Key Engineering Materials Vol. 809

Abstract: In the present study, a low viscous (complex viscosity between 200 to 500 mPas at 60 °C), flame retardant epoxy resin formulation is prepared and transferred to the carbon fiber reinforced plastic (CFRP) laminate using resin transfer molding (RTM) method. For the laminate production, a 12k carbon fiber fabric with an areal weight of 400 g/m2 is used to achieve a fiber volume content of approximately 60 vol % carbon fibers. Subsequently the unmodified laminate is produced, varying carbon fiber volume content to study its effect on flame retardant properties. As additives, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) modified epoxy resin and nanosilica particles delivered in an epoxy novolac masterbatch are added to the neat novolac resin system. The mixture is cured with isophorone diamine (IPDA) and polyetheramine hardener blend, resulting in a glass transition temperature of 104 °C for the unmodified laminate. Flame retardant properties of the materials are tested using cone calorimeter and thermal gravimetrical analysis. In addition, the mechanical behavior of the systems is evaluated via three-point bending method in static and dynamical loadings. In order to get deeper information on the resulting flame retardant mechanisms of the additives, the residual cone calorimeter char is analyzed with scanning electron microscopy, indicating the different flame retardant mechanisms of phosphorous and silica as well as the combination of both additives.

Abstract: This paper investigates the damage behavior of thermoset and thermoplastic fiber-reinforced composites. The specimens were subjected to low-velocity impacts (LVI) to produce barely visible impact damages (BVID). To compare the dependency of the matrix system and the laminate lay-up on the impact damage, four test series were set up. Therefore, laminates with an epoxy (EP) and a polyether ether ketone (PEEK) matrix in a quasi-isotropic (QI) [+45/0/-45/90]_2s and an orthotropic (OT) fiber lay-up [0/90]_4s were manufactured. To eliminate the influence of variant fiber systems, the thermoplastic tape and the thermoset prepreg contain similar carbon fibers (CF). After impact testing with three different impact energies, inner damages were investigated by using ultrasonic analyses. To get a deeper understanding of the interior damage mechanisms, cross sections of the damaged areas were examined via reflected light microscopy. By using these destructive and non-destructive test methods, significant differences in the damage behavior of composites with thermoplastic and thermoset matrix systems were identified for both laminate lay-ups.

Abstract: Silicone resins are manufactured and supply in various delivery forms since decades. The impregnation of large electrical machines demonstrates a main application in the field of electrical insulation technology at the moment. Silicone resins are characterized by outstanding resistance to heat stress at high temperatures in general. Experience has shown that they are also very stable against exposure to UV radiation. Based on the expected very good property profile, Wacker Chemie AG developed a solvent-free, low-viscosity silicone resin for use as a binder for castable and moldable products. The article presents the chemical composition of silicone resins, the resulting properties of the resin and selected results of the extensive work for the evaluation of possible fillers and filler mixtures as well as first experience with the processing.

Abstract: The fatigue behaviour of [0°/±45°] triaxial braided composites is investigated. Tension-tension and compression-compression loading is chosen to study main damage mechanisms and their effect on the stiffness degradation. In both cyclic loading cases a high stiffness degradation can be observed shortly after the start of cyclic loading. Inter-fibre fractures in the braid yarns with subsequent delaminations are responsible for this behaviour during tension-tension loading. Compression-compression loading leads to kink band formation in the axial yarns favoured by yarn undulations. In consequence of the observed material degradation, S-N curves based on a defined level of stiffness degradation are proposed for fatigue dimensioning instead of load cycles to fracture.

Abstract: In this paper experiments on the fatigue behavior of flat and tubular Glass Fiber Reinforced Plastics (GFRP) specimens with three different layups ([0/90₅/0], [0/90₂/0_1/2]_s and [0/±45/0_1/2]_s) are presented. The experiments are conducted to study the mechanics under cyclic tension-tension loading (R=0.1) until crack saturation (CDS). Fatigue testing is performed below the critical static load level where first matrix cracks can be observed. Therefore Load Levels (LLs) are derived from crack evolution curves obtained by static tests under usage of transmitted light photography. The shear lag model of Berthelot [1] is applied to the two cross-ply specimens to predict crack evolution. The results show good agreement between the prediction and the experimental data. Deviations can be found in prediction of crack evolution in [0/90₂/0_1/2]_s-specimens. For predicting fatigue stiffness degradation the phenomenological model of Adden [2] is used. The results show good capabilities for predicting stiffness degradation after crack onset.

Abstract: The wet compression molding (WCM) process enables short cycle times for production of fiber-reinforced plastics due to simultaneous infiltration, viscous draping and consolidation in one process step. This requires a comprehensive knowledge of occurring mutual dependencies in particular for the development of process simulation methods and for process optimization. In this context, it is necessary to develop suitable test benches to enable an evaluation of the outlined viscous draping behavior. In order to evaluate and suitably design the draping process, grippers are mounted on a surrounding frame, which enables targeted restraining of the local material draw-in during forming. In supporting the development of the new test bench, first experimental and simulation results are compared, which thereby enables a first validation of the simulation approaches. Results show a good agreement between experimental and numerical results in terms of shear deformation and final gripper displacement under dry and viscous conditions. Results recommend that future development for investigations of viscous draping effects should focus an enabling measurement of gripper displacement during the forming process. Beyond that, the modular test bench design enables experimental and virtual draping optimization and deduction of blank holder concepts for WCM tools.

Abstract: In this study, the quality of pre-consolidation of a PA6 based glass/carbon hybrid and a glass mono-material UD-tape laminate has been analyzed and the influence of the pre-consolidation step on flexural properties of end-consolidated laminates was determined. For this reason, three different pre-consolidation qualities were mechanically and optically analyzed. The investigations show that the quality of pre-consolidation is not critical for mono-material lay-ups considered in this work. However, the pre-consolidation step has a significant effect on the flexural properties of the hybrid-material laminate presented in this work. The flexural modulus is increased by more than 25 % and the flexural strength up to 9 % due to better welding of the distinct layers. Additionally, the influence of carbon fiber share on the flexural modulus of the UD hybrid-material was examined. It could be recognized that the flexural modulus can be increased up to 171 % by the exchange of glass with carbon fiber layers with a sandwich stacking compared to glass mono-material with constant thickness.

Abstract: The investigations deal with the experimental characterization of the structural deformation of type IV pressure vessels subjected to internal pressure. For the widespread use of hydrogen technology in transport industries, the development of cost-effective storage systems is a crucial step. State of the art in the field of hydrogen storage are type IV pressure vessels, which consist of a polymeric liner and an enforcing winding of carbon fiber-reinforced plastic (CFRP). For the development of material-optimized and high-safety pressure vessels, the acquisition of reliable experimental data in order to validate numerical simulations is a necessity. In a specially designed test chamber subscale vessels are clamped and subjected to internal pressure. At defined pressure stages the vessel’s deformation is recorded and analyzed. Consequently, the overall structural deformation is assessed with regard to the used structural mass, the burst pressure and the resulting failure. The results can be used for structure optimization purposes as well as for the optimization of numerical simulation models.

Abstract: Elastomer layers offer a wide range of surface functionalization options for fiber-reinforced polymer composites (FRPC), e.g. erosion protection or increased impact resistance. Goal of this study was to investigate if it is possible to prepare a textile-based semi-finished product with elastomeric surface, which can easily be used as outermost layer in different liquid composite molding (LCM) processes. For this purpose, different types of elastomer were pressed and vulcanized onto a biaxial glass fiber fabric. Target of this procedure was to reach partial immersion of the elastomer into the textile with remaining dry textile areas. The dry areas of the textile can later be impregnated with a thermoset resin system. The strategy is to have the transition region between elastomer and thermoset within one textile layer and to give a robust and easy to handle semi-finished-product in order to achieve a maximum bonding strength of the elastomer surface to the final composite part. It could be shown by micrographs and computer tomography that the elastomer only penetrates the textile at its boundary. A remarkable microimpregnation of individual filaments within the rovings does not take place. Concerning the manufacturing, since water evaporates during vulcanization, a sufficient process pressure must be maintained throughout the entire vulcanization process to ensure a pore-free elastomer. Peel-off tests similar to DIN EN 28510-1 on the finished composite showed a failure in the laminate and not in the boundary layer between laminate and elastomer, so that the desired high joint strength could be demonstrated.