Papers by Nektaria Barkoula
Materials
The present study demonstrates, for the first time, the ability of a 10-ply glass fiber-reinforce... more The present study demonstrates, for the first time, the ability of a 10-ply glass fiber-reinforced polymer composite laminate to operate as a structural through-thickness thermoelectric generator. For this purpose, inorganic tellurium nanowires were mixed with single-wall carbon nanotubes in a wet chemical approach, capable of resulting in a flexible p-type thermoelectric material with a power factor value of 58.88 μW/m·K2. This material was used to prepare an aqueous thermoelectric ink, which was then deposited onto a glass fiber substrate via a simple dip-coating process. The coated glass fiber ply was laminated as top lamina with uncoated glass fiber plies underneath to manufacture a thermoelectric composite capable of generating 54.22 nW power output at a through-thickness temperature difference οf 100 K. The mechanical properties of the proposed through-thickness thermoelectric laminate were tested and compared with those of the plain laminates. A minor reduction of approximate...
The influence of interfacial modification and relative fiber orientation (parallel, Pa and perpen... more The influence of interfacial modification and relative fiber orientation (parallel, Pa and perpendicular, Pe) on the solid particle erosion was investigated in unidirectional (UD) reinforced glass fiber (GF) epoxy (EP) composites. The interfacial modification was varied by GF sizing. The erosive wear behavior was studied in a modified sandblasting apparatus at three impact angles (30, 60 and 90). The surface topography of the eroded composites was investigated by a scanning electron microscope (SEM) and a non-contact 3D laser profilometer. The results showed a strong dependence of the erosive wear on the jet angle. The GF/EP systems presented a brittle erosion behavior, with maximum weight loss at 90 impact angle. It was established that good fiber/matrix adhesion improved the resistance to erosive wear. On the other hand, the relative fiber orientation had a negligible effect except the erosion at 30 impact angle. High roughness of the eroded surfaces indicated for high erosion rates, i.e. low resistance to solid particle erosion.
Applied Composite Materials, 2009
Many technologies presented in literature for the forming of self-reinforced or all-polymer compo... more Many technologies presented in literature for the forming of self-reinforced or all-polymer composites are based on manufacturing processes involving thermoforming of pre-consolidated sheets. This paper describes novel direct forming routes to manufacture simple geometries of self-reinforced, all-polypropylene (all-PP) composites, by moulding fabrics of woven co-extruded polypropylene tapes directly into composite products, without the need for pre-consolidated sheet. High strength co-extruded PP tapes have potential processing advantages over mono-extruded fibres or tapes as they allow for a larger temperature processing window for consolidation. This enlarged temperature processing window makes direct forming routes feasible, without the need for an intermediate pre-consolidated sheet product. Thermoforming studies show that direct forming is an interesting alternative to stamping of pre-consolidated sheets, as it eliminates an expensive belt-pressing step which is normally needed for the manufacturing of semi-finished sheets products. Moreover, results from forming studies shows that only half the energy was required to directly form a simple dome geometry from a stack of fabrics compared to stamping the same shape from a pre-consolidated sheet.
This study describes the fatigue behaviour of newly developed all-polypropylene (all-PP) tapes an... more This study describes the fatigue behaviour of newly developed all-polypropylene (all-PP) tapes and woven composites [1], with reference to the composite processing conditions, testing temperature and comparison to commercial alternatives. The possible damage development mechanisms are also part of the present paper.
Journal of Applied Polymer Science, 2007
The creation of highly oriented, coextruded polypropylene (PP) tapes allows the production of nov... more The creation of highly oriented, coextruded polypropylene (PP) tapes allows the production of novel, wholly thermoplastic, recyclable “all-polypropylene” (all-PP) composites, which possess both a large temperature processing window (>30°C) and a high volume fraction of reinforcement phase (highly oriented PP tapes: >90%). This large processing window is achieved by using coextruded, highly drawn PP tapes. To achieve coherent all-PP composites the interfacial characteristics following consolidation must be understood. This article investigates the interfacial characteristics of these coextruded tapes by using microcomposite models to create interfaces between tapes of varying draw ratios, drawing temperatures, skin/core ratios, and skin layer thicknesses. The tape drawing parameters are seen to control the interfacial properties in subsequent microcomposite models. The failure mode of these specimens, and hence bond strength, varies with consolidation temperature, and a model is proposed describing and explaining this behavior. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 118–129, 2007
Applied Composite Materials, 2009
Many technologies presented in literature for the forming of self-reinforced or all-polymer compo... more Many technologies presented in literature for the forming of self-reinforced or all-polymer composites are based on manufacturing processes involving thermoforming of pre-consolidated sheets. This paper describes novel direct forming routes to manufacture simple geometries of self-reinforced, all-polypropylene (all-PP) composites, by moulding fabrics of woven co-extruded polypropylene tapes directly into composite products, without the need for pre-consolidated sheet. High strength co-extruded PP tapes have potential processing advantages over mono-extruded fibres or tapes as they allow for a larger temperature processing window for consolidation. This enlarged temperature processing window makes direct forming routes feasible, without the need for an intermediate pre-consolidated sheet product. Thermoforming studies show that direct forming is an interesting alternative to stamping of pre-consolidated sheets, as it eliminates an expensive belt-pressing step which is normally needed for the manufacturing of semi-finished sheets products. Moreover, results from forming studies shows that only half the energy was required to directly form a simple dome geometry from a stack of fabrics compared to stamping the same shape from a pre-consolidated sheet.
Composites Science and Technology
The characterisation of the damage state of composite structures is often performed using the aco... more The characterisation of the damage state of composite structures is often performed using the acoustic behaviour of the composite system. This behaviour is expected to change significantly as the damage is accumulating in the composite. It is indisputable that different damage mechanisms are activated within the composite laminate during loading scenario. These ''damage entities'' are acting in different space and time scales within the service life of the structure and may be interdependent. It has been argued that different damage mechanisms attribute distinct acoustic behaviour to the composite system. Loading of cross-ply laminates in particular leads to the accumulation of distinct damage mechanisms, such as matrix cracking, delamination between successive plies and fibre rupture at the final stage of loading. As highlighted in this work, the acoustic emission activity is directly linked to the structural health state of the laminate. At the same time, significant changes on the wave propagation characteristics are reported and correlated to damage accumulation in the composite laminate. In the case of cross ply laminates, experimental tests and numerical simulations indicate that, typical to the presence of transverse cracking and/or delamination, is the increase of the pulse velocity and the transmission efficiency of a propagated ultrasonic wave, an indication that the intact longitudinal plies act as wave guides, as the transverse ply deteriorates. Further to transverse cracking and delamination, the accumulation of longitudinal fibre breaks becomes dominant causing the catastrophic failure of the composite and is expected to be directly linked to the acoustic behaviour of the composite, as the stiffness loss results to the velocity decrease of the propagated wave. In view of the above, the scope of the current work is to assess the efficiency of acoustic emission and ultrasonic transmission as a combined methodology for the assessment of the introduced damage and furthermore as a structural health monitoring tool.
Journal of Elastomers and Plastics, 2003
The influence of the newly developed binary accelerator 1-phenyl-2,4-dithiobiuret (DTB) in combin... more The influence of the newly developed binary accelerator 1-phenyl-2,4-dithiobiuret (DTB) in combination with sulphenamides like DCBS, MBS, TBBS on the cure characteristics, mechanical properties, viscoelastic behaviour and network characteristics of styrene butadiene rubber (SBR) is studied. Kinetics of vulcanisation is followed at three different temperatures. Mechanical properties are found to be improved upon DTB addition. Attempts have been made to correlate the observed properties with extent of crosslinking of DTB. Effect of aging on tensile properties of the vulcanisates was also done. The extent of crosslinking was also improved with DTB cured vulcanisates. Dynamic Downloaded from mechanical analysis has been done and the influence of the new accelerator systems on the modulus, glass transition and entanglement densities of the vulcanisates was investigated. The effect of fillers like carbon black and silica on DTB on technological properties of SBR vulcanisates and the extent of reinforcement caused by the fillers were also studied.
European Polymer Journal, 2009
The aim of the study was to develop an innovative processing method of magnetorheological elastom... more The aim of the study was to develop an innovative processing method of magnetorheological elastomers (MRE). This method comprises optimization of the MRE structure in the context of their performance in the magnetic field. The influence of the amount of ferromagnetic particles and their arrangement in relation to the external magnetic field was investigated. Urethane magnetorheological elastomers were manufactured using polyurethane gels, supplied by Dow Chemical Company. As the ferromagnetic carbonyl-iron powder with particles size from 6-9µm produced by Fluka was used. The amount of the carbonyl iron particles was varied from 1.5 to 33.0 vol. %. Magnetic field strengths used during the fabrication of MRE were 0.1 and 0.3 T. The samples with particle chains aligned or slopped at 45 degree to the long sample axis were produced. To evaluate the external magnetic field effect on the magnetorheological properties a deflection in the magnetic field was measured. Samples were placed parallel to the magnetic field lines and deflected prior to the application of a magnetic field. After the application of the magnetic field the sample tended to straighten which was measured by displacement sensor. Magnetic field in a range of 0-0.9 T has been applied. Also the compression tests were carried out without and within external magnetic field with the strength of 0.3 T. The experiment showed that application of the magnetic field increases stiffness of the material. The amount of iron particles and their arrangement have influence on the stress-strain curves course.
Plastics Rubber and Composites, 2003
Blends of natural rubber and styrene/butadiene rubber have been cured using a new binary accelera... more Blends of natural rubber and styrene/butadiene rubber have been cured using a new binary accelerator system containing 1-phenyl-2,4-dithobiuret (DTB). This new accelerator was synthesised and used in combination with sulphenamides such as dicyclohexyl benzothiazyl sulphenamide, tertiary butyl benzothiazole sulphenamide and morpholinothio benzothiazole sulphenamide. A signi cant reduction in cure time was observed with the addition of DTB. Network characterisation of the vulcanisates was carried out by the estimation of crosslink density and relative proportions of crosslinks. The mechanical properties of the vulcanisates improved upon addition of DTB. Dynamic mechanical analysis was carried out to study the eVect of the new accelerator on entanglement density and modulus. Based on the processing behaviour, mechanical properties and chemical characterisation it was found that DTB could be eVectively used as a secondary accelerator in the curing of NR-SBR blends.
Journal of Reinforced Plastics and Composites, 2010
ABSTRACT The present study focuses on short flax fiber, as well as long flax fiber-reinforced pol... more ABSTRACT The present study focuses on short flax fiber, as well as long flax fiber-reinforced polypropylene (flax/PP) composites, manufactured by the injection molding method. Compounding of flax with two different grades of PP (with and without maleic anhydride (MA-PP) grafting) is carried out by four methods: kneading process, Henschel kinetic mixer, extrusion compounding, and production of long fiber thermoplastic (LFT) granules through pultrusion. The effect of the compounding method and injection molding on the fiber length and mechanical properties of the composites is being investigated. Furthermore, the effect of fiber—matrix adhesion on the mechanical response is being discussed. It can be concluded that the reduction in fiber length, associated with injection molding, did not affect the tensile properties significantly for the studied systems due to improvements in fiber orientation along the polymer flow direction and increased fiber efficiency through dimensional changes due to fiber opening. The addition of MA-PP led to improvements in the tensile strength of injection-molded composites. Kneader compounded composites showed maximum tensile strength as well as stiffness when compared with other compounding methods.
Polymer Composites, 2005
The concept of “overheating” is one of the known methods for manufacturing single polymer composi... more The concept of “overheating” is one of the known methods for manufacturing single polymer composites. This concept is validated on two categories of semi-crystalline polymers: the drawable, apolar (i.e., isotactic polypropylene [iPP], ultra-high molecular weight polyethylene [UHMWPE]) and the less drawable, polar ones (i.e., polyethyleneterephalate [PET] and polyamides [PA]). The interchain interactions in apolar polymers are relatively weak and therefore a high degree of drawability can be obtained. Polar polymers on the other hand have relative strong interchain interactions, they are therefore less drawable. A shift higher than 20°C of the melting temperature can be obtained in case of highly extended iPP (draw ratios >14). Ultra-drawn PE shows only 10°C overheating upon constraining and this is mainly due to the change in chain mobility for PE in the hexagonal phase. In case of PET and PA6, only draw ratios of 4 could be reached; however, temperature shifts of about 10°C for constrained fibers compared to unconstrained fibers could be measured. A proof of principle of the potential of the constraining concept for the manufacturing of single polymer composites is obtained by the preparation of single fiber model composites. The effect of the post-drawing conditions on overheating is examined in details on the example of iPP. It is concluded that both post-drawing temperature and ultimate draw ratio have a significant influence on the degree of overheating. POLYM. COMPOS., 26:114–120, 2005. © 2004 Society of Plastics Engineers
Arthroscopy-the Journal of Arthroscopic and Related Surgery, 2010
The purpose of our study was to clarify the events that take place during anterior cruciate ligam... more The purpose of our study was to clarify the events that take place during anterior cruciate ligament (ACL) failure, focusing on the behavior of the ACL as a composition of multiple fibers, during uniaxial tension along the ligament.Ten fresh-frozen human cadaveric knee specimens were fixed in an Instron machine (Instron, Norwood, MA), and load was applied parallel to the ACL axis. Two cameras were used to detect the failure mode of the ACL and its different groups of fibers. The distinct bundles of fibers were marked in each specimen before testing. The macroscopic findings during the experiment were used for comparison with the biomechanical results.The ACL showed a non-monotonic response during testing. The load-elongation curve showed a plateau or a second peak after the initial drop in load. Macroscopically, some fibers were failing initially, whereas the intact fibers had a remaining load potential. In our setting, 3 different failure patterns were recognized, specifically, a midsubstance tear of the anteromedial or the posterolateral bundle with a subsequent failure of the intact bundle or an initial avulsion of the anteromedial attachment. Analysis of the video frames showed a direct connection between the failure patterns in the load-elongation curves and the macroscopic sequence of events during ACL failure.The ACL ligament acts as a multifiber construction. In our setting, rupture follows 3 specific patterns where a complete or partial tear of the fiber bundles occurs first and the remaining intact fiber bundles have a potential load resistance.Our study allows a better understanding of the mechanical properties of the ACL. An update on the biomechanics of ACL failure during uniaxial tension after the “double-bundle revolution” could provide data helpful for ACL reconstruction.
Construction and Building Materials, 2009
The present paper describes the Acoustic Emission (AE) behavior of concrete under four-point bend... more The present paper describes the Acoustic Emission (AE) behavior of concrete under four-point bending.
Composites Part A-applied Science and Manufacturing, 2006
The creation of highly oriented, co-extruded polypropylene (PP) tapes allows the production of re... more The creation of highly oriented, co-extruded polypropylene (PP) tapes allows the production of recyclable ‘all-polypropylene’ composites, with a large temperature processing window (>30°C) and a high volume fraction of highly oriented PP (>90%). These composites show little deviation of mechanical properties with compaction temperature. This paper introduces all-polypropylene composites and reports the tensile and compressive properties of unidirectional composites. These
Industrial Crops and Products, 2010
The present paper focuses on short flax fibre reinforced composites based on polyhydroxybutyrate ... more The present paper focuses on short flax fibre reinforced composites based on polyhydroxybutyrate (PHB) and its copolymer with hydroxyvalerate (HV). The effect of the fibre and copolymer content on the mechanical properties of the composites is being discussed. Furthermore, the influence of manufacturing method (compression moulding of non-woven mats and injection moulding of short fibre compounds) and processing conditions (cooling temperature and annealing) on the mechanical properties of the composites is being investigated. Finally, the biodegradability of the aforementioned composites expressed as weight, stiffness and strength loss as a function of burrier time is presented on the example of injection moulded flax/PHB/HV composites. It can be concluded that the addition of flax fibres along with controlled processing conditions seems to be a convenient way of toughening of the PHB matrix. Composites manufactured through injection moulding exhibited lower impact strength than those manufactured through compression moulding. Based on the biodegradation study of PHB/HV composites it can be concluded that the tensile properties drop significantly in the initial stage of degradation. The drop in tensile properties is more gradual in the later stages of biodegradation.
Composites Part B-engineering, 2008
... FSPs are machined steel cylinders which simulate projectiles from fragmentation grenades or s... more ... FSPs are machined steel cylinders which simulate projectiles from fragmentation grenades or shrapnel from explosions [18]. Fig. ... A tape with a skin:core:skin structure is produced by co-extrusion of a ethylene-propylene copolymer skin with a polypropylene homopolymer core. ...
European Polymer Journal, 2003
The influence of hygrothermally-degraded polyester urethane (HD-PUR) waste on chloroprene rubber ... more The influence of hygrothermally-degraded polyester urethane (HD-PUR) waste on chloroprene rubber (CR) has been studied giving special attention to curing behaviour, mechanical properties and dynamic mechanical behaviour. The presence of primary and secondary amines in HD-PUR, did not increase the cure rate of CR. The mechanical properties of chloroprene vulcanizates were improved upon HD-PUR addition. The strain-induced crystallisation of CR did not show any deviation upon the addition of HD-PUR. Crosslink densities calculated from swelling studies, stress-strain behaviour, and modulus measurements are found to increase upon HD-PUR addition and showed similar trend. The glass transition temperature (T g ) did not show any significant change, with the addition of HD-PUR. Scanning electron microscopic studies have been done in order to have an insight into fracture behaviour of the samples and to analyse the microstructure of the blends.
Composites Science and Technology, 2006
Highly oriented polypropylene (PP) tapes, with high tensile strength and stiffness achieved by mo... more Highly oriented polypropylene (PP) tapes, with high tensile strength and stiffness achieved by molecular orientation during solid state drawing are consolidated to create high performance recyclable “all-polypropylene” (all-PP) composites. These composites possess a large temperature processing window (>30°C) and a high volume fraction of highly oriented PP (>90%). This large processing window is achieved by using co-extruded, highly drawn PP
Journal of Composite Materials, 2002
In the present work the residual tensile strength of unidirectional (UD) glass fibre (GF) reinfor... more In the present work the residual tensile strength of unidirectional (UD) glass fibre (GF) reinforced thermoplastic polypropylene (PP) composites after oblique (30 ) solid particle erosion was investigated as a function of the impact time and relative fibre orientation (parallel, Pa and perpendicular, Pe).
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Papers by Nektaria Barkoula