Repair Of Composite Structures Research Articles

Experimental and numerical studies on compressive failure behaviors of stepped-scarf repaired composite stiffened panels

Composite materials have been widely used in aerospace and other fields due to their superior mechanical properties. The repair of composite structures has received increasing attention in recent decades. In this work, the compressive failure behaviors of intact, defective and stepped-scarf repaired composite stiffened panels are analyzed experimentally and numerically. The evaluation of repair schemes of composite stiffened panels based on finite element (FE) analysis is investigated by comparison with experimental results. The maximum absolute value of error in load-carrying capacity between the numerical and experimental results does not exceed 3.65%. Progressive failure analysis model and cohesive zone model are utilized to reveal the damage evolution of multilayered composites and adhesive film. The failure modes obtained from FE analysis are in good agreement with the experimental observations. Moreover, the influence of the stepped-scarf ratio and repair patch angle deviation on the compressive strength is investigated to determine optimum repair parameters based on the established FE models. On the basis of the conducted studies, the results provide valuable insights into the evaluation and validation of repair schemes for composite stiffened panels with high accuracy and cost-effectiveness.

Development and validation of a flexible fiber graphene laminated heater for composite adhesive repair

Flexible electric heaters are widely employed in the field of in-situ repair of composite structures for aircrafts. However, the existing traditional resistance wire heaters suffer from low damage tolerance, uneven heating, and high energy consumption. In this paper, a flexible Fiber Graphene Laminated heater(FGL heater) is proposed, which exhibits much better thermal efficiency in terms of heating rate, temperature uniformity, and energy consumption. The experimental results demonstrate that the traditional resistance wire heater achieves a heating rate of 0.58℃/s, with a maximum temperature difference of approximately 10℃under a 300W input power, which consumes 81.7×103J to heat from 24.6℃ to 100℃. In contrast, under the equal input power, the proposed FGL heater achieves a significantly higher heating rate of 3.8℃/s with a maximum temperature difference not exceeding 7℃. It only consumes 12.1×103J to reach the same temperature, reducing energy consumption by 85.2%. The silicone-encapsulated FGL heater also exhibits a heating rate of 1.1℃/s with a maximum temperature difference not exceeding 4℃. It consumes 45.7×103J, reducing energy consumption by 44%. Furthermore, the proposed FGL heater is capable of composite single-lap repair on the wing leading edge of aircraft, with a curing area temperature difference not exceeding 1°C.

Multi-matrix continuously-reinforced composites: A novel route to sustainable repair of composite structures

The worldwide increase of composites usage is accompanied by an equal increase of composite waste. While there are many reasons for discarding composite components, a primary reason is damage, with interlaminar failure being particularly common. A novel Multi-Matrix Continuously-Reinforced Composite (MMCRC) repair methodology is proposed, which incorporates repairable vitrimer resin. The procedure enables interlaminar failure repair without utilizing typical patch repair, that introduces additional damage. To achieve this a vitrimer matrix is incorporated into the damaged-prone region at the manufacturing stage. Interlaminar failure is then established and repaired using vacuum pressure and heating using an oven. With an eye to real-world application (where removing a part to be placed in an oven may not be possible), repair was also conducted utilizing localized induction heating. Both methods successfully repaired the interlaminar failure, recovering the initial strength and generating an average performance of 167% compared to the intact sample. The reasons for the increased performance are described in detail, which lead to the conclusion that the feasibility of incorporating reparable regions at the manufacturing stage has been demonstrated successfully. It is shown an interlaminar failure repair can be achieved using a less detrimental and time-consuming processing than traditional repair methodologies.

Injection repair of CFRP using a multi-Wall carbon nanotubes-modified epoxy resin: Flexural and compression behavior

This study presented the details of multi-wall carbon nanotubes (MWCNTs)-modified resin injection repair aiming to enhance the mechanical properties, considering the flexural and compression behavior. The resin injection of epoxy resin dispersed with MWCNTs (0.1, 0.3, and 0.5 wt.%) as low viscosity resin that delaminated composite structure repair was conducted using a developed vacuum-based resin injection system at 80°C with constant injection pressure. The quasi-static indentation (QSI) method with a circular window was applied to create the barely visible impact damage (BVID) in the laminate specimen and thus obtain the delamination damage with reproducibility. The flexural strength and compression after impact (CAI) test were conducted on repaired carbon fiber reinforced laminates to assess the effect of the dispersion of the MWCNTs in the epoxy resin injection approach compared to neat epoxy resin. The mechanical test results exhibited that the recovery rate was better improved in the case of the modified resin infiltration approach in laminate composites dispersed with nanoparticles. It was attributed to their more enhanced strengthening mechanisms under effective interaction in mixed interface of fiber-matrix-MWCNTs, mainly attributing to bridge connection and stronger interfacial adhesion properties.

In-situ analysis of cocured scarf patch repairs

To address the need for high-quality in-field repair of composite structures, a vacuum-bag-only (VBO) prepreg was designed, produced, and evaluated. The prepreg featured semi-preg formatting and a room-temperature-stable resin. The format provided a multitude of pathways with much shorter breathe-out distances relative to conventional, edge-breathing VBO prepregs, and thus enhanced through-thickness air permeability. A custom-built scarfed repair tool with an in-situ observation window was designed and employed to analyze the cure process during a repair. Microstructural quality, interlaminar shear strength, and glass transition temperature of semi-preg panels were compared to wet-laid epoxy panels processed with double vacuum debulking (DVD). The semi-preg formatting effectively reduced porosity for in-field scarf panels, and when used with the new material system, presents a viable alternative to DVD and wet layup.

Experimental and Numerical Study on the Efficiency of the Repair of Composite Structures Cracked and V-Notched By Different Types and Shapes of Composite Material Patch

Abstract The subject of this article around two studies, including experimental and numerical, aiming to know the efficiency of the repair of a composite plate notched (V-shape) and cracked by composite patches of different (types and shapes) . The experimental study to find out which composite materials and shapes are resistant to fracture and notch, We used the finite element method (ABAQUS) for the numerical study to follow the variation of the damaged area ratio DR, the stress intensity factor SIF and the stress distribution as a function of the crack size for located the most effective shape and type patch in repairing cracks and notch. From the results found we can deduce that the best type of patch used is Boron / epoxy and the full shape is the most efficient and resists structural failure, this is confirmed by the experimental test.

Efficient cocured scarf repair of composite structures through rheology modeling

To address the need for increased efficiency and high-quality in-field repair of composite structures, a vacuum bag only (VBO) semi-preg was produced, modeled, and evaluated against a conventional resin and format commonly used for repairs. The semi-preg featured a vinyl hybrid resin formulated for rapid processing with a discontinuous distribution of resin on the fiber bed. The format imparted high through-thickness air permeability relative to conventional out-of-autoclave (OoA) prepregs by virtue of abundant air evacuation pathways with short breathe-out distances. A model was developed to describe the rheological behavior of the resin, and then flow number analysis was employed to assess model accuracy and to guide the design of efficient cure cycles. A custom-built scarfed repair tool featuring an in situ observation window was employed to analyze the resin flow and cure process during a scarf repair. Microstructural quality and interlaminar shear strength were compared across the epoxy/vinyl hybrid and conventional/semi-preg panels. The results demonstrated that fast-cure resins can be used in conjunction with flow number analysis and semi-preg formats to design efficient VBO cure cycles that consistently yield patch repairs with low defect contents in repair environments.

РОЗВИТОК НОВИХ КОНСТРУКТИВНО-ТЕХНОЛОГІЧНИХ РІШЕНЬ КРИЛ ЛІТАКІВ І РЕМОНТ КОМПОЗИТНИХ КОНСТРУКЦІЙ

It is shown that in the complex problem of the permanent increase in the efficiency of using polymer composite materials in the construction of aircraft in the transport category, an important role is played by the rational choice of structural and technological solutions of their units and assemblies, which significantly contribute to the implementation of high flight-technological and economic characteristics of aircraft competitive on global market for sales and services. A review and analysis of the development of new wing composite structures discussed at the annual international exhibition Sampe 2019 under the NEWS project, formed in accordance with the STELIA Aerospace R&T strategy for the development of composite technologies with high technical and economic potential, is carried out. A detailed description of the experimental integral demonstrator of the wing compartment molded by infusion is given. The advantages of the selected integrated infusion technology are discussed, including minimizing the assembly steps, process automation, an economical autoclave-free molding process and repair of composite structures. The information provided by Airbus representatives on the repair of composite structures during the operation of the aircraft using portable equipment providing a high-quality curing regime for carbon fiber reinforced plastic. The types of structural repair are described. It is noted that structural repairs can be carried out by means of glue or bolt-on linings certified in accordance with EASA / FAA. The basic requirements of CS25.603 §8.8 are confirmation by analysis and / or test that repair methods and techniques will restore the structure to airworthiness. The justification for the repair is carried out by analysis confirmed by tests. Repairs must meet the same requirements as the original composite structure. Representative damage dimensions are performed in a full-scale test (maximum allowable damage size). A full-size test piece is also included in the evidence base. Repairing composite parts requires a clean environment, for which humidity, dust and temperature control are critical. Therefore, various portable portable “clean room” designs developed by various companies specifically for the repair of aircraft structures were presented at the exhibition.

Effect of curing condition on bonding quality of scarf-repaired composite laminates

Both single-face vacuum bag curing (SVC) and double-face vacuum bag curing (DVC) can be used in scarf repair of composite structures. But different curing conditions caused by the sealing state may affect the bonding quality of scarf-repaired structures. In this paper, the effect of curing condition on bonding quality of scarf-repaired laminates was experimentally investigated in terms of surface profiles, moisture absorption curves and section profiles. In order to further explore the moisture absorption mechanism, finite element model of the repaired laminates using DVC was established with moisture diffusion of both the adhesive and composite laminates considered. This model was verified by experimental results. Based on the model of DVC case, the model of SVC case was built by changing moisture absorption parameters of the adhesive. Results show that SVC reduces the bonding quality, mainly reflecting in more adhesive inner voids and patch-to-parent dislocation. And SVC increases moisture absorption rate and moisture equilibrium content of the adhesive, and its effect on the former is far greater than that on the latter.

Simulation and experiment on the effect of patch shape on adhesive repair of composite structures

In order to investigate the performance of the adhesive bonding repair in composite structures, static tensile test is carried out on the bonding structures numerically and experimentally. The tensile stress–displacement behaviors of T7901 composite adhesive bonding structure is studied under room temperature. Based on the three-dimensional progressive damage theory, the model of adhesive bonding structures with different patch shape has been constructed, and the final failure strength is predicted using APDL language. Moreover, experiment has been performed on each adhesive bonding structure with different patch shape. The results indicate a good agreement with numerical predictions. Furthermore, it is found that the repair effect of the adhesive bonding structure with the square patch is better than others. The final damage pattern and damage range of the patch repair structure are also tested by X-ray machine, which are consistent with the simulation results.

Modelling the size and strength benefits of optimised step/scarf joints and repairs in composite structures

Adhesive bonding offers a better load transfer between adherends, for the assembly and repair of composite structures, compared with mechanical fastening methods. One drawback of adhesive bonded repairs is the considerable amount of material which must be removed, around the damaged region, to ensure adequate load transfer. Optimised geometries that account for the highly-orthotropic properties of individual composite plies are investigated, including a novel ‘fibre-oriented’ scarf approach that is inspired by an existing fibre-oriented step design. These methods aim to reduce the length of joint and repair bonding regions by at least 36%, compared with conventional step and scarf geometries of a similar strength.Size-reduction benefits are predicted using a MATLAB tool that is applicable for any composite laminate, and parametric analysis used to assess the effect of ply thickness, the number of plies, stacking sequence and taper angle.Cohesive Zone Models of joints and repairs with conventional and fibre-oriented designs are used to predict and compare the ultimate strength of each configuration. The existing fibre-oriented step design appears to show no benefit over a conventional step design. However, the novel fibre-oriented scarf approach results in a 33–40% reduction in the size of the bonding region compared to a conventional scarf design with similar strength. Analysis further indicates a 17–22% increase in ultimate strength for joints and repairs with the same bonding region size that employ the optimised fibre-oriented scarf design.

Proof of a composite repair concept for aeronautical structures: a simplified method

This paper provides an illustration of all stages of primary aeronautical composite structure repair by using industrial tools and scientific methodologies, as well as numerical tools to simplify the cross-over analysis of the mechanical behaviour of the repaired area. Economically and scientifically speaking, one of the main challenges of composite repair (for monolithic long fiber composite parts) consists of promoting a bonded composite patch option without additional riveted doublers. To address this challenge, size reduction of the patch could be mandatory. A patent (jointly owned by ICA, Bayab Industries and CES), entitled “Method for repairing a wall consisting of a plurality of layers”, is devoted to reducing repair patch dimensions of monolithic composite parts provided the bonding zone has a stepped-lap geometry. This patent is based on a simple idea that no overlapping length is required between composite plies for load transfer except in the fiber directions of the plies (unidirectional or biaxial long fiber reinforcements with epoxy matrix). To prove this concept, we consider on one hand, a situation unusual in the literature by studying a composite specimen without fibers aligned along the main loading axis, and on the other hand, a classical situation of where the shape of the specimen is adapted to be studied by uniaxial tension tests. After different manufacturing steps, the studied specimen contains three zones representing both the influence of the total thickness of a repair patch, the stepped-lap area assembled with an adhesive film and the parent composite part. Basically, a simple parent structure consisting of 16 plies of UD Hexply® M21/35%/268/T700GC (close to Airbus composite raw materials on board in A380) is manufactured with a stacking sequence of [+45/−45/−45/+45/+45/−45/−45/+45]s. Then, the parent structure is machined by the Airbus Abrasive Water Jet machine and the final repair area has a stepped-lap geometry by overlapping successive plies of the same nature as the parent plate and after having previously applied an adhesive film (cured at 180 °C). Furthermore, 3 values of overlap length (respectively, 6, 8 and 13 mm) are investigated to include the mean value required by Airbus in the case of the use of the studied prepreg. After abrasive water jet machining of the composite parent part, repair patch manufacturing was performed according to Airbus requirements. The studied specimens were cut from the final plate (involving the parent plate, the stepped lap zone and the zone of the patch itself) and tested in an uniaxial tensile configuration with a loading direction shifted 45° with respect to the fiber direction. Furthermore, studying uniaxial tensile tests on multilayer-pasted interface is innovative in the literature. In this paper, it is shown that the stepped-lap area assembled with an adhesive film is not the weak link of the mechanical response but rather the parent area, i.e. the unrepaired monolithic composite. Numerical calculations confirm this proof of concept by underlying that the level of shear stress in the adhesive film, for these three overlapping values, is below the chosen limit value. These results show that the patch size reduction is possible.

Smart Patch Repair with Low Profile PVDF Sensors

Bonded repair of composite structures still remains a major concern for the airworthiness authorities because of the uncertainty about the repair quality. This work, investigates the applicability of conventional Structural Health Monitoring (SHM) techniques for monitoring of bonded repair with ring-shaped low profile sensors. A repaired composite panel has been sensorized with two Ring-Shaped Polyvinylidene fluoride piezopolymer Sensors (RSPS) and a piezoelectric (PZT) transducer. An electromechanical impedance (EMI) and Lamb wave analysis have been carried out to check the sensitivity of these sensors to detect an artificially introduced damage simulating a disbond of the repair. The state of the repair have been successfully monitored and reported by both methods.

Structural Health Monitoring of Bonded Patch Repaired Composite

Bonded repair of composite structures still remains a crucial concern for the airworthiness authorities because of the uncertainty about the repair quality. This works, investigates the applicability of Structural Health Monitoring (SHM) techniques for monitoring of bonded repair. Active sensing method has been applied to two case studies: a sensorised panel impacted to cause barely visible impact damage (BVID) and repaired afterwards, the tensile and fatigue testing of a composite strap repair. In the first case, the previous sensors have been used to detect an artificially introduced damage. In the second case the failure of the adhesive during the tensile testing is used as basis of the load levels in the tensile-tensile fatigue test. In both cases PZT transducers have been used to monitor the bonded patch. An electromechanical impedance (EMI) and Lamb wave analysis have been carried out to check the overall integrity of the repair patch between. In both cases the state of the repaired composite was monitored successfully and reported.

Experiment and design methods of composite scarf repair for primary-load bearing structures

A relative thick plate of composite being repaired by scarf bonding was substituted as the scarf joints and studied under tensile load in this paper. The experimental investigation told us the ultimate capability of load and failure mechanism of composite scarf joints. In order to explain failure mechanism for scarf bonding repair of composite structures distinctly, FEM based on continuum damage mechanics (CDM) was carried out. CDM equations were derived by adding the delamination mode and 3D Hashin criteria based on strain into the composite constitutive relation. Furthermore, a modified semi-analytical method (MAM) was developed to obtain shear stress distribution in theory. MAM can improve the accuracy of Harman and Wang method, solve the stress asymmetry, obtain secondary peak of shear stress, and increase the highest peak of stress. In the end, the discussions for the methodologies of MAM, CDM and linear FEM were executed to compare the errors of predicting ultimate load. The discussion revealed that MAM has sufficient accuracy as an analytical methodology which could be utilized to design composite scarf repairs in a short cycle simply.

Polyether–maleimide‐based crosslinked self‐healing polyurethane with Diels–Alder bonds

ABSTRACTThe Diels–Alder (DA) reaction is particularly desirable for the preparation of heat‐stimuli self‐healing polymeric materials because of its thermal reversibility, high yield, and minimal side reactions. Some attempts were conducted to synthesize polyether–maleimide‐based crosslinked self‐healing polyurethane with DA bonds (C‐PEMIPU–DA) through the reactions of the prepolymer (polymeric MDI/PBA‐1000) functionalized by furfuryl amine and polyether–maleimide without benzene in this study. The structures of intermediates and C‐PEMIPU–DA were first confirmed by 1H‐NMR, Fourier transform infrared spectroscopy, and differential scanning calorimetry. Next, the thermal reversibility and the self‐healing performance of C‐PEMIPU–DA were studied by 1H‐NMR, polarizing optical microscopy, tensile testing, and a sol–gel process. The results show that C‐PEMIPU–DA exhibited interesting properties of thermal reversibility and self‐healing. The polymers could be applied to self‐healing materials or recyclable materials in the fields of the repair of composite structures and aging parts because of their thermosetting properties at room temperature and thermoplasticity at higher temperatures. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41944.

On the Fabrication and Automation of Reliable Bonded Composite Repairs

For structures made of carbon fiber-reinforced plastics (CFRP), fast, robust, and reliable repair technologies are mandatory for economical usage. In this paper, the authors explain their strategy and experiences. An automated process is proposed to achieve the challenging goals. A general overview on the origin, effects, and analysis of contaminants in CFRP structures and the relationship to the achievable strength of adhesive bonds are given. For the repair of composite structures using adhesive bonding, surface pretreatment is a key factor in terms of reliability and strength. Different surface treatment processes such as grinding, grit blasting, plasma and pulsed lasers treatments are discussed. Furthermore, the possibilities and technical implementation of an automated milling process for the repair of composite structures are presented. This change from manual production to automation tremendously improved the quality and duration of the repair and allows the creation of a uniform surface for adhesive bonding. Further integration of novel technologies is discussed and will further support and enhance the repair in the near future.

Study of tensile failure mechanisms in scarf repaired CFRP laminates

Bonded scarf repairs are used in composite structures when high strength recovery is desired or when there is a requirement for a smooth surface to satisfy aerodynamic requirements. Experimental and finite element study is carried out to understand the damage propagation and ultimate strength of scarf patch repaired CFRP laminates under uni-axial tensile load here. The ultimate strength and damage propagation in scarf repaired laminates has been investigated with respect to change in scarf angle and patch diameter. It is revealed from the results that damage is initiated in adhesive film at the bonded interface of 0° plies of parent laminate and patch in and then propagates in circumferential direction around scarf patch. It is found that some damage occurs in the parent laminate before the damage initiation in the adhesive film. After the adhesive film failure, the damage in parent laminate quickly propagates transversely to the free edge sides of the laminate and failure occurs. The results of this investigation provide further insight into the damage mechanisms in scarf repairs of composite structures under tensile load. This study may be helpful in improving the design and analysis techniques for scarf patch repair of composite structures.

Ultrasonic activation of mendable polymer for self-healing carbon–epoxy laminates

Healing of matrix cracks or delamination in fibre-reinforced composites containing mendable polymers requires heating to melt the thermoplastic agents. This paper presents the first investigation into the use of ultrasonic welding to activate a mendable polymer, poly[ethylene-co-(methacrylic acid)] (EMAA), for healing a carbon–epoxy laminate. Mode I interlaminar fracture toughness tests were carried out on specimens containing two different concentration levels of interlaced EMAA fibres to quantify the healing efficiency of ultrasonic vibration. Experimental results reveal that bursts of short-duration, high frequency ultrasonic pulses are able to thermally activate the mendable polymer to repair delamination cracks. Examinations of the fracture surface indicate partial healing of delamination cracks, which were sufficient to completely recover delamination toughness (repair efficiency up to 130%). Furthermore, multiple repairs and recoveries of interlaminar fracture toughness of the composite were achieved with ultrasonic welding. The repair efficiency using ultrasonic welding, however, was found to be less than conventional heating by thermal oven. Nevertheless, the ultrasonic welding technique is portable and can be used for rapid in-field repair of composite structures containing mendable polymers.

Impact damage and repair of composite structures

AbstractThis paper gives an overview of the work carried out in a GARTEUR (Group for Aeronautical Research and Technology in Europe) program, under the chairmanship of the author, to develop and validate analytical and numerical methods to characterise real impact damage in composite structures, particularly those designed to sustain load in a postbuckled state, and to study the durability of bonded repairs. GARTEUR is an inter-governmental agreement between the seven European countries with the largest direct employment in the Aerospace industry, to mobilise scientific and technical knowledge between the member countries. A number of Action Groups have been launched, since GARTEUR’s inception in the early 1970s, to address specific technical issues of interest to the participating members. The research presented in this paper was performed under Action Group 28 with partners from ONERA, EADS-CCR (France), DLR, AIRBUS-Deutschland, EADS-M (Germany), CIRA (Italy), INTA (Spain), SICOMP, Saab, (Sweden), NLR (The Netherlands), QinetiQ, BAE Systems, Imperial College London and the University of Sheffield (United Kingdom). The Action Group tasks were divided into four Work Elements (WEs): WE1-Prediction and characterisation of impact damage, WE2-Postbuckling with delamination, WE3-Repair and WE4-Fatigue. This paper outlines the main developments and achievements within each Work Element.