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Item type:Publication, Access status: with files , Non-destructive assessment of strain and mechanical fatigue in neat and glass fibre-reinforced epoxy via NIR spectroscopyIn this study, changes in the near-infrared (NIR) spectrum in neat epoxy and glass fibre-reinforced plastic under tensile stress and cyclic fatigue loading are investigated. Specimens were fabricated using resin transfer moulding. During mechanical testing, NIR spectra were collected at regular intervals, enabling monitoring of characteristic absorption bands. Results demonstrate that both material systems show measurable and reproducible shifts in the wavenumber of the absorbed light in specific NIR bands as a function of applied mechanical load. These shifts closely track the progression of damage accumulation, particularly in the context of mechanical fatigue. Overall, the findings underscore the potential of NIR spectroscopy as a powerful, non-destructive, in-situ monitoring tool for assessing the integrity and damage evolution of fibre-reinforced plastic under mechanical stress. This approach provides valuable insights for the development of advanced structural damage monitoring systems and contributes to improving the safety and reliability of composite materials in engineering applications.Publicationtype: Journal Article31 12 - 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 , Smart dispersion: Validation of OCT and impedance spectroscopy as solutions for in-situ dispersion analysis of CNP/EP-composites© 2018 Acta Materialia Inc. In this study, we demonstrate the applicability of three-roll milling (TRM) process for carbon nanotube-epoxy dispersion in industrial scale and two methods to characterize the dispersion quality based on optical coherence tomography (OCT) and impedance spectroscopy were utilized. These methods are easily applicable for industrial continuous TRM dispersion process monitoring. Modeling impedance spectra with equivalent circuit enhances the possibility to quantify the grade of dispersion. We monitored the dispersion development over a complete seven step three-roll mill process for several batches. The gained results are in good accordance to the particle size measurements by grindometry. Results show, on the one hand, that the real part of impedance undergoes a minimum during processing. The most homogeneous dispersion state is not the favorable for gaining electrical conductive materials. On the other hand, imaginary part of impedance undergoes a maximum. High reactance correlates with small particle sizes as well as homogeneous distribution. A developed model from impedance spectra leads to an improved understanding of network formation. OCT results allow a qualitative analysis of the dispersion and detection of in-homogeneous agglomerate sizes.Publicationtype: Journal Article171 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of carbon nanoparticle geometry in epoxy matrix on mechanical properties and failure initiation regarding the size effect(Curran Associates, Inc., 2018-06); ; ; ; The influence of a carbon nanoparticle modification on the damage behaviour and resulting mechanical properties of an epoxy matrix is experimentally investigated. A comparison of different nanoparticle morphologies with regard to their potential use in FRP is carried out. Since the volumes between the fibres in FRP are very small, the focus is set on the investigation of size effects of polymer nanocomposites. The influence of local microdamage at the particles on the mechanical properties in a small volume is discussed by comparing the different energy dissipating damage mechanisms in dependence of the nanoparticle morphology. Crack initiation and damage mechanisms at the different nanoparticles can be clearly identified. For layered particles such as few layer graphene, it is found that the orientation of the graphene layers with regard to loading direction is critical for local damage mechanisms and hence mechanical properties.Publicationtype: Conference Paper49 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Compression fracture of CFRP laminates containing stress intensifications(Multidisciplinary Digital Publishing Institute (MDPI), 2017-09-05); ; ; ; For brittle fracture behaviour of carbon fibre reinforced plastics (CFRP) under compression, several approaches exist, which describe different mechanisms during failure, especially at stress intensifications. The failure process is not only initiated by the buckling fibres, but a shear driven fibre compressive failure beneficiaries or initiates the formation of fibres into a kink-band. Starting from this kink-band further damage can be detected, which leads to the final failure. The subject of this work is an experimental investigation on the influence of ply thickness and stacking sequence in quasi-isotropic CFRP laminates containing stress intensifications under compression loading. Different effects that influence the compression failure and the role the stacking sequence has on damage development and the resulting compressive strength are identified and discussed. The influence of stress intensifications is investigated in detail at a hole in open hole compression (OHC) tests. A proposed interrupted test approach allows identifying the mechanisms of damage initiation and propagation from the free edge of the hole by causing a distinct damage state and examine it at a precise instant of time during fracture process. Compression after impact (CAI) tests are executed in order to compare the OHC results to a different type of stress intensifications. Unnotched compression tests are carried out for comparison as a reference. With this approach, a more detailed description of the failure mechanisms during the sudden compression failure of CFRP is achieved. By microscopic examination of single plies from various specimens, the different effects that influence the compression failure are identified. First damage of fibres occurs always in 0°-ply. Fibre shear failure leads to local microbuckling and the formation and growth of a kink-band as final failure mechanisms. The formation of a kink-band and finally steady state kinking is shifted to higher compressive strains with decreasing ply thickness. Final failure mode in laminates with stress intensification depends on ply thickness. In thick or inner plies, damage initiates as shear failure and fibre buckling into the drilled hole. The kink-band orientation angle is changing with increasing strain. In outer or thin plies shear failure of single fibres is observed as first damage and the kink-band orientation angle is constant until final failure. Decreasing ply thickness increases the unnotched compressive strength. When stress intensifications are present, the position of the 0°-layer is critical for stability under compression and is thus more important than the ply thickness. Central 0°-layers show best results for OHC and CAI strength due to higher bending stiffness and better supporting effect of the adjacent layers.Publicationtype: Journal ArticleScopus© Citations 14 483 524 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Influence of carbon nanoparticle modification on the mechanical and electrical properties of epoxy in small volumes(Elsevier, 2017-07-25); ; ; ; The influence of nanoparticle morphology and filler content on the mechanical and electrical properties of carbon nanoparticle modified epoxy is investigated regarding small volumes. Three types of particles, representing spherical, tubular and layered morphologies are used. A clear size effect of increasing true failure strength with decreasing volume is found for neat and carbon black modified epoxy. Carbon nanotube (CNT) modified epoxy exhibits high potential for strength increase, but dispersion and purity are critical. In few layer graphene modified epoxy, particles are larger than statistically distributed defects and initiate cracks, counteracting any size effect. Different toughness increasing mechanisms on the nano- and micro-scale depending on particle morphology are discussed based on scanning electron microscopy images. Electrical percolation thresholds in the small volume fibres are significantly higher compared to bulk volume, with CNT being found to be the most suitable morphology to form electrical conductive paths. Good correlation between electrical resistance change and stress strain behaviour under tensile loads is observed. The results show the possibility to detect internal damage in small volumes by measuring electrical resistance and therefore indicate to the high potential for using CNT modified polymers in fibre reinforced plastics as a multifunctional, self-monitoring material with improved mechanical properties.Publicationtype: Journal ArticleScopus© Citations 42 281 411 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of carbon nanoparticle geometry in epoxy matrix on mechanical properties and failure initiation regarding the size effectThe influence of carbon nanoparticle morphology in epoxy on the mechanical properties is investigated regarding the size effect. Three types of particles are used, representing spherical, tubular and layered morphologies. For neat and carbon black modified epoxy, a clear size effect of increasing true failure strength with decreasing volume is found. Carbon nanotube modified epoxy shows high potential for strength increase, but dispersion and purity are critical. Few layer graphene particles are larger than statistically distributed defects in the epoxy and initiate cracks, counteracting any size effect. Different toughness increasing mechanisms on the nano- and micro-scale depending on particle morphology are discussed.Publicationtype: Conference Paper60 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , New test approach to determine the transverse tensile strength of CFRP with regard to the size effectThe subject of this work is the investigation of the size effect on transverse strength of CFRP. A new test approach uses the remaining parts of already tested specimens for a second transverse tensile test. With this method it is analysed, whether first failure is dominated by the most critical flaw inside the material or at a load that is equivalent to the maximum tensile strength of the matrix. Thickness of the specimens is varied to study the influence of specimen volume on transverse tensile strength. Test results show that strength follows the trend of defect distribution with increasing strength for decreasing volume. Second failure strength is higher than first failure strength, showing that first failure occurs at the most critical defect and gives no evidence of the maximum transverse tensile strength of the material. Stress at second failure showed less scatter, proving good applicability of the proposed test method.Publicationtype: Journal Article66 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Morphological influence of carbon nanofillers on the piezoresistive response of carbon nanoparticle/epoxy composites under mechanical load(Elsevier, 2016-10-18); ; ; ; In this study, we investigate the piezoresistive response of carbon black and multi walled carbon nanotube/epoxy composites films and bulk specimens. The morphology of the nanocomposites has been influenced by variation of particle geometry, aspect ratio, filler content and AC-field induced alignment of the particles. This allows to understand the influencing parameters so the electrical resistance change (ERC) of the material becomes adjustable. All nanocomposites show a reproducible characteristic ERC vs. strain under quasi-static unidirectional tension, with a resistance increase, a maximum and a subsequent resistance decrease at high strains. Due to the occurring maximum in ERC two resistance values correlate to one strain level. This ambiguity of ERC is unfavourable for sensing purposes. Higher aspect ratios and filler contents shift the maximum to a lower strain level, while the sensitivity is reduced. Alignment of particles leads to a shift of the maximum to a higher strain level, a linear ERC and a gauge factor of about six, but reduces the conductivity. In the composites films the alignment of particles in load direction enhances the strain sensing capabilities.Publicationtype: Journal Article69 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Size effect of graphene nanoparticle modified epoxy matrixThe size effect of unmodified and graphene nanoparticle modified matrix fibres is experimentally investigated. Neat matrix fibres show a clear size effect of increasing tensile strength with decreasing volume due to a statistical defect distribution. The nanoparticle modified matrix shows no significant size effect. Nanoparticles act as crack initiators and consume fracture energy. The size of the particles is independent of specimen volume, so that the failure initiating as well as energy absorbing mechanisms are available, independently of the volume. Fractography analysis of SEM images shows different energy dissipation mechanisms such as micro-damage at the graphene particles. Graphene pull-out, layer separation, layer shearing, formation of micro voids as well as crack separation and crack bifurcation are observed that depend on the orientation of the graphite layers to the fracture plane. These mechanisms dissipate energy and so that a graphene nanoparticle modification result in an increased fracture toughness and thus increased strength of an epoxy matrix system if the volume is large enough. The maximum stress in specimen of small volume depends on graphene layer orientation, so that ideally, the covalent bonds of the nanoparticles should be orientated in loading direction.Publicationtype: Journal ArticleScopus© Citations 20 275 425