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Modeling the anisotropic temperature-dependent viscoplastic deformation behavior of short fiber reinforced thermoplastics

( 2021)
Xu, H.
;
Kuczynska, M.
;
Schafet, N.
;
Welschinger, F.
;
Hohe, J.

Short fiber reinforced thermoplastics (SFRT) are widely used for automotive components. One of the significant challenges in designing industrial SFRT components is an efficient prediction of their mechanical response under mechanical and thermal loads. In this work, an anisotropic temperature-dependent elasto-viscoplastic model is implemented using the available macroscopic material models in commercial FE solver to describe polybutylene terephthalate with 30 wt.-% short glass fibers. The elastic behavior is described by the orthotropic linear elastic model generated through the mean-field homogenization method and the anisotropy in plastic region by Hill yield criterion dependent on fiber orientation. The rate-dependent plasticity is described by the unified viscoplasticity framework of Chaboche. To describe the continuous temperature dependency, model parameters are systematically determined as a function of temperature in the typical automotive temperature range, including regions below and above glass transition temperature. Further, an optimization method based on genetic algorithm is adopted for parameter optimization. The optimized model accurately describes the anisotropic material behavior observed in tensile and stress relaxation tests in a wide range of temperature, specimen orientation, and strain rate. The models prediction capability is validated by simulating tensile tests at three intermediate temperatures, which are not included during the calibration process.
Mechanical properties of a unidirectional basalt-fiber/expoxy composite

( 2020)
Plappert, David
;
Ganzenmüller, Georg
;
May, Michael
;
Beisel, Samuel

High-performance composites based on basalt fibers are becoming increasingly available. However, in comparison to traditional composites containing glass or carbon fibers, their mechanical properties are currently less well known. In particular, this is the case for laminates consisting of unidirectional plies of continuous basalt fibers in an epoxy polymer matrix. Here, we report a full quasi-static characterization of the properties of such a material. To this end, we investigate tension, compression, and shear specimens, cut from quality autoclave-cured basalt composites. Our findings indicate that, in terms of strength and stiffness, unidirectional basalt fiber composites are comparable to, or better than epoxy composites made from E-glass fibers. At the same time, basalt fiber composites combine low manufacturing costs with good recycling properties and are therefore well suited to a number of engineering applications.
A novel approach for segmenting and mapping of local fiber orientation of continuous fiber-reinforced composite laminates based on volumetric images

( 2020)
Schöttl, L.
;
Dörr, D.
;
Pinter, P.
;
Weidenmann, K.A.
;
Elsner, Peter
;
Kärger, L.

High stiffness and low density of continuous fiber-reinforced polymer (CoFRP) composites lead to increasing importance of this material class in today's lightweight components. One of the most important subgroups of CoFRP composites are thermoplastic UD-tapes. These consist of several unidirectional continuously fiber-reinforced layers which are aligned with different orientations. During the forming process, the initial fiber orientations of the laminate layers change individually. Since the mechanical properties like stiffness or damage behavior are significantly affected by the fiber orientation, methods for determining the fiber orientation distribution are essential to design composite components and validate process simulations. Modern X-ray computed tomography offers the opportunity to obtain high-resolution gray value volumetric images of fiber-reinforced structures. Methods to determine vectors aligned along the local fiber orientation are available in commercial and open-source software. In this paper, we present and compare several segmentation approaches based on layer thickness, fiber orientation angle and degree of fiber isotropy to separate each unidirectional tape layer and to analyze the layers individually. Moreover, we introduce mapping approaches, to transfer local fiber orientation of each tape layer to a discretized surface. The presented approaches can be applied to both plane and curved shell-shaped samples. Finally, the approaches are applied to a carbon fiber-reinforced polyamide 6 (PA6-CF) tape.
The effect of strain rate on the orientation of the fracture plane in a unidirectional polymer matrix composite under transverse compression loading

( 2020)
May, Michael
;
Ledford, Noah
;
Isakov, Matti
;
Hahn, Philipp
;
Paul, Hanna
;
Nagasawa, Sumito

Transverse compression tests on a unidirectional composite were performed under quasi-static and high-rate loading conditions using servo-hydraulic machines as well as a direct impact Hopkinson bar. Aside the expected increase of compressive strength with increasing loading rate, a change of fracture plane orientation was observed. For quasi-static loading conditions, the fracture angle was 54.5°, for high rate-loading conditions this increased to 65°. Assuming a Mohr-Coulomb type of fracture for unidirectional composites under transverse compression loading, the change of fracture plane orientation indicates a rate dependency of the internal friction angle f, which has not previously been reported for composite materials.
Fracture toughness measurement without force data - Application to high rate DCB on CFRP

( 2019)
Isakov, Matti
;
May, Michael
;
Hahn, Phillipp
;
Paul, Hanna
;
Nishi, Masato

The measurement of the fracture toughness of fiber reinforced composites at high rates of loading is still, despite years of research, not well established. This can be related to challenges in applying appropriate high rate loading on the specimen, accurately measuring the load, and in-situ determination of the crack length. In this work these challenges are addressed by using a direct wedge-on-specimen type loading of a double cantilever beam (DCB) specimen, high resolution optical deformation tracking, and a beam theory based analysis of the specimen deflection and crack length. This approach results in symmetric mode I opening of the crack and a robust analytical determination of the fracture toughness without the need to measure the external forces acting on the specimen nor to visually estimate the crack length. Tests carried out on carbon fiber reinforced epoxy composite at quasi-static and high rates (relative velocity up to 15 m/s) show the validity of the approach.
Pull-out testing of multiscale structured metallic z-reinforcements for CFRP laminates

( 2017)
Juergens, Michael
;
Hafe Pérez Ferreira da Silva, Manuel Tiago von
;
Heimbs, Sebastian
;
Lang, Holger
;
Ladstaetter, Elisabeth
;
Hombergsmeier, Elke

A testing method is presented to determine the pull-out behavior of interleaving metallic z-reinforcements for carbon fiber reinforced polymer (CFRP) laminates and joints. Energy absorbing mechanisms are described with respect to the metallic materials and means of surface pretreatment applied. Mechanical, wet-chemical and physical pretreatments result in milli-, micro- and nanoscaled oxide morphologies of titanium and stainless steel surfaces. Both, a high macro roughness of the reinforcement surface and a low difference in thermal expansion with regard to the surrounding CFRP is clearly proven to feature the highest level of energy absorption during pull-out tests. Surface analyses through scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) indicate laser-induced nanostructure's scale and morphology to provide good adhesion properties but not to allow macroscopic friction between metal surface and epoxy resin.
Investigations into the damage mechanisms of glass fiber reinforced polypropylene based on micro specimens and precise models of their microstructure

( 2017)
Fliegener, S.
;
Kennerknecht, T.
;
Kabel, M.

Fiber reinforced thermoplastics are considered as promising candidates to enable the mass production of lightweight components. To assure their structural application, accurate methods to describe their mechanical behavior are mandatory. However, the modeling of the damage behavior of the thermoplastic matrix under multiaxial stress states within the microstructure still poses enormous challenges. In order to capture the in situ behavior, a novel methodology is applied. Tensile tests are conducted on micro specimens, which are manufactured from 100 mm thin slices of the material's cross section. A corresponding finite element mesh which precisely depicts the individual microstructure of each sample is reconstructed based on microscope images and computer tomographic (CT) scans. Since the dimensions of the specimens are sufficiently small, the position and orientation of each fiber can be directly mapped to the model. Furthermore, the evolution of deformation and damage within the microstructure is visible in microscope images. Inverse simulations are performed to separate the mechanical behavior of the matrix, the fiber-matrix interface and the fibers. On a first specimen with approximately 5 vol-% fiber fraction, the constituent properties are calibrated by an automated parameter optimization procedure until the global stress-strain curve of the simulation agrees well with the experiment. The approach is validated by the simulation of a second specimen with a strongly different fiber fraction (15 vol-%) and thus, a strongly different damage behavior. It is shown that the stress-strain curves and the observed damage evolution of both specimens can be accurately captured applying the same material parameters. As an outlook for potential industrial applications of the method, mesh free fast Fourier transform (FFT) simulations are performed. Such models are based on raw CT voxel data, rendering the extensive work of geometrically reconstructing the microstructure obsolete.
Numerical simulation and experimental verification of hollow and foam-filled flax-fabric-reinforced epoxy tubular energy absorbers subjected to crashing

( 2017)
Sliseris, J.
;
Yan, L.
;
Kasal, B.

Numerical methods for simulating hollow and foam-filled flax-fabric-reinforced epoxy tubular energy absorbers subjected to lateral crashing are presented. The crashing characteristics, such as the progressive failure, load-displacement response, absorbed energy, peak load, and failure modes, of the tubes were simulated and calculated numerically. A 3D nonlinear finite-element model that allows for the plasticity of materials using an isotropic hardening model with strain rate dependence and failureis proposed. An explicit finite-element solver is used to address the lateral crashing of the tubes considering large displacements and strains, plasticity, and damage. The experimental nonlinear crashing load vs. displacement data are successfully described by using the finite-element model proposed. The simulated peak loads and absorbed energy of the tubes are also in good agreement with experimental results.
Rheological In-Mold Measurements and Characterizations of Sheet-Molding-Compound (SMC) Formulations with Different Constitution Properties by Using a Compressible Shell Model

( 2017)
Hohberg, Martin
;
Kärger, Luise
;
Bücheler, David
;
Henning, Frank

The rheological characterization of Sheet Molding Compound (SMC) and its modelling is crucial for reliable process simulations. In the past, characterization and material modelling were mainly focusing on SMC with low glass fiber content and a high filler fraction. Due to new application areas, SMC without fillers and with high glass fiber contents, and SMC with carbon fibers become more important. Therefore, these three types of SMCs are characterized in this work, using an inline rheological tool. Differences regarding their compressibility and their flow dependency are identified and considered in an analytical shell modelling. The comparison of the different materials leads to a better understanding of the phenomenological parameters related to the viscosity and friction in the models. Furthermore, the importance to properly consider all relevant material-specific effects becomes evident.
Numerical modelling of flax short fibre reinforced and flax fibre fabric reinforced polymer composites

( 2016)
Sliseris, J.
;
Yan, L.
;
Kasal, B.

The ever-increasing demand of flax short fibre-reinforced and flax fibre fabric-reinforced polymer composites in various engineering applications calls for accurate predictions of their mechanical behaviors. In this study, numerical methods to generate and simulate mechanical properties of flax short fibre-reinforced and flax fibre fabric-reinforced polymer composites are proposed. The microstructures of short flax fibres with different fibre length-to-diameter ratios are generated by algorithm taking fiber defects (e.g. kink band) and fiber bundles into account. Bidirectional flax fabric is generated and discretized by tetrahedron 4-node finite elements. A brittle material law for fibre defects and interfacial zones of fibre bundles is proposed. Flax short fibre/polypropylene and flax fabric/epoxy composites are modeled by a non-linear plasticity model considering an isotropic hardening law and non-local continuum damage mechanics. The numerical modelling results are compared with the experimental results of these composites. This study shows that the simulation can capture the main damage mechanisms of the composites such as fibre breakage initiated at the fiber defects, damage of polymer matrix and the fibre debonding at fibre/matrix interface accurately. In addition, the simulation results exhibit good agreements with the experimental results in the aspects of elastic properties and nonlinear tensile stress-strain behavior of the short fibre and fibre fabric reinforced polymer composites.

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