Key Engineering Materials Vol. 809

Abstract: Over the last decade, carbon fibre-reinforced composites (CFRP) are increasingly used as lightweight material for various industrial applications. Due to the anisotropic material structure and its corresponding properties, novel design concepts and processing technologies were developed to further harness the material inherent lightweight potential. However, the material degradation in long-term use and failure behaviour is still considered a challenging issue for material scientists and engineers in particular. Therefore, concepts for structural health monitoring and their suitable implementation is still a major research topic. Among others, one solution uses the conductivity of carbon fibre yarns and their suitability to act as in-situ strain sensors. In the present work, the measurement principle bases on the usage of the piezo-resistive effect, meaning that every mechanical strain of the roving filaments causes a correlative change of the measurable electrical resistance. Since, these sensory elements need shielding from their surrounding carbon filaments of the composite structure, suitable fibre-based dielectric jackets have been developed with a wide range of suitable materials and textile processing technologies. In this contribution, the influence of the integrated carbon fibre sensors on the resulting mechanical performance of the composite structure is evaluated using an analysis of variances approach. Beyond that, the local composite morphology is analysed to evaluate the composite microstructure.

Abstract: Hybrid laminates consisting of fibre-reinforced thermoplastic films and metallic thin sheets are successively replacing thermoset based systems due to their obvious advantages of higher formability and aptitude for mass production. In order to monitor the material under operating conditions, hybrid laminates need to be equipped with smart sensor units. Artifact-free integration of commercial strain gauges into hybrid laminates is almost impossible. Therefore, a new thin film strain sensor based on a PVD sputtering process was developed.The aim of this work was to evaluate the influence of the layer thickness as well as the elevated temperature during the sputtering process on the electrical performance of Ni-C strain sensors. The Ni-C films with different layer thickness and different sputtering temperatures manufactured by means of a magnetron sputtering process were investigated for the sheet resistance and the change of temperature coefficients of resistance. In addition, Raman spectroscopy was utilized to investigate the phase development with regard to different sputtering temperatures. It can be seen that the gauge factor gets doubled while optimizing the layer thickness. When the sputtering temperature was increased, the graphitic phase formation was preferred and the impurities were reduced. These results are discussed in this paper and appropriate solution concepts are provided.

Abstract: Fiber coatings for BN/SiC-and BN/Si₃N₄-bilayer systems were developed for the use in SiC/SiC composites. All coatings were produced with high process velocities of 500 m/h by a continuous roll-to-roll dip-coating process. The fiber surface was fully covered with a homogeneous coating and without fiber bridging. Tensile tests of fiber bundles were used to examine potential degradation of the fiber properties due to the application of the coatings. The coated fiber bundles showed a reduction of the maximum tensile load to 90.0 % for the BN/Si₃N₄ and to 86.7 % for the BN/SiC coating in comparison to the fiber bundle in the as-received state. A thermal treatment of the coated fiber bundles up to 1650 °C led to no reduction of their maximum tensile load. SiC/SiC composites were fabricated by polymer infiltration and pyrolysis. The flexural strength and strain of composites with BN/SiC fiber coating were improved to 467 MPa and 0.42 % in comparison to the composites without fiber coating. The composites with BN/SiC coating showed toughened fracture behavior with fiber pull-out effects.

Abstract: Ternary transition metal nitrides are commonly used as protective coatings on cutting tools, owing to their excellent mechanical and wear properties. While AlTiN is a very well-studied material, little is known about AlZrN, in part due to the large miscibility gap in the phase diagram of AlN-ZrN. In this study, AlZrN thin films were prepared using chemical vapor deposition. By the reaction of metallic aluminum and zirconium with HCl gas under elevated temperature, AlCl₃ and ZrCl₄ were produced in situ and subsequently transported into a heated coating reactor with a carrier gas. Due to the high temperatures and the separately introduced mixture of NH₃ and N₂, AlZrN coatings were deposited. By varying the experimental conditions, such as the ratio between ZrCl₄ and AlCl₃, we studied the influence of these parameters on the coating thickness and morphology as well as the microstructure. Additionally, the impact of different sample positions in the coating reactor on the deposited coatings was investigated. Furthermore, the generated samples were characterized by scanning electron microscopy, energy dispersive x-ray spectroscopy and transmission electron microscopy.

Abstract: An effective integration of natural fibers into engineering thermoplastics requires sufficient thermal stability of natural fibers during processing, since melting temperature of engineering thermoplastics lies above 200 °C. The aim of the work was to protect natural fibers from the heat of the molten thermoplastic via coating with a modified epoxy resin, thus enabling manufacture of natural fiber-reinforced engineering thermoplastic composites with minimized thermal degradation of the fibers. Processing temperature comprised the range of engineering thermoplastic polyamide 6 (PA6), which was 225 °C. Flax fabrics were spray coated with partially bio-based epoxy resin and incorporated via hot press technique into a PA6 matrix. The composite samples including spray coated flax fibers as well as the reference flax fibers without coating were characterized with regard to their mechanical properties, namely bending and tensile tests, thermal properties with differential scanning calorimetry (DSC) as well as thermogravimetric analysis (TGA) and optical via scanning electron microscopy (SEM) and computer tomography (CT). The results show that this approach enables manufacture of composites with reproducible mechanical properties, i.e. bending and tensile properties as well as enhanced thermal stabilities.

Abstract: An increase of the service life of tribological systems subjected to dynamic-mechanical loads is important for numerous mechanical applications. The present study deals with the impact of several micro-structured surface topographies of graded Cr/CrN_x/(Cr,W)C_y/a-C:H:W/a-C:H PVD hard coatings on their friction and wear behavior. The coatings were applied by reactive magnetron sputtering on a hardened 1.2379 steel substrates and subsequently micro-structured by laser ablation using a picosecond laser. Pin-on-disc tests were carried out against aluminum under both oil lubrication and dry conditions. The diameters of the micro-dimples were varied between 50 μm, 100 μm and 150 μm at a constant degree of coating coverage of about 60 %. The coefficients of friction and wear were determined after 20,000 cycles by confocal laser-scanning microscope (CLSM), scanning electron microscopy (SEM) and by energy-dispersive X-ray spectroscopy (EDX) to analyze possible transfer layer formations.

Abstract: Modeling failure and progressive damage of long fibre reinforced thermoplastics (LFT) presents a challenging task since local inhomogeneities, anisotropic fibre orientations, and strain-rate dependence must be taken into account also on the microscale. We show that for simple geometries, the material behaviour of the composite can be modelled using layered geometrical models. But for more complex geometries, this approach fails since the fibre orientation distribution is inhomogeneous. In this case, multiscale methods allow the accurate and efficient prediction of the material behaviour with the local fibre orientation taken from an injection molding simulation. This material model can be extended to viscoplasticity and integrated into the NTFA-TSO method from Michel & Suquet (2016). In this way, we can obtain an accurate and efficient multiscale method for the realistic modelling of LFT.

Abstract: The presented study emphasizes a favored design for carbon fiber reinforced plastic fiber patterns at intersection points of truss-like structures made with the Tailored Fiber Placement technology. Three different pattern types have been experimentally and numerically analyzed. A straight fiber crossing is the most simple design, but it cannot compete against a fanned out fiber pattern regarding structural stiffness, where fiber spacing increases with increasing crossing distance. A pattern design, where belt and web fiber paths merge, is the most preferred design due to a minimum of material waste, however it exhibits lower stiffness compared to the fanned out pattern.

Abstract: The simulation of load application elements requires the modeling of the contact point and a nonlinear analysis. This contact analysis is still time-consuming despite of powerful computers. A reduction of this contact by a simple load model would result in enormous time savings. The Hertzian contact theory provides an analytical approach to the contact problem. However, an isotropic material behavior is assumed, which is problematic especially with fiber reinforced structures. Nevertheless, a suitable load model can be developed for a simplified model of a bolt joint. The edge effects occurring at the edge of the hole are determined using an approximation function (parameterized polynomial approach). The anisotropic material behavior is represented by alternative models or it can also be integrated into the calculation by an extension of Hertzian theory. The different approaches are compared in respect of accuracy, complexity and computing time. For reference and verification of the results, a contact model is created using the FEM software HyperMesh and Optistruct from Altair. Besides the contact model can be used as an aid for creating the load model. Finally, a method is presented, which reduces a contact analysis to a purely linear static structural analysis and thus enables a significantly reduced computing time. The corresponding load model also gives a good representation of reality.

Abstract: The compression molding of sheet molding compounds (SMCs) is typically thought of as a fluid mechanics problem. The usage of CF-SMC with high fiber volume content (over 50%) and long fiber reinforcement structures (up to 50 mm) challenges the feasibility of this point of view. In this work a user-defined material model based on a solid mechanics formulation is developed in LS-DYNA^®. The material model is built on a modular principle where the different influence factors caused by the material characteristics form building blocks. The idea is that these blocks are represented by simple mathematical models and interact in a way that forms the overall behavior of the SMC material. To analyze the behavior of the SMC material and create input parameters for the material model it is necessary to perform some kind of material characterization experiment. This paper presents the press rheometry test which can be perform in two variations, differing in terms of specimen size and shape and degree of coverage in the tool. Here the material response to the compression molding can be analyzed and by the visualization of the flow front development the anisotropy and homogeneity of the material can be assessed. For a comparison between the material model and reality the two variations of the press rheometry test are simulated. The simulation results show a good prediction of the experiments. The differences between experiment and simulation can be used to further improve the model in a later process.