Implicit Finite Element Analysis Research Articles

Comparative dynamic analyses under HELB-induced jet impingement loads

While ANSI/ANS 58.2 has been widely adopted as a standard to protect nuclear power plants against effects of postulated piping rupture, there are still arguments in determination of dynamic fluid characteristics and application to structural integrity assessment. To solve non-conservativeness of jet impingement loads in the current standard, a numerical approach was proposed that is promising from computational fluid dynamics analyses standpoint but further elaboration seems necessary for FE (Finite Element) analyses due to its implicit feature. This study is to investigate appropriate dynamic FE methods and to quantify differences with the previous one under postulated high energy line break conditions. In detail, modal analyses were performed to assess the vibration characteristic of integrated component and structure as well as individual components. Subsequent vibration analyses were carried out by using the modal analyses results of the integrated full model and specific power spectral density data depend on the jet direction and target area. From the comparative study, the dynamic FE analyses provided approximately 47% higher von Mises stresses and 14% larger radial displacements than the implicit FE analyses at the critical location.

Formulation of a mixed-mode multilinear cohesive zone law in an interface finite element for modelling delamination with R-curve effects

A constitutive model for an interface finite element is proposed to enable simulation of delamination in composite materials with R-curve effects. The constitutive model is formulated in the framework of cohesive zone modelling (CZM). In essence, a multilinear CZ law with an arbitrary number of line segments is developed. The CZ law seeks to enable constitutive modelling of failure mechanisms on multiple scales within the fracture process zone and reduce conventional a priori assumptions regarding the shape of the CZ law. The CZ law relies on damage mechanics, an equivalent one-dimensional formulation, and criteria for mode interactions to simulate delamination under mixed-mode loading. Special emphasis is put on the derivation of interpolation formulas and a constitutive tangent stiffness tensor for the multilinear formulation. The constitutive model is implemented in the commercial FE program ANSYS Mechanical, for implicit finite element analysis (FEA), using user-programmable features. The implementation is verified through single interface element numerical studies, and its applicability is demonstrated by simulating an experiment of quasi-static delamination showing large-scale fiber bridging in pure mode I DCB glass-fiber epoxy specimens. Experimental measurements and simulation outputs using the novel cohesive element is compared to those of the conventional bi- and trilinear CZ laws.

Parallelized Finite Element Analysis of Knitted Textile Mechanical Behavior

Direct numerical simulations (DNS) of knitted textile mechanical behavior are for the first time conducted on high performance computing (HPC) using both the explicit and implicit finite element analysis (FEA) to directly assess effective ways to model the behavior of such complex material systems. Yarn-level models including interyarn interactions are used as a benchmark computational problem to enable direct comparison in terms of computational efficiency between explicit and implicit methods. The need for such comparison stems from both a significant increase in the degrees-of-freedom (DOFs) with increasing size of the computational models considered as well as from memory and numerical stability issues due to the highly complex three-dimensional (3D) mechanical behavior of such 3D architectured materials. Mesh and size dependency, as well as parallelization in an HPC environment are investigated. The results demonstrate a satisfying accuracy combined with higher computational efficiency and much less memory requirements for the explicit method, which could be leveraged in modeling and design of such novel materials.

Robust stress integration algorithms for implicit elastoviscoplastic finite element analysis of materials with yield-point phenomenon

In order to describe the yield-point phenomenon accurately in computational elastoviscoplastic analysis, several constitutive models have been proposed and studied in the field of metal plasticity. To precisely predict the behavior of materials with this phenomenon, the constitutive model should be able to depict both the yield-point phenomenon and the Bauschinger effect, rendering it difficult to obtain a converged solution in implicit numerical analysis when the conventional one-point Newton method is used. Here, we propose robust stress integration algorithms that can be used effectively in implicit finite element analysis. The two-point Newton method is employed and compared with other stress integration algorithms using the conventional one-point Newton method or the bisection method. Results demonstrate that they can be reliably used to calculate the solutions of YPP problems that cannot be obtained using conventional iterative methods although these algorithms may require longer computational times.

Role of microstructures in the compression response of three-dimensional foam-formed wood fiber networks.

High-porosity, three-dimensional wood fiber networks made by foam forming present experimentally accessible instances of hierarchically structured, athermal fiber networks. We investigate the large deformation compression behavior of these networks using fiber-resolved finite element analyses to elucidate the role of microstructures in the mechanical response to compression. Three-dimensional network structures are acquired using micro-computed tomography and subsequent skeletonization into a Euclidean graph representation. By using a fitting procedure to the geometrical graph data, we are able to identify nine independent statistical parameters needed for the regeneration of artificial networks with the observed statistics. The compression response of these artificially generated networks and the physical network is then investigated using implicit finite element analysis. A direct comparison of the simulation results from the reconstructed and artificial network reveals remarkable differences already in the elastic region. These can neither be fully explained by density scaling, the size effect nor the boundary conditions. The only factor which provides the consistent explanation of the observed difference is the density and fiber orientation nonuniformities; these contribute to strain-localization so that the network becomes more compliant than expected for statistically uniform microstructures. We also demonstrate that the experimentally manifested strain-stiffening of such networks is due to development of new inter-fiber contacts during compression.

Adjacent Disc Stress Following Floating Lumbar Spine Fusion: A Finite Element Study.

Study DesignExperimental study.PurposeThe study aimed to develop a finite element (FE) model to determine the stress on the discs adjacent to the fused segment following different types of floating lumbar spinal fusions.Overview of LiteratureThe quantification of the adjacent disc stress following different types of floating lumbar fusions has not been reported. The magnitude of the stress on the discs above and below the floating fusion remains unknown.MethodsA computer-aided engineering-based approach using implicit FE analysis was employed to assess the stress on the lumbar discs above and below the floating fusion segment (L4–L5) following anterior and posterior lumbar spine fusions at one, two, and three levels (with and without instrumentation).ResultsBoth discs suprajacent and infrajacent to the floating fusion experienced increased stress, but the suprajacent disc experienced relatively high stress level. Instrumentation increased the stress on the discs suprajacent and infrajacent to the floating fusion, but the magnitude of stress on the suprajacent disc remained relatively high.ConclusionsThe FE model was employed under similar loading and boundary conditions to provide quantitative data, which will be useful for clinicians to understand the probable long-term effects of floating fusions.

The generation of the forming path with the springback compensation in the CNC incremental forming

Aiming at the problem of the springback in the CNC incremental forming, a method for predicting the springback and generating the compensated forming trajectory was proposed based on the explicit and implicit finite element analysis. First, the middle surface of the simulated sheet metal part was reconstructed after the springback simulation with the coordinates of all nodes obtained from the numerical simulations, and the reconstructed middle surface was compared with the middle surface of the theory model to calculate the normal deviation values. Second, the theory model was offset with the unequal distances of the deviation values to generate the compensated facet and the forming path. Once more, the finite element analysis was done until the compensatory effect met the required precision. Meanwhile, aiming at the roughness of the compensated facet caused by the multi-compensation process, the smoothing method was given to guarantee the smooth of the forming path. According to the predicted result of the numerical simulations, three times compensation meets the required precision and the normal deviation value is decreased from 0.696 mm to 0.194 mm. The forming experiments indicate that the springback of the target part in Z direction is decreased from 1.107 mm to 0.427 mm. Therefore, the predicting and compensating method of the springback proposed in the paper is available, which can improve the forming precision of the sheet metal part.

Impact or blast induced fire simulation of bi-directional PSC panel considering concrete confinement and spalling effect

In recent years, numerous explosion and collision related tragedies due to military attack, terrorist bombing, and vehicle accident have occurred all over the world. However, researches on Prestressed Concrete (PSC) infrastructures such as Prestressed Concrete Containment Vessels (PCCVs) and LNG storage tanks under extreme loading such as impact, blast, and fire loading scenario are not being studied sufficiently. Especially, researches on possible secondary fire after bomb explosion or accidental collision on concrete structures has not been performed, while most of the past researches related to extreme loadings on structures focused on ideal isolated extreme loading event researches. Therefore, in this study, the PSC panel such as wall of PCCV and LNG storage tank is analytically evaluated under impact-blast-fire combined loading scenarios. For the analytical simulations of impact/blast and fire loaded behavior of the PSC panel, commercial finite element analysis program of LS-DYNA and MIDAS FEA, respectively, were used. Then, a simulation procedure coupling LS-DYNA and MIDAS FEA using element elimination algorithm is proposed to couple explicit and implicit finite element analyses (FEA) to perform structural analysis of impact or blast induced fire loading scenario. Then, the simulation results from the PSC and RC specimens applied with impact induced fire or blast induced fire loadings were compared with those of undamaged PSC and RC specimens. The results showed that PSC panel was more severely damaged from the fire due to the confining effect of prestressing forces. Confining effect increased the thermal conductivity of concrete due to compaction of concrete. Also, the spalling of concrete caused by impact loading followed by fire loading were implemented to the simulation model using the element elimination procedure proposed in the study. The simulation results obtained from the proposed simulation procedure agree well with the impact induced fire test results.

Revisiting the Numerical Convergence of Cohesive-Zone Models in Simulating the Delamination of Composite Adhesive Joints by Using the Finite-Element Analysis

Delamination is the dominating failure mechanism in composite adhesive joints. A deep insight into the delamination failure mechanism requires advanced numerical methods. Currently, cohesive-zone models (CZMs), in combination with the finite-element analysis (FEA), have become powerful tools for modeling the initiation and growth of delaminations in composites. However, ensuring the numerical convergence in the CZMs used for a delamination analysis of three-dimensional (3D) composite structures is always a challenging issue due to the “snap-back” instability in the nonlinear implicit FEA, which arises mainly from the cohesive softening behavior. Based on the midplane interpolation technique, first numerical techniques for implementing 3D bilinear and exponential CZMs by using ABAQUS-UEL (user element subroutine) are developed in this paper. In particular, a viscous regularization by introducing the damping effect into the stiffness equation is used to improve the convergence. Two examples, a single-lap composite joint and a composite skin/stiffener panel under tension, demonstrate the numerical technique developed. Then, the effect of cohesion parameters on the numerical convergence based on the viscous regularization is studied.

A Numerical Analysis on the Local Deformation of a Spacer Grid Structure for Nuclear Fuel Cells

The result of a preliminary numerical investigation on local deformation characteristics of a multi-layered spacer-grid structure with five guide tubes is reported based on implicit finite element analysis. For the numerical analysis, displacements of top and bottom cross sections of each guide tube in a single-layer model were constrained while a lateral displacement was imposed on the single layer. Unlike the impact hammer test that is generally employed to characterize the deformation characteristics of the space-grid structure, the buckling phenomenon occurs locally in this study; it takes place at the inner grids around each tube and the degree of bucking is more apparent for tubes near the lateral surface where the lateral displacement was imposed.

Sparse matrix factorization in the implicit finite element method on petascale architecture

The performance of the massively parallel direct multifrontal solver Watson Sparse Matrix Package (WSMP) for solving large sparse systems of linear equations arising in implicit finite element method on unstructured (free) meshes in solid mechanics was evaluated on one of the most powerful supercomputers currently available to the open science community—the sustained petascale high performance computing system of Blue Waters. We have performed full-scale benchmarking tests up to 65,536 cores using assembled global stiffness matrices and load vectors ranging from 11 to 40 million unknowns extracted from “real-world” commercial implicit finite element analysis (FEA) applications. The results show that a direct multifrontal factorization method with a hybrid parallel implementation in WSMP performs exceedingly well on a petascale high-performance computing (HPC) system, and delivers superior factorization time and parallel scalability, thus opening the door for the high fidelity modeling of complex industrial structures and assemblies in real scale.

Long-Term Effects of Segmental Lumbar Spinal Fusion on Adjacent Healthy Discs: A Finite Element Study.

Study DesignExperimental study.PurposeThe aim of the study was to develop a finite element (FE) model to study the long-term effects of various types of lumbar spinal interventions on the discs adjacent to the fused segment.Overview of LiteratureEarlier FE studies have been limited to one particular type of fusion and comparative quantification of the adjacent disc stresses for different types of surgical interventions has not been reported.Methods A computer aided engineering (CAE) based approach using implicit FE analysis assessed the stresses in the lumbar discs adjacent to the fused segment following anterior and posterior lumbar spine fusions at one, two and three levels (with and without instrumentation).ResultsIt was found that instrumentation and length of fusion were the most significant factors in increasing adjacent level stresses in the lumbar discs.ConclusionsIn the present study, a calibrated FE model that examined spinal interventions under similar loading and boundary conditions was used to provide quantitative data which would be useful for clinicians to understand the probable long-term effect of their choice of surgical intervention.

Finite element analysis of sliding distance and contact mechanics of hip implant under dynamic walking conditions.

An explicit finite element method was developed to predict the dynamic behavior of the contact mechanics for a hip implant under normal walking conditions. Two key parameters of mesh sensitivity and time steps were examined to balance the accuracy and computational cost. Both the maximum contact pressure and accumulated sliding distance showed good agreement with those in the previous studies using the implicit finite element analysis and analytical methods. Therefore, the explicit finite element method could be used to predict the contact pressure and accumulated sliding distance for an artificial hip joint simultaneously in dynamic manner.

A new approach to modeling isotropic damage for Mullins effect in hyperelastic materials

In this work we present a new approach to damage mechanics in hyperelastic materials and an efficient numerical procedure for modeling the Mullins effect in isochoric, isotropic materials. The formulation is based on the idea that both the virgin loading and the damaged unloading–reloading behavior may be measured, but only the unloading–reloading curve corresponds to hyperelastic behavior. The damaged unloading–reloading curve is the true hyperelastic behavior and may be described by any suitable hyperelastic constitutive model. We employ a spline-based formulation which is known to exactly capture the behavior. The virgin loading curve, which does not correspond to hyperelastic behavior and involves damage evolution is only employed to compute the energy release rate. The procedure does not employ any material parameter (and hence no parameter-fitting procedure) or any explicit damage evolution function. We highlight similarities and differences of the present model with usual damage mechanics models and with pseudo-elasticity. As a result of the detailed computational procedure which simply involves the solution of a nonlinear scalar function, the virgin loading curve and the damaged unloading–reloading curves are exactly captured. The computational algorithm for three-dimensional implicit finite element analysis is also addressed in detail. Examples show that there is no significant increase in computational effort respect to a pure hyperelastic model.

Creep–rupture model of aluminum alloys: Cohesive zone approach

In this article, a creep–rupture model of aluminum alloys is developed using a time-dependent cohesive zone law. For long-term creep rupture, a time jump strategy is used in a cohesive zone law. Stress–rupture scatter of aluminum alloy 4032-T6 is fitted with a power law form. Then, change in the slope of a stress-rupture line is identified on a log–log scale. Implicit finite element analysis is employed with a model containing a cohesive zone. Stress–rupture curves at various given temperatures are calculated and compared with experimental ones. Results show that a proposed method allows predicting creep–rupture life of aluminum alloys.

A study of intra-laminar damage in double-lap, multi-bolt, composite joints with variable clearance using continuum damage mechanics

A damage modelling approach, based on a continuum damage model (CDM) formulation, is proposed and applied to the problem of double-lap, multi-bolt, fibre-reinforced composite joints with variable clearances, subjected to quasi-static tensile loading. A new method of dealing with fibre failure is included in the CDM model, which is implemented in a commercial implicit finite element analysis code. The model is validated at element and structural levels by comparing with experimental data. It has been found that, for the joints examined in this paper, our formulation is capable of modelling development of damage from bearing failure onset all the way to ultimate catastrophic net-tension failure without numerical problems, which is an advance over previous work. The predictions from the CDM model of net-tension failure modes and ultimate loads are in good agreement with those from the experiments. Furthermore the model is capable of explaining some non-intuitive experimental findings, such as the larger energy absorption obtained in joints with higher clearances.

Low intrusive coupling of implicit and explicit time integration schemes for structural dynamics: Application to low energy impacts on composite structures

Simulation of low energy impacts on composite structures is a key feature in aeronautics. Unfortunately it involves very expensive numerical simulations: on the one side, the structures of interest have large dimensions and need fine volumic meshes (at least locally) in order to properly capture damage. On the other side, explicit simulations are commonly used to lead this kind of simulations (Lopes et al., 2009 [1]; Bouvet, 2009 [2]), which results in very small time steps to ensure the CFL condition (Courant et al., 1967 [3]). Implicit algorithms are actually more difficult to use in this situation because of the lack of smoothness of the solution that can lead to prohibitive number of time steps or even to non-convergence of Newton-like iterative processes. It is also observed that non-smooth phenomena are localized in space and time (near the impacted zone). It may therefore be advantageous to adopt a multiscale space/time approach by splitting the structure into several substructures with their own space/time discretization and their own integration scheme. The purpose of this decomposition is to take advantage of the specificities of both algorithms families: explicit scheme focuses on non-smooth areas while smoother parts (actually linear in this work) of the solutions are computed with larger time steps with an implicit scheme. We propose here an implementation of the Gravouil–Combescure method (GC) (Combescure and Gravouil, 2002 [4]) by the mean of low intrusive coupling between the implicit finite element analysis (FEA) code Zset/Zébulon (Z-set official website, 2013 [5]) and the explicit FEA code Europlexus (Europlexus official website, 2013 [6]). Simulations of low energy impacts on composite stiffened panels are presented. It is shown on this application that large time step ratios can be reached, thus saving computation time.

Total Lagrangian Finite Element Formulation of the Flory-Rehner Free Energy Function

The total Lagrangian finite element implementation of the Flory-Rehner free-energy function in the framework of a hyperelastic material model is addressed. It is explicitly given all the equations required to implement this material model in an implicit nonlinear finite element analysis, particularly, it is shown how to derive the so-called algorithmic or consistent linearized tangent moduli in the Lagrangian description. Some analytical and numerical results for different boundary-value problems are presented to validate the implementation.

Incrementally objective implicit integration of hypoelastic–viscoplastic constitutive equations based on the mechanical threshold strength model

The present paper focuses on the development of a fully implicit, incrementally objective integration algorithm for a hypoelastic formulation of \(J_{2}\)-viscoplasticity, which employs the mechanical threshold strength model to compute the material’s flow stress, taking into account its dependence on strain rate and temperature. Heat generation due to high-rate viscoplastic deformation is accounted for, assuming adiabatic conditions. The implementation of the algorithm is discussed, and its performance is assessed in the contexts of implicit and explicit dynamic finite element analysis, with the aid of example problems involving a wide range of loading rates. Computational results are compared to experimental data, showing very good agreement.

Failure analysis of carbon fiber/epoxy composite cylindrical laminates using explicit finite element method

Based on continuum damage mechanics, the progressive failure analysis using explicit finite element method is performed to predict the failure properties and burst strengths of aluminum–carbon fiber/epoxy composite cylindrical laminate structures in terms of three composite pressure vessels with different geometry sizes. The failure analysis employs the Hashin damage initiation criterion and the fracture energy-based damage evolution law for composite layers. The numerical convergence problem is solved by introducing viscous damping effect into finite element equations for strain softening phenomenon. Effects of the calculation time and mesh sizes on the failure properties of composite laminates are explored. In addition, the predicted failure strengths of composite laminates using explicit finite element analysis are also compared with those by experiments and implicit finite element analysis.