Impact load characterization for security barrier performance assessment through simulations using generic vehicle models

The threat stemming from the use of vehicles as a weapon in urban environments may be mitigated by employing properly designed protective structures such as bollards, street furniture or landscaping options. In order to assess the performance of a barrier resistance to a vehicle impact, the initial step involves characterizing the load on the barrier. To this aim, two recently developed generic vehicle models are utilized to conduct numerical simulations of vehicle impacts on a security barrier. Various impact configurations are examined and compared based on force-time functions. In addition to comparing the impact loadings in terms of peak forces, comparisons are also done in terms of equivalent static loads, determined by computing the dynamic load factors (DLF). The study provides new insights into the characterization of vehicle impact loads on security barriers, which could improve current engineering practices in the field

On the parametrization of velocity-based constraints in EUROPLEXUS

This report presents some notes on the treatment of constraints by Lagrange Multipliers in EUROPLEXUS, a computer code jointly developed by CEA DMT Saclay and by JRC. Recently, EPX is being used to simulate vehicle crash against obstacles such as road barriers for safety and security studies. These studies involve the treatment of complex contact-impact scenarios, which in EPX are typically modelled by the method of Lagrange Multipliers (LM), although other methods (e.g. penalty-based) are also available in the code. Recent investigations concerning (elastic) impact tests have shown that the LM-based mid-step velocity constraints strategy (the default one in EPX) leads to smooth solutions (few oscillations), but some energy is lost at the moment of the impact, namely each time some previously free nodes get into contact, so that the total energy of the system is not exactly conserved. On the contrary, the full-step velocity constraints strategy produces a lot of oscillations (if the material is elastic) but it conserves much better the energy of the system. Therefore, it has been proposed to implement a parametrization of the velocity constraints, allowing to choose an intermediate value of the time at which the constraints are imposed. The goal is to reduce the spurious energy dissipation observed with the mid-step strategy, but hopefully without triggering the large oscillations produced by the full-step strategy.JRC.E.3 - Safety and Security of Building

A revision of the SOLI model of rigid bodies in EUROPLEXUS

This report presents a revision and re-formulation of the so-called old model for the simulation of rigid bodies (SOLI model) in the EUROPLEXUS code. This model is based on the substitution ( decomposition ) of the discretized rigid body by a mechanically equivalent set of four points in 3D space (two points in 2D space, but the 2D version of the model has not been implemented yet). The rigidity of the equivalent mechanical system is enforced by a suitable set of constant-distance constraints on the equivalent points via the Lagrange multipliers method. Although EUROPLEXUS is primarily dedicated to the simulation of deformable mechanical sys ems (both uids and structures), it is not infrequent that at least some (solid) parts of the numerical model behave at least approximately as rigid, non-deformable bodies. A class of problems where a rigid model might be useful is that of crashes. For example, when treating the impact and crash of vehicles in the framework of protection of public spaces against terrorist attacks, e.g. by means of road barriers, some parts (typically the vehicle body and some components) are highly deformable while others (e.g. the engine) are comparatively much sti er and could be treated as rigid. Modelling such nearly-rigid parts as a very sti solid severely penalizes the transient explicit solution because of the drop it causes on the stability step of the time integration procedure. What is needed is the possibility of treating a rigid body in a more rigorous, but also in a more general manner than by simple blockages. The present report presents a possible approach to this task (the SOLI model) that was historically implemented in EUROPLEXUS and has been recently completely revised and enhanced by several new features (NODE, ELE2, HYBR, FEXT directives) in order to make it more functional to vehicle crash simulations for the protection of public spaces.JRC.E.3 - Built Environmen

Correction of friction implementation in the LM-based PINB contact model of EUROPLEXUS

This report presents some notes on the use of pinball-based contact-impact models with friction by the method of Lagrange Multipliers (LM) in EUROPLEXUS [1] (also abbreviated as EPX). Recently, EPX is being used to simulate vehicle crash against obstacles such as road barriers for safety and security studies. These studies involve complex contact-impact scenarios, which in EPX are typically modelled by the method of Lagrange Multipliers (LM), although other methods (e.g. penalty-based) are also available in the code. In some test cases, the effect of friction may be important and it must be included in the numerical simulations. Recent simulations concerning (elastic) impact tests with friction using the LM-based version of the pinball contact model have revealed some malfunctionings, which may be attributed to the friction model. Therefore, it has been proposed to completely review the formulation and implementation of the friction algorithm in the LM-based pinball contact model. The model used a single-tangent formulation that could lead to problems. It has now been replaced by the same friction model used by the sliding surface model (GLIS), which uses a more robust two-tangent formulation. Despite this improvement, and the correction of other minor issues, the LM version of the PINB contact model remains somewhat fragile in cases with friction, and the penalty-based version should preferably be used in industrial applications.JRC.E.3 - Safety and Security of Building

A Stochastic Multi-scale Approach for Numerical Modeling of Complex Materials - Application to Uniaxial Cyclic Response of Concrete

In complex materials, numerous intertwined phenomena underlie the overall response at macroscale. These phenomena can pertain to different engineering fields (mechanical , chemical, electrical), occur at different scales, can appear as uncertain, and are nonlinear. Interacting with complex materials thus calls for developing nonlinear computational approaches where multi-scale techniques that grasp key phenomena at the relevant scale need to be mingled with stochastic methods accounting for uncertainties. In this chapter, we develop such a computational approach for modeling the mechanical response of a representative volume of concrete in uniaxial cyclic loading. A mesoscale is defined such that it represents an equivalent heterogeneous medium: nonlinear local response is modeled in the framework of Thermodynamics with Internal Variables; spatial variability of the local response is represented by correlated random vector fields generated with the Spectral Representation Method. Macroscale response is recovered through standard ho-mogenization procedure from Micromechanics and shows salient features of the uniaxial cyclic response of concrete that are not explicitly modeled at mesoscale.Comment: Computational Methods for Solids and Fluids, 41, Springer International Publishing, pp.123-160, 2016, Computational Methods in Applied Sciences, 978-3-319-27994-

Methodology for numerical simulations of vehicle impact on security barriers considering soil-barrier interaction

Vehicle Security Barriers (VSBs) are frequently utilised in urban settings to defend against vehicular terrorist attacks. They may come in various forms, including bollards, street furniture and landscape features. The effectiveness of such barriers is usually assessed through a singular vehicle impact test, in accordance with various test standards and guidelines. The substantial cost of impact tests significantly restricts the amount of impact scenarios that can be analysed. Numerical simulation methods can substitute physical tests by virtually testing barriers. Various practical configurations can be assessed, such as different impact speeds, impact angles, and site conditions. As stated in several VSB-related impact test standards, such as the new ISO 22343, soil conditions can significantly influence the performance of VSBs. Nevertheless, test standards frequently lack comprehensive guidance on how to deal with soil conditions. However, it is crucial to be aware of the mechanical properties of the soil, especially when dealing with numerical simulations of the interaction between the soil and the embedded barrier foundation. This report investigates the relationship between soil conditions and the performance, underscores the importance of considering soil conditions during the VSB design, highlights the need for consistent soil assessments, and provides guidance for enhancing security in urban areas. While numerical vehicle models of specific vehicle categories are already available and can be utilized, the focus is on a methodology for numerically modelling the surrounding soil domain. The critical role of the soil domain dimensions, the influence of the finite element size in the meshed soil domain, and the soil modelling strategies are investigated. The basics on the characteristics of coarse-grained soil is presented, which are usually used in traffic infrastructure such as roadways or walkways. Sensitivity studies further highlight the role of soil material properties, such as the angle of internal friction, the angle of dilatancy, the cohesion and the Young's modulus on the VSB response. Furthermore, the influence of impacting vehicle’s type on the VSB response is demonstrated, i.e., EU-truck type versus US-truck type. While the primarily focus is on the soil conditions, the report also addresses the vital role of the surface courses of traffic infrastructure, such as asphalt surface courses. Furthermore, it discusses the interaction between the VSB-foundation and underground infrastructure, encompassing elements like sewers and gas networks. This report contributes towards the goal of further using numerical simulations to assess the performance of vehicle security barriers. It demonstrates that numerical simulations can be a useful tool for studying variations in the soil type, drawing inspiration from practical applications, e.g., due to different traffic infrastructure constructions. The findings emphasize the need for soil-focused guidelines for the testing and installation of vehicle security barriers.JRC.E.3 - Safety and Security of Building

Modélisation multi-échelles de structures hétérogènes aux comportements anélastiques non-linéaires

In design of civil and mechanical engineering structures it is very important to be able to adequately predict their failure mechanisms. However, materials used are often strongly heterogeneous, like for instance the concrete or metalic composites, and are consequently difficult to model when submitted to extreme loading. In this work we propose a new multi scale strategy adapted for this kind of problems. According to the approach we couple a finite element method (FEM) model at the structural scale (micro) with a very fine model at the scale of the microstructure (micro), also based on the FEM. It allows us to model the micro scale much more accurately than what is possible with phenomenological approaches or analytical homogenisation methods. Since in general the major incovenience of such an approach is a very high computational cost, we consider three aspects which enable its application to realistic situations. First of all, the variational formulation allows for using the fine model only in parts of the structure, where the scales are really strongly coupled, whereas we use a homogenised model elsewhere. In addition, we adapt the FEM on the micro scale by employing the so called structured approach, with which we can significanlty reduce the number of degrees of freedom by retaining the came accuracy. The third aspect concerns the implementation of the method in a finite element code, adapted for parallel machines, with which we could run analyses of a size comparable to realistic situations. Finally, we show in which way this approach can bc used for inclusion shape optimisation of a composite material.Lors de la conception des structures du génie civil et du génie mécanique il est très important de pouvoir prévoir comportement près de leur état ultime. Or, les matériaux utilisés sont souvent fortement hétérogènes, comme par exemple le béton et les métaux composites, et sont en conséquence difficiles à modéliser en cas de sollicitations extrêmes. Dans ce travail nous proposons une nouvelle stratégie multi échelles adaptée pour ce type des problèmes. Selon l'approche on couple un modèle basé sur la méthode des éléments finis (MEF) à l'échelle de la structure (macro) avec un modèle très fin à l'échelle de la microstructure (micro), lui aussi basé sur la MEF. Cela nous permet de modéliser l'échelle micro beaucoup plus fidèlement que ce qui est possible avec les approches phénoménologiques ou avec des méthodes d'homogénéisation analytique. L'inconvénient principal d'une telle approche étant en général le coût de calcul très élévé, nous considérons trois aspects rendant possible une application aux situations réelles. Premièrement, la formulation variationnelle permet de n'utiliser le modèle fin que dans les parties restreintes de la structure, là où les échelles sont vraiment fortement couplées, tout en gardant un modèle homogénéisé ailleurs. Deuxièment, nous adaptons la MEF sur l'échelle micro en employant l'approche dite structurée, avec laquelle on peut réduire le nombre de degrés de liberté d'une manière importante en gardant la même précision. Le troisième aspect porte sur l'implantation de la méthode dans un code de calcul, adaptée pour des machines parallèles, ce qui nous a permis de lancer les analyses d'une taille comparable à celle des situations réelles. Enfin, nous montrons de quelle manière cette approche peut être utilisée dans l'optimisation de forme des inclusions d'un matériau composite

Modélisation multi-échelles de structures hétérogènes aux comportements anélastiques non-linéaires

Lors de la conception des structures du génie civil et du génie mécanique il est très important de pouvoir prévoir comportement près de leur état ultime. Or, les matériaux utilisés sont souvent fortement hétérogènes, comme par exemple le béton et les métaux composites, et sont en conséquence difficiles à modéliser en cas de sollicitations extrêmes. Dans ce travail nous proposons une nouvelle stratégie multi échelles adaptée pour ce type des problèmes. Selon l'approche on couple un modèle basé sur la méthode des éléments finis (MEF) à l'échelle de la structure (macro) avec un modèle très fin à l'échelle de la microstructure (micro), lui aussi basé sur la MEF. Cela nous permet de modéliser l'échelle micro beaucoup plus fidèlement que ce qui est possible avec les approches phénoménologiques ou avec des méthodes d'homogénéisation analytique. L'inconvénient principal d'une telle approche étant en général le coût de calcul très élévé, nous considérons trois aspects rendant possible une application aux situations réelles. Premièrement, la formulation variationnelle permet de n'utiliser le modèle fin que dans les parties restreintes de la structure, là où les échelles sont vraiment fortement couplées, tout en gardant un modèle homogénéisé ailleurs. Deuxièment, nous adaptons la MEF sur l'échelle micro en employant l'approche dite structurée, avec laquelle on peut réduire le nombre de degrés de liberté d'une manière importante en gardant la même précision. Le troisième aspect porte sur l'implantation de la méthode dans un code de calcul, adaptée pour des machines parallèles, ce qui nous a permis de lancer les analyses d'une taille comparable à celle des situations réelles. Enfin, nous montrons de quelle manière cette approche peut être utilisée dans l'optimisation de forme des inclusions d'un matériau composite.In design of civil and mechanical engineering structures it is very important to be able to adequately predict their failure mechanisms. However, materials used are often strongly heterogeneous, like for instance the concrete or metalic composites, and are consequently difficult to model when submitted to extreme loading. In this work we propose a new multi scale strategy adapted for this kind of problems. According to the approach we couple a finite element method (FEM) model at the structural scale (micro) with a very fine model at the scale of the microstructure (micro), also based on the FEM. It allows us to model the micro scale much more accurately than what is possible with phenomenological approaches or analytical homogenisation methods. Since in general the major incovenience of such an approach is a very high computational cost, we consider three aspects which enable its application to realistic situations. First of all, the variational formulation allows for using the fine model only in parts of the structure, where the scales are really strongly coupled, whereas we use a homogenised model elsewhere. In addition, we adapt the FEM on the micro scale by employing the so called structured approach, with which we can significanlty reduce the number of degrees of freedom by retaining the came accuracy. The third aspect concerns the implementation of the method in a finite element code, adapted for parallel machines, with which we could run analyses of a size comparable to realistic situations. Finally, we show in which way this approach can bc used for inclusion shape optimisation of a composite material.CACHAN-ENS (940162301) / SudocSudocFranceF

Constitutive model of coupled damage-plasticity and its finite element implementation

International audienceIn this work we present a general theoretical framework for developing a constitutive model capable of coupling two basic types of inelastic behaviour, plasticity and damage. We elaborate upon the main novelty with respect to the previous models of this type, which pertains to a systematic use of criteria for defining the elastic domain, both for plasticity and damage, which can be adapted to a very wide variety of engineering materials, from metals with voids on one side to concrete compaction on the other side. The numerical implementation is first presented for a simple one-dimensional case, and subsequently extended to 2D and 3D criteria which are adequate for either metals or concrete