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High Speed Testing

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Numerical simulation of the laser scoring line behavior in airbag deployment

by Massimo Nutini5,704 views

The airbag door system is one of the most delicate aspects in the design phase of a car instrument panel: seamless systems are increasingly used, which combine styling criteria with good functional performances. These systems typically include a tear seam, which may be obtained through laser scoring, to pre-determine the location of the opening during airbag deployment. The design of the scoring line is currently validated through experimental tests on real life exemplars, submitted to airbag deployment, resulting in high development times and relevant costs. This is the main reason which suggests proposing numerical simulation in the design phase, not to substitute actual part homologation by testing but in order to limit the scope and complexity of the experimental campaign, thus reducing the development costs and the time to market. So far, modeling the scoring line has been difficult due to limitations in the testing methods and simulation codes available to the industry. The methodology proposed in this paper takes advantage from the availability of a material law as LS-Dyna SAMP-1, with polymer-dedicated plasticity, damage model and strain-rate dependent failure criteria, which is supported by local strain measurement used for material characterization. The method, here described in detail, is validated on a benchmark test, consisting in the real and virtual testing on a variety of scoring profiles obtained on a polypropylene box submitted to high speed impact test.

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Characterization of Polyolefins for Design Under Impact: from True Stress/ Local Strain Measurements to the F.E. Simulation with LS-Dyna Mat. SAMP-1

by Massimo Nutini5,891 views

Optical strain measurement for the mechanical characterization of polymers, and in particular of polyolefins, is becoming a common practice to determine the parameters to be used in a finite element analysis of crash problems. This experimental technique allows measuring the strain locally on the specimen, so that it is particularly suitable when the deformation is localized, as in the case of polymers: therefore a more accurate description of the behaviour of the material is obtained. By so doing, it is possible to describe the material constitutive law in terms of the true, local strain and of the true stress. As these data are those needed by the most complete material models developed for impact calculation, it is clear that this technique is particularly suitable for coupling with the most advanced material models currently available in the F.E. codes, as for instance with Mat 187 (SAMP-1) of LS-Dyna. The local measurement of the strain can also be used for evaluating the volume strain, whose evolution with the increasing strain shows that for PP-based material the deformation is not isochoric in most the cases. The observed increase in the material volume reflects the fact that voids generate and coalesce within the material, possibly resulting in fracture. The measure of the volume strain, computed as the trace of the strain tensor, is here used for determining the damage function utilized by the damage model implemented in SAMP-1. The effective stress is here estimated as the stress which would be measured if the deformation was isochoric, and it can be assessed on the basis of the measurement of the longitudinal local strain only. Corresponding to each value of longitudinal strain, the volume strain is then used to calculate the ratio between the effective and the true stress. Adopting this procedure, the damage function is thus determined without the needs of repeated loading-unloading tests used to derive the damage parameter from the unloading slope, which is furthermore difficult to be measured. As an application, the results of the numerical reproduction of a benchmark test, consisting in a drop test on a polypropylene box, are presented and discussed

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Simulating anisotropy with Ls-dyna in glass-reinforced, polypropylene-based components

by Massimo Nutini5,779 views

Glass-fiber-reinforced polypropylene (GF PP) materials are increasingly being used by customers to replace metal and engineering polymers in structural automotive applications. Like all glass-fiber reinforced thermoplastics, GF PP products can show anisotropy caused by fiber orientation that is induced by the injection process. Taking into account fiber orientation in the simulations enables designers to improve the accuracy of the analyses. This can help prevent arbitrary choices and assumptions when setting material parameters, which become mandatory when an isotropic material law is used. The method proposed in this paper takes advantage of the availability within Ls-dyna of an anisotropic material law (MAT_103), which allows simplified modeling to address critical issues. This law was not developed to address the problem discussed here. Therefore, this paper illustrates a simplified approach. The presence of glass reinforced fibers is taken into account by running a mold-filling analysis, and then transferring the material flow orientation in to the structural simulation as a material angle. The dependence of the material failure strain on the material orientation can be also easily modeled through a user subroutine. Finally, the approach only requires simple material data based on basic tensile tests; the material law parameters are then identified through optimization techniques. Although this approach is based on some simplifying assumptions, its application is quick and can help the designer obtain more accurate results with respect to the traditional isotropic approach. A selection of validation tests is then proposed that show reliable predictions using limited additional computational effort.

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AutomotiveHigh Speed TestingLS-DYNAMechanicalPlasticsRate DependencyResearch Papers

Caratterizzazione di materiali plastici: misure locali di deformazione per la simulazione ad elementi finiti di problemi di impatto

by Massimo Nutini5,363 views

Questo articolo si propone di illustrare l’importanza dell’utilizzo di metodi per la misura delle proprietà locali del materiale per determinarne la legge di comportamento. Vengono di seguito presentati alcuni esempi che evidenziano quanto più accurate e realistiche siano le simulazioni numeriche di test di trazione ad alta velocità su provini di poliolefine, quando vengano utilizzate proprietà dei materiali rilevate con misure locali, utilizzando metodi ottici. La disponibilità di misure locali e più accurate evidenzia come sia necessario che nei codici di calcolo commerciali vengano implementate delle leggi di materiale più sofisticate di quelle disponibili attualmente, che sono state per lo più originariamente sviluppate per materiali metallici, e dunque non riescono sempre a predire correttamente il comportamento dei componenti in materiali polimerici.

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Enhanced Failure Prediction in Sheet Metal Forming Simulations through Coupling of LS-DYNA and Algorithm Crach

by Helmut Gese5,343 views

"In sheet-metal-forming the forming limit curve (FLC) is used for ductile sheets to predict fracture in deep drawing. However the use of the FLC is limited to linear strain paths. The initial FLC cannot be used in a complex nonlinear strain history of a deep drawing process or a successive stamp and crash process including a significant change in strain rate. The CRACH software has been developed to predict the forming limit of sheets for nonlinear strain paths [1]. It has been validated to predict instability for bilinear strain paths with static loading in the first path and dynamic loading in the second path for mild steels [2]. As the postprocessing of strain paths from single finite elements in CRACH is not economic for industrial applications MATFEM initiated a project to couple CRACH directly with FEM-Code LS-DYNA using a userdefined material model. This allows a prediction of possible failure during the simulation for all elements with respect to their complete strain history. A special strategy has been developed to include CRACH without extensive increase in total CPU time. The developed interface to LS-DYNA allows also the implementation of other failure criteria demanding the history of deformation like for example a tensorial fracture criterion. In order to test the reliability of the calculated safety factor experimental tests for bilinear strain paths have been simulated [2]. In this case the experimental and numerical investigations have been made on two-stage forming processes (static in the 1st stage and both static/dynamic in the 2nd stage) . The static-static case should simulate a stamping process with bilinear strain path. The static-dynamic case should simulate a successive stamp and crash process. The simulation of a complex deep drawing problem including areas with significantly nonlinear strain paths has been simulated with LS-DYNA/CRACH-coupling. It can be shown that the prediction of CRACH can differ significantely from a “standard” prediction based on the initial FLC. The coupling of LS-DYNA and CRACH showed the potential to predict possible fracture in deep drawing and crash loading at an early design stage and allowed to optimise geometry and material quality to significantly reduce later problems in real components."

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A Systematic Approach to Model Metals, Compact Polymers and Structural Foams in Crash Simulations with a Modular User Material

by Helmut Gese5,899 views

"Today the automotive industry is faced with the demand to build light fuel-efficient vehicles while optimizing its crashworthiness and stiffness. A wide variety of new metallic and polymeric materials have been introduced to account for these increased requirements. Numerical analysis can significantly support this process if the analysis is really predictive. Within the numerical model a correct characterization of the material behaviour – including elasto-viscoplastic behaviour and failure - is substantial. The particular behaviour of each material group must be covered by the material model. The user material model MF GenYld+CrachFEM allows for a modular combination of phenomenological models (yield locus, strain hardening, damage evolution, criteria for fracture initiation) to give an adequate representation of technical materials. This material model can be linked to LS-DYNA when using the explicit-dynamic time integration scheme. This paper gives an overview on the material characterization of ultra high strength steels (with focus on failure prediction), non-reinforced polymers (with focus on anisotropic hardening of polymers), and structural foams (with focus on compressibility and stress dependent damage evolution) with respect to crash simulation. It will be shown that a comprehensive material model - including damage and failure behaviour - enables a predictive simulation without iterative calibration of material parameters. A testing programme has been done for each material group in order to allow a fitting of the parameters of the material model first. In a second step different component tests have been carried out, which were part of a systematic procedure to validate the appropriate predictions of the crash behaviour with LS-Dyna and user material MF_GenYld+CrachFEM for each material group."

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AutomotiveFoamsHigh Speed TestingLS-DYNAMechanicalMetalsPlasticsRate DependencyResearch PapersYielding/Failure Analysis

Improved Plasticity and Failure models for Extruded MgProfiles in Crash Simulations

by Helmut Gese5,638 views

"The Crash Simulation of Magnesium Structures with Finite Element Methods demands the use of suitable material and failure models. An associated plasticity model describing the complex asymmetric yield behaviour in tension and compression of Mg extrusions has been developed during the InMaK-project (Innovative Magnesium Compound Structures for Automobile Frames) supported by the German Federal Ministry for Education and Research (BMBF). Differences to the material model 124 in LS-DYNA are exposed. In order to describe the failure behaviour of Mg extrusions under multiaxial loading in FEM crash simulation this constitutive model has been combined with a fracture model for ductile and shear fracture. The fracture model has been added to the user defined constitutive magnesium model in LS-DYNA. The experimental investigations carried out on model components are compared with numerical derived results. Experimental methods for fracture parameter evaluation are shown and general aspects of metal failure due to fracture as well as different modelling techniques are discussed."

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Development, implementation and Validation of 3-D Failure Model for Aluminium 2024 for High Speed Impact Applications

by Paul Du Bois6,141 views

FAA William J Huges Technical Center (NJ) conducts a research project to simulate failure in aeroengines and fuselages, main purpose is blade-out containment studies. This involved the implementation in LS-DYNA of a tabulated generalisation of the Johnson-Cook material law with regularisation to accommodate simulation of ductile materials.

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Theoretical Development of an Orthotropic Elasto-Plastic Generalized Composite Material Model

by Paul Du Bois6,223 views

The need for accurate material models to simulate the deformation, damage and failure of polymer matrix composites is becoming critical as these materials are gaining increased usage in the aerospace and automotive industries. While there are several composite material models currently available within LS-DYNA, there are several features that have been identified that could improve the predictive capability of a composite model. To address these needs, a combined plasticity and damage model suitable for use with both solid and shell elements is being developed and is being implemented into LS-DYNA as MAT_213. A key feature of the improved material model is the use of tabulated stress-strain data in a variety of coordinate directions to fully define the stress-strain response of the material. To date, the model development efforts have been focused on creating the plasticity portion of the model. The Tsai-Wu development efforts have focused on creating the plasticity portion of the model. The Tsai-Wu composite failure model has been generalized and extended to a strain-hardening based orthotropic material model with a non-associative flow rule. The coefficients of the yield function, and the stresses to be used in both the yield function and the flow rule are computed based on the input stress-strain curves using the effective plastic strain as the tracking variable. The coefficients in the flow rule are computed based on the obtained stress-strain data. The developed material model is suitable for implementation within LS-DYNA for use in analyzing the nonlinear response of polymer composites.

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Verification and Validation of a Three-Dimensional Generalized Composite Material Model

by Paul Du Bois5,840 views

"A general purpose orthotropic elasto-plastic computational constitutive material model has been developed to accurately predict the response of composites subjected to high velocity impact. The three-dimensional orthotropic elasto-plastic composite material model is being implemented initially for solid elements in LS-DYNA® as MAT213. In order to accurately represent the response of a composite, experimental stress-strain curves are utilized as input, allowing for a more general material model that can be used on a variety of composite applications. The theoretical details are discussed in a companion paper. This paper documents the implementation, verification and validation of the material model using the T800-F3900 fiber/resin composite material."

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Aerospace and DefenseAutomotiveCompositesHigh Speed TestingLS-DYNAMechanicalPlasticityResearch PapersValidationYielding/Failure Analysis