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Item type:Publication, Access status: without files , General influence of the environmental temperature on the matrix strength under tensile and compressive loading - A comprehensive study on high performance matricesThis comprehensive experimental study investigates the influence of operating temperatures ranging from −50 °C to 256 °C on the mechanical properties of various high-performance thermosets regularly used as matrix polymers for composites. The resins are either crucial for specific industry sectors or challenging to obtain because they are part of commercial prepreg systems. The research focuses on the temperature-dependent tensile and compressive strength of those materials and provides a wide database of mechanical properties. This paper summarizes the mechanical properties on the one hand in terms of a strength versus temperature scale, which allows an easy comparison for engineering applications. And on the other hand, it demonstrates that the thermo-mechanical properties are mainly dependent on the polymer's glass transition temperature. More precisely, three approaches to estimate the temperature dependence of the strength of thermosets with reduced testing effort are applied and presented. While one is based on the assumption that strength depends linearly on the environmental temperature, the others focus on the distance or the ratio of the environmental temperature to the glass transition temperature. This data set and the resulting correlations provide the community with valuable material data and allow to forecast the thermo-mechanical behavior of most thermosets in the future with significantly reduced effort.Publicationtype: Journal Article70 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Data-driven inverse uncertainty quantification in the transverse tensile response of carbon fiber reinforced composites(2021-07-28); ; Uncertainty quantification is critical for the full exploitation of composite materials’ potential. Inverse methods offer the possibility of indirectly characterizing the uncertainty of microscopic parameters by employing data sets from standard structural tests in higher scales. Two crucial requirements though, are the efficient modeling especially for the nonlinear prediction, and the measurement error availability from the tests which affects the updated scatter. This study employs effective stiffness and strength experimental data in order to quantify uncertainties of a carbon fiber UD composite in the microscale. A polynomial chaos surrogate model is trained from finite element simulations, able to efficiently predict the homogenized stiffness and strength for the uncertainty quantification procedure. The random parameters which are influential enough to be updated, are identified via a variance-based global sensitivity analysis. The inverse problem is solved with the Bayesian inference method, which updates any prior estimation of the probability models of the input parameters, based on output observations from the tests. Results show significant uncertainty reduction in comparison with typically used variance values in the literature and can be used to enrich the composite material databases. The proposed methodology is applied for the transverse tensile load case, although its non-intrusive nature allows applications for more load cases and various setups.Publicationtype: Journal Article103 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Damage tolerance of few-layer graphene modified CFRP: From thin-to thick-ply laminates(2021-06-16); ; ; ; Low impact resistance and high notch sensitivity are significant drawbacks of carbon-fibre-reinforced plastics (CFRP). The addition of carbon nanoparticles (CNP), a third phase, reduces this deficit through additional energy-consuming mechanisms. The influence of CNP modification on strength and damage behaviour cannot be separated from the effect of layer thickness because it controls the stress state in the entire laminate. This study investigates the influence of few-layer graphene (FLG) modification on CFRP with layer thicknesses varying from ultra-thin-ply (28μm, 30g/m ) to thick-ply (220μm, 240g/m ). The mode I and mode II inter-laminar energy release rates increased significantly with the FLG modification. It also increased the damage initiation stress for notched laminates of all layer thicknesses, thereby increasing the useable design space. The FLG modification also increased the ultimate notched strength for the ultra-thin ply laminates, thereby reducing the high notch sensitivity which is the limiting factor for their application. In the case of low-velocity impact damage, the modification of thick-layer laminates improves the residual compressive strength. FLG modification leads to a considerable improvement in the compressive strength. The selection of CNP was based on a comprehensive investigation of the fracture toughness of the prepreg resin/CNP nanocomposites. The particle loading and type was selected for FLG with a 25μm sheet size and 0.05 wt %.Publicationtype: Journal Article76 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Ultra-thin and ultra-strong organic interphase in nanocomposites with supercrystalline particle arrangement : mechanical behavior identification via multiscale numerical modeling(Elsevier, 2020-09-29); ; ; ; © 2020 The Authors. A key challenge in the development of inorganic-organic nanocomposites is the mechanical behavior identification of the organic phase. For supercrystalline materials, in which the organic phase ranges down to sub-nm areas, the identification of the organic materials' mechanical properties is however experimentally inaccessible. The supercrystalline nanocomposites investigated here are 3D superlattices of self-assembled iron oxide nanoparticles, surface-functionalized with crosslinked oleic acid ligands. They exhibit the highest reported values of Young's modulus, nanohardness and strength for inorganic-organic nanocomposites. A multiscale numerical modeling approach is developed to identify these properties using supercrystalline representative volume elements, in which the nanoparticles are arranged in a face-centered cubic superlattice and the organic phase is modeled as a thin layer interfacing each particle. A Drucker-Prager-type elastoplastic constitutive law with perfectly plastic yielding is identified as being able to describe the supercrystals' response in nanoindentation accurately. As the nanoparticles behave in a purely elastic manner with very high stiffness, the underlying constitutive law of the organic phase is also identified to be Drucker-Prager-type elastoplastic, with a Young's modulus of 13 GPa and a uniaxial tensile yield stress of 900 MPa, remarkably high values for an organic material, and matching well with experimental and DFT-based estimations. Furthermore, a sensitivity study indicates that small configurational changes within the supercrystalline lattice do not significantly alter the overall stiffness behavior. Multiscale numerical modeling is thus proven to be able to identify the nanomechanical properties of supercrystals, and can ultimately be used to tailor these materials' mechanical behavior starting from superlattice considerations.Publicationtype: Journal ArticleScopus© Citations 12 268 360 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Reaching maximum inter-laminar properties in GFRP/nanoscale sculptured aluminium ply laminatesThe aim of the present work is to reach maximum inter-laminar properties in fibre metal laminates (FML) consisting of glass fibre reinforced polymer (GFRP) and aluminium (Al) plies. The Al plies are AA5019 and AA5754 alloys and pre-treated by nanoscale sculpturing before the FMLs are manufactured by resin transfer moulding. The nanoscale sculpturing of the Al plies leads to the formation of cubical hook-like structures on the surface giving rise to a three-dimensional mechanically interlocking surface. The inter-laminar properties of the FML are investigated by double-notch shear as well as double cantilever beam (Mode I) and end notched flexure (Mode II) testing methods and compared to untreated Al plies and conventional GRFP laminates as reference. As result the nanoscale sculptured Al plies show drastically increased inter-laminar mechanical properties due to highly improved inter-ply bonding between metal surface and resin. For all FMLs with nanoscale sculptured Al plies the delamination appears in the transition zone between glass fibres and matrix due to the lower adhesion of the glass fibre/matrix interface compared to the nanoscale sculptured Al ply/matrix interface. This proves that the maximum necessary inter-laminar properties are achieved.Publicationtype: Journal Article185 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Thinner and better: (Ultra-)low grammage bacterial cellulose nanopaper-reinforced polylactide composite laminatesOne of the rate-limiting steps in the large-scale production of cellulose nanopaper-reinforced polymer composites is the time consuming dewatering step to produce the reinforcing cellulose nanopapers. In this work, we show that the dewatering time of bacterial cellulose (BC)-in-water suspension can be reduced by reducing the grammage of BC nanopaper to be produced. The influence of BC nanopaper grammage on the tensile properties of BC nanopaper-reinforced polylactide (PLLA) composites is also investigated in this work. BC nanopaper with grammages of 5, 10, 25 and 50 g m−2 were produced and it was found that reducing the grammage of BC nanopaper from 50 g m−2 to 5 g m−2 led to a three-fold reduction in the dewatering time of BC-in-water suspension. The porosity of the BC nanopapers, however, increased with decreasing BC nanopaper grammage. While the tensile properties of BC nanopapers were found to decrease with decreasing BC nanopaper grammage, no significant difference in the reinforcing ability of BC nanopaper with different grammages for PLLA was observed. All PLLA composite laminates reinforced with BC nanopapers possessed similar tensile modulus of 10.5–11.8 GPa and tensile strength of 95–111 MPa, respectively, at a BC loading fraction vf,BC = 39–53 vol.-%, independent of the grammage and tensile properties of the reinforcing BC nanopaper.Publicationtype: Journal ArticleScopus© Citations 24 228 374 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Improvement of mechanical properties by a polydopamine interface in highly filled hierarchical composites of titanium dioxide particles and poly(vinyl butyral)(Elsevier, 2017-04-15); ; ; ; In this work, the preparation and properties of hierarchical composites of titanium dioxide (TiO2) particles (rutile modification) and poly(vinyl butyral) (PVB) are discussed. The volume fraction of the ceramic particles was approximately 60%. Two types of composites with different fillers were examined, i.e. TiO2 particles with and without a thin coating of polydopamine (PDA). A variety of characterization methods was applied in order to analyze the properties of the particles and the composites. Infrared spectroscopy is used to verify the functionalization of the particles with a thin polydopamine layer. Thermal analysis provides information on the thermal stability and the degree of functionalization of the coated particles and the composites. Scanning electron microscopy investigations reveal that the functionalized TiO2 particles with PDA form larger agglomerates which enable the coating of the TiO2 particles with PVB via the spouted bed technique. Nanoindentation experiments show that the final hierarchical composite material with the use of non-coated TiO2 particles exhibits a hardness of 0.75 ± 0.04 GPa and a Young's modulus of 29.5 ± 1.0 GPa. The composites containing polydopamine coated TiO2 particles show an increase of approximately 40% in hardness and 25% in Young's modulus in comparison to the composites with uncoated TiO2.Publicationtype: Journal Article163 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Electro-mechanical piezoresistive properties of three dimensionally interconnected carbon aerogel (Aerographite)-epoxy composites(Elsevier, 2016-08-28); ; ; ; Aerographite (AG) is a carbon aerogel consisting of three-dimensionally (3D) interconnected graphitic microtubes. This study characterizes the electrical and mechanical properties of Aerographite/epoxy composites under tensile load. Aerographite can be used as a highly tailorable filler in polymer nanocomposites (PNCs) where the carbon filler and the matrix form an interpenetrating structure, contrary to particle filled systems. Aerographite networks with densities ranging from 3.0 to 13.9 mg/cm3 were produced in a chemical vapour deposition (CVD) process. An infiltration with epoxy leads to Aerographite/epoxy composites with filler contents in the range of 0.26–1.24 wt%. Their electrical conductivity is in the range of 2–13.6 S/m, thus, orders of magnitude higher compared to CNT-based PNCs at comparable filler contents. Although a large amount of direct interconnections of the graphitic tubes is given, interestingly the Aerographite/epoxy composites show a piezoresistive behaviour comparable to PNCs filled with carbon nanotubes (CNT) or graphene. Unexpected shifts between external mechanical strain and electrical signal have been observed in incremental piezoresistive experiments. Young's moduli and tensile strengths of the PNCs are not affected by embedding Aerographite networks. Fractographic observations identify graphitic wall slippage as the dominating failure mechanism. Both, piezoresistive characterization and fractography studies have been correlated and a model for the observed piezoresistive response is derived.Publicationtype: Journal Article74 - 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 - 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