Carbon Fiber Reinforced Polymer Patch Research Articles

Evaluating the bonding effectiveness of CFRP patches in strengthening concrete structures

This study offers a thorough analysis of the efficiency of adhesively bonded Carbon Fiber-Reinforced Polymer (CFRP) patches in reinforcing concrete structures. In the dynamic field of civil engineering and infrastructure maintenance, utilizing modern materials and innovative techniques is crucial. Given the diverse stresses on concrete structures, sustainable reinforcement strategies like CFRP patching are vital. CFRP patches, with their high tensile strength and lightweight properties, reinforce concrete, enhancing its load-bearing capacity. The study reviews extensive literature on CFRP use, emphasizing its effectiveness in various environments. However, prevailing analyses often overlook shear-induced failures at the interface of bonded CFRP patches. To address this, the study introduces a novel fracture analytical technique, consolidating material parameters for assessing CFRP patch bonding efficiency. Experimental investigations compare epoxy, polyurethane, and silyl-modified polymers under mode-I loading conditions, revealing performance variations. The study underscores the importance of adhesive selection, advocating for a balance between strength and fracture energy absorption for optimal CFRP patch performance in concrete structures. It urges future design applications to consider these findings for enhanced selection and application of CFRP patches.

Investigation on bonding behavior between CFRP patch and corrosion-damaged steel associated with surface preparation techniques

To assess the impact of corrosion on steel and the effectiveness of surface preparation techniques on the bonding behavior between carbon fiber reinforced polymer (CFRP) and steel, thirty bonded joint specimens were examined. These specimens were based on three distinct corrosion levels of steel and three conventional surface preparation techniques. Furthermore, various evaluation methods were employed to understand the steel surface characteristics, encompassing macro and microscale observations, analysis of topographical structures, and quantification of surface evaluation indices. These were performed to determine their correlations with the interfacial stress behavior. The findings revealed that when the target for repair shifted from a new steel substrate to a corroded one, the synergistic performance at the steel-adhesive interface decreased. This led to a decrease in the maximum shear stress at the plate end, and there was also an undesirable shift in the mode of failure. Both test parameters (i.e., the corrosion level and surface preparation), synergistically determined the surface cleanliness, topography, and bonding defects, all three of which directly influenced the bonding behavior. Finally, an improved model for predicting the debonding capacity of CFRP-corroded steel bonded joints was introduced, incorporating surface damage (k1) and surface topography (k2) coefficients. This model's applicability was also verified by collecting other test data from available literature.

Gate‐to‐Gate Study for Collaborative Robot‐Assisted Composite Parts Manufacturing Using a Work Effort Unit Approach

AbstractThis work presents one example from a set of technologies that aim at improving the efficiency of manufacturing processes of aircraft components in one of Airbus Defence and Space plants in regards to environmental aspects. Among the proposed technologies, one relates to the automation of two key manufacturing steps of the reference technology in the composite manufacturing of omega stringers: 1) lamination of main omega stringer structure through an Automated Fiber Placement robotic process; and 2) cobotic pick‐and‐place process for Carbon Fiber Reinforced Polymer patches. To evaluate potential environmental benefits, Fraunhofer performs a gate‐to‐gate study using as main parameter the “Work effort unit,” in this case representing the average energy consumption per productive hour in a typical assembly factory. When comparing the results for the composite part manufacturing by hand lay‐up with the use of collaborative robots, the key technical parameter is the reduction in lead time. While production using robots consume additional electricity when compared to human workers, automation significantly shortens lead times (‐74%), resulting in lower energy consumption per manufactured unit. The results indicate that this new technology from Airbus Defence and Space can potentially improve the overall environmental performance of composite parts during their manufacture.

Flexural performance of squared one-sided CFRP patches: modelling and experimental study

ABSTRACT Three-point bending analysis, experimental and numerical, is conducted for three different sizes, 30*30 mm, 40*40 mm and 60*60 mm, of one-sided square cross-ply TEXIPREG HS 160 RM carbon-fiber-reinforced polymer (CFRP) patches as well as intact and notched specimens. The tests are carried out with the aim to study the influence of the patch’s size on the flexural properties of the specimen. Hashin-2D damage initiation criterion is adopted to investigate the failure in each ply, and the damage evolution is represented by the stiffness degradation. The intact and notched laminates flexural properties are investigated as well as notch sensitivity. The bonded patch repairs efficiency is investigated in the light of ultimate load, flexural modulus, peel and shear stresses along the bond-line as well as the damage initiation coefficient at critical zones. A significant advancement in the flexural modulus and ultimate load for the repaired laminates is noticed with increasing the patch’s size. A Parametric numerical study is conducted to investigate the influence of the patch size on the damage initiation coefficient at the critical zones. The recommended patch’s size is 44*44 mm in order to avoid failure initiation at the patch corner overlap on the parent laminate.

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.

Fatigue crack growth modelling for cracked small-scale structural details repaired with CFRP

This paper presents an analytical model to predict the mode I fatigue crack growth of steel Central Cracked Tensile (CCT) specimens repaired with Carbon Fiber Reinforced Polymer (CFRP) bonded on the two sides of the plate. A review of the behaviors observed in past experimental tests is provided. The CFRP-steel stiffness ratio, as well as the level of the initial crack length and the location of the CFRP patch, influence the performance of the CFRP crack repair. A total of 69 experimental and 7 numerical results of CFRP repaired specimens were collected from the literature and a database is created. Seventy percent of the data (Experimental only) were used for the development of the model and thirty percent for verification. The fatigue crack propagation of each pair of unrepaired and repaired experimental specimens was calibrated for the development of the model. The effect of the CFRP on the fatigue crack propagation is represented by a SIF reduction factor, β. An analytical law based on nonlinear regression analysis is proposed to evaluate β which depends on the CFRP-steel ratio, level of damage of the steel plate, and patch location. The model is fairly validated with experimental and numerical results. A parametric study is conducted for further verification, the model shows reasonable trends.

Enhancing fatigue performance of damaged metallic structures by bonded CFRP patches considering temperature effects

This paper reports an experimental and theoretical investigation on the fatigue performance enhancement of damaged steel structures by bonded carbon fiber reinforced polymer (CFRP) systems considering the temperature effects. First, thermomechanical properties of the epoxy adhesive and CFRPs were examined via dynamic mechanical analysis (DMA) and tensile tests. The appropriate curing procedure for the structural adhesive was obtained. Next, the fatigue behavior of notched steel plates strengthened by bonded CFRPs was investigated at different temperatures. During the fatigue tests, beach marking and back-face strain techniques were used to monitor the fatigue crack growth on steel components and the damage evolution within the bonding interface. The results show that the high-temperature resistance of the epoxy adhesive can be effectively improved by increasing the curing temperature and duration. The properly cured bonded CFRP patches can effectively enhance the fatigue performance of damaged steel structures. However, and the elevated temperatures significantly degrade the fatigue behavior of the CFRP-strengthened steel components.

Reinforcement of Notched PBX Simulant Beams with CFRP Patches-Experimental Study

Strength degradation induced by geometrical discontinuities is a primary cause of structural failure for polymer bonded explosive (PBX). To restore structural integrity and extend service life, reinforcement of weakened PBX structures is urgently demanded. In the present work, carbon fiber reinforced polymer (CFRP) patches are employed to notched PBX simulant beams. By using digital image correlation (DIC) technique, the influences of CFRP patches on mechanical behavior of the beams under bending load are explored. Results show that the CFRP patches endows the beams with increased strength and stiffness. Due to the load-sharing and deformation-confining of the patches, stress concentration at the notch is relieved, and crack propagation in PBX simulant matrix is prevented. These discoveries may provide important enlightment for the reinforcement and repair of the weakened PBX structures, and in addition, provide basis to the potential application of CFRP patches in energetic materials.

Performance evaluation of cracked aluminum alloy repaired with carbon fiber reinforced polymer for aerospace application

Composite patch repairing technique of damaged structure has a substantial potential in aircraft maintenance and crack repair in Aluminium alloy. In this work unidirectional Carbon Fiber Reinforced Polymer (CFRP) patch separation from Aluminium alloy surface is studied through interfacial shear stress point of view whereas earlier studies focused their attention on fracture toughness parameters like stress intensity factor (K) and J-integral. These studies elaborated the importance of patch shape, size and material on fracture toughness. To study interfacial shear stress inclined crack is made in Aluminium alloy sheet and CFRP patch is applied by epoxy adhesives. When the repaired Aluminium alloy model is simulated in Abaqus the numerical solution showed reduction in the J-integral but in actual experiments patches tend to delaminate from the leading edge and the crack face. It is observed that out of plane shear stresses 23 play an important role in delaminating the patch from the Aluminium alloy surface. To suppress the delamination by reducing interfacial shear stress, ply thickness is varied by suggested ply drop technique. Design of experiment is conducted for optimum sequence of CFRP ply. Three configurations show reduced interfacial shear stress. This is validated by testing the specimen on universal testing machine.

Shape memory alloy-carbon fiber reinforced polymer system for strengthening fatigue-sensitive metallic structures

This paper provides an overview of the basic elements, course of development, experimental evaluation, and numerical simulation of a thermally activated shape memory alloy (SMA) and carbon fiber reinforced polymer (CFRP) composite system for fatigue repair or retrofit of metallic structures. Nickel titanium niobium (NiTiNb) SMA wires, which are able to generate 400 MPa recovery stress upon thermal loading and maintain that stress level at a wide range of temperatures, was adopted to apply compressive stresses near the crack. Monotonic bond behavior of single and multiple SMA wires to CFRP was investigated; the debonding onset load and maximum capacity were quantified. The fatigue behavior of patches consisting of multiple wires bonded to CFRP was studied. Results indicated that the system could maintain 80% of the recovery stress, after up to 2 million load cycles, so long as the maximum applied stress was below the debonding onset level. A fatigue strengthening system, using such multiple SMA wire system as underlay and CFRP patch as overlay, was applied to fatigue sensitive steel plates for fatigue life improvement evaluation. The average fatigue life of the patched steel plates was over 26 times longer than that of the unpatched plates tested at the same load range. Finally, a numerical framework was developed to simulate the fatigue crack growth in steel plates patched with such strengthening system and was validated by the experimental data. The findings suggest that the proposed system could be a promising alternative to traditional techniques for fatigue crack repair.

Blast Response of Cracked Reinforced Concrete Slabs Repaired with CFRP Composite Patch

In the present study, a numerical model is developed for investigating the capacity of the Carbon Fiber Reinforced Polymer (CFRP) patch to improve the blast response of cracked reinforced concrete (RC) slab. The model uses a finite element method adopted by LS-DYNA. To achieve this aim, a 3D nonlinear Finite-element (FE) model is developed to study the blast resistance of cracked RC slab with and without external reinforcement by CFRP. After model validation, the blast response of the cracked RC slabs with several different crack parameters (e.g. orientation, width and depth) under the blast loading is firstly studied. Then to improve the blast resistance, the cracked RC slabs are stiffened by CFRP composite patches. Numerical results show that the repaired patches can significantly reduce the mid-span deflection and improve the damage distribution of the cracked RC slab. The CFRP is proved to be a valid solution for repairing of cracked RC slab and can increase the blast resistance of RC structures under blast loading. Finally, a parametric analysis is further conducted to investigate the influences of the layer number and layer size of the CFRP patch on the blast resistance of the retrofitted model.

Development of a testing methodology for the design and quality control of carbon fiber reinforced polymer (CFRP) anchors

Using CFRP anchors to prevent the premature debonding failure of CFRP strips from concrete substrate is gaining acceptance. Although adequate CFRP anchors can fully develop the strength of CFRP strips, few recommendations for designing CFRP anchors are available. Based on experimental results obtained from 38 tests, a testing methodology using design strip strength is presented to evaluate the quality of CFRP anchors and to identify the design parameters including the strength ratio of CFRP anchor to CFRP strip, the strip width, the concrete strength, the fan angle, the embedment length, the bend radius, the hole diameter, the CFRP patch, the bond length and the bond condition. Design recommendations and installation procedures are also presented to ensure the desired performance of the anchorage system.

Rehabilitation of notched circular hollow sectional steel beam using CFRP patch

The application of carbon fiber reinforced polymer (CFRP) composites for rehabilitation of steel structures has become vital in recent years. This paper presents an experimental program and a finite element (FE) modelling approach to study the effectiveness of CFRP patch for repair of notch damaged circular hollow sectional (CHS) steel beams. The proposed modeling approach is unique because it takes into account the orthotropic behavior and stacking sequence of composite materials. Parametric study was conducted to investigate the effect of initial damage (i.e., notch depth) on flexural performance of the notched beams and effectiveness of the repair system using the validated FE models. Results demonstrated the ability of CFRP patch to repair notched CHS steel beams, restoring them to their original flexural stiffness and strength. The effect of composite patch repair technique on post-elastic stiffness was more pronounced compared to the elastic stiffness. Composite patch repair becomes more effective when the level of initial damage of beam increases. Copyright © 2018 Techno-Press, Ltd.

Experimental and numerical disbond localization analyses of a notched plate repaired with a CFRP patch

Through the use of finite element analysis and acoustic emission techniques we have evaluated the interfacial failure of a carbon fiber reinforced polymer (CFRP) repair patch on a notched aluminum substrate. The repair of cracks is a very common and widely used practice in the aeronautics field to extend the life of cracked sheet metal panels. The process consists of adhesively bonding a patch that encompasses the notched site to provide additional strength, thereby increasing life and avoiding costly replacements. The mechanical strength of the bonded joint relies mainly on the bonding of the adhesive to the plate and patch stiffness. Stress concentrations at crack tips promote disbonding of the composite patch from the substrate, consequently reducing the bonded area, which makes this a critical aspect of repair effectiveness. In this paper we examine patch disbonding by calculating the influence of notch tip stress on disbond area and verify computational results with acoustic emission (AE) measurements obtained from specimens subjected to uniaxial tension. The FE results showed that disbonding first occurs between the patch and the substrate close to free edge of the patch followed by failure around the tip of the notch, both highest stress regions. Experimental results revealed that cement adhesion at the aluminum interface was the limiting factor in patch performance. The patch did not appear to strengthen the aluminum substrate when measured by stress-strain due to early stage disbonding. Analysis of the AE signals provided insight to the disbond locations and progression at the metaladhesive interface. Crack growth from the notch in the aluminum was not observed until the stress reached a critical level, an instant before final fracture, which was unaffected by the patch due to early stage disbonding. The FE model was further utilized to study the effects of patch fiber orientation and increased adhesive strength. The model revealed that the effectiveness of patch repairs is strongly dependent upon the combined interactions of adhesive bond strength and fiber orientation.

Nonlinear three-dimensional FE analyses of RC beams retrofitted using externally bonded CFRP sheets with or without anchorages

In this study, behavior of reinforced concrete (RC) beams retrofitted using carbon fiber-reinforced polymer (CFRP) strips and anchorages is investigated by performing nonlinear finite-element (FE) analyses. The main aim of the study is testing the applicability of current FE techniques in the field of CFRP-retrofitted concrete elements. In the study, effect of CFRP strip width, presence of fan-type anchorages and CFRP patches were considered as main variables. In the study, eight RC beams with varying cross sectional properties along beam length are simulated. The simulated beams were tested in a former experimental study. The nonlinear FE analyses are conducted using the ANSYS FE software. The results obtained from the former experimental study are compared with the FE analyses results. From the results, it is observed that when properly modeled, FE simulations are efficient tools to estimate the nonlinear behavior of CFRP-retrofitted RC beams and similar structural elements.

Notch stress intensity factor for center cracked plates with crack stop hole strengthened using CFRP: A numerical study

It is well known that drilling a hole ahead of crack tip is one of the most common techniques to prevent crack propagation in structures subjected to fatigue load. An adequate size crack stop hole is necessary to convert a sharp crack into a blunt notch thereby preventing crack propagation. However, fatigue cracks typically occur at locations where drilling a crack-stop hole of required dimensions may not be possible due to geometrical constraints. In such situations, the crack may initiate again from the hole within its service life. Hence, there is a need to strengthen undersized crack-stop holes. In the present study, a combination of crack-stop hole and carbon fiber reinforced polymer (CFRP) overlays under static loads are studied numerically using finite element analysis (FEA) to evaluate its potential as a viable repair technique. A steel plate with an initial central crack subjected to static tensile loading (mode-I) is considered. A hole is modelled ahead of a crack tip and CFRP patches are applied on either side of the crack. The numerical analysis is performed using general purpose FEA ANSYS. A total of 1008 models, i.e 112 unstrengthened and 896 strengthened specimens are used to evaluate the stress intensity factor at notch tip. The material behaviour is assumed to be elastic in case of linear analysis and as a multi linear isotropic hardening material type for nonlinear analysis. The results from linear analysis are used to compare Crack Stress Intensity Factor (CSIF,KI/ρ) and Notch Stress Intensity Factor (NSIF, KI/ρα). The results indicate the need to include the stress gradient α in arriving at adequate crack stop hole radius for both bare steel and CFRP patched specimens. Nonlinear FEA, which takes into account the post yield material behaviour, is carried out to propose a modified NSIF expression by including a Reduction Factor (RF) that is a function of the ratio of the radius of crack stop hole to crack length (ρ/2a), the ratio of stiffness of CFRP to steel (SR) and the ratio of applied stress to yield stress (σapplied/σys). Numerical examples are provided to demonstrate the applicability of the proposed equation.

Experimental study of CFRP patches bonded on a cracked aluminum alloy panel

An experimental study was conducted to arrest a crack in 1.0-mm-thick 6061-T6 aluminum single-edge-notch tension specimen. The specimen was repaired by symmetrical bonding of carbon fiber-reinforced polymer patches made of several plies. Although J-integral was reduced drastically, the specimen failed at one of the leading edges through separation at the interface. The separation was suppressed by tapering all the four leading edges of the patches. The specimens did not fail in the patched area when loaded up to the far field stress of 272 MPa, which was higher than the yield stress, 265 MPa of the aluminum plate.

Fatigue crack growth study of CFRP patch repaired Al 2014-T6 panel having an inclined center crack using FEA and DIC

In this work, the fatigue life of unrepaired and repaired Al-2014-T6 panels with an inclined center crack is investigated. Cracked panels are repaired through single- and double-sided adhesively bonded carbon fiber reinforced polymer (CFRP) patch. The fatigue crack growth is monitored experimentally using digital image correlation and numerically using 3D finite element analysis. The adhesive-interface between the panel and patch is modeled using bilinear cohesive law. The CFRP/Al-2014-T6 adhesive-interface properties are obtained from the baseline tests. Fatigue life of double-sided repaired panel is observed to be twice that of single-sided. And non-uniform crack front is observed in single-sided repaired panels.

Tests on Cracked Steel Plates with Different Damage Levels Strengthened by CFRP Laminates

Strengthening steel structures using carbon fiber reinforced polymer (CFRP) materials has attracted much attention in recent years owing to their potential for fatigue crack repair and their convenience in construction. However, little is known about the efficiency of this strengthening method when applied to steel plates at different crack propagation stages. An experimental study was carried out on notched steel plates strengthened using CFRP laminates. 20 specimens were tested to evaluate the fatigue performance of the strengthened steel plates with emphasis on various degrees of initial damage, simulated by different lengths of slots, including 2%, 10%, 20%, 30% and 40% of the plate width. The effects of the retrofitted configuration and CFRP stiffness were also investigated. The "beach marking" technique and crack propagation gauges were adopted to monitor the fatigue crack propagation. The experimental results were very encouraging, demonstrating that the CFRP patches could effectively slow crack growth and extend fatigue life, regardless of the initial damage levels. More effective strengthening was found by using ultra-high modulus CFRP laminates, covering the initial cracks with CFRP and repairing at an earlier stage (i.e. smaller damage level).

CFRP-Strengthening and Long-Term Performance of Fatigue Critical Welds of a Steel Box Girder

Empa’s research efforts in the 1990s provided evidence that a considerable increase of the fatigue strength of welded aluminum beams can be achieved by externally bonding pultruded carbon fiber reinforced polymer (CFRP) laminates using rubber-toughened epoxies over the fatigue-weak welding zone on their tensile flange. The reinforcing effect obtained is determined by the stiffness of the unidirectional CFRP laminate which has twice the elastic modulus of aluminum. One can therefore easily follow that an unstressed CFRP laminate reinforcement of welded beams made of steel will not lead to a substantial increase in fatigue strength of the steel structure. This consideration led to the idea of prestressing an external reinforcement of the welded zone. The present investigation describes experimental studies to identify the adhesive system suitable for achieving high creep and fatigue strength of the prestressed CFRP patch. Experimental results (Wöhler-fields) of shear-lap-specimens and welded steel beams reinforced with prestressed CFRP laminates are presented. The paper concludes by presenting a field application, the reinforcement of a steel pendulum by adhesively bonded prestressed CFRP laminates to the tensile flanges of the welded box girder. Inspections carried out periodically on this structure revealed neither prestress losses nor crack initiation after nine years of service.