Fiber Reinforced Polymer Laminates Research Articles

Inverse of initial stress in carbon fiber reinforced polymer laminates using lamb waves and deep neural network

The prediction of the initial stress in composites is essential for the non-destructive testing (NDT) and structural health monitoring (SHM) of carbon fibre reinforced polymer (CFRP). This paper examines the potential of Lamb waves in the inverse of initial stress by calculating the influence of initial stress on the dispersion characteristics of Lamb waves propagating in multilayered CFRP laminates. By introducing the mechanics of incremental deformation into the linear three-dimensional elasticity theory, the Legendre orthogonal polynomial expansion (LOPE) method is used to mathematically model the Lamb wave propagating in multilayered CFRP laminates subjected to horizontal and vertical homogeneous initial stresses. Then, a three-hidden-layers Feed Forward Deep Neural Network (DNN) with Back Propagation (BP) algorithm is constructed to invert the magnitude and direction of the initial stresses. The input features are the phase velocities of fundamental Lamb wave A0 mode at five different frequencies. Both training and testing samples are obtained by LOPE forward calculation. An ablation experiment is presented to compare the two different activation functions. Finally, the accuracy of the inverse is verified by comparing with the available outcomes of LOPE forward calculation.

Ultrasonic imaging of damage in plates in spectral ripple frequency domain

The ultrasonic C-scan method is a well-known non-destructive inspection method which typically analyzes ultrasound echoes in time domain. Although demonstrated for a variety of defects and materials, it has certain challenges in correctly assessing complex and distributed defects in multi-layer heterogeneous materials, e.g. barely visible impact damage in composite laminates.This paper proposes a C-scan imaging technique in the spectral ripple frequency domain by exploiting interference phenomena between multiple ultrasonic echoes in the response signal. The spectral ripple frequency domain imaging method processes the full A-scan signal without having to define an evaluation time gate. As such, it is highly suited for inspection of curved or tilted parts, or parts having different thickness sections. The method is demonstrated on several 5.5 mm thick carbon fiber reinforced polymer laminates with barely visible impact damage, revealing its capability to resolve neighboring delaminations and small defect fragments over the entire depth of the sample. The spectral ripple frequency domain imaging method shows good performance on datasets with high noise levels and/or low sampling rates, and is computationally very efficient, allowing real-time defect imaging.

Numerical Investigation of Axial Force–Bending Moment Interaction for FRP-Confined Reinforced Concrete Columns with Internal Steel Transverse Reinforcement

Externally bonded fiber-reinforced polymer (FRP) laminates are widely used in the retrofitting and rehabilitation of reinforced concrete (RC) columns because they can increase the axial and bending moment capacities of these columns through a confinement mechanism. The confinement produced by FRP laminates acts in addition to that exerted by the internal transverse steel, although the latter is often neglected in current design standards and guidelines. In this study, a recently developed FRP-and-steel-confined concrete constitutive model was employed to numerically investigate the axial force–bending moment interaction for FRP-confined RC columns modeled using finite-element (FE) analysis. The proposed model was first validated against experimental data available in the literature and then used to quantify, through an extensive parametric study, the effects of transverse steel confinement, FRP strength, FRP stiffness, FRP reinforcement ratio, column diameter, concrete compressive strength, and load eccentricity ratio on the strength of FRP-confined RC columns subject to combined axial compression and bending moment. It was found that the internal steel confinement can substantially enhance the strength of these columns, especially for low concrete compressive strengths, large cross sections, and small eccentricities. When design code provisions limiting concrete and FRP deformations were considered for eccentrically loaded columns, the contribution of the steel confinement increased for increasing FRP reinforcement ratio. Based on the parametric study results, this investigation proposed an extension to eccentric axial compression of two relative confinement coefficients, which were previously developed to describe the contribution of transverse steel confinement to the peak strength of FRP-confined RC columns subject to pure compression.

Effect of concrete strength and tensile steel reinforcement on RC beams externally bonded with fiber reinforced polymer composites: A finite element study

Due to the continual deterioration of concrete infrastructures, the construction industry faces challenges in terms of their serviceability, structural strength, and durability. Therefore, efficient rehabilitation strategies are required to enhance their performance. Externally bonding fiber reinforced polymers (FRP) have evolved as a simple and convenient retrofit technique when compared to conventional techniques of strengthening. FRPs can be powerful toolsfor strengthening concrete structural members in shear and flexure because of their exceptional mechanical characteristics. Flexural strengthening is generally achieved by externally bonding FRP strips or laminates in the soffit side of beams, resulting in improved ultimate strength and bending capacity. This study aims to conduct a numerical parametric investigation on reinforced concrete (RC) beam specimens flexurally strengthened by externally bonding FRP. Three-dimensional finite element models of four RC beams strengthened with FRP and with different concrete compressive strength and flexural steel reinforcement were developed to predict their load capacity. Models were developed and simulated using finite element software called ABAQUS. The beam behavior was evaluated with the help of different material models available in the ABAQUS library. Study considerations included concrete damage plasticity, Hashin's damage in CFRP, and perfect bonding between CFRP and concrete interfaces. The parametric study included two groups of beams: one with varying concrete compressive strength (i.e., 25 MPa and 50 MPa) and the other with varying tensile steel reinforcement (5#14 mm diameter bars and 2# 22 mm diameter bars). The ABAQUS results were verified using experimental data from the previousliteratures.The computed numerical resultsand experimental results demonstrated reasonable accuracy for load–displacement curves and ultimate load capacity. It was found that the ultimate load increased significantly in high-strength (M50) beams with a higher number and smaller diameter (5#14 mm) tensile bars. In conclusion, RC beams with more compressive strength and tensile steel reinforcement boost the effectiveness of the external bonding technique.

Guided wave propagation and skew effects in anisotropic carbon fiber reinforced laminates

Guided ultrasonic waves provide a promising structural health monitoring (SHM) solution for composite structures as they are able to propagate relatively long distances with low attenuation. However, the material anisotropy results in directionally dependent phase and group velocities, in addition to energy focusing, wave skewing, and beam spreading phenomena. These effects could lead to inaccurate damage localization if not accounted for. In this contribution, the guided wave propagation behavior (A0 mode) for a highly anisotropic, unidirectional carbon fiber reinforced polymer laminate is systematically investigated through both finite element analysis and non-contact laser measurements and compared to theoretical predictions. The directional dependency of phase and group velocity measured for a point and line source shows good agreement with theoretical predictions, once a correction for wave skew effects is applied. Wave skew angles were evaluated from the experimental and numerical wave propagation in multiple directions and matched theoretical predictions based on the phase slowness curve. Significant guided wave beam spreading from a line source was observed and quantified from both experiments and simulations and compared with theoretical predictions using the anisotropy factor. The impact of anisotropic guided wave propagation behavior on SHM is discussed.

Durability of bond of EBR CFRP laminates to concrete under real-time field exposure and laboratory accelerated ageing

The durability of bond between carbon fibre-reinforced polymer (CFRP) laminates and concrete with the externally bonded reinforcement (EBR) technique was investigated under real-time field exposure (RTFE) and accelerated ageing. The experimental program, over two years, includes four outdoor environments inducing carbonation, freeze–thaw attack, extreme temperatures, and airborne chlorides from the ocean. A laboratory environment (20 °C/55% RH) was used as reference environment. Additionally, a water-immersion environment (20 °C) was also considered. Relatively low values of bond degradation were observed, where the maximum pullout force varied between −4 % and +16 % under RTFE, while on water–immersed specimens, the maximum pullout force decreased by ∼8 %.

Study on Dynamic Mechanical Properties of Carbon Fiber-Reinforced Polymer Laminates at Ultra-Low Temperatures

This study uses experimental methods, theoretical research, and numerical prediction to study the dynamic mechanical properties and damage evolution of CFRP laminates at ultra-low temperatures. Based on the Split Hopkinson Pressure Bar (SHPB) device, we set up an ultra-low temperature dynamic experimental platform with a synchronous observation function; the dynamic mechanical properties of laminates were tested, and the damage evolution process was observed. The experimental results are as follows: The compression strength and modulus increase linearly with the increase in strain rate and show a quadratic function trend of increasing and then decreasing with the decrease in temperature. The damage degree of the dynamic bending sample increases obviously with the impact velocity and decreases first and then increases with the decrease in temperature. Based on the low-temperature dynamic damage constitutive, failure criterion, and interlayer interface damage constitutive of the laminates, a numerical model was established to predict the dynamic mechanical properties and damage evolution process of CFRP laminates at ultra-low temperatures, and the finite element analysis (FEA) results are consistent with the experimental results. The results of this paper strongly support the application and safety evaluation of CFRP composites in extreme environments, such as deep space exploration.

Characterization of nonlinear viscoelastic behaviors of CF/EP laminates with consideration to physical aging effect under thermo-mechanical loading

The characterization of the time-dependent deformation of fiber-reinforced polymer laminates is critical when evaluating the reliability of composite structures over long-term service lifetime. Several isothermal tensile creep tests under different temperatures and mechanical load levels for two types of carbon fiber-reinforced epoxy composite laminates are conducted in this work. The results indicate that the composites deformed in a time-dependent way and their axial strains are non-monotonic, which implies that both the creep temperature effect and physical aging effect come into play and the creep deformation was affected by the physical aging within the range of loads and temperatures involved in the tests. To formulate this kind of time-dependent behavior, a phenomenological nonlinear viscoelastic constitutive model of single-integral form was proposed and the effective time theory was introduced to describe the effects of temperature, stress, and physical aging. The measured strain was departed into two parts, one is the creep strain and the other is caused by the physical aging effect. The model was implemented within a finite element software by employing a recursive-iterative algorithm and was verified by comparing the simulation results with the experimental ones.

A Modified Strain-Rate-Dependent Spring-Mass Model for Response Prediction of Composite Laminates Subjected to Low-Velocity Impact

A modified strain-rate dependent (SRD) spring-mass model was first developed to capture the strain rate effect on response prediction of fiber-reinforced polymer (FRP) laminates subjected to low-velocity impact (LVI). The constitutive relations of FRP material were modified with the impact-induced strain rate using Yen-Caiazzo’s function in space and time dimensions simultaneously. The stiffness coefficients of the spring-mass model were updated step-by-step during solving the Bubnov-Galerkin equation by the variational method, allowing the SRD contact history to be obtained via recursive integration. The SRD expressions of stiffness coefficients under four typical boundary conditions were presented. Drop-weight tests on FRP laminates and corresponding VUMAT-based LVI simulation with the finite element method (FEM) were provided to prove the validity of the proposed SRD model in evaluating the strain rate effect on the LVI response of composites. Further parameter studies were carried out to investigate the reliability of the proposed SRD model for evaluating the influence of reinforced fibers with different strain rate dependency on impact response. The proposed lumped parameter model has been proven to be more efficient than the traditional FEM, which can be combined with some existing damage models to accurately analyze the delamination evolution process of FRP laminates under LVI in further studies.

Experimental study of notched tensile strength of large open-hole carbon fiber reinforced polymer laminates at low temperature

Carbon fiber reinforced polymer composites used in the aircraft structures often face the problems of open-hole and low temperature. A better understanding of the effects of low temperature and large open-hole on the tensile behaviors of carbon fiber reinforced polymer laminates is highly desirable. In this study the open-hole tensile tests on the carbon fiber/epoxy laminate-T700/SYE20005 with the stacking sequence of [0/90]4S were conducted at both room temperature and low temperature of −60 °C, where the specimens with three different ratios of hole diameter D over the specimen width W were considered. The results showed that the notched strength of the laminates decreases by 54.1% with the increase of the ratio D/W from 0.3 to 0.7 at room temperature. One interesting result is that low temperature greatly improves the strength of laminates with the ratios of D/W = 0.3 and 0.5, producing the increase of 13% in the notched strength, while it has an unobvious effect on the laminate with the ratio of D/W = 0.7. The observations of fracture surface of specimens show that the failure of the laminate with small open hole is mostly dominated by the fiber breakage and axial splits, and the interlaminar delamination and intralaminar matrix cracks are also found for the laminated specimen with large open hole.

A nonlinear prediction model of the debonding process of an FRP-concrete interface under fatigue loading

Externally bonded Fiber Reinforced Polymer (FRP) strengthening has been proven to be an efficient and reliable method for structural strengthening of reinforced concrete (RC) members. However, the beneficial effects of this method can be diminished due to the debonding of the FRP laminates. The mechanism of FRP debonding still requires further research, especially for strengthened members under fatigue loading. To understand and predict the FRP fatigue debonding process better, eleven FRP-concrete joint specimens were tested under static or fatigue loading. Both the theoretical derivation and the experimental study indicated that the debonding growth rate of the FRP laminate depended not only on the mean level (S¯), but also the amplitude (ΔS) of the applied fatigue load. In addition, the debonded portion of the FRP laminate had a significant impact on the following debonding process due to the friction and mechanical interaction between the debonded FRP and the concrete surface. Therefore, a new nonlinear prediction model is proposed in this paper. The proposed model explicitly took into account the amplitude and the mean level of the fatigue loading, which enabled the effect of both to be modelled. Meanwhile, a correction term was also introduced into the model to account for the influence of the previously debonded FRP laminate. The predicted results of the debonding growth rate and the debonding length agreed well with the experimental results.

Evaluation of a climbing drum laminate peel test to determine the interlaminar mode I fracture toughness of thin CFRP laminates—Comparison with the standard mode I DCB test and a mandrel laminate peel test proposed by ESIS TC4

A recently proposed mandrel laminate peel (MLP) test for quantification of delamination propagation in Fiber-Reinforced Polymer (FRP) composites is compared with a climbing drum laminate peel (CDLP) test, and the standard quasi-static Mode I fracture test with Double Cantilever Beam (DCB) specimens. MLP and CDLP both are applicable to thin laminates, for which the Mode I DCB test is not suitable. MLP and CDLP, however, do not yield delamination initiation values. Delamination propagation resistances from the three tests performed with different types of FRP, one partly and one fully cured epoxy laminate, and one thermoplastic laminate, all with unidirectional fiber lay-up agree within about 20%. Reduction of the diameter of the climbing drum (100 mm for the standard climbing drum peel test for adhesives) indicates a minimum diameter on the order of 50 mm for the CDLP test. Additional tests with selected laminates investigated potential effects of specimen width. It is concluded that FRP laminate specimens 20 mm wide and 180 mm long are sufficient for consistent results from CDLP and MLP tests. The CDLP test yielded less scatter (around 10%), i.e., better repeatability than the MLP (round 13%) and the Mode I DCB test (around 17%). Hence, the CDLP test is considered advantageous for industrial application, also due to the simplicity of the test set-up and of the data analysis.

Nonlinear Finite Element Analyses of Prestressed Concrete Beams Strengthened with CFRP Laminate

Nonlinear finite-element (FE)-based quasi-static and vibration analyses of prestressed concrete beams with carbon fiber-reinforced polymer (CFRP) laminate are conducted. To establish nonlinearities in concrete and prestressing steel material properties and concrete–steel interfaces in FE modeling, numerical models of reinforced concrete (RC) and prestressed concrete (PSC) beams are developed, and their response is assessed with experimental results. The established FE model is then extended so as accurately to model CFRP laminate and the nonlinear interaction at concrete–CFRP interfaces between RC and PSC beams. Cohesive interface elements capable of showing delamination failure are introduced to model the adhesion between a concrete beam and the CFRP laminate at its soffit. Ignoring such interaction overestimates the load-carrying capacity of the beam apart from its inability to capture the delamination. The effect of the prestressing force, location and profile of prestressing tendons on the modal frequencies of unstrengthened and CFRP-strengthened beams is studied. Strengthened beams have higher fundamental frequencies, and their modal frequencies are independent of prestressing force, varying significantly with the location and profile of tendons.

Flexural behavior of strengthened concrete beams with multiple retrofitting systems

Due to material deterioration, structure age, changes in building function, fault design, and decreased structural reliability brought on by natural catastrophes, it may require to strengthening the reinforced concrete (RC) beams. The entire carbon footprint is increased by the usage of building materials like cement. Therefore, restoring/improving their overall functionality may be accomplished more sustainably by strengthening and repairing damaged members. In this study, a series of RC beams were tested in three-point bending to determine the ability of various strengthening techniques to improve the beams' flexural capacity. Three strengthening techniques were proposed: External bonding with glass fiber-reinforced polymer (GFRP) laminates, near-surface mounted (NSM), and U-jacket. The experimental study compared the various strengthening methods in terms of ultimate loads, crack loads, maximum deflections, gains in ductility, and energy absorption. The finite element program (ABAQUS) was used for the numerical computations because it is capable of properly simulating experimental research on the flexural behaviour of reinforced concrete beams. Compared to all techniques, the strengthening using GFRP in the different techniques (GFRP laminate, NSM, and U-jacket) propagation of cracks before failure occurred in the RC beam. The sample B3-St. plate-NSM, which used the NSM technique, had the highest peak load of 77.2 MPa compared to all samples and a 36.64% increase over the control sample B0-Control. Moreover, the sample B3-St. Plate-NSM has improved stiffness which contributed to reducing the deflection at peak load by 21.12% than the control sample. The strengthening system using steel plate as NSM showed the lowest cost ($/m`) among the other strengthening systems. The specimen B1-GFRP, which used the GFRP technique, had the highest toughness of 1099.2 kN.mm by an increase of 40.25% over the control sample. Finally, the results obtained by using numerical FE models show good agreement with the experimental results.

Meshfree simulation of progressive damage in composite laminates using discrete element analysis

This study presents a Discrete Element Method (DEM) for a meshfree progressive failure analysis of IM7/8552 carbon fibre reinforced polymer laminates at the macroscale. By implementing a cohesive contact model to describe progressive damage, experimental results from over-height compact tension (OCT) tests are used to calibrate the required input parameters in the DEM contact model. The simulation of various open-hole tension (OHT) test specimens validates the presented DEM model with minimal errors in strength predictions of up to 4%. Discrete Element Method results are directly compared to those obtained from finite element (FE) models. It is demonstrated that DEM is able to seemingly incorporate damage evolution leading to realistic macroscopic damage patterns, however the computational cost is increased by a factor of 100 compared to FE simulations.

An efficient parametrized optical infrared thermography 3D finite element framework for computer vision applications

Matching infrared thermography (IRT) with deep learning-based computer vision has recently gained a lot of interest for automated defect assessment in materials. One of the remaining bottlenecks concerns the necessity of a large and diverse experimental and/or virtual training dataset in order to achieve a sufficiently generalizable computer vision algorithm. This paper presents a parametrized 3D finite element (FE) framework, implemented in Fortran90, for efficiently simulating optical infrared thermographic inspection of multi-layer anisotropic media and establishing large-scale virtual dataset with sufficient diversity. The interface element is introduced for the modelling of an imperfect thermal contact, allowing to simulate a variety of defect types. The flexibility of the interface element makes it possible to simulate delaminations with different thickness using the same discretized model. Validation is done for two benchmark cases which are representative for a fiber reinforced polymer laminate with delamination-like defects. In order to achieve true-to-nature thermographic simulation data, non-uniform heating conditions are adopted from experiment, and a stochastic morphology generator is introduced for modelling realistic irregular defect geometries. To demonstrate the added value of a large, diverse and true-to-nature virtual database for computer vision applications, a Faster-RCNN model was trained on a generated virtual dataset for the detection of delamination-like defects in fiber reinforced polymer laminates. Application of the trained Faster-RCNN on experimental thermographic data yields excellent inference results, illustrating the high generalization ability of the virtually trained object detector.

Investigation of quasi-static indentation on sandwich structure with GFRP face sheets and PLA bio-inspired core: Numerical and experimental study

Composite materials incorporated in naval, aerospace, and automobile structures are exposed to unforeseen damages, which seriously damage their mechanical performance. In particular, the existence of hardly visual impairment on the in-service composite structures is a critical issue that highlights the severity of structural safety in an aircraft. The ability of these materials to sustain multiple impacts requires an understanding of the impact energy absorption to improve the material’s damage tolerance. The present study investigates the energy absorption, load response, failure modes, and damage mechanics under the quasi-static indentation of a sandwich structure. The structure comprises bio-inspired Poly-lactic Acid (PLA) core and Glass Fibre Reinforced Polymer (GFRP) laminates, used in real naval and aeronautical structures, under repeated low-velocity impact with reduced energies and quasi-static indentation. In addition, a lightweight sandwich structure’s damage performance is greatly influenced by its core geometry. Therefore, choosing an appropriate core structure is critical. A Numerical model is developed for the composite structure under quasi-static indentation loads, considering suitable material properties and damage models for the core and face sheets. These results are then compared with the experimental investigation. The load–displacement response and the energy absorption characteristics of the sandwich structure showed good agreement for the experimental and numerical analysis results.

Enhanced thermographic inspection of woven fabric composites by [formula omitted]-space filtering

Infrared thermography is a well-known non-destructive testing technique for detecting defects in unidirectional fiber reinforced polymer laminates. When applied to woven fabric reinforced polymers, however, the weave structure causes strong disturbances and patterns in the background of the thermal images, making accurate and reliable defect assessment a challenging task. In this paper, the concept of k-space filtering is introduced for an improved evaluation of thermographic images obtained from woven fabric composites. An algorithm is introduced to automatically decompose a thermographic image into an image which contains the structured thermal background related to the weave pattern, and a residual image representing other features, e.g. defects. The proposed k-space filtering approach is demonstrated on thermographic data from various woven fabric composites with different weave patterns and defect scenarios, clearly showing an enhanced performance in terms of defect detection and sizing.

Behavior of fiber reinforced polymer laminates strengthening prestressed concrete beams

Pre-stressed concrete constructions' service life may be extended considerably by using fibre reinforced polymer (FRP) reinforcement. It is necessary to have knowledge of the real performance of the strengthened members in order to deploy the technology properly. Determining the static response of pre-stressed concrete beams with externally bonded fiber-reinforced polymer is the main objective of this work. A total of 14 beams of 3 m long, 150 mm × 250 mm cross-sectional beams were cast and put to the test in the lab. Twelve other beams had Glass Fiber Reinforced Polymer laminates added to their soffits to strengthen them, while the other two post-tensioned beams that weren't bonded functioned as reference beams. Three distinct GFRP laminates of two different thicknesses (3 mm and 5 mm) were used to reinforce seven M 35 concrete beams. The strengthened beams were put to the test with loads that increased monotonically as manual readings were collected. The remaining seven M60 concrete beams were reinforced using three different GFRP laminates of two different thicknesses 3 mm and 5 mm, and they were put to test with monotonically increasing loads while having hand readings obtained. All of the variables—concrete grade, GFRP laminate type, and GFRP laminate thickness—were assessed. Uni-Directional Cloth, Woven Roving, and Chopped Strand Mat were some of the GFRP laminates with various configurations (UDC). According to strength, stiffness, ductility, the combined action of the concrete and external reinforcement, and associated failure, all the beams that were tested under static stress underwent evaluation. Deflection ductility, yield load, deflection at yield load, ultimate load, and deflection at ultimate load were the main subjects of this work. According to static testing, pre-stressed concrete beams reinforced with externally bonded GFRP laminates have increased strength, flexural stiffness, ductility, and composite action until failure. The experiment's outcomes were discovered to be rather closely connected. Laminates were suggested for pre-stressed concrete beams with and without externally bonded GFRP in order to forecast the crucial performance factors.

Effects of Small Deviations in Fiber Orientation on Compressive Characteristics of Plain Concrete Cylinders Confined with FRP Laminates

The effectiveness of concrete confinement by fiber-reinforced polymer (FRP) materials is highly influenced by the orientation of fibers in the FRP laminates. In general, acceptable deviation limit from the intended direction is given as 5° in most design guidelines, without solid bases and reasoning. In this paper, a numerical study using finite element modeling was conducted to assess the effects of small deviations in fiber orientation from the hoop direction on compressive behavior of concrete cylinders confined with FRP. Different fiber angles of 0°, 2°, 5°, 8°, 10° and 15° with respect to hoop direction, unconfined concrete compressive strengths of 20, 35 and 50 MPa, FRP thicknesses of 0.2, 0.5 and 1.0 mm and FRP moduli of elasticity of 50 and 200 GPa were considered. The results showed that total dissipated energy (Et), ultimate axial strain (εcu') and compressive strength (fcu') exhibited the most reduction with deviation angle. For 5° deviation in fiber orientation, the average reduction in fcu', εcu' and Et were 2.4%, 2.8% and 4.5%, respectively. Furthermore, the calculated allowable limit of deviation in fiber orientation for a 2.5% reduction in fcu', εcu' and Et were 6°, 3° and 2°, respectively, with a 95% confidence.