Key Engineering Materials Vol. 809

Abstract: Nowadays, carbon fiber reinforced plastics (CFRP) are present in a variety of applications. However, it is still possible to maximize the potential of CFRP by creating multi-material designs of CFRP and metal. The key to success for multi-material designs is the joining technology. In this work a metal/carbon fiber connection module was develop. Carbon fibers (CF) were integrated with stainless steel by using a powder metallurgy approach. After this, the created connection module was integrated in a fiber layup, which was infiltrated with epoxy resin by a Resin Transfer Molding (RTM) process. Leveraging from this technology, a M6 thread-forming screw was chosen and added in the sintered body. The screw press out test indicated that the strength between the screw and the sintered body was above 11 kN, which can be still enhanced for future by thread optimization. Microscopic cut images and computer tomography (CT) were used to characterize the CF in the sintered steel body and to examine the border area between the two materials.

Abstract: In this study, multi-material nonwovens were produced using a wet laying nonwoven batch process. The aim of this work is to investigate and develop nonwoven material solutions that can be used for a substitution of pure glass fibre (GF)-applications and also provide a more cost-sensitive option compared to nonwovens purely made from recycled carbon fibres (rCF). The multi-material-nonwovens of this study consisted of the functional components rCF and GF as well as a thermoplastic matrix, built by the admixture of PA6-fibres. All three fibre types were mixed directly within the nonwoven manufacturing process, respectively in the course of the initial weighed portions. Six different material compositions with individual amounts of rCF and GF were produced, but a constant overall fibre volume content (FVC) as well as a uniform grammage was defined. A hot pressing technique was used to consolidate these multi-material-nonwoven layers. Subsequently, the sheet materials were examined using tensile and 4-point bending tests, as well as wet-chemical fibre volume content determinations and micrographic sections. With regard to the mechanical performance, a near-linear increase is observed for increased proportions of recycled carbon fibres. In this context a potential for the use of multi-material-nonwovens consisting of rCF and GF can be found. It is demonstrated that rCF might be an adequate substituent for classical GF-applications. The results contribute to broadening the performance spectrum of rCF and thus to its substantial recycling route.

Abstract: Aim of this study is to investigate the influence of welding temperature and weathering on bond strength of induction welded hybrid joints made of steel and thermoplastic fiber reinforced polymer composites (TP-FRPC). The used TP-FRPC are continuous glass fiber reinforced-polyamide 6 (PA6), -polypropylene (PP), -polycarbonate (PC) and-polyphenylenesulfide (PPS). The metal sheets are mild steel (1.0338 and 1.0330). The surface pretreatments of the metal sheets are either laser structuring in line pattern, perpendicular to the load direction, or two different types of functional coating adhesives (Köratac HL 400 and Köratac HL 403) currently used in coil-coating processes. First the process parameters were optimized for each material combination by investigating the bond strength at different welding temperatures. In a second step, for each material combination, specimens were welded using the determined welding temperatures with the highest bond strength in order to investigate the influence of weathering on hybrid joints. Therefore the cataplasma test (DIN EN 13523-27: 2009) as well as alternating climatic change (BMW PR 308.2) were used.

Abstract: There is evidence that the fiber-matrix interaction in fiber-reinforced cementitious composites is determined by the wettability and roughness of the fibers. Due to the high ionic strength in the hydrated cement, also the surface charge or acid/base behavior of the fibers is assumed to play a role. To create fibers with alkaline and acidic surface functionalities, water-insoluble poly (vinyl alcohol) fibers were permanently modified by adsorption of various polyelectrolytes. X-ray photoelectron spectroscopy, zeta potential, and contact angle measurements revealed acidic, alkaline or amphoteric fiber surfaces with advancing water contact angles between 34° and 58°. In a first step to study the interaction with cementitious materials, the interaction of these fibers with pore solution (the liquid phase of hydrated cement) and the adsorption of calcium ions on the fiber surface were investigated. The work will be continued by studying the fiber-matrix interaction in cementitious matrices and its influence on the composite strength.

Abstract: The formability of fiber-metal laminates (FML) produced in autoclave process is limited due to processing restrictions. In addition, the industrial application of these hybrid laminates is usually unattractive, as the production time leads to high production costs. A new manufacturing process has been developed, which combines the deep-drawing process with a thermoplastic resin transfer molding process (T-RTM). Three-dimensional sandwich components were produced. They consist of 1 mm DC04 steel face sheets and a varying number of intermediate layers of thermoplastic reinforced glass fiber fabrics. Due to the manufacturing process, the mechanical properties differ locally depending on the location-dependent degree of sheet metal forming and the location-dependent fiber draping. The mechanical properties are therefore investigated by three-point flexural tests on miniaturized samples in order to map these location-dependent properties.

Abstract: Ultrasonic welding is a suitable solid-state joining technique for producing high strength joints of similar or dissimilar materials, even of material combinations that were previously considered as not weldable. Several varieties of transmitting the ultrasound into the joining partners exist whereas the investigated torsional welding principle utilizes a ring shaped sonotrode for transmitting ultrasonic vibrations tangentially to the welding force into the workpiece. Due to the specific sonotrode geometry ultrasonic torsional welding is a remarkably gentle welding technique, allowing to join even most sensitive components e.g. sensors or brittle elements. Nevertheless, ultrasonic torsional welded joints show high tensile strengths and helium-tightness. Current investigations focus on the realization of metal/glass ceramics joints. In this project two metals with different thermal expansion coefficients have been utilized as the metal joining partner. The glass joining partner was the commercially available Li₂O-Al₂O₃-SiO₂ CERAN. For examining the microstructure light as well as scanning electron microscopy have been performed. Additionally, mechanical characterization has been carried out through tensile shear tests.

Abstract: Nowadays, orthoses are made from fibre reinforced thermoset based composites with a high manual labor input. These thermoset based orthoses are no longer formable, which brings forth a significant disadvantage. Hence, hybrid laminates consisting of fibre reinforced thermoplastic films and thin metal sheets can replace successive thermoset based systems due to their advantages of higher formability and the suitability for mass production. In the present work, various surface treatment methods like pickling or mechanical blasting have been used on thin metal sheets to increase the adhesive and shear strength of the produced thermoplastic based hybrid laminates. The modified metal sheets were further combined with basalt fibre reinforced interlayers to manufacture overlap samples, which were used to determine the tensile shear strength. In addition, the roughness of the modified metal sheets has also been investigated. Moreover, the consolidation parameters such as pressure, holding time and temperature have been varied for the production of hybrid laminates using hot-pressing process and then the microstructural images have been recorded. Finally, the mechanical properties of the produced hybrid laminates have been tested by means of a three-point bending test and the interlaminar shear strength has been analyzed.

Abstract: Forging of different steel grades is called Damascus technique and results in a layered composite material termed “Damascus steel”, but forging of different copper alloys is termed “mokume gane”. In this paper the joining of copper and iron plates by forging is described. Metallographic investigations showed well bonded interfaces of copper and iron. A very small diffusion zone was observed. To study the diffusion between copper and iron two heat treatments were performed in Ar atmosphere. After 30 minutes at 1000 °C a marginal Cu-Fe interaction took place. Above the melting point of Cu at 1100 °C an intense Cu-Fe interaction was observed, which significantly changes the interface of both metals. Cu penetrated Fe along the grain boundaries and Fe droplets were formed sporadically. This correlates with the typical morphologies of liquid metal embrittlement (LME). Moreover, Fe is dissolved in Cu at 1100 °C and after cooling fine Fe precipitates in the Cu phase were detected.

Abstract: Fibre-metal-elastomer laminates offer the possibility of using material combinations which often have to deal with premature delamination, for example due to different coefficients of thermal expansion or galvanic corrosion due to different electronegativities. The present study deals with laminates made of layers of CFRP and aluminum, each of which is bonded together by an elastomer layer. The shear-soft elastomer also allows the much stiffer aluminum and CFRP layers to be sheared off against each other under bending stress. This leads to complex deformation behavior. The shear of the elastomer also plays a crucial role in the damping behavior of the laminate. Due to large shear deformations in the elastomer layer, the combination of rigid layers and soft elastomer layers shows very good damping behavior according to the principle of constrained layer damping. Since bending vibrations that occur during normal use usually have only small amplitudes, the deformation behavior is of particular interest in the elastic range. Since this deformation behavior is strongly dependent on the shear modulus of the elastomer used and this in turn is strongly influenced by temperature, the deformation behavior is characterized at different temperatures. Within the scope of this investigation, quasi-static 3-point bending tests are carried out on different laminate lay-ups in the temperature range from -40 °C to +80 °C. The laminates are consolidated by compression molding and contain two different EPDM elastomers in varying layer thicknesses, unidirectional CFRP prepreg in biaxial layer lay-up and aluminum 2024 sheets. The deformation behavior is analyzed by digital image correlation. This is used to measure both the bending line of the overall composite and strains over the layer thickness. In particular, the shear in the elastomer layers is evaluated and set in relation to the bending lines. Finally, the ability of the laminate lay-up to damp bending vibrations is evaluated.

Abstract: 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.