Study of the possible relationships between tramway front-end geometry and pedestrian injury risk

OBJECTIVES: The aim of this article is to report on the possible relationships between tramway front-end geometry and pedestrian injury risk over a wide range of possible tramway shapes. METHODS: To study the effect of tramway front-end shape on pedestrian injury metrics, accidents were simulated using a custom parameterized model of tramway front-end and pedestrian models available with the MADYMO multibody solver. The approach was automated, allowing the systematic exploration of tramway shapes in conjunction with 4 pedestrian sizes (e.g., 50th percentile male or M50). RESULTS: A total of 8,840 simulations were run, showing that the injury risk is more important for the head than for other body regions (thorax and lower extremities). The head of the M50 impacted the windshield of the tramway in most of the configurations. Two antagonist mechanisms affecting impact velocity of the head and corresponding head injury criterion (HIC) values were observed. The first is a trunk rotation resulting from an engagement of the lower body that can contribute to an increase in head velocity in the direction of the tram. The second is the loading of the shoulder, which can accelerate the upper trunk and head away from the windshield, resulting in lower impact velocities. Groups of design were defined based on 2 main parameters (windshield height and offset), some of which seem more beneficial than others for tramway design. The pedestrian size and tramway velocity (30 vs. 20?km/h) also affected the results. CONCLUSIONS: When considering only the front-end shape, the best strategy to limit the risk of head injury due to contact with the stiff windshield seems to be to promote the mechanism involving shoulder loading. Because body regions engaged vary with the pedestrian size, none of the groups of designs performed equally well for all pedestrian sizes. The best compromise is achieved with a combination of a large windscreen offset and a high windscreen. Conversely, particularly unfavorable configurations are observed for low windshield heights, especially with a large offset. Beyond the front-end shape, considering the stiffness of the current windshields and the high injury risks predicted for 30?km/h, the stiffness of the windshield should be considered in the future for further gains in pedestrian safety

Sensitivity analysis of complex engineering systems: Approaches study and their application to vehicle restraint system crash simulation

Restricted by high calculation cost of single engineering model run and large number of model runs for sampling-based Sensitivity Analysis (SA), qualitative SA are used for parameter study of the Vehicle Restraint System (VRS) and quantitative SA of such models has always been a challenge. Sequential approaches are proposed for SA of complex systems and the SA of a VRS is realized: sampling-based SA methods are discussed; SA of a simple three points dynamic bending test model is realized, the aims are to compare different two-level screening methods and put into practice the sequential SA; crash test FE model of a VRS is created and used for SA; influential uncertain parameters of the VRS are identified qualitatively through screening analyses (SA with Two-level screening and Morris Analysis), and Sobol' indices are used to quantify the influence of influential parameters with Kriging metamodeling. The uncertain parameters which contribute the most to robustness of the VRS are identified and their influences are quantified by combining screening analyses and Sobol' Indices

Incertitudes de mesures sur les lois de comportement obtenues avec SHPB (S05b)

Les barres de Hopkinson permettent de caractériser des matériaux à vitesse de sollicitation élevée. La détermination de la loi de comportement repose sur un certain nombre d'hypothèses et sur un post-traitement important des données collectées lors de l'essai. L'erreur de mesure sur les lois de comportement obtenues figure très rarement. Cet article présente une méthode et une première estimation de l'incertitude de mesure associée à un tel essai

Random dynamical system in time domain: A POD-PC model

Propagating uncertainties through mechanical systems has been widely studied for the last thirty years. In particular metamodels based on polynomial chaos expansion (PCE) have been successfully developed in the context of both intrusive and non-intrusive methods. However, modelling random dynamical systems is much more challenging and requires increasing computational resources when the time integration becomes longer. Therefore separating the time aspect of the dynamical response and the random contributions is an appealing approach, which has been used in this paper. Thus, a non-intrusive method is proposed by associating a proper orthogonal decomposition (POD) and a PCE. The POD-PC model was applied on three examples. On two examples, the method was very efficient not only in calculating the first two statistical moments, but also in estimating the responses corresponding to several samples of the random parameters. As to the last example, the Kraichnan-Orszag three-mode problem, the model was able to estimate well the evolution of the first two statistical moments for the first part of the time duration, but a noticeable discrepancy occurred for the remaining part. It seems that the model is more appropriate to estimate the transient response of a random dynamical system than the steady-state response

Multi-Objective and Non-deterministic Optimization of Vehicle Restraint Systems

Vehicle Restraint Systems (VRS) are specially designed to restrain an errant vehicle by dissipating or absorbing the impact energy and redirecting the vehicles to reduce crash accident severity and protect the roadside equipment. Before being installed on the roadside, a VRS must be tested by crashing with vehicles to evaluate its performances for severity reduction in traffic accident. Europe Norm, EN1317, normalized the crash test conditions, and defined the qualitative and quantitative performance criteria of the device. Dynamic simulation of the crash test has been used for development of new VRS. Uncertainties exist in the VRS: a crash test of the VRS can't be repeated even under the same impact condition; as for the numerical simulations, in fact a model cannot be validated to ?have simulated the crash test accurately'; what's more, uncertainty is inevitable and it may significantly influence the reliability or robustness of a design. The crash test of a steel VRS has been carried out by LIER-TRANSPOLIS according to the norm EN1317. Considering the existence of uncertain factors, the device is optimized with Multi-Objective Non-deterministic Optimization (MONO) approach. The challenges for MONO of complex engineering systems such as the VRS include: high calculation cost of model simulation; numerous uncertain factors. In the previous studies: The crash test of VRS was simulated by LS-DYNA with a simplified VRS & Vehicle model. The numerical model has been used for Sensitivity Analysis (SA) of the VRS: uncertainties in mechanical properties of material, in tolerances of fabrication, in installation conditions of the device were considered and eleven uncertain factors were chosen. SA helps to identity the factors whose uncertainties have great influence on the robustness of the VRS. The two influential uncertain factors were identified and their influences were quantified after the SA. The MONO of the VRS will be discussed in the article: The ?objectives' of the optimization are to increase the capability of the VRS in reducing crash accident severity, and in the same time to minimize the deformation of the VRS during the crash process; Dimensions of the VRS components are chosen as the ?design variables' Robustness of the VRS is mainly influenced by the two influential uncertain factors identified after the SA. Considering influences of the two uncertain factors, along with the mass of device (i.e. cost of production), the robustness of the device are defined as the constraints of the design; Kriging interpolation is used to create the surrogate model of the crash test. Genetic Algorithm is used for the multi-objective optimization; Generalization of impact conditions: the device is optimized under the impact condition specified by EN1317. Performances of the optimized design are evaluated under different impact conditions. Simulations considering the variations of the uncertain factors help to evaluate the robustness of a design and give a cloud of results in which the result of an experimental test could be contained; SA helps to identity the influential uncertain factors; with robustness of the device defined as the constraints of design, the non-deterministic approach optimized the performances of the VRS with their variations controlled under the defined thresholds; the norm EN1317 provides a guideline for the design of VRS, robust analysis and generalization analysis provide useful information about the performance of a design and the norm EN1317 could be enriched

Contrôle des vibrations de charge utile sur lanceur spatial

Les lanceurs spatiaux sont soumis à un certain nombre d’excitations complexes durant les différentes phases de vie du produit. Ces excitations sont transmises à la charge utile par voie solidienne ou aérienne. Pour assurer la protection de la charge utile, l’architecture du lanceur étant figée au début du projet, l’amélioration des comportements dynamiques passe par l’introduction de systèmes secondaires. La partie essentielle des travaux de thèse est donc consacrée à l’implantation optimale de systèmes capables de diminuer les réponses vibratoires en utilisant des modèles adaptés. C’est pourquoi une méthode de double synthèse modale est mise en place, permettant ainsi de calculer la réponse vibratoire de la structure à l’aide de bases réduites et offrant des performances améliorées par rapport aux méthodes classiques. L’ajout d’un dispositif amortissant local nécessite la prise en compte d’une ou plusieurs modifications structurales dans le modèle, une méthode dédiée est alors développée. Le choix du dissipateur se porte sur un dissipateur frottant. Un prototype est conçu et réalisé. Il est dans un premier temps caractérisé seul et le modèle de comportement identifié est un modèle constitué d’un ressort en série avec un patin ; la loi de frottement adaptée est une loi de Coulomb simple. En parallèle, une maquette représentative du dernier étage d’un lanceur est dimensionnée et réalisée. Le frotteur est alors monté en pied de propulseur de la maquette et permet une diminution significative des vibrations de la charge utile au passage du mode de propulseur.Space launchers undergo a certain amount of complex excitations during their lifecycle. These excitations are transmitted to the payload in a structure-born or air-born way. To improve the dynamic behaviour and thus ensure the protection of the payload, secondary systems must be added to the launcher – indeed, the architecture of the launcher is fixed at the beginning of the project. The essential part of this thesis work is dedicated to the optimal fitting of a system capable of reducing the vibration response of the payload, using appropriate models. Therefore a double modal synthesis method is implemented, allowing to calculate the vibrational response of the structure with reduced bases and offering improved performances over conventional methods. The addition of a local damping device requires the consideration of one or more structural modifications in the model, a dedicated method is thus developped along with a specific continuation algorithm. A friction damper is retained, a prototype is designed and built. It is first characterized alone ; the identified behaviour is that of a spring in series with a dry friction element, a simple Coulomb friction law enables to reproduce the experimental curves. A scale model of the launcher’s last stage is designed and built. The friction device is then mounted inside the scale model and leads to a significant reduction of the payload vibration levels

An Impact Test to Determine the Wave Speed in SHPB: Measurement and Uncertainty

Post-processing the strain waves in split Hopkinson pressure bars to get the stress-strain in the sample requires the knowledge of the characteristic wave speed c0 in the measuring bars. In the context of metrology, the measurement uncertainty in the value of c0 must be assessed. The aim is to minimize this uncertainty, which depends on the way c0 is determined, as it has an impact on the uncertainty in the final stress and strain in the sample. The frequency domain method we introduce is based on an impact test on a single bar. The frequency spectrum of the impact response of the bar clearly exhibits the longitudinal resonant frequencies of the bar. The experimental dispersion curve is deduced from the spectrum and an optimization procedure was applied to determine the wave speed c0 along with the Poisson's ratio and the uncertainty in the wave speed. This method gives a relative uncertainty in c0 lower than 0.05%: it is mainly related to the uncertainty in the length of the bar (which is hard to reduce when using standard meter tape), which prevails over the uncertainty in the resonance frequencies of the bar. A precise value of the wave speed c0 and of the associated measurement uncertainty is an important step in the context of split Hopkinson pressure bars if we want to precisely assess the final uncertainty in Split Hopkinson pressure bars test results, which is at present scarcely done

Uncertainty quantification and global sensitivity analysis of longitudinal wave propagation in circular bars. Application to SHPB device

The experimental characterisation of materials at intermediate strain rates often implies the use of split Hopkinson pressure bars. Shifting the measured pulses in the bars requires to take dispersion into account. However, the dispersion correction, in the case of linear elastic bars, relies on physical parameters which are measured with a given accuracy (bar velocity c0, bar radius r0, Poisson's ratio ? and propagation distance x). The object of the present article is to evaluate the influence of the uncertainty on these parameters and quantify the uncertainty on the resulting propagated pulse. A common dispersion correction method is based on a nonlinear curve fitting approach of the real dispersion curve. The accuracy of this approximate method is first assessed and we show that it can lead to non negligible errors on the computation of the propagated pulse in the context of SHPB (typically a few percent). The numerical solving of the dispersion equation is therefore preferred. We then use Latin hypercube sampling to perform an uncertainty quantification (UQ) on the propagated pulse. The next step is a Sobol' sensitivity analysis (SA) to identify the most influential parameters on the error on the propagated pulse. For an identical relative uncertainty on the parameters of 0.1%, the maximum uncertainty on the propagated pulse is 3% of the pulse amplitude. Therefore, even with carefully measured parameters, the UQ on the dispersion correction procedure for SHPB tests cannot be neglected. The SA gives the order of decreasing importance of the parameters: c0, r0; x and ?. This can be helpful if one wants to reduce the uncertainty as it indicates on which parameter(s) the measurement accuracy must be improved in priority

Estimating Measurement Uncertainty on Stress-Strain Curves from SHPB

Split Hopkinson Pressure Bar tests are commonly used to determine material stress-strain relationship at high deformation rates. Obtaining this relationship is dependant both on certain assumptions and substantial post-processing of the data recorded during the test. Measurement uncertainty rarely appears on the resulting curves. This article introduces a simple method of estimating the measurement uncertainty associated with SHPB tests

Uncertainty quantification of nonlinear stochastic dynamic problem using a Kriging-NARX surrogate model

3rd International Conference on Uncertainty Quantification in Computational Sciences and Engineering, ECCOMAS, HERSONISSOS (CRETE), GRECE, 24-/06/2019 - 26/06/2019Uncertainty quantification of nonlinear stochastic dynamic problem is always a challenging task due to the complexity of the systems. In this paper, a hybrid surrogate modelling approach is proposed for the uncertainty quantification of nonlinear stochastic dynamical systems in the time domain. The proposed hybrid surrogate model is constructed using a nonlinear system identification tool, the Nonlinear AutoRegressive with eXogenous (NARX) input model, and the Kriging approach for uncertainty propagation. Further, to increase the computational efficiency, least angle regression (LARS) is utilized in the hybrid framework. The method is applied on a nonlinear stochastic dynamic oscillator to check its applicability. The time dependent mean and standard deviation are predicted using the proposed approach, and all the results are compared with the Monte Carlo simulation (MCS) results. A high-level accuracy is noticed using the proposed approach as compared to other state-of-the-art methods. This accuracy is achieved using a very limited number of model evaluations which is suggesting the efficiency of the proposed approach. Moreover, an excellent accuracy and efficiency is achieved using the proposed approach in predicting the probability density function (PDF) at several time instances for the nonlinear stochastic dynamic oscillator