FRP strengthening of steel and steel-concrete composite structures: an analytical approach

A recent technique for strengthening steel and steel-concrete composite structures by the use of externally bonded Fiber Reinforced Polymer (FRP) sheets, to increase the flexural capacity of the structural element, is described. Several researches developed FRP strengthening of reinforced concrete and masonry structures, but few experimental studies about steel and steel-concrete composite elements are available. Some examples of guidelines for the design and construction of externally bonded FRP systems for strengthening existing metal structures are available, but the method used to predict the flexural behaviour of FRP strengthened elements is usually based on the hypothesis of elastic behaviour of materials and FRP laminate is mainly considered only under the tensile flange. In this paper, an analytical procedure to predict the flexural behaviour of FRP strengthened steel and steel-concrete composite elements, based on cross-sectional behaviour and taking into account the non-linear behaviour of the materials with any configuration of FRP reinforcement, is given. Analytical predictions are compared with some experimental results available in the literature on the flexural behaviour of FRP strengthened steel and steel-concrete composite elements, showing good agreement of the results, even in the non-linear phase, until failure.     

A systematic analysis and design approach for single-span FRP deck/stringer bridges

There is a concern with worldwide deterioration of highway bridges, particularly reinforced concrete. The advantages of fiber reinforced plastic (FRP) composites over conventional materials motivate their use in highway bridges for rehabilitation and replacement of structures. In this paper, a systematic approach for analysis and design of all FRP deck/stringer bridges is presented. The analyses of structural components cover: (1) constituent materials and ply properties, (2) laminated panel engineering properties, (3) stringer stiffness properties, and (4) apparent stiffnesses for composite cellular decks and their equivalent orthotropic material properties. To verify the accuracy of orthotropic material properties, an actual deck is experimentally tested and analyzed by a finite element model. For design analysis of FRP deck/stringer bridge systems, an approximate series solution for orthotropic plates, including first-order shear deformation, is applied to develop simplified design equations, which account for load distribution factors under various loading cases. An FRP deck fabricated by bonding side-by-side box beams is transversely attached to FRP wide-flange beams and tested as a deck/stringer bridge system. The bridge systems are tested under static loads for various load conditions, and the experimental results are correlated with those by an approximate series solution and a finite element model. The present simplified design analysis procedures can be used to develop new efficient FRP sections and to design FRP highway bridge decks and deck/stringer systems, as shown by an illustrative design example.     

Analysis and design of fiber reinforced plastic composite deck-and-stringer bridges

A comprehensive study on analysis and design of fiber reinforced plastic (FRP) composite deck-and-stringer bridges is presented. The FRP decks considered consist of contiguous thin-walled box sections and are fabricated by bonding side-by-side pultruded thin-walled box beams, which are placed transversely over FRP composite stringers. In this study, we review the modeling and experimental verification of FRP structural beams, including micro/macro-mechanics predictions of ply and laminate properties, beam bending response, shear-lag effect, and local and global buckling behaviors. A simplified design analysis procedure for cellular FRP bridge decks is developed based on a first-order shear deformation macro-flexibility (SDMF) orthotropic plate solution. The present approach can allow the designers to analyze, design and optimize material architectures and shapes of FRP beams, as well as various bridge deck configurations, before their implementation in the field. Experimental studies of cellular FRP bridge decks are conducted to obtain stiffness coefficients, and an example of a cellular FRP deck on optimized winged-box FRP stringers under actual track-loading is presented to illustrate the analytical method. The experimental-analytical approach presented in this study is used to propose simplified engineering design equations for new and replacement highway FRP deck-and-stringer bridges.     

The restrained torsional response of open section carbon fibre composite beams

The analysis procedure outlined in this paper essentially makes use of the existing isotropic theories of torsion suitability modified to account for the non isotropic nature of typical carbon fibre composite material.

The warping and St Venant torsional stiffnesses of the beams are determined using the appropriate equivalent engineering elastic constants of the composite material which correspond to the membrane and bending modes of action respectively.

The differential equation governing the constrained torsional equilibrium of the open section beams is solved exactly in the paper for Z and channel sections with some emphasis being given to the influence of ply stacking sequence.

Theoretical results are presented in graphical form and these depict the variations in warping displacement, warping shear flow and longitudinal or axial constraint force intensity with applied torque for the cantilever beam condition with torque applied at the free end.

The paper also gives details of finite element studies of the composite beams and of an experimental programme of work pertaining solely to the behaviour of composite Z beams.

Comparisons between theory, finite element and experiment are presented and these are seen to give exceptionally close agreement.

It is clearly indicated that fibre orientation significantly influences the restrained torsional behaviour of thin-walled open-section composite beams. It is also clear that the use of the appropriate equivalent engineering elastic material constants in the theory is able to closely predict actual behaviour.     

Development of the composite bumper beam for passenger cars

The fuel efficiency and emission gas regulation of passenger cars are two important issues in these days. The best way to increase the fuel efficiency without sacrificing safety is to employ fibre reinforced composite materials in the body of cars.

In this work, a new composite bumper that has two pads at the ends of the bumper was developed. The two pads were designed to hit the front two tyres of the car when the bumper brackets collapsed during collision. The end of the bumper beam was designed to have a tapered section to absorb energy by progressive buckling when the pads hit the rims of wheels after collapsing tyres.

The composite bumper beam was made of glass fibre fabric epoxy composite material except the elbow section. The elbow section was made of carbon fibre epoxy composite material to increase bending stiffness. From the static bending test of the prototype composite bumper, it was found that the weight of the composite bumper beam was only 30% that of the steel bumper beam without sacrificing the static bending strength.     

Development of the composite flexspline for a cycloid-type harmonic drive using net shape manufacturing method

The flexspline for a harmonic drive must be flexible in the radial direction for the elastic deformation, but must be stiff in the torsional direction for accurate transmission of rotational motion. Since these requirements cannot be satisfied simultaneously with conventional metals such as steel or aluminium, in this work the carbon fiber epoxy composite material was employed for the flexspline material in order to increase the torsional stiffness by tailoring the stacking sequence and to improve the manufacturing productivity by moulding rather than machining.

The toothed composite flexspline was manufactured with the elastomeric cascade tooling that is composed of the steel tooth die, the silicon rubber mandrel and the cone-shaped steel core. Also, the steel flexspline with the same dimensions of the composite flexspline was manufactured by CNC wire cutting method.

The static and dynamic performances of the composite flexspline and the steel flexspline were experimentally tested. From the test results, it was found that the developed composite flexspline had better flexibility in the radial direction and high damping capacity at the fundamental natural frequency.     

Structural solution for a composite glider wing with suction boundary layer control for 100% laminar flow

The possibility of airfoil drag reduction by active laminar flow control is well known. The reduced airfoil drag compared to conventional laminar flow airfoils can improve the performance of a glider significantly. A dedicated airfoil designed to operate with boundary layer control could be used on a very high performance glider if a structurally sound design can be created and manufactured.

This paper deals with the structural design, production and subsequent testing of a 1 m long wing test specimen, containing a spanwise orientated suction slit. The novel feature of this design is a composite truss web for the rear spar with very small individual members. This design permits an undisturbed internal flow of the sucked away air mass. The specimen is designed to meet stiffness and strength requirements approximately equivalent to the ASW 25 glider at the 1/4 semispan position.

The test wing box was loaded, and met the torsional stiffness and strength requirements.     

Ultimate strength prediction for RC beams externally strengthened by composite materials

A simple and efficient computational analysis to predict the nominal moment capacity of RC beams strengthened with external FRP laminates is presented. It considers the determination of the limits on the laminate thickness in order to assure tensile failure due to steel yielding and to avoid tensile failure due to FRP laminate rupture. The study presents the design of laminate thickness to attain a specified moment capacity in a given beam. Furthermore, the study affords the approach to determine the laminate thickness of any type of composite material available that is equivalent to FRP laminate required to achieve the desired moment capacity. This approach helps in studying comparative costs of rehabilitation using different FRP materials. The analytical and experimental results of series of beams strengthened with different number of layers of glass/epoxy or carbon/epoxy FRP laminates are presented. The results show that the design guidelines presented in this study performed well in prediction of experimental results.     

Development of the anthropomorphic robot with carbon fiber epoxy composite materials

The material for the robot structure should have high specific stiffness (stiffness/density) to give positional accuracy and fast maneuverability to the robot. Also, the high material damping is beneficial because it can dissipate the structural vibration induced in the robot structure. This cannot be achieved through conventional materials such as steel and aluminum because these two materials have almost the same specific stiffnesses which are not high enough for the robot structure. Moreover, steel and aluminum have low material dampings.

Composites which usually consist of very high specific modulus fibers and high damping matrices have both high specific stiffnesses and high material dampings. Therefore, in this work, the forearm of an anthropomorphic robot which has 6 degrees of freedom, 70 N payload and 0·1 mm positional accuracy of the end effector was designed and manufactured with high modulus carbon fiber epoxy composite because the magnitudes of the mass and moment of inertia of the forearm of an anthropomorphic robot are most important due to its farthest position from the robot base.

Two power transmission shafts which deliver the power of the motors positioned at the rear of the robot forearm to the wrist and the end effector were also designed and manufactured with high modulus carbon fiber epoxy composite to reduce weight and rotational inertia. The mass reduction of the manufactured composite forearm was 15·9 kg less than the steel forearm.

The natural frequencies and damping capacity of the manufactured composite arm were measured by the fast Fourier transform method and compared to those for the steel arm. From the test, it was found that both the fundamental natural frequency and damping ratio of the composite arm of the robot were much higher than those of the steel arm.     

Evaluation of the built-in stresses and residual distortions on cured composites for space antenna reflectors applications

Manufacturing and thermal distortion RMS is an important dimensional parameter which can be correlated with the antenna performances. For this reason it is used to characterize their allowed dimensional stability in order to guarantee the mission requirements.

The antenna reflectors on board of space platforms are generally manufactured with composite materials. The RMS of these structures is very tightly connected with the technologies like curing cycles, kind of materials, lay up tipology, mould configuration etc. The maximum effort is produced to minimize this parameter due to the manufacturing, while the prediction methods able to correlate the residual distortions versus the applied technology can be very useful to optimize the hardware performances.     

Novel applications of composite structures to robots, machine tools and automobiles

Mechanical design can be classified into stiffness design and strength design. In the stiffness design, the stiffness or deformation of members is concerned, and the enhancement of dynamic characteristics such as natural frequency or damping capacity of members or systems is also important. While, in the strength design, the primary concern is the enhancement of load carrying ability of members or systems.

Fiber reinforced composite materials offer a combination of strength and modulus that are either comparable to or better than many traditional metallic materials. Because of their low specific gravities, the strength-weight ratios, and modulus-weight ratios of these composite materials are much superior to those metallic materials. Composite materials can be tailored to meet the specific requirements of each particular design. Available design parameters are the choice of materials (fiber, matrix), the volume fraction of fiber and matrix, fabrication method, number of layers in a given direction, thickness of individual layers, type of layer (unidirectional or fabric), and the layer stacking sequence.

The greatest disadvantages of composite materials are the costs of the materials and the lack of well-defined design rules, therefore, composite materials should be applied in the right place with appropriate design rules. Up to now, the fiber reinforced composite structures are mainly employed in the strength design such as aircraft, spacecraft and vehicles.

In this paper, the novel application examples of composite structures to components for the robots, machine tools and automobiles are addressed considering the stiffness design issues of composite structures.     

ARALL: A materials challenge for the next generation of aircraft

The development of new aircraft is significantly influenced by the introduction of new structural materials. A driving force behind such developments is the perpetual incentive of reducing the aircraft operating costs of which the most important aspects are fule consumption and maintenance.

The structural weight of the aircraft has a high impact on the fuel consumption. Reduction of the weight can be achieved by the introduction of advanced structural design techniques together with new materials which have low density and high (static and dynamic) stress level capabilities. Maintenance costs can be reduced mainly by enlarging the inspection periods of the aircraft. These two aspects, together with the new damage tolerance requirements (FAR and JAR) and durability questions in view of increasing aircraft lives, make the design of an aircraft with low operating costs very complex and difficult.

In this whole process the selection of materials becomes a critical issue. This is illustrated by considering three potential families of materials:

1. - aluminium alloys (both new and conventional)

2. - composites

3. - aramid aluminium laminate (ARALL).

Special attention is given to the development and possible applications of the ARALL material.     

Effect of natural stitched composites on the crashworthiness of box structures

In the present paper the effects of stitching on the energy absorption and crashworthy behaviour of composite box structures will be studied. The combination of unidirectional carbon fibre-reinforced polymer (CFRP) and glass fibre-reinforced polymer (GFRP) composite materials are used to laminate the composite boxes. Delamination study in Mode-I with the same lay-up was carried out to investigate the effect of stitching on delamination crack growth on energy absorption of stitched and non-stitched composite box structures. The double cantilever beam (DCB) standard test method was chosen for delamination studies. For non-stitched and stitched composite boxes the lamina bending and brittle fracture crushing modes were observed. It was found that the stitched composite boxes which show higher fracture toughness in Mode-I delamination tests, are not necessarily able to absorb more crushing energy in comparison with non-stitched composite boxes. It was also observed that the position of stitched area can affect the crushing mode and consequently energy absorption capability of composite box structures. The main reason can be related to other mechanisms such as bending, friction and bundle fracture which significantly contribute to energy absorption. The analytical model based on energy balance approach is proposed to estimate the mean crushing force, F_m, in axial crushing of square composite box.     

Impact analysis of fiber reinforced polymer honeycomb composite sandwich beams

Large scale fiber reinforced polymer (FRP) composite structures have been used in highway bridge and building construction. Recent applications have demonstrated that FRP honeycomb sandwich panels can be effectively and economically applied for both new construction and rehabilitation and replacement of existing structures. This paper is concerned with impact analysis of an as-manufactured FRP honeycomb sandwich system with sinusoidal core geometry in the plane and extending vertically between face laminates. The analyses of the honeycomb structure and components including: (1) constituent materials and ply properties, (2) face laminates and core wall engineering properties, and (3) equivalent core material properties, are first introduced, and these properties for the face laminates and equivalent core are later used in dynamic analysis of sandwich beams. A higher-order impact sandwich beam theory by the authors [Yang MJ, Qiao P. Higher-order impact modeling of sandwich beams with flexible core. Int J Solids Struct 2005;42(20):5460–90] is adopted to carry out the free vibration and impact analyses of the FRP honeycomb sandwich system, from which the full elastic field (e.g., deformation and stress) under impact is predicted. The higher order vibration analysis of the FRP sandwich beams is conducted, and its accuracy is validated with the finite element Eigenvalue analysis using ABAQUS; while the predicted impact responses (e.g., contact force and central deflection) are compared with the finite element simulations by LS-DYNA. A parametric study with respect to projectile mass and velocity is performed, and the similar prediction trends with the linear solution are observed. Furthermore, the predicted stress fields are compared with the available strength data to predict the impact damage in the FRP sandwich system. The present impact analysis demonstrates the accuracy and capability of the higher order impact sandwich beam theory, and it can be used effectively in analysis, design applications and optimization of efficient FRP honeycomb composite sandwich structures for impact protection and mitigation.     

Development of a pultruded composite material highway guardrail

Fiber-reinforced polymer (FRP) composite materials are being used to develop products for use as highway appurtenances, such as, sign supports, luminaire supports and guardrails (crash barriers). These structures, that are located alongside highways and roads, are subjected to vehicular impacts and must be designed to be ‘crashworthy’ to ensure the safety of the driving public. This paper reviews an ongoing 10-year research and development program funded by the United States federal highway administration (FHWA) and the United States department of transportation (DOT) to produce a crashworthy composite material highway guardrail system. An overview of the research and development leading to a patented pultruded composite material guardrail is provided.     

Performance of reinforced concrete beams strengthened by hybrid FRP laminates

In the last two decades, the use of advanced composite materials such as Fiber Reinforced Polymers (FRP) in strengthening reinforced concrete (RC) structural elements has been increasing. Research and design guidelines concluded that externally bonded FRP could increase the capacity of RC elements efficiently. However, the linear stress–strain characteristics of FRP up to failure and lack of yield plateau have a negative impact on the overall ductility of the strengthened RC elements. Use of hybrid FRP laminates, which consist of a combination of either carbon and glass fibers, or glass and aramid fibers, changes the behaviour of the material to a non-linear behaviour. This paper aims to study the performance of reinforced concrete beams strengthened by hybrid FRP laminates.

This paper presents an experimental program conducted to study the behaviour of RC beams strengthened with hybrid fiber reinforced polymer (HFRP) laminates. The program consists of a total of twelve T-beams with overall dimensions equal to 460 × 300 × 3250 mm. The beams were tested under cyclic loading up to failure to examine its flexural behaviour. Different reinforcement ratios, fiber directions, locations and combinations of carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) laminates were attached to the beams to determine the best strengthening scheme. Different percentages of steel reinforcement were also used. An analytical model based on the stress–strain characteristics of concrete, steel and FRP was adopted. Recommendations and design guidelines of RC beams strengthened by FRP and HFRP laminates are introduced.     

Fracture Resistance of a Cracked Concrete Beam Post-strengthened with FRP Sheets

Fibre-reinforced plastic (FRP) composites have been increasingly used in rehabilitation and strengthening of concrete structures. Significant increases in stiffness and strength have been achieved by applying this technique. However, there is concern about the ductility or toughness performance of FRP/concrete hybrid structures, which is critical in the application of this technology. This paper presents a new theoretical method to predict the fracture resistance behaviour of FRP post-strengthened concrete flexural beams. No slip between the FRP and plain concrete matrix is assumed and Mode I fracture propagation is considered. The model is valid for a wide range of span-to-depth ratios and any crack length. The influence of the bridging stresses provided by the fracture process zone (FPZ) at the tip of a fictitious fracture is examined. The effect of various material and geometric parameters on the resistance curve and toughness of the hybrid structure is discussed, based on the numerical results from the developed theoretical formulae. The results provide a useful insight into the strengthening/toughening and the design of FRP sheet/concrete beam structures.     

Effect of delamination failure in crashworthiness analysis of hybrid composite box structures

In the present paper the effects of delamination failure of hybrid composite box structures on their crashworthy behaviour will be studied and also their performance will be compared with non-hybrid ones. The combination of twill-weave and unidirectional CFRP composite materials are used to laminate the composite boxes. Delamination study in Mode-I and Mode-II with the same lay-ups was carried out to investigate the effect of delamination crack growth on energy absorption of hybrid composite box structures. The end-loaded split (ELS) and double-cantilever beam (DCB) standard test methods were chosen for delamination studies. In all hybrid composite boxes the lamina bending crushing mode was observed. Regarding the delamination study of hybrid DCB and ELS the variation of the specific energy absorption (SEA) versus summation of G_IC and G_IIC were plotted to combine the effect of Mode-I and Mode-II interlaminar fracture toughness on the SEA. From this relationship it was found the hybrid laminate designs which showed higher fracture toughness in Mode-I and Mode-II delamination tests, will absorb more energy as a hybrid composite box in crushing process. The crushing process of hybrid composite boxes was also simulated by finite element software LS-DYNA and the results were verified with the relevant experimental result.     

Behaviour of hybrid FRP–UHPC beams subjected to static flexural loading

This paper provides the experimental results of a new hybrid beam intended for use in bridge applications. The hybrid beams were made up of pultruded Glass Fibre Reinforced Polymer (GFRP) hollow box section beams strengthened with a layer of Ultra-High-Performance-Concrete (UHPC) on top and either a sheet of Carbon FRP (CFRP) or Steel FRP (SFRP) on the bottom of the beam. Four hybrid FRP–UHPC beams were tested along with one control GFRP hollow box beam under four-point static flexural loading. Two types of beams were tested (Phase I and Phase II), which incorporated different connection mechanisms at the GFRP–UHPC interface. It was concluded that the hybrid beams had higher flexural strength and stiffness than the control beam, where the beams reinforced with SFRP showed greater percent cost effectiveness than beams reinforced with CFRP. In addition, the improved connection mechanism used in Phase II beams was found to provide adequate interface bond strength to maintain full composite action until ultimate failure.     

Optimizing the performance characteristics of beams strengthened with bonded CFRP laminates

Ductility is of fundamental importance in the design of concrete structures. With structures using conventional materials such as concrete and steel, ductility of a member as a whole can be satisfactorily defined in terms of deflection, curvature or energy absorption capacity as examplified by the area under the load-deflection curve. However, when structural members strengthened with externally bonded fibre reinforced polymer (FRP) laminates are considered, conventional definitions of ductility become less precise because of the brittle behaviour of FRP materials, and their resultant effect on the performance of the strengthened beam. Furthermore, such beams when stregthened without the provision of external anchorages will fail very suddenly, with abrupt debonding of the laminate and substantial loss of load capacity. This paper intends to propose a new criterion to evaluate the structural performance of such strengthened composite beams, and the efficiency of the external anchorage system. The design criterion termed, Peformance Factor, incorporates both the deformability and strength of composite beams. Unlike the concept of toughness as applied to materials, the Performance Factor incorporates the effect of numerous parameters which influence structural design. To examine the reliability of this parameter a series of eleven reinforced concrete beams were tested to evaluate the structural performance of beams strengthened with and without externally bonded carbon fibre reinforced polymer (CFRP) laminates, and with different types of internal reinforcement and external anchorage systems. The structural behaviour of these beams was then evaluated using the Performance Factor.