Defense Special Weapons Agency Research Articles

DYNA-modelling of the high-velocity impact problems with a split-element algorithm

The present work addresses the implementation of a split-element algorithm for modelling fracture in terminal effects (TE) problems. The algorithm is incorporated within Vec-Dyna3D hydrocode (Technical report DSWA-TR-96-95. Alexandria (VA): Defense Special Weapons Agency, 1998), which is a prototype of LS-DYNA3D (Version 950. Livermore: Livermore Software Technology Corporation, May 1999). This algorithm has also been implemented in LS-DYNA2D (UCID-18756, Rev. 2. Livermore: Lawrence Livermore National Laboratory, 1984.) and it is verified numerically in the present paper. In doing so, tensile and shear modes of fracture due to high velocity impact are analysed in detail. Plate collision (the spallation problem) may be considered as a test problem for achieving the tensile mode of fracture. Encounter of a compact projectile with a plate (the plugging problem) plays similar role for the shear mode. Influence of choice of effective (equivalent) stress involved in a 3D-extension of a fracture criterion is analysed from two points of view: (i) the mesh effects, and (ii) a role of the complex stress state. The strain-rate sensitive Maxwell-type model (J. Appl. Mech. Tech. Phys. 13(6) (1972) 868.) is employed as a constitutive model. Workability of the algorithm in 3D case is illustrated with a numerical example for the plugging problem. The calculations being conducted show an appropriateness of the present approach for TE problems.

Characterization of reflected hydroacoustic signals for ctbt localization

The sparse nature of the proposed IMS hydroacoustic network implies that maximizing the information from each sensor is critical to overall network performance for nuclear event localization. Performance may be enhanced using information contained in late-time reflected arrivals from continental shelves, islands, and seamounts. The primary focus here is to characterize reflected signals and determine the conditions under which reflections are likely to occur. Several data sets, including a 1970 ship-scuttling explosion conducted in the North Atlantic (CHASE 21 event) are analyzed. Travel-time ellipses and bearing estimates generated from measured arrival times of waveform envelopes are used to determine possible geographic locations of acoustic reflections. Environmental characteristics at these locations are examined in order to understand the conditions under which reflections are likely to occur. Time- and frequency-domain characteristics of both the direct arrivals and reflections are measured and discussed. [Work sponsored by Defense Special Weapons Agency Contract Number DSWA01-97-C-0164.]

Seismic experiments, nuclear dismantlement go hand in hand in Kazakhstan

Unique seismic experiments involving large chemical explosions at different depths have been conducted in Kazakhstan, thanks to nuclear dismantlement activity there. Collaborative efforts of several bodies have provided this creative, cost‐efficient extension of the dismantling work, improving technical monitoring and verification of the Comprehensive Test Ban Treaty (CTBT).For the past several years, the Defense Special Weapons Agency (DSWA) has been closing the nuclear test tunnels and bore‐holes at the former Soviet nuclear test site nearSemipalatinsk, eastern Kazakhstan, as part of the Nunn‐Lugar Cooperative Threat Reduction (CTR) Program. The existence of this program and the infrastructure that was in place to implement it made it possible to conduct the seismic experiments. As a result, benchmark data have been collected on the variations in seismic signals from explosions at different burial depths.

Inductive energy technology for pulsed intense X-ray sources

Defense Special Weapons Agency (DSWA) has been developing inductive energy storage technology (IES) for applications requiring short pulses of X-rays with peak radiation power in the terawatt range. The purpose of this program is to develop more compact and affordable pulsed power sources and power flow technology needed for efficient conversion of the electromagnetic energy into radiation. Performance characteristics of present generators and power conditioning for future pulsed power IES designs are discussed. Because of the complex physics governing the interaction among the power conditioning, power flow, and load performance, DSWA supports studies of power flow to the load that converts the electromagnetic energy to X-ray radiation. Experiments performed at high power levels and resulting improvements in performance are reported.