Fiber-reinforced Polymer Research Articles

Evaluation of Mechanical and Thermal Properties of Alkali-Treated Sisal, Bamboo, and Hybrid Fiber-reinforced Polymer Composites

Evaluation of Mechanical and Thermal Properties of Alkali-Treated Sisal, Bamboo, and Hybrid Fiber-reinforced Polymer Composites

Tough, high-strength, flame-retardant and recyclable polyurethane elastomers based on dynamic borate acid esters

With the wide application of polyurethane elastomers, it is necessary to develop recyclable, fire-retardant polyurethane elastomers with great mechanical properties to comply with industrial requirements. Herein, we fabricated a flame-retardant, recyclable, strong yet tough polyurethane elastomer (PIDB-1) based on dynamic borate acid esters. The introduction of phosphaphenanthrene and boron-containing groups endow PIDB-1 with great flame retardancy, as reflected by it achieving the vertical burning (UL-94) V-0 rating. The PIDB-1 film shows high visible light transmittance, and its transmittance reaches 90 % at the wavelength of 800 to 900 nm. The tensile strength of PIDB-1 is 54.9 MPa, and its toughness reaches 207.8 kJ/m3, indicative of superior mechanical properties. Meanwhile, the dynamic borate acid esters allow the PIDB-1 elastomer to possess physical and chemical recyclability. When using PIDB-1 as a polymer matrix for carbon fiber-reinforced polymer composites, the carbon fibers can be fully recycled. This work provides an integrated design strategy for creating transparent, flame-retardant, recyclable polyurethane elastomers combining high strength and toughness based on dynamic borate ester bonds, which is expected to find wide applications in different industries.

Flexural performance of narrow RC beams hybrid strengthened by TRECC plate

This paper proposes a novel hybrid strengthening method for narrow reinforced concrete (RC) beams that combines fiber-reinforced polymer (FRP) textile-reinforced engineered-cementitious composites (TRECC) plates along with a self-locking anchorage system. To evaluate the effectiveness of the proposed system, a series of four-point bending tests were conducted. These tests were designed to understand the effects of different strengthening methods, number of FRP grid layers, and TRECC bond lengths. The results showed that the hybrid strengthening system could effectively prevent the end debonding failure and impede the development of the intermediate debonding in the TRECC plate. Consequently, the TRECC plate can continue to carry the force, leading to a significant increase in strengthening efficiency, quantified as the increased ratio of the peak load of the strengthened beams to that of the reference beam. Furthermore, increasing the number of FRP grid layers can also improve the strengthening efficiency. However, longer TRECC bond length in hybrid strengthened beams will reduce the strengthening efficiency since the intermediate debonding will be more susceptible to occur; the design for appropriate bond length is necessary. Finally, an analytical model to predict the flexural capacity was derived and verified against the experimental results.

Experimental and numerical investigation of bias performance of steel fiber-reinforced GRC-CFFT columns

Geopolymer concrete-filled glass fiber-reinforced polymer (GFRP) tube columns show significant potential in marine engineering due to their excellent durability and mechanical properties. This study investigated the bias performance of steel fiber-reinforced geopolymer recycled aggregate concrete-filled GFRP tube (SGRC-CFFT) columns, considering variables such as load eccentricity, GFRP tube thickness, fiber winding angle, and recycled coarse aggregate (RCA) replacement rate. The failure modes, load-displacement behaviors, and strain distribution were analyzed and discussed. Furthermore, a high-precision finite element model was established based on the test data. The test results indicated that increasing load eccentricity significantly reduced both the load-bearing and axial deformation capacity of CFFT columns. When the load eccentricity reached 0.40, the peak load of the specimen was only 39 % of that of the corresponding axial compression specimen. A higher RCA replacement rate would cause a more pronounced reduction in bias-bearing capacity, with a maximum decrease of 24.6 %. Before 80 % of the peak load, the axial deformation of the column mid-span section accorded to the plane section assumption. Due to the eccentric load, the hoop deformation of the CFFT column was concentrated on the compression side. The fiber winding angle was the critical factor influencing both the peak load and the peak moment of the biased SGRC-CFFT column.

Bond strength prediction of externally bonded reinforcement on groove method (EBROG) using MARS-POA

The Externally Bonded Reinforcement on Grooves (EBROG) method represents an advancement in externally bonded reinforcement (EBR) techniques, specifically addressing the challenge of premature debonding often encountered in conventional Fiber Reinforced Polymer (FRP) applications directly bonded to concrete. This article introduces a novel and straightforward mathematical equation for predicting bond strength in the EBROG method using soft computing techniques for the first time. The study delves into the combined potential of the Multivariate Adaptive Regression Spline (MARS) model and the Pelican Optimization Algorithm (POA) for bond strength prediction in this method. The input parameters include FRP width, FRP thickness, elasticity modulus of FRP, concrete strength, groove length, groove width, and groove depth, while the output is EBROG bond strength. The study demonstrates exceptional accuracy with R2 values of 0.9629 for training and 0.9623 for testing, highlighting the model’s precision. The proposed bond strength prediction equation for the EBROG method undergoes validation against existing models, encompassing thirteen equations for EBR and a recent one specific to EBROG. Statistical metrics confirm the accuracy and reliability of the proposed equation. Notably, FRP stiffness emerges as the parameter with the highest relative importance, while groove width exhibits the lowest impact on bond strength.

Experimental and analytical investigations on debonding response of embedded through-section ribbed GFRP bar-to-concrete joints at elevated temperatures

Embedded Through-Section (ETS) technique has gained a great deal of increasing interest in the shear strengthening of existing reinforced concrete members, which is based on using fibre-reinforced polymer (FRP) bars installed through predrilled holes into concrete via adhesive. One of the key issues associated with the bonding of ETS FRP bars to concrete is the debonding of FRP bar-to-concrete interfaces which leads to premature and brittle failure of strengthened members, particularly under elevated temperatures. However, little research has been conducted on the influence of elevated temperatures on the bond behavior between ETS FRP bars and concrete. This paper presents experimental and analytical evaluations of the bond response of ETS-ribbed glass FRP (GFRP) bar-to-concrete joints subjected to elevated temperatures. A total of 90 pullout specimens were tested to quantify the influences of the temperature, bar diameter, and concrete strength on the failure mode, load response and bond strength. An analytical model to reproduce the full-range debonding process of ETS-GFRP bar-to-concrete interfaces was presented, which allowed simultaneously considering both long and short embedded lengths, interfacial friction, and snap-back behavior. The load-slip response, stress field, and peak load were solved analytically, resulting in a closed-form solution for the critical length for snap-back and a non-closed-form solution for the effective embedment length. After an inverse analysis procedure was proposed for determining the bond parameters, the accuracy of the analytical solution was verified through comparisons with the self-conducted test results and existing experimental data. A new temperature-dependent bond strength model was also developed. The findings revealed a remarkable decline in the bond strength of up to 92.3 % as the temperature increased from 20 °C to 160 °C, with about 60–70 % of this degradation occurring close to the glass transition temperature (Tg) of the adhesive. At temperatures well above Tg, the bond strength decreased with the bar diameter and displayed insignificant improvement with higher concrete strength. The results also demonstrated that the proposed analytical approach and bond strength model can predict the bond response of ETS ribbed GFRP bars embedded in concrete with reasonable accuracy.

Self-healing carbon fiber/epoxy laminates with particulate interlayers of a low-melting-point alloy

In order to prolong the service life of fiber-reinforced polymer composites, the implementation of self-healing ability with the micro-encapsulated healing agent has been extensively studied. However, such microcapsule-based self-healing composites typically suffer from degraded mechanical properties due to the liquid-phase inclusions, thereby limiting their proliferation. Here, a low-melting-point alloy is utilized as the particulate inclusions of carbon fiber/epoxy laminated composites. Field's Metal particles (melting point: 62 °C) are distributed between woven carbon fiber preforms followed by the resin impregnation to realize laminated composites with a Field's Metal-enhanced interlayer(s). The resulting laminated composites demonstrate the autonomic repair of interlaminar failure with a 40 % of healing efficiency. Most of all, the mechanical properties of these self-healing laminated composites are comparable to the conventional laminated composites attributed to the rigid inclusions that can be compressed to increase the fiber volume. Since the Field's Metal particle inclusions can bestow polymer composites with self-healing ability and the potential increase in mechanical properties, Field's Metal-enhanced fiber-reinforced polymer composites are expected to unlock the practical utility of self-healing composites.

Microstructural strain localisation phenomena in fibre-reinforced polymer composites: Insights from nanoscale digital image correlation and finite element modelling

Multiscale models for fibre-reinforced polymer composites currently lack experimentally validated microscale damage descriptors as input parameters. This work demonstrates the occurrence of strain localisation phenomena at the fibre/matrix level using nanoscale digital image correlation. Unidirectional carbon-fibre reinforced epoxy and glass-fibre reinforced PMMA composites were loaded in transverse compression in a scanning electron microscope. Radial and shear strain maps were extracted and compared with finite element simulations based on a conventional elastoplastic model. Near the interface, an interphase layer is present in the matrix, presumably due to locally different polymerisation conditions. A skin-core structure was found in carbon fibres, corresponding to an increased transverse modulus towards the interface.

Compressive behaviour of concrete columns confined with textile reinforced concrete composites

The enhancement of strength and deformation capacity in confined concrete members could be achieved through the application of advanced composite materials. Textile reinforced concrete (TRC), a novel cement-based composite material, is considered as a viable alternative to fiber reinforced polymer (FRP) sheets, addressing the limitations associated with organic resins. This study delved into the impact of various parameters, such as cross-section shapes, concrete strengths, and the number of TRC layers, on the confinement effect. The results reveal a consistent augmentation in both axial compressive strength and deformation capacity of confined concrete with an escalation in the number of TRC layers, irrespective of the cross-sectional configurations. Notably, the maximum enhancements in axial peak stress for rectangular, square, and circular specimens are recorded at 45.96 %, 49.98 %, and 90.06 %, respectively, accompanied by corresponding increases in axial strains of 56.28 %, 53.74 %, and 86.00 %, respectively. The effectiveness of TRC confinement exhibits a substantial correlation with the geometry of the cross-sections. The circular ones have higher utilization rates followed by the square ones, and the rectangular ones with section aspect ratio greater than 1.0 have the lowest utilization rates. Furthermore, an evaluation of existing confinement identification models indicates that both the Mirminan Model and Wu Model could effectively delineate the hardening and softening characteristics of circular and rectangular specimens. However, there exists scope for improving the accuracy of these models. Therefore, a novel modified identification model, predicated on the Mirminan model, was proposed to refine the confinement assessment process. The revised modified confinement ratio (MCR) is recommended at a value of 0.045, aiming to elevate the precision and reliability of confinement evaluations in concrete structures.

Flexural behavior of coral aggregate concrete beams reinforced with BFRP bars under seawater corrosion environments

The combined utilization of fiber-reinforced polymer (FRP) composites with coral aggregate concrete (CAC) in remote island areas contributes to the reduced construction cost and construction period, offering a promising application. However, the evolution pattern and deterioration mechanism of flexural behavior for FRP bars reinforced CAC beams in marine service environments remains unclear. Thus, this paper investigates the flexural behavior of basalt-FRP (BFRP) bars reinforced CAC beams under seawater immersion and dry-wet cycle conditions using laboratory-accelerated aging methods. The research considers the effects of exposure temperature and corrosion age on their failure modes, flexural stiffness, ultimate loading capacity, and deformation deflection. The experimental results revealed that with the increase in corrosion age and exposure temperature, the amount of vertical cracks of CAC beams decreased and their crack depth also exhibited a slight reduction, but their crack width and spacing at failure significantly increased. After being attacked by seawater environments, the load-deflection curves of CAC beams experienced an enhanced slope of the ascending section (i.e., flexural stiffness), but this strengthened flexural stiffness did not result in an enhancement in the ultimate load capacity. Instead, with prolonged corrosion and higher temperatures, the ultimate load of CAC beams displayed significant degradation, accompanied by reduced mid-span deflection. After 12 months of seawater exposure to wet-dry cycle conditions at 60 °C, the ultimate load and mid-span deflection of CAC beams degraded by approximately 25.0 % and 56.6 %, respectively. The study also predicted the flexural bearing capacity of CAC beams utilizing existing specifications for FRP-reinforced normal aggregate concrete (NAC). It was concluded that the formulae for flexural loading capacity for FRP-reinforced NAC beams were still suitable for CAC beams.

Fiber-reinforced polymer waste in the construction industry: a review

Fiber-reinforced polymer waste in the construction industry: a review

Shear strengthening of simply-supported deep beams with openings incorporating combined steel-reinforced engineered cementitious composites and externally bonded carbon fibre-reinforced polymer sheets

Shear strengthening of simply-supported deep beams with openings incorporating combined steel-reinforced engineered cementitious composites and externally bonded carbon fibre-reinforced polymer sheets

A new finite element formulation for the lateral torsional buckling analyses of orthotropic FRP-externally bonded steel beams

Abstract The present study develops a new finite element (FE) formulation for the lateral torsional buckling (LTB) analyses of steel beams exteriorly bonded with orthotropic fiber-reinforced polymer (FRP) layers. The formulation considers shear deformations, partial material interaction, local and global warping deformations, and orthotropic FRP properties. The buckling responses of multispan FRP-bonded steel beams predicted by the present solutions are excellently validated against experiment and numerical solutions. As observed, the FRP strengthening is highly effective for the LTB resistance of steel beams. However, the LTB responses are strongly dependent on the orthotropic properties and strengthening lengths of FRP layers. The effects of shear deformations, span ratios, and loading conditions on the LTB responses are also quantified in the present study.

Flexural behavior of reinforced concrete beams externally strengthened with ECC and FRP grid reinforcement

To prevent premature delamination of FRP sheets in strengthened concrete structures, this study explores a novel method of laying a composite layer of fiber reinforced polymer (FRP) grid reinforced with engineering cementitious composite (ECC), designated as FGRE, and applied at the soffit of reinforced concrete (RC) beams to improve its flexural performance. Experimental studies were conducted on RC beams with different strengthening configurations to validate the effectiveness of the proposed strengthening method. In addition, analysis on the working mechanism and a parametric analysis were performed for the composite strengthened beam using nonlinear finite element modeling. The experimental results showed that FGRE composite layers can significantly improve the bearing capacity of RC beams. Compared with the control RC beam, the FGRE composite strengthening technique significantly increased the cracking load, yield load, and ultimate load of the beam specimens up to 28, 11, and 12 %, respectively. The ECC grout layer maintained a good bonding performance with the concrete substrate until beam failure. The flexural capacity of RC beams with the proposed FGRE strengthening systems is determined by the thickness of the ECC layer. In addition, the effect of strengthening on the flexural capacity was more significant when the yield and tensile strength of the steel reinforcement were lower. Moreover, the prediction deviation from the experimental data of the FE models and analytical models for the ultimate load-bearing capacity were less than 2 and 7 %, respectively. It can be concluded that the proposed FGRE system is a viable solution to enhance the flexural capacity of RC beams.

Creep assessment of thermoplastic materials for non-structural components in marine engines

The substitution of metals with fiber-reinforced polymers represents a well-established practice in the automotive and aerospace industries, driven by advantages such as cost reduction, weight savings, and environmental benefits. In the marine engineering sector, there is a growing interest in this metal replacement trend, particularly for non-structural components of marine engines that can be produced by adopting proper fiber-reinforced thermoplastic polymers. However, evaluating the suitability of such materials for marine applications necessitates a thorough understanding of their creep behavior, given the demanding operational environments and strict safety standards. The material selection process must intricately consider the material's susceptibility to creep through tailored material design methodologies. Moreover, redesign activities should aim to leverage the material's creep-resistant properties while ensuring adequate strength and simplifying installation procedures. Unfortunately, unlike the automotive industry, testing innovative plastic components in real environments on working engines is quite impossible. Neither engine manufacturers nor owners would allow jeopardizing their machinery by installing technologies not yet certified in a delicate environment such as a ship's engine room. In this context, finite element simulations offer a valuable tool to assess the material's creep performance and validate the proposed design when experimental measurements cannot be performed, hence predicting the component behavior over its intended lifetime. This study aims to exploit finite element analysis to evaluate the creep behavior and suitability for marine engine applications of a fiber-reinforced thermoplastic material, focusing on a camshaft cover of a four-stroke marine engine currently manufactured from aluminum alloy. Through numerical simulations, a commercial 30 % wt GFs/PA6,6 thermoplastic composite emerges as a promising candidate, demonstrating adequate creep resistance while significantly reducing weight, processing costs, and energy consumption. The results obtained from the present study lay the foundations for the adoption of such material-based technology also in the marine engine sector despite the difficulties coming from the peculiarities of this industry.

The performance of waste glass aggregate concrete GFRP-steel double-skin tubular columns under axial compression: Experimental study

In an attempt to alleviate the problem of growing waste glass and fully utilize the excellent material properties of fiber-reinforced polymer (FRP), a novel composite structure, waste glass aggregate concrete glass fiber reinforced polymer (GFRP)-steel double-skin tubular column (WGAC-DSTC), was proposed based on the FRP-concrete-steel double-skin tubular column (DSTC). In the present research, eight WGAC-DSTCs and four waste glass aggregate concrete-filled GFRP tubular columns (WGAC-CFFTs) were investigated to explore their mechanical performance under axial compression. Effects of diverse replacement ratio of waste glass aggregate, GFRP tube thickness, and hollow ratio of column on the mechanical performance of twelve columns were investigated. Meanwhile, this research analyzed and discussed the failure characteristics of columns, load-displacement relationship, ductility coefficient, stiffness coefficient, parameter analysis, ultimate bearing capacity, load-strain relationship and constraint factor. Besides, predicted formulae for estimating the ultimate bearing capacity of WGAC-CFFTs and WGAC-DSTCs were proposed. The research results indicated that (1) Failure mode of the WGAC-DSTCs displayed axial compression failure or shear failure and failure mode of WGAC-CFFTs exhibited mainly axial compression failure; (2) WGAC-DSTCs with 10 % replacement ratio of waste glass aggregate and hollow ratio of 0.4 indicated an increase in the ultimate bearing capacity by 17.24 % and 45.78 %, respectively, as the GFRP tube thickness increased from 1.5 mm to 2 mm and 3 mm. As the column's hollow ratio enhanced from 0.27 to 0.68, the column's ultimate bearing capacity significantly decreased by 7.08 %-60.55 %. The ultimate bearing capacity of WGAC-CFFTs showed a trend of first increasing by 8.33 %-9.41 % but then decreasing by 1.52 % when the replacement ratio of waste glass aggregate rose from 0 % to 30 %; (3) WGAC could replace natural aggregate concrete in traditional DSTC and the bearing capacity could be guaranteed. Besides, it's recommended to utilize 20 % replacement ratio of waste glass aggregate for the WGAC-DSTC whose ultimate bearing capacity enhanced by 15.17 % and possessing optimal mechanical performance; (4) The results calculated utilizing predicted formulae matched well with results of experiment. The application of waste glass aggregate concrete in DSTC can contribute to economic growth and environmental protection and meet the concept of sustainable development.

Shear behavior of pre-damaged RC beams strengthened with UHPFRC-CFRP grid layer

Ultra-high performance fiber reinforced concrete (UHPFRC), incorporating recycled tyre fibers, and carbon fiber-reinforced polymer (CFRP) grid were utilized to strengthen the pre-damaged reinforced concrete beams caused by coupling action of sustained loads and marine environment. The failure modes, characteristic loads, deflection features, and strain behavior of the strengthened beams with various types of UHPFRC, thicknesses of UHPFRC layers, CFRP grid ratios, and CFRP grid layout angles were investigated. The results indicated that they exhibited the typical shear failure, and their characteristic loads increased by 102 %–205 % compared to the undamaged beam. By increasing the thickness of the UHPFRC layer and the CFRP grid ratio, and adopting 45° CFRP layout angle, the cracking stiffness, energy absorption capacity, and shear force sharing ability would be further improved, while adopting the UHPFRC incorporating recycled tyre fibers would decrease the ductility. Moreover, analytical models were developed to calculate the ultimate loads, manifesting reasonable accuracy.

Optimization and statistical analysis on mechanical, thermal, wear and water-absorption characteristics of fragrant screwpine fiber-reinforced polymer biocomposites

Optimization and statistical analysis on mechanical, thermal, wear and water-absorption characteristics of fragrant screwpine fiber-reinforced polymer biocomposites

Damage detection and classification of carbon fiber-reinforced polymer composite materials based on acoustic emission and convolutional recurrent neural network

Carbon fiber-reinforced polymer (CFRP) composite laminates generate acoustic emission signals during the tensile process, which can be used to identify the type of acoustic emission (AE) signal at a given moment and determine the current damage condition. However, due to the large number and complexity of AE signals generated during the tensile process of carbon fiber composite laminates, it is challenging to manually identify and annotate them. To address this issue, we propose a low-complexity classification and detection network model based on the convolutional recurrent neural network (CRNN) architecture. First, we construct a dataset of composite damage signals for a single damage category and use log-mel spectrogram features to train the model for that specific category of damage signals. Then, we utilize the trained model to recognize the AE signals of CFRP composite laminates. The proposed method achieves an accuracy of 99.02% in classifying single damage modes in the test set and effectively distinguishes the types of AE signals generated during the damage process of CFRP composite laminates. Compared with the classic classification model using AE signals, the number of parameters in our proposed low-complexity CRNN model is reduced by 94.42%. It also demonstrates that 19.4% of the AE signals during the damage process are in a superimposed state, indicating the simultaneous occurrence of multiple damage modes in CFRP composite laminates.

Effect of MICP-recycled GFRP fiber on the self-repairing properties of concrete

The recycling of waste glass fiber-reinforced polymer (GFRP) holds paramount importance in realizing sustainability goals. This study aims to enhance the efficacy of fiber-reinforced self-healing concrete by employing recycled GFRP (rGFRP) fibers as carriers for Microbiologically Induced Calcite Precipitation (MICP). Preparations involved the development of pretreatment of rGFRP using microbial mineralization. Examination encompassed the assessment of interactions between microorganisms and fibers, microstructural characterization of self-healing specimens, and the strength of repaired specimens. This was conducted through the quantification of CaCO3 precipitation, strength tests, scanning electron microscopy (SEM), X-ray computed topography (CT) analysis, and mercury intrusion porosimetry (MIP). The results revealed that mineralized rGFRP fibers had little impact on the flexural and compressive strength of the reinforced mortar, while increased the tensile strength by up to 34.1 %. The mineralized rGFRP fiber-reinforced mortar achieved a repair rate of 84.5 % for cracks within 0.3 mm after 28 days of water curing. Comparatively, the mineralized rGFRP fiber bacterial mortar exhibited higher water absorption than its non-mineralized counterpart. Using untreated rGFRP fibers for self-healing can reduce the porosity and water absorption of the mortar to some extent, while adding mineralized rGFRP fibers for self-healing increased the crack area repair rate and also increased the porosity of the mortar. Therefore, using untreated rGFRP fibers as a carrier for microorganisms to repair mortar may present a more favorable choice. The results of this study will contribute to further improvement in the application of fiber-reinforced self-healing concrete in the engineering field.