Fiber Reinforced Polymer Laminates Research Articles

A novel electromagnetic nondestructive testing method for carbon fiber reinforced polymer laminates based on power loss

Carbon fiber reinforced polymer (CFRP) is electrically conductive, so eddy-current testing (ECT) method can be used for non-destructive testing (NDT) of CFRP. However, due to the low bulk conductivity of CFRP, the high-frequency systems are considered more suitable in conventional ECT method. But high-frequency systems are more difficult to build and implement. Concerning this issue, a novel CFRP non-destructive testing method with working frequency up to 750 kHz is proposed in this paper. The method is based on power loss of the micro-probe, which makes use of Joule heat generated by the eddy-current in carbon fibers. The method is combination of ECT and thermography principle, which not only maintains higher signal-to-noise ratio at low frequency but also avoids the problems caused by high frequency. An experimental setup using C-Scan technique was built. Scanning experiments on orthogonally woven laminates, two-directional laminates, four-directional laminates and unidirectional laminates at various working frequencies including 750 kHz were carried out. Scanning results prove that the method can detect the cracks and fiber distribution of CFRP at low frequency up to 750 kHz, and has a good signal-to-noise ratio.

Analytical study on the effectiveness of hybrid FRP strengthening on behaviour of slender reinforced concrete square columns

Strengthening of reinforced concrete (RC) columns using external bonded (EB) fibre-reinforced polymer (FRP) composites has become popular since last three decades. The effectiveness of EB confinement reduces for slender columns due to the secondary moment arising from applied axial loads. The effectiveness of the hybrid FRP strengthening system for enhancing the performance of RC slender square columns is analytically investigated. Hybrid FRP strengthening uses a combination of EB FRP wrapping for confinement and longitudinal near-surface mounted (NSM) FRP laminates for bending. Hybrid FRP strengthening aims to reduce the second-order lateral displacement of the slender RC column and increase its axial capacity and flexural stiffness. A numerical modelling approach is presented to capture second-order effects in hybrid FRP strengthened slender RC square column based on Newmark's central difference scheme. Both material and geometric nonlinearities are accounted for in the calculations. The predictions are validated with test results available in the literature. After validation, a parametric study is carried out to evaluate the influence of slenderness ratio, the number of EB FRP layers, NSM laminate ratio, modulus of NSM laminate, and buckling of FRP composites. Results show that hybrid FRP strengthening is effective in improving the performance of slender RC columns.

Numerical flexural strengthening investigation of timber-CFRP composite beams

Timber is among the most important construction materials with unique characteristics in comparison with other materials for instance steel and concrete. It is one of the first sustainable materials and continues to be a favoured selection in the current infrastructure. In the last decade, to refine the mechanical properties of timber, structural elements reinforcing fiber polymers have developed. In this study, to improve the flexural properties of timber beams, carbon fiber-reinforced polymer (CFRP) laminates are attached externally and internally between timber layers. For numerical simulations, first finite element model was validated according to the data obtained from experimental tests, then a series of simulations with different parameters were executed to investigate the flexural behaviour. The considered parameters include CFRP laminates’ existence and position. The linear elastic orthotropic material model was used to represent linear behaviour while the 3D anisotropic Hill material model was assumed to define the nonlinear behaviour of the material. Finite element simulations are executed by commercial software ABAQUS.

Post-fire analysis and numerical modeling of a fire-damaged concrete bridge

Extreme fire hazard is not a typical design consideration for highway bridges because of the low probability of occurrence. However, historically, fire events caused higher numbers of bridge collapse than earthquakes on the nation’s highways, showing the criticality of fires on highway bridges. Despite this fact, the during and post-fire performance of highway bridges is still not well-understood. The current study evaluated the post-fire performance of a fire-damaged concrete bridge in Irving, Texas, through theoretical modeling and in-situ testing. The bridge sustained significant damage when a fuel tanker crashed and caught fire in May 2005, and was then repaired and strengthened with carbon fiber reinforced polymer laminates. A three-tiered calibrated numerical modeling scheme involving computational fluid dynamics-based fire model, heat transfer and stress analysis was developed. The accuracy of the scheme was validated through static bridge load testing. It was found that the modulus of elasticity of the fire damaged section of the bridge was reduced by more than 50%. The numerical model represented the bridge fire with reasonable accuracy. The model thermal profiles showed good correlation with actual post-fire observations. The concrete modulus of elasticity decreased by 10% and 75% for the girder and the deck, respectively. The actual strain and displacements from the load testing closely matched those from the calibrated model.

A Methodology for Evaluating the Progression of Damage in a Glass Fibre Reinforced Polymer Laminate Subjected to Vertical Weight Drop Impacts.

This study describes a methodology that allows evaluating the behavior of a glass fibre reinforced polymer (GFRP) laminate impacted by a vertical weight drop, analyzing the damage that occurred inside. The purpose of the designers was, by means of characterization tests of the curing processes, evaluation of the cohesion of a particular laminate, application of vertical tests by weight drops and with the use of the readings of an accelerometer in a single direction, know the trend of how intralaminar breaks in the matrix and interlaminar breaks between layers occur. It is proposed to establish the behavior of the laminate before the tests by analyzing curing times, for after the tests by observations with penetrating fluorescent inks. This allows researchers to know the response of the laminate to the loads imposed on the applied structure. For the tests, prepreg material cured outside the autoclave in an oven was used and qualitative quantification of the damage by observing sections of the tested material infiltrated with penetrating fluorescent ink exposed to ultraviolet light.

Shear and Flexural Strengthening of Steel Beams with Thick Carbon Fiber Reinforced Polymer Laminate

In this paper, shear and flexural behavior of structural steel beams strengthened by high modulus carbon fiber reinforced polymer (CFRP) laminates are presented. Totally, 18 steel specimens including 6 un-strengthened beams as control specimens and 12 strengthened steel beams with simple supports were tested under 3-point bending test set-up. All specimens were strengthened using the bonded system. Influence of different parameters including length of steel beams, section size of specimens, number of CFRP laminates, and location of CFRP laminates were studied. Based on anticipated failure modes, the bonded laminates were implemented on the surface of tension flange, compression flange, and web of beams. Three failure modes of flexural, shear, and lateral-torsional buckling failures were observed in the tested beams. The main goal of these experiments was to evaluate the enhancement in load capacity, beam ductility, and initial stiffness. The results showed that the yield load, ultimate load capacity, and energy absorption of strengthened steel beams improved up to 15, 29 and 28 percent, respectively. Finally, in order to predict test results and compare the actual and predicted valves, analytical and numerical studies were carried out.

Multi-objective optimization of adhesively bonded single-lap joints of carbon fiber reinforced polymer laminates based on genetic algorithm

To improve its structural performance, the multi-objective optimization of adhesively bonded single-lap joints of carbon fiber reinforced polymer (CFRP) laminates was carried out based on genetic algorithm. Firstly, finite element (FE) models were constructed using 3D Hashin damage criteria and triangle cohesive zone model (CZM), those well capture the intra-laminar, inter-laminar and adhesive damages during the tensile loads, respectively. And its effectiveness was verified through experiments. Secondly, using the Latin hypercube sampling (LHS) method and polynomial response surface method (RSM), a multi-objective optimization agent model with tensile strength and shear strength as the objective function was established based on lap the length, the adhesive thickness and the width of the bonded parts. Finally, the tensile strength and shear strength agent model was optimized to obtain Pareto solution set based on genetic algorithm (GA), and the Pareto solution set was sequenced by technique for order preference by similarity to ideal solution (TOPSIS) method to obtain the optimized single-bonded joint structure design scheme. The result shows that the experimental measurements of tensile load tests concur with the numerical predictions and validate the FE models. The tensile strength and shear strength of the adhesively bonded single-lap joints of CFRP laminates have a significant correlation with the lap length, the thickness of the adhesive layer and the width of the bonded parts. Compared with the numerical simulation results, the error of the quadratic response surface proxy model results is less than 2.3%. Compared with the conventional single lap bonding structure, the tensile strength and shear strength are increased by 2.65% and 17.24%, respectively.

Parametric study of carbon fiber reinforced polymer laminates geometry on the mechanical behavior of strengthened reinforced concrete beams under standard four‐point bending test

AbstractThis article presents the results of an experimental parametric study on reinforced concrete (RC) beams strengthened by an external bonding of carbon fiber reinforced polymer (CFRP) laminates to their tensile faces. This experimental work investigates the effects of both CFRP laminate sizes and concrete compressive strength on the flexural behavior of strengthened RC beams. Accordingly, a total of 10 RC beams have been cast, eight of which were strengthened in flexure by external bonding of CFRP laminate by varying its sizes, such as length, width, and thickness, and two control beams. All prepared beams were tested under standard four‐point bending until failure. The load–deflection response curves and failure modes were recorded and compared. Results show that failure modes of strengthened beams varied with the CFRP laminate characteristics from steel yielding, CFRP peeling off, and interfacial debonding laminate, Also, the load–deflection curves showed a considerable increase in the load capacity by up to 1% and 112%, compared to the unstrengthen beam, if the CFRP laminate length ratio passes from 0.40 to 0.94, respectively. In addition, the load capacity contribution increases from 56% to 112% when the CFRP laminate width increases from 50 to 100 mm. On the other hand, when laminate thickness increases from 1.2 to 2.4 mm, the load capacity contribution increases from 56% to 76%. Finally, it is concluded that the mechanical behavior of strengthened beam is strongly affected by the CFRP laminate sizes.

SCFs in tubular X-joints retrofitted with FRP under out-of-plane bending moment

This paper presents a parametric study on the stress concentration factors (SCFs) on the chord member in tubular X-connections reinforced with fiber reinforced polymer (FRP) under out-of-plane bending moment. For this aim, a FE model was generated and validated using several available experimental tests. After that, a set of 276 FE models was created to evaluate the effect of the FRP (layer number, orientation, and material) and joint geometry (γ, τ, and β) on the SCFs. In these FE models, the contact between the FRP sheets and the steel members (chord, weld, and braces) was modeled. Results indicated that the rise of the FRP laminate number causes a notable drop of the SCFs, especially in the connections with big γ. Moreover, the increment of the elastic modulus of FRP along the fibers causes a notable decrease of the SCF. Results showed that, for certain geometrical parameters set, the SCF in an X-connection retrofitted with carbon fiber reinforced polymer (CFRP) can be down to 23% of the SCF in the associated un-retrofitted connection. Despite the notable efficacy of the FRP sheets on the drop of the SCFs in the X-connections, there is not any study or equation on the X-connections with FRP. Therefore, an equation was proposed for quantifying the SCFs in the X-connections with FRP.

Computational Investigation of Square Embedded Delamination of a Composite Laminate using Surface based Cohesive Contact Behavior

The fiber reinforced polymer laminates have found extensive applications because of its advantages over other materials in terms of thrust to weight ratio, strength to weight ratio, manufacturing benefits such as tailoring, resistance to erosion and corrosion and so on. In the transverse direction, strength, stiffness and stability are comparatively less so that a failure mechanism called interface delamination comes into picture due to poor manufacturing or when tools are dropped that would create an impact load. In this paper, Surface based Cohesive contact behavior is implemented at the interface between base and sub laminate to investigate for 60mm square embedded buckling driven delamination growth. The computational prediction of delamination growth initiation is obtained by solving a HTA/6376C composite laminate specimen for geometric non linearity using SC8R continuum shell elements of Abaqus CAE and by plotting the inplane loads versus out of plane displacements.

Computational Investigation of through the Width Delamination of a Composite Laminate using Surface based Cohesive Contact Behaviour

The fiber reinforced polymer laminates have found extensive applications because of its advantages over other materials in terms of strength to weight ratio, manufacturing flexibility and so on. But in the transverse direction, strength is comparatively less so that a failure mechanism called delamination will occur in case of poor manufacturing or when tools are dropped. In this paper, Surface based Cohesive contact behavior is implemented at the interface between base and sub laminate to investigate for 60mm through the width buckling driven delamination growth. The computational prediction of delamination growth initiation is obtained by solving a HTA/6376C composite laminate specimen for geometric non linearity using SC8R continuum shell elements of Abaqus CAE and by plotting the inplane loads versus out of plane displacements.

Computational Investigation of through the Width Delamination of a Composite Laminate using VCCT

The fiber reinforced polymer laminates have found extensive applications because of its advantages over other materials in terms of strength, stiffness, stability, weight saving features, resistance to corrosion and erosion and many more. But due to poor transverse direction strength, a failure mechanism called delamination will occur in case of poor manufacturing or when tools are dropped which would make an impact. In this paper, VCCT is implemented at the interface between base and sub laminate to investigate for 20mm through the width buckling driven delamination growth. The computational prediction of delamination growth initiation is obtained by solving a T300/976 specimen for geometric non linearity using SC8R continuum shell elements of Abaqus CAE and by plotting the required energy release rate at the edge of delamination geometry.

Structural analysis of FRP laminate with triangle convexities (FLTC) using the VAM-based equivalent model

To comprehensively study the static and dynamic behaviors of fiber-reinforced polymer (FRP) laminates with triangle convexities (FLTC) under different boundary conditions and avoid structural damage, an equivalent model of the FLTC was developed based on the variational asymptotic method. A constitutive model of the unit cell was developed to obtain the constitutive relations that could be used as effective plate properties of the two-dimensional equivalent plate model (2D-EPM). The accuracy of the equivalent model and the obtained equivalent stiffness were verified by comparing with the global displacement, buckling modes, critical loads, free-vibration modes, and frequencies of a three-dimensional finite element model (3D-FEM). The influence of different layup configurations, structural arrangements and geometric parameters on the effective performance of the FLTC are discussed. The results showed that the critical load and natural frequencies obtained using the equivalent model were in good agreement with those obtained from the 3D-FEM, and the local field distributions within the unit cell of the FLTC were well captured. The free vibration and global buckling of the FLTC were closely related to the geometric sizes and layup configurations. Most importantly, the calculation efficiency was greatly improved. The research results provide a reference for the practical application of the FLTC in civil engineering.

Computational Investigation of Square Embedded Delamination of a Composite Laminate using VCCT

The fiber reinforced polymer laminates are widely implemented in aviation industry due to its advantages and applications other materials in terms of strength to weight ratio, dsign features and many more. The strength of the interface compared to longitudinal and lateral directions of the plies are comparatively less and give rise too poor transverse direction strength. Hence a failure mechanism called delamination will occur in case when tools are dropped or due to poor manufacturing which would give rise to interface delamination. In this paper, VCCT is employed at the interface between base and sub laminate to investigate for a square shape delamination geometry of 20mm buckling driven delamination growth. The computational prediction of delamination growth initiation is obtained by solving a T300/976 specimen for geometric non linearity using SC8R continuum shell elements of Abaqus CAE and by plotting the required energy release rate at the delamination geometry.

Monotonic and cyclic compression characteristics of CFRP confined masonry columns

The lateral confinement of masonry columns using composites have shown to improve their strength and ductility. Although, several research studies were focused on investigating the monotonic compression behaviour of confined masonry columns in the past, their cyclic compression characteristics, which are necessary for seismic and dynamic analyses, are not well investigated. Thus, an attempt has been made to experimentally characterise the confined masonry columns under monotonic and cyclic axial compression in this research. In total, 36 masonry columns were built and tested under monotonic and cyclic compression. Out of 36 columns, twelve columns were unconfined and tested under monotonic compression, while the rest of the columns were confined with Carbon Fibre Reinforced Polymer (CFRP) laminates and tested under monotonic and cyclic compression. The experimental results are presented in terms of observed failure modes, compressive strengths and stress–strain curves. Cyclic loading protocol was displayed to marginally reduce the compressive strength of CFRP confined masonry columns by 6% to 13% compared to the compressive strengths obtained through monotonic compression testing. The analytical models available to predict the monotonic stress–strain curves were used to predict the cyclic envelop stress–strain relationship of confined masonry columns. Finally, the best fit analytical model to predict the cyclic envelop compression behaviour of CFRP confined masonry columns has been proposed.

Shear behavior of RC T-beams externally strengthened with anchored high modulus carbon fiber-reinforced polymer (CFRP) laminates

This work presents the results of an experimental study on the effect of shear strengthening of reinforced concrete (RC) T-beams with high modulus carbon fiber-reinforced polymer (CFRP) laminates and anchors. The parameters considered were the anchor insertion angle and dowel diameter. Experimental results showed that the addition of U-wraps led to a 61% increase in shear strength compared to the control specimen, but failure was dominated by the brittle debonding of the CFRP laminates, with no ductility exhibited prior to failure. Anchoring the CFRP laminates with CFRP spike anchors further enhanced the shear capacity of the U-wrapped specimen by 14–51% and improved the shear contribution of the FRP by 36–136%, respectively. In addition, the CFRP anchorage system highly utilized the strain in the CFRP laminates and substantially enhanced the ductility of the specimens. A comparison between the shear resistance provided by the FRP reinforcement of the tested specimens and predicted capacity using current design guidelines showed that the current provisions could be safely applied to design RC T-beams strengthened in shear with anchored CFRP U-wraps.

Low-velocity impact behaviors of glass fiber-reinforced polymer laminates embedded with shape memory alloy

Shape memory alloy wires embedded glass fiber-reinforced polymer (SMA-GFRP) laminates have great potential in engineering applications. In this paper, low-velocity impact behaviors of SMA-GFRP laminates are investigated under different initial impact energies. Firstly, tensile tests are conducted on a single SMA wire and SMA-GFRP laminates to obtain their mechanical parameters. Then, finite element models are established to describe the mechanical behaviors of SMA-GFRP laminates. Finally, experiments and simulations are carried out to explore the low-velocity impact behaviors and damage mechanisms of SMA-GFRP laminates. The results show that, due to their excellent superelastic deformation and shape recovery ability, SMA wires can improve the damage tolerance and impact resistance of GFRP laminates. The damage patterns and mechanisms of SMA-GFRP laminates vary with the increase of initial impact energy. Under low and medium initial impact energies, deformation can be mostly recovered, while under high impact energy, laminates are almost penetrated and deformation cannot be recovered because of breakage of SMA wires. The damage area of laminates increases first and then decreases as the increase of impact energy. The findings provide a guidance for design and evaluation of SMA-GFRP laminates with low-velocity impact resistance.

Numerical modeling of two-dimensional delamination growth in composite laminates with in-plane isotropy

The two-dimensional (2D) delamination growth in fiber-reinforced polymer (FRP) laminates with in-plane isotropy under Mode I loading condition was numerically investigated using finite element analyses. Two sizes of plate models were developed, focusing on different fracture stages. Cohesive elements were employed to simulate the fracture behavior in the presence of large-scale bridging (LSB). The influences of the pre-crack shape/area, loading zone shape/area and fracture resistance were parametrically studied. It was found that either a flatter pre-crack shape or a flatter loading zone shape could result in higher initial structural stiffness and less uniform distribution of the strain energy release rate (SERR) along the pre-crack perimeter during crack initiation and early propagation. However, they had only a minor effect on the stiffness after full fiber bridging development in all directions. The plates finally achieved constant stiffness, which increased linearly with the fracture resistance. The final crack shape was dependent on the loading zone shape and area, but the effects were relatively weak.

The Improvement of the Tensile Behavior of CFRP and GFRP Laminates at Elevated Temperatures Using Fire Protection Mortar

In spite of many benefits, FRP materials are susceptible to elevated temperatures. On the other hand, because FRP laminates are different from other FRP materials, data acquired from investigations concerning FRP materials cannot be suggested for FRP laminates. An assessment of the tensile performance of fibers impregnated by epoxy resin as binder is needed. In recent decades, many methods have been presented to protect fiber reinforced polymer (FRP) composites against high temperatures. The application of fire protection mortar is a low-cost and easy technique among all methods. In this investigation, the influence of fire protection mortar on the improvement of the tensile strength of glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) laminates was evaluated. For this object, over 200 FRP laminates with or without fire protection mortar were tested at various elevated temperatures. Investigated temperatures varied from 25°C to 500°C. According to the results obtained from this study, the strength of FRP laminates considerably reduced following the laminates experienced the temperatures higher than 400°C. However, the samples covered with fire protection mortar underwent lower the tensile strength decrements. Eventually, a linear model was presented to estimate the strength of FRP laminates including or excluding protective mortar at elevated temperatures on the basis of linear regressions carried out on test data.

Strain rate‐dependent deformation behaviors of multi‐layer carbon fiber reinforced polymer laminates

AbstractThis paper focuses on the contributions of diversities of strain rate and orientations for aggravating the diversities of micro failure behaviors on carbon fiber reinforced polymer (CFRP) laminates. A miniature horizontal type tensile tester is employed to conduct experiments with strain rate ranging from 2.6 × 10−6 s−1 to 2.6 × 10−3 s−1. The CFRP laminates are obtained based upon a thermoset toughened epoxy matrix (termed CF/Epoxy) with ply orientations of (0°/0°) and (0°/90°). Significant differences in deformation behaviors of CFRP laminates are determined through tests. The study clearly reveals the strain rate‐dependent deformation modes of CFRP laminates, involving pure fiber fracture, epoxy crack with stepped surface and interface failure with residual voids, determines the “low‐high‐low” variation tendency of Young's modulus and strength as a function of strain rate. Ply orientation‐dependent differences in deformation behaviors are also investigated via severe interfacial shearing effect. A unified model consisted of four deformation modes to is clarified to analyze the complexity of CFRP laminates failure mechanism.