The effect of disbond on the efficiency and durability of a composite patch repair in mode I and mixed mode considering different patch materials

In this paper, considering different parameters and various patch materials, the effect of disbond on the efficiency and durability of a composite patch repair is investigated in mode I and mixed-mode. One of the most important aspects of the composite patch repair is the bond strength. Repair patch disbond may occur at the patch edges or the crack site. At first, the effect of different parameters such as repair patch material and Young’s modulus and thickness of the adhesive on the efficiency and durability of the patch is investigated. Then, the effect of the disbond site on the stress intensity factor (patch efficiency) and adhesive stress (patch durability) is analyzed in both modes I and II. The results show that disbond at the crack site leads to a further reduction in patch efficiency compared to the patch edge disbond, but when separation occurs at the patch edge, the adhesive stress and the disbond growth rate are higher. Also, when 15% of the patch is separated in the crack site, for the longitudinal and transverse disbond modes, the mean KI is increased by 8 and 4%, respectively, compared to the state without disbond. Thus, the longitudinal disbond mode is more critical.

Fatigue Behavior of Al 7075-T6 Plates Repaired with Composite Patch under the Effect of Overload

Repair of aeronautical structures by composite patch bonding has shown its effectiveness in several studies during the last few decades. This repair technique leads to a retardation in the propagation of repaired cracks via load bridging across the patch throughout the adhesive layer, interfacing it with the repaired structure. The purpose of this study is to analyze the behavior of patch-repaired cracks present in thin plates made of aluminum alloy 7075-T6 and subjected to a single tensile overload. The sequence of application of overload on the fatigue behavior was also studied. Fatigue tests were conducted on Al 7075-T6 notched specimens where crack growth and number of cycles to failure were monitored for different patching/overload scenarios. A detailed fractographic study was performed on failed specimens to analyze the micromechanical behavior of the crack growth related to each scenario. The obtained results showed that the application of the overload before bonding the patch leads to an almost infinite fatigue life of the repaired plates.

Effects of the Temperature–Time Regime of Curing of Composite Patch on Repair Process Efficiency

Repair procedures with the use of composite patches are considered to be the most effective among the current technologies of repair of the structures of various applications. In the process of moulding-on of a patch made of polymeric composite material by means of curing, technological stresses arise in the patch. Determination of residual technological stresses is a priority task for the modelling of the repair process. Reduction of residual stresses can be achieved by optimization of the mode of repair patch curing. For meeting this objective, the method for determination of technological stresses, which arise in the structure under repair in the process of curing of a composite patch, has been developed. The method takes into account the shrinkage, change in physico-mechanical characteristics, rheological processes occurring in the binder during moulding process, and determination of stresses in the structure under repair at any time. Therefore, premature failure of the repair joint at the stage of repair can be avoided. It is shown that the method adequately describes the level of deformations and stresses in the structure being repaired at the stage of heating and holding of the composite patch. Increase in the moulding temperature leads to a reduction in residual stresses in the structure under repair. However, current stresses at the stages of heating and temperature holding are increased significantly. Reliability of assumptions and developed method is confirmed by the comparison with the experimental data. The obtained experimental graph of total deformation of the composite patch allowed us to clearly determine the moment of residual stress occurrence in the structure under repair. This moment matches quite exactly (with the discrepancy not exceeding 5 min) the gel point determined analytically based on dependence of the degree of curing on the moulding mode. Consequently, the research together with the results previously obtained allows making an integrated choice of geometric parameters of the repair composite patch and temperature–time regime of its curing in order to ensure the specified level of strength and stiffness of the structure under repair.

Curing monitoring of bonded composite patch at constant temperature with electromechanical impedance and system parameters evaluation approach

As one of cost-effective maintenance methods, bonded composite patch repair has been receiving more and more attention in the engineering community since past decades. However, realizing real-time monitoring for curing process of bonded repair patch is difficult for most current techniques. In our work, a method based on electromechanical impedance and system parameters evaluation for structural health monitoring issues was developed, which could implement the online monitoring throughout whole curing process. Compared with the dynamic thermomechanical analysis results, the experiment data matches well. It demonstrates that the proposed approach can effectively monitor the curing process of composite repair patch at a constant temperature of 120 °C. Hence, the presented approach in this paper is expected to be a novel, robust, and real-time monitoring method for structural maintenance with the composite patch.

Stress Redistribution Near a Crack in Maraging Steel using Composite Patch

The presence of a crack significantly reduces the load bearing capacity of a structure made of fracture prone material. The conventional process of repairing a defect is gouging and filling the gouged location by rewelding. It is not only time consuming but also constrained by the number of repairs that can be done as material properties degrade with each round of rewelding. In an attempt to overcome the limitation of the conventional repair process, repairing a defect using composite patch is proposed. The study is carried out on Maraging steel (M250) and the defect considered is a crack. Stress intensity factor (SIF), being an important parameter in fracture-based design, it is evaluated in Abaqus. Extended finite element method is used to model the crack. SIF is used to predict the failure load of a surface cracked tension specimen and the same is compared with the values reported in literature. Composite patch is modelled using woven ply properties. A separate layer of adhesive is also modelled to predict the properties adhesive properties. Failure analysis of each component namely, the Maraging steel plate, the composite patch and the adhesive is carried. It was observed that the addition of a composite patch completely nullifies the presence of a crack. The patch with thickness 1mm and woven ply properties is having minimal damage initiation and likely to survive. The adhesive properties required is also obtained from the finite element analysis. Thus, it was observed that a composite patch with woven ply properties and thickness 1mm is able to completely nullify the effect of a crack when bonded with a suitable adhesive as predicted by the analysis.

Development of Hybrid Piezoelectric-Fibre Optic Composite Patch Repair Solutions

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.

Theoretical and Experimental Investigation of Bonded Patch Repairs of a Rubber Reinforced Composite Conveyor Belt

The present study proposes a reparation method for designing and optimizing a rubber to rubber and rubber to textile reinforcement. The present application is the conveyor belt used in the transport industry. The tensile behavior of the repaired specimens was studied using experimental results. A bidirectional linear analysis allows us to predict the effect of geometric parameters on the stress concentration zone of the repaired belt under hygro-thermo mechanical loading and its consequence on the integrity of the structure. A tensile test was carried out in order to investigate the behavior of a repaired specimen made with a rubber cover patch and an inner composite patch. Two stacking sequences of an inner composite patch and the material properties are considered in the parametric study in order to reduce the stress concentration in the parent belt. The correlation between the theoretical and experimental results allows us to define a strength tool to understand the load transfer from rubber to a textile rubber patch.

Fatigue Life Improvement of Cracked Aluminum 6061-T6 Plates Repaired by Composite Patches

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.