Efficient simulation of low-velocity impacts on composite sandwich panels

For efficiently simulating the damage resistance of sandwich panels subjected to low-velocity impacts, the finite element based damage tolerance tool CODAC has been enhanced. While sandwich structures are very weight efficient and provide integrated acoustic and thermal insulation, impact damage can provoke a significant strength and stability reduction. Therefore, the objective of CODAC is to provide methodologies which reliably simulate impact events and predict impact damage sizes. Since frequent design loops require a quick analysis, efficient deformation and failure models are desired. To achieve a rapid and accurate stress analysis, a recently developed three-layered finite shell element is applied. Failure analysis is based on a progressive damage mechanics approach: Damage initiation is detected by stress-based failure criteria. Material resistance is reduced by appropriate, step-wise linear degradation models. An experimental impact test program on honeycomb sandwich panels is used to validate the impact simulation of the FE-tool CODAC. Comparisons between impact tests and simulations showed that CODAC is capable of accurately and rapidly simulating impact events, which induce barely visible damage. Furthermore, the onset of clearly visible damage is correctly predicted.     

Solid modeling aspects of three-dimensional fragmentation

The ways in which the topology and geometry of a three-dimensional finite-element model may evolve as a consequence of fracture and fragmentation are enumerated, and the actions which may be taken in order to update the boundary representation of the solid so as to faithfully reflect that evolution are described. Arbitrary topological and geometrical evolution of a three-dimensional solid, not necessarily restricted to an evolution of its surface, are addressed. Solids are described by their boundary representation (BRep) and a surface and volume triangulation. Fracture processes are modeled by the introduction of cohesive elements at element interfaces. Simple rules are shown to enable the simulation of strikingly complex crack patterns. The scope and versatility of the approach is illustrated with the aid of selected examples of application.     

Feature based object decomposition for finite element meshing

Performing a finite element analysis requires overlaying an object with a mesh of varying density based on the expected stress levels within the part. Attempts have been made in the past to automat the finite element meshing procedure. The method presented here is intelligent in the sense that it examines the complete part for potential stress gradients and decomposes the part into hexahedral regions according to the geometry gradients in the part. High geometry gradients are regions of high curvature, especially edges. The algorithm segregates high gradient features into isolation volumes. It then continues to decompose each isolation volume dependent on the particular geometry contained in the feature. The result is a set of hexahedral bricks suitable for passing to an automatic meshing routine.     

Efficient stochastic generation of special quasirandom structures

We present a new algorithm to generate Special Quasirandom Structures (SQS), i.e., best periodic supercell approximations to the true disordered state for a given number of atoms per supercell. The method is based on a Monte Carlo simulated annealing loop with an objective function that seeks to perfectly match the maximum number of correlation functions (as opposed to merely minimizing the distance between the SQS correlation and the disordered state correlations for a pre-specified set of correlations). The proposed method optimizes the shape of the supercell jointly with the occupation of the atomic sites, thus ensuring that the configurational space searched is exhaustive and not biased by a pre-specified supercell shape. The method has been implemented in the “mcsqs” code of the Alloy Theoretic Automated Toolkit (ATAT) in the most general framework of multicomponent multisublattice systems and in a way that minimizes the amount of input information the user needs to specify and that allows for efficient parallelization.     

Feature-preserving adaptive mesh generation for molecular shape modeling and simulation

We describe a chain of algorithms for molecular surface and volumetric mesh generation. We take as inputs the centers and radii of all atoms of a molecule and the toolchain outputs both triangular and tetrahedral meshes that can be used for molecular shape modeling and simulation. Experiments on a number of molecules are demonstrated, showing that our methods possess several desirable properties: feature-preservation, local adaptivity, high quality, and smoothness (for surface meshes). We also demonstrate an example of molecular simulation using the finite element method and the meshes generated by our method. The approaches presented and their implementations are also applicable to other types of inputs such as 3D scalar volumes and triangular surface meshes with low quality, and hence can be used for generation/improvement of meshes in a broad range of applications.     

非线性动态有限元碰撞仿真技术的工程应用研究

该文简要介绍了非线性动态显式有限元碰撞仿真技术的相关理论和算法 ,涉及到实际应用中的一系列关键问题及解决方案。最后文中以薄板冲压、汽车碰撞等工程实际问题为例运用所述理论进行了仿真计算研究 ,取得了令人满意的效果 ,因而也为其它有关碰撞问题的研究解决提供了一种强有力的方法和思路     

A statistical atlas based approach to automated subject-specific FE modeling

Subject-specific modeling is increasingly important in biomechanics simulation. However, how to automatically create high-quality finite element (FE) mesh and how to automatically impose boundary condition are challenging.This paper presents a statistical atlas based approach for automatic meshing of subject-specific shapes. In our approach, shape variations among a shape population are explicitly modeled and the correspondence between a given subject-specific shape and the statistical atlas is sought within the “legal” shape variations. This approach involves three parts: (1) constructing a statistical atlas from a shape population, including the statistical shape model and the FE model of the mean shape; (2) establishing the correspondence between a given subject shape and the atlas; and (3) deforming the atlas to the subject shape based on the shape correspondence. Numerical results on 2D hands, 3D femur bones and 3D aorta demonstrate the effectiveness of the proposed approach.     

Fidelity in visualizing large-scale simulations

Computer simulations are powerful tools frequently used today in many important applications, for example to build safer buildings, to crash-test an automobile before it is built, to stabilize the Pisa tower, to design artificial joints that are comfortable and durable, or to investigate what-if scenarios to avoid and best recover from natural or man-made disasters. The simulation codes have reached a very high-level of sophistication and, by running on powerful computing machinery, can accurately track with infinitesimal time steps dozens of physical properties of millions of interacting elements under extreme conditions. In order to take fully advantage of the bounty of information concealed in the data produced, visualization is a uniquely powerful tool since it caters to the sense that provides our highest bandwidth connection to the surrounding world.Unfortunately, simulation results are usually examined with graphics and visualization tools that are one or several steps behind the state-of-the-art. We describe our efforts of producing high-fidelity visualizations of the results of large-scale simulations using the latest commercial rendering and animation systems. To this effect we built a scalable and reusable link between the software worlds of animation and simulation. Our system also offers a set of tools that allow integrating the results of the simulation in the surrounding scene, of great importance when the intended audience extends beyond the researchers that designed the simulation. We built our system as part of the efforts of a larger, interdisciplinary team to produce a high-quality, physically accurate visualization of the September 11 attack on the Pentagon.     

Numerical modeling of electrochemical–mechanical interactions in lithium polymer batteries

This paper presents a multi-scale finite element approach for lithium batteries to study electrochemical–mechanical interaction phenomena at macro- and micro-scales. The battery model consists of a lithium foil anode, a separator, and a porous cathode that includes solid active materials and a liquid electrolyte. We develop a multi-scale approach to analyze the surface kinetics and electrochemical–mechanical phenomena within a single spherical particle of the active material. Homogenization techniques relate parameters in the micro-scale particle model to those in the macro-scale model describing the lithium ion transport, electric potentials and mechanical response based on porous electrode theory.     

A modeling strategy for hybrid systems based on event structures

This paper considers hybrid systems which are continuous/discrete-time systems interacting with a decision maker which oversees the control and structure of the continuous/discrete-time system. These two segments combine with a processor which evaluates data to produce a three-segment model of a hybrid system having sufficient flexibility to represent a broad range of real-time situations and sufficient generality to incorporate the essential aspects of other models into a single framework. In particular the paper uses a graphically expressive controlled Petri net formulation of the decision maker and any of the usual models for the continuous/discrete-time system. Interaction between the systems occurs via three types of events: continuous/discrete-time events, decision-making events, and processor events. These types of events and their composition are rigorously defined to produce event structures and event histories. These events and event histories are used for the domain of interaction functions which specify the channels of communication between the three essential segments of the hybrid system. The event-based domains allow the disassociation of these communication channels from dependence on particular kinds of models or applications. The range of the interaction functions are binary vector-valued indicating the activation/deactivation processes in the respective segments. The entire modeling strategy is motivated by applications and models found in the literature especially flexible manufacturing systems and the C-net model of a hybrid system.This work was partially supported by the National Science Foundation under Grant No. ECS-8800910.     

An analysis of the effects of the leg-spacing on spectral response of offshore structures

The authors have applied a computer-based model to perform spectral analysis of jacket platforms. The analysis is carried out in a consistent manner using the finite element method. The proper estimation of the phasing effects of the ocean waves on structural responses is dependent, inter alia, on the accurate calculation of nodal forces. Using the finite element approach the vector of nodal forces can be most accurately computed taking into account the varying distributed loads along the structural members. The model, developed by the first author, calculates cross-receptances of response for all degrees of freedom that enable the phase effects due to the spatial extent of the structure to be properly accounted for. This model includes the phase effects of wave loadings on the platform and it can thus give a more accurate estimation of response spectra which are generally required for design calculations; this is far more accurate than calculating wave loadings on an equivalent cantilever as is often done.     

Three dimensional modeling and finite element simulation of a generic end mill

The geometry of cutting flutes and the surfaces of end mills is one of the crucial parameters affecting the quality of the machining in the case of end milling. These are usually represented by two-dimensional models. This paper describes in detail the methodology to model the geometry of a flat end mill in terms of three-dimensional parameters. The geometric definition of the end mill is developed in terms of surface patches; flutes as helicoidal surfaces, the shank as a surface of revolution and the blending surfaces as bicubic Bezier and biparametric sweep surfaces. The proposed model defines the end mill in terms of three-dimensional rotational angles rather than the conventional two dimensional angles. To validate the methodology, the flat end milling cutter is directly rendered in OpenGL environment in terms of three-dimensional parameters. Further, an interface is developed that directly pulls the proposed three-dimensional model defined with the help of parametric equations into a commercial CAD modeling environment. This facilitates a wide range of downstream technological applications. The modeled tool is used for finite element simulations to study the cutting flutes under static and transient dynamic load conditions. The results of stress distribution (von mises stress), translational displacement and deformation are presented for static and transient dynamic analysis for the end mill cutter flute and its body. The method described in this paper offers a simple and intuitive way of generating high-quality end mill models for use in machining process simulations. It can be easily extended to generate other tools without relying on analytical or numerical formulations.     

Probabilistic fiber element modeling of reinforced concrete structures

A computational model based on a stochastic fiber element model is developed in this study. This model can be utilized for probabilistic evaluation of reinforced concrete (RC) members. The stochastic fiber element model is developed by combining the conventional fiber element formulation and the midpoint method for random field representation, to account for spatial variability of material and geometrical properties within a structural member. Three verification examples show the capability of the developed model in estimating the nonlinear structural behavior including softening. As an application of the developed computational model, a probabilistic strength analysis of a RC column is conducted in terms of the axial load-bending moment interaction. An approach of evaluating RC structural systems using the developed probabilistic computational model is also presented.     

Free-form deformation of constructive shell models

Free-form deformation (FFD) is known to be a powerful technique for deforming an object independent of its representation. A point on a solid is deformed by specifying the point relative to a coordinate system defined with a lattice of control points. Adjusting the control points of the lattice deforms the object. The deformed object can be visualized by sampling points on the object surfaces, or by approximating the object with a polyhedral model. However, a conversion of the polyhedral model to the underlining solid representation is required if further solid operations (e.g. Boolean operation) is to be applied. This paper presents a technique for applying FFD on constructive shell models (CSR). A CSR object is constructed by subtracting a set of depression (negative) trunctets from the union of a set of outer (positive) trunctets and a polyhedron core. FFD is applied to the polyhedron core and the trunctets of the model. A trunctet is deformed by applying FFD to a set of selected surface points on the trunctet. The deformed trunctet is obtained by interpolating a surface through the deformed surface points. In the deformation of a trunctet, an outer trunctet may become a depression trunctet (or vice versa). By using the concept of mating trunctet, and an approach for classifying the shape of a trunctet, shape changes of a trunctet in a deformation can be determined. Deformation of a trunctet may also result in an invalid trunctet that bulges out of its enclosing tetrahedron. Besides, the degree of the algebraic surface used determines the size of a trunctet relative to the distance between adjacent lattice control points. A subdivision of a trunctet may have to be performed to maintain the validity of a deformation.     

Variation modeling of aeronautical thin-walled structures with multi-state riveting

Assembly variation is inevitable in aeronautical thin-walled structure (ATWS) assembly, especially in structures joined with “C-type” automated riveting system (CARS). The variation propagates along the assembly process flow and will influence final product performance such as dimensional quality and fatigue durability. This paper represents a new variation model of ATWS with multi-state riveting. Firstly, a novel multi-state process of ATWS riveting with CARS called PDJR to PDRR (P to P) is developed and it contains two stages and eight states. Secondly, based on the P to P process, the variation model is divided into three sub-models (feature, displacement and propagation) to represent the assembly variation of ATWS multi-state riveting. For the feature sub-model, three important features are discussed, and the geometric and topological information is represented by a hierarchical method. For the displacement sub-model, the variation of eight-state in P to P is divided into four types, and the displacement of each type is analyzed separately according to the coordinate transformation and finite element method (FEM). For the propagation sub-model, a translation matrix considering the disturbance factor of every state is developed to obtain the final variation. Lastly, a multi-state riveting process of a wing panel which is made up of a skin and four stringers is modeled as a case study. The FEM is integrated into the Monte Carlo simulation (MCS) to analyze the variation, and the result proves that the proposed variation modeling of ATWS multi-state riveting can solve the problem of variation analysis in ATWS multi-state riveting efficiently.     

Automated modeling of modular robotic configurations

This research presents an automated method to build kinematic and dynamic models for assembling modular components of modular robotic systems. By comparison with other approaches, the proposed method is applicable to any robotic configuration with serial, parallel, or hybrid structures. In addition, it is object oriented so that each modular component is an element with a submodel and the overall model can be assembled from submodels subject to the connection constraints.     

Recurrent neural network technique for behavioral modeling of power amplifier with memory effects

A new technique for behavioral modeling of power amplifier (PA) with short‐ and long‐term memory effects is presented here using recurrent neural networks (RNNs). RNN can be trained directly with only the input–output data without having to know the internal details of the circuit. The trained models can reflect the behavior of nonlinear circuits. In our proposed technique, we extract slow‐changing signals from the inputs and outputs of the PA and use these signals as extra inputs of RNN model to effectively represent long‐term memory effects. The methodology using the proposed RNN for modeling short‐term and long‐term memory effects is discussed. Examples of behavioral modeling of PAs with short‐ and long‐term memory using both the existing dynamic neural networks and the proposed RNNs techniques are shown. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:289–298, 2015.     

Efficient verification of timed automata with BDD-like data structures

We investigate the effect on efficiency of various design issues for BDD-like data structures of TA state space representation and manipulation. We find that the efficiency is highly sensitive to decision atom design and canonical form definition. We explore the two issues in detail and propose to use CRD (Clock-Restriction Diagram) for TA state space representation and present algorithms for manipulating CRD in the verification of TAs. We compare three canonical forms for zones, develop a procedure for quick zone-containment detection, and present algorithms for verification with backward reachability analysis. Three possible evaluation orderings are also considered and discussed. We implement our idea in our tool Red 4.2 and carry out experiments to compare with other tools and various strategies of Red in both forward and backward analysis. Finally, we discuss the possibility of future improvement.     

Nonlinear topology optimization of layered shell structures

Topology stiffness (compliance) design of linear and geometrically nonlinear shell structures is solved using the SIMP approach together with a filtering scheme. A general anisotropic multi-layer shell model is employed to allow the formation of through-the-thickness holes or stiffening zones. The finite element analysis is performed using nine-node Mindlin-type shell elements based on the degenerated shell approach, which are capable of modeling both single and multi-layered structures exhibiting anisotropic or isotropic behavior. The optimization problem is solved using analytical compliance and constraint sensitivities together with the Method of Moving Asymptotes (MMA). Geometrically nonlinear problems are solved using iterative Newton–Raphson methods and an adjoint variable approach is used for the sensitivity analysis. Several benchmark tests are presented in order to illustrate the difference in optimal topologies between linear and geometrically nonlinear shell structures.     

Gradient-based minimax optimization of planar microstrip structures with the use of electromagnetic simulations

In this contribution we show how the method in [1] for the calculation of sensitivities with respect to geometrical parameters in a method-of-moments-based electromagnetic simulation is applied for the automated design of microwave filters. A key step in the method is the definition of a velocity vector. We explain how these velocity vectors are generated and the consequences on the total efficiency. The application of the calculation of these sensitivities, in automated optimal design is shown through the example of a double-folded stub band-stop filter, a hairpin bandpass filter, and a seventh-order elliptic low-pass filter. © 1997 John Wiley & Sons, Inc. Int J Microwave Millimeter-Wave CAE 7: 29–36, 1997.