Parts made from phenolic molding compounds reinforced with glass fibers have a good heat resistance, a high dimensional accuracy, and an excellent chemical resistance. However, these parts typically have a low elongation at break and a low impact toughness. Conventional glass fiber-reinforced phenolic molding compounds have an average fiber length which is below the critical fiber length for this material combination. To increase the fiber length in the molded parts, a new injection molding process was developed, which allows the direct feeding of long glass fibers into the plasticizing unit of the injection molding machine. For opening and dispersing long fiber bundles, a thermoset-specific screw mixing element was developed and compared to the conventional conveying screw geometry. The process stability was quantified by analyzing the plasticizing work and injection work. Despite the increased fiber shortening when using the mixing element, the dynamical mechanical properties were improved compared to the standard conveying screw geometry, which is attributed to the better fiber-matrix homogenization.

To quantify the homogeneity of fiber dispersion in short fiber-reinforced polymer composites, a method for image texture analysis of 3-dimensional X-ray micro computed tomography (µCT) images is presented in this work. The adaption of the method to the specific requirements of the composite material is accomplished using a statistical region merging approach. Subsequently, the method is applied for evaluating the homogeneity of specimens from an intermediate step of the long fiber thermoset injection molding process as well as molded parts. This new injection molding process enables the manufacturing of parts with a flexible combination of short and long glass fibers. By using a newly developed screw element based on the Maddock mixing element design, the material homogeneity of parts molded in the long fiber injection molding process is improved.

Programmable materials are a novel development, in which specialized production processes are used to introduce a framework of information capabilities into the inner structure of materials. Since the design and fabrication of programmable materials are still challenging, this aims to introduce a design and fabrication concept to pave the way toward industrial application. Herein, complex shape morphing has been implemented in the sense that the shape changes in response to external conditions, following a predefined program. First, the feasibility of a fabrication concept for uniform metamaterials with auxetic behavior is presented. A material with a predetermined nonuniform inner structure that deforms to a symmetrical shape has been developed and fabricated according to this concept. More complex behavior can be implemented by facilitating optimization methods to find inner structures according to a target shape. Lastly, an optimized and producible design for asymmetrical shape morphing is described to demonstrate the applicability of the approach.

Glass fiber-reinforced phenolic resins are well suited to substitute aluminum die-cast materials. They meet the high thermomechanical and chemical demands that are typically found in combustion engine and electric drive train applications. An injection molding process development for further improving their mechanical properties by increasing the glass fiber length in the molded part was conducted. A novel screw mixing element was developed to improve the homogenization of the long fibers in the phenolic resin. The process operation with the mixing element is a balance between the desired mixing action, an undesired preliminary curing of the phenolic resin, and the reduction of the fiber length. The highest mixing energy input leads to a reduction of the initial fiber length L0 = 5000 μm to a weighted average fiber length of Lp = 571 μm in the molded part. This is an improvement over Lp = 285 μm for a short fiber-reinforced resin under comparable processing conditions. The mechanical characterization shows that for the long fiber-reinforced materials, the benefit of the increased homogeneity outweighs the disadvantages of the reduced fiber length. This is evident from the increase in tensile strength from σm = 21 MPa to σm = 57 MPa between the lowest and the highest mixing energy input parameter settings.

For a newly developed thermoset injection molding process, glass fiber-reinforced phenolic molding compounds with fiber contents between 0 wt% and 60 wt% were compounded. To achieve a comparable remaining heat of the reaction in all compound formulations, the specific mechanical energy input (SME) during the twin-screw extruder compounding process was used as a control parameter. By adjusting the extruder screw speed and the material throughput, a constant SME into the resin was targeted. Validation measurements using differential scanning calorimetry showed that the remaining heat of the reaction was higher for the molding compounds with low glass fiber contents. It was concluded that the SME was not the only influencing factor on the resin crosslinking progress during the compounding. The material temperature and the residence time changed with the screw speed and throughput, and most likely influenced the curing. However, the SME was one of the major influence factors, and can serve as an at-line control parameter for reactive compounding processes. The mechanical characterization of the test specimens revealed a linear improvement in tensile strength up to a fiber content of 40–50 wt%. The unnotched Charpy impact strength at a 0° orientation reached a plateau at fiber fractions of approximately 45 wt%.

ABSTRACT Thermoset molding compounds based on glass fiber-reinforced phenolic resins are an attractive material for thermo-mechanically demanding applications. Usually, the average fiber length in commercially available molding compounds is significantly shorter than the critical fiber length. In order to investigate the potential for increasing the fiber length in the molded part, molding trials using a conventional short fiber and an experimental long fiber-reinforced compound are carried out and the fiber shortening during the process is analyzed. An improvement to the fiber length measurement is described, consisting of a repeatable and accurate down-sampling and fiber dispersion method. The nondestructive measurement of the fiber length distribution by means of X-ray computed tomography is evaluated regarding its suitability for molding compounds with a high glass fiber content. By using the introduced fiber length measurement method, a fiber shortening during the injection molding process from 370µm initial weighted average fiber length to 300µm is detected for conventional phenolic molding compounds. In contrast, the experimental long fiber phenolic compound is shortened from 11,800µm to 970µm. Analytical calculations indicate that the asymptotic fiber length, until which the fluid forces during processing are sufficiently high for breaking the fibers, is between the two measurements. This leads to the conclusion that the commercially available, state-of-the-art short fiber molding compounds leave a strong potential for improving the fiber length in the molded part unused. GRAPHICAL ABSTRACT

In practice, a dominant failure mode of brittle materials, such as fiber-reinforced thermosets is the initiation and propagation of cracks under cyclic loading. In general, the damage in composites with irregular microstructure do not occur in regular patterns. There are several state-of-the-art continuum mechanical models which take the anisotropic damage for predicting the material behavior into account. Since damage generally occurs in an anisotropic way, methods that experimentally characterize the damage anisotropy are needed.Micro-Computed Tomography systems (μCT) acquire volumetric images in a non-destructive way. By combining μCT scanning and mechanical in-situ testing, detailed microstructure data of specimens under load are generated. Based on image processing methods, the damage propagation from crack initiation to fracture is analyzed. In practice, cyclic load is a common load case for many components. Consequently, fatigue and cycle load tests are essential for a comprehensive material characterization.In this contribution interrupted in-situ μCT tests with cyclic tensile load are performed on Sheet Molding Compounds (SMC), a discontinuous glass fiber-reinforced thermset composite. Image processing methods are introduced for the experimental determination of the anisotropic damage characteristic based on volumetric images. The methods enable analyzing the spatial crack orientation distribution. In order to quantify the damage anisotropy, empirical formulations of the crack density distribution and second-order crack orientation tensor are applied. The presented work in this contribution extends the results of preliminary experimental damage investigations. In addition to previous studies, the damage anisotropy is quantified and analyzed.

The mechanical properties of plastic-based additively manufactured specimens have been widely discussed. However, there is still no standard that can be used to determine properties such as the interfacial strength of adjacent tracks and also to exclude the influence of varying manufacturing conditions. In this paper, a proposal is made to determine the interfacial strength using specimens with only one track within a layer. For this purpose, so-called single-wall specimens of polylactide were characterised under tensile load and the interfacial area between the adjacent layers was determined using three methods. It turned out that the determination of the interfacial area via the fracture surface is the most accurate method for determining the interfacial strength. The measured interfacial strengths were compared with the bulk material strength and it was found that the bulk material strength can be achieved under optimal conditions in the FFF process. It was also observed that with increasing nozzle temperature, the simultaneous printing of specimens influences the interfacial strength. To conclude, this method allows to measure the interfacial strength without superimposing the influence of voids. However, for example, the interfacial strength within a layer cannot be determined.

The use of biobased materials in additive manufacturing is arising as a promising approach to modernize the polymer industry reducing its environmental impact. Herein, novel sustainable formulations are developed for digital light processing (DLP) using five vegetable oils-sunflower, canola, soybean, olive, and sesame oil-as feedstock. These vegetable oils are successfully modified incorporating photopolymerizable groups, i.e., acrylates, enabling printability. The oil-based formulations consisting of a functionalized oil and a photoinitiator are employed as inks for DLP without the need for further additives. The rheology and curing behavior of all the inks and printed materials are carefully investigated. The values obtained for their critical curing energy (E-c) range from 14.52 to 18.49 mJ cm(-2), allowing for fast printing. Interestingly, it is found that Ec not only correlates with the average number of acrylate groups per molecule but also the viscosity plays a key role. Additionally, the thermal and mechanical properties are studied and compared. In summary, sunflower and canola oil derivatives offer a better cost-performance ratio than the state-of-the-art soybean oil inks and can be employed for 3D printing of complex geometries with high speed and resolution. This work demonstrates the potential of using biobased and inexpensive materials as high performance inks for DLP 3D printing and opens new possibilities for the next generation of sustainable 3D printing.

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.

Fiber-reinforced polymers combine the aspects of stiff fibers and light polymer matrix. The arrangement of fibers on the microstructural level significantly affects the mechanical properties on the macroscopic scale, such as stiffness and fracture toughness. Micro-computed tomography systems (μCT) enables a detailed volumetric view into microstructures in a none-destructive way. By performing mechanical test inside a μCT system, the effects of mechanical load onto the microstructure, such as damage initiation and propagation are observed in-situ. Since cyclic load is a common use case for many components, fatigue and cycle load tests are essential for material characterizing. In this contribution, the authors introduce an in-situ μCT setup for cyclic load tests. The test device is designed to generate high-resolution images, in order to observe cracks through different propagation stages. In addition, cracks are three-dimensionally segmented and a crack volume fraction definition is introduced. The presented in-situ test setup and crack characterization method are demonstrated on sheet molding compound specimen, a glass-fiber reinforced thermosetting material system.

Discontinuous fiber reinforced polymers (DicoFRP) like Sheet Molding Compounds (SMC) are frequently applied in modern lightweight designs, due to their good formability and mechanical properties at low density. The DicoFRP microstructure affects the mechanical properties and has to be taken into account in the design process. X-ray Micro-Computed Tomography (μCT) systems acquire volumetric images of microstructures in a non-destructive way. The fiber orientation is determined by using state-of-the-art image processing tools. The resolution of volumetric images and the specimen size are directly coupled due to the cone-beam μCT geometry. In order to identify all individual fibers, high resolution images are needed and consequently, only small specimen volumes are scanned. The present contribution makes use of the property that fibers are arranged as bundles within SMC. Consequently, fiber bundles instead of individual fibers are used to analyze microstructures in order to overcome the μCT-cone-beam-related conflict between sample size and image resolution. The fiber bundles are determined by means of orientation data and an introduced tracking method, facing the challenge of crossing fiber bundles. Subsequently, the tracked fiber bundles, which are related to the same mesoscopic fiber bundle are merged by using a hierarchical agglomerative clustering method. The presented method is applied to a typical SMC microstructure. This contribution introduces an image processing method for analyzing SMC microstructures based on fiber bundles, opening up the possibility to investigate large specimen volumes. This enables to characterize representative SMC microstructures and utilize the data for art modeling approaches.

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 this paper, the bending stiffness-to-weight-ratio of novel hybrid sandwich structures is investigated. The build-up of the sandwich panels consisted of face sheets made from carbon fibre reinforced polymer, aluminium foam cores and an interface of foamed polyurethane. The sandwich panels were produced in a single step, infiltrating the face sheet fibres and connecting the face sheets to the core simultaneously. By means of mechanical characterization, specimens with several variations of face sheet architecture and thickness, core structure and interface properties were examined. Quasi-static four-point bending and flatwise compression tests of the sandwich composites were conducted, as well as tensile tests of the face sheets. The results of the tensile and compressive tests were integrated in analytical models, describing the sandwich stiffness depending on the load case and the face sheet volume fraction. The effective Young's modulus of the composite, measured in the four-point bending test, correlates well to the modelled effective bending modulus calculated from the single components face sheet and core. The model underestimates the effective density of the bending specimens. It could be shown that this underestimation results from the polyurethane foam connecting the face sheets to the core, as the mass of this polyurethane is not included in the model.

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.

Fiber reinforced plastics show a wide range of different damage mechanisms such as matrix cracking, fiber breakage and interface failure. These can be observed in damaged specimens by means of volumetric images acquired by computed tomography (CT). As each failure mechanism causes a characteristic acoustic emission (AE) signal, AE analysis is a promising tool to identify damage mechanisms and offers the advantage that a real-time observation of the damage evolution during the testing period is possible. For a correlation of damage mechanisms and AE events, AE analysis was combined with in- situ CT measurements. This combined approach was validated by means of a 3-point-bending test on a discontinuous glass fiber reinforced sheet molding compound (GF-SMC) in which AE signals were acquired during loading using two high frequency piezoelectric sensors. At times of increasing AE activity, the test was interrupted in order to carry out a CT-scan of the specimen under load. AE events could subsequently be linked with the damage mechanisms observed in the CT-scans at different stages of damage to identify signal features that are characteristic for a certain mechanism. The sources of the signals could be localized and were in line with the actual location of damage.

This work focuses on failure mode maps of sandwich panels exposed to bending load, which were produced using a polyurethane spraying process. This process allows for an automated production of sandwich panels omitting a separate bonding step of the face sheets to the core. The investigated sandwich panels consisted of carbon fiber reinforced face sheets in various configurations, and four different core structures of aluminum foam or Nomex honeycomb. After production, measurements of the pores inside the core foam structures, the fiber package thickness inside the face sheets, and the density homogeneity of the core structure were made using X-ray computed tomography. The failure mode maps were based on the individual mechanical properties of the face sheets and the core, determined by mechanical testing. The critical forces determining the failure modes were partially modified to fit the application on foam core structures and face sheets with a porous matrix. The verification of the failure modes was performed with four-point bending tests. Since all tested configurations of sandwich specimens were produced using the same process route, the applied models for the creation of the failure mode maps could be verified for numerous parameter combinations. Except for two parameters with inconstant properties, the failure modes determined by the failure mode maps matched the observed failure modes determined by the bending tests.

To achieve high levels of lightweight design in automotive or aerospace industry, it is necessary to optimize certain composite material systems concerning their lightweight potential. With sandwich composites, which generally consist of a core which is coated with two face-layers, it is possible to reduce the overall part weight while increasing the specific mechanical properties under inhomogeneously distributed loads. Hereby the sandwich core ensures the load transmission while the face layers absorb the tensile and compressive loads occurring at bending stress. The aim is to increase the efficiency of said sandwiches while minimizing the weight per area by using for example (fiber-) reinforced face layers and foamed core materials. Polyphenylene sulfide (PPS) is an important high temperature, engineering thermoplastic polymer. Because of its properties it is used in the automotive, aerospace and electronics industry. As polyphenylene sulfide has a high processing temperature and requires processing know-how, there a few scientific studies on its foaming behavior. To understand the process-structure-property relationship of this material system, an experimental study on the foaming behavior of neat polyphenylene sulfide and long glass fiber reinforced polyphenylene sulfide using the high-pressure foam injection molding process is conducted. As it is also attractive from a manufacturing point of view to reduce the number of manufacturing steps and to operate with the source materials from the beginning of the process an in-line compounding and foam injection molding process is used. This allows the manufacturing of integral sandwich structures with connected, fiber reinforced face and foamed core layers in one shot via “direct thermoplastic foam injection molding”. With the findings of this study researchers and manufacturers of neat and fiber reinforced PPS parts are able to identify process parameter boundaries and the influence of these parameters on the sandwich structure and mechanical properties.

As the demand of the automotive and aerospace industries for lightweight and cost effective materials increases, it is necessary to combine different materials with respect to their lightweight and functional properties. The combination of polymer-steel-sandwich composites - which consist of a polymer core structure (transferring shear loads) and two metal face-layers (absorbing tensile and compression loads occurring at bending) - suite the need of minimizing weight per area under bending loads. The reduction of process steps can be achieved by connecting the face layers and core in-situ via an in-mold assembly process using variothermal processing. The injection mold hereby is heated near the melt temperature of the polymer with a variothermal water processing unit. Via contact heating inserted steel blanks are heated to the same temperature as the mold. This process enables the combination of the metal surface with the polymer core by infiltrating the micro or nano scale structure, which is generated by laser structuring or nano coating. Through the increased mold/blank surface temperature induced via variothermal heating the melt viscosity is lowered. This decreasing viscosity of the polymer melt hereby enables a higher degree of infiltration of the laser structured and nano coated blanks. This improved infiltration behavior is a key factor for the adhesion of the sandwich components and beneficial for the composites strength. Within this work two steel blanks are inserted into the mold to manufacture sandwich structures with steel face layers and a polymer (here: polylactidacid; PLA) core. As these sandwich composites are prone to bending failure, the 4-point-bending test is used to characterize the mechanical properties of this hybrid structure. The two surface treatments will also be compared concerning their mechanical interface properties with a shear edge test. The additional reduction on the polymer melt viscosity by means of gas inducing with chemical blowing agent is investigated on the laser structured surfaces only. To investigate the influence of the polymers melts viscosity on the bonding properties chemical blowing agent is added for some blanks.

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.