Dale S. Preece

Discrete element modeling of sand production

This paper describes the application of Discrete Element Methods (DEM), [5, 14, 11, 12], to model the mechanics of sand production in oil recovery processes. Sand production occurs when a weak sandstone matrix disintegrates due to changes in fluid pressure gradient and/or effective stress. The principal goal of this work is to investigate possible mechanisms contributing to sand production that occur at the microscopic scale. Experimental results indicate that sanding processes are localized in nature, episodic, hysteretic, and depend on the fluid saturation and mobility in the porous matrix, [8, 3]. It is the coupled motion of the fluid and solid phases that we seek to model in this work. The sandstone matrix is modeled as an assemblage of bonded particles, providing a mechanism to simulate the tensile strength in the bulk material, attributable to cementation or capillary forces. A flow model based on Darcys Law is incorporated in an existing DEM system to investigate the effect of flow rate on the collapse of the matrix.

Bullet Impact on Steel and Kevlar®/Steel Armor - Computer Modeling and Experimental Data*

Computer hydrocode analyses and ballistic testing have been used to investigate the effectiveness of steel plate armor against lead/copper bullets commonly available in the U.S. and across the world. Hydrocode simulations accurately predict the steel plate thickness that will prevent full penetration as well as the impact crater geometry (depth and diameter) in that thickness of steel armor for a 338 caliber bullet. Using the hydrocode model developed for steel armor, studies were also done for an armor consisting of a combination of Kevlar® and steel. These analyses were used to design the experiments carried out in the ballistics lab at Sandia National Laboratories. Ballistics lab testing resulted in a very good comparison between the hydrocode computer predictions for bullet impact craters in the steel plate armor and those measured during testing. During the experiments with the combination armor (Kevlar®/steel), the steel became a witness plate for bullet impact craters following penetration of the Kevlar®. Using the bullet impact craters in the steel witness plate it was determined that hydrocode predictions for Kevlar® armor are less accurate than for metals. This discrepancy results from the inability of the hydrocode (Eulerian) material model to accurately represent the behavior of the fibrous Kevlar®. Thus, this paper will present the hydrocode predictions and ballistics lab data for the interaction between a lead/copper bullet and several armoring schemes: 1) steel, 2) Kevlar®, and 3) a Kevlar®/steel combination.Copyright

Kevlar and carbon composite body armor - analysis and testing.

Kevlar materials make excellent body armor due to their fabric-like flexibility and ultra-high tensile strength. Carbon composites are made up from many layers of carbon AS-4 material impregnated with epoxy. Fiber orientation is bidirectional, orientated at 0° and 90°. They also have ultra-high tensile strength but can be made into relatively hard armor pieces. Once many layers are cut and assembled they can be ergonomicically shaped in a mold during the heated curing process. Kevlar and carbon composites can be used together to produce light and effective body armor. This paper will focus on computer analysis and laboratory testing of a Kevlar/carbon composite cross-section proposed for body armor development. The carbon composite is inserted between layers of Kevlar. The computer analysis was performed with a Lagrangian transversely isotropic material model for both the Kevlar and Carbon Composite. The computer code employed is AUTODYN. Both the computer analysis and laboratory testing utilized different fragments sizes of hardened steel impacting on the armor cross-section. The steel fragments are right-circular cylinders. Laboratory testing was undertaken by firing various sizes of hardened steel fragments at square test coupons of Kevlar layers and heat cured carbon composites. The V50 velocity for the various fragment sizes was determined from the testing. This V50 data can be used to compare the body armor design with other previously designed armor systems. AUTODYN [1] computer simulations of the fragment impacts were compared to the experimental results and used to evaluate and guide the overall design process. This paper will include the detailed transversely isotropic computer simulations of the Kevlar/carbon composite cross-section as well as the experimental results and a comparison between the two. Conclusions will be drawn about the design process and the validity of current computer modeling methods for Kevlar and carbon composites.Copyright

Blastwall effects on down range explosively-induced overpressure.

Blastwalls are often assumed to be the answer for facility protection from malevolent explosive assault, particularly from large vehicle bombs (LVBs). The assumption is made that the blastwall, if it is built strong enough to survive, will provide substantial protection to facilities and people on the side opposite the LVB. This paper will demonstrate through computer simulations and experimental data the behavior of explosively induced air blasts during interaction with blastwalls. It will be shown that air blasts can effectively wrap around and over blastwalls. Significant pressure reduction can be expected on the downstream side of the blastwall but substantial pressure will continue to propagate. The effectiveness of the blastwall to reduce blast overpressure depends on the geometry of the blastwall and the location of the explosive relative to the blastwall.

Design of Conical Shaped Charges for Prompt Initiation of TNT Chemical Munition Bursters

The Explosive Destruction System (EDS) has been designed at Sandia National Laboratories for the disposal of chemical munitions (phosgene, mustard gas, sarin etc.), many dating back to World War I. EDS is a portable system that is trailer mounted and consists of a vessel into which a chemical munition can be loaded and neutralized with linear and conical shaped charges. Gases are contained within the sealed chamber. The linear shaped charges split the munition in two and the conical is aimed at the explosive burster, in each munition, which is detonated by the shaped charge jet. Toxic chemicals remaining in the vessel following detonation are neutralized and disposed of. This paper documents the development of a new conical shaped charge (CSC) needed to reliably detonate explosive bursters in an expanding array of chemical munitions that are beyond what the device was originally designed to neutralize. Design of this new CSC was controlled by the need to deliver energy above the detonation threshold into the explosive after penetrating the outer steel casing, fluid, the burster casing and finally the explosive. Design considerations were driven by jet conditions at the steel/explosive interface inside the burster. Parameters to consider in CSC design include: 1) diameter, 2) liner thickness, 3) liner position in body, 4) explosive weight, and 5) liner shape or interior angle. The effects of these parameters on final CSC performance are examined in detail. CSC’s meeting the design specifications have been manufactured and tested. The performance of these charges is compared with the original design requirements.Copyright

Modeling of Munitions Fragmentation and Fragment Interaction With Containment Vessels and Shielding Systems

Design of pressure vessels intended to contain explosive blast and high velocity fragments can present several potential difficulties. The stresses and velocities resulting from explosive events generally result in highly non-linear material behavior, thereby limiting the applicability of standard design techniques. As a result, extensive testing is usually required to verify a containment vessel’s structural integrity. Computer simulation can be utilized to decrease the cost and time associated with vessel development. The Explosive Destruction System (EDS) was created by Sandia National Laboratories to safely dispose of aged chemical weapons. Development of the EDS system has provided a wealth of test data, quite useful for verification and improvement of computer-based predictive capabilities. The computer simulation code AUTODYN (currently being used at Sandia National Laboratories) provides an excellent basis for prediction of munition behavior as a result of explosive effects. Through comparison and refinement, appropriate simulation methods can be determined and integrated into future modeling efforts. Another computer code, CTH, has successfully predicted much of the physical behavior observed in EDS development and testing. Models created in AUTODYN 2-D can be compared with EDS data as well as results of the CTH modeling efforts, further refining the predictive capabilities of AUTODYN.© 2002 ASME

Analysis of Dynamic Loading of a Simple Structure to a Blast Wave

An object in the path of a blast wave generated by an explosion will experience a certain level of structural damage. The degree of destruction seen in a structure from a explosive blast wave is effected by three main parameters, (1) the force applied to the structure, (2) how long the force is acting on the structure, and (3) the specific geometric and material properties of the structure, or architectural surety. Structures capable of large lateral loads can be used for defense against explosions (terrorist threats). However, in order to fully predict the architectural surety of a structure, further investigation of the interaction of explosive blast waves with structures is required. The purpose of our analysis is to determine the efficiency of coupling energy from a blast wave to a simple structure. We performed some explosives tests and computer simulations to provide this analysis. In our experiments, the structures consisted of several free hanging steel plates at various distances from an explosion. The blast wave was generated by a sphere of TNT. We used a standard model to calculate the overpressure incident on this plates, we then calculated the shock energy coupled to the plates, we measured the overpressure at points near the plates (for calibration), we measured the effects of the blast wave on the plates (measured their displacement due to the blast), and we performed computer code calculations to predict the effect of the blast wave on the plates. The computational code Autodyn is currently being used at Sandia National Laboratories for various impact and blast loading problems. The code contains several simulation methods, including ALE (Arbitrary Langrangian Eulerian) simulation. Because explosive blast in air involves both expanding gases as well as solid/solid impacts, ALE codes typically provide better predictive capabilities.Copyright

An assessment of ore waste and dilution resulting from buffer/choke blasting in Surface Gold Mines

A discrete element computer program named DMC{underscore}BLAST (Distinct Motion Code) has been under development since 1987 for modeling rock blasting (Preece {ampersand} Taylor, 1989). This program employs explicit time integration and uses spherical or cylindrical elements that are represented as circles in two dimensions (2-D). DMC{underscore}BLAST calculations compare favorably with data from actual bench blasts (Preece et al, 1993). Buffer Choke blasting is commonly used in surface gold mines to break the rock and dilate it sufficiently for ease of digging, with the assumption of insignificant horizontal movement. The blast designs usually call for relatively shallow holes benches ({lt} 11 m) with small blastholes (approx. 165 mm), small burdens and spacings ({lt}5 m), often with 50% or more of the hole stemmed. Control of blast-induced horizontal movement is desired because the ore is assayed in place from the blasthole drill cuttings and digging polygons of ore and waste are laid out before the blast. Horizontal movement at the ore waste boundary can result in dilution of the ore or loss of ore with the waste. The discrete element computer program DMC{underscore}BLAST has been employed to study spatial variation of horizontal rock motion during buffer choke blasting. Patterns of rock motion can be recognized from the discrete element simulations that would be difficult or impossible to recognize in the field (Preece, Tidman and Chung, 1997). Techniques have been developed to calculate ore waste and dilution from the horizontal movement predicted by DMC{underscore}BLAST. Four DMC{underscore}BLAST simulations of buffer blasting have been performed. The blasts are identical except that the burden and spacing are systematically varied which also changes the powder factor. Predictions of ore waste or dilution are made for each burden in the blast, assuming no horizontal movement, to illustrate the spatial variation observed.