An efficient numerical approach for simulating contact in origami assemblages

Origami-inspired structures provide novel solutions to many engineering applications. The presence of self-contact within origami patterns has been difficult to simulate, yet it has significant implications for the foldability, kinematics and resulting mechanical properties of the final origami system. To open up the full potential of origami engineering, this paper presents an efficient numerical approach that simulates the panel contact in a generalized origami framework. The proposed panel contact model is based on the principle of stationary potential energy and assumes that the contact forces are conserved. The contact potential is formulated such that both the internal force vector and the stiffness matrix approach infinity as the distance between the contacting panel and node approaches zero. We use benchmark simulations to show that the model can correctly capture the kinematics and mechanics induced by contact. By tuning the model parameters accordingly, this methodology can simulate the thickness in origami. Practical examples are used to demonstrate the validity, efficiency and the broad applicability of the proposed model.

An Improved Analytical Algorithm on Main Cable System of Suspension Bridge

This paper develops an improved analytical algorithm on the main cable system of suspension bridge. A catenary cable element is presented for the nonlinear analysis on main cable system that is subjected to static loadings. The tangent stiffness matrix and internal force vector of the element are derived explicitly based on the exact analytical expressions of elastic catenary. Self-weight of the cables can be directly considered without any approximations. The effect of pre-tension of cable is also included in the element formulation. A search algorithm with the penalty factor is introduced to identify the initial components for convergence with high precision and fast speed. Numerical examples are presented and discussed to illustrate the accuracy and efficiency of the proposed analytical algorithm.

EFESTS: Educational finite element software for truss structure – Part 3: Geometrically nonlinear static analysis

This article covers the modeling, formulation, solution, and software development of the geometrically nonlinear static finite element method of truss structure. Firstly, we summarize the total Lagrange bar elment formulation, which includes the tangent stiffness matrix and the internal force vector. Secondly, static class diagrams and dynamic sequence diagrams assist students in designing software architecture. Thirdly, the analytical example of the 2-bar truss structure and the numerical example of the 10-bar truss structure are presented to promote students’ understanding of geometrically nonlinear finite element theory and application. Finally, the developed software is free for educational research and can be downloaded from the website: http://mach.jlu.edu.cn/hb_images/xygk/xssz_sz_js.php?id=395 .

ANÁLISE QUALITATIVA DE SÓLIDOS ELASTOPLÁSTICOS SOB DEFORMAÇÃO FINITA UTILIZANDO ELEMENTOS DE BARRA BI-ARTICULADOS 2D E 3D

RESUMO: Neste trabalho apresenta-se uma descrição Lagrangeana Total para retratar sólidos elastoplásticos sob deformação finita. Discretiza-se estes sólidos com elementos de treliça 2D e 3D com o intuito de obter analiticamente o vetor de forças internas e a matriz de rigidez tangente. Assume-se um modelo constitutivo hiperelástico para o estado uniaxial de tensão-deformação, utilizando a tensão de Kirchhoff e a deformação logarítmica, que formam um par energeticamente conjugado. Para descrever as deformações plásticas finitas utiliza-se a hipótese da decomposição multiplicativa do estiramento uniaxial do elemento de treliça. Por fim, apresentam-se algumas simulações numéricas de sólidos 3D discretizados com elementos de treliça 2D e 3D. ABSTRACT: This paper presents a Lagrangian Total description to describe elastoplastic solids under finite deformation. These solids are discretized with truss elements 2D and 3D aiming to obtain analytically internal force vector and the tangent stiffness matrix. Are assumed a hyperelastic constitutive model for the state of uniaxial stress-strain using the Kirchhoff's stress and logarithmic strain, to form a conjugate pair energetically. To describe the finite plastic deformation using the hypothesis of the multiplicative decomposition of uniaxial stretching of the truss element. Finally, we present some numerical simulations of solid 3D discretized with 2D and 3D truss elements.

Uma abordagem analítica para detecção de pontos limites e de bifurcação

RESUMO: Neste trabalho descreve-se analiticamente de maneira detalhada a detecção e a classificação de pontos críticos na trajetória primária de equilíbrio de sistemas estruturais. Utiliza-se a Formulação Lagrangiana Total para descrever a cinemática de um elemento de barra biarticulado 2D. Através desta formulação obtém-se o vetor de forças internas e a matriz de rigidez tangente que levam em conta os efeitos da não linearidade geométrica. Assume-se um modelo constitutivo linear elástico para o estado uniaxial de tensão-deformação, usando a deformação de Green-Lagrange e a tensão axial do segundo tensor de Piola-Kirchhoff que são energeticamente conjugados. Como estudo de caso apresenta-se uma treliça plana hiperestática composta com 3 elementos biarticulados 2D e com dois graus de liberdade. Por fim, determinam-se as condições geométricas e físicas para a coalescência entre os pontos limites e de bifurcação. A principal contribuição deste trabalho é demonstrar a necessidade de compreender melhor os fenômenos não lineares para projetar sistemas estruturais mais seguros. ABSTRACT: Using an analytical this paper describes in detail the detection and classification of critical points in the primary equilibrium path of structural systems. The Total Lagrangian formulation is employed to describe the kinematics of a 2D bar element. With this formulation, the internal force vector and the tangent stiffness matrix including the geometric nonlinearity effects are obtained. An elastic linear constitutive model is assumed for the uniaxial stress-strain state. Such model uses the Green-Lagrange strain tensor and the second Piola-Kirchhoff axial stress tensor which are energetically conjugate tensors. As a study case, the article presents a statically inderminate plane truss discretized with three 2D bar elements. Finally, the geometrical and physical conditions for the coalescence between limit and bifurcation points are determined. The main contribution of this work is to demonstrate the need to better understanding the non linear phenomena. Such understanding is necessary for designing safer structural systems.

AN EFFICIENT CO-ROTATIONAL FEM FORMULATION USING A PROJECTOR MATRIX

Co-rotational finite element (FE) formulations can be seen as a very efficient approach to resolving geometrically nonlinear problems in the field of structural mechanics. A number of co-rotational FE formulations have been well documented for shell and beam structures in the available literature. The purpose of this paper is to present a co-rotational FEM formulation for fast and highly efficient computation of large three-dimensional elastic deformations. On the one hand, the approach aims at a simple way of separating the element rigid-body rotation and the elastic deformational part by means of the polar decomposition of deformation gradient. On the other hand, a consistent linearization is introduced to derive the internal force vector and the tangent stiffness matrix based on the total Lagrangian formulation. It results in a non-linear projector matrix. In this way, it ensures the force equilibrium of each element and enables a relatively straightforward upgrade of the finite elements for linear analysis to the finite elements for geometrically non-linear analysis. In this work, a simple 4-node tetrahedral element is used. To demonstrate the efficiency and accuracy of the proposed formulation, nonlinear results from ABAQUS are used as a reference.

Beam element for large displacement analysis of elasto-plastic frames

The present paper develops a non-linear beam element for analysis of elastoplastic frames under large displacements. The finite element formulations are derived by using the co-rotational approach and expression of the virtual work. The Gauss quadrature is employed for numerically computing the element tangent stiffness matrix and internal force vector. A bilinear stress-strain relationship with isotropic hardening is adopted to update the stress. The arc-length technique based on the Newton-Raphson iterative method is employed to compute the equilibrium paths. A number of numerical examples is employed to assess the performance of the developed element. The effects of plastic action on the large displacement behavior of the structures as well as the expansion of plastic zones in the loading process are discussed.

An Explicit Scheme for Structural Dynamics Analysis

The purpose of this paper is to introduce a new simple explicit single step time integration method with controllable high-frequency dissipation. As opposed to the methods generally used in structural dynamics, with a consistency experimentally chosen of second order, the new method is only first-order-consistent but yields smaller numerical errors in low frequencies and is therefore very efficient for structural dynamic analysis. The new method remains explicit for any structural dynamics problem, even when a non-diagonal damping matrix is used in linear structural dynamics problem or when the non-linear internal force vector is a function of velocities. Convergence and spectral properties of the new algorithm are discussed and compared to those of some well-known algorithms. Furthermore, the validity and efficiency of the new algorithm are shown in a non-linear dynamic example by comparison of phase portraits.