Materials Science Forum Vols. 825-826

Abstract: Modern architecture is dominated by the tendency to design organically shaped filigree buildings. The resource and energy efficient construction of multifunctional buildings is as important as a broad variety of possible shapes. Multi-material support structures and shell constructions in lightweight design that also take over e. g. lighting and monitoring are needed for these purposes. Textile reinforced lightweight shell structures have been developed at Technische Universität Chemnitz within the scope of research projects. They consist of a hybrid material from carbon-fiber-reinforced concrete and glass-fiber-reinforced plastic. Thanks to the coupling of the positive material characteristics, the combination of two different composite materials results in a hybrid material with a total thickness of 15 mm, which has a high fatigue strength (XF4) and surface quality (exposed concrete). Furthermore, the hybrid is characterized by excellent compressive strength (120 MPa) and bending tensile strength (150 MPa), low susceptibility to corrosion and free formability. Therefore, it is highly suitable for thin-walled filigree lightweight shell structures. A research pavilion with a size of 4 x 4 x 3 m3 (l x w x h), made from textile reinforced lightweight shells, was built on the campus of TU Chemnitz, to test the theoretical investigations. Specially developed tensile sensors for the active lighting and determination of the elongations were integrated into the different layers. This aimed at an online-monitoring of the shell support structure.

Abstract: The increasing implementation of fibre-reinforced plastics (FRP) leads to a broad utilization of metal/FRP joints. The examination of adhesively bonded metal/FRP joints is focusing on the maximum joint strength and the surface pretreatment used to promote it. A deeper understanding of the mechanisms behind the correlation is lacking. Along with the chemical condition of the surface (free chemical bonds, polarity, etc.) the surface morphology plays an important role. This study addresses the connection between the surface morphology and joint strength. The investigation of the surface morphology of the metallic joining area and the fractured surface of both joining partners by means of imaging and tactile methods and the results are presented.

Abstract: A currently common method to design high-performance workpieces is to combine two or more materials to one compound. This way, workpieces can be composed of the most qualified materials according to local loads.When machining high-performance workpiece compounds (e.g. cylinder crankcases), high quality requirements concerning the accuracy of dimension and shape as well as the surface roughness must be fulfilled. However, machining of workpiece compounds leads to unfavorable changes of the workpiece quality in comparison to machining of the single materials. Significant shape deviations occur when different materials are machined alternately in one cutting operation. This is due to unequal material properties, cutting characteristics, chip formation mechanisms as well as characteristic interactions between the single components.The focus of this research is on the process understanding as well as on the identification of measurable shape deviation indicators that describe the surface finish of hybrid structures. Here, the indicators material height deviation, transition deviation at the material joint, surface roughness deviation as well as surface defects (e.g. scratches) on the surface are presented. The overall aim of this research is to predict the surface finish resulting from face milling an aluminum/cast iron compound or a polyurethane/cast iron compound.

Abstract: Hybrid laminates combine the positive properties of metals and fibre reinforced plastics. Thereby, the relatively free selectable components provide further benefits. Especially thermoplastic matrices offer positive aspects like the possibility of deformation, recyclability as well as the possibility of mass production. To obtain such hybrid laminates the first step is the production of pre-consolidated unidirectional endless fibre reinforced thermoplastic foils. In a second step, these pre-impregnated fibre-foil tapes were alternating thermally pressed with metallic layers in tailored compositions. To use the full capacity of the hybrid laminates an adequate interface between the fibre reinforced thermoplastics and the metallic foil is essential. Different investigations of the authors display the principle possibility to produce hybrid laminates with carbon endless fibre reinforced thermoplastics and aluminium alloy foils. Nevertheless, load free delamination’s occurs. The reason for these delaminations within the interface of the fibre reinforced thermoplastics and the metallic foil are the differences in the thermal expansion coefficient of the components. Caused by the consolidation at elevated temperatures these differences become more significant and reduce the reproducibility of the hybrid laminates. To minimize these thermal induced stresses the graduation of the thermal expansion coefficient is one possibility. This graduation is possible by utilising glass fibre thermoplastic tapes between the aluminium alloy foil and the carbon fibre reinforced thermoplastics. Further investigations are dealing with so called expansion alloys to adapt the thermal expansion coefficient. The latter approach provides the benefit to utilize the full mechanical properties of the carbon fibre reinforced thermoplastics and to economize the glass-fibre tapes. Nevertheless, these expansion alloys are characterized by a high density. Hence, within this contribution the specific mechanical properties as well as the advantages and disadvantages of hybrid laminates with expansion alloys or aluminium alloys with glass-fibre thermoplastics interlayers are discussed and assessed. These specific mechanical properties display the potential of the expansion alloy in spite of the high density by means of comparable values. The sample only consisting of carbon fibre reinforced plastics highlights the great variety and possibilities of different hybrid laminate structures and combinations regarding the thickness and positioning of the component layers.

Abstract: The present paper approaches possible advantages of hybrid constructions compared to monolithic design. Hybrid constructions represent multi-material composites where each of the materials employed are optimally utilized. Therefore, materials consumption decreases which leads to material, energy and cost efficiency and finally contributes to sustainability.The investigations targets on a possible substitution of a heavy iron casted pump housing by a metal-polymer hybrid light weight construction and on the achievable total mass reduction. Multi-material composites are prerequisite for lightweight design and promise a huge mass reduction potential.

Abstract: The following work was focused on the analysis of adaptable pressurized sandwich components. The investigations were carried out to study the influence of internal pressure on mechanical characteristics. The bonding quality between core material and outer layers is of particular importance. For analyzing the bonding quality peel tests were conducted. In order to investigate the influence of the internal pressure and also of a bonding technique on bending properties four-point bending tests were carried out. In addition, the pressure characteristics were studied with compression tests during which a compression die was pressed into the component. After the compression tests, the rebound properties of pressurized and standard components were observed and compared.

Abstract: This paper focuses on the reduction of process-related thermal residual stress in fiber metal laminates and its impact on the mechanical properties. Different modifications during fabrication of co-cure bonded steel/carbon epoxy composite hybrid structures were investigated. Specific examinations are conducted on UD-CFRP-Steel specimens, modifying temperature, pressure or using a thermal expansion clamp during manufacturing. The impact of these parameters is then measured on the deflection of asymmetrical specimens or due yield-strength measurements of symmetrical specimens. The tensile strength is recorded to investigate the effect of thermal residual stress on the mechanical properties. Impact tests are performed to determine the influence on resulting damage areas at specific impact energies. The experiments revealed that the investigated modifications during processing of UD-CFRP-Steel specimens can significantly lower the thermal residual stress and thereby improve the tensile strength.

Abstract: Lightweight solutions and functional integration become more and more important in different fields of industry. In order to achieve a sensor and actuator functionality of shaped sheet metal parts, today a generally manual application step of the piezomodule is necessary. This subsequent process is time consuming and leads to high costs. In earlier studies a method was presented allowing the fabrication of a formable compound with an integrated sensor and actuator functionality. The formability of the compound is achieved using a viscous adhesive, surrounding the piezomodule during the forming operation. The low viscosity of the adhesive allows a relative movement between the piezomodule and the sheet metals and drastically reduces the transfer of critical strains to the piezomodule. Curing of adhesive takes place after the forming operation. To improve the efficiency of the process chain an advanced adhesive system with robust application properties has to be used. Furthermore, the productivity of several fabrication steps and their sequence in the process chain have to be verified and improved. The paper presents the process chain designed for automated production of formable sandwich sheets with integrated piezomodules, including the production steps for the fabrication of the semi-finished part as well as the forming operation. Aiming on a good formability during the shaping operation and on a stiff connection between the piezomodule and the sheet metal in the finished sandwich part, one focus is set on the adhesive properties required during the different process steps.

Abstract: The conflict of targets between mass reduction, strength and costs of a multi-material-design module is addressed by the example of a multi-material hybrid leaf spring. A rather simple model is defined such that one portion of the spring is made by glass fiber reinforced plastic (GFRP) and the other portion by a high strength spring steel.In a rather basic approach the leaf spring is exposed to uniaxial bending. The mass of this module is discussed as a function of the strength of the joint. Subsequently, the leaf spring is exposed to a multi-axial bending, e.g. as an effect of side loads. Hence, the relative strength of the anisotropic portion (GFRP) of the leaf spring is diminished whereas the strength of the isotropic portion (high strength spring steel) is only slightly affected. The mass of the module is discussed in the same way. It is shown up by this analysis that the conflict of targets can be solved in different ways by considering the specific strength of the joint.It is the target of this basic study to derive the mechanical requirement of strength of this tailored joint which has to be met by its design in order to solve the addressed conflict of targets in a preferable optimal way.

Abstract: To enlarge the field of application of aluminum matrix composites (AMC) suitable joining technologies are necessary. Especially dissimilar joints like between AMC and stainless steel are of interest. In this work the arc brazing of this material combination is investigated. A new filler based on the ternary system Al-Ag-Cu is used. The results of wetting tests of the base filler and adapted variants are described and discussed. Wetting angles and microstructure of the interfaces are regarded.