Fiber-Reinforced Injection-Molded Plastic Parts: Efficient Calculation and Lifetime Prediction

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1 Fiber-Reinforced Injection-Molded Plastic Parts: Efficient Calculation and Lifetime Prediction Michael Parrott e-xstream engineering, LLC MA5941 Class Description: Fiber-reinforced composites are increasingly replacing classical metal construction of lightweight structures. To accurately predict the anisotropy caused by the fiber reinforcement, the material nonlinearities of the material matrix, as well as strain rate and temperature dependencies, have to be taken into account. This requires in-depth material modeling that goes beyond the macroscopic level and enters the microscopic regime. Material behavior understood and calibrated on this level enables multiscale techniques to integrate processing simulation into the simulation of the structural behavior. This class deals with the prediction of the overall behavior of polymer matrix composites and structures using mean-field homogenization theory as implemented in the DIGIMAT software platform. Learning Objectives At the end of this class, you will be able to: Describe joint benefits of material and structural engineering Know how Moldflow can be used to enhance accuracy of structural simulation Understand the influence of a material s microstructure on it s performance Describe case studies where anisotropic, non-linear material behavior is crucial to accurate simulation Perform multi-scale analysis of heterogeneous materials

2 About the Speaker Michael Parrott, BS in Mechanical Engineering from Northwestern University (USA) MS in Materials Engineering from the Catholic University of Louvain (Belgium) Thesis: Nanocomposites for the Aerospace Industry Awards: National Merit Scholar, magna cum laude, Erasmus Mundus Scholar, valedictorian Researched experience at Northwestern University (USA), University of Augsbug (DE) and the Catholic University of Louvain (BE) Previous work experience at SGL Carbon GmbH, Munich Reinsurance Summary Fiber-reinforced composites are increasingly replacing classical metal construction of lightweight structures. To accurately predict the anisotropy caused by the fiber reinforcement, the material nonlinearities of the material matrix, as well as strain rate and temperature dependencies, have to be taken into account. This requires in-depth material modeling that goes beyond the macroscopic level and enters the microscopic regime. Material behavior understood and calibrated on this level enables multi-scale techniques to integrate processing simulation into the simulation of the structural behavior. This class deals with the prediction of the overall behavior of polymer matrix composites and structures using mean-field homogenization theory as implemented in the DIGIMAT software platform. DIGIMAT Software developed by e- Xstream engineering e-xstream engineering develops and commercializes the DIGIMAT suite of software, a state of the art multi-scale material modeling technology that speeds up the development of optimal composite materials and parts for material suppliers and end users in the automotive, aerospace, electronic and electrical, consumer goods and industrial equipments industries. Our solutions are used by CAE engineers, materials scientists, chemists, specialists in manufacturing processes of composite materials to accurately predict the nonlinear micromechanical behavior of complex multi-phase composite materials and structures (PMC, RMC, MMC, nanocomposites... ) DIGIMAT, The nonlinear multi-scale materials and structures modeling platform, consists in a complete set of complementary interoperable software. This is an efficient and predictive tool that helps our customers designing and manufacturing innovative and optimal composite materials and parts, time and costs efficiently. With major customers in Europe, America and Asia, we have added to our deep expertise in numerical simulation the business understanding of a large variety of materials such as reinforced plastics, rubber, hard metals, nanocomposites and honeycomb sandwich panels used across the automotive, aerospace, electronic and electrical, consumer goods and industrial equipments industries. 2

3 While this class handout contains detailed information about micromechanical modeling, the DIGIMAT software suite and its integration with Moldflow, the online course complements this with a number of case studies to give an understanding of the real world application of the product. Micromechanical modeling We consider heterogeneous materials whose microstructure consists of a matrix material and multiple phases of so-called inclusions, which can be short fibers, platelets, micro-cavities or micro-cracks: see Figure 1. There are several examples of such composite materials, such as: thermoplastic polymers reinforced with short glass fibers (GFRP), rubber matrix composites (RMC), metal matrix composites (MMC) such as titanium reinforced with carbon or ceramic fibers, short steel fiber reinforced concrete, biomaterials, etc. The objective of micromechanical modeling is to predict the interaction between the microstructure and the macroscopic (or overall or effective) properties. Figure 1 - Matrix material reinforced with multiple phases of inclusions Consider a heterogeneous solid body whose microstructure consists of a matrix material and multiple phases of so-called inclusions, subjected to given loads and boundary conditions (BCs). The objective is to predict the influence of the microstructure on the body s response. It would be computationally prohibitive to solve the mechanical problem at the scale of the microstructure. Therefore, we distinguish two scales: the microscopic one (that of the heterogeneities) and the macroscopic one, where the solid can be seen as locally homogeneous. 3

4 The link between the two scales is made via the concept of representative volume element (RVE). At macro scale, each material point is supposed to be the center of a RVE, which should be sufficiently large to represent the underlying heterogeneous microstructure, and small with respect to the size of the solid body; see Figure 2. A two-scale approach which enables a transition between the two scales, both ways, is summarized in the following steps: 1. Micro à Macro transition: a. Macro material point: center of a representative volume element (RVE) b. At micro scale: RVE contains a finite number of constituents c. Need a constitutive model for each of the constituents. d. Micro/macro transition: homogenization method to find the macro constitutive response of RVE. e. Continuum mechanics at macro scale with macro constitutive equation. 2. Macro à Micro transition: a. At each time and at each macro material point, do a numerical zoom in order to see what happens at the micro level (e.g. stresses and strains in each phase). Figure 2 - Micro-macro transition. Upper left: microscopic scale, upper right: macroscopic scale, bottom: representative volume element (RVE). Nemat-Nasser and Hori (1993). By definition, a composite implies the combination of two or more phases. These phases can refer to the same material in different molecular configurations, but most of the times they are made out of different materials. When dealing with composites in a micromechanical approach, a good definition of the phases that constitute the whole microstructure is as important as a good definition of the materials themselves. The microstructure definition refers to many parameters like the content of each inclusion phase, the aspect ratio and the orientation definition of the inclusions, or the presence or not of coatings around an inclusion phase. 4

5 Four of the six DIGIMAT packages are highlighted below. These packages contain everything needed to create a digital material model that is coupled with the output of a moldflow simulation to give more accurate FEA results. Digimat-MF is a mean-field homogenization based software that aims at predicting the nonlinear constitutive behavior of multi-phase materials based on the constitutive properties of the base materials and the composite morphology (filler content, length and aspect ratio, orientation). Digimat-MF is accurate, efficient and very easy to learn and use. Digimat-MX is a composite materials database giving access to experimental data at different strain rates, temperatures and so on. It also contains DIGIMAT material files already reverse engineered based on experimental data. This software clearly helps sharing data between stakeholders or simply internally in a company. It also provides optimization tools to perform parametric identification and to reverse engineer easily and efficiently DIGIMAT material models for a whole range of composite materials. Digimat-CAE centralizes the upstream and downstream interfaces between Digimat-MF, injection molding codes (if necessary) and structural Finite Element Analysis (FEA) solvers. Digimat-CAE is used to define a nonlinear micromechanically-based material model of a multiphase material within the FEA code. Such an approach accounts for the effect of manufacturing processes on the material end performance, thanks to its multi-scale approach. In the case of injection molding, Digimat-CAE bridges the gap between injection molding simulation and the FEA by accounting for changes in the material microstructure throughout the injected part (e.g. fiber orientation, content and shape). Digimat-Map is a 3D mapping software used to transfer fiber orientations residual stresses, temperatures and weld lines between dissimilar injection molding and structural FEA meshes. Digimat-Map helps structural engineers to generate the optimal mesh refinement and make the appropriate element choice to capture changes in the composite material microstructure properties. Prediction of the composite microstructure by Moldflow The Digimat-CAE/Moldflow interface enables taking into account the results computed from an injection molding simulation in a coupled FE analysis. The usual data used for coupled FE analyses is the fibers orientation, but for midplane meshes, the residual stresses and the weld lines location can also be processed in the structural simulation. Four types of Moldflow results can be used in a Digimat-CAE computation: fiber orientation, temperature, residual stress and weld lines. Moldflow can provide fiber orientation and residual stress for shell and solid model. 5

6 Fiber orientation Two formats must be distinguished: the *.xml and the *.ele format. This first is dedicated to fiber orientation results for solid and shell elements meshes, and the latter is on the other hand dedicated to those for shell elements meshes. Residual stresses Residual stresses are computed by Moldflow based on the thermoelastic material properties defined inside Moldflow. Those are computed after the cool down and the packing phase processes and can be used in coupled DIGIMAT to ABAQUS computations only, defining them as initial boundary conditions. Residual stress tensor can also be imported and exported from Moldflow in.xml format (for shell and solid elements) in Digimat-MAP. The.xml format for residual stresses is the same as for.xml orientation file format. The initial stress are given in global axis and thus cannot be used as it is in Abaqus Temperature Two formats must be distinguished: the *.xml and the *.ele format. This first is dedicated to temperature results for solid and shell elements meshes, and the latter is on the other hand dedicated to those for shell elements meshes The temperatures for each layer are saved in a specific file that has for extension.ele.00y where y is the number of the interface. If you have 20 layers, you will have files with extension from ele.001 to.ele.021. The position of the interface of each layer is always given between -1 and +1 and is stored in each.ele file. The thickness of the layers is obtained by taking the absolute value of the difference between the positions of the interface defining the layers. Weld lines DIGIMAT also can account for weld lines when working with shell elements meshes. This Moldflow midplane data actually contains a flag telling which nodes participate to the weld lines located at the end of the simulation. This data is exported in a.nod format and when it is mapped onto the structural mesh in Digimat-MAP, the data is actually transferred on the elements surrounding the indicated nodes. From that are created new element sets on the structural mesh, making it therefore easier to apply different material properties on the elements located in the weld lines areas. References S. Nemat-Nasser and M. Hori. Micromechanics: overall properties of heterogeneous solids. Elsevier Science, , 88 6