Carbon Fiber Reinforced Polymer Repair Research Articles

Implementation and precision of CFRP geometric configurations before adhesive bonding repair based on 1064 nm infrared pulsed fiber laser ablation

Before the adhesive bonding repair of the aircraft’s CFRP (Carbon Fiber Reinforced Polymer) structure, the common practice is to remove the damaged area material and form a predetermined repair geometric configuration using the grinding method according to the repair design shape. The geometric accuracy, dimensional accuracy, and surface characteristics of the geometric configuration have a significant impact on the adhesive bonding and repair effectiveness in CFRP structure repair. In the study, a 1064 nm infrared pulse laser was used to selectively and controllably remove the three layers of cured prepreg (fiber and resin) on the surface of the CFRP laminate structure. Based on the analysis of geometric accuracy and surface morphology, the effectiveness of CFRP adhesive bonding repair was verified. The results showed that, under the condition of undamaged CFRP fibers and thorough removal of the surface resin, the laser achieved a dimensional accuracy of ± 50 μm and a thickness accuracy of ± 20 μm for CFRP geometric configuration. The angular accuracy of rectangular configuration was less than ± 0.15°, and the roundness accuracy of circular configuration was greater than 0.995. The mechanical validation of the adhesive bonding repair indicated that the tensile strength recovery rate of CFRP repair structure after laser treatment was 84.7 %, and the failure mode was cohesive failure.

Debonding quantification in adhesive bonded joints by the inverse finite element method

In the past two decades, the aerospace industry has massively shifted from aluminum-made components to composite materials such as carbon fiber reinforced polymers (CFRP), striving for more fuel efficient and lighter aircrafts. Consequently, traditional joints have been replaced by adhesive bonded interfaces, which are also the most common choice to repair damaged components. Although adhesive bonding is the most efficient choice for permanent connections, it is not free of disadvantages: one of the most common failure modes, the debonding of the two laps, is very problematic to detect and predict in practice. Therefore, frequent inspections must be performed to ensure structural safety, increasing maintenance costs, and lessening the availability of the platforms. The development of innovative sensing technologies has allowed for a close monitoring of structural interfaces, and several structural health monitoring techniques have been proposed to monitor adhesive bonded connections. Sensitivity and correlation between measurements and debonding entity has been demonstrated in the literature: nevertheless, hardly any technique has been proposed and quantitively evaluated to estimate the debonding entity independently of the applied loads, such as misalignment-induced torsion, which is a major confounding influence in the traditional backface strain gauge technique. This paper proposes the inverse finite element method (iFEM) as a load and material independent approach to infer the debonding entity from strain measurements in adhesive-bonded joints. Two approaches to estimate the debonding entity with the iFEM are compared on cracked leap shear specimens representative of CFRP repair patches: one is based on anomaly indexes, the other on performing a model selection with multiple iFEM models including different damages. The latter demonstrates satisfactory performances; thus, it is considered a significant scientific advancement in this field.

Impact of design parameters and cycles of damage loads on CFRP repair effectiveness of Shear-Deficient RC structures

• The study utilize experimental static load test to assess the effectiveness of CFRP sheets as externally bonded repair system for shear-deficient RC structures. • The study investigates different shear damage locations, different design parameters, and different pre-repair damage levels. • The CFRP sheets were found to be effective as a shear repair system regardless of the pre-repair damage level, shear damage location and existence of shear stirrups. • The results show no contribution from the shear stirrups to the load capacity at the pre-repair stage for shear damage closer to the support. • The failure mode was a combination of flexural cracks and CFRP delamination of a concrete layer for shear damage at quarter-span, and a combination of shear cracks and CFRP debonding for shear damage close to the support. An experimental static load test was used to assess the shear damage and ascertain the effectiveness of an externally bonded carbon-fibre-reinforced polymer (CFRP) repair system. The static test-based evaluation covers load against deflection, load against steel strain, load against CFRP strain and failure mode. The study investigates different shear damage locations, different shear design parameters and different pre-repair damage levels. A total of eight RC beams were prepared and categorised into two groups with different loading conditions. The first group of beams was tested under a concentrated point load at quarter-span to induce shear damage, while the second group was for shear damage with load positions at 1.5 times the beam depth from the support. The CFRP sheets were found to be effective as a shear repair system regardless of the pre-repair damage level, shear damage location and existence of shear stirrups. The failure mode is a combination of flexural cracks and CFRP delamination of a concrete layer for shear damage at quarter-span and a combination of shear cracks and CFRP debonding for shear damage close to the support.

Numerical study of surface fatigue crack growth in steel plates repaired with CFRP

This paper presents a numerical study of the fatigue behaviour of carbon fibre-reinforced polymer (CFRP) repaired steel plates with a surface crack under uniaxial cyclic tension. A simulation programme for surface fatigue crack propagation has been developed by means of Abaqus-Python scripting. Validation of models was first performed analytically and experimentally, through evaluation of the stress intensity factor (SIF) and fatigue life of unrepaired specimens, respectively. The developed programme was then used to simulate the fatigue behaviour of surface cracked specimens repaired with CFRP sheets. Different aspect ratios of surface cracks, numbers of CFRP layers, CFRP sheet widths and types and CFRP repair schemes (i.e. single- and double-sided reinforcement) were considered, to assess their influence on the CFRP repair efficiency. The results showed that applying CFRP sheets could effectively reduce the SIF of the surface cracks, thereby prolonging the fatigue life of the steel plates. Further, the repair performance improved with more and wider CFRP sheets and higher equivalent modulus but remained almost unchanged for different crack aspect ratios, while the single-sided CFRP repair outperformed the double-sided repair in efficiency. The present study is of instructive value for application of CFRP materials to the repair of surface cracked metallic structures.

Experimental Study of the Fatigue Performance of the Bonding Surfaces and Load-Bearing Capacity of a Large-Scale Severely Damaged Hollow Slab Strengthened by CFRP

In recent years, carbon fiber reinforced polymer (CFRP) has been widely used in bridge repair, retrofitting, rehabilitation and strengthening to improve the bearing capacity. Although many studies have been conducted to explore the strengthening efficiencies of CFRP, the test specimens were small and the results were difficult to apply to full-scale bridges. Investigations into the strengthening effects of CFRP on real life structures rely on field load tests (without damaging the structures), making it difficult to understand actual improvements in load carrying capacity and strengthening effect. Moreover, there have been few experimental studies on the fatigue performances of CFRP-strengthened structures, especially on the large-scale structures with real wheel moving loads. In this study, the feasibility and efficiency of CFRP strengthening and repair was investigated on a large-scale, prestressed concrete hollow slab decommissioned from a real-life concrete bridge. The hollow slab was first put through a destructive test to test the ultimate load-bearing capacity. Then, CFRP strips were installed on the surface of the severely damaged slab to repair and strengthen it. Fatigue load test—including the moving load test and single point sinusoidal load—and load-bearing capacity tests were conducted on the CFRP-strengthened hollow slab after the destructive test to evaluate the strengthening performance. This study could help us to understand the actual load-bearing capacities of severe damaged concrete structures strengthened by CFRP, reduce waste, save resources and improve the utilization of our infrastructures.

Full-scale experiment and numerical simulation of prestressed concrete cylinder pipe with broken wires strengthened by prestressed CFRP

Prestressed concrete cylinder pipe (PCCP) is widely used in large-scale water supply projects. The most common damage mode of PCCP is caused by rupture of prestressed wires. To strengthen a damaged PCCP, the technology of wrapping prestressed carbon fiber reinforced polymer (CFRP) externally is proposed. A full-scale test on a PCCP with an inside diameter of 1400 mm and a length of 6000 mm is conducted. In the test, three pipe sections are selected with broken wires, unbroken wires, and broken wires but repaired by prestressed CFRP. The strain response of the material in each section is analyzed by applying stepwise increased internal water pressure. Then, a finite element model is established and calibrated. Compared with the section containing broken wires, the material strain of the section repaired by prestressed CFRP decreases with almost no damage, indicating that prestressed CFRP is an effective repair method for damaged PCCP. Additionally, the influence area of broken wires is rather limited, showing negligible effect beyond a distance of 1300 mm from the broken wire area. When a PCCP reaches the serviceability limit state, prestressed CFRP repair and non-prestressed CFRP repair can increase the bearing capacity of the pipe by 87.3% and 34.9%, respectively.

Ultimate strength of a cracked stiffened panel repaired by CFRP and stop holes

An analysis of the ultimate strength of a cracked stiffened panel repaired by carbon fibre reinforced polymer (CFRP) under axial compression is presented. The analysis method adopts the nonlinear mechanics of adhesives in the interaction of CFRP with the steel structure. The numerical model is validated by tests of cracked plates repaired by CFRP. The collapse of the cracked stiffened panel repaired by CFRP revealed that the debonding of CFRP happens after buckling. The CFRP repair can recover the ultimate strength of the cracked structure to the one of the intact panel. A series of analysis is performed with the validated numerical model to investigate the effects of geometry, configuration of CFRP and crack length on the ultimate strength. Crack treatment with stop holes is also studied, and the results show that the repair of the CFRP can recover the ultimate strength of the cracked stiffened panel with the occurrence of the stop holes, at the same time as it may reduce the stress levels significantly.

Finite Element Analysis of Composite Repair for Damaged Steel Pipeline

Carbon fiber reinforced polymer (CFRP) matrix composite overwrap repair systems have been introduced and accepted as an alternative repair system for steel pipeline. This paper aimed to evaluate the mechanical behavior of damaged steel pipeline with CFRP repair using finite element (FE) analysis. Two different repair strategies, namely wrap repair and patch repair, were considered. The mechanical responses of pipeline with the composite repair system under the maximum allowable operating pressure (MAOP) was analyzed using the validated FE models. The design parameters of the CFRP repair system were analyzed, including patch/wrap size and thickness, defect size, interface bonding, and the material properties of the infill material. The results show that both the stress in the pipe wall and CFRP could be reduced by using a thicker CFRP. With the increase in patch size in the hoop direction, the maximum von Mises stress in the pipe wall generally decreased as the maximum hoop stress in the CFRP increased. The reinforcement of the CFRP repair system could be enhanced by using infill material with a higher elastic modulus. The CFRP patch tended to cause higher interface shear stress than CFRP wrap, but the shear stress could be reduced by using a thicker CFRP. Compared with the fully bonded condition, the frictional interface causes a decrease in hoop stress in the CFRP but an increase in von Mises stress in the steel. The study results indicate the feasibility of composite repair for damaged steel pipeline.

Effect of web opening diameter on performance and failure mode of CFRP repaired RC beams

Abstract This paper presents an experimental assessment of the repair effectiveness of Carbon Fibre Reinforced Polymer (CFRP) laminate for damaged Reinforced Concrete (RC) beams with a circular web opening at the shear zones. The study highlights the effect of the opening diameter on CFRP repair effectiveness by examining the effect on the ultimate load capacity, deflection, steel strain, CFRP laminate strain and failure modes. In the experimental programme, a total of four beams were used inclusive of one solid beam used as a control beam. The other three beams were designed with different opening diameters and were exposed to damage at the pre-repair stage, up to their ultimate capacity. Damaged beams were repaired using externally bonded CFRP laminate around the openings. The results concluded that the CFRP laminate repair system was effective regardless of the web opening diameter. It was found that the increase in the opening diameter resulted in a higher deflection as well as higher strain values for the mid-span reinforcement and shear stirrups. The CFRP repaired beams behaviour transformed from brittle at the pre-repair stage to ductile at the post-repair stage. The failure mode of the CFRP repaired beams with a smaller opening diameter changed from shear failure to flexural failure, while the larger opening diameter beams failure mode was governed by CFRP deboning at the shear zone.

Linking seismic resilience into sustainability assessment of limited-ductility RC buildings

While sustainability addresses the economic, social and environmental impacts distributed over the life cycle of a structure; commonly, ordinary events such as construction, normal operation and ageing are considered and the resilience of the structure to extreme events (natural disasters and man-made hazards) is often ignored. This study aims to introduce a framework to systematically integrate seismic-resilience into sustainability assessment of structures/infrastructures. A case study of a multi-story limited-ductility reinforced-concrete (RC) building with soft-story collapse mechanism is presented. This mode of failure is often expected in regions of low to moderate seismicity, where the detailing is poor. A numerical model is constructed based on the results of a number of multi-axis hybrid simulations allowing to realistically capture the three-dimensional response and the damage states of the RC building first-story columns from linear-elastic range to collapse. In addition, the application of carbon-fiber reinforced polymers (CFRP) as a cost-effective and rapid repair strategy in restoring the building columns is studied. The CFRP repair is proposed as an alternative to the demolition of the full building when subjected to large deformations; thereby, significantly enhancing post-disaster reparability and resilience of the structure. The developed numerical models are then used to quantify the sustainability metrics including repair cost, repair downtime and repair environmental impacts. These metrics are quantified by consecutive hazard, structural response, damage state, and recovery loss analyses in accordance to the PEER PBEE methodology. The results are then used to quantify the resilience parameters, including loss of performance and recovery, using the probabilistic resilience-based earthquake engineering (RBEE) approach.

Finite element analysis of fatigue crack growth in CFRP-repaired four-point bend specimens

This paper presents a finite element (FE) study on fatigue crack growth in carbon fiber–reinforced polymer (CFRP) repaired four-point bend (FPB) specimens with an edge crack subjected to various cyclic loadings (pure mode I, pure mode II and mixed mode I/II). The crack propagation rate was described using NASGRO model together with Tanaka formula, while the crack growth direction was determined by maximum circumferential stress criterion (MCSC). A simulation program was developed using Python scripting in conjunction with FE software ABAQUS to handle the automated fatigue crack growth modelling. The effects of different shear-tension loading ratios (KII/KI) and CFRP layer numbers on CFRP repair performance were first investigated by a comparative analysis of numerical and experimental results. The FE results appeared to be in reasonable agreement with test data in terms of specimens’ failure modes, crack growth trajectories and fatigue lives. Based on the developed program, a parametric study was conducted to gain a deeper insight into the effects of KII/KI ratios, the patching layer number, Young’s modulus and width of CFRP on FPB specimens’ fatigue behavior. The results indicated that the above-mentioned parameters have a significant influence on fatigue crack growth to varying degrees. This work provides an effective way of predicting the fatigue life and mixed mode I/II crack growth trajectories with high computational efficiency and accuracy.

Experimental Study of Aged and Seriously Damaged RC Beams Strengthened Using CFRP Composites

This paper presents results from experiments on aged and seriously damaged reinforced concrete (RC) beams strengthened with different arrangements of external carbon fiber‐reinforced polymer (CFRP) laminates and end anchorages. Seven RC beams from an old bridge, measuring 250 × 200 × 2300 mm, were tested. All specimens were loaded to yield load to evaluate initial mechanical properties. Then, these seriously damaged specimens were repaired using different CFRP‐reinforcing schemes and reloaded to failure. The yield load growth due to CFRP reinforcement ranged from 5% to 36%. Different parameters including CFRP dimension and position, bonding length, and end anchorage were investigated and facilitated conclusions on beam ductility, load‐midspan deflection response, and failure mode. This research contributes to knowledge about the CFRP repair of aged and seriously damaged beams to ensure better performance in overloaded conditions.

Cost Effectiveness of CFRP Repair Technique for Cold Formed Steel Beams

Objectives: Structural steel can be broadly classified into hot rolled steel and cold formed steel (CFS). CFS manufactured from roll forming or press braking operation. The main advantage of CFS sections are thin wall and light weight. These thin wall leads to buckling of structural member. This paper aims to study the capacity of Carbon Fibre Reinforced Polymer (CFRP) strengthened CFS channel beams, buckling behaviour, deflection and cost studies on CFRP strengthening. Methods/Statistical analysis: In this study three different length of CFS channel sections are selected for testing in which beams are tested for bending. All the beams were tested in plain section and CFRP strengthened series in universal testing machine. Findings: Experimental results shows the enhancement in capacity of beam due to CFRP strengthening. The local and distortional buckling modes were observed as failure modes. Experimental results were compared with maximum load resistance calculated by AISI specification which are in good agreement. Cost studies were compared on replacing a new member with CFRP strengthened channel member. Improvements/Applications: CFRP strengthening is suitable for enhancement of capacity of beams and repair. This CFRP strengthening technique can applied to reinforced concrete members, hot rolled steel, in filled steel tubes and bridges etc.

Experimental investigations of repair of pre-damaged steel-concrete composite beams using CFRP laminates and mechanical anchors

This paper investigates the flexural performance of pre-damaged steel-concrete composite beams repaired using externally-bonded carbon fiber-reinforced polymer (CFRP) laminates with and without mechanical anchors. A total of 10 beams were prepared, one beam was left undamaged, whereas nine beams were artificially damaged by cutting different U-shaped notches in the bottom flange at the beams’ mid-span that resulted in 45%, 73% and 100% losses in the flange thickness. Three damaged beams were not strengthened, whereas six damaged beams were repaired in flexure using externally-bonded CFRP laminates with and without mechanical anchors. The load-carrying capacities of the unstrengthened beams with damage states of 45%, 73% and 100% were approximately 11%, 23%, and 50% lower than that of the control-undamaged beam, respectively. The CFRP repair schemes were capable of restoring the original load capacity of the damaged beams with the lower damage state of 45%. For the beams with the higher damage states of 73% and 100%, the repair schemes could restore a maximum of 81% of the original load capacity. The inclusion of mechanical anchors in the repair regime improved the strength gain from 15% to 19% and from 46% to 63% for the beams with 45% and 100% damage states, respectively, relative to the strength of the corresponding damaged-unstrengthened beam.

Fatigue Crack Propagation of Notched Steel Rebar in RC Beams Repaired with Externally Bonded CFRP

AbstractThis paper presents the results of an experimental study on the fatigue performance of RC beams strengthened with different externally bonded carbon fiber-reinforced polymer (CFRP) systems. Seven specimens were fabricated; three had no CFRP, while the remaining four had one of two CFRP systems. The objective of the research reported in this paper was to determine the effect the CFRP repair had on the growth rate of a fatigue crack initiating from a notch in the tensile rebars. The steel reinforcement was selected for focus because similar studies have found this reinforcement to be the limiting fatigue component in RC beams. The experimental results reported in this paper showed an extended fatigue life and a slowed crack growth rate in specimens repaired with both CFRP systems. The crack growth rates were used to determine material constants for the Paris law, which describes growth of a stable fatigue crack. These results are then used to propose recommendations for design of CFRP repair systems...

Behavior of Laterally Damaged Prestressed Concrete Bridge Girders Repaired with CFRP Laminates Under Static and Fatigue Loading

Many bridges are subject to lateral damage for their girders due to impact by over-height vehicles collision. In this study, the optimum configurations of carbon fiber reinforced polymers (CFRP) laminates were investigated to repair the laterally damaged prestressed concrete (PS) bridge girders. Experimental and analytical investigations were conducted to study the flexural behavior of 13 half-scale AASHTO type II PS girders under both static and fatigue loading. Lateral impact damage due to vehicle collision was simulated by sawing through the concrete of the bottom flange and slicing through one of the prestressing strands. The damaged concrete was repaired and CFRP systems (longitudinal soffit laminates and evenly spaced transverse U-wraps) were applied to restore the original flexural capacity and mitigate debonding of soffit CFRP longitudinal laminates. In addition to the static load tests for ten girders, three more girders were tested under fatigue loading cycles to investigate the behavior under simulated traffic conditions. Measurements of the applied load, the deflection at five different locations, strains along the cross-section height at mid-span, and multiple strains longitudinally along the bottom soffit were recorded. The study investigated and recommended the proper CFRP repair design in terms of the CFRP longitudinal layers and U-wrapping spacing to obtain flexural capacity improvement and desired failure modes for the repaired girders. Test results showed that with proper detailing, CFRP systems can be designed to restore the lost flexural capacity, sustain the fatigue load cycles, and maintain the desired failure mode.

Repair of Prestressed Concrete Beams with Damaged Steel Tendons Using Post-Tensioned Carbon Fiber- Reinforced Polymer Rods

Research implementing unibody clamp anchors and a simple mechanical stressing device to post-tension external, unbonded carbon fiber-reinforced polymer (CFRP) rods is presented. The experiments described in the paper concern three prestressed concrete beams: one was used as the control beam and the other two were damaged. Damage consisted of cracked concrete that was removed and internal steel tendons that were cut to simulate vehicle collision, corrosion, or both. The repair system was then applied to the two damaged concrete beams. The CFRP repair system performed well, increasing the ultimate strength and flexural capacity of the damaged beams to meet or exceed the strength capacity of the control. An analytical model considering the tendon stress at ultimate and the distribution of internal forces was developed to explore design recommendations for the use of the unibody clamp anchors and stressing device for post-tensioning CFRP rods.

Assessment of Carbon Fiber Reinforced Polymer Repair for Laterally Damaged Prestressed Concrete Girders

This paper presents an investigation of the most efficient configuration of carbon fiber reinforced polymers (CFRP) for repairing laterally damaged prestressed concrete (PS) bridge girders. The flexural behavior of 10 half-scale AASHTO type II 20-ft-long PS girders is presented in this paper. Lateral damage due to vehicle collision impact was simulated by sawing through the concrete of the bottom flange and slicing through one of the prestressing strands. The damaged concrete was repaired, and different CFRP repair configurations (longitudinal soffit strips and U-wrapping) were applied. Both the control and repaired PS girders were tested in flexure until failure using a four-point loading setup. Measurements of the applied load, the deflection at five different locations, strains along the cross-section height at mid-span, and multiple strains longitudinally along the bottom soffit were recorded. We also compared the performance of girders with full wrapping of CFRP to that of girders with longitudinal CFRP strips applied to the bottom of girders combined with discrete, spaced U-wrapping strips. Conclusions are drawn regarding the most efficient CFRP design configuration for avoiding CFRP debonding and undesired failure modes.

Fatigue behaviour of CFRP strengthened steel plates with different degrees of damage

An experimental and analytical study was conducted to further investigate the effectiveness of the carbon fibre reinforced polymer (CFRP) plates in extending fatigue life of steel structures. Different lengths of artificial cracks were introduced to represent different degrees of fatigue damage. The experimental results demonstrated that the CFRP patches could effectively slow down the crack growth and prolong the fatigue life. A theoretical model was developed to predict the fatigue life of tested specimens. Thereafter, a parametric study was carried out to investigate the fatigue behaviour of steel plates with a wider range of damage degrees. This study extends the understanding of CFRP repair at different stages of crack propagation and provides some useful suggestions for the strengthening method.

Limits of Application of Externally Bonded CFRP Repairs for Impact-Damaged Prestressed Concrete Girders

Collisions between overheight vehicles and bridges are becoming more commonplace. Generally, collision damage is far from catastrophic, although sound repair techniques are critical if additional damage (typically related to corrosion) is to be mitigated. Although the decision to repair or replace a collision-damaged prestressed concrete structure must ultimately be made on a case-by-case basis, basic criteria for making this decision may be established. This paper describes parameters affecting the efficacy of external carbon fiber-reinforced polymer (CFRP) composite repair methods for restoring the flexural capacity of damaged prestressed concrete bridge girders. The study focuses on impact damage requiring structural repair (rather than simply aesthetic measures). The physical limitations of each CFRP repair method are described to define an initial design space, followed by a parametric study using two prestressed girder shapes (box and I-girder) and three CFRP repair techniques (externally bonded, prestressed, and near-surface mounted) in which the goal was to identify the design space of each technique. The effective design space for each method is then described by the spectra of damage to which the method may be effectively applied. The spectra of damage may be envisioned as having two thresholds: the no repair—repair threshold and the repair—replace threshold.