26 research outputs found
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Peacemaker: Fracture assessment of a 155mm cannon barrel
A single crack 30 mm or deeper which is 75 mm long is sufficient to fracture a typical 155 mm cannon barrel with a pressure at or above two-thirds (206 MPa -- 30 ksi) of the maximum operating pressure (310 MPa -- 45 ksi). Longer and deeper flaws reduce the critical pressure required to initiate fracture. For the monolithic barrel design considered in this work, the postulated 30 mm deep by 75 mm long crack should propagate through the entire wall and, depending upon the new ``fractured`` geometry, may propagate axially down the cannon barrel. Numerical analyses conducted with straight through-thickness crack fronts propagated axially at pressures below the maximum operating pressure while those with curved crack fronts required pressures in excess of the working pressures to extend axially. (Experiments on actual 155 mm barrels with flaws similar to the one generated by the tested shape charge show appreciable axial crack extension at approximately equivalent pressures.) In either case, a through-thickness ``hole`` will be formed in the barrel`s side and a reduction in firing pressure should result. Finally, debris deposited within the barrel can greatly assist the fracture process, especially at lower operating pressures. Overall, a single deep and long interior crack appears the most effective way to fracture a cannon barrel. Unless clustered very closely together, multiple ``shallow`` cracks require higher pressures to fracture than does a single deep crack. Flaws introduced on the barrel`s exterior are less efficient since no crack-face pressures exist and the overall stresses on the barrel`s exterior are much lower than on its interior. Thus, very deep exterior cracks would be required to fail the barrel from internal pressure
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DYNA3D Non-reflecting Boundary Conditions - Test Problems
Two verification problems were developed to test non-reflecting boundary segments in DYNA3D (Whirley and Engelmann, 1993). The problems simulate 1-D wave propagation in a semi-infinite rod using a finite length rod and non-reflecting boundary conditions. One problem examines pure pressure wave propagation, and the other problem explores pure shear wave propagation. In both problems the non-reflecting boundary segments yield results that differ only slightly (less than 6%) during a short duration from their corresponding theoretical solutions. The errors appear to be due to the inability to generate a true step-function compressive wave in the pressure wave propagation problem and due to segment integration inaccuracies in the shear wave propagation problem. These problems serve as verification problems and as regression test problems for DYNA3D
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DYNA3D Material Model 71 - Solid Element Test Problem
A general phenomenological-based elasto-plastic nonlinear isotropic strain hardening material model was implemented in DYNA3D for use in solid, beam, truss, and shell elements. The constitutive model, Model 71, is based upon conventional J2 plasticity and affords optional temperature and rate dependence (visco-plasticity). The expressions for strain hardening, temperature dependence, and rate dependence allow it to represent a wide variety of material responses. Options to capture temperature changes due to adiabatic heating and thermal straining are incorporated into the constitutive framework as well. The verification problem developed for this constitutive model consists of four uni-axial right cylinders subject to constant true strain-rate boundary conditions. Three of the specimens have different constant strain rates imposed, while the fourth specimen is subjected to several strain rate jumps. The material parameters developed by Fehlmann (2005) for 21-6-9 Nitronic steel are utilized. As demonstrated below, the finite element (FE) simulations are in excellent agreement with the theoretical responses and indicated the model is functioning as desired. Consequently, this problem serves as both a verification problem and regression test problem for DYNA3D
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Performance of Rank-2 Fortran 90 Pointer Arrays vs. Allocatable Arrays
The computational performance of two-dimensional Fortran 90 arrays defined with the pointer attribute were compared to identically sized arrays defined with the allocatable attribute. The goal of this work was to quantify the computational cost of using each array type within a high-performance finite element setting
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Shell Element Verification & Regression Problems for DYNA3D
A series of quasi-static regression/verification problems were developed for the triangular and quadrilateral shell element formulations contained in Lawrence Livermore National Laboratory's explicit finite element program DYNA3D. Each regression problem imposes both displacement- and force-type boundary conditions to probe the five independent nodal degrees of freedom employed in the targeted formulation. When applicable, the finite element results are compared with small-strain linear-elastic closed-form reference solutions to verify select aspects of the formulations implementation. Although all problems in the suite depict the same geometry, material behavior, and loading conditions, each problem represents a unique combination of shell formulation, stabilization method, and integration rule. Collectively, the thirty-six new regression problems in the test suite cover nine different shell formulations, three hourglass stabilization methods, and three families of through-thickness integration rules
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Energy and momentum conserving algorithms for rigid body contact
Energy-momentum conserving methods are developed for rigid body dynamics with contact. Because these methods are unconditionally stable, they are not time step dependent and, hence, are well suited for incorporation into structural mechanics finite element codes. Both penalty and Lagrange multiplier methods are developed herein and are the extension of the energy-momentum conserving integration schemes for rigid bodies given by Simo and Wong [1]
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Elastic properties of large tow 2-D braided composites by numerical and analytical methods
The homogenized extensional and flexural properties of a large tow, two- dimensional, braided carbon-fiber composite lamina were evaluated using analytical and numerical methods. The plane-stress composite lamina was assumed to be periodic in its plane and was modeled with a single representative volume element. The homogenized elastic properties were analytically estimated using beam-theory concepts and upper and lower bound techniques as well as using three-dimensional finite element analyses. The homogenized extensional and bending lamina properties are, in general, distinct properties and are not simply related to each other as in monolithic beams and plates or in composites with very fine and highly periodic microstructures. The importance and cause of distinct homogenized extensional and flexural elastic properties is briefly discussed
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A New Stabilized Nodal Integration Approach
A new stabilized nodal integration scheme is proposed and implemented. In this work, focus is on the natural neighbor meshless interpolation schemes. The approach is a modification of the stabilized conforming nodal integration (SCNI) scheme and is shown to perform well in several benchmark problems
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Crashworthiness simulation of composite automotive structures
In 1990 the Automotive Composites Consortium (ACC) began the investigation of crash worthiness simulation methods for composite materials. A contract was given to Livermore Software Technology Corporation (LSTC) to implement a new damage model in LS-DYNA3DTM specifically for composite structures. This model is in LS-DYNA3DTM and is in use by the ACC partners. In 1994 USCAR, a partnership of American auto companies, entered into a partnership called SCAAP (Super Computing Automotive Applications Partnership) for the express purpose of working with the National Labs on computational oriented research. A CRADA (Cooperative Research and Development Agreement) was signed with Lawrence Livermore National Laboratory, Oak Ridge National Laboratory, Sandia National Laboratory, Argonne National Laboratory, and Los Alamos National Laboratory to work in three distinctly different technical areas, one of which was composites material modeling for crash worthiness. Each Laboratory was assigned a specific modeling task. The ACC was responsible for the technical direction of the composites project and provided all test data for code verification. All new models were to be implemented in DYNA3D and periodically distributed to all partners for testing. Several new models have been developed and implemented. Excellent agreement has been shown between tube crush simulation and experiments
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A Tow-Level Progressive Damage for Simulating Carbon-Fiber Textile Composites: Interim Report
A numerical approach to model the elasto-plastic and tensile damage response of tri-axially braided carbon-fiber polymeric-matrix composites is developed. It is micromechanically based and consists of a simplified unit cell geometry, a plane-stress tow-level constitutive relationship, a one-dimensional undulation constitutive law, and a non-traditional shell element integration rule. The braided composite lamina is idealized as periodic in the plane, and a simplified three-layer representative volume (RV) is assembled from axial and braider tows and pure resin regions. The constituents in each layer are homogenized with an iso-strain assumption in the fiber-direction and an iso-stress condition in the other directions. In the upper and lower layers, the fiber-direction strain is additively decomposed into an undulation and a tow portion. A finite-deformation tow model predicts the plane-stress tow response and is coupled to the undulation constitutive relationship. The overall braid model is implemented in DYNA3D and works with traditional shell elements. The finite-deformation tow constitutive relationship is derived from the fiber elasticity and the isotropic elasto-plastic power-law hardening matrix response using a thermodynamic framework and simple homogenization assumptions. The model replicates tensile damage evolution, in a smeared sense, parallel and perpendicular to the fiber axis and is regularized to yield mesh independent results. The tow-level model demonstrates reasonable agreement, prior to damage, with detailed three-dimensional FE (finite element) elasto-plastic simulations of aligned, periodically arranged, uni-directional composites. The 3-layer braid model response is compared with predictions obtained from detailed micromechanical simulations of the braid's unit cell in uni-axial extension, shear, and flexure for three braid angles. The elastic properties show good agreement as does the non-linear response for loadings dominated by the axial tows. In loadings dominated by the braider tow response, the absence of a non-linear undulation model deteriorates the agreement. Nonetheless, the present approach is applicable to a broad range of tri-axially braided composites as well as for unidirectional composites, but presently lacks any compressive failure mechanisms and an adequate non-linear undulation model