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This study examines the combined effects of patch stiffness, adhesive type, and composite patch material on the fatigue life of repaired structures. Simulations were conducted with four repair configurations: Boron epoxy with FM-73 and 3 M AF163-2 K adhesives, and Graphite epoxy with FM-73 and 3 M AF163-2 K adhesives, with patch stiffness varying from 70 to 120%. Results show that increasing patch stiffness significantly enhances fatigue life, with the best performance achieved with a 120% stiffness patch combined with 3 M AF163-2 K adhesive, extending fatigue life to nearly 200,000 cycles. The study also demonstrates that patches with higher stiffness improve load distribution and reduce stress concentrations around cracks compared to those with lower stiffness. In variable loading tests, 120% stiffness patches show a greater delay in crack propagation and better overload absorption, validating the Willenborg model. Additionally, the analysis of applied stresses reveals that higher stiffness patches better maintain stresses beyond 10 6 cycles, highlighting the importance of selecting appropriate patch stiffness to maximize repair durability.

Old airplanes produced in the 1970s are still flying, while being exposed to various new types of detriments, leading to a need to repair them to enable the safe use of the airborne body. The present state of the art advocates the use of laminated composite to repair aluminum parts due to their effective durability. The studies presented in the literature mainly focused on bodies under tensile loads. It seems that shear-type loading appearing in the fuselage of airplanes when being under torsion has been ignored in literature. Therefore, to fill this gap, the present study investigates the behavior of defective aluminum panels under pure shear. The present investigation uses a novel finite element (FE) method of modelling the loaded body by 2D and 3D elements. Then, the model is used to calculate the influence of various parameters, like the size of the repair patch, overlaps, sequences of the laminated composite plate, and other structural properties on the stability and strength of the examined part. To validate the numerical predictions, tests were performed on typical elements. Based on the experimental results, the fidelity of the FE model was assessed and the method approach of repairing using composite patches was validated. The main conclusion from the present study is the use of solid (3D) elements, over shell (2D) elements, due to their high-fidelity results.

Since the implementation of repair processes by composite patch bonding, this process has consistently demonstrated high performance across various industrial sectors, especially in the fields of aeronautics, aerospace and civil engineering. Consequently, there are situations in which the riveting process becomes the sole solution, particularly when the structure is subjected to severe mechanical or thermo-mechanical stresses, since adhesives have low mechanical strength after aging. Each method has its own set of advantages and disadvantages. The current trend is to combine these two processes to minimise their drawbacks as much as possible. The objective of this work is to present an experimental study on the repair of an aluminium plate AL2024-T3 with a central circular notch using a patch of different nature (metal or composite), under tensile loading conditions. The repair composite considered is a carbon/epoxide. The results of the tensile tests showed that the repair by the combination of the two processes improves the mechanical strength of the damaged structure. A comparison of the results of the experimental curves obtained on riveted, bonded and hybrid assemblies has been taken into consideration.

Over the last four decades, numerous studies have been conducted on the use of bonded composite repairs for aircraft structures. These studies have explored the repair of damaged plates through experimental, numerical, and analytical methods and have found that bonded composite repairs are effective in controlling crack damage propagation in thin plates. The use of double-sided composite repairs has been found to improve repair performance within certain limits. This study focuses on these limits and optimizes double-sided composite repairs by varying adhesive bond and composite patch parameters. The optimization process begins with a finite element analysis to determine the stress intensity factor (SIF) for various variables and levels, followed by the application of the Taguchi method to find the optimal combination of parameters for maximizing the normalized SIF. In conclusion, we successfully determined the stress intensity factor (SIF) for various variations and normalized it for optimization. An optimization study was then performed using the Taguchi design and the results were analyzed. Our findings demonstrate the repair performance of bonded composite patches using a cost-effective and energy-efficient approach.

This paper proposes a hybrid structural health monitoring (SHM) solution for a smart composite patch repair for aircraft structures based on piezoelectric (PZT) and fibre optic (FO) sensors to monitor the integrity of a the bondline and detect any degradation. FO sensors are used to acquire guided waves excited by PZT transducers to allow the advantages of both sensor technologies to be utilised. One of the main challenges of guided wave based detection methodologies is to distinguish the effect of temperature on the propagating waves, from that of an existing damage. In this research, the application of the hybrid SHM system is tested on a composite step sanded repair coupon under operational condition (temperature variation) representative of an aircraft for the first time. The sensitivity of the embedded FO sensor in recording the strain waves is compared to the signals acquired by PZT sensors under varying temperature. A novel compensation algorithm is proposed to correct for the effect of the temperature on the embedded FO sensor spectrum in the hybrid set-up. The repaired specimen is then impacted with a drop mass to cause barely visible impact damage (BVID). The hybrid SHM system is then used to detect the damage, and its diagnosis results are compared to a PZT only based smart repair solution. The results show promising application of the hybrid solution for monitoring bondline integrity as well as highlighting challenges of the embedding of FO sensors for a reliable and repeatable diagnosis.

Bonded patches are widely used in several industry sectors for repairing damaged plates, cracks in metallic structures, and reinforcement of damaged structures. Composite patches have optimal properties such as high strength-to-weight ratio, easiness in being applied, and high flexibility. Due to recent rapid growth in the aerospace industry, analyses of adhesively bonded patches applicable to repairing cracked structures have become of great significance. In the present study, the fatigue behavior of the aluminum alloy, repaired by a double-sided glass/epoxy composite patch, is studied numerically. More specifically, the effect of applying a double-sided composite patch on the fatigue life improvement of a damaged aluminum 6061-T6 is analyzed. 3D finite element numerical modeling is performed to analyze the fatigue performance of both repaired and unrepaired aluminum plates using the Abaqus package. To determine the fatigue life of the aluminum 6061-T6 plate, first, the hysteresis loop is determined, and afterward, the plastic strain amplitude is calculated. Finally, by using the Coffin-Manson equation, fatigue life is predicted and validated against the available experimental data from the literature. Results reveal that composite patches increase the fatigue life of cracked structures significantly, ranging from 55% to 100% for different applied stresses.

Wrapping glass fiber-reinforced polymer (GFRP) composites around damaged pipes are effective method and accepted in practices. The main aim of this paper is to propose new composite repair geometry to optimize the wrap repair thickness for cost-effective repair system using finite using finite element analysis (FEA). Two different repair geometries were considered: one where the defect cavity was completely filled with putty (resin) material and a new proposed geometry where the cavity was partially (50%) filled with putty and remaining with a GFRP patch. The numerical results show that using a 50% GFRP patch above putty material in the defect cavity reduces the composite wrap thickness from 8.4 to 5.4 mm, decreasing material usage by 40%. Replacing the GFRP sleeve with a high-strength carbon fiber composite further reduced the wrap thickness from 5.4 mm to 1.6 mm but with additional material cost. Composite repair system demonstrated marginal improvement of 6% rise, when the stiffness of putty material was increased from 3500 to 7000 MPa. This study highlights the potential of tailored repair geometries for effective load transfer between pipe and composite wrap through putty composite patch to reduce composite wrap material while maintaining repaired structural integrity. The proposed method offers a cost-effective solution, particularly suitable for repairs in confined or restricted areas.

Cylindrical shells are extensively employed in fluid transport, pressure vessels, and aerospace structures, where they endure mechanical and environmental stresses. However, under high pressure or external loading, circumferential cracks may develop, threatening structural integrity. Composite patch reinforcement is an effective method to mitigate crack propagation and restore structural performance. This study presents a finite element model using p-refinement techniques to analyze cylindrical shells with circumferential cracks reinforced by composite patches. The approach integrates equivalent single-layer (ESL) and layer-wise (LW) theories within a unified single-element mesh, significantly reducing the degrees of freedom compared to conventional LW models. Fracture analysis is conducted using the virtual crack closure technique (VCCT) to evaluate stress intensity factors. The model’s accuracy and efficiency are verified through benchmark and patch reinforcement simulations. Additionally, a parametric study examines how patch material, thickness, and adhesive properties affect reinforcement efficiency across varying crack angles. This study provides an effective methodology for analyzing composite-reinforced thin-walled cylindrical shells, offering valuable insights for aerospace, marine, and pipeline engineering.

For the repair of a crack in thin aluminium sheets, polymer composite patching is one of the better repair techniques. During service, when the load acts, the patch separates from the substrate due to the stresses developed at the interface. This separation of the patch from the substrate largely depends on the geometrical factors of the patch, i.e. length, width and thickness. The optimum geometrical factors need to be incorporated for the effective and economical repair of the cracks. In this work, the optimum combination of the geometrical factors, i.e. length, width and thickness of the polymer composite patch, is obtained using the Taguchi technique with the help of the results generated in the numerical analyses.

Composite patches are widely accepted as a useful practice for the repair of cracked aircraft components and the repair method is of vital importance to the final performance of the repaired structures. The present research experimentally studied the repair efficiency and processing stability of pre-cured, prepreg (including unidirectional and plain weave prepregs) and wet-layup methods for use on cracked Ti-alloy panels through the configuration of a butt joint bonded with a one-sided composite patch. The efficiency and stability of these repair methods were elaborately evaluated and compared via the load bearing behavior, the microstructure of the bonding interface, and the structural failure morphology through two batches of testing specimens. Typical patterns were found in load-displacement curves where the initial damage and ultimate bearing load points divided them into elastic-linear, damage propagation and complete fracture phases. Although the co-cure process of both unidirectional prepreg and wet-layup methods can form a jigsaw-like demarcation interface between the adhesive layer and the composite patch to achieve a good bonding force and a high recovery of loading performance, the latter presents porous patches with a high coefficient of variation in load-carrying capacity. Conversely, the pre-cured laminate and the plain weave prepreg patches failed to restore the mechanical properties owing to the weak bonding interface and the low axial patch strength, respectively. The unidirectional prepreg patch was proven to be the optimal repair method for the cracked metallic structures when balancing repair efficiency and processing stability.