Sikhanda Satapathy

Edge Elements and Current Diffusion

Current flow in the neighborhood of sharp conducting edges and corners is encountered in many applications of computational electromagnetics. Some examples include resonant cavities and waveguides of rectangular shape, cracks in metal components in nondestructive testing, and the edges and corners of the rail-armature contact in an electromagnetic launcher. Although the number of analytical solutions to such problems is small, it is generally believed that such solutions often involve singularities, high gradients, or discontinuities in one or more field components near the edge or corner. It has been demonstrated that the so-called edge elements can provide much more accurate predictions of global quantities (like resonant frequencies and scattering parameters) in the case of full-wave electromagnetics when field discontinuities or singularities are present. They have also been shown to better represent field discontinuities at material interfaces in magnetostatics and reentrant corners in low-frequency electromagnetics. This feature of edge elements is due to their ability to allow necessary jumps in field components at geometric or material discontinuities, as opposed to nodal elements, with the Coulomb gauge enforced, which can overconstrain the fields in certain situations. The main question of interest in this paper is whether edge elements give a similar advantage in the representation of current diffusion around a sharp edge. Both edge- and node-based formulations are considered, and the results are discussed.

Strain Evolution in Metal Conductors Subjected to Short-Duration Current Pulses

The behavior of metallic materials subjected to short-duration heating resulting from the passage of high currents is investigated. Specifically, direct experimental measurements are made of the stress drop and recovery engendered by the application of a 1-ms-long current pulse that raises the temperature of the specimen to about 450 K. The experimental results are used to calibrate an adiabatic viscoplastic model describing the material behavior

Studies of Asperity-Scale Plasma Discharge Phenomena

A combined experimental and computational simulation study of direct-current plasma discharge phenomena in small-length-scale geometries (<; 10 μm) is described. The primary goal is to study discharge breakdown characteristics in small-length-scale geometries as quantified by a modified Paschen breakdown curve and the quench characteristics in these discharges. A modified mesoscale friction tester apparatus is used for the experiments. A self-consistent nonequilibrium plasma model is used for the simulation studies. The model includes field-emission effects, which is a key process in determining small-length-scale breakdown behavior. The breakdown and quench curves obtained from the experiments and simulations showed the same general trends. Quantification of the heat fluxes from the simulations shows higher erosion at the cathode and a highly nonlinear heating behavior with applied overvoltages above the breakdown threshold.

Effect of Helmet Pads on the Load Transfer to Head under Blast Loadings

Recent wars have highlighted the need to better protect dismounted soldiers against emerging blast and ballistic threats. Current helmets are designed to meet ballistic performance criterion. Therefore, ballistic performance of helmets has received a lot of attention in the literature. However, blast load transfer/mitigation has not been well understood for the helmet/foam pads. The pads between the helmet and head can not only absorb energy, but also produce more comfort to the head. The gap between the helmet and head due to the pads helps prevent or delay the contact between helmet shell and the head. However, the gap between the helmet shell and the head can produce underwash effect, where the pressure can be magnified under blast loading. In this paper, we report a numerical study to investigate the effects of foam pads on the load transmitted to the head under blast loading. The ALE module in the commercial code, LS-DYNA was used to model the interactions between fluid (air) and the structure (helmet/head assembly). The ConWep function was used to apply blast loading to the air surrounding the helmet/head. Since we mainly focus on the load transfer to the head, four major components of the head were modeled: skin, bone, cerebrospinal fluid (CSF) and brain. The foam pads in fielded helmets are made of a soft and a hard layer. We used a single layer with the averaged property to model both of those layers for computational simplicity. Sliding contact was defined between the foam pads and the helmet. A parametric study was carried out to understand the effects of material parameters and thickness of the foam pads.Copyright

Jump conditions for Maxwell equations and their consequences

We derived the jump conditions for Faradays induction law at the interface of two contacting bodies in both Eulerian and Lagrangian descriptions. An algorithm to implement the jump conditions in the potential formulation of Maxwell equation is presented. Calculations show that the use of the correct jump conditions leads to good agreement with experimental data, whereas the use of incorrect jump conditions can lead to severe inaccuracies in the computational results. Our derivation resolves the jump condition discrepancy found in the literature and is validated with experimental results.

Numerical Study of Head/Helmet Interaction Due to Blast Loading

Recent wars have heightened the need to better protect dismounted soldiers against emerging blast and ballistic threats. Traumatic Brain Injury (TBI) due to blast and ballistic loading has been a subject of many recent studies. In this paper, we report a numerical study to understand the effects of load transmitted through a combat helmet and pad system to the head and eventually to the brain during a blast event. The ALE module in LS-DYNA was used to model the interactions between fluid (air) and the structure (helmet/head assembly). The geometry model for the head was generated from the MRI scan of a human head. For computational simplicity, four major components of the head are modeled: skin, bone, cerebrospinal fluid (CSF) and brain. A spherical shape blast wave was generated by using a spherical shell air zone surrounding the helmet/head structure. A numerical evaluation of boundary conditions and numerical algorithm to capture the wave transmission was carried out first in a simpler geometry. The ConWep function was used to apply blast pressure to the 3D model. The blast pressure amplitude was found to reduce as it propagated through the foam pads, indicating the latter’s utility in mitigating blast effects. It is also shown that the blast loads are only partially transmitted to the head. In the calculation where foam pads were not used, the pressure in the skin was found to be higher due to the underwash effect in the gap between the helmet and skin, which amplified the blast pressure.Copyright

The Multiaxial Failure Response of Porcine Trabecular Skull Bone Estimated Using Microstructural Simulations

The development of a multi-axial failure criterion for trabecular skull bone has many clinical and biological implications. This failure criterion would allow for modeling of bone under daily loading scenarios that typically are multi-axial in nature. Some yield criteria have been developed to evaluate the failure of trabecular bone, but there is a little consensus among them. To help gain deeper understanding of multi-axial failure response of trabecular skull bone, we developed 30 microstructural finite element models of porous porcine skull bone and subjected them to multi-axial displacement loading simulations that spanned three-dimensional (3D) stress and strain space. High-resolution microcomputed tomography (microCT) scans of porcine trabecular bone were obtained and used to develop the meshes used for finite element simulations. In total, 376 unique multi-axial loading cases were simulated for each of the 30 microstructure models. Then, results from the total of 11,280 simulations (approximately 135,360 central processing unit-hours) were used to develop a mathematical expression, which describes the average three-dimensional yield surface in strain space. Our results indicate that the yield strain of porcine trabecular bone under multi-axial loading is nearly isotropic and despite a spread of yielding points between the 30 different microstructures, no significant relationship between the yield strain and bone volume fraction is observed. The proposed yield equation has simple format and it can be implemented into a macroscopic model for the prediction of failure of whole bones.

A Sensitivity Study of the Porcine Head Subjected to Bump Impact

Understanding load transfer to the human brain is a complex problem that has been a key subject of recent investigations [4–6]. Because the porcine is a gyrencephalic species, having greater structural and functional similarities to the human brain than other lower species outlined in the literature, it is commonly chosen as a surrogate for human brain studies [7]. Consequently, we have chosen to use a porcine model in this work. To understand stress wave transfer to and through the brain, it is important to fully characterize the nature of the impact (i.e. source, location, and speed) as well as the response of the constituent tissues under such impact. We suspect the material and topology of these tissues play an important role in their response.In this paper, we report on a numerical study assessing the sensitivity of model parameters for a 6-month old Gottingen mini-pig model, under bump loading. In this study, 2D models are used for computational simplicity. While a 3D model is more realistic in nature, a 2D representation is still valuable in that it can provide trends on parameter sensitivity that can help steer the development of the 3D model. In this work, we investigate the variation of skull and skin thickness, evaluate material variability of the skull, and consider the effects of nasal cavities on load transfer. Eighty simulations are computed in LS-DYNA and analyzed in MATLAB. The results of this study will provide useful knowledge on the necessary components and parameters of the porcine model and therefore provide more confidence in the analysis. This is an essential first step as we look toward bridging the gap between correlates of injury in animal and human models.Copyright

Microstructural Analysis of Porcine Skull Bone Subjected to Impact Loading

Skull fracture can be a complex process involving various types of bone microstructure. Finite element analysis of the microscopic architecture in the bone allows for a controlled evaluation of the stress wave interactions, micro-crack growth, propagation and eventual coalescence of trabecular fracture. In this paper, the microstructure and mechanics of small-volume sections of a 6-month-old Gottingen Minipig skull were analyzed. MicroCT scans were used to generate finite element models. Various computational methods were investigated for modeling the intricacies contained within the porous microstructure of the trabecular bone. Pores were explicitly meshed in one method, whereas in the second, a mesh was created from a microCT image-informed mapping algorithm that mapped the trabecular porosity from an image stack to a solid volume mesh of the model. From here, all models were subject to uniaxial compression simulations. The output of the simulations allowed for a detailed understanding of the failure mechanics of the skull structure and allowed for comparison between the methods. Fracture typically occurs in the weakest areas where the bone is highly porous and forms a fracture surface throughout the material, which causes the bone to collapse upon itself.Copyright

Numerical Analysis of Electrochemical Erosion for Functionally Graded Tungsten/Copper Materials

One innovative approach for producing functionally graded tungsten/copper composites involves infiltrating a porous tungsten structure with molten copper. The porous structure is created by sintering tungsten powder to create an initial porous preform and then connecting the preform to the anode of an electrochemical cell. The porosity increases with time as the tungsten oxidizes in the alkaline electrolyte. The rate of porosity increase depends on the local value of electric-potential difference between the electrode and electrolyte, while the local electrical properties are a function of the porosity. Therefore, the porosity distribution, which determines the material-property gradient of the resulting material, can be modeled by a system of coupled equations describing porosity and electric potential. The present work develops a numerical tool capable of predicting the evolution of porosity in the rectangular-slab geometry (the extension to cylindrical-shell and elliptical-shell geometries is straightforward). Analysis of the results suggests that the shape of the gradient can be varied by adjusting parameters such as initial porosity and current density.