Key Engineering Materials Vol. 809

Abstract: Fiber reinforced plastics show a wide range of different damage mechanisms such as matrix cracking, fiber breakage and interface failure. These can be observed in damaged specimens by means of volumetric images acquired by computed tomography (CT). As each failure mechanism causes a characteristic acoustic emission (AE) signal, AE analysis is a promising tool to identify damage mechanisms and offers the advantage that a real-time observation of the damage evolution during the testing period is possible. For a correlation of damage mechanisms and AE events, AE analysis was combined with in- situ CT measurements. This combined approach was validated by means of a 3-point-bending test on a discontinuous glass fiber reinforced sheet molding compound (GF-SMC) in which AE signals were acquired during loading using two high frequency piezoelectric sensors. At times of increasing AE activity, the test was interrupted in order to carry out a CT-scan of the specimen under load. AE events could subsequently be linked with the damage mechanisms observed in the CT-scans at different stages of damage to identify signal features that are characteristic for a certain mechanism. The sources of the signals could be localized and were in line with the actual location of damage.

Abstract: The quality of composite materials based on natural fibres is highly influence by humidity content of the fibres. For a high product quality in the resin-transfer-moulding (RTM) process a constant humidity content has to be achieved. As the humidity content of the fibres can change relative quickly depending on the humidity, measuring humidity content in the mould is beneficial. Near-Infrared-Spectroscopy (NIR) is a widely used tool for humidity content measurement allowing determination of the moisture content within seconds. To do so a calibration model with good accuracy is required. To generate the calibration model a dry flax woven fabric is placed in a climate chamber and weight change is recorded as well as NIR-Spectra. By correlating the spectra with the weight increase a model can be developed allowing to assign the spectra with unknown weight. This allows not only monitoring the moisture content of natural fibres with in the mould. Also can the moisture content reduced to an aspired value by applying vacuum to the preheated mould, before starting the resin infusion.

Abstract: Carbon fiber reinforced Plastics are materials with high strength and stiffness at low weight compared to metals. These properties make the materials ideal candidate for structures in aerospace applications, where they are often used under bending conditions. Due to the strongly anisotropic composition the CFRP typically fail in compression by fiber buckling. In order to improve this weakness, nanotube and nanofiber reinforced matrix was used to build CFRP. Four-point bending tests showed that stiffness and strength could be improved by the fillers, whereas negative influence was found on fracture energy.

Abstract: Composite materials in structural applications that are subjected to static loads for several decades tend to change material performance over their lifetime. Classical creep tests with constant static loading are quite simple tests with low demands on the test equipment. Unfortunately, these tests require uneconomically long test times, which is why a shortening of the test times with various accelerated approaches is being researched. Within this work two approaches for reduction of the testing time were investigated. On the one hand a fatigue test with the variation of R-ratio and following extrapolation to an R-ratio of 1 was done. On the other hand a Stress Rate Accelerated Creep Rupture Test (SRCR) was developed, where a defined initial stress σ_i is applied at the beginning of the loading process, followed by an increase load with a constant rate instead of the static stress segment of the classic creep rupture tests. Changing the load rate in several individual tests leads to stress rate-dependent fracture strengths with associated fracture times, which allows extrapolation to a fracture time at a load rate of zero. In particular, the approach of the SRCR offers great potential for greatly reducing test times with an acceptable prediction quality.

Abstract: To reduce the emission of carbondyoxide (CO₂) of combustion engines, liquefied naturalgas (LNG) is used as an alternative fuel. LNG is transported via truck, ship or railway for longdistances. Double walled stainless steel tanks are used for transportation, which are heavy and expensive.The vacuum insulation between the two walled structure ensures that the LNG stays liquid overthe transportation time (boiling point of LNG: −162 °C). This causes a high temperature differencebetween the transported good and the ambient air. A simplified tank construction is used to reduce the weight and price of the tank. Instead of stainlesssteel, glass fiber reinforced plastic (GFRP) is used. The designed is changed to a single walledconstruction with a solid insulation material outside on the GFRP structure. Goal of this work is the characterization of a suitable insulation material and configuration as wellas the analysis of the mechanical properties of GFRP under cryogenic conditions. Several experimentsare carried out. Numerical models of these experiments can then be used for parameter studies.

Abstract: In the past, the focus for the development of modern vehicle structures was very much on lightweight construction. However, there are increasing aspirations to develop not only light but also sustainable solutions which use resources efficiently. As a result, natural materials become more attractive compared to conventional lightweight construction materials. The "For (s) tschritt" research project investigates the use of veneer-based multi-material systems in vehicle structures. For this purpose, various concepts were developed, ranging from a use of the material to reinforce thin sheet metals to structural components which are produced completely from wood and are only reinforced locally. In order to evaluate the aspired solutions, generic components were derived, manufactured at the Department for Cutting and Joining Manufacturing Processes of the University of Kassel (TFF) and the Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut (WKI), and tested at the Institute of Vehicle Concepts of the German Aerospace Center (DLR). The advantages of the use of wood are particularly evident in structures which are subjected to bending stress and pressure loads: As a result of the lower density, they can be designed with reinforcement. This allows the second moments of inertia to be increased without affecting the weight. The disadvantages of the natural material, such as reduced reproducibility and the complex failure behaviour, are offset by systematic hybridisation of wood and the use of veneer multilayer composites.

Abstract: Basalt fibres are inorganic man-made fibres which, in fibre-reinforced plastic composites (FRP), represent a cost-effective alternative to carbon fibres. Within petrochemical thermoplastic matrices, basalt fibres are established in technical applications. The use of biobased polyamides as matrix material is an innovative approach for the production of FRP with a high proportion of renewable raw materials. In addition to material selection, the compound manufacturing process also defines the properties of the FRP. At the Institute of Lightweight Structures, extensive investigations are carried out into material development and optimization of the associated process technology. As a result, basalt fibre-reinforced, biobased polyamides are produced whose mechanical relevant properties are above those of petrochemical based glass fibre-reinforced polyamides.

Abstract: Veneer plywood is established in building construction, interior finishing and vehicle construction. Particularly in automotive or ship interior applications, the requirements with regard to strength and stiffness properties are increasing. At the same time, the weight of the panel materials used is to be reduced. The development presented here is a new type of lightweight basalt fibre-reinforced poplar veneer plywood and at the same time rigid alternative to the established birch veneer plywood. By adapting the adhesive and the reinforcing semi-finished products, the material and manufacturing costs are comparatively low. The bonding of the fibre reinforcement to the carrier material is achieved by means of adapted wood adhesives (e.g. polyvinyl acetates), which are also used as matrix material for the fibre reinforcement. An application of the reinforcement layer is integrated into the coating process (e.g. with High Pressure Laminate HPL decorative fabrics) of the carrier material. Essential advantages compared to conventional board materials are shown in this paper. The research results were achieved at the Institute for Structural Lightweight Construction of the TU Chemnitz in cooperation with the company Toms Gerber GmbH within the ZiM cooperation project FuBa.

Abstract: Sheet Molding Compound (SMC) was developed in the 1960s and initially enabled the production of glass fiber reinforced polymer composite (GFRPC) in mass production. Nowadays, both material and process are well established for the production of semi-structural components in various applications from construction industry to automotive components. Currently, approximately 20% of all glass fibers produced in Europe are processed to SMC. In this paper, the use of renewable filler materials in SMC is described. By using those alternative fillers, a density reduction of 20%, while maintaining same processability and mechanical properties of SMC, can be achieved.

Abstract: Natural fiber reinforced polymer composites (NFRPC) are conventionally processed in compression molding process to shell-shaped components, which are mostly used in automotive industry. Therefore, natural fibers (NF) and polypropylene (PP) as matrix system are processed to a thermoplastic organic sheet. Organic sheets are heated in a contact heating press to temperatures above melting temperature of the matrix and then transferred to a forming press. In this paper, Acrodur^®Power 2750 X (AP) is used as an alternative to PP. This thermoplastic matrix system enables an increase of fiber content to 75 wt.-% in NFRPC. Furthermore, infrared radiation (IR) with adapted wavelength was used as an alternative to conventional contact heating. To improve the heating behavior of NF/AP infrared and heat transfer additives were used. By adding heat transfer additives and using IR heating with adjusted wavelength, both heating time can be reduced from 45 seconds (NF/PP) to 25 seconds (NF/AP) and a contact heating press can omitted.