The physical foundations and domain of applicability of the Kayenta constitutive model are presented along with descriptions of the source code and user instructions. Kayenta, which is an outgrowth of the Sandia GeoModel, includes features and fitting functions appropriate to a broad class of materials including rocks, rock-like engineered materials (such as concretes and ceramics), and metals. Fundamentally, Kayenta is a computational framework for generalized plasticity models. As such, it includes a yield surface, but the term "yield" is generalized to include any form of inelastic material response including microcrack growth and pore collapse. Kayenta supports optional anisotropic elasticity associated with ubiquitous joint sets. Kayenta supports optional deformation-induced anisotropy through kinematic hardening (in which the initially isotropic yield surface is permitted to translate in deviatoric stress space to model Bauschinger effects). The governing equations are otherwise isotropic. Because Kayenta is a unification and generalization of simpler models, it can be run using as few as 2 parameters (for linear elasticity) to as many as 40 material and control parameters in the exceptionally rare case when all features are used. For high-strain-rate applications, Kayenta supports rate dependence through an overstress model. Isotropic damage is modeled through loss of stiffness and strength. Penetration project led first by Jim Hickerson and later by Danny Frew; several DOE Accelerated Strategic Computing Initiative (ASCI) Design and Qualification (DQ) Materials & Physics Models (M&PM) projects led by Mike McGlaun and Justine Johannes, a Hard and Deeply Buried Target (HDBT) project led by Paul Yarrington and Shawn Burns; a Model Accreditation Via Experimental Sciences For Nuclear Weapons (MAVEN) laboratory testing project led by Moo Lee, two LDRD projects, one led by Larry Costin and the other by Rich Regueiro; and an Army project under John Rowe and Scott Schoenfeld.
A technique is formulated for projecting vector fields from one unstructured computational grid to another grid so that a constraint condition such as a conservation property holds at the cell or element level on the ‘receiving’ grid. The approach is based on ideas from constrained optimization and certain mixed or multiplier‐type finite element methods in which Lagrange multipliers are introduced on the elements to enforce the constraint. A theoretical analysis and estimates for the associated saddle‐point problem are developed and a new algorithm is proposed for efficient solution of the resulting discretized problem. In the algorithm a reduced Schur's complement problem is constructed for the multipliers and the projected velocity computation reduces to a post‐processing calculation. In some instances the reduced system matrix can be factored so that repeated projections involve little more than forward and backward substitution sweeps. Numerical tests with an element of practical interest demonstrate optimal rate of convergence for the projected velocities and verify the local conservation property to expected machine precision. A practical demonstration for environmental simulation of Florida Bay concludes the study. Copyright © 2001 John Wiley & Sons, Ltd.
This article describes the use of fractal simulations of African design in a high school computing class. Fractal patterns-repetitions of shape at multiple scales-are a common feature in many aspects of African design. In African architecture we often see circular houses grouped in circular complexes, or rectangular houses in rectangular complexes. Typically the accompanying ceremonies, cosmologies, and other traditions make use of scaling and recursion in their conceptual models. African scaling designs include textiles, sculpture, adornment, and other forms; in many cases there are explicit geometric algorithms and other formal aspects (e.g., pseudorandom number generation in divination systems) embedded in the associated indigenous knowledge system. Thus African fractals provide a strong counter to stereotypes of African culture as primitive or simplistic. Following this fieldwork, we developed a Web site which uses Java simulations of these African designs to teach computational perspectives on fractals to high school students. 1 We hypothesized that this combination of anti-primitivist "ethnocomputing" and design-based creative learning would enhance both the engagement and performance of under-represented students in computing. A quasi-experimental study used two 10th grade computing classes, both taught by the same instructor, and both including more than 50% under-represented students (Latino and African American). The control class received six days of instruction using a popular Web site (with Java applets but no cultural content or design activities) for high school fractal lessons; the experimental class received the same amount of instruction using our Web site. Pre/post differences on both achievement and attitude tests indicate statistically significant improvement for the students in the experimental class. Potential implications for improving participation and achievement of under-represented students in computing education are discussed.
The feasibility of simulating dynamic fracture in quasi-brittle material using a dual particle computational method with a smeared-crack representation of material failure is explored. The computational approach utilized is dual particle dynamics, which incorporates a moving least squares interpolation of field variables between two sets of particles that discretize the spatial domain, and a Lagrangian description of the moving least squares weight function. Material failure is represented by an inelastic continuum strain contribution obtained from smearing the effect of a cohesive failure model over a discrete volume of material. A threedimensional simulation of the initiation and development of a dynamic mode I failure is performed for the case of approximate plane wave propagation. Post failure wave interaction with the resulting global failure surface replicates the behavior of a stress-free boundary condition. The computational material failure approach is applied to problems of spalling in split Hopkinson pressure bar tests. Experimental failure trends are reproduced successfully.The use of zero thickness interface elements placed along inter-element boundaries is a widely used approach for simulation of material failure [2,3]. In this failure representation, the discrete constitutive model (traction versus displacement discontinuity) is applied across the interface element to model localization and fracture. The downside of using interface elements is that the macro-crack trajectory is restricted to follow the element boundaries. Consequently, the results are always mesh dependent. A summary of interface elements is provided by Rots [4].Computational failure methods based on automatic mesh generation have also been utilized [5,6]. Like the interface element method, cracks are confined to element boundaries, but the mesh structure changes throughout the computation to accommodate the changing crack path. The discontinuity is the result of the changing mesh structure, and remeshing only needs to be performed locally near the crack tip. The drawback of this approach is the complicated implementation of the automated meshing procedure and the potentially high computational cost of remeshing many times throughout the course of the simulation.The smeared-crack representation of material failure utilizes a classical continuum description of the problem. In this method, also known as a crack band model [7], the fracture process zone is idealized as a band of finite width, as opposed to a discrete surface. The effect of localization within the band is represented by an inelastic strain contribution. Consequently, the effect of failure is "smeared" across the width of the band. The smeared-crack approach has been utilized to model failure of concrete structures [8,9]. A thorough overview of smeared-crack models is provided by Rots [4,10,11]. Implementation of smeared-crack failure representation is straightforward and can be done with minimal changes to existing codes. Limitations of the method in FEM are mesh orien...
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