Fiber Reinforced Plastic Rods Research Articles

Understanding field failures of composite insulators

The failure mechanism of in-service fiber reinforced plastic (FRP) rod used in composite insulators is studied experimentally; the integrity of Electrical/Chemical Resistance (ECR) of glass fibers in an epoxy-resin matrix of FRP rod used in transmission composite insulator is the subject of study. FRP Rods are subjected to extreme conditions of aging: the results of leakage current analysis, physical inspection, and corresponding Fourier Transform Infrared Spectroscopy (FTIR) strongly support the reason for the failure of FRP rods under operating condition. The entry of water inside the rod-silicone rubber interface, accompanied by arcing, rise in temperature cause considerable damage and hence failure in the operation of rods. Some interesting results of laboratory aging/failures are compared with field failure samples, also a laboratory method to evaluate failures of FRP rods is proposed.

Discussions on Fracture Mechanisms of Composite Insulator

During actual operation, the fiber reinforced plastic (FRP) rod of composite insulator was usually cracked with major characteristics of brittle and decay-like. The made a systematical expatiation of the failure modes of FRP rod, and the mechanisms of these two common fracture types were analyzed. At last, as far as possible to avoid the occurrence of FRP rod fracture accidents in the transmission lines, from the point of view of operation and running, advice about promoting the manufacture level and installation quality of composite insulators were given.

Study on Abnormal Heating Mechanism of FRP Rod of Composite Insulator

In recent years, abnormal heating phenomenon has occurred in composite insulators in China, which have brought potential trouble to the safe operation of power grid. The abnormal heating of fiber reinforced plastic (FRP) rod is an important reason for the temperature rise of composite insulators. In order to study the mechanism of abnormal heating of the FRP rod of composite insulator, three analysis methods of material including scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) are combined to make a microscopic analysis of abnormal heating phenomenon and sample of mandrel, which find that the abnormal heating mandrel of epoxy resin matrix and glass fiber were all deteriorated and accompanied by the invasion of moisture and the produce of HNO3. The experimental results show that polarization loss, partial discharge and resistance loss caused by deterioration of mandrel are the main reasons for abnormal heating of FRP core of composite insulator.

Stress corrosion cracking and brittle failure in a fiber-reinforced plastic (FRP) insulator from a 400 kV transmission line in humid environment

Abstract A fiber-reinforced plastic (FRP) suspension insulator belonging to the Israel Electric Corporation (IECo) failed catastrophically on a 400 kV transmission line during service at the humid south Israeli coastline. The failure occurred during the summer of 2016 after 15 years of service. The suspension had a declared life expectancy of 30 years. The failure resulted in a local outage at a high cost. An investigation was commissioned. The insulator includes a FRP rod with a housing and weather-sheds made of silicon rubber (SiR); the FRP rod is fitted into a metal connector at its end. The fracture occurred 30–40 mm above the metal fitting/grading ring area near the first weather-shed. Macroscopically, the fracture surface was partially characterized by the features of brittle fracture mode in FRP, while the rest of the fracture surface featured severe fiber pullout and debonding. It was also possible to observe that a large crack in the SiR housing led directly to the fracture surface area that included stained and crushed fibers. EDS analysis revealed that the housing around the crack and the stained fibers themselves were polluted with a significant amount of nitrogen, in addition to other foreign pollutants. After surveying similar cases, it was strongly suggested that the source of nitrogen was nitric acid formed as a result of corona discharges on the insulator's surface. The acid caused the housing, fiber and resin to undergo corrosion. SEM imaging revealed fiber fracture surfaces with a morphology typical of stress corrosion cracking, showing that stress was part of the failure mechanism. The fracture of fibers and loss of resin led to a decrease in FRP rod strength and to failure.

Study on Graphite-Electrode Gas Switch Applied for Pulsed Power Supply With a 700-kA Peak Current

A 700-kA gas switch with graphite electrodes for pulsed power supply module has been developed. This paper describes the design, construction, and testing of the high-current gas switch and its trigger system. The two-electrode gas switch is selected, which suffers high peak and fast rise time currents. The trigger generator based on resonant charging of a pulse transformer is constructed to trigger the gas spark. An electrodynamic force model of switch electrodes is established. The electrodynamic force of connecting wires is also calculated. By comparison of the mechanical strengths of polyformaldehyde, epoxy, and fiber-reinforced plastic (FRP) rods, where FRP rods are selected as switch rods, the experimental results show that the mechanical strength of the switch increases by 200%. Extensive electric testing has proven the operating parameters of the switch and its trigger system. The experimental results show that the measured discharge current waveform is demonstrated to be over a 700-kA peak current and a 500- $\mu \text{s}$ pulsewidth. The transfer charge per shot is more than 500 C.

Strengthening RC beams with composite fiber cement plate reinforced by prestressed FRP rods: Experimental and numerical analysis

This paper deals with the development of a new strengthening system for reinforced concrete beams with externally-bonded plate made of composite fiber cement reinforced by rebars made of fiber-reinforced plastic (FRP) [1]. The proposed strengthening material involves the preloading of FRP rod before mortar casting. The paper presents experimental and numerical analysis carried out on many large-scale beams strengthened by well-known reinforcement techniques, such as externally bonded Carbon Fiber-Reinforced Plastic (CFRP) plate and the Near Surface Mounted (NSM) technique, which are compared to the proposed new strengthening material through four-point bending tests. Results are analyzed with regard to the load–displacement curve, bending stiffness, cracking load, yield strength and failure load. The developed numerical model is in agreement with the experimental results. It clearly shows the effects of prestressed FRP rod on cracking mechanisms and internal strength distribution in the analyzed beams.

Surface micro-crack initiated brittle fracture in fiber reinforced plastic rod of composite insulator

Previous studies on brittle fracture of composite insulator were concluded, and unanswered questions were analyzed. Almost all the previous studies focus on the source of corrosion medium (the acid) and the mechanical load in brittle fracture, but the stress concentration of stress corrosion fracture was neglected. Sand paper polishing in the manufacturing process of composite insulator may lead to surface micro-cracks with the depth deeper than 30 ?m-40 ?m on the fiber reinforced plastic (FRP) rod surface. Stress concentration caused by surface micro-crack in FRP rod and its influence on brittle fracture was noticed in experimental study. The importance of the surface micro-crack in FRP rod was verified by a series of experimental studies. An explanation of the randomness of brittle fracture in composite insulator was put forward. Finally, the actions that should be taken to prevent brittle fracture of composite insulator were emphasized.

Analytical solution for the pull-out response of FRP rods embedded in steel tubes filled with cement grout

Fiber-reinforced plastic (FRP) tendons have been widely used for ground anchors in civil engineering. Although various pull-out tests of FRP rods from grout-filled steel tubes have been conducted to simulate ground anchors in rock, there are relatively few theoretical studies reported in the literature for this type of bonded anchorages. The intention of this paper is to present an analytical solution for predicting the maximum pull-out load of FRP rods embedded in steel tubes filled with cement grout. First, the expression of the shear stress along the thickness direction of the grout layer is obtained analytically. The tensile stress in the rod and the interfacial shear stress at the rod–grout interface are formulated at different loading stages. By modeling interfacial debonding as an interfacial shear crack, the pull-out load is then expressed as a function of the interfacial crack length. Finally, based on the Lagrange multiplier method, the maximum pull-out load and the critical crack length are determined. The validity of the proposed analytical solution is verified with the experimental results obtained from literature. It can be concluded that the proposed analytical solution can predict the maximum pull-out load of spiral wound and indented rods embedded in steel tubes filled with cement grout with reasonable accuracy. The proposed solution can be also applied in predicting the pull-out capacity of steel bars from concrete.

The Effect of Fiber Winding Angles on the Bending Strength of Fiber-Reinforced Plastic Rods

Fiber-reinforced plastics consist of fibers of high strength and modulus embedded in, or bonded to a matrix with distinct interfaces between them. Because fiber configuration plays a key role in determining mechanical strength of fiber-reinforced plastic rods, especially bending strength of fiber-reinforced plastic rods was measured and simulated numerically in variation with winding angles. Also, stress distribution in fiber-reinforced plastic rods was simulated numerically under the condition of constant bending load to fiber-reinforced plastic rods. The measured bending strength of fiber-reinforced plastic rods in variation with winding angles was different from that of simulated. The difference between measured and simulated results was due to the effect of shear stresses on the strength of fiber-reinforced plastic rods.

Effect of the braiding angle on the energy absorption properties of a hybrid braided FRP tube

Energy absorption is achieved by the combination of various fracture mechanisms such as fibre fracture, delamination, and central crack. However, serious problems would arise if this energy absorption ability were compromised by brittle crack propagation of the cross-sectional central part. In a previous study, the use of flexible resin with lower stiffness and higher toughness than the resin generally used was suggested as a method to restrain brittle crack propagation. In this study, hybrid braided fibre reinforced plastic (FRP) tubes were fabricated according to the previous study involving FRP rods. In this case, the flexible resin was applied to middle-end-fibre. The energy absorption characteristics and crushing mechanisms based on precise cross-sectional observation of the crush zone of the braided FRP tubes with or without the presence of flexible resin in middle-end-fibre were investigated. It was found that braided FRP tubes with or without the presence of flexible resin in middle-end-fibre were investigated. It was found that braided FRP tube with a 30° braiding angle, together with the presence of flexible resins, shows significant improvement in terms of energy absorption ability. The added flexibility of the tubes owing to the addition of flexible resin in turn causes short cracks, more fibre breakage, and consequently enhanced energy absorption properties.

Energy Release in Fiber-Reinforced Plastic Reinforced Concrete Beams

A set of 30 concrete beams reinforced with carbon/epoxy FRP (fiber-reinforced plastic) and four reinforced with comparable size steel rebars were subjected to static bending tests. Adequate bond between the FRP and the concrete was obtained, due to the use of carbon fiber overwrap on the smooth pultruded FRP rods. With adequate bond, the large strain to failure (>2%) of the FRP determines the ductility and failure mode of the FRP reinforced beams. An analytical evaluation of the fracture energy in these experiments shows that there is ductility due to the large fraction of the total strain energy that is absorbed in the concrete, because of the formation of distributed cracking. Variations in overwrap configuration, addition of steel stirrups, addition of polypropylene fibers, and comparison with four beams reinforced with equivalent steel reinforcement were also analyzed.

Fracture Analysis of FRP Reinforced Concrete Beams

Carbon/epoxy FRP (Fiber Reinforced Plastic) rebars were produced with the pultrusion technique. Concrete beams reinforced with these rebars were subjected to static and cyclic 3-point bending. Flexural cracking is arrested by an adequate bond between the FRP and the concrete because of the use of a carbon fiber overwrap on the otherwise smooth pultruded rods. In spite of the brittle nature of the FRP rods and the concrete, their combined behavior demonstrate ductility in excess of what is typically expected from reinforced concrete. An analytical evaluation of the fracture energy shows that such ductility is due to the large fraction of the total strain energy that is consumed in the formation of distributed cracking in concrete. Therefore, if an adequate bond can be provided, the strain-to-failure of the FRP determines the ductility and failure mode of FRP reinforced beams.

Tensile properties and pullout behaviour of AFRP and CFRP rods for grouted anchor applications

Fiber-reinforced-plastic (FRP) rods have been introduced in the market for grouted ground anchors. The resistance to corrosion and chemical attack, high strength-to-weight ratio, and ease of handling of these rods make them a better alternative to steel tendons in some applications of ground anchors. However, to fully utilise FRP rods as tendons for cement grouted anchors, some aspects of their behaviour have to be determined, including tensile properties, tensile capacity, bond strength in cement grout, long-term strength and durability in alkaline environments. In this paper, the tensile characteristics, bond strength and pullout behaviour of AFRP and CFRP rods embedded in cement grout are discussed. The pullout test program involved four types of rods, four types of cement grouts and three types of anchorage tubes with three bonded lengths. The experimental results have shown that the tensile properties of the FRP rods are close to the manufacturers’ data. The surface geometry of FRP rods, the properties of the filling grout and the stiffness of the host medium (anchorage tube) influence the pullout behaviour, pullout capacity and maximum bond stress of cement grouted FRP anchors.

Micro-mechanical analysis of splitting failure in concrete reinforced with fiber reinforced plastic rods

The present investigation provides a micro-mechanical model for the splitting failure analysis of fiber reinforced plastic (FRP) reinforced concrete members subjected to longitudinal tensile stresses. The model consists of three co-axial cylinders: (a) the inner elastic FRP rod; (b) the mid cracked part of concrete; and (c) the outer elastic part of concrete. The anisotropic properties of reinforcement, the compatibility of longitudinal strain at interface and the effect of Poisson's ratio of concrete are taken into account in the analysis. The method can be used to predict the stress distributions in the hybrid structure and the relations between the growth of cracks and the applied end forces. It is found that the number of splitting cracks and the material properties of the anisotropic FRP rods are not the dominant factors in splitting failure. It is also observed that neglecting Poisson's ratio of cracked concrete may under-estimate stresses in the hybrid structure.

Modeling of bond behavior of hybrid rods for concrete reinforcement

Fiber reinforced plastic (FRP) rods are used as reinforcement (prestressed or not) to concrete. FRP composites can also be combined with steel to form hybrid reinforcing rods that take advantage of the properties of both materials. In order to effectively utilize these rods, their bond behavior with concrete must be understood. The objective of this study is to characterize and model the bond behavior of hybrid FRP rods made with epoxy-impregnated aramid or poly-vinyl alcohol FRP skins directly braided onto a steel core. The model closely examines the split failure of the concrete by quantifying the relationship between slip of the rods resulting transverse stress field in concrete. The model is used to derive coefficients of friction for these rods and, from these, their development length requirements. More testing is needed to confirm this model, but in the interim, it may serve as a design aide, allowing intelligent decisions regarding concrete cover and development length. As such, this model has helped to explain and predict some experimental data from concentric pull-out tests of hybrid FRP rods.

Experimental bond behavior of hybrid rods for concrete reinforcement

Fiber reinforced plastic (FRP) rods provide certain benefits over steel as concrete reinforcement, such as corrosion resistance, magnetic and electrical insulation, light weight, and high strength. FRP composites can be combined with a steel core to form hybrid reinforcing rods that take advantage of properties of both materials. The objective of this study was to characterize the bond behavior of hybrid FRP rods made with braided epoxy-impregnated aramid or poly-vinyl alcohol FRP skins. Eleven rod types were tested using two concrete strengths. Specific topics examined were bond strength, slip, and type of failure in concentric pull-out tests from concrete cubes. From analysis of identical pull-out tests on both hybrid and steel rods, information on relative bond strength and behavior were obtained. It is concluded that strength is similar but slip in hybrid rods is much higher. Hybrid rods failed either by pull-out or splitting the concrete block (with or without yielding of the steel core). Experimental data showed consistency with similar test results presented in the literature.

Effect of shear stiffness and concrete wedge cone failure on fracture of a fiber-reinforced plastic rod at an intersection with a concrete crack

Experimental results of model speciments in which FRP rods fractured due to local deformation at a crack intersection in a concrete member were analyzed by a 3D nonlinear finite element method in which orthogonal anisotropy of the FRP rod was considered. The analytical results indicated that accurate prediction of shear modulus of the FRP rod and size of concrete wedge cone failure around the FRP rod was significant to predict deformation and fracture of the FRP rod. FRP rods as reinforcement in concrete members, the small shear modulus, because of the orthogonal anisotropy and the wedge cone failure, may prevent the FRP rod from fracturing at a very low tensile stress due to the local deformation at the crack intersection.

Transient Response of a Pultruded Fiber-Reinforced Plastic Rod Subject to Impact Loading

This work is concentrated on investigating the transient response of a pul truded fiber-reinforced plastic rod subject to impact loading. Both impact tests and corre sponding numerical simulation are performed. The target rod made of S-glass/epoxy, with a diameter of 9.7 mm, is clamped on one end. The other end of this rod is struck by a cy lindrical flat-ended steel rod weighing 16 gm. The diameter of the impactor is equivalent to the target rod. A pneumatic gun is utilized to launch the impactor with a velocity up to 60 m/s. The dynamic response of the composite rod is detected by using a pair of strain gages (TML, type FLK-6-11, 120 Ω) and associated facilities. A digital oscilloscope (Nico let 4094C) is used here to record transient signals. Several tests have been conducted in order to obtain the longitudinal waveform of the composite rod due to impact loading. Corresponding numerical simulation based on explicit Lagrangian finite element method is also performed to check against experimental observation. This code is capable of han dling the orthotropic constitutive relationship, large displacement mechanism, and also the lateral effect. Numerical results show that the pulse duration agrees well with test findings. However, the computed peak longitudinal strain is greater than that of the test result. This discrepancy is due to employing the static mechanical properties in numerical analysis. It is found that the computed peak longitudinal response agrees more closely with the test result when corresponding mechanical properties, such as Young's modulus, are increased. This increment is expected for composite material under high strain rate loading.

Strength and Failure Modes in Notched Pultruded Rod Tension Specimens

Failure mode in a notched pultruded rod containing unidirectional E-glass fibers depends on the ratio of gage length L to specimen diameter d at the notch. For a tensile failure, a large L/d ratio is required. At smaller L/d ratios, long pull-out or crushing at the grips occurs. Based on the failure mode observation and tensile strength evaluation, development of a V-notched tension test with large L/d ratio is suggested as a possible replacement for the current ASTM tension test (ASTM Test Method for Tensile Properties of Pultruded Glass Fiber-Reinforced Plastic Rod [D 3916]).

High-Strength End Fittings for FRP Rod and Rope

Improved structural end fittings of two basic types were developed for use with fiber-reinforced plastic (FRP) rod and rope materials. Each type of end fitting is essentially a metal sleeve that is attached to the FRP material by a thick layer of potting material. Tensile tests demonstrated that the new end fittings can equal or approach the full tensile strength of E-glass-reinforced-plastic rod and rope. In tensile strength, the new fittings are superior to commercially available end fittings. An axisymmetric finite element analysis was developed to study the transfer of load from a rod or rope, through the potting material, to the metal fitting. In the analysis the FRP material and the potting material are assumed to be transversely isotropic. The results of both the analytical study and the laboratory tests indicate the importance of using a relatively thick layer of low-stiffness potting material with these fittings.