Pultruded Fiber-reinforced Polymer Research Articles

Enhancement of pin‐bearing behavior of pultruded FRP profiles by multi‐directional fiber architecture design

AbstractThe low resistance of bolted joints in pultruded unidirectional (UD) fiber‐reinforced polymer (FRP) profiles limits the load grade of truss bridges due to shear damage along UD fibers. To address this issue, this study proposes multi‐directional (MD) fiber architecture in FRP profiles to improve the ultimate bearing strength of bolted joints. The digital image correlation shows that the addition of off‐axis fibers in MD FRP profiles prevents shear failure in the UD FRP profile. As a result, the ultimate bearing strengths of MD FRP profiles are enhanced attributed to “off‐axis fiber tie action”. Micro‐computed tomography shows that fiber damage in MD FRP profiles emerged from outer plies, including fiber breakage in 90° and ±45° fibers and fiber kinking in 0° and ±45° fibers. The off‐axis fibers break due to tension when resisting the bearing load in MD FRP profiles. The fiber kinking indicated strength contribution from 0° to ±45° fibers. The fiber breakage and fiber kinking in each ply are dominant in bearing failure instead of delamination, which increases the bearing strength of the pultruded MD FRP profiles. These findings facilitate controlling damage evolution in the pultruded MD FRP profiles and designing high‐resistance bolted joints.Highlights Multi‐directional fibers improved bearing strengths of FRP profiles by 28.5%–33%. Off‐axis fibers bent under pin‐bearing, forming a tie action until breakage. Fiber breakage and kinking in each ply contributed to the bearing failure. Strain localization and redistribution were observed using DIC.

A novel crimped composite spline joint for pultruded FRP tubes: Conceptual design and tensile properties

Creating an effective composite joint between fiber-reinforced polymers (FRPs) and metallic profiles poses a significant challenge for heavy-loaded and lightweight emergency truss bridges. A novel crimped composite spline joint (CCSJ) was developed to achieve satisfactory engineering performance. Comparative tensile tests were conducted to evaluate the tensile properties and load-carrying mechanism of the CCSJ in comparison to those of joints without splines. Finite element (FE) analysis considering the crimping process and tensile loading was simultaneously executed and calibrated by experiments. Parametric simulation was supplemented to explore the key factors influencing the ultimate bearing capacity. The results indicated that the novel CCSJ features a simple preparation process, high connection efficiency, and large bearing capacity. A 60×6 mm hybrid FRP tube can withstand a maximum tensile load of 804 kN, achieving a connection efficiency of 61.3 % without the need for any secondary processing of the FRP tube. The CCSJ demonstrates a significantly higher connection efficiency of approximately 20 % compared to joints without splines. The ability of the CCSJ to effectively transfer significant axial loads through the interfacial friction effect is attributed to the increase in the interfacial contact area and preloading radial pressure resulting from the incorporation of pre-crimping splines. A typical failure mode involves interfacial slippage, resulting in satisfactory ductile behavior. The ultimate bearing capacity of the CCSJ is positively correlated with the crimping variables, friction coefficient, spline length, and number of splines. The FE model demonstrates sufficient accuracy in predicting the key behavioral indicators of the composite joint.

Buckling critical load prediction of pultruded fiber-reinforced polymer columns and feature analysis by machine learning

Buckling critical load prediction of pultruded fiber-reinforced polymer columns and feature analysis by machine learning

Experimental and finite element investigation on bolted connection in monolithic 3-dimensional cuff with pultruded box section

This article presents a concise and practical research study on the application of pultruded GFRP (Glass Fiber Reinforced Polymer) hollow sections in structural engineering. Experimental and computational investigations are conducted on the utilization of GFRP cuff components for joining tubular pultruded fiber-reinforced polymer (PFRP) beams and columns. Both adhesive bonding and bolted connections are explored for connecting PFRP box sections of beam-column interfaces. The study investigates twelve series of beam-to-column connections under monotonic loading conditions, varying cuff geometry. A finite element model is developed to represent the pultruded fiber-reinforced polymer box sections, columns, and cuff sections. The model’s accuracy in depicting frame stiffness and cuff damage patterns, considering different thicknesses and lengths of GFRP cuff connections, is validated against experimental test frame behavior. This research fills a gap in scientific inquiry regarding GFRP hollow profiles, providing valuable insights for structural engineering applications.

Effects of off‐axis load direction on tensile properties of pultruded fiber‐reinforced polymer bolted joints with multi‐directional fiber lay‐ups

AbstractThe limited joint performance of unidirectional (UD) pultruded fiber‐reinforced polymer (FRP) composites poses a critical issue that constrains their application in structural engineering. Moreover, UD pultruded FRP joints display a notable reduction in strength and stiffness as the load angle diverges from the fiber pultrusion direction. This paper presents a numerical study of bolted pultruded FRP joints with different off‐axis loading directions. Two types of fiber lay‐ups, including UD and multi‐directional (MD), were investigated. Static tensile tests were conducted with the loading direction parallel to the pultrusion. A finite element model was validated by the test results. Studied geometric parameters includes the end‐to‐diameter (e/d) ratio, width‐to‐diameter (w/d) ratio, and bolt arrangement. Compared to UD joints, MD joints exhibited significant advantages in strength and stiffness under off‐axis loading with an increasing of 78.3% at a loading angle of 90°. Compared to the end distance, plate width has a more significant impact on the off‐axis behaviors of MD joints, especially between 30° and 75°. Multi‐bolted joints were more susceptible to net‐tension failure than single‐bolted joints. Furthermore, classical laminate theory and Hashin failure criterion were employed to predict the resistance of single‐bolted FRP joints with different loading angles, achieving high accuracy.Highlights Under increased off‐axis loading angles, compared with unidirectional (UD) joints, multi‐directional (MD) joints display a less reduction in both strength and stiffness. At a 90° angle UD joints displayed only 21.7% of the joint resistance of MD joints. Compared to the end distance, plate width has a more significant impact on the off‐axis behaviors of MD joints, especially between 30° and 75°. Classical Laminate Theory, Hashin failure criterion were effectively utilized to predict the ultimate strength of pultruded joints under various tensile loading angles, yielding high accuracy. The error between theoretical and simulated values is within 20%.

Failure analysis of curved interface in pultruded fiber-reinforced polymer laminations using phase-field approach

Pultruded fiber-reinforced polymer (FRP) composites frequently encounter fabric misalignment perpendicular to the pultrusion direction, resulting in the formation of a curved interface. For predicting delamination in pultruded fabric with curved interfaces, the phase field model is employed in the commercial software ABAQUS® through the UMAT and HETVAL subroutines. In this study, the Mode-I, Mode-II, and mixed-Mode delamination behavior are successfully simulated to validate the efficiency of the proposed phase field method. A parametric study is conducted to investigate the impact of mesh size and phase field length on the simulation results. The recommended mesh size and interface length of the phase-field model are given. Finally, the validated phase-field model is used to predict the delamination behavior exposed to Mode-I, Mode-II, and mixed-Mode loading for the curved interface. The results indicate that specimens with a curved interface exhibit greater bearing capacity compared to those with a planar interface.

Numerical Evaluation of Lateral Torsional Buckling of PFRP Channel Beams under Pure Bending

The use of pultruded fiber reinforced polymers (PFRPs) in strengthening and sustainable design of bridges and other structures exposed to corrosion and resistance reduction factors is growing rapidly. However, a comprehensive understanding of the structural behavior of these materials under various loading conditions is crucial to unlock their full potential and promote their wider use in diverse structural and industrial applications. Pultrusion profiles can be also used as beams in bridges. One important aspect of the structural behavior of PFRPs is their buckling behavior, particularly in thin-walled open cross sections. Lateral torsional buckling is a probable instability mode for beams with thin-walled open cross sections that are not laterally restrained along their span. Therefore, research on the buckling behavior of PFRP members is essential. In this study, the analytical responses of channel-shaped PFRP beams in bridges under pure bending are calculated using an equation in the Eurocode 3 regulation. The buckling behavior of these beams is then investigated through numerical modeling using the finite element package Abaqus. A total of 75 specimens of PFRP channel profiles with different thicknesses in various spans and lateral restraint conditions are studied for their lateral-torsional buckling behavior. This study uniquely explores the behavior of PFRP beams with lateral restraints, a novel aspect in the field of lateral-torsional buckling research of PFRP beams. The results show that the analytical equation used for these beams needs to be modified to more accurately estimate the buckling loads of FRP beams under the conditions studied in this paper.

Built-Up Fiber-Reinforced Polymers (FRP) Profiles with Improved Shear Performance for FRP–Concrete Hybrid Section

Fiber-reinforced polymer (FRP)–concrete hybrid sections, composed of FRP profiles and a concrete slab, have gathered attention in construction due to their lightweight, easy installation, and high durability. However, the low shear strength and brittle behavior of commercially available pultruded FRP profiles often leads to brittle shear failure at low load levels. To enhance the shear strength and ductility, this study proposes a novel H-shaped FRP profile that is built from two U-shaped pultruded FRP profiles and a hand lay-up sandwiched core of multi-directional fibers. Direct shear tests showed that the built-up FRP profiles failed in pseudo-ductile mode while the U-shaped pultruded FRP profiles failed in brittle mode. Built-up FRP had 1.5 times the capacity and 2.8 times the ultimate redundancy compared to pultruded FRP. Additionally, flexural tests of FRP–concrete hybrid beams revealed that the webs of the built-up FRP profiles failed in a higher shear capacity with smeared cracks.

Determination of local buckling resistance of Fiber-reinfoced composite column having material imperfection by nonlinear buckling analysis

The pultruded Fiber Reinforced Polymer (PFRP) material manufactured by the “pultrusion” process has many applications in the construction industry. However, the production process involves several difficult-to-control stages, which can lead to material defects. This paper proposes a method to determine the local buckling resistance of FRP composite I column having material imperfection on the flanges and web. The research has shown that the buckling load determined in nonlinear buckling analysis is 15.5% higher than that obtained from eigenvalue solution (linear perturbation analysis). In addition, the post-buckling resistance of the column is 66.4% higher compared to the local buckling load. The author suggests that this modelling and determination method be widely applied to PFRP structures when studying the instability of columns or beams under various loadings and boundary conditions and different combinations of defects.

Cyclic compressive behavior and load–strain model of FRP–concrete​ double tube composite columns

A novel fiber-reinforced polymer (FRP)–concrete double tube composite column, consisting of an outer filament winding FRP tube, an inner pultruded FRP tube and infilled core concrete and ring concrete, has been proposed and experimentally investigated under monotonic compression by the authors. It exhibited obviously improved deformability than the traditional FRP-confined concrete columns. In this study, cyclic compression tests were conducted on the composite column to examine its structural behavior under cyclic loadings. Effects of different outer and inner FRP tube thicknesses and ring concrete types were investigated. Failure modes, cyclic load–strain responses and hoop strain behavior were presented and analyzed. Cyclic load–strain model, including the envelope model, unloading and reloading models, plastic strain equation and stress deterioration equation, was proposed to predict the cyclic compressive behavior of the FRP–concrete double tube composite columns. The proposed model was verified against the test results and exhibited good performance.

Numerical Parametric Study and Design of Pultruded GFRP Composite Channel Columns

This article reports the finite element (FE) investigation of the axial capacities of pultruded fiber-reinforced polymer (PFRP) composite channel columns. The nonlinear finite element model (FEM) was developed by using the ABAQUS package for glass fiber-reinforced polymer (GFRP) composite channel columns, which included geometric and initial geometric imperfections. The developed FEMs were verified against an experimental result available in the literature for GFRP channel columns. The validated FEMs were used to carry out the parametric study comprising 61 FE models to investigate the effect of different geometries, plate slenderness and the length of members on the axial capacities of GFRP pultruded channel columns. The results obtained from the parametric study were used to examine the accuracy of the current Italian guidelines, American pre-standard and the Direct Strength Method (DSM) proposed in the literature for GFRP channel profiles. Based on the obtained results, the suitability of the current design guidelines is assessed and, also, a new set of design equations is proposed to estimate the axial capacity of the pultruded GFRP channel columns. The new proposed set of reliable design equations witnessed a less scattered and a high degree of accuracy in determining the axial load capacity of the pultruded GFRP composite channel columns.

Local buckling resistance of Pultruded FRP columns: Theoretical predictions vs. Experimental study

Fiber-Reinforced Polymer (FRP) in the form of thin-walled plate structures are widely used in different engineering applications due to their outstanding features such as: high strength-to-weight ratio, excellent fatigue and corrosion resistance, etc. Due to the low stiffness, design of FRP material is usually governed by deformations or instabilities, rather than strengths. The development of a technical specification for design of FRP composite structures requires the understanding on buckling behaviour of FRP components. Local buckling of pultruded FRP column is a type of structural instabilities, occurs when the compressive stress reaches to the level that form waves in the flanges or web plates, causing the plastic deformation in the fiber which ultimately reduces its strength and stiffness. This study focuses on the local buckling resistance of pultruded FRP columns by experimental work. The test data were compared to current theoretical and numerical predictions. It is found that the difference of <15% between experiment and predictions can be acceptable. The author recommends that the Kollar’s equations to be used when predicting local buckling of Pultruded FRP column of I or channel shapes.

An experimental study of FRP truss side plate joint

Ultra-high performance concrete (UHPC) slab - fiber reinforced polymer (FRP) truss hybrid beam has been proved as an effective and advanced bridge system. However, the limited shear strength and the significantly lower transverse properties compared with its longitudinal counterparts result in low rigidity and load carrying capacity of bolted FRP truss joints, which governs the overall performance of the UHPC slab- FRP truss hybrid beam. A FRP truss side plate joint was proposed which integrates a steel U-shaped gusset plate (SUGP) to reinforce the traditional bolted joint. The proposed joint outperformed the conventional bolted joints in terms of the failure mode, load carrying capacity, and stiffness. The load carrying capacity of the proposed joint can be improved by up to 63% and the ultimate deflection was reduced by up to 33%. The effects of a variety of key design parameters, including adhesive, bolt diameter, pre-torque, and length and thickness of the gusset plate, on the performance of the SUGP joint were evaluated. Test results showed that: (i) the adhesive between the pultruded FRP profiles and the steel gusset plate could increase both the initial stiffness and ultimate load carrying capacity of the steel SUGP joint; (ii) length and thickness of the steel gusset plate did not result in notable difference in the behavior of the steel SUGP joint for the range of parameters investigated herein; and (iii) application of pre-tightening torque within the thread of the bolts slightly improved the load carrying capacity but significantly improved the ductility of the steel SUGP joint.

Influence of geometry on failure modes of PFRP single bolted connections

This paper investigates the ultimate strength, stiffness, and failure modes of pultruded fibre-reinforced polymer (PFRP) single bolted connections subjected to tension. An experimental programme of 72 PFRP connections was conducted. The parameters of the test specimens include plate thickness, bolt diameter, edge distance, end distance and loading angle. A total of 10 different failure modes, including three new combinations of the failure modes were reported. The mechanics reasoning for those new failure modes were discussed. The strength, stiffness and failure modes were interpreted combinedly for determining the effective geometry. The test results were correlated with currently available design guidelines and existing literature for formulating the minimum geometry limitations. A new geometric ratio is proposed for connection characterisation. The appropriateness of the current design standards for minimum geometry is evaluated.

The Effects of Eccentric Web Openings on the Compressive Performance of Pultruded GFRP Boxes Wrapped with GFRP and CFRP Sheets.

Pultruded fiber-reinforced polymer (PFRP) profiles have started to find widespread use in the structure industry. The position of the web openings on these elements, which are especially exposed to axial pressure force, causes a change in the behavior. In this study, a total of 21 pultruded box profiles were tested under vertical loads and some of them were strengthened with carbon-FRP (CFRP) and glass-FRP (GFRP). The location, number and reinforcement type of the web openings on the profiles were taken into account as parameters. As a result of the axial test, it was understood that when a hole with a certain diameter is to be drilled on the profile, its position and number are very important. The height-centered openings in the middle of the web had the least effect on the reduction in the load-carrying capacity and the stability of the profile. In addition, it has been determined that the web openings away from the center and especially the eccentric opening significantly reduces the load carrying capacity. Furthermore, when double holes were drilled close to each other, a significant decrease in the capacity was observed and strengthening had the least effect on these specimens. It was also determined that the specimens reinforced with carbon FRP contribute more to the load-carrying capacity than GFRP.

Compressive Behavior of Pultruded GFRP Boxes with Concentric Openings Strengthened by Different Composite Wrappings.

Web openings often need to be created in structural elements for the passage of utility ducts and/or pipes. Such web openings reduce the cross-sectional area of the structural element in the affected region, leading to a decrease in its load-carrying capacity and stiffness. This paper experimentally studies the effect of web openings on the response of pultruded fiber-reinforced polymer (PFRP) composite profiles under compressive loads. A number of specimens have been processed to examine the behavior of PFRP profiles strengthened with one or more web openings. The effects of the size of the web opening and the FRP-strengthening scheme on the structural performance of PFRP profiles with FRP-strengthened web openings have been thoroughly analyzed and discussed. The decrease in load-carrying capacity of un-strengthened specimens varies between 7.9% and 66.4%, depending on the diameter of the web holes. It is observed that the diameter of the hole and the type of CFRP- or GFRP-strengthening method applied are very important parameters. All CFRP- and GFRP-strengthening alternatives were successful in the PFRP profiles, with diameter-to-width (D/W) ratios between 0.29 and 0.68. In addition, the load-carrying capacity after reinforcements made with CFRP and GFRP increased by 3.1–30.2% and 1.7–19.7%, respectively. Therefore, the pultruded profiles with openings are able to compensate for the reduction in load-carrying capacity due to holes, up to a D/W ratio of 0.32. The capacity significantly drops after a D/W ratio of 0.32. Moreover, the pultruded profile with CFRP wrapping is more likely to improve the load-carrying capacity compared to other wrappings. As a result, CFRP are recommended as preferred composite materials for strengthening alternatives.

Developing a hybrid FRP-concrete composite beam

Current materials engineering trends put forward the development of efficient structural solutions. The steel replacement with fiber-reinforced polymers (FRP) exemplifies the key to the corrosion problem. However, the relatively low deformation modulus of typical FRP materials raises the deformations of the structural components. Together with the self-weight reduction increasing the kinematic displacements, the latter issue makes developing hybrid structures comprising compression-resistant concrete and high-performance in tension FRP profiles important. Although such hybrid systems are applicable for bridge engineering, the uncertainty of the inter-component bonding properties complicates developing these innovative structures, including the design models. The typical solution focuses on the local bond improvement, e.g., employing FRP profile perforation and mechanical anchorage systems. However, this study introduces an alternative solution, using the stress-ribbon bridge structural system for creating the hybrid beam prototype, which combines the synthetic fiber-reinforced concrete slab and pultruded FRP profile fixed on the supports. This work exemplifies the structural development concept when the finite element (FE) modeling outcome defines the target reference of the design procedure. Thus, on the one hand, this innovative structure simplifies the corresponding numerical (FE) model, which assumes the perfect bond between the components of the hybrid beam system. On the other hand, the solution to the support problem (resulting from a low resistance of pultruded FRP profiles to transverse loads) improves the structural performance of the bridge prototype, doubling the structure’s flexural stiffness and load-bearing capacity regarding the weak concrete supports’ system. The bending tests proved the adequacy of this solution in describing the design reference for further development of the proposed structural concept.

Design optimisation of hollow box pultruded FRP profiles using mixed integer constrained Genetic algorithm

Hollow Pultruded Fibre-Reinforced Polymer (FRP) profiles are increasingly used as structural members in many infrastructure applications. However, there is still a lack of coherent design methodology considering local buckling. This research presents a fast-converging numerical approach combining the Finite Element Modelling (FEM) and the Genetic Algorithm (GA) to design the optimal configuration of the geometry and layup design parameters against local buckling under two separate loading conditions of axial compression and four-point bending. The FEM model was validated experimentally. The mixed-integer constrained optimisation GA code (MI-LXPM) was used to solve the problem. The optimisation objective was to minimise the manufacturing cost per metre of pultrusion while maintaining the same stiffness and strength properties of the control profile. The Kriging model was used to interpolate the design space based on the intermediate optimisation data output and produce a practical design chart linking the profile geometry to the ultimate load capacity. An experimental case study on the design of a hollow rectangular pultruded FRP girder demonstrated the proposed optimisation approach. The new design saved 10.6% of the material cost and enhanced the ultimate strength by 41%.

Influence of elevated temperatures on the mechanical properties of glass fibre reinforced polymer laminates produced by vacuum infusion

One of the main concerns about the use of fibre reinforced polymer (FRP) materials in civil engineering is their behaviour when subjected to elevated temperatures and under fire exposure. In fact, considerable reductions in the mechanical properties of FRP materials have been reported even at moderately elevated temperatures (e.g. 50–150 °C), due to the glass transition of their polymeric matrices. Most previous studies on this topic have focused on the mechanical characterization at elevated temperatures of quasi-unidirectional pultruded FRP composites, namely profiles, rebars and strips; much less data is available about FRP materials produced with different methods with more balanced fibre architectures. This paper aims at contributing to fulfil this knowledge gap by presenting experimental and analytical investigations about the effects of elevated temperatures on the mechanical properties of glass-FRP (GFRP) laminates produced by vacuum infusion, with a balanced fibre architecture, representative of typical GFRP face sheets of sandwich panels used in civil engineering structures. The experimental programme included (i) tensile tests up to 300 °C, (ii) compressive tests up to 250 °C, and (iii) shear tests up to 200 °C, all under steady state conditions, as well as dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). The results obtained confirm that the mechanical properties of the GFRP laminates are severely affected by the temperature increase, especially those that are matrix-dependent: compared to room temperature, at 200 °C, the shear modulus was reduced by 88%, whereas the compressive strength and modulus were reduced by 96% and 67%, respectively. The tensile properties (fibre-dominated) were much less affected - at 200 °C, the tensile strength and modulus were reduced by 40% and 48%, respectively. Finally, the suitability of four empirical models and of a design-oriented temperature conversion factor to take into account the variation of the GFRP mechanical properties with temperature was assessed. Overall, the empirical models presented good agreement with the test data, and the temperature conversion factor provided conservative estimates of the material properties, attesting its suitability for design purposes.

Sensitivity analysis of shear modulus on Lateral – Torsional Buckling of PFRP Beam

An elastic constant of Pultruded Fiber Reinforced Polymer (PFRP) has a wide range of value, especially for the shear modulus which is generally ranging from 3 to 5 GPa. Lateral – torsional buckling (LTB) is a type of global buckling occurs when the beam is under bending about its major axis. The instability happens when the beam twists and move laterally. This research analyses and evaluates the sensitivity of changing the shear modulus on the critical LTB load of PFRP beam using a numerical approach by ABAQUS. It is found that the critical LTB load increases 17% for short beam and 27% for long slender beam when shear modulus increases from 3 to 5 GPa. The influence of changing of shear modulus is reduced when load position moves from top to bottom flange. It is suggested that the determination of proper shear modulus (upper and lower bound of values) is of crucial in design and in validating the compatibility between experimental work and numerical or theoretical analysis.