐SCALE COMPUTATIONAL STUDY OF THE SHOCK‐COMPRESSION OF COLD‐ROLLED Ni‐Al LAMINATES

Meso‐scale computational analysis is used to study the shock‐compression response of cold‐rolled Ni‐Al laminates which represent a fully dense reactive system with continuous interparticle contacts. The laminates were prepared through rolling multiple Ni and Al foils with initial thicknesses of 127 μm and 178 μm, respectively. Simulations of shock wave propagation through the laminates are performed using CTH on heterogeneous, imported microstructures obtained through optical microscopy. Uniaxial strain experiments are utilized to validate the simulated responses in order to draw comparisons to previously obtained results for Ni‐Al powder compacts. The Baer‐Nunziato nonequilibrium multiphase granular mixture model is then used to model the 1D reaction response of the materials during dynamic loading.