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Item type:Publication, Access status: with files , Double-sided incremental forming of continuous fiber reinforced thermoplasticsContinuous fiber reinforced thermoplastics (CFRTP) combine excellent mechanical properties with recyclability, weldability, and reshapeability. However, their conventional processing requires part-specific tooling, imposing high initial cost and lead times. This limits their applicability to small lot sizes. A flexible alternative could be Incremental Sheet Forming (ISF), which creates parts by moving a small, part-agnostic tool along numerically controlled paths. While ISF is established for metals, its adoption for CFRTP started only recently. In particular, the Double-Sided Incremental Forming (DSIF) process, which employs a second forming tool to improve quality and flexibility, has not been investigated in detail for CFRTP. Therefore, this work presents the first extensive study into DSIF of CFRTP, with a focus on establishing a baseline process. The conducted process characterization includes temperature and force assessments. Forming strategies were compared by forming cone frusta, yielding a multi-stage strategy combining spiral and fiber-following toolpaths, which produced wrinkle-free parts with good accuracy. Micrographs indicate good consolidation for the tested material and process conditions. Furthermore, a preliminary process parameter analysis, including temperature, support force, and step size, was conducted. The results indicate that DSIF can become a viable, flexible manufacturing route for CFRTP components and provide a sound foundation for future research.Publicationtype: Journal Article53 19 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Damage monitoring of GFRP laminates via electrical measurements: experiments and predictionsIn this work, an experimental investigation is presented on the early-stage fatigue damage of conductive glass/epoxy cross-ply laminates. The study is specifically designed to generate a comprehensive and consistent experimental dataset suitable for the validation of an analytical model previously developed by the authors. Fatigue tests were performed under controlled loading conditions, while transverse matrix cracking in the 90° plies was continuously monitored together with stiffness degradation and changes in electrical resistance. These measurements provide a detailed dataset that enables correlation between mechanical damage and electrical response. The data were then used to validate the analytical model developed by the authors, which predicts stiffness degradation as a function of the resistance change induced by transverse cracking. The good agreement between model predictions and experimental observations confirms the suitability of the adopted experimental approach and supports the use of electrical resistance measurements as a quantitative tool for model-based assessment of early-stage damage in conductive GFRP laminates.Publicationtype: Journal Article87 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Flame-retardant composites and analysis of their residual strengths after tailored flame testsIn recent decades, fibre-reinforced polymers (FRPs) have become a key factor to realise lightweight design, e.g., to safe fuel in mobility applications. As FRPs are usually combustible, flame retardants (FRs) are necessary to fulfil critical requirements. On the one hand, FRs can negatively affect thermo-mechanical properties of composites and should therefore used as little as possible. On the other hand, frequently used infusion processes to manufacture FRPs are no longer possible due to filtration effects, if the FRs are present as particles. However, novel in this study is the use of a combined prepreg and infusion process to solve both challenges by implementing FRs only in the most exposed plies. Simultaneously, this method enables the manufacturing of large parts with implemented FR particles, without compromising the lightweight potential or causing adhesion challenges through the use of FR gel-coats. As part of this study, tailored flame tests with adequate reproducibility were developed for qualitative comparisons between different configurations and the residual compressive strength was determined afterwards, because the structural integrity in fire scenarios is of special importance in mobility applications. It was found, that despite a very low global amount of ≈ 1.5 - 3 % red phosphorus leads to significant improvements in the residual strength compared to unmodified configurations.Publicationtype: Journal Article62 21 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Feasibility study on ultrasonic-assisted processing techniques for the value-retention of hybrid thermoplastic–thermoset compositesThe integration of high-performance thermoplastics in fiber-reinforced polymers with thermoset matrices offers the possibility of using existing manufacturing processes and at the same time incorporating the advantages of thermoplastics. These include higher fracture toughness, weldability and reprocessability. Among existing high-performance thermoplastics, polyetherimide (PEI) stands out due to its amorphous structure, which provides good compatibility with thermosets. This study investigates the integration of a PEI interlayer within carbon fiber-reinforced epoxy prepregs to evaluate the feasibility of value-retention and multiple circular use of hybrid thermoplastic–thermoset laminates via power ultrasonics. A novel ultrasonic-assisted separation method employing a cutting tool enabled controlled pre-crack initiation and propagation along the PEI interface, resulting in clean and repeatable separation. Subsequent displacement-controlled ultrasonic reconsolidation of the separated laminates, facilitated by a PEI film as energy director, led to uniform melt generation and a 55% increase in lap-shear strength compared to time-controlled reconsolidation experiments. Furthermore, double cantilever beam (DCB) testing revealed a 250% enhancement in the critical energy release rate (GI,C) for laminates with PEI interlayer, demonstrating significantly improved fracture toughness. These findings underscore the dual functionality of the PEI interlayer, not only as a toughening agent but also as an enabler for reprocessing and reusing by power ultrasonics. The demonstrated approach offers a compelling pathway for sustainable composite design, particularly for aerospace applications where mechanical performance and end-of-use circularity options are critical.Publicationtype: Journal Article81 25 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Modelling of low-velocity impact and compression after impact of CFRP at elevated temperatures(2021-08); ; In this study, temperature-dependent material properties are implemented into a continuum-damage-based (CDM) model to describe the progressive failure of CFRP under temperature influence. Low-velocity impact (LVI) simulations showed that the material model could accurately represent the damage resulting from LVI at different temperatures and impact energies. The simulations show that an increase of the interlaminar ERR does not cause the reduction in delamination size, but the change in intralaminar damage and overall structural response. Compression after impact (CAI) simulations are in good agreement with experimental data. The results emphasise that an increased temperature during compression has a more decisive influence on residual strength than the impact of energy. The proposed approach and CDM model can predict the material behaviour of CFRP under temperature influence and complex load cases.Publicationtype: Journal Article65 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Mechanical properties and failure mode of thin-ply fiber metal laminates under out-of-plane loadingIn this study, fiber metal laminates (FMLs), which consisted of thin-ply prepreg and stainless steel layers with a ply thickness of 0.04 mm, were fabricated with the aim of suppressing the fiber breakage that decreases the compression-after-impact (CAI) strength. Then, static flexural tests of FMLs were conducted to investigate the effects of the number of metal layers on the failure mode and mechanical properties during static out-of-plane loading, and the results were compared with those of our previous impact and CAI tests. The results confirmed that the plastic deformation of metal layers improved the energy dissipation and thus failure occurred in a limited range of locations. Furthermore, the investigation of the in situ stress during flexural tests by finite element method (FEM) analysis revealed that a hybrid material that both suppresses ply failure and has a high modulus of the metal can be realized owing to the constraining effect of thin-ply laminates.Publicationtype: Journal Article34 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Stochastic modeling techniques for textile yarn distortion and waviness with 1D random fieldsWithin the concept of simulation approaches for manufacturing-induced imperfections of composite structures, this work proposes modeling frameworks for the consideration of stochastic deviations concerning the yarns of textile composite materials. The random distortion of a yarn's cross-section, is addressed by flexible 1D Fourier-based random fields, with the potential to be calibated from measurements of the deviations from the nominal yarn shape and their statistical characteristics. Furthermore, a Kriging-based modeling approach is presented, able to randomize any nominal yarn path in short or long range problems, considering data for the correlation and variance in a straightforward manner. The effects of defects due to stochastic yarn distortion and waviness, are investigated by simulating a forward uncertainty propagation problem of a triaxially braided composite material. The response variability concerning stiffness and strength for different uncertainty levels is highlighted, while several comments are offered regarding numerical issues and potential surrogate modeling techniques.Publicationtype: Journal Article151 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Effect of fabric architecture, compaction and permeability on through thickness thermoplastic melt impregnationTo reduce the cycle time of structural, automotive thermoplastic composites, we investigated the potential of direct thermoplastic melt impregnation of glass fabrics using an injection moulding process. At the high pressures that occur during the process, the effect of the fabric architecture on the impregnation, compaction, volume fraction and permeability of two unidirectional fabrics was studied. Using impregnation experiments with a low viscosity PA6 melt, we identified a favourable processing window resulting in an impregnation time of 5 min. The impregnation experiments with thermoplastic melts demonstrate that textile architectures promoting dual scale flow during impregnation are favourable for complete filling. Based on our findings, thermoplastic compression resin transfer moulding is an efficient processing route for automated production of composite parts with a high fibre volume fraction, if the fabric architecture is adapted for higher processing pressures and by fully utilising dual scale flow.Publicationtype: Journal Article185 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Effect of infrared laser surface treatment on the morphology and adhesive properties of scarfed CFRP surfacesThe bonded scarf joint is a common design for the repair of carbon fibre reinforced polymer (CFRP). Surface treatment is an essential factor for the performance of such joints. Infrared (IR) laser surface treatment is of particular interest, as it has the potential for a fast and automatable process. The effect of the IR laser surface treatment on the morphology was investigated by scanning electron microscopy in combination with ion beam milling. Change of surface chemical composition was assessed by X-ray photoelectron spectroscopy. Additionally, IR laser treated CFRP scarf joints were tested under tensile load. Surface analysis revealed a stripping of carbon fibres in combination with a roughening. On the other hand, a decrease of oxygen and nitrogen on the surface was detected. The increase of the bonding strength was attributed to the increase of the effective surface and mechanical interlocking due to the roughening of the CFRP surface.Publicationtype: Journal Article263 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Maximizing bearing fatigue lifetime and CAI capability of fibre metal laminates by nanoscale sculptured Al pliesThis paper presents an experimental investigation of multifunctional fibre metal laminates (FMLs). Metal sheets are embedded into the hybrid composites by means of glass fibre reinforced polymer (GFRP) ply substitution, avoiding any laminate thickening and providing high bearing and compression after impact (CAI) capabilities. The Al sheets (AA6082) are nanoscale sculptured to prevent premature delamination between the metal and the matrix, which commonly occurs in conventional FMLs. The laminates are manufactured by resin transfer moulding. As a result the FMLs show drastically increased mechanical properties compared to conventional GFRP laminates for structural applications, achieving values for static pin-bearing (up to 21%), static bolt-bearing (up to 2%), resistance under fatigue pin-bearing (up to 100 times), resistance under fatigue bolt-bearing (up to 28 times) and CAI (up to 9%). The pre-treatment leads to a three-dimensional mechanical interlocking surface structure with highly improved inter-ply bonding between metal surface and resin. The fracture surface demonstrates that the adhesive bond between the nanoscale sculptured surface structure of the Al and matrix remains intact.Publicationtype: Journal Article224