Modeling brittle fracture, slip weakening, and variable friction in geomaterials with an embedded strong discontinuity finite element.

Localized shear deformation plays an important role in a number of geotechnical and geological processes. Slope failures, the formation and propagation of faults, cracking in concrete dams, and shear fractures in subsiding hydrocarbon reservoirs are examples of important effects of shear localization. Traditional engineering analyses of these phenomena, such as limit equilibrium techniques, make certain assumptions on the shape of the failure surface as well as other simplifications. While these methods may be adequate for the applications for which they were designed, it is difficult to extrapolate the results to more general scenarios. An alternative approach is to use a numerical modeling technique, such as the finite element method, to predict localization. While standard finite elements can model a wide variety of loading situations and geometries quite well, for numerical reasons they have difficulty capturing the softening and anisotropic damage that accompanies localization. By introducing an enhancement to the element in the form of a fracture surface at an arbitrary position and orientation in the element, we can regularize the solution, model the weakening response, and track the relative motion of the surfaces. To properly model the slip along these surfaces, the traction-displacement response must be properly captured. This report focuses on the development of a constitutive model appropriate to localizing geomaterials, and the embedding of this model into the enhanced finite element framework. This modeling covers two distinct phases. The first, usually brief, phase is the weakening response as the material transitions from intact continuum to a body with a cohesionless fractured surface. Once the cohesion has been eliminated, the response along the surface is completely frictional. We have focused on a rate- and state-dependent frictional model that captures stable and unstable slip along the surface. This model is embedded numerically into the element using a generalized trapezoidal formulation. While the focus is on the constitutive model of interest, the framework is also developed for a general surface response. This report summarizes the major research and development accomplishments for the LDRD project titled 'Cohesive Zone Modeling of Failure in Geomaterials: Formulation and Implementation of a Strong Discontinuity Model Incorporating the Effect of Slip Speed on Frictional Resistance'. This project supported a strategic partnership between Sandia National Laboratories and Stanford University by providing funding for the lead author, Craig Foster, during his doctoral research

Proof-of-Principle Experiment for FEL-Based Coherent Electron Cooling,”

Abstract Coherent electron cooling (CEC) has a potential to significantly boost luminosity of high-energy, highintensity hadron-hadron and electron-hadron colliders. In a CEC system, a hadron beam interacts with a cooling electron beam. A perturbation of the electron density caused by ions is amplified and fed back to the ions to reduce the energy spread and the emittance of the ion beam. To demonstrate the feasibility of CEC we propose a proof-of-principle experiment at RHIC using SRF linac. In this paper, we describe the setup for CeC installed into one of RHIC's interaction regions. We present results of analytical estimates and results of initial simulations of cooling a gold-ion beam at 40 GeV/u energy via CeC

Scanning tunneling microscopy studies of the surfaces of a-Si:H and a-SiGe:H films

The report contains a detailed description of the experimental complexities encountered in developing scanning tunneling microscope (STM) probing of atomic structure on the surface of freshly-grown hydrogenated-amorphous semiconductors. It also contains a speculative microscopic film-growth model that explains differences between the disorder in CVD grown a-Ge:H versus a-Si:H films. This model is derived from prior results obtained in the chemical analysis of GeH{sub 4} plasmas, combined with surface reaction and thermodynamic considerations. The neutral radical fragments of silane, disilane and germane dissociation in discharges, which dominate the vapor and film-growth reactions, have been deduced from detailed analysis of prior data and are reported. 4 refs., 7 figs

Passivation of Interfaces in High-Efficiency Photovoltaic Devices

Solar cells made from III-V materials have achieved efficiencies greater than 30%. Effectively ideal passivation plays an important role in achieving these high efficiencies. Standard modeling techniques are applied to Ga0.5In0.5P solar cells to show the effects of passivation. Accurate knowledge of the absorption coefficient is essential (see appendix). Although ultralow (<2 cm/s) interface recombination velocities have been reported, in practice, it is difficult to achieve such low recombination velocities in solar cells because the doping levels are high and because of accidental incorporation of impurities and dopant diffusion. Examples are given of how dopant diffusion can both help and hinder interface passivation, and of how incorporation of oxygen or hydrogen can cause problems

Synoptic radio meteorology /

"October 1962."Includes bibliographical references (p. 27-30).Mode of access: Internet