Key Engineering Materials Vol. 809

Abstract: Sandwich structures consisting of fibre-reinforced plastic (FRP) facings and core are ideally suited as substitution materials for reducing component masses. The endless fibre reinforcement has the greatest performance potential. Both thermoset and thermoplastics are already being processed into endless fibre-reinforced sandwich facings according to the state of the art. The 3D endless fibre reinforcement of cores is a current research topic. This paper describes the development of a hybrid sandwich consisting of thermoplastic composite facings and an innovative core composite. This is made of polyurethane (PUR) rigid or flexible foam, which is reinforced with spacer fabric. The sandwich manufacturing in Reaction Injection Moulding (RIM) includes the original forming of the core and the simultaneous bonding of the facings. This efficient process offers the potential for the production of such complex structures in medium or large series. The sandwich structures and their individual components were characterised in the standardised compression and bending test. The lightweight potential of spacer fabric reinforcement is demonstrated by comparing the specific mechanical properties of sandwich structures with and without core reinforcement. In comparison to reinforced and unreinforced foams, the effect of sandwich design is also shown.

Abstract: o accurately simulate the foam core in composite parts on a macroscopic scale the morphology, the characterisation, and the nonlinear behaviour of thefoam must be understood properly. Accounting for the heterogeneity and the mechanical properties of the foam core affects the dimensioning of the final part.In the present study the microstructure of the foam samples were characterized using scanning electron microscopy. To determine the bulk material behavior and the strength limitations of the nonlinear foam, shear and compressions tests are performed. All numerical calculations were carried out on the macroscopic level.A basic challenge in the finite element modelling of hyperelastic materials by means of test data is the identification of material model coefficients which are appropriate to describe the behaviour of the considered foam.

Abstract: In the present study, Organosandwich structures consisting of a folded polypropylene (PP) honeycomb core and glass-fiber reinforced PP cross-ply face sheets are investigated with the aim of developing a valid method for structural simulation of Organosandwich structures. On the one hand, effective material properties are gained from a mesoscopic model on the basis of a representative volume element (RVE) and compared with data from experimental characterization. The morphology of the folded honeycomb structure was investigated via X-Ray computed tomography and 3d image analysis. On the other hand, the differences between experimental and effective core properties are shown, and the correlation between the mesoscopic honeycomb core structure and the entire sandwich behavior is validated by bending tests and a corresponding finite element model. A comparison shows that results can be achieved with the RVE model as well as with the homogenized core FE model with good agreement with the experimental data.

Abstract: For Automated Tape Placement process, degree of bond varies with variation in process parameters and material. Interlaminar bond strength characterization is one of the most important criteria in determining the quality of bond between two layers of thermoplastic tapes. Depending on the bond strength achieved using different process parameters, a process window is defined. Based on the process window an iterative procedure is adopted to find optimum parameters to realize maximum bond strength. This paper aims to investigate the interlaminar bond strength of thermoplastic CF-PA6, during Automated Tape Placement process. A fairly new heating source, a pulsed light solution, i.e. a humm3^TM system, which delivers uniform, highly controllable heat to the nip point is used. Experiments were conducted for different process parameters and results obtained using wedge peel test were analyzed. Results acquired help in assessing the material and the heating source in terms of capabilities and efficiency.

Abstract: To increase product quality injection molding tools are equipped with innovative tempering technologies. The customers strive for the technology with the lowest energy consumption. Ceramic materials like yttria-stabilized zirconia (YSZ) are able to thermally insulate tool surfaces providing a more precise temperature regulation with intent to shorten cycle times as well as to decrease energy demands during the molding process. High quality ceramic thin films could be applied by metalorganic chemical vapor deposition (MOCVD). Laser machining technologies have been developed for machining the ceramic materials. In this work we demonstrate the fabrication of zirconia based thin films on steel tools via MOCVD using solid metalorganic precursors. Shorter coating times and a solvent free process are some of the advantages of our new developed coating process. The ultrashort pulse laser processing (USPLP) was used to structure the developed MOCVD coating. Using this technology the ceramic material undergoes no thermal stress cracks, because USPLP is characterized by the preference of cold material removal. The laser processing procedure was developed by working out machining parameters for the different materials. The difference between steel and ceramic in the removal behavior was determined immediately so that a machining strategy for the ceramic CVD coating could be designed successfully. The implementation of defined roughness and a carbon fiber like structure in the coating were realized. Coated and laser-structured injection molding tools were tested regarding their desired properties under production conditions.

Abstract: Freestanding diamond foils are an exceptionally strong material. A major problem that prevents industrial use is their inherent brittleness. We here present a first approach to introduce metallic interlayers into a diamond matrix by brazing stacks of diamond foils. This represents a potential route to toughen the material. Laminates of two and four layers of diamond were produced from the same batch of diamond foils. A first attempt to approximate the bending strength of this new material was made using a Ball-on-three-Balls (B3B) setup. Substantially higher strength values were achieved for the laminates compared to the freestanding (monolithic) foil.

Abstract: To foster a sustainable deployment of the innovative composite material ‘carbon concrete composite’ in the building sector, it is necessary to ensure its resource efficiency and environmental compatibility. The Institute for Building Materials Research of the RWTH Aachen University is therefore investigating the leaching behavior of this material, especially for the case of irrigated façade elements. Laboratory and outdoor exposure tests are run to determine and assess the heavy metal and trace element emissions by leaching. Feasible interconnections between laboratory and outdoor examinations can be used to develop a faster testing of future composite materials. Current results show no critical release of environmental harmful substances from carbon concrete composite.

Abstract: Due to the process, metal foams produced by powder metallurgy have an inhomogeneous pore structure. The pore size variations from a few tenths of a millimeter up to the centimeter range is so typical. This leads to inconstant properties and makes the predictability of metal foam based components difficult. The present work deals with the approach to precondition the feedstock powder for the production of metal foams by mechanical alloying. Thereby, the homogeneous distribution of the blowing agent TiH₂ is already present in the so-called composite powder. In addition, there is the possibility to add reinforcement particles with regard to a further improvement of the mechanical properties. Overall, this leads to a rather wide range of possible influencing factors and parameters to be varied. Accordingly, the present work is only a preview of a broad experimental program, taking into account the design of experiments and is currently being processed. In this first study, an alloy AA6060 has been used as the matrix material. The evaluated method provides a finer pore structure associated with a modified foaming behavior.

Abstract: The trend towards multi material design is strongly driven by improved functionality and decreased total weight of hybrid parts. Conventional joining techniques for metals and polymers usually require a complex process and are thus time consuming and expensive. A novel technique addressing these shortcomings is ultrasonic assisted thermal direct joining of metals and thermoplastic polymers. The metallic joining partner is laser pre-treated to generate a specific surface topology. The subsequent joining process is a combination of thermal direct joining and ultrasonic joining. This hybrid joining process results in short cycle times, and the maximum heat input is localized to the joining area. The joint performance was measured by lap shear tests, resulting in strength values exceeding 18 MPa, while the duration of the joining process was about 1.5 seconds. The relevant joining parameters were identified and a process window was obtained. The results indicate that there may be an optimum energy range for successful joining. An appropriate energy map may allow a deeper understanding of the process and enables prediction of process windows for various material combinations.

Abstract: Additive manufacturing of endless carbon fiber-reinforced composites is a technology which produces parts with mechanical properties similar to those of additively-manufactured metallic parts. In this work, the influence of layer height and width on mechanical properties of additively-manufactured carbon fiber-reinforced polymer composites has been studied. Two different 3k carbon fibers have been used as reinforcement. The composites are printed by material extrusion technology with layer heights of 0.2, 0.3, and 0.4 mm and layer widths of 1.0, 1.2, and 1.7 mm. The composites possess higher flexural strength at smaller layer height and the flexural modulus is dependent on the fiber volume content. The formation of voids/defects decreases the mechanical properties of composite and should be optimized.