Editor’s Choice Articles

The use of steel fiber reinforced concrete (SFRC) in structures with high physical-mechanical characteristics allows engineers to reduce the weight and costs of the structures, to simplify the technology of their production, to reduce or completely eliminate the manual labor needed for reinforcement, at the same time increasing reliability and durability. Commonly accepted technology is exploiting randomly distributed in the concrete volume fibers with random each fiber orientation. In structural members subjected to bending, major loads are bearing fibers located close to outer member surfaces. The majority of fibers are slightly loaded. The aim of the present research is to create an SFRC construction with non-homogeneously distributed fibers. We prepared layered SFRC prismatic specimens. Each layer had different amount of short fibers. Specimens were tested by four point bending till the rupture. Material fracture process was modelled based on the single fiber pull-out test results. Modelling results were compared with the experimental curves for beams. Predictions generated by the model were validated by 4PBT of 100 × 100 × 400 mm prisms. Investigation had shown higher load-bearing capacity of layered concrete plates comparing with plate having homogeneously distributed the same amount of fibers. This mechanism is strongly dependent on fiber concentration. A high amount of fibers is leading to new failure mechanisms—pull-out of FRC blocks and decrease of load-bearing capacity. Fracture surface analysis was realized for broken prisms with the goal to analyze fracture process and to improve accuracy of the elaborated model. The general conclusion with regard to modelling results is that the agreement with experimental data is good, numeric modelling results successfully align with the experimental data. Modelling has indicated the existence of additional failure processes besides simple fiber pull-out, which could be expected when fiber concentration exceeds the critical value. Full article

Environmentally benign layer-by-layer (LbL) deposition was used to obtain flame-retardant and antimicrobial cotton. Cotton was coated with 8, 10, and 12 phytic acid (PA) and chitosan (CH)-urea bilayers (BL) and then immersed into copper (II) sulfate (CuSO₄) solution. Our findings were that 12 BL of PA/CH-urea + Cu²⁺ were able to stop flame on cotton during vertical flammability testing (VFT) with a limiting oxygen index (LOI) value of 26%. Microscale combustion calorimeter (MCC) data showed a reduction of peak heat release rates (pHRR) of more than 61%, while the reduction of total heat release (THR) was more than 54%, relative to untreated cotton. TG-IR analysis of 12 BL-treated cotton showed the release of water, methane, carbon dioxide, carbon monoxide, and aldehydes, while by adding Cu²⁺ ions, the treated cotton produces a lower amount of methane. Treated cotton also showed no levoglucosan. The intumescent behavior of the treatment was indicated by the bubbled structure of the post-burn char. Antibacterial testing showed a 100% reduction of Klebsiella pneumoniae and Staphylococcus aureus. In this study, cotton was successfully functionalized with a multifunctional ecologically benign flame-retardant and antibacterial nanocoating, by means of LbL deposition. Full article

Cellulose nanofibrils (CNF) were produced by high-pressure homogenization from kraft pulp in the presence of carboxymethyl cellulose (CMC) of varying molecular weights. CNF pretreated with 250 kD CMC exhibited the maximum specific surface area (SSA) of 641 m²/g, which is comparable to that of CNF pretreated by 2,2,6,6-tetramethyl-piperidinyl-1-oxyl (TEMPO)-meditated oxidation with a high degree of fibrillation. Rheological and microscopic analyses also indicated a high level of fibrillation for the CMC-pretreated CNF. In contrast, the reference CNF without the CMC pretreatment showed a lower level of fibrillation, which was reflected in decreased viscosity and the reduction of SSA by a factor of 19. With the high-degree fibrillation and low toxicity, the CMC pretreatment is a promising method for the production of high-quality CNF in an environmentally friendly way. Full article

This paper presents an optimized washing protocol for as-received natural fibers, prior to large-scale composite manufacturing, for the structural strengthening of historic masonry. The aim was to achieve a simple protocol for standard cleaning of fiber surfaces from low molecular weight constituents that may be detrimental towards interfacial strength without damaging the fibers. The proposed procedure employs the application of the solvent sequence: ethanol, acetone, hexane, with optimized incubation times and stirring conditions. Additionally, this procedure may change the surface of the fiber, thereby enhancing the durability of the fiber-matrix interface. The washing protocol resulted in an increase of tensile strength by 56%, 52% and 22% for flax, hemp and sisal fibers, respectively, as compared to the corresponding non-washed fibers, without loss of elongation. The static contact angle measurements confirmed exposure of a higher fraction of the hydrophilic crystalline cellulose, with a higher wettability observed after washing protocols. Full article

In the last decades, the uses of fiber reinforced polymer (FRP) composites in the structural strengthening of reinforced concrete (RC) structures have become the state of the art, providing a valid alternative to the traditional use of steel plates. These relatively new materials present, in fact, great advantages, including high corrosion resistance in aggressive environments, low specific weight, high strength-to-mass-density ratio, magnetic and electric neutrality, low axial coefficient of thermal expansion and sustainable costs of installation. In flexural and shear strengthening of RC members, the effectiveness of the epoxy bonded FRP strongly depends on the adhesion forces exchanged with the concrete substrate. When the flexural moment is present, the FRP strengthening is activated through the stress transfer on the tension side, which is guaranteed by the contact beam region to which the adhesive is bonded to the beam itself. Hence, the determination of the maximum forces that cause debonding of the FRP-plate becomes crucial for a proper design. Over the years, many different analytical models have been provided in the scientific literature. Most of them are based on the calibration of the narrow experimental database. Now, hundreds of experimental results are available. The main goal of the current study is to present and discuss an alternative theoretical formulation for predicting the debonding force in an FRP-plate, epoxy-bonded to the concrete substrate by using an artificial neural networks (ANNs) approach. For this purpose, an extensive study of the state of the art, reporting the results of single lap shear tests, is also reported and discussed. The robustness of the proposed analytical model was validated by performing a parametric analysis and a comparison with other existing models and international design codes, as shown herein. Full article

The effectiveness of externally applied fiber-reinforced polymer (FRP) ropes made of carbon fibers in X-shape formation and in both sides of the joint area of reinforced concrete (RC) beam–column connections is experimentally investigated. Six full-scale exterior RC beam–column joint specimens are tested under reverse cyclic deformation. Three of them have been strengthened using carbon FRP (CFRP) ropes that have been placed diagonally in the joint as additional, near surface-mounted reinforcements against shear. Full hysteretic curves, maximum applied load capacity, damage modes, stiffness and energy dissipation values per each loading step are presented and compared. Test results indicated that joint sub assemblages with X-shaped CFRP ropes exhibited improved hysteretic behavior and ameliorated performance with respect to the reference specimens. The effectiveness and the easy-to-apply character of the presented strengthening technique is also discussed. Full article

The research described in this paper is dedicated to the use of continuous fibers as reinforcement for additive manufacturing, particularly using Shotcrete. Composites and in particular fiber reinforced polymers (FRP) are increasingly present in concrete reinforcement. Their corrosion resistance, high tensile strength, low weight, and high flexibility offer an interesting alternative to conventional steel reinforcement, especially with respect to their use in Concrete 3D Printing. This paper presents an initial development of a dynamic robot-based manufacturing process for FRP concrete reinforcement as an innovative way to increase shape freedom and efficiency in concrete construction. The focus here is on prefabricated fiber reinforcement, which is concreted in a subsequent additive process to produce load-bearing components. After the presentation of the fabrication concept for the integration of FRP reinforcement and the state of the art, a requirements analysis regarding the mechanical bonding behavior in concrete is carried out. This is followed by a description of the development of a dynamic fiber winding process and its integration into an automated production system for individualized fiber reinforcement. Next, initial tests for the automated application of concrete by means of Shotcrete 3D Printing are carried out. In addition, an outlook describes further technical development steps and provides an outline of advanced manufacturing concepts for additive concrete manufacturing with integrated fiber reinforcement. Full article

Oily wastewaters are considered as one of the most dangerous types of environmental pollution. In the present study, the microfiltration (MF) process of model emulsions and real oily wastewaters was investigated. For this purpose, capillary polypropylene (PP) membranes were used. The experiments were conducted under transmembrane pressure (TMP) and feed flow rate (V_F) equal to 0.05 MPa and 0.5 m/s, respectively. It was found that the used membranes ensured a high-quality permeate with turbidity equal to about 0.4 NTU and oil concentration of 7–15 mg/L. As expected, a significant decrease in the MF process performance was noted. However, it is shown that the initial decline of permeate flux could be slightly increased by increasing the feed temperature from 25 °C to 50 °C. Furthermore, Hermia’s models were used to interpret the fouling phenomenon occurring in studied experiments. It was determined that cake formation was the dominant fouling mechanism during filtration of both synthetic and real feeds. Through detailed studies, we present different efficient methods of membrane cleaning. Results, so far, are very encouraging and may have an important impact on increasing the use of polypropylene MF membranes in oily wastewater treatments. Full article

Plant fibres and especially flax can be distinguished from most synthetic fibres by their intricate shape and intrinsic porosity called lumen, which is usually assumed to be tubular. However, the real shape appears more complex and thus might induce stress concentrations influencing the fibre performance. This study proposes a novel representation of flax fibre lumen and its variations along the fibre, an interpretation of its origin and effect on flax fibre tensile properties. This investigation was conducted at the crossroads of complementary characterization techniques: optical and scanning electron microscopy (SEM), high-resolution X-ray microtomography (µCT) and mechanical tests at the cell-wall and fibre scale by atomic force microscopy (AFM) in Peak-Force Quantitative Nano-Mechanical property mapping (PF-QNM) mode and micromechanical tensile testing. Converging results highlight the difficulty of drawing a single geometric reference for the lumen. AFM and optical microscopy depict central cavities of different sizes and shapes. Porosity contents, varying from 0.4 to 7.2%, are estimated by high-resolution µCT. Furthermore, variations of lumen size are reported along the fibres. This intricate lumen shape might originate from the cell wall thickening and cell death but particular attention should also be paid to the effects of post mortem processes such as drying, retting and mechanical extraction of the fibre as well as sample preparation. Finally, SEM observation following tensile testing demonstrates the combined effect of geometrical inhomogeneities such as defects and intricate lumen porosity to drive the failure of the fibre. Full article

We show numerically that ultra-short pulses can be generated in the mid-infrared when a gas filled hollow-core fiber is pumped by a fundamental pulse and its second harmonic. The generation process originates from a cascaded nonlinear phenomenon starting from a spectral broadening of the two pulses followed by an induced phase-matched four wave-mixing lying in the mid-infrared combined with a dispersive wave. By selecting this mid-infrared band with a spectral filter, we demonstrate the generation of ultra-short 60 fs pulses at a 3–4 µm band and a pulse duration of 20 fs can be reached with an additional phase compensator. Full article

The present paper proposes a useful closed-form solution for a wide class of mechanical problems, among which one of the most relevant and debated is the deboning process of Fiber-Reinforced Polymer (FRP) strips glued to generic materials and possibly intended as a mode-II fracture process. Specifically, after outlining well-known equations, a novel piecewise analytical formulation based on a cascading solution process is proposed with the aim of keeping the mathematical expressions of the relevant mechanical quantities as simple as possible. Although other analytical solutions and numerical procedures are already available in the literature, the present one is capable of handling the softening or snap-back response deriving from the full-range simulation of the depending process with no need for complex numerical techniques. This is obtained by considering the slip at the free end of the strip as the main displacement control parameter. After some comparisons between the proposed closed-form solution and experimental results available in the literature, some mechanical considerations are highlighted by elaborating on the results of a parametric study considering the variation of the main geometric and mechanical quantities. The numerical code implemented as part of the present study is available to readers in Open Access. Full article

In this study, an artificial neural network is designed and trained to predict the elastic properties of short fiber reinforced plastics. The results of finite element simulations of three-dimensional representative volume elements are used as a data basis for the neural network. The fiber volume fraction, fiber length, matrix-phase properties, and fiber orientation are varied so that the neural network can be used within a very wide range of parameters. A comparison of the predictions of the neural network with additional finite element simulations shows that the stiffnesses of short fiber reinforced plastics can be predicted very well by the neural network. The average prediction accuracy is equal or better than by a two-step homogenization using the classical method of Mori and Tanaka. Moreover, it is shown that the training of the neural network on an extended data set works well and that particularly calculation-intensive data points can be avoided without loss of prediction quality. Full article

The study of natural fiber-based composites through the use of computational techniques for modelling and optimizing their properties has emerged as a fast-growing approach in recent years. Ecological concerns associated with synthetic fibers have made the utilisation of natural fibers as a reinforcing material in composites a popular approach. Computational techniques have become an important tool in the hands of many researchers to model and analyze the characteristics that influence the mechanical properties of natural fiber composites. This recent trend has led to the development of many advanced computational techniques and software for a profound understanding of the characteristics and performance behavior of composite materials reinforced with natural fibers. The large variations in the characteristics of natural fiber-based composites present a great challenge, which has led to the development of many computational techniques for composite materials analysis. This review seeks to infer, from conventional to contemporary sources, the computational techniques used in modelling, analyzing, and optimizing the mechanical characteristics of natural fiber reinforced composite materials. Full article

This study investigated the effect of acid hydrolysis of cellulose on its dissolution under mild conditions in ionic liquid, 1-butyl-3-methylimidazolium acetate/N,N-dimethylacetamide (BMIMAc/DMAc). Acid hydrolysis of high molecular weight (MW) cotton cellulose (DP > 4000) was carried out to produce hydrolyzed cotton (HC) samples for dissolution. The HC samples were characterized using gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA), and the dissolution process was monitored using polarized light microscopy (PLM). It was found that the drastic decrease of the MW of cellulose did not result in improvement of its dissolution at room temperature. As compared to original cotton cellulose, the high amount of undissolved fibers in HC solutions led to unstable rheological behavior of HC solutions. Agglomeration and inhomogeneous dispersion of HC, and increased crystallinity, in this case, likely made the diffusion of BMIMAc/DMAc more difficult to the inside of the polymeric network of cellulose at ambient temperature, thereby hindering the dissolution. However, increasing the temperature from room temperature to 35 °C and 55 °C, led to a significant improvement in cellulose dissolution. This phenomenon implies that reducing the MW of cellulose might not be able to improve its dissolution under certain conditions. During the dissolution process, the physical properties of cellulose including fiber aggregation status, solvent diffusivity, and cellulose crystallinity may play a critical role compared to the MW, while the MW may not be an important factor. This finding may help further understand the mechanism of cellulose dissolution and seek better strategies to dissolve cellulose under mild conditions for industrial applications. Full article

Fabric structures are prone to contamination with microorganisms, as their morphology and ability to retain moisture creates a proper environment for their growth. In this work, a novel, easily processed and cheap coating for a nylon fabric with antimicrobial characteristics was developed. After plasma treatment, made to render the fabric surface more reactive sites, the fabric was impregnated with chitosan and silver nanoparticles by simply dipping it into a mixture of different concentrations of both components. Silver nanoparticles were previously synthesized using the Lee–Meisel method, and their successful obtention was proven by UV–Vis, showing the presence of the surface plasmon resonance band at 410 nm. Nanoparticles with 25 nm average diameter observed by STEM were stable, mainly in the presence of chitosan, which acted as a surfactant for silver nanoparticles, avoiding their aggregation. The impregnated fabric possessed bactericidal activity higher for Gram-positive Staphylococcus aureus than for Gram-negative Pseudomonas aeruginosa bacteria for all combinations. The percentage of live S. aureus and P. aeruginosa CFU was reduced to less than 20% and 60%, respectively, when exposed to each of the coating combinations. The effect was more pronounced when both chitosan and silver were present in the coating, suggesting an effective synergy between these components. After a washing process, the antimicrobial effect was highly reduced, suggesting that the coating is unstable after washing, being almost completely removed from the fabric. Nevertheless, the new-coated fabric can be successfully used in single-use face masks. To our knowledge, the coating of nylon fabrics intended for face-mask material with both agents has never been reported. Full article

Ultra-high performance (UHP) concrete is a special type of fibrous cementitious composite that is characterized by high strength and superior ductility, toughness, and durability. This research aimed to investigate the resistance of ultra-high performance fiber-reinforced concrete (UHPFRC) against repeated impacts. An adjusted repeated drop mass impact test was adopted to evaluate the impact performance of 72 UHPFRC disc specimens. The specimens were divided into six mixtures each of 12 discs. The only difference between the mixtures was the types of fibers used, while all other mixture components were the same. Three types of fibers were used: 6 mm micro-steel, 15 mm micro-steel, and polypropylene. All mixtures included 2.5% volumetric content of fibers, however with different combinations of the three fiber types. The test results showed that the mixtures with the 15 mm micro-steel fiber absorbed a higher number of impact blows until cracking compared to other mixtures. The mixture with pure 2.5% of 15 mm micro-steel fiber exhibited the highest impact resistance, with percentage increases over the other mixtures ranging from 25 to 140%. In addition, the Weibull distribution was used to investigate the cracking impact resistance of UHP at different levels of reliability. Full article

Expanded clay concrete (ECC) is a promising structural material for buildings due to its light weight and heat- and sound-insulating properties. Adding basalt fibers (BFs) in ECC reduces its brittleness and enhances its mechanical properties. The heat treatment (HT) of BF-reinforced ECC can significantly accelerate the strength growth during cast-in-situ construction, which allows the reduction of the turnover of the formwork and the construction period, as well as leading to lower construction costs. This paper presents an HT technology for load-bearing structures, containing a BF-reinforced ECC mix and using infrared rays for cast-in-situ construction. The issue of the strength growth of BF-reinforced ECC during HT has been studied. Microsilica and fly ash were added to the ECC mix to obtain a compressive strength of more than 20 MPa. Four different mixes of ECC with chopped BFs in the ratios of 1:0, 1:0.0045, 1:0.009 and 1:0.012 by weight of cement were studied. Test specimens were heated by infrared rays for 7, 9, 11, 13, 16 and 24 h. Then, the heat-treated specimens were tested for compressive strength after 0.5, 4, 12 and 24 h cooling periods. The analysis and evaluation of the experimental data were carried out based on probability theory and mathematical statistics. Mathematical models are proposed for forecasting the strength growth of BF-reinforced ECC during cast-in-situ construction. Full article

Processing parameters in electrospinning allow us to control the properties of fibers on a molecular level and are able to tailor them for specific applications. In this study, we investigate how relative humidity (RH) affects the mechanical properties of electrospun polyvinylidene fluoride (PVDF). The mechanical properties of single fibers were carried out using a specialized tensile stage. The results from tensile tests were additionally correlated with high-resolution imaging showing the behavior of individual fibers under tensile stress. The mechanical characteristic is strongly dependent on the crystallinity, chain orientation, and fiber diameter of electrospun PVDF fibers. Our results show the importance of controlling RH during electrospinning as the mechanical properties are significantly affected. At low RH = 30% PVDF fibers are 400% stiffer than their counterparts prepared at high RH = 60%. Moreover, the vast differences in the strain at failure were observed, namely 310% compared to 75% for 60% and 30% RH, respectively. Our results prove that humidity is a crucial parameter in electrospinning able to control the mechanical properties of polymer fibers. Full article

The ever-increasing demand for environmentally friendly biocomposites for use in various engineering applications requires a strong understanding of these materials properties, especially in automotive applications. This study focused on investigating how the stacking sequence and fibre orientation impacts the damping properties of hybrid flax/carbon fibre-reinforced composites. Different hybrid carbon fibre/flax fibre-reinforced composites using epoxy resin as the matrix were manufactured using vacuum-assisted resin infusion moulding technique. Each composite material was then tested for tensile properties using a universal testing machine, and the damping experiment was conducted using an impulse hammer and a Laser Doppler Vibrometer. The tensile study found out that adding a flax layer to the external layers of carbon fibre laminate reduced Young’s modulus by 28% for one layer and 45% for two layers. It was noted that when the fibre orientation of the internal layer of [C/F2/C]s was replaced with two ±45° layers, this had a very little effect on Young’s modulus but reduced the ultimate tensile strength by 61%. This experimental study also showed that the most important layer when it comes to damping properties is the external layers. By adding an external flax layer into an epoxy/carbon fibre-reinforced composite considerably enhanced its damping ratio by 53.6% and by adding two layers increased it by 94%. The results indicated a high potential for the automotive semi-structural applications to improve damping properties of the vehicle. Full article

This study presents a non-linear cracked-hinge model for the post-cracking response of fiber-reinforced cementitious composites loaded in bending. The proposed displacement-based model follows a meso-mechanical approach, which makes it possible to consider explicitly the random distribution and orientation of the reinforcing fibers. Moreover, the model allows for considering two different fiber typologies whereas the cement matrix is modelled as a homogeneous material. The proposed mechanical model combines a fracture-based, stress-crack opening relationship for the cementitious matrix with generalized laws aimed to capture the crack-bridging effect played by the reinforcing fibers. These laws are derived by considering both the fiber-to-matrix bond mechanism and fiber anchoring action possibly due to hooked ends. The paper includes a numerical implementation of the proposed theory, which is validated against experimental results dealing with fiber-reinforced cement composites reinforced with different short fibers. The excellent theory vs. experiment matching demonstrates the high technical potential of the presented model, obtained at a reasonable computational cost. Full article

The use of fiber-reinforced polymer (FRP) composite materials are continuously growing in civil infrastructure due to their high strength, low weight, and manufacturing flexibility. However, FRP is characterized by sudden failure and lacks ductility. When used in construction, gradual failure of FRP components is desired to avoid catastrophic structural collapse. Due to its mechanical orthotropy, the behavior of FRP relies significantly on fiber orientation and stacking sequence. In this paper, a novel multi-angled glass fiber reinforced polymer (GFRP) composite laminate showing pseudo ductile behavior is produced using 3D-printing. This is accomplished by varying fiber orientation angles, stacking sequence, and thickness of lamina. Single-angled GFRP composite specimens were 3D-printed with different fiber orientation angles of 0°, 12°, 24°, 30°, 45°, and 90° using continuous and fused filament techniques. The tension test results of the single-angled specimens were then used to aid the design of multi-angled laminate for potential progressive failure behavior. A 3D finite element (FE) model was developed to predict the response of the experimental results and to provide insight into the failure mechanism of the multi-angled laminate. The experimental observations and the FE simulations show the possibility of producing pseudo ductile FRP-by-design composite using 3D-printing technology, which leads the way to fabricate next-generation composites for civil infrastructure. Full article

A large part of the European building Heritage is dated back over centuries. Consequently, its structural and thermal performances are often inadequate. Commonly, different interventions are proposed for solving these issues separately. However, reasonable drawbacks arise when the structural retrofitting requires a direct contact with the target-member while the insulation layer is potentially interposed in between. In this scenario, the present research proposes a novel and unique system able to guarantee both the energetic and the structural retrofitting. Inorganic Matrix Composites (IMCs) are a promising solution in this sense. Among them, the Fabric Reinforced Cementitous Matrix (FRCM) is one of the most used; or rather a composite made of a fabric (open grid or mesh) within an inorganic matrix (lime or cement based). Even if the inorganic matrix has a relevant thickness (if compared with the one of the fabric), its thermal resistance is insufficient. The novelty of this work consists in assessing a new geo-polymeric FRCM-system by combining fly-ash binder (reused material) and expanded glass aggregate (recycled material). Direct tensile tests, for measuring the tensile strength, ultimate strain and elastic modulus, were performed in addition to thermal conductivity tests. The results were compared with those of traditional FRCM (commercially available). The potentiality of the proposal for structural and energy retrofitting is discussed and examples of its possible application are also reported. Full article

Replacing greenhouse gas-intensive building materials with wood products from sustainable forestry contributes to the implementation of current climate conventions such as the Paris Agreement. Hardwood products, such as laminated veneer lumber made of beech (e.g., BauBuche), are an alternative to conventional building materials. For the application of wood products in the construction sector, a precise knowledge of the mechanical and physical properties is essential. Therefore, the aim of the present study was to investigate the sorption behavior and associated dimensional changes of the product BauBuche. This was done by applying a manual testing procedure (climatic chamber, balance and caliper) as well as a dynamic vapor sorption analyzer equipped with a camera. During initial moistening after production, due to the irreversible spring back (approximately 2 mm at 50 mm; i.e., 4%), Baubuche shows an extremely strong swelling in the radial direction. Once the maximum spring back is reached, Baubuche shows sorption behavior in the radial and tangential direction, which is comparable to that of solid beech wood in the radial direction. Consequently, the dimensional changes caused by moisture changes must be taken into account in the dimensioning of Baubuche components in order to avoid damage to building structures. Full article

In this paper, experimental investigations for strengthening reinforced concrete (RC) continuous beams were performed. Eighteen T-beams were cast, twelve of which were inverted T-beams where the flange portion of the T-beam was subjected to positive flexure to represent the support region of a continuous beam. Six of the T-beams were non-inverted where the web is subjected to positive flexure. Carbon fiber reinforced polymer (CFRP) sheets with different widths were considered, and different strengthening configurations with the same area of CFRP were investigated. The use of one-layer, multiple layers, or multiple strips of CFRP were evaluated to investigate the effect of these configurations on the ultimate capacity and ductility of the strengthened beams. From the experimental observation of the non-inverted beams, it was found that the ultimate load capacities of the CFRP-strengthened beams were enhanced by 4% to 90% compared to the control beam. Using multiple layers of CFRP sheets enhanced the stiffness of the beams by 4% to 46%, depending on the CFRP area and configurations. The debonding of CFRP before the ultimate failure provided additional ductility to the tested beams. For the strengthening of the inverted beams, it was found that the addition of CFRP strips did not increase the strength of the beams when the width of CFRP to beam width ratio was less than 0.25, but the ductility of the beam was enhanced slightly. The use of multiple strips was found to be a more effective way for the strengthening of the negative moment region than using multiple layers. This can also provide more desirable modes of failure than when applying CFRP in multiple layers. Ductility was found to be lower if multiple layers were used compared to other configurations. Moreover, it was observed that as the compressive strength of concrete increased the addition of the CFRP improved the beams ductility. Full article

The cyclic performance of non-seismically designed reinforced concrete (RC) columns, strengthened with carbon fiber reinforced polymer (CFRP) jackets, was analytically and experimentally investigated herein. Three cantilever column specimens were constructed, incorporating design parameters of the period 1950s–1970s, namely with concrete of a low compressive strength, plain steel bars, widely-spaced ties and inadequate lap splices of reinforcement. The specimens were strengthened using CFRP jackets and were subsequently subjected to cyclic inelastic lateral displacements. The main parameters examined were the length of the lap splices, the acceptable relative bar slipping value and the width of the jackets. The hysteresis behaviors of the enhanced columns were compared, while also being evaluated with respect to those of two original columns and to the seismic performance of a control specimen with continuous reinforcement, tested in a previous work. An analytical formulation was proposed for accurately predicting the seismic responses of the column specimens, comparing the actual shear stress value with the ultimate shear capacity of the concrete in the lap splice region. The test results verified the predictions of the analytical model, regarding the seismic performance of the strengthened columns. Moreover, the influences of the examined parameters in securing the ductile hysteresis performance were evaluated. Full article

The present work is focused on improving mode I and mode II delamination resistance of glass/epoxy composite laminates (50 wt.% of glass fibers) with milled glass fibers, added in various amounts (2.5, 5, 7.5 and 10% of the epoxy weight). Including fillers in the interlayer enhances the delamination resistance by providing a bridging effect, therefore demanding additional energy to initiate the crack in the interlaminar domain, which results in turn in enhanced fracture toughness. The maximal increase of mode I and mode II fracture toughness and of flexural strength was obtained by the addition of 5% milled glass fiber. The mechanism observed suggests that crack propagation is stabilized even leading to its arrest/deflection, as a considerable amount of milled glass fiber filler was oriented transverse to the crack path. In contrast, at higher filler loading, tendency towards stress concentration grows due to local agglomeration and improper dispersion of excess fillers in inter/intralaminar resin channel, causing poor adhesion to the matrix, which leads to reduction in fracture toughness, strength and strain to failure. Fractured surfaces analyzed using scanning electron microscopy (SEM) revealed a number of mechanisms, such as crack deflection, individual debonding and filler/matrix interlocking, all contributing in various ways to improve fracture toughness. Full article

This paper discusses approaches to the numerical integration of the coupled nonlinear Schrödinger equations system, different from the generally accepted approach based on the method of splitting according to physical processes. A combined explicit/implicit finite-difference integration scheme based on the implicit Crank–Nicolson finite-difference scheme is proposed and substantiated. It allows the integration of a nonlinear system of equations with a choice of nonlinear terms from the previous integration step. The main advantages of the proposed method are: its absolute stability through the use of an implicit finite-difference integration scheme and an integrated mechanism for refining the numerical solution at each step; integration with automatic step selection; performance gains (or resolutions) up to three or more orders of magnitude due to the fact that there is no need to produce direct and inverse Fourier transforms at each integration step, as is required in the method of splitting according to physical processes. An additional advantage of the proposed method is the ability to calculate the interaction with an arbitrary number of propagation modes in the fiber. Full article

In this work the flexural rigidity of individual large diameter multi-walled carbon nanotubes (MWCNTs) was investigated. The bending modulus were obtained by detecting the resonance frequencies of mechanically excited cantilevered carbon nanotubes using the so-called dynamic scanning electron microscopy technique, and applying the Euler–Bernoulli beam theory. For the nanotubes studied, we determined a modulus of up to 160 GPa. This agrees with values reported by other authors for MWCNTs produced by catalytic chemical vapor deposition, however, it is 6-8 times smaller than values reported for single and multi-walled carbon nanotubes produced by arc-discharge synthesis. Toxicological studies with carbon nanotubes have been showing that inhaled airborne nanofibers that reach the deep airways of the respiratory system may lead to serious, asbestos-like lung diseases. These studies suggested that their toxicity critically depends on the fiber flexural rigidity, with high rigidity causing cell lesions. To complement the correlation between observed toxicological effects and fiber rigidities, reliable and routinely applicable measurement techniques for the flexural rigidity of nanofibers are required. Full article

In recent years, huge progress has been made in the development of rare-earth ion doped tellurite glass laser sources, ranging from watt- and multiwatt-level fiber lasers to nanowatt level microsphere lasers. Significant success has been achieved in extending the spectral range of tellurite fiber lasers generating at wavelengths beyond 2 μm as well as in theoretical understanding. This review is aimed at discussing the state of the art of neodymium-, erbium-, thulium-, and holmium-doped tellurite glass fiber and microsphere lasers. Full article

The fast and precise fabrication of three-dimensional (3-D) structures made of nanofibers is an important development trend in the electrospinning technique. This paper describes a new and facile method of electrospinning to fabricate nanofibrous 3-D structures. The nanofibrous 3-D structures can be engineered to have the desired layer thicknesses, where the fiber spacing, density (i.e., fiber volume/unit volume), as well as shape of the structure may be controlled. While innumerable structural variations are possible with this method, this paper discusses, as proof-of-concept, a few cases that illustrate how 3-D nanofiber webs can be made for filtration application. Computerized automation of the method will make it possible to build almost any 3-D web structure suitable for a myriad of applications including ultra-light-weight insulation and scaffolds for hydrogel preparation and tissue. Full article

We present new type of silica graded index laser-optimized multimode optical fibers (LOMF) with extremely enlarged core diameter up to 100 μm and “typical” “telecommunication” cladding diameter 125 μm. This optical fiber was designed for harsh environment Gigabit onboard cable systems and industrial networks. It differs by special optimized graded refractive index profile, providing low differential mode delay (DMD) for selected guided modes. We present some results of tests, performed for manufactured pilot 520 m length of described LOMF 100/125, concerned with its geometry properties as well as key transmission parameters—attenuation and DMD map. Full article

Multiscale characterization of the textile preform made of natural fibers is an indispensable way to understand and assess the mechanical properties and behavior of composite. In this study, a multiscale experimental characterization is performed on three-dimensional (3D) warp interlock woven fabrics made of flax fiber on the fiber (micro), roving (meso), and fabric (macro) scales. The mechanical tensile properties of the flax fiber were determined by using the impregnated fiber bundle test. The effect of the twist was considered in the back-calculation of the fiber stiffness to reveal the calculation limits of the rule of mixture. Tensile tests on dry rovings were carried out while considering different twist levels to determine the optimal amount of twist required to weave the flax roving into a 3D warp interlock. Finally, at fabric-scale, six different 3D warp interlock architectures were woven to understand the role of the architecture of binding rovings on the mechanical properties of the dry 3D fabric. The results reveal the importance of considering the properties of the fiber and roving at these scales to determine the more adequate raw material for weaving. Further, the characterization of the 3D woven structures shows the preponderant role of the binding roving on their structural and mechanical properties. Full article

The application of externally bonded fiber-reinforced polymer (EB-FRP) as shear transverse reinforcement applied in vulnerable reinforced concrete (RC) beams has been proved to be a promising strengthening technique. However, past studies revealed that the effectiveness of this method depends on how well the reinforcement is bonded to the concrete surface. Thus, although the application of EB-FRP wrapping around the perimeter of rectangular cross-sections leads to outstanding results, U-jacketing in shear-critical T-beams seems to undergo premature debonding failures resulting in significant reductions of the predictable strength. In this work, five shear-critical RC beams with T-shaped cross-section were constructed, strengthened and tested in four-point bending. Epoxy bonded carbon FRP (C-FRP) sheets were applied on the three sides and along the entire length of the shear-strengthened T-beams as external transverse reinforcement. Furthermore, the potential enhancement of the C-FRP sheets anchorage using bolted steel laminates has been examined. Test results indicated that although the C-FRP strengthened beams exhibited increased shear capacity, the brittle failure mode was not prevented due to the debonding of the FRP from the concrete surface. Nevertheless, the applied mechanical anchor of the C-FRP sheets delayed the debonding. Moreover, the design provisions of three different code standards (Greek Code of Interventions, Eurocode 8 and ACI Committee 440) concerning the shear capacity of T-shaped RC beams retrofitted with EB-FRP jackets or strips in U-jacketing configuration are investigated. The ability of these code standards to predict safe design estimations is checked against 165 test data from the current experimental project and data available in the literature. Full article

A hollow core antiresonant photonic crystal fiber (HC-ARPCF) with metal inclusions is numerically analyzed for transmission of terahertz (THz) waves. The propagation of fundamental and higher order modes are investigated and the results are compared with conventional dielectric antiresonant (AR) fiber designs. Simulation results show that broadband terahertz radiation can be guided with six times lower loss in such hollow core fibers with metallic inclusions, compared to tube lattice fiber, covering a single mode bandwidth (BW) of 700 GHz. Full article

This paper deals with the experimental and numerical study of the bond behavior of two steel reinforced grout (SRG)-strengthened masonry systems. Ten shear bond tests were carried out on prismatic masonry specimens. The data of experimental tests are recorded and results are given in terms of load/stress-global slip curves, failure modes, tables, graphs and photographic reports, comparing the results of the two strengthening systems. Two kinds of steel fibers available in marketplace were used: ultra-high tensile strength steel galvanized micro-cords and stainless-steel strands. The main target is to obtain information on the behavior of the bond between masonry surface and the two types of SRG composites, which are characterized by two substantial differences: tensile strength with a ratio of 2.4 and the corresponding surface mass density with a ratio of 0.30. Finally, the influence of the matrices coupled with the two systems is critically analyzed. The characterization of the bond behavior is necessary in order to confirm the performance of the SRG systems that have become increasingly used and attractive. It also aims to make a contribution to the existing knowledge especially in relation to the use of low resistance steel fibers (stainless steel) which are still few studied today. Furthermore, using a suitable interface law proposed in the literature, a numerical model is defined and employed to simulate the behavior of the specimens tested in the laboratory. The comparisons show a good agreement between numerical and experimental results in terms of the maximum load, load versus global-slip curves, and crack patterns. Full article

Industrial hemp (Cannabis sativa) is one of the most available and widely produced bast fibers with high cellulose content. Interest in these fibers is warranted due to environmental protection challenges as well as their inherent properties such as low density, high specific strength, and stiffness. In addition, advanced research and progress have gone into increasing their mechanical performance through surface treatments and in the development of new materials. The most promising application for hemp fibers is as reinforcement in polymeric composites or through hybridization. Nonetheless, more research is needed to improve their properties and expand their range of applications. The biodegradability issue is one problem that must be addressed when considering long life-cycle applications as the reproducibility of these composites’ final properties. This review is a comprehensive literature review on hemp fibers. It includes hemp fibers’ chemical and mechanical properties, surface modifications, hybrid composites, as well as current and future applications. Full article

For years, scientists have been looking for different techniques to make glasses perfect: fully amorphous and ideally homogeneous. Meanwhile, recent advances in the development of particle-containing glasses (PCG), defined in this paper as glass-ceramics, glasses doped with metallic nanoparticles, and phase-separated glasses show that these “imperfect” glasses can result in better optical materials if particles of desired chemistry, size, and shape are present in the glass. It has been shown that PCGs can be used for the fabrication of nanostructured fibers—a novel class of media for fiber optics. These unique optical fibers are able to outperform their traditional glass counterparts in terms of available emission spectral range, quantum efficiency, non-linear properties, fabricated sensors sensitivity, and other parameters. Being rather special, nanostructured fibers require new, unconventional solutions on the materials used, fabrication, and characterization techniques, limiting the use of these novel materials. This work overviews practical aspects and progress in the fabrication and characterization methods of the particle-containing glasses with particular attention to nanostructured fibers made of these materials. A review of the recent achievements shows that current technologies allow producing high-optical quality PCG-fibers of different types, and the unique optical properties of these nanostructured fibers make them prospective for applications in lasers, optical communications, medicine, lighting, and other areas of science and industry. Full article

For shear-critical structural elements where the use of stirrups is not desirable, such as slabs or beams with reinforcement congestion, steel fibers can be used as shear reinforcement. The contribution of the steel fibers to the shear capacity lies in the action of the steel fibers bridging the shear crack, which increases the shear capacity and prevents a brittle failure mode. This study evaluates the effect of the amount of fibers in a concrete mix on the shear capacity of steel fiber-reinforced concrete beams with mild steel tension reinforcement and without stirrups. For this purpose, 10 beams were tested. Five different fiber volume fractions were studied: 0.0%, 0.3%, 0.6%, 0.9%, and 1.2%. For each different steel fiber concrete mix, the concrete compressive strength was determined on cylinders and the tensile strength was determined in a flexural test on beam specimens. Additionally, the influence of fibers on the shear capacity was analyzed based on results reported in the literature, as well as based on the expressions derived for estimating the shear capacity of steel fiber-reinforced concrete beams. The outcome of these experiments is that a fiber percentage of 1.2% or fiber factor of 0.96 can be used to replace minimum stirrups according to ACI 318-14 and a 0.6% fiber volume fraction or fiber factor of 0.48 to replace minimum stirrups according to Eurocode 2. A fiber percentage of 1.2% or fiber factor of 0.96 was observed to change the failure mode from shear failure to flexural failure. The results of this study support the inclusion of provisions for steel fiber-reinforced concrete in building codes and provides recommendations for inclusion in ACI 318-14 and Eurocode 2, so that a wider adoption of steel fiber reinforced concrete can be achieved in the construction industry. Full article

The aim of the present paper is to assess the bearing performance evolution of pinned, glass-composite laminates due to environmental aging in salt-spray fog tests. Glass fibers/epoxy pinned laminates were exposed for up to 60 days in salt-spraying, foggy environmental conditions (according to ASTM B117 standard). In order to evaluate the relationship between mechanical failure mode and joint stability over increasing aging time, different single lap joints, measured by the changing hole diameter (D), laminate width (W) and hole free edge distance (E), were characterized at varying aging steps. Based on this approach, the property-structure relationship of glass-fibers/epoxy laminates was assessed under these critical environmental conditions. Furthermore, an experimental 2D failure map, clustering main failure modes in the plane E/D versus W/D ratios, was generated, and its cluster variation was analyzed at each degree of aging. Full article

Comparing experimental results of the shear capacity of steel fiber-reinforced concrete (SFRC) beams without stirrups to the capacity predicted using current design equations and other available formulations shows that predicting the shear capacity of SFRC beams without mild steel shear reinforcement is still difficult. The reason for this difficulty is the complex mechanics of the problem, where the steel fibers affect the different shear-carrying mechanisms. Since this problem is still not fully understood, we propose the use of artificial intelligence (AI) to derive an expression based on the available experimental data. We used a database of 430 datapoints obtained from the literature. The outcome is an artificial neural network-based expression to predict the shear capacity of SFRC beams without shear reinforcement. For this purpose, many thousands of artificial neural network (ANN) models were generated, based on 475 distinct combinations of 15 typical ANN features. The proposed “optimal” model results in maximum and mean relative errors of 0.0% for the 430 datapoints. The proposed model results in a better prediction (mean V_test/V_ANN = 1.00 with a coefficient of variation 1 × 10⁻¹⁵) as compared to the existing code expressions and other available empirical expressions, with the model by Kwak et al. giving a mean value of V_test/V_pred = 1.01 and a coefficient of variation of 27%. Until mechanics-based models are available for predicting the shear capacity of SFRC beams without shear reinforcement, the proposed model thus offers an attractive solution for estimating the shear capacity of SFRC beams without shear reinforcement. With this approach, designers who may be reluctant to use SFRC because of the large uncertainties and poor predictions of experiments, may feel more confident using the material for structural design. Full article

We discuss the fabrication of hollow-core optical fibers made of borosilicate glass. We show that, despite the high attenuation of the glass relative to silica, the fiber optical losses can be of the same order of magnitude of those obtained by using ultrapure silica glass. Short lengths of the fabricated fibers, used in combination with incoherent optical sources, provide single-mode optical guidance in both near and mid-infrared spectral ranges without any additional optical components. Full article

Electrospinning and polymer blending have been the focus of research and the industry for their versatility, scalability, and potential applications across many different fields. In tissue engineering, nanofiber scaffolds composed of natural fibers, synthetic fibers, or a mixture of both have been reported. This review reports recent advances in polymer blended scaffolds for tissue engineering and the fabrication of functional scaffolds by electrospinning. A brief theory of electrospinning and the general setup as well as modifications used are presented. Polymer blends, including blends with natural polymers, synthetic polymers, mixture of natural and synthetic polymers, and nanofiller systems, are discussed in detail and reviewed. Full article

Natural fibers, such as bamboo, flax, hemp, and coir, are usually different in terms of microstructure and chemical composition. The mechanical properties of natural fibers strongly depend on the organization of cell walls and the cellulose micro-fibril angle in the dominant cell wall layers. Bamboo, flax, and hemp are known for high strength and stiffness, while coir has high elongation to failure. Based on the unique properties of the fibers, fiber hybridization is expected to combine the advantages of different natural fibers for composite applications. In this paper, a study on bamboo/coir fiber hybrid composites was carried out to investigate the hybrid effect of tough coir fibers and brittle bamboo fibers in the composites. The tensile behavior of unidirectional composites of bamboo fibers, coir fibers, and hybrid bamboo/coir fibers with a thermoplastic matrix was studied. The correlation between the tensile properties of the fibers and of the hybrid composites was analyzed to understand the hybrid effects. In addition, the failure mode and fracture morphology of the hybrid composites were examined. The results suggested that, with a low bamboo fiber fraction, a positive hybrid effect with an increase of composite strain to failure was obtained, which can be attributed to the high strain to failure of the coir fibers; the bamboo fibers provided high stiffness and strength to the composites. Full article

TiO₂ is a semiconductor that is commonly used in dye-sensitized solar cells (DSSCs). However, the necessity of sintering the TiO₂ layer is usually problematic due to the desired temperatures of typically 500 °C in cells that are prepared on polymeric or textile electrodes. This is why textile-based DSSCs often use metal fibers or metallic woven fabrics as front electrodes on which the TiO₂ is coated. Alternatively, several research groups investigate the possibilities to reduce the necessary sintering temperatures by chemical or other pre-treatments of the TiO₂. Here, we report on a simple method to avoid the sintering step by using a nanofiber mat as a matrix embedding TiO₂ nanoparticles. The TiO₂ layer can be dyed with natural dyes, resulting in a similar bathochromic shift of the UV/Vis spectrum, as it is known from sintered TiO₂ on glass substrates, which indicates an equivalent chemical bonding. Our results indicate a new possibility for producing textile-based DSSCs with TiO₂, even on textile fabrics that are not high-temperature resistant. Full article

A cellulose-cadmium (Cd)-tellurium (TE) quantum dots (QDs) composite film was successfully synthesized by incorporating CdTe QDs onto a cellulose matrix derived from waste cotton linters. Cellulose-CdTe QDs composite film was characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and X-ray diffraction (XRD). The antibacterial activity of the prepared composite film was investigated using the multidrug-resistance (MTR) Staphylococcus aureus bacteria. In vitro antibacterial assays demonstrated that CdTe QDs composite film can efficiently inhibit biofilm formation. Our results showed that the cellulose-CdTe QDs composite film is a promising candidate for biomedical applications including wound dressing, medical instruments, burn treatments, implants, and other biotechnology fields. Full article

With the advent of wearable electronic devices in our daily lives, there is a need for soft, flexible, and conformable devices that can provide electronic capabilities without sacrificing comfort. Electronic textiles (e-textiles) combine electronic capabilities of devices such as sensors, actuators, energy harvesting and storage devices, and communication devices with the comfort and conformability of conventional textiles. An important method to fabricate such devices is by coating conventionally used fibers and yarns with electrically conductive materials to create flexible capacitors, resistors, transistors, batteries, and circuits. Textiles constitute an obvious choice for deployment of such flexible electronic components due to their inherent conformability, strength, and stability. Coating a layer of electrically conducting material onto the textile can impart electronic capabilities to the base material in a facile manner. Such a coating can be done at any of the hierarchical levels of the textile structure, i.e., at the fiber, yarn, or fabric level. This review focuses on various electrically conducting materials and methods used for coating e-textile devices, as well as the different configurations that can be obtained from such coatings, creating a smart textile-based system. Full article

Although considerable research has been directed at developing materials for ballistic protection, considerably less has been conducted to address non-firearm threats. Even fewer studies have examined the incorporation of particle-laden elastomers with textiles for spike, knife, and needle protection. We report on a new composite consisting of ultra-high-molecular-weight polyethylene (UHMWPE) fabric impregnated with nanoparticle-loaded elastomer, specifically designed for spike- and needle-resistant garments. Failure analysis and parametric studies of particle-loading and layer-count were conducted using a mixture of SiC and polyurethane at 0, 30, and 50 wt.%. The maximum penetration resistance force of a single-layer of uncoated fabric increased up to 218–229% due to nanoparticle loading. Multiple-layer stacks of coated fabric show up to 57% and 346% improvement in spike puncture and hypodermic needle resistance, respectively, and yet were more flexible and 21–55% thinner than a multiple-layer stack of neat fabric (of comparable areal density). We show that oxygen-plasma-treatment of UHMWPE is critical to enable effective coating. Full article

Due to the high specific surface area, high mechanical strength and broad possibility of surface modification, nanocellulose has obtained much attention as a new class of bio-based nanomaterials with promising potential in a wide variety of applications. Recently, a considerable amount of research has been aimed to the fabrication of nanocellulose based hybrid membranes for water treatment. However, nanocellulose based hybrid gas separation membrane is still a new research area. Herein, we force on recent advancements in the fabrication methods and separation performances of nanocellulose-based hybrid membranes for CO₂ separation, the transport mechanisms involved, along with the challenges in the utilization of nanocellulose in membranes. Finally, some perspectives on future R&D of nanocellulose-based membranes for CO₂ separation are proposed. Full article

Excellent water-absorbing nanofiber meshes were developed as a potential material for removing excess fluids from the blood of chronic renal failure patients toward a wearable blood purification system without requiring specialized equipment. The nanofiber meshes were successfully fabricated from poly(acrylic acid) (PAA) under various applied voltages by appropriately setting the electrospinning conditions. The electrospun PAA nanofibers were thermally crosslinked via heat treatment and then neutralized from their carboxylic acid form (PAA) to a sodium carboxylate form poly(sodium acrylate) (PSA). The PSA nanofiber meshes exhibited a specific surface area 393 times that of the PSA film. The PSA fiber meshes showed a much faster and higher swelling than its corresponding film, owing to the higher capillary forces from the fibers in addition to the water absorption of the PSA gel itself. The proposed PSA fibers have the potential to be utilized in a new approach to remove excess water from the bloodstream without requiring specialized equipment. Full article

A fiber-optic strain sensor that can show strain via color change and which can be viewed using human eyes has demand in the civil engineering field for alerting purposes. A previous sensor was fabricated using PMMA (Poly(methyl methacrylate)), which had the exceeding hardness to exhibit satisfactory sensor performance. In this research, an elastomer-based fiber-optic structure was fabricated to enhance the elastic response of such sensors and to enlarge the waveguide cross section. Various organic fluorophores were added to the core and cladding regions of the elastic waveguide to induce energy flow from the core to the cladding when stress is applied to the waveguide. Elastomer pairs suitable for the core and cladding were selected from among several candidate materials having high transparency. A method of dispersing fluorophores to each host elastomer and constructing an excellent core–cladding interface using the selected materials was proposed. To investigate the time-dependent changes in the fluorescence of the doped elastomer waveguide, the absorption and emission spectra were monitored after the host elastomers were cured. Full article