Fiber Reinforced Polymer Materials Research Articles

Application of iterative reconstruction algorithms to mitigate CT-artefacts when measuring fiber reinforced polymer materials

In general, fiber reinforced polymer materials are being used in case of challenging mechanical requirements. Important parameters determining performance of such polymer composites are properties of the components as well as the quality of the fiber-polymer interface. For example, porosity or voids particularly at the interface will influence the overall performance of the composite negatively. Therefore, it is of importance to know the actual porosity value to e.g. calculate and predict the mechanical properties. In principle, a CT scan can provide the means to determine the amount, size, and shape of the voids in an X-ray transparent material. The difficulty, which arises in fully organic based i.e. carbon fiber reinforced polymer composites, is the small difference between absorption of X-rays in the carbon fibers versus the surrounding polymer matrix. At certain conditions, when single fibers need to be resolved and voids surrounding the fibers may exist, CT reconstruction artefacts introduced by standard algorithms are visible as “virtual” voids in large quantities severely limiting the correct determination of the actual porosity. On the other hand, in glass fiber reinforced polymer materials, the large density difference between fibers and matrix causes artefacts to appear, too. Specific experimental settings necessary to deal with the high absorption of the glass fibers (beam hardening enforces the use of X-ray filters) cause very low absorption differences between air and the polymer matrix and thus hardly provide information on the existence of voids. Iterative reconstruction techniques, however, are more robust against the encountered CT-reconstruction artifacts and may enable accurate determination of important sample features in polymer composite materials, and especially allow for accurate determination of voids or porosity. This work explores the use of two iterative versus standard reconstruction algorithms (CGLS and SIRT versus FDK) and validates the results with cross-sectional Optical Microscopy. The paper shows that iterative reconstruction algorithms outperform the standard reconstruction algorithm with respect to mitigating the influence of the CT reconstruction artefacts.

Finite element modeling of reinforced concrete beams externally strengthened in flexure with side-bonded FRP laminates

Reinforced concrete (RC) beams are often strengthened through externally bonded (EB) fiber-reinforced polymer (FRP) systems. In such applications, improving the bending capacity of RC beams is of main interest and which is achieved via bonding EB-FRP systems to the soffit of beams. Access to soffit of beams can be restricted by existing structural members or limited (in case a RC beam span over neighboring compartments). In those scenarios, RC beams can still potentially be strengthened in flexure through longitudinal bonding of EB-FRP systems to the sides of beam's web. This paper examines critical parameters that influence the effectiveness of side bonded EB-FRP systems through a newly developed finite element (FE) model. This model utilizes state-of-art simulation techniques and is capable to trace the flexural behavior of RC beams externally strengthened with side-boned FRP laminates throughout all loading stages till failure. The model was validated by comparing with experimental data and the predicted and measured results were in good agreement. The validated model was then utilized to study the effect of concrete compressive strength, FRP material type, FRP size, and steel reinforcement ratio on the behavior of strengthened RC beams. Outcomes of this numerical investigation show the effectiveness of side-bonding FRP systems as an alternative to conventional soffit strengthening systems to improve the flexural capacity of RC beams.

Debonding Damage Detection in CFRP Plate-Strengthened Steel Beam Using Electromechanical Impedance Technique.

Carbon fiber reinforced polymer materials are widely applied in structure strengthened engineering because of the many advantages of carbon fiber reinforced polymer. However, the debonding damage between the carbon fiber reinforced polymer and host structures occurs frequently, which might lead to the brittle failure of structure components, especially flexural ones. In this paper, an electromechanical impedance-based method, an important technique in structural health monitoring, was adopted to detect the debonding damage of carbon fiber reinforced polymer plate-strengthened steel beam by using lead zirconate titanate (PZT) transducers. A carbon fiber reinforced polymer plate-strengthened steel beam specimen was fabricated in the laboratory and two PZT sensors were attached at different locations on the carbon fiber reinforced polymer plate. The impedance signatures with variation of the different degrees of the debonding damage were measured by an impedance analyzer. The root-mean-square deviation method and the cross-correlation coefficient method were used to quantify the correlation between the electromechanical impedance and the debonding damage degree. The results reflect that an electromechanical impedance-based structural health monitoring technique can serve as a good method to detect the debonding damage of carbon fiber reinforced polymer plate-strengthened steel structures.

Mesoscale experimental investigation of thermomechanical behaviour of the carbon textile reinforced refractory concrete under simultaneous mechanical loading and elevated temperature

In comparison with carbon fiber reinforced polymer material, the carbon textile reinforced concrete (TRC) is usually coted to have a better fire behaviour. The aim of this paper is to analyse the elevated temperature behaviour of TRC. This paper presents the experimental results conducted on carbon TRC at mesoscale in the range of temperature from 25 °C to 700 °C. The carbon TRC consists of refractory matrix and carbon textile which has been treated with amorphous silica products for a good bond with the cement matrix. As a result, carbon TRC showed the strain-hardening behaviour with different phases and brittle behaviour depending on the temperature. The evolution of carbon TRC properties with elevated temperature (the ultimate stress, the crack stress, the initial stiffness) was experimentally identified. The mechanical property evolution of carbon textile and refractory concrete (the ultimate strength, the Young’s modulus) depending on temperature was also presented. The thermal tests on carbon TRC cylinder specimens were also performed to identify the diffusivity coefficient of this composite. All results obtained were analyzed in order to find the contribution of carbon textile and refractory matrix to fire resistance of carbon TRC. Its ultimate strength has been improved about 2 times compared with non-reinforced matrix at elevated temperature ranging from 25 °C to 400 °C. The efficiency coefficient of carbon textile could be calculated in this study.

Stress-strain model of an FRP-confined concrete filled steel tube under axial compression

An FRP (fiber-reinforced polymer) -confined concrete filled steel tube is formed by the combination of FRP material and a concrete filled steel tube, and it combines the advantages of FRP-confined concrete and a concrete filled steel tube (CFT); however, there is still no effective model to generally describe the complete stress-strain curve of a concrete filled FRP-confined steel tube. The stress-strain response of a concrete filled FRP-confined steel tube features the dual characteristics of both FRP confined concrete and steel confined concrete. In this paper, ninety-six test results of FRP confined circular CFT columns under axial compression were collected from different researchers. The steel tube confinement index is taken as an additional key influencing parameter on the strength and deformation, and calculation methods for the ultimate strain, ultimate stress, peak strain and peak stress of a concrete filled FRP-confined steel tube are established by taking into account the steel tube confinement index. Moreover, the stress-strain model was developed to describe the complete stress-strain curve consisting of the four stages of a concrete filled FRP-confined steel tube. The proposed model reflects the characteristics of the stress-strain behavior of a concrete filled FRP-confined steel tube with good accuracy and simplicity.

Nonlinear behavior analysis of flexural strengthening of RC beams with NSM FRP laminates

This paper proposes an analytical method for predicting the nonlinear behavior of reinforced concrete (RC) beams after flexural strengthening with near surface mounted (NSM) fiber reinforced polymer (FRP) laminates. With a gradual increase in the beam curvature and a step-by-step numerical integration method, an algorithm model was designed for predicting the load-deflection response of RC beams strengthened with the NSM technique. Based on the bond mechanism between the NSM FRP laminates and the surrounding concrete, the available calculation methods of the load bearing capacity of RC beams with NSM FRP laminates were derived according to the bearing capacity model for the strengthened RC structure with FRP materials. Using the proposed model, the load-deflection responses of five flexural strengthened RC beams with NSM carbon FRP (CFRP) laminates were analyzed and verified against the experimental results. The presented results indicate that the proposed model based on the available test data can accurately analyze the tensile stresses of the NSM CFRP laminates. Moreover, the results prove the feasibility of the designed algorithm model in precisely predicting the nonlinear behavior of RC beams strengthened with NSM CFRP laminates. The proposed algorithm model could provide a much needed link between structural design and nonlinear behavior analysis of RC structures strengthened with the NSM technique.

Flexural rehabilitation of steel beam with CFRP and BFRP fabrics – A comparative study

This paper compares the performance of structural steel beams retrofitted with two different fiber reinforced polymer (FRP) fabrics: Carbon FRP (CFRP) and Basalt FRP (BFRP) fabrics. A total of eight steel beams with and without corrosion defect in the flexural tension zone were tested under 4-point bending load. The study found that the use of both FRP fabrics resulted in reduced ductility, however, ductility of beams retrofitted with BFRP fabric is much higher than that of the beams retrofitted with CFRP fabric of similar thickness. The study also found that both FRPs fabrics are effective in increasing the ultimate load, yield load, and elastic stiffness of beams, however, number of BFRP fabric layers required is higher than the number of CFRP fabrics. The structural behavior of steel beams including the complex behavior of rupture in the FRP fabrics were successfully modeled using a commercially finite element software and a good correlation was obtained between the finite element models and the lab specimens. Validated finite element model was used to obtain additional information that could not be obtained from the experimental study. This study concludes that the Basalt fabric offers a competitive and green alternative to the Carbon fabric.

FRP-FLEXURAL STRENGTHENING OF STEEL BEAMS: A FINITE ELEMENT ANALYSIS STUDY

Fiber reinforced polymer (FRP) strengthening has been widely used for concrete structures. However, limited studies have been conducted on its application to steel structures. The use of FRP materials to retrofit steel members is an effective replacement for welded or bolted steel plates and offer high strength-to-weight ratio, resilience to environmental degradation, and a robust fatigue performance. Past studies have shown that the application of FRP to steel is a promising technical solution to help against corrosion and also to increase the load carrying capacity of steel members. In the present study, finite element analysis (FEA) model of a steel beam strengthened with carbon fiber reinforced polymer (CFRP) laminates was developed using ANSYS Mechanical finite element software program. The model was validated by comparing the FEA and experimental results of an existing study in the literature and they were in good agreement. A parametric study was conducted on the validated steel beam model with the aim to investigate the effects of basalt fiber reinforced polymer (BFRP) and two types of adhesives on its performance under static loading. The FRP laminates provided an increase in flexural capacity and pseudo ductility of the beams. The optimum bond length was found to be 2/5 of the span length for the beam in this study.

Seismic performance of reinforced concrete beam–column joint strengthening by frp sheets

Reinforced concrete beam-column joints are generally recognised as critical regions in frame structures which experience severe inelastic deformations during earthquakes. This paper presents the results of an experimental campaign to investigate the optimal seismic performance of a method of strengthening reinforced concrete beam-column joints. Ten reinforced concrete beam-column joints, at a scale of one to three (1/3), were subjected to reverse cyclic loading under constant axial load to simulate an earthquake, and tested with the deflection being monitored. The joints were strengthened with fibre-reinforced polymer (FRP) systems, made up of a combination of carbon and fibreglass fabric, and a hybrid braided FRP fabric. Of these, four samples were strengthened with different FRP systems, while two were used as benchmarks. Once damaged, four specimens were repaired with FRP and re-tested. Strain and cracking fields were monitored using an Aramis digital video camera. The test results provided useful information on the strengthening configurations adopted, in relation to strength, ductility and energy dissipation capacity. The results establish the efficiency of glass fibre GFRP in upgrading deficient beam-column joints in an equivalent proportion than carbon fibre CFRP. It was observed that using of a hybrid sheet (Glass-Carbon) improves the ductility and dissipation energy of the RC joints to a great extent, with a highly competitive cost.

The tensile performance of FRP bars embedded in concrete under elevated temperatures

In this research, the mechanical properties of glass and carbon fiber reinforced polymer (FRP) bars with epoxy resin matrices embedded in concrete were investigated under an extensive range of elevated temperatures (i.e., 25–800 °C). Embedded FRP bars with various bar diameters were studied in order to determine bar diameter influence on the results. In addition, analysis of variance (ANOVA) was performed on the experimental results to investigate the contribution of exposure temperature and bar diameter to the tensile behavior of embedded in concrete FRP bars at elevated temperatures. The results show that the tensile strength of embedded FRP bars generally decreases with increasing temperature; however, the rate of decrease of the tensile strength varies within different temperature ranges due to different failure modes and states of the FRP material. The influence of the bar diameter was not significant in the tensile behavior of embedded FRP bars at various elevated temperatures. However, the influence of the bar diameter increased within the temperature range of 150–500 °C compared to the temperatures lower than 150 °C and above 500 °C. The results also show that the concrete cover prevented direct heat and oxygen from reaching the bars; as a result, embedded FRP exhibited improved tensile performance at elevated temperatures compared to bare bars that were directly exposed to heat. Two Bayesian regression models were developed for predicting the tensile performance of embedded GFRP and CFRP bars at elevated temperatures. The models have been shown to be in good agreement with the experimental results.

A state-of-the-art review: Near-surface mounted FRP composites for reinforced concrete structures

Abstract The use of fibre reinforced polymer (FRP) in civil construction applications with near-surface mounted (NSM) method has gained considerable popularity worldwide and can produce confident strengthening and repairing systems for existing concrete structures. The response of concrete structural members rehabilitated using FRP with externally-bonded (EB) method under monotonic and fatigue loading has been well reported. On the other hand, the monotonic and fatigue response of NSM FRP-rehabilitated concrete structures is less known. This paper reviews current research on concrete members retrofitted by NSM FRP system and exposed to monotonic and fatigue loading. It provides an outline of FRP composites and the fatigue behaviour of concrete, steel and FRP materials. In addition, the review focuses on the bond characteristics between NSM FRP, adhesive and concrete substrate, and on the flexure behaviour of NSM FRP-strengthened and repaired reinforced concrete (RC) beams. Furthermore, the paper reviews the failure modes, factors affecting NSM FRP systems, guidelines and codes for the use of FRP in concrete structures, damage accumulation and residual strength, stress limits in steel reinforcement, and the limitations of using epoxy and polymer-based cementitious adhesives for the strengthening of concrete members.

Experimental study of hybrid strengthening technique using carbon fiber laminates and steel plates for reinforced concrete slabs

Abstract External bonding (EB) technique has been used widely to strengthen reinforced concrete (RC) structures in civil engineering over several decades. Steel and fiber reinforced polymers (FRP) are the two most common materials used for strengthening RC structures. Although each material has its advantages for EB application, they also have their own weaknesses. Steel plates are heavy and poor in corrosion resistance, and the thickness of steel plates is also limited due to lack of shape flexibility for strengthening RC structures. The EB-FRP technique is restricted for broader applications because the bond interface is the weakest link in such retrofitted or strengthened concrete members, resulting in sudden and brittle debonding failure, and the strength of FRP has not been fully utilized. To overcome these weaknesses, an innovative strengthening method combining FRP and steel materials was explored in this research. Carbon fiber laminates (CFL) and thin steel plates (SP) were combined in different ways to determine the most effective hybrid strengthening configuration. A total of nineteen RC slabs strengthened by different configurations were tested under four-point bending. Failure modes, load versus deflection curves, strains of internal reinforcement bars, CFL and SP, and stiffness of the strengthened specimens were analyzed. The test results proved that the CFL-SP hybrid strengthening technique was effective. The average maximum increases in flexural capacity and stiffness were 204.2% and 91% over the control slab, respectively. In addition, the cracking and steel yielding loads of slabs strengthened with CFL-SP composite material were higher than those of slabs strengthened by FRP or steel plate only. By combining with steel plate, FRP material was used more efficiently and the stiffness of the strengthened RC slabs was improved. The overlap-type hybrid configuration of bonding CFL as inner plates while steel as outer plates was proved as the most effective hybrid strengthening method within all configurations tested in this research.

Microwave imaging of multilayered structures using ultrawideband noise signals

A microwave ultrawideband (UWB) radar system is developed and used for non-destructive testing (NDT) of multilayered structures. The effectiveness of this approach lies in the choice of critical parameters such as waveform type, operational frequency range, and stand-off distance. Matched filtering of the received waveform with a replica of the transmitted signal is used to locate hidden anomalies or flaws within a multilayered structure, most of which are not apparent through common visual-inspection techniques. Due to the impulse-like autocorrelation properties of white noise, noise waveforms provide the ability to locate material discontinuities as well as, or better than, other common waveforms such as Gaussian-modulated sinusoidal pulses (GMSP) or linearly frequency modulated chirp signals. Experimental data are gathered using a specially developed microwave imaging system which operates in the X-band frequency range. Subsequently, two-dimensional (2-D) and three-dimensional (3-D) images are rendered following a raster scanning approach to reveal internal boundaries, hidden inclusions, and defects due to impact damages or corrosion in fiber-reinforced polymer (FRP) materials and other dielectric materials.

A bond-based peridynamic modeling of machining of unidirectional carbon fiber reinforced polymer material

As an alternative theory of solid mechanics, the peridynamic theory is an effective method to analyze the damage process of materials. In this paper, a bond-based peridynamic modeling for unidirectional carbon fiber reinforced polymer material(UD-CFRP) orthogonal cutting process is proposed, and the corresponding composite material bond failure criterion is also investigated for better revealing the machining mechanism of UD-CFRP machining. From comparing between simulation and experimental results, it can be indicated that the peridynamic modeling is capable for predicting the chip formation and surface damages in UD-CFRP machining. The investigation about effect of critical stretch and mass density on the machined surface crack is also conducted.

Experimental investigation on the accuracy of XCT measurement of fiber length in fiber reinforced polymers

X-ray computed tomography (XCT) allows performing non-destructive measurements of fibers in fiber-reinforced polymeric materials. The accurate evaluation of fiber orientation and fiber length distribution is particularly useful as these factors have a strong influence on physical and mechanical properties of such materials. XCT data is already successfully used for fiber orientation analysis, whereas the fiber length measurement is more complex as it requires the individual fibers to be identified and segmented. Software tools able to cope with such tasks are available, but the accuracy of XCT fiber length measurement has not been thoroughly investigated so far. In this work, an experimental methodology was developed to determine the accuracy of XCT based fiber length measurements, using a reference object that was specifically designed, produced and calibrated for this purpose.

Mechanical and durability performance of FRP confined and unconfined strain hardening cementitious composites exposed to sulfate attack

Abstract In this study, the performance of Fiber Reinforced Polymer (FRP) confined and unconfined Strain Hardening Cementitious Composite (SHCC) specimens exposed to sulfate attack under static and cyclic loading were investigated. Two types of FRP fabrics (Basalt (BFRP) and Carbon (CFRP)) and two types of fly ash (Low calcium (LCFA) and high calcium (HCFA)) were studied. In addition, FRP fabrics as a rehabilitation material was also investigated for the sulfate deteriorated specimens. LCFA-SHCC specimens showed superior performance than HCFA-SHCC specimens in the sulfate environment. In addition, confined specimens with FRP fabrics significantly improved compressive strength, ductility, and durability of the specimens.

Development of a longitudinal–torsional ultrasonic vibration-aided drilling system for drilling carbon fiber-reinforced polymer materials

This study aimed to explore the motion properties of longitudinal–torsional ultrasonic vibration-aided drilling, the feature of drilling dynamics, and an efficient path of implementing longitudinal–torsional component conversion. It also aimed to propose a spiral chute horn with high performance and low cost based on the propagation characteristics of ultrasonic waves. The finite element software ANSYS was used for modal analysis and the geometric parameters of helical flutes were achieved. The dynamics analysis of the longitudinal–torsional ultrasonic drilling system revealed that the ultrasonic vibration could improve the rigidity of the drill and that the components existed in longitudinal and torsional directions. Drilling the T300-12K/AG80 unidirectional carbon fiber-reinforced polymer using the longitudinal–torsional ultrasonic drilling system revealed that the results of theoretical analysis and numerical simulation were comparable. This drilling system could change the longitudinal–torsional ratio according to the geometrical parameters of the modal converter, improve the surface quality of the carbon fiber-reinforced polymer hole, and reduce the exit delamination.

Probabilistic fatigue life modelling of FRP composites for construction

Abstract Fiber reinforced polymer (FRP) composites have been used extensively for aerospace, marine transportation, and wind energy applications for several decades prior to attracting widespread attention in the field of civil engineering. While limited data is available on the long term fatigue performance of FRP materials for construction and building projects, it is worthwhile to review the numerous comprehensive works done in other engineering disciplines and consider the lessons learned. In particular, the stochastic nature of the fatigue life of composite materials has been captured by probabilistic models presented in literature, but until now has not been evaluated in the context of construction materials. In this paper, various simple fatigue life models are presented and the Sendeckyj wear-out model based on the strength-life-equal-rank assumption is applied to fatigue data from a variety of material types and configurations intended for civil engineering structures. This approach uses maximum likelihood estimates to optimize the Weibull distribution parameters fitting experimental fatigue data to develop S-N curves for any probability of failure. The results presented show that the model is versatile and can be calibrated to describe the probabilistic nature of both the static and fatigue response of FRP composite materials for construction and building applications.

Digital image correlation (DIC) for measurement of strains and displacements in coarse, low volume-fraction FRP composites used in civil infrastructure

Accurately measured strains are critical when investigating the application of Fiber Reinforced Polymer (FRP) materials, but traditional mechanical strain measurement methods have several critical drawbacks related to the installation process and the recording capabilities of the devices. FRP materials typically used in the civil engineering field feature large asymmetries and heterogeneity originated from the manual installation procedures, as opposed to the highly controlled FRP fabrication methods used in other fields that result in more homogeneous materials. The feasibility of using an optical full-field Digital Image Correlation (DIC) technique for measurement of strain fields on FRP materials used in the civil engineering industry has been investigated and the level of error in the DIC method when using more traditional methods was determined. The main advantage of using DIC over more traditional methods, which is the capacity of DIC to measure full field strains instead of strains at isolated points, has been demonstrated by providing exemplar measurements of various specimens of FRP materials. The reported strain fields are examples of what was obtained during an experimental campaign to understand the behavior of FRP anchors and other materials. The main conclusions drawn from the observation of those strain fields are discussed.

Flexural response of continuous unbonded post-tensioned beams strengthened with CFRP laminates

The sudden brittle rupture of carbon fiber-reinforced polymer (CFRP) materials is often confused with the redistribution capacity of statically indeterminate CFRP-strengthened structures. Few investigations have been carried out regarding the flexural behavior of continuous CFRP-strengthened unbonded post-tensioned (UPT) beams, despite such beams having been extensively applied in practice, owing to retrofitting alternatives. This study emphasizes the flexural response of continuous UPT beams strengthened with CFRP laminates subjected to monotonic loading to failure. Progressive fracture of the CFRP laminates appears in the hogging and sagging regions, accompanied by concrete crushing in the sagging regions. The degree of moment redistribution (MR) and ductility generally increase with decreasing compressive zone depth of the critical section over the inner support. When the ratio of the global reinforcement index of the inner support to that of the mid-span increases, the MR and ductility decrease.