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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: 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: without files , Hierarchical analysis of the degradation of fibre-reinforced polymers under the presence of void imperfectionsThe subject of this work is the investigation of the influence of voids on the mechanical properties of fibre-reinforced polymers (FRPs) under compression loading. To specify the damage accumulation of FRPs in the presence of voids, the complex threedimensional structure of the composite including voids was analysed and a reduced mechanical model composite was derived. The hierarchical analysis of the model composite on a micro-scale level implies the description of the stress and strain behaviour of the matrix using the photoelasticity technique and digital image correlation technology. These studies are presented along with an analytical examination of the stability of a single fibre. As a result of the experimental and analytical studies, the stiffness of the matrix and fibre as well as their bonding, the initial fibre orientation and the fibre diameter have the highest impact on the failure initiation. All these facts lead to a premature fibre-matrix debonding with ongoing loss of stability of the fibre and followed by kink-band formation. Additional studies on the mesoscale of transparent glass FRPs including a unique void showed that the experiments carried out on the model composites could be transferred to real composites. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.Publicationtype: Conference Paper121 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of graphene nanoparticles on the size effect of epoxy matrix(2016-06); ; ; The size effect of unmodified and graphene nanoparticle modified epoxy matrix is experimentally investigated. A significant size effect due to a statistical defect distribution is identified for the neat matrix. The graphene modified matrix shows almost no size effect. Failure initiates at the nanoparticles, counteracting any size effect due to a statistical defect distribution. In a bulk volume, energy dissipation mechanisms play a more important role, therefore higher values for tensile strength, compared to the neat matrix, are obtained with a nanoparticle modification.Publicationtype: Conference Paper34 - 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 Paper53 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Fracture, failure and compression behaviour of a 3D interconnected carbon aerogel (Aerographite) epoxy compositeAerographite (AG) is a mechanically robust, lightweight synthetic cellular material, which consists of a 3D interconnected network of tubular carbon [1]. The presence of open channels in AG aids to infiltrate them with polymer matrices, thereby yielding an electrical conducting and lightweight composite. Aerographite produced with densities in the range of 7-15 mg/cm3 was infiltrated with a low viscous epoxy resin by means of vacuum infiltration technique. Detailed morphological and structural investigations on synthesized AG and AG/epoxy composite were performed by scanning electron microscopic techniques. The present study investigates the fracture and failure of AG/epoxy composites and its energy absorption capacity under compression. The composites displayed an extended plateau region when uni-axially compressed, which led to an increase in energy absorption of ~133% per unit volume for 1.5 wt% of AG, when compared to pure epoxy. Preliminary results on fracture toughness showed an enhancement of ~19% in KIC for AG/epoxy composites with 0.45 wt% of AG. Observations of fractured surfaces under scanning electron microscope gives evidence of pull-out of arms of AG tetrapod, interface and inter-graphite failure as the dominating mechanism for the toughness improvement in these composites. These observations were consistent with the results obtained from photoelasticity experiments on a thin film AG/epoxy model composite.Publicationtype: Journal Article73 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of voids on the compressive failure behaviour of fibrereinforced compositesThe subject of this work is the micromechanical investigation of the influence of voids on the compressive failure behaviour of fibre-reinforced polymers (FRP). Although, the effect of void content on the mechanical properties was investigated in previous literature, a local examination of the failure mechanisms is essential to understand how the presence of voids reduces the compressive strength in FRP. Therefore, the complex three dimensional structure of a composite, including several voids, were analysed and a reduced mechanical model composite (focusing on one unique void) was derived. The experimental investigation of the model composite included the description of the stress- and strain behaviour of the matrix using photoelasticity. Additionally, a numerical examination of a parameterised model composite was conducted. From the experimental studies the failure initiation and propagation could be observed. Supported by the findings from numerical simulation, foundation and alignment of fibres as well as fibre-matrix bonding has the most impact on the failure initiation.Publicationtype: Journal Article122