Session Y3: ED-4b: Convergent Loading and Other Loading Paths

Damage Experiments in a Cylindrical Geometry

Using a cylindrical configuration to study spallation damage allows for a natural recollection of the damaged material under proper driving conditions. In addition, the damaged material is able to come to a complete stop without the application of further forces. Specific areas of research include the damage initiation regime in convergent geometry, behavior of material recollected after damage, and effects of convergent geometry on the material response. These experiments challenge existing computational material models and databases and provide motivation to improve these models and increase the predictive capabilities of codes, as numerical modeling of such experiments requires the consideration of the effect of convergence and two-dimensional strains and shear stresses on the spallation profile of a material. A series of 3 experiments (R-Damage-0, -1 and -2) provided data about failure initiation of a well-characterized material (aluminum) in a cylindrical geometry. A second series of three experiments (R-Damage-3, -4 and -5) studied the behavior of material recollected after damage from pressures in the damage initiation regime. A third series of two experiments (R-Damage-6 and -7), scheduled for March 2009, will study the behavior of material recollected after complete failure. In addition to post-shot collection of the damaged target material for subsequent metallographic analysis, dynamic in-situ experimental diagnostics include velocimetry and transverse radial radiography. This presentation will cover the design, experimental results and numerical simulations for these experiments.   

Peculiarities of high-rate deformation of copper at convergence of cylindrical channels under effect of shock waves

This work presents a new method for study of peculiarities of metal deformation at micro- and meso scale levels at strain rate of 105-107s-1. A copper liner loads test copper samples pressed into copper guide rings behind which one can find a substrate made of the same material. The loading parameters, such as the intensity SX, and the pulse duration t are specified by the liner's thickness and speed. Cylindrical holes of various diameters (D0=0.5-2mm) in the test samples are produced beforehand. Coarse-grain annealed copper M1 was chosen as the subject of the study. The loading parameters were chosen in such a way as to realize the decrease of the holes diameter and complete compaction in the recovered samples.   

Collapsing of Thick-Walled Cylinders Using Electro-Magnetic Driving Forces

The Thick-Walled Cylinder technique, reported in the literature, uses explosive loading to enforce collapsing of the cylindrical sample. This experimental set-up has been established as a controlled and repeatable technique to create and study multiple adiabatic shear bands. Searching to establish a simpler experimental platform to perform large sets of experiments, we have designed an Electro-Magnetic (EM) set-up for the collapsing of thick walled cylinders. The EM set-up is based on a pulsed current generator using a capacitor bank system. The specimen is an assembly of coaxial cylinders, where the inner and outer cylinders, each attached to an opposite pole, are short-circuited. Upon discharge, a high current flows through the cylinders, in \textit{opposite} directions, creating repulsive magnetic forces between them. This work presents the design procedure of the specimens using numerical simulations and some experimental results for SS304L thick-walled samples, using this set-up. The spatial distribution of the multiple adiabatic shear bands in these experiments is in good agreement with that reported in the literature for the explosive driven experiments with a similar material.   

Investigation of dynamic dry friction between stainless steel and aluminium alloy

Previous workers studied dynamic friction by using an impacting copper plate to drive a tapered aluminium alloy plug into a matching hole in a stainless steel outer sheath. The velocity of the back surface of the plug was measured using velocity interferometry. We have performed experiments on a version of this basic configuration that has been enlarged so that the sliding surfaces remain in contact for a longer time than with the original configuration. By comparing our results with computer simulations we conclude that the frictional forces between the inner cone and the steel outer are initially high but decrease significantly as the sliding proceeds. This effect is assumed to result from thermal softening of the material at the sliding interface. The study is supported by metallography of the recovered components.   

Shock Waves in Converging Geometries

Plate impact experiments are a powerful tool in equation of state development, but are inherently limited by the range of velocities accessible to the gun. In an effort to dramatically increase the range of pressures which can be studied with available impact velocities, a new experimental technique is being developed. The possibility of using a confined converging target to focus shockwaves and produce a large amplitude pressure pulse is examined. When the planar shock resulting from impact enters the converging target the impedance mismatch at the boundary of the confinement produces reflected shock waves with a radial component of velocity. Once the reflected shocks converge at the center of target, a high pressure shock forms, and with proper target design this pressure pulse will overtake the initial planar shockwave. The use of radial density gradings on the impactor and a target buffer can be used to further facilitate shock focusing by creating a mismatch in shock velocities. If the shock velocity is graded such that it is lowest towards the center of the buffer, continuity conditions force the shock wave to converge prior to even entering the target. Numerical simulations indicate a significant increase in pressure as the shock converges and show promise for the proposed concept. Experimental results on aluminum for validating the concept will be presented and discussed.   

Experimental Analysis of High Velocity Impact Phenomena with an Ultra High Speed Camera

High velocity impacts of aluminum sphere against aluminum target have been performed at impact velocity of 4000 m/s with the two stage light gas gun HERMES at THIOT-INGENIERIE laboratory. The specificity of this launcher is the use of a gas breech for the first compression stage, which avoids the use of explosive powder. The visualization of the impact phenomena has been realized with the ultra high speed camera SIM8 developed by SPECIALISED IMAGING LIMITED. This very sophisticated camera can give eight frames with a minimum exposure time of 5 ns and a minimum time between frames of 5 ns. Results for impact on planar and tilted target are presented and analyzed. The high resolution of the monitored frames allows the distinction of very fine details from the cloud debris after the impact. This information is essential for the understanding of the physics associated with the impact phenomena at these velocities.