Fibers, Volume 10, Issue 8 (August 2022) – 8 articles

Computation-based mathematical models of tissue indentation are capable of predicting the distribution of forces and mechanical properties of soft tissues. This paper presents a three-dimensional mathematical model of anisotropic tissue indentation developed using the mechanical bidomain model. The mechanical bidomain model hypothesizes that the relative displacement between intra- and extracellular spaces triggers a force on the mechanosensitive proteins in the membrane: integrins. Some soft tissues, such as cardiac muscle, are anisotropic, a property which arises from the fibrous structure of the tissue. The degree of anisotropy in intra- and extracellular spaces can be different. Tissue indentation for different anisotropy ratios that indicate isotropy, equal anisotropy and unequal anisotropy, were tested using the model. Results of the tissue indentation analysis compared the spatial distribution of the magnitude of bidomain displacement for different anisotropy conditions between monodomain and bidomain models. The proposed mathematical model predicted unexpected spatial patterns of cardiac mechanotransduction for unequal anisotropy ratios of mechanical modulus. Full article

Steel fibres provide ductility to concrete structures. This, in turn, gives possibility to replace or reduce conventional reinforcement in structural elements. In this study, the focus is on structural walls and the fibres as potential replacements for horizontal reinforcement in areas where vertical rebars are needed. An experimental study was conducted, in which prismatic specimens with longitudinal rebars were subjected to centric loading. Ten samples with 12 specimens in each were tested. The parameters considered were: fibre content, concrete cover for the longitudinal bars, and presence of stirrups. Self-compacting concrete with 30 and 60 kg/m${}^3$ steel fibres was used. Relative and normalised values of the test results were calculated; correlation and analysis of variance was used to estimate the effect of fibres. The results show that the fibres eliminated brittle collapse and spalling of concrete at failure. A strong negative correlation (−0.72 to −0.92) between amount of fibres and load-bearing capacity was found. On average, the reduction of the capacity was 8% to 16% if compared to the specimens with no fibres. However, a positive effect of the fibres on the ductility was observed. Specimens with 30 kg/m${}^3$ fibres showed the same post-peak behaviour as specimens with minimum horizontal reinforcement required by Eurocode 2. The study suggests that combination of steel fibres and conventional rebars can lead to less qualitative compactness of the self-compacting concrete, which in turn may reduce load-bearing capacity and stiffness of the structure. Special attention on concrete cover and distance between rebars should be paid if self-compacting concrete structures with steel fibres are designed. Full article

The strengthening of existing reinforced concrete (RC) structures by means of steel-fabric reinforced cementitious matrix (Steel-FRCM) systems has been universally recognized in the academic literature as an effective method. Several types of steel fibres can be found in the marketplace, and they are classified according to mass per unit area and tensile strength. In the flexural strengthening design of RC beams, a fundamental parameter is the effective tensile strain level in the Steel-FRCM system attained at failure. Some authors and guidelines suggest evaluating this strain value using the results of bond tests. As is well highlighted in many works, the debonding strain in Steel-FRCM composites applied on concrete beams is usually higher than that from single-lap shear tests. At this point, it can be easily obtained by applying an appropriate amplification coefficient. This study experimentally investigates the difference in the debonding strain between Steel-FRCM composites bonded to concrete blocks in single-lap shear tests (end strain) versus the debonding strain in concrete beams (intermediate strain). The results were used to critically discuss the variability of the amplification coefficient, significantly affected by the mechanical and geometrical properties of the steel fibres. Moreover, a simple predictive formula to evaluate the intermediate strain debonding was used, and the results were compared with the experimental evidence. Finally, a large database of direct shear and flexural tests was used to confirm the experimental and theoretical data obtained herein. Full article

Atmospheric plasma treatment can modify fabric surfaces without affecting their bulk properties. One recently developed, novel variant combines both plasma and UV laser energy sources as a means of energising fibre surfaces. Using this system, the two most commonly used fibres, cotton and polyester, have been studied to assess how respective fabric surfaces were influenced by plasma power dosage, atmosphere composition and the effects of the presence or absence of UV laser (308 nm XeCl) energy. Plasma/UV exposures caused physical and chemical changes on both fabric surfaces, which were characterised using a number of techniques including scanning electron microscopy (SEM), radical scavenging (using 2,2-diphenyl-1-picrylhydrazyl (DPPH)), thermal analysis (TGA/DTG, DSC and DMA), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). Other properties studied included wettability and dye uptake. Intermediate radical formation, influenced by plasma power and presence or absence of UV, was key in determining surface changes, especially in the presence of low concentrations of oxygen or carbon dioxide (20%) mixed with either nitrogen or argon. Increased dyeability with methylene blue indicated the formation of carboxyl groups in both exposed cotton and polyester fabrics. In the case of polyester, thermal analysis suggested increased cross-linking had occurred under all conditions. Full article

One of the main challenges of using a high fiber volume content in a cement composite is the narrow margin of fiber volume content beyond which fibers can cause an adverse effect on the mechanical properties. In this paper, the significance of fiber size distribution and fiber volume content of different proportions of chopped and milled carbon microfibers are investigated. The mixes’ flowability showed improvement with altering the fiber size distribution despite having a high fiber content. Uniaxial compression cylinders and unnotched and notched beams were cast and then tested at 7 and 28 days of age. It was found that the compressive strength is significantly affected by fiber size distribution more than fiber volume content. On the other hand, the modulus of rupture and fracture toughness are proportional to the fiber volume content with little effect of fiber size distribution. Finally, neither high fiber volume content nor altered fiber size distribution significantly affected the elastic modulus of the fiber cement composites. Full article

The incorporation of reinforcements is a necessity to compensate for the deficiency that concrete presents with its fragile behavior and low deformation capacity. One of the solutions to improve tensile performance is the addition of fiber in random distributions throughout the volume. However, this strategy can compromise the compressive strength of concrete; consequently, the purpose of this study was to analyze the compressive strength of conventional concrete with hybrid fiber reinforcement. A behavioral equation of compressive strength as a function of the hybridization of three types of fibers (steel, polypropylene, and carbon) was determined. This equation accounted for the proportions, as well as the binary and tertiary combinations, of fibers. Results showed that the effective participation of metallic fibers and their combination with synthetic fibers contributed positively to the performance of fiber-reinforced concrete. The gain in axial compression strength reached values in the range of 10% to 19% depending on the content of total fibers and their combination, without problems in the production process. Full article

In this study, elementary kenaf fibres were separated from fibre bundles using two different treatments. The first involved treating with nitric acid (HNO₃) while the second used a mixture of hydrogen peroxide (H₂O₂) and acetic acid (CH₃COOH). Both treatments were successful in isolating the elementary fibres but the H₂O₂/CH₃COOH gave a better fibre yield and required a shorter treatment time. The fibres treated with HNO₃ had an average length of 0.2 mm, an aspect ratio of 15 and a defect density of 21 defects per mm. In contrast, the H₂O₂/CH₃COOH treated fibres had a length of 2.3 mm, an aspect ratio of 179 and a defect density of 14 defects per mm. Both treatments removed lignin, pectin, and waxes. They also increased cellulose crystallinity in the fibres, especially for HNO₃ treatment. However, they resulted in some oxidation of cellulose. The H₂O₂/CH₃COOH treatment gave a substantial improvement in the thermal stability of the fibres while a marked decrease was observed for the HNO₃ treatment. Full article

The feasibility of using epoxidized waste cooking oils as a partial replacement for synthetic resins in the manufacture of lignocellulosic composites where the reinforcement is comprised of mechanically ground wood from civil construction waste wood (CCWW) was investigated. For this study, the wood-epoxy composite was prepared using the thermo-curing technique, and wood particle contents of 20 and 30% (m/m) were studied with a matrix comprised of 50% epoxidized vegetable oil and 50% petroleum-based epoxy resin. The specific mass of the composites was in the range of 1130 to 1380 kg/m³, with the lowest value for the highest content of wood particles. Fourier transform infrared spectroscopy was successfully used to monitor the epoxidation of the vegetable oils and the subsequent curing of the epoxy resins and particleboards. Thermal stability of the composite was dictated by its lignocellulosic content, and significant mass losses occurred at temperatures higher than 300 °C, regardless of the wood particles content. The introduction of CCWW particles into the polymeric matrices did not promote the desired effect of improving the mechanical properties in regard to those of the cured blend of epoxy resins. However, the produced particleboards still met the standards of the American National Standards for general purpose boards in regard to their physical and mechanical properties (e.g., density, tensile strength). Hence, the use of wood waste and waste cooking oil to produce particleboards was deemed justified within the framework of a cascading lifecycle-extended service for both wastes. Full article