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Item type:Publication, Access status: with files , Coelacanth-scale inspired thin-ply composites for load-bearing applications(Elsevier, 2025-10-18); ; ; ; Thin-ply composites are known for their superior in-situ strength and manufacturing quality, offering higher unnotched tensile and compressive strengths compared to conventional laminates. However, their damage suppression capability leads to increased notch sensitivity, where the delamination and matrix cracking mechanisms are suppressed. As a result, thin-ply laminates are limited in their use in critical load-bearing applications. To address this, bio-inspired Bouligand structures, defined by their helical fibre arrangements, have shown promise in reducing notch sensitivity through helicoidal matrix cracking and stress redistribution. This study explores the mechanical performance of partial Bouligand layups derived from biological fibre architectures observed on coelacanth fish scales, where fibrils reorient under load. An analytical stiffness-based optimization was performed to match the mechanical properties of the conventional [0◦, ±45◦, 90◦] (50%, 40%, 10% load introduction layup used in bolted and riveted aircraft structures, while integrating the partial Bouligand structure. The weights of the two-layer fibres (30 gsm and 60 gsm) were investigated, resulting in different pitch and stack angles. Tensile and bearing tests were conducted to evaluate the influence of the partial Bouligand structure on bearing sensitivity. The results indicate that bio-inspired fibre orientation can improve load redistribution and damage tolerance in thin-ply laminates, making them compatible for off-axis and notched applications.Publicationtype: Journal Article49 16 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Influence of the position of fibre misalignment in glass fibre-reinforced polymers on mechanical properties, damage behaviour and traceability via non-destructive tests(Elsevier, 2025-10-01); ; ; ; Defects in glass fibre-reinforced polymers (GFRP), especially fibre misalignments, are a well-known issue affecting structural performance. This paper investigates how the position of such misalignments influences the mechanical behaviour to deepen the understanding of their influence. GFRP cross-ply laminates with 11 fibre layers were fabricated using the resin transfer moulding process, containing four different types of fibre misalignments. Subsequently, the specimens were evaluated by computed tomography scans and ultrasonic testing and experimentally investigated under tensile, compressive, and bending loading. The obtained mechanical properties were also compared with simulation data. The results of the mechanical tests show that the strength reduction varies greatly depending on the load case, the type of misalignment and its position. Undulations in tensile tests can lead to a strength reduction as low as 8 %, while a single fold located outside the middle layer can reduce the compressive strength by up to 37 %. In service life tests, the effect is even more pronounced. In the worst case, the presence of multiple defects can reduce the service life by up to 94 %. The simulation results showed that a local displacement of individual layers does not represent the extent of undulations and that existing simulation approaches need to be revised and extended. The results of the experiments and simulations demonstrate that both the presence and the position of fibre misalignments significantly affect the material behaviour and are likely underestimated in current research.Publicationtype: Journal Article28 26 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Reversible and irreversible effects on the epoxy GFRP fiber-matrix interphase due to hydrothermal aging(Elsevier, 2023-08-29); ; ; ; Aouissi, Hani AmirEpoxy R-Glass Fiber-Reinforced Polymer (GFRP) composite plates were hydrothermally aged at 60 °C for 23, 75, and 133 days. The water content reached 0.97 wt%, 1.45 wt% and 1.63 wt%, respectively. The studied GFRP matrix was inert to hydrolysis or chain scission, allowing for investigation of irreversible changes in the fiber-matrix interphase due to hydrothermal aging upon re-drying. During each period, a subset of the specimens was removed from the water bath and dried in a chamber. The weight loss upon drying was explained with epoxy leaching (impurities), sizing-rich interphase hydrolysis, glass fiber surface hydrolysis, accumulated degradation products escaping, and water changing state from bound to free. The influence of hydrothermal aging on the fiber-matrix interfacial properties was investigated. Lower interfacial strength of hydrothermally aged (wet) samples was attributed to plasticization of the epoxy, plasticization and degradation of the sizing-rich interphase (including formation of hydrolytic flaws), and hydrolytic degradation of the glass fiber surface. The kinetics of epoxy-compatible epoxysilane W2020 sizing-rich interphase hydrolysis provided an estimate of ca. 1.49%, 4.80%, and 8.49% of the total composite interphase degraded after 23, 75, and 133 days, respectively. At these conditions, the interface lost 39%, 48%, and 51% of its strength. Upon re-drying the specimens, a significant part of the interfacial strength was regained. Furthermore, an upward trend was observed, being 13%, 10% and 3% strength, respectively; thus, indicating a possibility of partial recovery of properties.Publicationtype: Journal ArticleScopus© Citations 6 45 108 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Strain sensing in GFRP via fully integrated carbon nanotube epoxy film sensors(Elsevier, 2021-10); ; ; ; Structural health monitoring of composite structures enables early damage detection to prevent critical component failure. Surface-mounted strain gauges are commonly applied to monitor the integrity of composite structures in highly loaded areas. However, strain gauges can only measure strain on the structure's surface and are exposed to environmental influences. Within this paper, fully integrated carbon nanotube thin-film sensors for strain and damage sensing in glass fibre reinforced polymers (GFRP) via electrical resistance measurements are presented. Single wall carbon nanotube epoxy thin-films were manufactured using a manual film applicator, partially pre-cured, placed on dry glass fabrics and infused in a resin transfer moulding process. The mechanical properties of the composites and strain sensing capabilities of the integrated sensors were studied for various load cases and different laminate lay-ups. Results of quasi-static and step-wise three-point bending and tensile tests show that the integrated films allow for localised strain measurements in GFRP without significant loss of mechanical properties. Open hole tensile tests proof the ability to monitor local strain and damage in highly loaded areas enabling failure prediction via threshold resistance change values. The proposed thin-films enable a tailored strain and damage monitoring of GFRP offering the possibility for measurements at different material depths, over large sections or selectively in highly loaded areas. The manufacturing process is easily automatable and suitable for large scale manufacturing.Publicationtype: Journal ArticleScopus© Citations 7 83 277 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Fatigue and fatigue after impact behaviour of Thin- and Thick-Ply composites observed by computed tomography(Elsevier, 2021-07); ; ; ; This study investigates the influence of load ratio and impact damage on the fatigue behaviour of high-performance carbon fibre reinforced polymers (CFRP) with areal fibre weights between 30 gsm and 360 gsm. For undamaged samples, the ultimate tensile and compressive strength, as well as the fatigue properties, are evaluated with regard to their layer thicknesses. The fatigue tests were performed under tension-tension (R=0.1), tension-compression (R=-0.5) and compression-compression (R=10) regime. The results are illustrated as a constant-life diagram, and a piecewise linear interpolation examines a first prediction. The results show that static and fatigue performance improves with decreasing layer thickness. Particularly under tension-compression loading, significant improvements are observed, due to the suppression of matrix cracks and delaminations with thinner layers. In addition, the effect of low-energy impact on the fatigue behaviour of Thin- and Thick-Ply laminates is investigated. The tests demonstrate that although the delamination area is larger, Thin-Ply laminates can sustain higher stresses and still reach the same number of load cycles in contrast to Thick-Ply laminates. Computed tomography measurements visualize 3-dimensional the damage progression after various cycles and prove that the Thin-Ply composites show no increase in the damaged area during fatigue. The interlaminar stress at the delamination is not sufficient for expansion. In contrast, in the case of thicker layers, the damage growths progressively throughout the whole sample with increasing number of cycles.Publicationtype: Journal ArticleScopus© Citations 19 182 304 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Damage tolerance and notch sensitivity of bio-inspired thin-ply Bouligand structures(Elsevier, 2021-07); ; ; ; Different bio-inspired Bouligand thin-ply Carbon-Fibre-Reinforced Plastic (CFRP) laminates with a pitch angle as low as 2.07∘ are realised, which is the smallest pitch angle realised in literature. The angle is therefore close angles found in biological microstructures. Low-Velocity Impact (LVI) and residual compressive strength tests determined the damage tolerance of the structures. Investigated were two different interlaminar fracture toughnesses and two different metal-Bouligand-CFRP-layups. The low pitch angle results in significantly higher residual strengths than 45∘ quasi-isotropic (QI) layups, despite the significantly lower proportion of 0∘ fibres. Higher fracture toughness and hybridisation with steel layers lead to reduced matrix damage without increasing residual compressive strength. In-plane plane tension properties are determined with a pitch angle of 2.59∘. The results reveal, that the unnotched tensile strength is significantly lower. However, only helicoidal, sub-critical matrix cracking and no delaminations occur before final failure. The sub-critical matrix cracking leads to almost no notch sensitivity and a similar open-hole-tensile strength to 45∘-QI layups despite the low number of 0∘-fibres.Publicationtype: Journal ArticleScopus© Citations 14 92 423 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Steel foil reinforcement for high performance bearing strength in Thin‐Ply composites(Elsevier, 2021-03); ; ; ; This study investigates the influence of local hybridization of Thin and Thick-Ply CFRP laminates on the open-hole and bearing properties. The area weight of the CFRP unidirectional prepregs used is 40 gsm in the case of Thin-Ply layers and 160 gsm in the case of Thick-Ply layers. The steel used is a 1.4310 stainless-steel foil with the same layer thickness as the prepregs. In the hybrid area, 90 layers were locally replaced by stainless steel patches. The local metal foil content varies from 6.25%, 12.5% to 25.0%. For notched laminates, the open hole tensile strength is significantly decreased with thinner layer thicknesses. The failure behavior changes from complex delamination dominated failure to brittle failure. By using stainless steel foils in the regions of stress concentrations, energy can be dissipated by plastic deformation of the steel foil and stresses can be deflected to neighbouring areas. For Thin-Ply samples with a local steel content of 25% the open hole tensile strength increases by 64% in comparison to the reference Thin-Ply specimens and the sensitivity towards stress concentrations decreases. The bearing strength of the hybrid CFRP laminates is increased by up to 54.6% in comparison to the reference material, due to the confinement of the steel foil and the resulting higher compressive strength and the plastic deformation at high stresses. The stress–strain diagrams and micrographs of the fibre metal samples reveal that damage is initiated before the maximum bearing strength. However, the damage offset bearing strength increase concerning the specific density of the material significantly.Publicationtype: Journal ArticleScopus© Citations 13 157 423 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Impact of temperature on LVI-damage and tensile and compressive residual strength of CFRPThis study investigates the influence of temperature and impact-energy on low-velocity impact damage in CFRP and the resulting residual tensile and compressive strength. Impacts were introduce d at −20 ◦C , 20 ◦C and 80 ◦C , which are moderate temperature compared to the glass transition temperature of 203 ◦C of the used CFRP. A change in temperature leads to a substantial change in damage behaviour. With increasing temperature, the delamination area is reduced, and at 80 ◦C fibre-failure occurs on the impacted side. The residual tensile strength was tested at 20 ◦C utilising a new jig, which counteracts the stresses resulting from free-edge effects and thus prevents that edge delaminations are overshadowing the effect of the impact damage due to the specimen size. The fibre failure reduces the residual tensile strength significantly compared to the damage resulting from impaction at 20 ◦C . The compressive residual strength was determined at 20 ◦C and 80 ◦C . The results point out that the temperature mainly determines the residual compressive strength. Consequently, it is essential to identify the material’s behaviour in the range of the in-service temperatures, even if they are far away from the glass transition temperature, to evaluate the damage tolerance and performance.Publicationtype: Journal ArticleScopus© Citations 4 152 232