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Item type:Publication, Access status: without files , Strength of scarf-bonded CFRP repairs containing disc-shaped zones of weak bonding considering hot-wet conditioning(2020-10); ; ; ; A commonly used design for the repair of carbon fibre reinforced polymers (CFRP) is the bonded scarf joint. As contaminations on the adherend surface or bondline flaws can reduce the bonding strength drastically, the effect of such strength reducing phenomena has to be known. This is also the case for bonded joints that had to sustain hot-wet conditions. Therefore, the aim of this study was to investigate the influence of weak bonds and hot-wet conditioning on the mechanical properties of scarf bonded CFRP joints. Circular zones of weak bonding have been artificially produced by application of release agent on the adherend surface. Different sizes and locations were studied. Additionally, two scarf ratios and two lay-ups have been investigated, to account for different stress states in the bondline. The investigation showed that the effect of a weak bond on bond strength is strongly dependent on the location, size and lay-up of the adherend, while hot-wet conditioning reduces the bond strength in all cases.Publicationtype: Journal Article138 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Near-infrared laser surface treatment of milled CFRP aircraft structures for bonded repair applications(2020); Carbon fibre reinforced polymers (CFRP) are widely used as a material for aircraft structures. During their lifetime, aircraft structures can be subject to damages that have to be repaired to maintain aircraft safety. For a lightweight and material-efficient repair, the scarf bonded joint is the preferred solution. The scarf bonded joint is produced by removing the damaged material by milling and the application of a repair patch through adhesive bonding. A critical step during the adhesive bonding process is surface treatment since it significantly influences the bond strength. This work investigates the effects of near-infrared (N-IR) surface treatment on the adhesive properties of milled CFRP. The laser treatment is able to remove disruptive milling debris from the surface. As a result, a significant increase in bond strength is achieved. Furthermore, the laser treatment is able to strip the fibres. The fibre stripping improves the bond strength compared to the already high standard of the reference process. The increase of the roughness and surface area is seen as the driving mechanism behind the further improvement. Opposed to the reference process, laser treatment is additionally capable of removing contaminations. The removal of contaminations is a considerable advantage of laser treatment, given the severe risk that is posed by contaminations. Though no clear evidence of a functionalisation was found, the chemical composition shows a high functionality without any contaminations and is therefore suitable for adhesive bonding. The main downside of the process is the physically induced interaction between the epoxy resin matrix and the N-IR laser and the consequential formation of cavities under matrix-rich areas. The cavities, though comparably small, can only be partially filled and could induce failure in the bond. Nevertheless, for all tested scarf ratios and lay-ups, an increase of the bond strength compared to the reference process was achieved. This shows the high potential of N-IR laser treatment for composite repair applications.Publicationtype: Doctoral Thesis395 968 - 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: with files , Comparison of analytical approaches predicting the compressive strength of fibre reinforced polymers(Multidisciplinary Digital Publishing Institute (MDPI), 2018-12-11); ; ; ; Common analytical models to predict the unidirectional compressive strength of fibre reinforced polymers are analysed in terms of their accuracy. Several tests were performed to determine parameters for the models and the compressive strength of carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP). The analytical models are validated for composites with glass and carbon fibres by using the same epoxy matrix system in order to examine whether different fibre types are taken into account. The variation in fibre diameter is smaller for CFRP. The experimental results show that CFRP has about 50% higher compressive strength than GFRP. The models exhibit significantly different results. In general, the analytical models are more precise for CFRP. Only one fibre kinking model’s prediction is in good agreement with the experimental results. This is in contrast to previous findings, where a combined modes model achieves the best prediction accuracy. However, in the original form, the combined modes model is not able to predict the compressive strength for GFRP and was adapted to address this issue. The fibre volume fraction is found to determine the dominating failure mechanisms under compression and thus has a high influence on the prediction accuracy of the various models.Publicationtype: Journal ArticleScopus© Citations 11 349 343 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Effect of laser surface treatment on the adhesive properties of bonded CFRP joints(Curran Associates, Inc., 2018-06); ; ; ; In this work, the effect of an Ytterbium fibre laser treatment on the adhesive properties of scarf bonded carbon fibre reinforced plastics (CFRP) joints has been investigated. The surfaces were scarfed and afterwards laser treated. The effect of laser treatment on the surface morphology was investigated with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The laser treatment resulted in a morphological and chemical change on the CFRP surface. The introduced thermal energy was able to partly strip and roughen up the carbon fibres. The laser treatment increased the carbon and decreased the oxygen, nitrogen and sulphur content on the surface. With the laser treatment the static and fatigue strength of the scarf bonded joints could be improved.Publicationtype: Conference Paper98 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of 90°-layer thickness on damage initiation and propagation in CFRP cross-ply laminatesThe influence of layer thickness on failure initiation and damage propagation as matrix cracks in the 90°-layers of cross-ply laminates is investigated experimentally and simulated by using FEM. Decreasing the 90°-layer thickness leads to a shift of damage initiation to higher loads. The crack density and the shape of inter fibre failure also depend on the layer thickness. With increasing 90°-layer thickness, the total amount of cracks decreases, whereas the amount of cracks running through the layer thickness and through the whole specimen width increases. Cracks exhibit a faster growth rate and larger crack opening with increasing layer thickness. This case is more severe for initiating delaminations. A reduction of ply thickness in cross-ply laminates increases the resistance against inter-fibre-fracture and thus offer great potential for increased fatigue lifetime.Publicationtype: Conference Paper52 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of ply thickness on the fibre matrix debonding(2015-07); ; ; In this work, the influence of ply thickness in the 90° layer in cross-ply laminates on the initiation of fibre matrix debonding and inter-fibre-fracture under tensile loads is investigated experimentally by using model composites as well as in a FEM simulation. Decreasing the ply thickness leads to a shift of the onset of inter-fibre-fracture to higher loads. The failure propagation at higher loads is less severe with decreasing layer thickness. Explanations for this behaviour like the probability of a critical defect in the reduced volume or the in-situ effect are discussed. The FEM model is able to represent the ply thickness effect and simulation results are qualitatively in good agreement with the experiments. In the simulation, first fibre-matrix-debonding was found at higher global strain and matrix cracks were more but less severe for thinner plies, whereas an earlier damage onset and fewer cracks, running through the whole layer thickness are found for thicker layers. A reduction of ply thickness in cross-ply laminates increases the resistance against inter-fibre-fracture and failure propagation.Publicationtype: Conference Paper27