Fiber-reinforced Structures Research Articles

An experimental investigation on freeze-thaw resistance of fiber-reinforced red mud-slag based geopolymer

This study aims to investigate the influence of different fiber types, lengths, and contents on their performance in freeze-thaw environments. Indoor freeze-thaw tests were conducted to evaluate the flexural strength, compressive strength, and mass loss of fiber-reinforced geopolymer materials. Additionally, SEM and XRD analyses were performed to examine the internal structure of the geopolymer composites. The results demonstrated that the incorporation of different fibers significantly improved various properties, particularly the flexural strength of the specimens. With an increasing number of freeze-thaw cycles, surface cracks became more prominent in the geopolymer specimens, leading to a gradual decrease in frost resistance. However, the addition of an appropriate amount of fiber effectively enhanced frost resistance. Following freeze-thaw exposure, an increase in fiber content initially resulted in a decrease and subsequently an increase in relative flexural strength, compressive strength, and mass loss. Notably, studies revealed that polypropylene fibers with a length of 9 mm and a content of 0.9 % exhibited minimal mass and strength loss. SEM analysis indicated uniform distribution of fibers within the geopolymer matrix, contributing to improved flexural strength and enhanced frost resistance in fiber-reinforced structures. XRD results indicate that red mud-slag based geopolymers produce high-strength hydrated calcium (aluminosilicate) silicate and hematite phases, which enhances the mechanical properties and durability of the specimens. In conclusion, this study highlights that incorporating suitable fibers can enhance the durability and performance of red mud-slag based geopolymers under freeze-thaw conditions. The findings provide valuable insights for optimizing fiber type, length, and content to achieve superior mechanical properties and frost resistance in geopolymer composites.

The effect of span length on the flexural properties of glass and basalt fiber reinforced sandwich structures with balsa wood core for sustainable shipbuilding

The current research aims to analyze the mechanical characterization of sandwich materials through a three-point flexural test. The sandwich structures in question composed of balsa wood as a core and four different types of fiber reinforced vinyl ester composite facesheets, namely Glass, Basalt, Glass/Carbon, and Basalt/Carbon. The sandwich panels were prepared using the vacuum infused processing method. The primary objective of this investigation is to explore the feasibility of utilizing basalt fibers for the production of structural parts in shipbuilding and replacing the already existed glass fibers. The flexural test was carried out with using three-point flexural test with varying the span length from 120 mm, 180 mm, 220 mm. Additionally, an analysis of variance (ANOVA) was then carried out to compare the mean values of properties deduced from these tests. The results showed that using basalt fibers instead of glass fiber reinforced enhanced the flexural stiffness of the sandwich structure. The flexural strength and modulus are shown to depend on span length and fiber type. The flexural modulus increases with an increase in span length. Similarly, flexural strength increases in glass fiber-based structures, while a slight reduction is observed in basalt fiber-reinforced structures the larger span length. The findings of this research suggest that basalt fibers hold potential as a replacement for glass fibers in producing structural components in shipbuilding. These results offer valuable information that can aid in the design and optimization of sandwich materials in shipbuilding.

Study of different initiation and propagation criteria in the XFEM modelling of scarf adhesive joints

Scarf adhesive joints, characterized by an end-to-end connection between tapered adherents at a given scarf angle (α), are applicable in structural applications, e.g., fiber-reinforced structures, owing to their efficiency. This work presents an analysis on the tensile strength of scarf adhesive joints, considering a and the adhesive type as design parameters. A detailed numerical evaluation is presented, based on the eXtended Finite Element Method (XFEM), following the technique validation with experimental data, for adhesives with different characteristics. The XFEM technique was tested with varying damage initiation and propagation criteria. The XFEM technique was positively validated for specific damage initiation and propagation criteria.

Investigation of Tensile Strength of Long-Fiber Thermoplastic Composites Based on Evaluation of Fiber-Reinforced Structure

Investigation of Tensile Strength of Long-Fiber Thermoplastic Composites Based on Evaluation of Fiber-Reinforced Structure

A nodal-based optimization method for the design of continuous fiber-reinforced structures

The mechanical performance of fiber-reinforced structures highly relies on fibers' paths in the matrix material. Unlike the level set method that offsets the zero-level contours of a higher-dimensional function, this work shifts the nodes of a truss network within an optimization framework to design individual paths specifically. All intersection manners of a single fiber with the rectangular continuum element are considered in the finite analysis to retrieve mechanical responses within an acceptable error margin numerically. The design variables, namely node coordinates, are updated via the method of moving asymptotes under the guide of the sensitivity of the objective function, which is smoothed using a radial basis function. After each optimization iteration, the paths are slightly adjusted to avoid twisting problems and control fiber gaps using a polynomial interpolation scheme. Numerical examples illustrate that the proposed method can generate elegant fiber paths within a few steps. In the meantime, the optimization efficiency is improved as the design variables are significantly reduced. Compared with existing methods, we found that lower objective values can be achieved as allowing nodes to move freely in the design domain guarantees a global minimum theoretically.

Size-scale effects and modelling issues of fibre-reinforced concrete beams

This paper compares numerical and analytical predictions for the shear capacity of fibre-reinforced concrete beams based on experimental literature tests. The authors compared the outcomes of a FE model using the DamageTC3d constitutive model, a literature formulation, and the current proposal of the Eurocode 2 draft for fibre-reinforced structures. The paper evaluates the sensitivity of the beam response to the fracture energy Gf, modified after the Model Code 2010 formulation. The investigation reveals a dependence of the estimated fracture energy on the beam size. Furthermore, the comparison between the numerical estimates and the analytical predictions using the MC2010 and the current EC2 draft proves that the error is substantially independent of the model selection but is strongly affected by the specific case study. This fact confirms the absence of weaknesses in the numerical modelling and highlights the aleatoric uncertainties of the experimental data.

Eco-Friendly Design and Sustainability Assessments of Fibre-Reinforced High-Strength Concrete Structures Automated by Data-Driven Machine Learning Models

In recent years, adding fibres into brittle concrete to improve ductility has gained momentum in the construction industry. Despite the significant momentum, limitations do exist in design and industrial applications, contributing to the complexity of shear behaviours in fibre-reinforced concrete and the existing empirical models that can hardly provide a reasonable prediction, especially for high-strength concrete applications. A critical review reveals that current research mostly focuses on single eigenvalue analysis and pay less attention to the different synergetic effect of fibres on high-strength concrete and normal-strength concrete. This study aims to fill the research gap by the unprecedented use of reliable models for the prediction and evaluation of structural and sustainable properties of high-strength fibre-reinforced concrete beams. To this end, this study establishes three novel deep learning (ANN, BNN, and Xgboost) models for designing and optimising the shear capacity of ‘high-strength’ fibre-reinforced concrete beams towards the circular economy. In addition to introducing a new type of novel machine learning (BNN) model, which is capable of structural design and takes into account complex design features, our study also enhances sustainability by reducing greenhouse gas (GHG) emissions. The novel prediction models unprecedentedly elicit flexural capacity, structural stiffness, carbon emission, and price, together with the shear strength for high-strength fibre-reinforced structures. Firstly, this study focuses on multiple parameters for forecasting high-strength fibre-reinforced concrete beams. In addition, the models provide more comprehensive insights into the design and manufacture of high-strength steel fibre-reinforced concrete structures in a more environmentally friendly manner. With the help of the proposed models, it will be more cost-benefit and time-efficient for the researchers to obtain the optimum design with the consideration of both structural and sustainable performance. The established models exhibit excellent prediction accuracy, and the Bayesian neural network (BNN) is found to have the best performance: R2 is 0.937, MSE is 0.06 and MAE is 0.175 in shear strength prediction; R2 = 0.968, MSE is 0.040, and MAE is 0.110 in flexural capacity prediction; R2 is 0.907, MSE is 0.070, and MAE is 0.204 in shear stiffness prediction; R2 is 0.974, MSE is 0.022, and MAE is 0.063 in carbon emission prediction; and R2 is 0.977, MSE is 0.020, and MAE is 0.082 in price prediction.

Efficient spline design via feature-mapping for continuous fiber-reinforced structures

We present a feature-mapping topology optimization approach, in which curved features are parametrized as piecewise linear splines smoothly rounded by arcs. The motivation for our contribution to the tool set of feature-mapping methods is the optimization of structures manufactured by variable angle continuous fiber-reinforced filaments. For this reason, the feature’s geometry should be able to represent long, curved fiber objects satisfying manufacturing constraints, such as minimum turning radius. The proposed model has been chosen with special care for rigorous continuous differentiability, as well as an efficient analytical evaluation of the signed distance field to the spline. The geometrical description and sensitivity analysis of the spline model are developed fully analytically and then mapped to a discretized pseudo-density field for finite element analysis. For the fiber-reinforced material formulation, we also present a new combine step for individual features, in which the best possible angle for the combined features is searched. The model and results are presented in a two-dimensional setting.

Bioinspired NiTi-reinforced polymeric heart valve exhibiting excellent hemodynamics and reduced stress

The durability of the current polymeric heart valves remains to be improved, which has proven to be highly dynamics and species dependent. Herein, inspired by the fiber-reinforced structure of native valve leaflets, we developed a valve prototype with the polymer matrix reinforced by NiTi wires in the radial direction, to achieve both desirable dynamics control and stress reduction. Textile technology and computer modeling were primarily utilized for integrated construction and biomimetic geometry. The NiTi-reinforced textile composites had suitable properties with valve leaflets. The pulsating flow tests confirmed that the systolic hemodynamics and leaflet kinetics were significantly optimized through the stiffness-domination of NiTi wires. Particle imaging velocimetry measurements revealed the satisfactory flow field characteristic for VP40-RA6 when compared with other reinforced valves. Furthermore, the finite element analysis indicated that the maximum principal stress and the area percentage of that were significantly reduced. As a preliminary proof-of-concept, this novel valve prototype exhibited excellent comprehensive performance which would be beneficial for the further improvement of long-term durability and provide new insight into the design modification of heart valve prosthesis.

In-plane and out-of-plane quasi-static compression performance enhancement of 3D printed re-entrant diamond auxetic metamaterial with geometrical tuning and fiber reinforcement

Auxetic materials are cellular materials with a unique property of negative Poisson's ratio. The auxeticity and performance of these metamaterials utterly depend on the geometrical parameters and loading direction. For the first time, the quasi-static uniaxial compression performance of fused filament fabricated re-entrant diamond auxetic metamaterial is evaluated in the x-direction (in-plane) and z-direction (out-of-plane). The most commonly used thermoplastic feedstock, Acrylonitrile butadiene styrene, is considered a material of choice. The effect of influential geometrical parameters of the re-entrant diamond structure and printing parameter is systematically studied using Taguchi's design of experiments. Grey-based multi-objective optimisation technique has been adopted to arrive at the optimal structure. Efforts are made to improve the stiffness and strength of the structure with fibre reinforcements. Micro glass fibre reinforcements have enhanced specific strength and stiffness in both in-plane and out-of-plane directions. A sevenfold and thirteen times increase in specific strength and energy absorption is evident for glass fibre-reinforced structures in out-of-plane directions compared to in-plane ones. Proper tuning of geometrical parameters of the re-entrant diamond structure can result in a Poisson's ratio of up to −3.49 when tested in the x-direction. The parametric study has illustrated the tailorability of the structure according to the application requirements. The statistical study has signified each considered parameter's contribution to the compression performance characteristics of the 3D printed re-entrant diamond auxetic metamaterial.

Mechanically robust bamboo node and its hierarchically fibrous structural design.

Although short bamboo nodes function in mechanical support and fluid exchange for bamboo survival, their structures are not fully understood compared to unidirectional fibrous internodes. Here, we identify the spatial heterostructure of the bamboo node via multiscale imaging strategies and investigate its mechanical properties by multimodal mechanical tests. We find three kinds of hierarchical fiber reinforcement schemes that originate from the bamboo node, including spatially tightened interlocking, triaxial interconnected scaffolding and isotropic intertwining. These reinforcement schemes, built on porous vascular bundles, microfibers and more-refined twist-aligned nanofibers, govern the structural stability of the bamboo via hierarchical toughening. In addition, the spatial liquid transport associated with these multiscale fibers within the bamboo node is experimentally verified, which gives perceptible evidence for life-indispensable multidirectional fluid exchange. The functional integration of mechanical reinforcement and liquid transport reflects the fact that the bamboo node has opted for elaborate structural optimization rather than ingredient richness. This study will advance our understanding of biological materials and provide insight into the design of fiber-reinforced structures and biomass utilization.

Fiber-reinforced flexible joint actuator for soft arthropod robots

The flexible joints are the fundamental building blocks of modular soft robots. However, the investigation on flexible joints made entirely of soft material is still in its infancy, with complex structure, small rotation range and insufficient driving force output. Bio-inspired from the hydraulic joints of spider legs, a dexterous fiber-reinforced flexible joint actuator was proposed. For the irregular shape joint actuator with fiber-reinforced structure, its fabrication process and modeling method are discussed. And the advantages of fiber reinforcement are demonstrated in terms of rotation angle, driving torque and efficiency experiments. Compared with the joint actuator without fiber layer, the actuator with a fiber angle of 30° can increase the rotation angle by up to 36% and the driving efficiency by up to 19.6%. The maximum inflation pressure can reach 120 kPa, and the force density 3.56 mN/mm3, which demonstrates extremely high pressure tolerance capacity and force density, and shows the wide applicability of the fan-shaped flexible actuator to enable lightweight, mobile, multifunctional soft robots.

ДЕФОРМИРОВАНИЕ ФИБРОБЕТОНА ПРИ ОДНОКРАТНОМ ДИНАМИЧЕСКОМ ВОЗДЕЙСТВИИ С УЧЕТОМ ВЛИЯНИЯ НАЧАЛЬНЫХ НАПРЯЖЕНИЙ ОТ СТАТИЧЕСКОЙ НАГРУЗКИ

An analysis of the experimental studies presented in the scientific literature indicates that the strength and deformability of fiber-reinforced concrete under dynamic additional loading may depend on the initial level of stresses caused by a preliminarily applied static load. In this regard, the purpose of this study was to build improved models of fiber-reinforced concrete deformation of operated fiber-reinforced concrete structures of bearing systems of buildings and structures during their dynamic additional loading, taking into account the nonlinearly elastic law of material deformation at the stage of loading with a static load, as well as changes in the parameters of viscous resistance during loading. As a model of the force resistance of fiber-reinforced concrete of loaded fiber-reinforced concrete structures during their dynamic additional loading caused by an emergency situation, the Kelvin-Voigt model for an elastic-viscous body was adopted. On the basis of the adopted model, an analytical solution of the differential equation of uniaxial compression of fiber-reinforced concrete was obtained under its single dynamic additional loading from an arbitrary level of initial stresses acting in it from a previously applied static load. A step-iterative approach to determining the parameters of state diagrams of fiber-reinforced concrete under static-dynamic loading conditions is proposed. An analysis of the obtained dependences between stresses and strains shows that an increase in the level of initial stresses acting in a compressed concrete element at the moment a dynamic load is applied to it leads to a decrease in the dynamic strength of concrete with the same impact parameters.

Phase-field based robust topology and orientation optimization under fiber orientation uncertainty

Recently, there has been an increased usage of fiber-reinforced composites in aerospace and automobile due to their celebrated mechanical properties. Thus, a massive research effort has hitherto been done to develop a method for concurrent optimization of structural topology and fiber orientation. Topology-optimized continuous fiber-reinforced structures tend to show complex morphologies with spatially varying fiber orientation. Due to this nature, it is nearly possible to manufacture the results only through additive manufacturing technology However, a gap must exist between prescribed optimized fiber orientation and fiber orientation of manufactured composites under a limited degree of precision of the manufacturing process. Robust anisotropic topology optimization under such uncertainty, therefore, needs to be developed. This presentation aims to present a novel robust topology optimization methodology for fiber-reinforced materials under fiber orientation uncertainty. Our method is built upon Phase-Field based topology optimization which is suitable for this type of optimization method. The effectiveness of our method is validated through the stiffness maximization problem and the fundamental eigenfrequency maximization problem.

Three-dimensional crack bridging model of biological materials with twisted Bouligand structures

Twisted fiber-reinforced structures that resemble plywood, also called Bouligand structures, are widely observed biological materials in organisms such as lobsters, crabs, mantis shrimp and scorpions, where they exhibit outstanding fracture toughness and damage resistance. In this paper, we develop a three-dimensional crack twisted-bridging model to correlate fracture toughness with Bouligand microstructures and reveal the underlying toughening mechanisms. Depending on their orientation, some nanofibers bridge the crack surfaces in a twisting arrangement within the fracture process zone at the crack tip. The crack resistance of the structures assembled by soft biopolymers and physical cross-links is parameterized in terms of the nanofiber pitch angle and interfacial properties. Bouligand structures exhibit high and direction-independent fracture toughness in the nanofiber rotating plane, a notable advantage that endows material with superior load-bearing capacity in all directions. The crack resistance of Bouligand structures increases with the pitch angle of nanofibers, and the highest value is achieved at 90 degrees. Within a reasonable structural motif, an increase in the nanofiber length or interfacial strength results in an enhancement of the fracture toughness. Compared with the finite deformation of biopolymers, the facile formation and reformation mechanism of physical cross-links at the interfaces is a more efficient toughening strategy for materials with network-like structures of nanofibers. Our theoretical predictions agree well with relevant experimental measurements. This work can aid in designing biomimetic structural materials with high performance.

Evaluating and Minimizing Induced Microbending Losses in Optical Fiber Sensors Embedded Into Glass-Fiber Composites

Conventional silica optical fibers can be embedded into composite structures or packaging to provide structural monitoring capabilities. In this paper, the microbending optical losses induced by the packaging of a sensing optical fiber into a sandwiched glass-fiber reinforced structure are investigated experimentally and by simulations. Results show that the positioning of the optical fiber within a plain-weave glass fiber-reinforced structure has a critical effect on microbending optical losses due to potentially induced microdeformations of the optical fiber's cylindrical shape. Similar analysis is also carried out with the use of a 1D glass-fiber layer packaging, demonstrating lower residual microbending losses after manufacturing. By the proper positioning of an optical fiber containing a series of wavelength-multiplexed fiber Bragg grating (FBG) sensors over a 2D plain-weave glass fabric, a 10-fold improvement in the induced optical losses is demonstrated, increasing the optical power that reaches the interrogating unit and allowing the monitoring of hundreds of meters over glass-fiber structures. Although all the analysis is focused and verified using a series of FBG sensors, the results and conclusions related to the induced microbending losses are also valid and useful for distributed optical fiber sensing approaches.

Calibration of piezoresistive shape-memory alloy strain sensors

Continuous strain measurement on fibre-reinforced structures demands mechanical sensors with superior fatigue resistance. Shape-memory alloy wires are predestined for strain sensors utilising their strong piezo-resistance. Calibration of these sensors is necessary in order to extract mechanical data. Therefore, four-point bending of glass-fibre reinforced plastic specimens with applied strain sensors and an optical reference measuring system is used to calibrate and compare shape-memory alloy sensors and standard strain gauges. The gauge factor and its standard deviation is successfully measured by this calibration method. Shape-memory alloy sensors show strain-dependent gauge factor whilst standard strain gauges show a constant strain sensitivity, both with a narrow stochastic distribution. Shape-memory alloy mechanical sensors are reliable to determine strain of fibre-reinforced structures. This offers the possibility to use them in structural health monitoring applications of such structures. Consequently, the four-point bending calibration using glass-fibre reinforced specimens represents a suitable possibility for calibration of strain sensors exposed to higher strain amplitudes.

HYIMFO: Hybrid method for optimizing fiber orientation angles in laminated piezocomposite actuators

Numerous engineering applications employ composite materials due to the advantages that the combination of their components can provide. Fiber-reinforced composites are a particular group of composites which allows designing optimized structures by tailoring the orientation of the fiber. For this reason, new additive manufacturing technologies have been developed nowadays. These tools allow the manufacturing of fiber-reinforced structures designed by using optimization algorithms. Consequently, there are several works in recent literature determining the optimized fiber orientation in composite structures, where the methods based on gradient stand out due to their better efficiency. There are two main approaches to determine the fiber orientation by using gradient-based methods, Continuous Fiber Angle Optimization (CFAO), where the angle is the design variable and commonly present local minima problems, and Discrete Fiber Angle Interpolation Models, which consider candidate angles a priori and by using weighting functions choose one of them as the optimized angle. This work proposes a method named HYMFO, which uses a combination of both approaches. To verify the efficiency of the method, two design problems of Laminated Piezocomposite Actuators (LAPA) are solved by using several mesh discretizations with the discrete methods DMO, HPDMO, BCP, and NDFO, the continuous method SPIMFO and the newly proposed hybrid method. Therefore, the method is not evaluated with the traditional problem of maximizing the stiffness in structural design. A new fiber orthogonality regularization is presented, facilitating the appearance of continuous paths in fiber distribution by imposing a specified maximum angle between adjacent fibers. A comparative analysis based on the performance of the structure and the computational cost is carried out to determine the most appropriate method for each situation.

Experimental and numerical investigations on the failure behavior of metal skin and carbon fiber-reinforced polymer core structures considering layup and manufacturing process conditions

In this paper, the failure mechanism of metal skin (MS) and long carbon fibers reinforced core (CFRP) structures is investigated by means of tensile and bending tests, scanning electron microscope image analysis, and finite element simulations. For the failure modeling, a ductile damage model has been utilized for the MS, a traction separation model for the adhesive layers, whereas a modified Tsai-Hill criterion under the orthotropic assumption has been employed for the CFRP core. To verify the reliability of the proposed approach in predicting the failure of more complex MS-CFRP structures, experimental and numerical load-stroke curves relevant for two hat-shaped specimens, manufactured with two different MS, have been compared, showing a maximum deviation in the maximum load equal to 3.5%. The methodology proposed in this paper shows a systematic and reliable approach for the investigation of the fracture behavior of metal-composite structures, to be utilized for the design phase.

Hybrid Polymer Composites Used in the Arms Industry: A Review.

Polymer fiber composites are increasingly being used in many industries, including the defense industry. However, for protective applications, in addition to high specific strength and stiffness, polymer composites are also required to have a high energy absorption capacity. To improve the performance of fiber-reinforced composites, many researchers have modified them using multiple methods, such as the introduction of nanofillers into the polymer matrix, the modification of fibers with nanofillers, the impregnation of fabrics using a shear thickening fluid (STF) or a shear thickening gel (STG), or a combination of these techniques. In addition, the physical structures of composites have been modified through reinforcement hybridization; the appropriate design of roving, weave, and cross-orientation of fabric layers; and the development of 3D structures. This review focuses on the effects of modifying composites on their impact energy absorption capacity and other mechanical properties. It highlights the technologies used and their effectiveness for the three main fiber types: glass, carbon, and aramid. In addition, basic design considerations related to fabric selection and orientation are indicated. Evaluation of the literature data showed that the highest energy absorption capacities are obtained by using an STF or STG and an appropriate fiber reinforcement structure, while modifications using nanomaterials allow other strength parameters to be improved, such as flexural strength, tensile strength, or shear strength.