This paper presents different modeling approaches and technical challenges for the discretization of anisotropic elastic-viscoplastic materials in secondary structural parts for the automotive sector. In terms of accuracy, complex geometries based on reinforced plastics in secondary load paths need to factor in the manufacturing process and the resultant local anisotropies within correspondent CAE models. However, during the early phase of product development, integrating reinforced plastics, a robust numerical basis throughout the concept evaluation is required. The basic idea is to maintain the correspondent level of complexity through the subcomponent design within certain limits, in order to improve, speed up and adequately handle complexity in CAE concepts for the automotive sector. Finally, a benchmark analysis of possible options and modeling techniques is introduced, as a contribution to evaluate the balance between the dimensioning of structural load paths and the required material characterization within an acceptable effort.
Impact damage induced by hailstone impact, tool, or equipment dropping can lead to severe reductions in composite structures’ load-carrying capacity. Aerospace companies and manufacturers of other products, in which composite materials are extensively used, spend considerable resources to determine the level of degradation of composite parts’ load-bearing capacity that have received impact damage during operation or during assembly, as well as the permissible degree of damage at which the replacement of an expensive structural member is unnecessary. Usually, such assessments are based on the integrated application of experimental destructive and non-destructive methods, which, in turn, also requires considerable financial and time investments. Understandably, the availability of a verified simulation approach capable of predicting the residual load-carrying capacity of composite parts with impact damage would provide significant costs savings and accelerate the decision making when such assessments are required. This preliminary study represents the first steps aimed at developing such a simulation approach using LS-DYNA software and is focused on the load-bearing capacity of damaged composite structural members designed to work primarily under the action of compressive loads.
The compression molding of Sheet Molding Compounds (SMCs) is typically thought of as a fluid mechanics problem. The simulation of such materials is at present based on the background of compression or injection molded short fiber reinforced materials. The usage of CF-SMC consisting of high fiber volume content (over 50%) and long fiber reinforcement structures (up to 50 mm) challenges the feasibility of this point of view. The goal of this work is the development of a user-defined material model based on a solid mechanics formulation for SMC materials in LS-DYNA®. To allow for large deformations in the simulation an Arbitrary Lagrangian-Eulerian (ALE) approach is used. As a first step, a material characterization is carried out in a so-called press rheometer test where the mechanical behavior of the SMC material is analyzed during the compression molding process. The resulting stress response of the material then serves as input information for the material model. The material model itself is based on a modular building-block approach. The individual modules describe certain aspects of the material behavior (e.g. compaction, plastic flow behavior or fiber orientation) and interact with one another through the passing of parameters between the respective modules. This procedure allows for the flexible development of the mathematical description for each part of the material behavior. Initially, a simple mathematical model describes every module. In the further development of the model, each module is expanded by more complex mathematical descriptions. As the overall goal is a work in progress, this paper shows the current implementation of several of these modules including the characterized compression and flow behavior as well as a description for the fiber orientation based on the Folgar-Tucker equation. By simulating the press rheometer test itself using the developed user defined material model, a comparison between simulation and experiments is performed to check the accuracy of the various mathematical models used. The stress response and the flow front development provide the basis for the comparison and provide clues on how to proceed with the further development.
Composites subjected to out-of-plane stresses due to impact loading can suffer from multiple delaminations, but modelling these in large scale structures is a challenging problem. To address this, a methodology is proposed for modelling dynamic delamination initiation and propagation in composites. It adaptively segments the mesh with additional nodes which model the discontinuities in the displacement field caused by delamination. Besides, it also introduces cohesive segments between the newly created nodes so that delamination propagation is controlled by an energy criterion. These adaptations are performed ‘on-the-fly’ in a dynamic explicit Finite Element solution without the need for user intervention, and the mesh segmentation technique does not reduce the time increment size for solution stability. A technique to initialise cohesive tractions with minimal disturbances to the surrounding stress field is also presented. This methodology is described here in detail and demonstrated in the commercial finite element software LS-Dyna. Finally, it is validated against experimental data from the existing literature.
When a quasi-static axial compressive load is applied to a Fiber Reinforced Plastics (FRP) tube, a continuous and stable fracture phenomenon called “Progressive Crushing” which shows highly effective energy absorption appears. The authors have constructed a cohesive element FEM model that can reproduce the process to this phenomenon. The purpose of this paper is to investigate the most stable chamfer shape for progressive crushing of the FRP tube, by using Cohesive Zone modeling technique. In the study, cross-sectional shapes of triangle type, chevron type and M-type were selected for the simulation of axial crushing test to confirm crush mode. Five geometric shapes of flat plate FEM model were considered to conducting a fundamental investigation. Furthermore, the 3D finite-element models of FRP tube using reasonably cross-sectional shapes were intended to obtain a well-balanced chamfer shape, therefore, providing useful suggestions for FRP tube design and/or manufacture.
In the last years the demands of the automotive industry have led to a strong interest in a more detailed virtual description of the material behavior of thermoplastics. More and more complex material models, including damage and failure, have to be characterized, while keeping the importance of gaining material data quickly in mind. Currently material and failure modeling in crash simulations typically deal with simple von Mises visco-plasticity (*MAT_024) and equivalent strain failure criteria, which cannot describe the complex material behavior of plastics. Past developments have focused on the yield behavior under different load situations (tension, shear, compression), which are implemented in more complex material models like *MAT_SAMP-1 for thermoplastics as well as *MAT_215 for fiber reinforced thermoplastics.
Part and component design with Polypropylene Compounds can create several challenges for simulation methods. When Short Glass Fibers Polypropylene (SGF-PP) is considered, fiber orientation prediction, process-induced anisotropy and rupture criteria must be properly addressed in the structural analyses. The time frame is also relevant, as industrial environment simulations often need to provide fast solutions to designers in order to limit the time to market. Responding to the needs of a simulation tool for an early stage design, this paper describes a methodology based on an orthotropic material law (Ls-dyna MAT_157), embedded interactive criteria and a mapping tool (LS-DYNA ENVYO). This approach has been applied in the design of a part used in the building and construction industry, for which an experimental validation on an impact test has been also carried out. This study is here reported.
The obvious question is how to combine models and data to create a virtual prediction tool? There is a long history with measured data to adjust finite element models representing the geometry and material properties of a tested component. It seems to be a good starting point to represent the initial geometry with correct stiffness and challenge the element formulations to maintain realistic stiffness even when the elements are severely deformed. A proper discretisation is crucial when building a finite element model, and most metallic parts are represented as continuums although we know about the elementary particles. It is likely that this simplification is acceptable as long as the strains are small and the strain hardening is sufficient to compensate for local variation in material properties, but remember that brittle behaviour may be the result when the elastic energy stored in the component is allocated into the first local area that fails.
JSOL Corporation has developed J-Composites®, a set of tools, which works in cooperation with LS-DYNA®, to facilitate the complex manufacturing process and process-chain simulation of fiber reinforced composite materials. The J-Composites series consists of “Form Modeler”, a tool to set up a press forming analysis model, and “Fiber Mapper”, a tool to map a resin flow simulation result on to a structural mesh. Additionally, “Compression Molding”, a tool for compression molding simulation is in development. This paper introduces some new capabilities of J-Composites/Form Modeler version 2.0 and demonstrates composite forming simulations.
Composite materials like fiber reinforced plastics (FRP) are becoming more widely used in the automotive industry and have been found very effective in reducing vehicle weight. Recently, long carbon fiber reinforced thermoplastics are increasingly being used for lightweight structural parts with high stiffness, strength and energy absorption performance. Compression molding is considered one of the most efficient manufacturing processes to mass produce FRP parts for automotive applications. Compression molding can form FRP into complex shapes with relatively low manufacturing cost and short process time. However, this often generates unwanted fiber orientation, uneven distribution of fibers and fillers, weld lines and matrix rich regions. These forming effects strongly affect mechanical strength. To analyze these complex phenomena, LSTC and JSOL developed new compression molding simulation techniques for long fiber reinforced plastics using a beam-in-adaptive EFG coupling function in LS-DYNAⓇ. In this paper, a compression molding simulation for long carbon fiber reinforced thermoplastics is introduced and new component strength analysis method with a beam-in-SPG coupling model using deformed beams calculated in the compression molding simulation is presented.
For efficient vehicle development there is a strive to reduce prototypes and shorten development times which leads to the need to rely on CAE methods for continuous evaluation of product performance. This puts demands on the CAE methods, not only in terms of predictability but also in terms of how well they integrate in the development process. Model preparation, material characterization and computational costs are important aspects for successful integration. New materials and production methods are other drivers for CAE method development as current methods may not be adequate. Short fiber reinforced polymers (SFRP) have found their way into more automotive applications in recent years. Weight, the geometrical possibilities, part production cycle times and cost are some of the potential benefits. The injection molding process, however, leads to an inhomogeneous distribution of fiber orientation throughout a part. As the fiber orientation distribution has significant impact on the mechanical properties it causes anisotropy and spatial variations of the material response. This paper addresses the modeling of an SFRP part which is produced by gas assisted injection molding leading to a porous, microcellular, material consisting of three phases, i.e. matrix-, fiber- and pore phases.
Design and Material Characterization of Reinforced Plastics for Secondary Structural Load Paths in an Early Development Phase