Composite Patch Repair Research Articles

Some Analytical Solutions of the Kamal Equation for Isothermal Curing With Applications to Composite Patch Repair

In this paper, we derive some analytical solutions of the Kamal cure rate differential equation. The Kamal model is a first order quasilinear ordinary differential equation, describing the progress of the curing reaction of several thermosetting polymers. All the examined cases refer to isothermal curing processes. The solutions obtained in this paper are all of implicit form. The derived solutions are applied to a repair technique based on the adhesive bonding of polymer matrix composite patches onto damaged or corroded areas. Critical duration times of realistic cure cycles corresponding to composite patch repair are estimated. The practical importance of the proposed analytic solutions is demonstrated through the presented engineering application.

Modeling and validation of composite patch repair to cracked thick and thin metallic panels

A two-dimensional finite element analysis is presented to predict crack growth behavior of cracked panels repaired with bonded composite patch. Fatigue experiments were conducted with precracked aluminum specimens of two thicknesses (1 and 6.35 mm), with and without debond, and repaired asymmetrically. Fatigue lives of thick and thin repaired panels extended four and ten times relative to unrepaired cases, respectively. The predicted fatigue crack growth rates were in agreement with experimental values at the unpatched face but not at the patched face. Thus, the present analysis provides a conservative assessment of durability and damage tolerance of repaired thin and thick panels.

The effects of structural variations on the health monitoring of composite structures

Composite repairs have been demonstrated to be an effective and efficient method of repairing or reinforcing aircraft structures. However, the implementation of this repair scheme gives rise to the difficulty in monitoring of the growth of the defect or crack beneath. In-situ monitoring methodology can be developed to monitor the fatigue crack development after the implementation of this repair on the structure. This paper presents a set of numerical results on the use of Lamb waves for the monitoring of crack growth in the lower wing skin adhesively bonded with a composite repair patch. These results demonstrate that the monitoring of defect growth beneath a repair patch can be made more challenging if severe geometry changes occurs in the vicinity of the defect and the multi-layered construction (with aluminum, adhesive and boron/epoxy) of the repaired structure.

Health monitoring of bonded composite repair in bridge rehabilitation

Structural rehabilitation with carbon fiber reinforced composite materials has proven to bean excellent way to enhance/repair steel reinforced concrete structures and prolongtheir service lives. However, disbonds between composite repair patches and hoststructures continue to be a great concern of this technology. In this paper, a built-inpiezoelectric sensor network based structural health monitoring system is introduced formonitoring the disbonds between composite repair patches and the host structures.This diagnostic system combines the sensor network, diagnostic hardware anddata analysis software allowing for real-time monitoring of the integrity of thebonded repair. The effectiveness of detecting disbonds using the system has beendemonstrated on a full scale bridge model in a laboratory setting. The bridge model wasloaded incrementally to failure, and disbond monitoring was carried out during theloading intervals. Test results showed that the system could detect the disbondsbefore they have a noticeable effect on the global stiffness of the bridge model.

Modelling of a cracked aluminium plate repaired with composite octagonal patch in mode I and mixed mode

Adhesively bonded composite patch repair technique has been successfully applied in military aircraft repair and has recently been expanded to commercial aircraft industry. This technique is applied to extend the service life of cracked aluminium components. In this paper, the finite element method is applied to analyse the central crack’s behaviour repaired by a boron/epoxy composite patch. The effects of the mechanical and geometrical properties of the patch on the variation of the stress intensity factor at the crack tip were highlighted. The obtained results show that the stress intensity factor at the crack tip, repaired by an octagonal patch of height 2 c/3, is reduced by 5% with regard to the one repaired by an octagonal patch of size ‘ c’. For a height patch of c/3 the reduction is about 7%. The maximum reduction of composite patch of fibres in y-direction is about 30% compared to the aluminium patch. This reduction doubles when a composite patch of fibres in x-direction is used. The adhesive properties must be optimised to increase the performance of the repair of structures by such reinforcement.

Closed Form Solutions for Nonlinear Analysis of Single-Sided Bonded Composite Patch Repairs

This paper presents novel analytical solutions for nonlinear analysis of single-sided adhesively bonded composite patch repairs to cracked structures in tension. In the present formulation, large deflections of adherends are taken into account in the equilibrium equations of the adhesively bonded patch repairs. In addition, large deflections of the overlap adherends, shear, and peel strains of the adhesive are included simultaneously in the derivation of the governing equations. By employing the boundary and continuity conditions, closed-form solutions are obtained for adherend displacements, adhesive stresses, and edge-moment factors for the repairs with isotropic adherends and composite adherends with symmetrical layups. The novel solutions are then simplified for practical uses. The geometrically nonlinear finite element analysis is conducted using MSC/NASTRAN. The numerical results predicted by the present closed-form solutions and their simplifications are compared with those of the nonlinear finite element analysis using MSC/NASTRAN. Remarkable agreement among the numerical results indicates that the present formulation and solutions are capable of capturing the geometrical nonlinearity of the single-sided bonded composite patch repairs to cracked structures.

Low recurrence rate after Gore-Tex/Marlex composite patch repair for posterolateral congenital diaphragmatic hernia

Purpose Many different prosthetic materials have been used for repair of large posterolateral congenital diaphragmatic hernias (CDH), with high recurrence rates for hernias that cannot be primarily repaired. Since 1993, we have used a composite patch of Gore-Tex/Marlex to repair large CDHs and hypothesized that this repair leads to fewer recurrences. Methods This is a retrospective review of 137 consecutive patients with CDH cared for at a single institution from 1993 to 2004. Data collected include timing and method of repair and use of extracorporeal membrane oxygenation. Outcomes include hernia recurrence, complications, and death. Results One hundred thirty-seven patients with CDH were analyzed. Repair was not attempted in 12 because of disease severity. Primary repair was accomplished in 79 and 46 required patch repair. Of the 46 patients with patch repairs, 32 required extracorporeal membrane oxygenation, and 18 died before discharge. Of the 28 patch repair survivors, 1 (3.57%) developed a recurrence over a median follow-up of 47 months (range, 2-115 months). Overall survival was 77%. Conclusions Gore-Tex/Marlex composite patch repair of large CDHs in this complex patient population results in a lower recurrence rate than has been reported for other types of prosthetic diaphragm repair.

Debonding monitoring of composite repair patches using embedded small-diameter FBGsensors

Small-diameter fiber Bragg grating (FBG) sensors were applied to the debondingmonitoring of composite repair patches. The target of the monitoring consists of carbonfiber reinforced plastic (CFRP) patches and an aluminum substrate. The specimens wereprepared through simple means: the tabular patches adhered to both sides of the aluminumsubstrate using epoxy adhesive films. Under cyclic loading conditions, the debondingprogressed between the aluminum substrate and the adhesive layers. Before theembedding of the FBG sensors, the embedding location was determined usingthe calculation of the sensor signals, also called the reflection spectra. When thecyclic loading was stopped at a pre-determined numbers of cycles, the reflectionspectra were measured at stress conditions of both 0 and 200 MPa. The newlyappearing peak in the reflection spectrum indicated the arrival of debonding atthe FBG sensors. As the debonding length increased, the intensity of the peakwas found to increase. The relationship between the spectral change and thedebonding length was verified by the simulated spectra and the strain distributionscalculated by finite element (FE) analysis. As a result, the debonding lengthcould be evaluated quantitatively by the spectral changes in the FBG sensors.

The effects of disbonds on the pure mode II stress intensity factor of aluminium plate reinforced with bonded composite materials

Adhesively bonded composite patch repair has been widely used to restore or extend the service life of cracked structural components due to its effectiveness to mechanical repair technique. In this work, the finite element method is applied to analyse the performance of the bonded composite patch for repairing cracks emanating from semicircular notch root in pure mode II. The stress intensity factor was computed at the crack tip repaired using a boron/epoxy patch for different orientation of fibers, taking into account the disbond. In this case, the increase of a patch thickness reduces the negative effects of disbond. When this effect is significant between the patch and the plate, it reduces the repair effectiveness. The maximum reduction obtained by using a boron/epoxy of fibers in the x-direction is of the order of 20% more important compared to a patch having its fibers in the y-direction. The stress intensity factor exhibits an asymptotic behaviour as the disbond size increases.

Fatigue Crack Growth Monitoring by Optical Fiber Sensors in Smart Composite Patch Repairs

The fiber optic smart structures allow engineers to add nerve systems to their designs, giving structures capabilities that would be very difficult to achieve by other means, including continuous assessment of damage processes. In this study, we evaluated the potentiality of the application of the optical fiber sensors to the monitoring of the fatigue crack growth behavior of composite patch repaired structures. The composite patch with embedded optical fiber sensors can be considered as a smart patch which has both repairing and monitoring functions. We used recently developed Transmission-type Extrinsic Fabry-Perot Interferometric (TEFPI) optical fiber sensors for the monitoring of fatigue crack growth behavior of cracked thick aluminum plate repaired with bonded composite patch. The sensing principle and the senor construction of the optical fiber sensor are presented. The experimental results show that it is possible to monitor the fatigue crack growth behavior of structures repaired with composite patch using the optical fiber sensor

Comparison of the effectiveness of boron/epoxy and graphite/epoxy patches for repaired cracks emanating from a semicircular notch edge

The adhesively bonded composite patch repair technique has been used to restore or extend the service life of the cracked aluminium structural components because of its efficiency. In this study, the finite element method is used to analyse the performance of the different bonded composite patches at a semicircular lateral notch and the repair of cracks emanating from this kind of notch. The knowledge of the stress distribution in the neighbourhood of the cracks is important for the analysis of their repair according to the geometry of the patch. The effects of the mechanical and geometrical properties on the variation of the stress intensity factor in the crack tip were highlighted. The effects of the adhesive properties and of the patch size on the stress intensity factor variation at the crack tip in mode I were also highlighted. The comparison between the double and single patch repairs is also given in this study. The results obtained show that the stress intensity factor of the crack tip repaired by two composite patches, is reduced to a half compared to the one that is repaired only by one patch. The orientation of fibres possessing a higher rigidity perpendicularly to the crack propagation considerably influences the reduction of the stress intensity factor. The adhesive properties must be optimised in order to increase the performance of the patch repair or the reinforcement.

Can stress waves be used for monitoring sub-surface defects in repaired structures?

Following the discovery of fatigue crack in the F111 lower wing skin, the RAAF (Royal Australian Air Force) implemented a boron/epoxy composite repair to restore its strength. This was carried out by bonding the composite repair patch to the lower wing skin. It was proposed that an in situ monitoring methodology be developed to monitor the fatigue crack development after the implementation of this repair on the structure. This paper presents a set of numerical results on the use of Lamb waves for the monitoring of crack growth in the lower wing skin adhesively bonded with a composite repair patch. These results show the significance of a good understanding of the propagation characteristics of the stress wave over the defective region. This task is made more challenging because of the severe geometry changes in the region of interest and the multi-layered construction (with aluminum, adhesive and boron/epoxy) of the repaired structure. This paper will outline the efficacy of this stress wave based monitoring methodology.

Design and analysis of adhesively bonded thick composite patch repair of corrosion grind-out and cracks on 2024 T3 clad aluminum aging aircraft structures

Many military and commercial aging aircrafts flying beyond their design life may experience severe crack and corrosion damage, and thus lead to catastrophic failures. In this paper, the design, fabrication and analysis of adhesively bonded thick composite patch repair of circular corrosion grind-out and a crack propagating on the periphery of the corrosion grind-out on thick 2024 T3 clad aluminum aircraft panel is presented. Thick orthogonal composite patch configurations of 7–25 plies were designed separately for crack and corrosion grind-out using CRAS. Using the principles of superimposition a single patch was designed to repair both the crack and corrosion grind-out. Finite element analysis (FEA) was performed on the test specimen subjected to uniaxial tensile loading. Stress distribution and displacements were obtained and analyzed. Dog-bone shaped tensile test panels were fabricated with damage and repaired with boron/epoxy patch of 11 plies. The patched and unpatched panels were subjected to tensile tests. The experimental and the FEA results show that the maximum skin stress decreases significantly and shifted away from damaged area after the application of composite patch. The load carrying capacity of patched specimen significantly increased over that for unpatched specimen.

Fatigue crack growth analysis of stiffened cracked panel repaired with bonded composite patch

Abstract Fatigue crack growth behavior in a stiffened thin 2024-T3 aluminum panel repaired with one-sided adhesively bonded composite patch was investigated through experiments and analyses. The patch had three plies of unidirectional boron/epoxy composite. 2024-T3 aluminum stiffeners were riveted as well as bonded on the panel. Stiffeners were oriented in the loading direction and were spaced at either 102 mm or 152 mm with a crack centered between them. Also, un-repaired cracked panel with and without stiffeners were studied. Experiment involved tension–tension fatigue at constant amplitude with maximum stress of 120 MPa and stress ratio of 0.05. Bonded composite patch repair increased fatigue life about five-fold in the case of stiffened panels while it increased about ten fold in the case of un-stiffened panels. Fatigue life also increased with decrease of the distance between the stiffeners for both repaired and un-repaired panels. A three-dimensional finite element method was used to analyze the experiments. Residual thermal stresses, developed during patch bonding, requires the knowledge of temperature at which adhesive becomes effective in creating a bond between the structure and patch in the analysis. A simple method to estimate the effective curing temperature range is suggested in this study. The computed stress intensity factor versus measured crack growth relationships for all panel configurations were consistent and in agreement with the counterpart from the test material. Thus, the present approach provides a means to analyze the fatigue crack growth behavior of stiffened structures repaired with adhesively bonded composite patch.

3-D modelling of cyclically loaded composite patch repair of a cracked plate

A three-dimensional finite element model is utilized to capture the effects of plasticity-induced crack closure (PICC) on an adhesively bonded patch repair of a cracked plate. To accurately capture the PICC process the choice of material model employed is of significant importance, therefore this paper considers a relatively new model, the Ellyin–Xia model. The paper shows that the PICC phenomenon is beneficial to the repair.

Design, analysis and performance of adhesively bonded composite patch repair of cracked aluminum aircraft panels

During its service life, an aircraft is subjected to sever structural and aerodynamic loads. These loads can cause damage or weakening of the structure especially for aging military and civilian aircraft thereby affecting its load carrying capabilities. Hence, a repair or reinforcement of the damaged or weakened part of the structure to restore the structural efficiency and thus assure the continued airworthiness of the aircraft has become an important issue in recent years to military and civilian aircraft operators. The US Air Force in recent years has shown considerable interest in the use of advanced composites to repair cracked metallic aircraft structures to enhance their life. One issue preventing using bonded composite patches, as a standard means of repairing damaged metallic aircraft structures is the fact that the integrity of the repairs is unknown. In this paper the design, analysis and durability of adhesively bonded composite patch repairs of cracked aircraft aluminum panels is reported. Pre-cracked 2024-T3 clad aluminum panels of 381 × 89 × 1.6 mm (15 × 3.5 × 0.063 in.) repaired with octagonal single sided boron/epoxy composite patch were used as test specimen. Two different composite ply configurations, 5- and 6-ply were investigated. Linear and non- linear finite element analyses were performed on the test specimen using 8-noded 24 degree of freedom (DOF) hexagonal elements for the aluminum panel, boron/epoxy patch and adhesive material subjected to uni-axial tensile loading. The stress distributions obtained were used to predict the increase in strength and durability of the repaired structure. A comparison of the stress values at critical points was made. The analysis also was used to validate various assumptions made in the design of the composite patch. Experimental investigations were conducted on the cracked aluminum panel repaired with a 5-ply composite patch as well as on two baseline-unpatched panels (one with a crack and one with no crack) by uni-axial tensile testing to validate the analytical results. The experiment was conducted on the Instron tension-testing machine. It was found that the maximum skin stress decreases significantly after the application of the patch and the region of maximum skin stress shifts from the crack front for an unpatched panel to the patch edges for a patched one.

Oil platforms, destroyers and frigates—case studies of QinetiQ's marine composite patch repairs

Composite patch repair solutions offer many advantages over conventional repairs to marine structures such as avoidance of hotwork (welding) and the ability to seal cracks. QinetiQ has been carrying out patch repairs to marine structures for over 20 years. The following case studies of QinetiQ's marine patch repair work will show how these advantages have produced clear benefits for the operators of the vessels involved. QinetiQ has patch repaired Type 21 frigates, Type 42 destroyers and offshore oil platforms as well as developing a number of other composite repair techniques to marine structures. During this work QinetiQ has trialed the patch fabrication techniques of hand layup, resin infusion and prepreg, and the advantages and disadvantages of the methods are discussed. QinetiQ is also trialling the Alternating Current Potential Difference (ACPD) crack monitoring technique on board two Royal Navy vessels and this technique is also discussed. The wider use of composite patch repairs to marine structures is held back by a lack of repair schemes approved by the classification societies. QinetiQ is working with those classification societies and operators of marine vessels to increase our understanding of the effectiveness of composite patches and to develop effective non-destructive examination (NDE) techniques to inspect patch repairs. This will increase confidence in patch repairs leading to their wider acceptance on marine structures. Progress on this work is reported here.

Curvature Effect on Fracture Toughness of Cracked Cylindrical Shells Bonded with Patches

Adhesively bonded patch repair has been widely used as an efficient and economical method to extend the service life of cracked structural components. Most of the currently available analysis methods and empirical databases for composite bonded patch repair to flat structures are computationally efficient and easy to use. However, the current knowledge on composite bonded repair for flat structures cannot be directly applied to curved repairs. A novel adhesive element developed by the authors in conjunction with a shell element is employed to investigate the fracture toughness at the crack tip of a cracked cylindrical shell bonded with a composite patch. To validate the present finite element model for curved composite patch repairs, the stress intensity factors in a flat composite patch repair are first computed by the strain energy release rate analysis method and compared with those available in the literature. For the curved patch repairs, a full three-dimensional finite element analysis is also conducted to validate the proposed numerical model. Some selected numerical examples are given to demonstrate the effect of curvature on the fracture toughness of a cracked cylindrical shell bonded with a composite patch subjected to different types of loading.

A Standard Analysis Methodology for the Stress Analysis of Repaired Aircraft Structures with the Method of Composite Patch Repair

The method of composite patch repair is a very modern method of repairing damaged aircraft structures, which presents many advantages over the traditional methods of repair. Many analytical as well as numerical methods have been suggested, from time to time, for the stress analysis of such bonded repairs. The engineer (especially one with not very much experience or theoretical background) who will be asked to design a repair may find it difficult to choose the most appropriate method for the specific problem that he has to deal with, among the large number of available analysis methods. The scope of this paper is to suggest a very specific, standardized, step by step analysis methodology for damaged/repaired aircraft structural components.

Fracture toughness analysis of inclined crack in cylindrical shell repaired with bonded composite patch

Adhesively bonded composite patch repair has been widely used to restore or extend the service life of cracked structural components due to its efficiency and cost-effectiveness compared to mechanical repair technique. Current available knowledge on patch repair mainly focus on flat damaged structures and the corresponding analysis methods and empirical databases are computationally efficient. In contrast, only limited work has contributed to studying patch repair to curved damaged structures. Authors have developed an adhesive element in conjunction with a shell element to investigate the effect of curvature on the adhesive stresses and mode I fracture toughness of the cracked host shell in the curved repairs. In this paper, this technology is again employed to model an adhesively bonded composite patch repair to a cylindrical shell embedded with an inclined through-thickness crack. The total strain energy release rate (SERR), calculated by the modified virtual crack closure technique (VCCT), is used to evaluate the mix-mode fracture toughness of the damaged structure and further to estimate the efficiency of patch repair. An automatic mesh generation scheme is proposed to conduct a quick parametric analysis, which can also be used to structural optimization design of composite patch repair. The numerical results are presented to show the effect of curvature and inclined angle of the through-thickness crack on fracture toughness of the repaired structure subject to different loads.