This study presents an approach to characterize thermoplastic generative designed parts and compares the usability of different material models in LS-DYNA. For using 3D printed parts in prototypes it is at first necessary to be able to predict the deformation behaviour of the printed part itself. The deformation behaviour of thermoplastics and especially of thermoplastic generated parts depends on a variety of material properties. In general the parts have a composed anisotropy consisting of the process and material anisotropy. The process anisotropy is reflected to different mechanical properties due to the building directions of the 3D printer. The material anisotropy includes divergent tension and compression behaviour and approximately orthotropic behaviour due to particle reinforcement. The main task therefore is to evaluate current material routines and modeling techniques to ensure the predictability of the parts behaviour with available and implemented material cards. The performed characterization consists standard specimen tests for a non-reinforced and a carbon particle reinforced thermoplastic, which is produced in the selective laser sinter process. The conducted tests are a tensile, a compression and a shear test. The test specimen were built in different construction directions. In addition, component tests were executed in order to evaluate the predictability of the generated material cards in multiaxial stress states.
A new method to model damage properties of Randomly-Oriented Thermoplastic Composites (RO-FRTP) was proposed for finite element analysis (FEA). The materials are composites of thin sheets in which carbon fibers of approximately one inch in length are distributed randomly in thermoplastics resin matrix. While their material properties are intrinsically isotropic in plane from a macro perspective, RO-FRTP have complex nature of damage initiation and progression that depends on the deformation mode. In this study, the damage model that based on Continuum Damage Mechanics (CDM) was developed and the modelling method with 3D shell element for RO-FRTP was proposed. The multifunctional feature of *MAT_ADD_GENERALIZED_DAMAGE in LS-DYNA® has been utilized in order to reproduce damage properties of the material. Furthermore, several numerical studies are conducted and compared to experiments for the purpose of validation of the modelling method. Simulation results show that the modelling method can capture complex damage characteristics of the material in detail and predict deformation of structures accurately.
For automotive suppliers, it is essential to model the behavior of thermoplastics under crash loading and for a large range of temperature typically from -30° until 85°c. Thermoplastics are very sensitive to both strain rate and temperature with an inverse relation: hardening with strain rate and softening with temperature. Generally, a large experimental campaign has to be carried out to identify different behavior laws of the material, each of them for a specific range of strain rate and temperature. Then, according to the characteristics of the loading case, e.g. impact, corresponding behavior laws are chosen in the database to run the numerical simulations. This results in an important experimental cost and a large database to manage. It is then interesting to explore the time-temperature equivalence of thermoplastics to act on both aspects. Relations between strain rate and temperature sensitivities are identified through dynamic mechanical analysis (DMA) in the viscoelastic domain and described through the Williams, Landel and Ferry model or the Arrhenius model for example. For that, a shift factor is experimentally determined and introduced to modify the time step in the behavior model for the finite element simulation, thus simulating an adapted strain rate. As a novelty, the time-temperature equivalence is here extended to the viscoplastic domain by keeping the same shift factor. It therefore becomes possible to cover all the scope of temperature and strain rate of automotive applications from only DMA and tensile tests at room temperature and different strain rates. This approach is implemented in association with viscoelastic, viscoplastic with non-associative plasticity constitutive laws and non-local damage model [1][2][3] and applied to the case of a polypropylene. The time temperature equivalence is validated for the viscoelastic as well as for the viscoplastic parts of the behavior with good experimental/numerical correlation. As a result, the number of material cards required in Ls Dyna is reduced to only one to cover all the simulations. This approach is also under investigation to be applied to the failure model.
Polymers have been often used as structural materials under mechanically severe conditions instead of metals. We usually use FEM simulation when we design polymer products. The failure prediction of impact is especially important because its effect for reduction of development duration and number of trial products cannot be disregarded. The failure prediction has been investigated for a long time [1][2]. We generally start impact simulations using elasto-plastic material such as MAT_024 by giving stress-strain curves with strain rate dependency. We often add our own research achievement about material model by the user defined material models which LS-DYNA offers us to improve prediction accuracy. We developed the isotropic material model based on damage of polymers and introduced it into LS-DYNA by using user subroutine “*MAT_041-050”. We found many good coincidences between experimental impact test and numerical results with our material model. After that, we found the reason why we got good coincidence by simulation with isotropic material model was that glass fibers in the structural specimen of these experimental tests align well at the impact area. Therefore, we decided to start simulations for structural specimens with various fiber distributions. Differences between the isotropic simulation results and anisotropic results are recognized. The importance for taking fiber orientation into account in impact simulations is known [3]. Then, we conducted the experimental impact tests using structural specimen made of Polyamide 66 with 35 weight% short fiber (ASAHI-KASEI LeonaTM 14G35). We set different fiber distribution by giving two gate types in injection molding. In this paper, the effect of introducing fiber distribution is discussed.
The use of thermoplastics in structural applications requires that engineers can reliably predict their mechanical behaviour. Depending on the intended use, a component must withstand various load cases and environmental factors. This paper seeks to investigate the capabilities of a phenomenological material model to represent polypropylene (PP) plates subjected to a dropped weight impact at low temperatures. The dropped weight tests were performed with an Instron CEAST 9350 Drop Tower Impact System, shown in Figure 1. An incorporated environmental chamber injected with liquid nitrogen enabled sub-room temperature conditions. A total of 11 drop tests were made at five different impact velocities. The material was found to experience moderate plastic deformations until failure through plugging. By comparison of force displacement curves, the tests are found to show good repeatability. Some variations are found with respect to initiation of failure, possibly caused by small variations between the plates or misalignments during tests.
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.
In this contribution, a finite element implementation of a micromechanically based constitutive model describing several inelastic effects of filled rubbers in multiaxial deformation states is presented. The model describes the elastic and inelastic effects of filled rubbers and is based on the network decomposition concept. Accordingly, the rubber network is decomposed into an isotropic elastic network E, responsible for the polymer matrix, and two anisotropic permanent damage networks (M and H) which are responsible for the filler-polymer interaction. The anisotropic damage networks M and H are capable of capturing the Mullins effect, hysteresis, permanent set and induced anisotropic stress softening. This model is implemented into LS-DYNA® by means of a subroutine within *MAT_USER_DEFINED _MATERIAL_MODELS (UMAT). The user can easily switch between different combinations of elastic and inelastic models by activating and deactivating each network. The user can also select the number of directions to be considered depending on the complexity of the loading history. The model with appropriate material constants demonstrates good agreement with experimental data.
This contribution discusses experiments necessary to describe the behavior of filled elastomers under large strains. Filled elastomers show a variety of inelastic phenomena such as Mullins effect, hysteresis, induced anisotropy, and permanent set. While uniaxial tension tests to rupture provide virgin loading curves, other tests are necessary to gather information about the inelastic effects mentioned above. In order to capture these phenomena experimentally, cyclic uniaxial tension tests with stepwise increasing load amplitudes are carried out. Since experimental stress-strain curves characterize some restricted set of defined loading conditions (such as uniaxial test, pure shear, or equi-biaxial tension), additional investigations of two-dimensional strain data from arbitrary deformations states are necessary. This strain data can be obtained for example by digital image correlation. In particular, we demonstrate how the previously described constitutive model can be calibrated using a variety of experiments. We verify the calibration comparing the results of our subroutine implemented as UMAT in LS-DYNA with two-dimensional strain data obtained from digital image correlation of a plate with a hole subjected to tension. The described model shows good agreement with the obtained data.
Approach for Modelling Thermoplastic Generative Designed Parts