Materials Science Forum Vols. 825-826

Abstract: The lightweight potential of components made of fiber-reinforced plastic can be enhanced by use of sandwich composites. So far, limited dynamic properties of plastic-based foams have prevented the use of sandwich composites in machine applications. The combination of closed-cell aluminum foam (ALF) and carbon fiber reinforced plastic (CFRP) provides a solution to this obstacle. Aluminum foam is characterized by favorable damping properties with minimum weight and CFRP provides high strength and stiffness at similarly low density. This paper deals with the design of a hybrid sandwich composite and its interpretation by using customized FEM simulations.Producing this kind of a sandwich composite in an economic production process presents a major challenge. Thus, a method has been developed that prevents excessive penetration of the resin into the pores of the aluminum foam. A high volume fraction of the resin in the foamed sandwich core would increase density and negatively influence damping properties. The implementation of a barrier layer will avoid this penetration. A DoE was developed and RTM process parameters were varied with the objective of achieving the highest specific bending stiffness. In preliminary experiments the appropriate range of injection pressure, mold temperature, and pressure force was determined. Tests with a nonwoven fabric could prevent the resin from infiltrating into the aluminum foam. Mechanical properties of the sandwich composite are only marginally affected.A model was developed to calculate the obtainable sandwich composite properties. The calculation method considers both the characteristics of the aluminum foam and the CFRP anisotropy. Based on this model a reliable calculation of the applied load could be accomplished. The design of the sandwich composite was targeting at high stiffness and determination of the natural frequency. Parallel to calculations, tests on specimen were performed and the obtained results were included into the calculation as part of the material model.

Abstract: A modern lightweight design with dissimilar materials such as metals in combination with fiber-reinforced polycaprolactam represents a possible lightweight variant in the automotive body shop. One of the issues arising from the use of polycaprolactam is its hygroscopy. The collected moisture evaporates out during the curing of the adhesive in the paint drying process and causes the adhesive to foam up. This leads to a decrease of the structural bonding strength. Many different factors, such as waterjet cutting processes, transportation and storage in undefined environmental conditions before the gluing process, as well as the flushing bath of the painting pretreatment, can contribute to the component humidity.Based on the common lap-shear test, this essay presents the influence of the substrate humidity on the bonding strength as well as possible pretreatment methods to avoid or decrease the negative influence of the substrate humidity. Using infrared radiation or atmosphere plasma localized at the bonding spot, the substrate begins to dry resulting in a bond with better bonding strength. For the future it is planned to test other options like laser substrate drying, adhesive modifications with regard to viscosity and moisture catcher enrichment.

Abstract: Metal-matrix composite materials, based on a metastable austenitic stainless steel reinforced with a magnesia partially stabilised zirconia have been prepared by a ceramics-derived extrusion technology. Using this powder metallurgical method enables the shaping of lightweight cellular structures as well as bulk specimens with a variety of steel/ceramic ratios at room temperature. However, the extrusion of composite structures is limited by the uniform cross section throughout its entire length. Joining of these metal-matrix composite preforms after sintering by conventional welding techniques is a challenging task. The presence of ceramic fractions may lead to several complications and the subsequent heat exposure during joining may initiate phase transformations in both metastable components resulting in a deterioration of the mechanical properties of the composite material. An adapted ceramics-derived joining technology allows the combination of varying TRIP-steel/zirconia composite materials. The main features are the machining and joining of the parts in their dry green state at room temperature before their thermal treatment. Thus, the material’s consolidation and the formation of the joint take place simultaneously. The ability of joining different parts offers the possibility to create structures for complex applications and testing conditions. The key to advanced properties of the joining zone are the base materials, the surface treatment of the parts, and the paste used for joining. The joining process of different base materials, the mechanical properties, and the microstructure of sinter-joint samples are presented.

Abstract: The use of fiber-reinforced-plastics (FRP) contributes to an efficient implementation of lightweight design due to their outstanding specific mechanical properties. The RTM process offers great design freedom and allows the integration of functional elements during manufacturing. Embedded metal elements, so-called inserts, can be used to deal with the load transfer to structural parts. These elements have distinctive characteristics in comparison to other joining technologies. For example, detachable connections can be established with the help of inserts. Due to the fiber continuity not being interrupted and, subsequently, the FRP parts not having to be drilled, there is no local bearing stress. This paper aims at the characterization of metal inserts in FRP parts. The parts are manufactured using the RTM process with a specially adapted RTM mold with exchangeable cartridges for different insert geometries. The inserts are made of metal sheets with welded bushings and are embedded during preforming. The cured FRP specimens are tested under different load conditions to evaluate their suitability for various fields of application. Furthermore, the diameter and thickness of the metal sheet of the insert as well as the thickness of the FRP are varied to identify their influence on the failure behavior and load capacity under tensile loads.

Abstract: In regard to the realization of innovative lightweight constructions, textile reinforced composites show outstanding mechanical properties, e. g. adjustable high specific stiffness and strengths. Furthermore, these materials enable a process immanent integration of functional elements (f. e. conductors, sensors, actuators or electronic components) directly into the composite structure due to their layered built up. Currently, no joining technologies for such function integrative composite parts exists, which enable a simultaneous mechanical and electrical load transfer from part to part by only one joining element type. Therefore, the paper focuses on detachable functional interfaces, which enable the mechanical connection of two join partners and the transfer of electrical signals by contacting the composite embedded conductors. Investigations in regard to the selection of suitable joining elements and their behavior under mechanical and thermal loads are performed. The investigations show that the electrical resistances are low and not affected significantly by rising tensile loads or repetitive joining operations. Tensile tests using both single lap shear and double lap shear specimens show that the electrical contact almost exists until the mechanical failure of the joints occurs.

Abstract: The use of lightweight structures is a major trend in the reduction of fuel consumption and CO2 emissions, especially in transport. Metal plastic hybrid structures are an efficient solution to use the best material at every point in the design space. In the state of the art production technologies, the metal parts are produced separately from the plastic parts. The injection moulding process is only used for forming the plastic parts and for joining. These process chains are very extensive. The article shows the development of new process combinations. The aim is a combination of metal forming and injection moulding in one die and one process. One part should be produced with every single stroke of the press.In the first step, deep drawing, injection molding and media based forming with the plastic melt were successfully merged in one tool and one process. It was possible to integrate the injection moulding process into a deep drawing machine. In the next step, it was possible to successfully combine hydroforming and injection molding. For this process combination the hydroforming process is integrated into an injection moulding press. Different surface structures of the metal tubes, such as sandblasting, knurling and laser structuring, were systematically tested regarding to their properties as an adhesion promoter. The target is to establish a purely mechanical connection between the hydroformed metal component and the injection moulded component from glass fibre reinforced plastic instead of the chemical bonding agents often used previously, such as Vestamelt^®.

Abstract: The application of hybrid yarns and their further processing to textile preforms enforce adapted manufacturing processes. Furthermore, a consolidation of cross-section varying parts requires an adapted mold and core system for a reproducible production process. Similarly, the application of hybrid yarns facilitates low consolidation pressures and thus favors the integration of electronic components in fibre-reinforced thermoplastic parts. In this paper, the analyses of the boundary conditions for a process integration of sensor elements in complex fibre-reinforced spacer structures are presented.

Abstract: The electrical contacting of an embedded sensoryarn in a unidirectional glass fiber polypropylene (GF/PP) composite realizedby laser processing was investigated. It was shown that compared to thecircular drilled hole the same electrical contacting quality can be achieved atan increased mechanical performance level.

Abstract: Hybrid laminates with thermoplastic matrices offer clear advantages over laminates based on thermosetting resins. These include the formability, recyclability as well as the suitability for mass production, to name a few. The inline integration of smart systems like sensors and actuators in the hybrid laminates during the hot-pressing process is one of the long-term objectives of the Federal Cluster of Excellence MERGE. This work aims to deposit Ni-C thin films by dc magnetron sputtering on polyimide substrates for the application as strain sensors in hybrid laminates. During the first step hybrid laminates containing different polyimide foils were prepared by hot-pressing, this was followed by the mechanical testing and selection of the most suitable polyimide substrate for the Ni-C thin film deposition. The second part consisted of depositing Ni-C thin films by means of dc magnetron sputtering and utilising different Ni-C plug targets. The films were characterised regarding their growth rate, composition, structure and temperature coefficient of electrical resistance.

Abstract: Regarding the economical use of fiber-reinforced plastics (FRP) as a construction material for structural components whose additional value caused not only in their high specific mechanical properties. Due to the layerwise structure definition of continuous fiber reinforced composites the corresponding production technologies offers a high potential for integration of additional functional elements. Past efforts to the integration of functions in fiber-reinforced composites usual provide in front of a passive use of the piezoelectric effect (eg. structural-health monitoring). Through efficient and structurally defined using of piezoceramic actuators, the planar structure topology of cylindrical hollow FRP profiles can be actively influenced. Based on experimental studies on the definition of basic concepts for the integration of thermoplastic compatible piezoceramic modules (TPM) in fiber composite tubular segments, this paper deals with the understanding and performance capabilities of such actuarical hollow frp structures. The selective excitation and manipulation of the vibration behavior of such rotationally symmetric structures serves for generation of wave effects with radial translational characteristic. The performed experimental studies on the structural behavior of active piezo integral pipe segments are abstracted and compared by means of numerical simulations using multi-physical elements.