For Fiber-reinforced Polymer Research Articles

Pullout Behavior of Connections Using Self-Drilling Screws for Pultruded Fiber-Reinforced Polymer Composites in Construction

Self-drilling screws provide an efficient and cost-effective way of joining lightweight structural members. In this work, the performance of self-drilling screw connections for fiber-reinforced polymer (FRP) structural members is investigated, for use in joining panels or plates to square hollow sections (SHSs). To investigate the resistance to pullout loads, tensile tests were conducted on connection specimens consisting of an FRP plate joined to an FRP SHS at a right angle. The specimens were prepared in five configurations, with differences in pultrusion directions of the FRP plate, screw gauge size, and coarseness of the screw threads. The specimens having fibers oriented transversely in the FRP plate exhibited a flexural failure of the plate and the lowest connection strength. The specimens having fibers oriented longitudinally in the FRP plate failed by the screw being pulled out from the FRP SHS profiles; shearing out of the thread in the screw hole. An increase in screw gauge size and thread coarseness was found to be beneficial to the connection strength. Further analysis was carried out to estimate the initial stiffness and the connection strength, by finite-element and analytical modeling, respectively. In addition, these analyses are able to correlate the average shear stress at failure of the connection specimens to the screw gauge size, thread coarseness, and effective contact area of the screws.

Numerical modelling of progressive failure in fiber-reinforced polymer composites under quasi-static loading

The demand for fibre-reinforced polymer (FRP) composites has been increasing in the last decades due to their excellent mechanical properties when compared to traditional materials. However, this class of materials has a more complex stress-strain response, which requires improved design methodologies. To take advantage of these unique characteristics and efficiently design structures, the usage of finite element analysis to predict progressive failure can be a powerful tool. Hence, in the present study, a method for predicting the initialization and progression of matrix and fibre damage in fibre-reinforced polymer composites was implemented using Puck's failure criterion and the Element Weakening Method (EWM). This numerical approach was validated using Digital Image Correlation (DIC) measurements and monotonic tensile tests to evaluate the ultimate load. The results show that the implemented method is able to predict the strain field at the specimen surface as well as the ultimate load level.

A Review of Recycling Methods for Fibre Reinforced Polymer Composites

This paper presents a review of waste disposal methods for fibre reinforced polymer (FRP) materials. The methods range from waste minimisation, repurposing, reusing, recycling, incineration, and co-processing in a cement plant to dumping in a landfill. Their strength, limitations, and key points of attention are discussed. Both glass and carbon fibre reinforced polymer (GFRP and CFRP) waste management strategies are critically reviewed. The energy demand and cost of FRP waste disposal routes are also discussed. Landfill and co-incineration are the most common and cheapest techniques to discard FRP scrap. Three main recycling pathways, including mechanical, thermal, and chemical recycling, are reviewed. Chemical recycling is the most energy-intensive and costly route. Mechanical recycling is only suitable for GFRP waste, and it has actually been used at an industrial scale by GFRP manufacturers. Chemical and thermal recycling routes are more appropriate for reclaiming carbon fibres from CFRP, where the value of reclaimed fibres is more than the cost of the recycling process. Discarding FRP waste in a sustainable manner presents a major challenge in a circular economy. With strict legislation on landfill and other environmental limits, recycling, reusing, and repurposing FRP composites will be at the forefront of sustainable waste-management strategies in the future.

Ply-stacking effects on mechanical properties of Kevlar-29/banana woven mats reinforced epoxy hybrid composites

Development of new hybrid laminated composites of Kevlar-29 (K-29)/banana fiber ( Musa acuminata) mats to meet future demand for fiber reinforced polymer (FRP) composites has been investigated. The different ply-stacking sequenced Kevlar (K)/natural (N) banana reinforced epoxy polymeric hybrid composite samples were designated as KN1, KN2, KN3, KN4, KN5 and KN6, in addition to NN7 and KK8 for single or non-hybrid FRP (control) composite samples. The ply-stacking effects on mechanical properties of all the laminated composite were investigated. The maximum tensile, flexural, impact and interlaminar shear strengths (ILSS) were obtained with sample KN4, because of the stacking of its Kevlar and natural banana mats, which was K2/N4/K2 of 8 layers and different from other stacking sequences. The percentage improvements on tensile strength of sample KN4 when compared with other hybrid composite samples KN1, KN2, KN3, KN5 and KN6 were 6.3, 4.4, 3.6, 13.1 and 11.3%, respectively. While, same optimum sample KN4 recorded highest flexural strength among hybrid samples with percentage improvements of 122.19, 70.97, 31.03 and 83.68% when compared with other hybrid samples KN2, KN3, KN5 and KN6, respectively. Similar trend of results was obtained for their tensile and flexural moduli. But, both hybrid composite samples KN3 and KN4 recorded higher impact strengths of 3.0 and 2.8 J, respectively, when compared with other hybrid counterparts. The tensile and flexural strengths of sample KN4 were 147.48 and 223.69 MPa, respectively. The tensile properties of various theoretical model were compared with experimental values.

Evaluation of aluminum to composite bonded lap joints

Adhesive bond is one of the best suitable joining technique for fiber reinforced polymer (FRP) composite structures used in aerospace applications. The weight of the composite structure greatly reduces by using structural adhesives for the joining processes. The surface preparation plays a major role for achieving the better bond strength. The present work deals with the validation of the bond strength of single lap joint(SLJ) with dissimilar adherends. The Al adherend has been considered for the surface treatments such as sand paper, NaOH and Resin pre-coating (RPC) for SLJs. The angle of contacts for the different surface treated specimens has been measured using the Young–Laplace method. X-Ray radiography and ultrasonic testing has been carried to verify the internal defects of bonded joint. The bonding strength was evaluated using a tensile and flexural test rig in polymer UTM. The bond strength of the RPC surface treated bonded joints showed better performance in terms of peak load and shear strength. The maximum shear strength has been increased up to 154.63 percentage when compared to untreated surface specimens, similar trend was observed in flexural strength. A numerical simulation model has been developed for validating the results. The comprehensive analysis for the de-bonded joints at maximum loading conditions has been done with morphological studies.

A study on the FRP composite’s matrix damage resistance by means of elastic wave propagation analysis

In the study, selected materials used for fiber reinforced polymer (FRP) composites’ matrices were subjected to Compact Tension (CT) test. During the test acoustic emission (AE) parameters were registered which let to introduce AE technique as a valuable and helpful method to monitoring and identifying polymer failures. Gathering of elastic waves propagation data was conducted with AMSY-5 acoustic emission system produced by Vallen Systeme GmbH, in accordance with Vallen’s user manual. Next, the analysis was done using dedicated software based on Fast Fourier Transform (FFT) and Wavelet Transform (WT). Four cured epoxy resins with different chemical composition were chosen to the investigation. Epidian 5 and Epidian 53 epoxy resins were hardened using the two following curing agents: Z1 triethylenetetramine curing agent and PAC polyaminoamide curing agent. The paper was prepared as a continuation of authors’ previously conducted research on evaluating FRP composites’ matrices strength and damage growth resistance, for the same four materials. Obtained AE results were then discussed in comparison to mechanical tests results, such as fracture toughness and stress/strain characteristics. Therefore, some interesting relations were observed between AE frequency and number of hits/counts which were affected by material composition and its fracture toughness. Moreover, microscopic analysis was conducted to present phenomena appeared on fracture surfaces of broken specimens, including microcracks, “crack arrest” as well as so called river patterns.

Nonlinear modeling on stiffness properties of diatomite-incorporated BFRPs with experimental strength and toughness evaluations

As the siliceous sedimentary rock, Diatomite (DT) with a unique mesoporous structure exhibits outstanding properties like easy accessibility, environment-friendly, nontoxicity-to-human, high mechanical properties, high resistance to heats and chemicals. However, the employment of DT as the additive for fiber-reinforced polymer (FRP) was rarely reported. Furthermore, to our best knowledge, the micromechanical modeling of DT regarding the special mesoporous structure has not been investigated. Therefore, this study proposed a nonlinear modeling method to investigate the stiffening behavior by incorporating DT into basalt fiber reinforced polymer (BFRP). The relationship between the stiffening behavior and the DT geometries was studied by implementing Cox model and Eshelby-Mori-Tanaka model. Experimental works were also performed to evaluate the strength, stiffness, and toughness of DT-incorporated BFRPs. The enhanced mechanical properties of DT-incorporated BFRPs revealed DT as the promising mesoscale additive for FRPs.

Hygrothermal Aging History of Amine-Epoxy Resins: Effects on Thermo-Mechanical Properties

Epoxy systems are widely used as matrix resins for fiber reinforced polymers (FRP) and, therefore, often have to withstand harsh environmental conditions. Especially in marine and offshore environments, moisture or direct water contact leads to water absorption into the epoxy resin. As a result, the mechanical properties change during application. Since diffusion at room or colder temperatures is slow, industry and academia typically use accelerated aging methods at elevated temperatures for durability prediction. However, as the water-polymer interaction is a complex combination of plasticization, physical aging, and molecular interaction, all of these mechanisms are expected to be affected by the ambient temperature. To reveal the impact of aging time and temperature on the thermo-mechanical properties of an amine-epoxy system, this publication includes various hygrothermal aging conditions, like water bath and relative humidity aging at temperatures ranging from 8°C to 70°C and relative humidity from 20% to 90%. Thus, it is demonstrated via long-term aging, DMTA and FTIR investigations that, e.g., strength, stiffness, strain to failure, and the glass transition temperature (Tg) can differ significantly depending on aging time and temperature. For example, it can be shown that water absorption at cold temperatures leads to the strongest and longest-lasting reduction in strength, although the maximum water absorption amount is lower than at higher temperatures. For the application, this means that strength differences of up to 26% can be obtained, depending on the aging method selected. Furthermore, it can be shown that conventional prediction models, such as Eyring correlation, which consider the mobility of the molecular structure for the prediction of thermo-mechanical properties, can only be used to a limited extent for prediction in hygrothermal aging. The reasons for this are seen to be, in particular, the different characteristics of the water-polymer interactions depending on the aging temperature. While plasticization dominates in cold conditions, relaxation and strong water-molecule bonds predominate in warm conditions.

Integrating mixture experiments and six sigma methodology to improve fibre‐reinforced polymer composites

AbstractThis article illustrates a Six Sigma project aimed at reducing manufacturing‐induced visual deviations for fibre‐reinforced polymer (FRP) composites. For a European composites manufacturer, such visual deviations lead to scrapping of cylindrical composite bodies and subsequent environmental impact. The composite bodies are manufactured through vacuum infusion, where a resin mixture impregnates a fibreglass preform and cures, transforming from liquid to solid state. We illustrate the define‐measure‐analyse‐improve‐control (DMAIC) steps of the Six Sigma project. Specific emphasis is placed on the measure and analyse steps featuring a 36‐run computer‐generated mixture experiment with six resin mixture components and six responses. Experimental analysis establishes causal relationships between mixture components and correlated resin characteristics, which can be used to control resin characteristics. Two new resin mixtures were developed and tested in the improve step using the understanding developed in previous steps. Manufacturing‐induced visual deviations were greatly reduced by adjusting the resin mixture to induce a slower curing process. Further refinement of the mixture was made in the control step. A production scrap rate of 5% due to visual deviations was measured during a monitoring period of 5 months after the resin mixture change. The scrap rate was substantially improved compared to the historical level (60%). The successful experimental investigation integrated in this Six Sigma project is expected to generate increased quality, competitiveness, and substantial savings.

Evaluation of Existing Bond‐Slip Relations for CFRP‐Steel Joints and New Model for Linear and Nonlinear Adhesives

Existing bond‐slip (τ‐s) relations for fibre‐reinforced polymer (FRP)‐steel joints employ different shapes and mathematical expressions, inferring that their predictions of failure load and mode, and other interface properties, might be inconsistent or inaccurate. In this study, predictions of four widely used τ‐s relations are evaluated using a large experimental database of 78 double‐lap FRP‐steel specimens. To facilitate the evaluation process, a robust finite element (FE) model is developed for each test, implementing data from either of the existing τ‐s relations to define the FRP‐steel interface. Comparisons between test and FE results indicated that the existing τ‐s models were unable of predicting the ultimate load (Pu) and effective bond length (Leff) of FRP‐steel joints, or the relation between Pu and bond length and that between Leff and FRP modulus (Ef). A new τ‐s model is developed based on an inverse FE simulation, comparison with experimental results, and regression analysis. It considers the effects of Ef, the type of FRP reinforcement (sheet or plate), and applies to both linear and nonlinear adhesives. The model predictions were validated by comparing with results from small bond tests and large FRP‐strengthened steel beams tested under bending, yielding excellent results for Pu, failure mode, and all other interfacial properties.

An investigation of hygrothermal effect on fatigue behavior of adhesively bonded anchorages for FRP tendon

The present study is concerned with the fatigue performance of the adhesively bonded anchorage for fiber reinforced polymer (FRP) tendons under hygrothermal environment. Series of static and fatigue tests were carried out to investigate the performance aging of the conditioned anchorage. The results show that the increase of the immersion duration and immersion temperature aggravates the fatigue performance's degradation. The tendon slippage is increased and the fatigue life is shortened. The extension of the immersion makes the anchorage’s fatigue life more sensitive to the stress amplitude. The initial stiffness is degraded as well. A decrease of the stress ratio increases the tendon slippage and speeds up the stiffness degradation during cyclic loading, regardless of the moisture conditions. For the specimens suffering from the same immersion conditions, the constant life diagram with a longer fatigue life is always below the CLD with a shorter fatigue life. But the CLDs may become distinctly different for the specimens subjected to different immersion conditions. Under any immersion conditions, it is applicable for the anchorage that the fatigue life can be improved by lowering the stress amplitude or the stress level.

Three-dimensional finite element simulations of milling carbon/epoxy composites

Mechanical milling is an extensively used finishing process for fiber-reinforced polymer (FRP) composites. To better understand the characteristics in machining FRPs for higher quality and efficiency, various studies have been conducted based on the finite element (FE) method as it can give insight on the mechanisms of material removal during cutting. Yet there is still a lack of three-dimensional FE models suitable for studying the mechanisms involved in the milling of FRPs. This paper developed a 3D meso-scale FE model for simulating CFRP milling by considering the damage behavior of the composites in 3D stress state. A progressive damage model was proposed and implemented using the VUSDFLD subroutine to predict the continuous damage evolution and stiffness degradation of the laminae. The simulated cutting forces and sub-surface damage agreed with the experimental observations. Numerical studies also showed that the failure criterion affected the simulated cutting forces and sub-surface damage. Upon increasing the tool edge radius, the simulated sub-surface damage affected a larger zone beneath the machined surface; however, the simulated cutting force and the sub-surface post-failure-initiation damage area experienced no obvious changes for the studied edge radius range.

Numerical simulation and ultimate deformation model of FRP-plated RC beams using H-type end anchorage

A ductile end anchorage (EA) system for fiber-reinforced polymer (FRP) sheets using H-type anchors has been recently proposed to compensate for the loss of deformation capability of externally-bonded FRP strengthened reinforced concrete (RC) beams. The effectiveness of the novel anchorage system has been verified by existing experimental results, while there still lack of deep investigations on its strengthening mechanisms and quantitative evaluations of the deformation capability. Against this background, high-fidelity finite element (FE) models of FRP-plated concrete members and RC beams with H-type end anchors are developed using LS-DYNA in this paper. The most proper simulation approach is identified through comparing with the experimental results. Based on the validated FE model, the mechanical behaviors and strengthening mechanisms of the end anchorage system are investigated thoroughly. Extended parametric studies are conducted to study the influences of FRP thickness and the width of the anchorage deformation section on the strengthening effectiveness. Based on the findings in the numerical simulation, the strength model of EA-strengthened beams is improved, and an analytical model for predicting the ultimate deflection of EA-strengthened beams is established.

Flexural strengthening of RC slabs using textile reinforced mortar improved with short PVA fibers

Textile reinforced mortar (TRM) composite is a viable and effective solution for fiber reinforced polymer (FRP) in some limited environments. Short and dispersible polyvinyl alcohol (PVA) fibers were applied in TRM matrix to remedy the lack of cracking strength of TRM composite, which was conducive to improve the cracking capacity and serviceability of flexural members. The effects of the matrix improved with and without short fibers and the reinforcing ratio of textiles on the flexural behavior of reinforced concrete (RC) slabs were studied in this work. Six RC slabs were fabricated and tested under four-point flexural loading, including one control slab, one with one layer of carbon TRM composite without short fibers, one with a matrix improved by short fibers and three with carbon TRM composite improved with short fibers. The results showed that short PVA fibers could increase the cracking capacity of RC slabs by 90 ~ 105%. The flexural capacity of strengthened slabs was increased with the increasing of reinforcing ratio of textiles. Based on the strain compatibility and force equilibrium, an analytical model was given. Moreover, the model was further improved according to the predicted formula of tensile strength for TRM. The theoretical and experimental results showed a very good agreement.

Development of ensemble machine learning approaches for designing fiber-reinforced polymer composite strain prediction model

Over the past few decades, it has been observed a remarkable progression in the development of computer aid models in the field of civil engineering. Machine learning models provide a reliable and robust alternative modeling design for solving complex engineering issues. The current study introduced three versions of newly explored ensemble machine learning models [extreme gradient boosting (XGBoost), multivariate adaptive regression spline (MARS) and random forest (RF)] for fiber-reinforced polymer (FRP) composite strain prediction. An experimental dataset were collected from the literature with total number of 729 experiments. The dataset is presented the FRP strain and its influential parameters including material geometry, strength properties, strain properties, FRP properties, and confinement properties. Five different input combination were built for the prediction process of the FRP composite strain. The current research results were validated against the well-established literature review empirical formulations and the machine learning models. In general, the modeling results confirmed the capacity of the proposed new ML models in predicting the strain enhancement ratio. The fifth input combination incorporated all the predators attained the best modeling accuracy results. However, the developed MARS model could achieve acceptable and superior prediction results using only strain properties parameters. Overall, the research finding confirmed the significant of the proposed ensemble ML as reliable alternative computer aid model for solving strain enhancement ratio problem.

ASCE 41 Seismic Assessment of FRP-Repaired Concrete Columns

Abstract Current standards establish methods for seismic assessment and objectives for retrofit of existing building structures and provide guidance for fiber-reinforced polymer (FRP)-based seismic...

Analysis-oriented stress-strain model for FRP-confined predamaged concrete

In the area of structural rehabilitation, extensive research has been conducted on stress-strain models for fiber-reinforced polymer (FRP)-confined concrete. Most of the existing models are developed from tests conducted under laboratory conditions. However, structural repair work on concrete structures is usually applied to damaged concrete, which has different mechanical properties compared with those of normal concrete. An analysis-oriented stress-strain model of FRP-confined predamaged concrete columns was studied in this paper for the first time. Two state-of-the-art databases, including the cross-sectional variables, concrete damage degree, and FRP-confined stiffness, were established by collecting 313 test data points from the literature. Parameters in the analysis-oriented stress-strain model were well developed for FRP-confined predamaged concrete. The proposed models are unified in terms of column cross sections, which can predict both FRP-confined circular and square columns by considering the factor of the corner radius ratio. The proposed model shows a good performance by comparison with test data.

A new formula of punching shear strength for fiber reinforced polymer (FRP) or steel reinforced two-way concrete slabs

This study is to propose a new punching shear strength formula for either FRP or steel reinforced two-way concrete slabs based on deterministic and probabilistic methods. Former is to employ the test to prediction ratio and latter is newly suggested approach that can guarantee more reliable prediction considering the significant uncertainties to minimize the possibility of over-estimating punching shear strength. For comparative study, the selected test results and the previous formulas are employed. The proposed new formula exhibits the most accurate test to prediction ratio among the existing formulas. Furthermore, it renders the lowest probability of exceedance that the predicted punching shear strength exceeds the nominal punching shear strength. The proposed new formula can minimize the probability of over-estimating the punching shear strength. Consequently, it can lead to enhance the accuracy, safety and reliability of punching shear strength design of either FRP or steel reinforced two-way concrete slabs.

Strengthening of unreinforced masonry walls against out-of-plane loads using carbon textile reinforced mortar optimized by short PVA fibers

Textile reinforced mortar (TRM) is a potential substitute for fiber reinforced polymer (FRP) and exhibits multi-cracking and strain-hardening behaviors under tensile loads. In this work, short polyvinyl alcohol (PVA) fibers were added into the TRM matrix to improve the toughness of matrix and serviceability of the composite. The effectiveness of the optimized TRM composite on strengthening masonry walls against out-of-plane loads was studied through 10 groups of unreinforced masonry (URM) walls, including 3 groups of reference specimens and 7 groups of TRM-strengthened specimens. The effects of textile reinforcing ratio, masonry strength and wall thickness on the deflections and bearing capacity were analyzed in this work. The failure mode, load–deflection response, ductility factor and energy absorbed capacity of specimens under out-of-plane loads are presented in the paper. The experimental results show that the optimized TRM composite can significantly improve the out-of-plane deformation and bearing capacity of masonry walls. Finally, the analytical models associated with the flexural and shear failures are proposed to predict the out-of-plane bearing capacity.

Confining Stress Path-Based Compressive Strength Model of Axially Loaded FRP-Confined Columns

Abstract The accuracy of existing compressive strength models for fiber–reinforced polymer (FRP)-confined concrete might vary significantly since most of them were established empirically, without ...