Fiber reinforced polymers, for example sheet molding compounds (SMC), have gained significant importance as lightweight materials. In this work, a SMC composite based on unsaturated polyester-polyurethane hybrid resin (UPPH) reinforced with discontinuous long glass fibers is considered. The mechanical behavior of this composite is highly governed by its microstructure. Based on samples extracted from compression molded sheets, the underlying microstructure is characterized by means of X-ray computed tomography (µCT) scans. Tensile tests are performed to investigate and characterize the elastic properties of this composite. A mean-field method is presented to approximate the effective elastic behavior. In this context, the tensile tests performed serve as validation data for the simulation results. The effective Young’s modulus is in good agreement with the experimentally obtained data with a deviation less than 15%.

A micromechanics model that utilizes microstructural characteristics for predicting the bending stress-strain response of a SMC composite is presented. The model integrates matrix, interphase and fibre damage models to form a framework that is able to accurately represent the damage response of the SMC using a single parameter damage model that is equivalent to the von mises interfacial stress. This is a valuable capability given the inherent random nature of SMC properties. Before being subjected to three-point bending tests, three SMC samples from different areas of a molded plate were characterized using X-ray computed tomography. The through thickness properties such as fibre orientation and volume fraction were characterized for multiple layers and then used as input data for a finite element model to predict the stress-strain behaviour. It was found that prediction accuracy increased with more layer properties. The model was able to represent the stress strain results within 2% of the experimentally measured response using 12 microstructural layers for a 2.8 mm thick specimen.

Purpose The purpose of this paper is to understand the relationship between defect properties and the tool path used for generating additively manufactured parts. The correlation between processing strategy and porosity architecture is one of the key aspects for a precise understanding of defect formation and possibilities for defect reduction. Design/methodology/approach The authors present a new combined geometry, processing path and porosity analysis procedure based on the use of x-ray computed micro tomography image data and numerical control programming code. The procedure allows for a covisualisation of the track of the respective processing head with the three-dimensional microstructure data. Findings The presented method yields statistical results about defect distribution and morphologies introduced by the respective process characteristics in parts. The functionality of the proposed procedure is demonstrated on an aluminum (AlSi10Mg) and a polylactide test sample to show the additional insight found for both additive manufacturing processes and the resulting microstructural properties. Originality/value The novelty of this paper is the analysis of the porosity with respect to the underlying additive process zone and the sample geometry.

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.

No AccessContinuous–Discontinuous Fiber-Reinforced PolymersAug 2019Compression Molding of the Demonstrator StructureJohannes Görthofer, Nils Meyer, Ludwig Schöttl, Anna Trauth, Malte Schemmann, Pascal Pinter, Benedikt Fengler, Sergej Ilinzeer, Martin Hohberg, Tarkes Dora Pallicity, Luise Kärger, Kay A. Weidenmann, Peter Elsner, Frank Henning, Andrew Hrymak, Thomas BöhlkeJohannes GörthoferSearch for more papers by this author, Nils MeyerSearch for more papers by this author, Ludwig SchöttlSearch for more papers by this author, Anna TrauthSearch for more papers by this author, Malte SchemmannSearch for more papers by this author, Pascal PinterSearch for more papers by this author, Benedikt FenglerSearch for more papers by this author, Sergej IlinzeerSearch for more papers by this author, Martin HohbergSearch for more papers by this author, Tarkes Dora PallicitySearch for more papers by this author, Luise KärgerSearch for more papers by this author, Kay A. WeidenmannSearch for more papers by this author, Peter ElsnerSearch for more papers by this author, Frank HenningSearch for more papers by this author, Andrew HrymakSearch for more papers by this author, Thomas BöhlkeSearch for more papers by this authorhttps://doi.org/10.3139/9781569906934.006SectionsAboutPDF ToolsAdd to FavoritesDownload CitationTrack CitationsCopy LTI LinkPDF key 'share (en)' returned an object instead of string.FacebookTwitterEmailLinkedIn previous chapternext chapter FiguresReferencesRelatedDetails 2019Pages: 297-314Print ISBN: 978-1-56990-692-7eISBN: 978-1-56990-693-4 Copyright & Permissions© 2019 Carl Hanser Verlag GmbH & Co. KGPDF downloadLoading ...

In shape memory alloy metal matrix composites manufactured by continuous composite extrusion the strategies of active property tuning and active strain energy tuning are used for the improvement of the mechanical properties of the components. Due to the thermal activation of the embedded NiTi wires (SM495), compressive stresses are transferred to the surrounding aluminum matrix (AA6060). At elevated temperatures tensile tests, three-point-bending tests and notch impact test show the influence of temperature, prestrain and reinforcing volume on the component performance. In tensile testing, a simultaneous increase of strength and ductility can be found, leading to an increase of the energy abortion capacity. Results of the three-point-bending test and notch impact test also show an increase of the required work depending on the thermomechanical treatment of the specimens.

A virtual process chain for sheet molding compound (SMC) composites is established and validated by means of experimental investigations on a demonstrator structure. The flow in the compression molding step is simulated via a Coupled-Eulerian-Lagrangian approach using an anisotropic non-Newtonian fluid flow model. Evolution of the fiber orientation distribution (FOD) is described by Jeffery's equation. The predicted FOD is mapped to structural simulations employing a neutral data format. A mean-field anisotropic damage model is used to predict the damage evolution in the demonstrator. Simulated FOD at the end of the compression molding is validated by computer tomography. Structural simulations are validated by means of a cyclic four-point bending test on the demonstrator. The predicted results show increased accuracy with the experiments by transferring FOD data within the virtual process chain. Critical points of high damage concentrations leading to failure agree with the experimental observations.

This contribution presents a physical process chain and the corresponding virtual process chain for sheet molding compound (SMC) composites. Here, focus lies on the physical process chain as a motivation for the virtual process chain as discussed in the authors' publication [1]. The key steps of the virtual process chain are the identification of initial and boundary conditions, the compression molding simulation, the mapping of data and the structural simulation. The so established virtual process chain is validated via experimental investigations on a demonstrator structure. Both, the predicted results of the compression molding simulation, as well as the results of the structural simulation are in good accordance with the experiments.

Fibre orientation represents one of the most important micro-structural characteristics for fibre reinforced composite materials. Its influence on mechanical behaviour, material anisotropy and damage evolution has, especially with the modern trend to mass production of structural composite parts, moved back into the focus of micro-structural analysis. Following this development and the enormous improvements of X-ray computed tomography resolution and evaluation techniques in the field of composites the 3D analysis of fibre orientations throughout macroscopic components has become possible. Therefore, the complex connection between manufacturing process and manifested fibre architecture can be determined and correlated. Various image analysis methods exist in the material science and image analysis community which can be employed to extract fibre orientations of an image. In this work we compare three methods based on anisotropic Gaussian filtering, Hessian matrix calculation or structure tensor calculation in their ability to describe correct fibre orientations and their error-proneness with respect to imaging modalities and material composition. Those methods have become popular also in commercial software platforms for micro-structure analysis in user-friendly forms. All three algorithms were implemented with the open source imaging tool-kit ITK. In order to compare those methods, error estimates and testing procedures will be described herein.

For predicting the mechanical response of mis aligned short fibre composite materials a two step homogenization procedure coupled with orientation averaging of the elastic material properties is often used. Orientation averaging is not valid for long fibre composites due to non-symmetric strain localization tensors. A method to homogenize the stress for mis aligned long fibre composites using the Functionally Graded Interphase (FGI) approach is presented and the results are compared to two step Mori Tanaka homogenization. The microstructure of long fibre Sheet Moulding Compound (SMC) composite is evaluated using X-ray micro computed tomography. The measured fibre orientation distributions are used to evaluate the number of representative fibre orientations required to predict the stress state of the SMC due to applied strains. It is found that 60 orientations are required to capture the stress response of the long fibre SMC composite using the FGI model and two step Mori Tanaka model.

Characterization of 9% to 25% weight fraction compression molded carbon fibre LFT-D polyamide-6 was completed with orientations of 0°, ±45°, and 90°. A key finding is that tensile stiffness/strength and flexural stiffness were higher in the +45° direction compared to −45° (tensile modulus: 20%, strength: 10%, flexural modulus: 8%). Correspondingly, engineering strain at failure for uniaxial tensile tests was 18% lower in the +45° direction. This is hypothesized to be the result of fibre orientation asymmetry in the compression molding charge. Fibre orientation was measured by CT, fibre length was measured by matrix incineration/optical microscopy, and micromechanics models were employed to model elastic characteristics. An effectiveness was established for each process configuration by comparing the experimental modulus to the Generalized Self Consistent model. For the 0° direction, this effectiveness is approximately 90%. However, fibre length is more critical for the specimen geometry studied in the ±45° and 90° directions and this effectiveness is much lower (approximately 70%). Measurements of fibre length by an industrial partner without experience with direct compounding yielded data, that when input into the micromechanics models, poorly predicted mechanical characteristics for 0°. A length measurement methodology yielding accurate data is critical in modelling basic mechanical properties.

Based on two artificial microstructures representing a long fiber reinforced thermoset material, the effective linear elastic material properties are calculated by both a mean and a full field homogenization method. Concerning the mean field method, the effective elastic material properties are approximated using the homogenization scheme by Mori and Tanaka, formulated explicitly in terms of orientation averages. This allows to use orienation tensors of 2nd and 4th order describing the orientation information on the micro level. The full field method is based on the fast Fourier transformation (FFT), for which the effective material properties are determined by volume averaging. The comparison between both methods show good agreements, the deviations are in the range between 2% and 12%. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Additive manufacturing provides the ability to produce structural components featuring complex shapes in one step, compared to traditional methods of production. Therefore, additive manufacturing has recently gained attention for the direct production of parts. Using fibre reinforced filaments offers the opportunity to improve the mechanical properties of FFF printed components. In order to dimension them correctly, the mechanical properties of additive manufactured samples based on glass fibre reinforced filaments were determined. Additionally, the influence of extrusion paths resulting in a distinct fibre orientation were taken into account. Samples were produces by FFF-method (Fused Filament Fabrication) from three materials: Bulk ABS and short glass fibre reinforced ABS featuring 5 wt% and 10 wt% fibre content. Additionally, samples were printed in two different raster orientations of 0° and 90°. Three different sample types were manufactured in order to perform tension, flexural and impact tests. Prior to printing the samples, the slicer parameters were optimized for usage with the fibre reinforced filament. To determine the FOD (Fibre Orientation Distribution) and FLD (Fibre Length Distribution), the samples were scanned using a CT. Results show that fibre reinforced filaments used in this contribution can increase stiffness to 150 % of the bulk material in printing direction with a fibre weight content of 10 %. CT investigations have shown that the orientation of fibres is primary aligned to the printing path.

Sheet molding compounds (SMC) are discontinuously fiber-reinforced thermosets, attractive to the automotive industry due to their outstanding specific strength and stiffness, combined with a cost efficient manufacturing process. Increasingly important for structural components, a structural SMC-based improved resin formulation featuring no fillers is investigated in this study. The influence of fiber volume content, fiber length, and manufacturing induced fiber orientation on quasi-static and dynamic mechanical properties of vinylester-based SMC is characterized. Stiffness and strength increased with increasing fiber volume content for tensile, compression, and flexural loadings. Fiber length distribution did not significantly influence the mechanical properties of the material. The movement of the conveyor belt leads to an anisotropic fiber orientation and orientation-dependent mechanical properties. Acoustic emission coupled with machine learning algorithms enabled the investigation of the damage mechanisms of this discontinuous glass fiber SMC. The acoustic emission analysis was validated with micro computed tomography of damaged specimens. The dominant failure mechanisms of the SMC exposed to bending loading were matrix cracking and interface failure.

Carbon-fiber-reinforced plastics (CFRPs) are gaining increasing applicability to lightweight structures (e.g., automotive applications) due to their outstanding mechanical properties. High-performance parts can be fabricated from CFRPs, but they have the disadvantages of low shear and bearing strength. To achieve detachable connections and introduce loads without decreasing the load-bearing capacity of the composite, it is important to use mechanical fasteners without drilling into the parts. To accomplish this, metal elements called inserts are embedded in the CFRP laminate. Damage behavior in a CFRP under tensile conditions has several different mechanisms, depending primarily on the deformation of the insert. This research investigates the in-situ failure behavior of the composite under tensile loads by investigating the deformation of the insert via computed tomography (CT). The results are also used for validation of the insert’s deformation using a finite-element model (FEM).

Failure of fiber reinforced polymers is a complex interaction of different microstructural mechanisms. In order to assign those mechanisms to the macroscopic material response, in-situ methods as acoustic emission can be applied. This allows for the detection of initiation and growth as well as for the localization of damage in mechanically loaded materials. In this study, mechanical material testing of continuous and discontinuous fiber reinforced polymers was coupled with acoustic emission. Results have shown that different failure mechanisms resulting from different reinforcement architectures can be distinguished due to their acoustic emission signal. Based on experimentally captured acoustic emission signals, machine learning algorithms were applied to differentiate various failure mechanisms. This offers the possibility to investigate damage of hybrid continuous-discontinuous Sheet Molding Compounds exposed to bending loads.