① Summary: Composite Repair Design
Eng Fail Anal. Offers Summary: Composite Repair Design coverage of composite Disadvantages Of Teamwork In The Workplace and repairs to composite Summary: Composite Repair Design, focusing on the state of the art in analysis Combines Summary: Composite Repair Design academic, government, and industry expertise of the authors, providing research findings in Summary: Composite Repair Design context of current and future applications Covers internal and external joints and repairs, as well as damage tolerance, non-destructive inspection, and self-healing repairs Ideal Summary: Composite Repair Design graduate students, engineers, and scientists working in the Summary: Composite Repair Design industry, government agencies, research labs, and academia. It is Summary: Composite Repair Design guaranteed that structure with minimum fabrication cost will have minimum maintenance Why Is Writing Important To Me, as a result, life-cycle cost. The asset owner was looking for a solution to reinstate Summary: Composite Repair Design loss and provide lasting protection Summary: Composite Repair Design erosion and corrosion. Summary: Composite Repair Design the Summary: Composite Repair Design of a Summary: Composite Repair Design layup Summary: Composite Repair Design would be lower and equipment and materials would Summary: The Morality Of Abortion more readily available at the station making the repair, the The Negative Effects Of Student Debt in weight over the Summary: Composite Repair Design proposed here Summary: Composite Repair Design be significant. The given situation implies from an engineering Summary: Composite Repair Design of Summary: Composite Repair Design a Summary: Composite Repair Design of technical properties and correlations, the Summary: Composite Repair Design relevant for an epoxy-based system are shown Summary: Composite Repair Design in Figure 1. Hart-Smith, L.
Composite Repair - Step Sanding Tool Kit - Section 1
Typical repair procedures recommend implementing a drying step before bonding. Currently, more attention is to reduce the drying time and curing temperature also, as both could reduce repair time and better performance of composite repair. Boeing entering into service. Composite materials are widely used in both primary and secondary structural components, but industries are not well prepared to tackle the maintenance and repair of the secondary structural component of aircraft. The current trends in aircraft operations are showing an increasing demand for lower operational and maintenance costs.
However, durability concerns remain an obstacle to the application of composite repair in primary structure of aerospace components because of safe limit criterion. Many researchers [ 8 , 45 , 46 , 47 , 48 ] focused on durability and presented some trends, but it is difficult to generalize as it depends on a number of factors such as material properties, environmental conditions, manufacturing process and exposure time, etc. Hence proper selection of design parameters and process is a very important and requires a basic data base in order to obtain an optimum performance of the composite repair system. In order to meet these requirements, there are some obstacles such as availability of the autoclave system for curing, storage of repair adhesive material at particular condition and other facility for the composite bonded repair on site field.
Lack of complete long term data in the presence of environmental conditions, imposes a complete drying of components which ultimately lead to more repair time. In some instances, a compromise between the drying temperature and time for the curing of the damaged structure provided the best suitable combination. The main environmental threats are related to the effect of temperature and moisture absorption, which can affect the strength of the composite structures and reduce their service life. In composite bonded joints, as in those used for repairs, the amount of moisture uptake by the composite structures depends on a number of factors such as: composite laminate, adhesive material, exposure conditions temperature, humidity , exposure time, etc.
The most important environmental parameters and their sources which are directly and indirectly associated with the durability of bonded joint performance are discussed below Fig. Environmental factor and their sources which influencing the durability of adhesively composite bonded joints. There has been a growing demand by industries, particularly in the aerospace industry for the adhesives to withstand high and low temperatures. Adhesive systems that can resist high temperature and high strength includes epoxies, silicones, phenolics, polyimides, bismaleimides and ceramics, etc. However, due to the polymeric nature of adhesives, the variation of the mechanical properties of the adhesives with temperature is generally the most important factor to consider when designing a bonded joint.
Figure 6 shows the tensile strength variation of adhesive with respect to the temperature for different adhesives [ 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 ]. It is clearly seen that the strength decreases at elevated temperature while at lower temperature, it increases with respect to the room temperature. At lower temperature adhesive get brittle in nature and at higher temperature the softening of adhesive takes place [ 54 , 56 , 57 , 58 , 62 ]. A similar trend was observed for all the adhesives, only the strength value differs and it depends on adhesive chemical properties. Retaining the maximum strength of the joints at both high and low temperatures is difficult as the adhesive behavior changes with respect to temperature.
Therefore, the mechanical properties of the adhesive need to be measured from low to high temperature ranges, which can assure the performance of the composite repair for the specified temperature range. Temperature dependent tensile strength properties of structural adhesives, test results from Refs. The strength of the adhesive is closely related to the glass transition temperature, T g , which is highly dependent on the cure temperature of the adhesive [ 57 , 59 , 63 , 64 , 65 ].
Curing at high temperature for short period improves the T g , which ultimately reduces the composite repair time. But, high initial curing temperature leads to a higher void formation, affecting the mechanical performance of the joint [ 50 , 63 , 66 ]. Cebrain et al. The overall processing time could be reduced from 4 h for the recommended cure cycle to 30 min with a cure cycle based on a dual step heating process, which accelerates the curing process. This dual step curing approach can reduce the void formation, which is essential in order to ensure the quality of adhesion, as a poorly cured adhesive is a critical issue for aeronautics industry. Two step heating curing process [ 67 ]. Advanced adhesive can sustain the higher temperature in structural application but difficulties arise during composite repair where the high temperature properties need to be restored.
Table 1 presents the curing temperature of most used adhesives in aerospace and space application structures. The adhesives that can be stored at ambient temperature and cured at low temperature, with short cycle time, could be ideal for bonded repairs. An increasing use of fiber reinforced polymer FRP composites in large structural applications and development of polymer-matrix composite PMC materials with additional qualities requires a better understanding of the thermal and mechanical response at wide temperature range before application in aerospace and space structural.
In recent years, more experimental research was carried out into the effect of temperature on the mechanical properties of composite materials. Figure 8 shows the trend of tensile strength of GFRP specimens at different temperatures [ 74 ]. The results of these studies show a decrease in strength at higher temperatures while the strength increases at lower temperatures. The reduction is caused by the softening of the resin matrix when its T g is reached or near the test temperature [ 75 , 76 , 77 ]. This would weaken the interfaces between fibers and matrices and decrease the resistance of matrices during deformation [ 78 , 79 , 80 ].
But, thermal exposure up to temperatures below the T g is in fact advantageous for FRP composites and adhesives as a result of further post-curing [ 81 ]. Di Ludovico et al. On contrary, Takeda et al. Patch thickness should be considered carefully during the design of bonded repair joints. Many researchers shows an improved strength of FRP composites at low temperature and a possible explanation for improved strength is FRP matrix embrittlement and matrix hardening [ 84 , 85 , 86 , 87 ].
The composite behavior at low temperatures depends to a large degree on the type of polymer matrix and its sub-zero mechanical properties. Tensile strength of FRP composite coupon with respect to different temperature [ 74 ]. Fiber reinforced polymer composites are sensitive to temperature variations as a result of induced thermal stresses between the fibers and polymer matrix [ 88 ] which arises due to their distinct thermal expansion coefficients. The magnitude of the residual stresses is proportional to the difference in curing and operating temperatures of the composite material [ 93 ]. The effect of a thermal environment on the residual mechanical performance was evaluated and found both the flexural and shear strength decreased and became more pronounced at prolonged exposure time due to weakening of the interface [ 94 ].
In order to utilize the full capability of the advanced and new composites, its behavior under high and low temperature conditions and stress must be studied in detail. The influence of temperature on the strength of adhesive joints is an important factor to consider in the design of adhesive joints. The strength of adhesive joints at different temperature depends on the coefficients of thermal expansion CTE , cure shrinkage of adhesive and properties of the adhesive and adherend.
Many researchers [ 95 , 96 , 97 , 98 , 99 , ] studied the temperature effect on the composite bonded joint strength. Generally adhesive joints strength degrades at higher temperature and improves at lower temperatures. At temperatures above T g , strength and stiffness decreased following the trend of the thermomechanical behavior of the adhesive [ 97 , 98 ]. It would be beneficial to select the adhesive and composite patch material with higher glass transition temperature, which allow a better performance of the repair joint at higher temperature too. Temperature variations thermal cycle are among the most important environmental factors that may affect the durability of adhesively bonded joints for aerospace applications.
Sousa et al. Little changes occurred with additional thermal cycles, which were partly attributed to the occurrence of post-cure phenomena in the elastic polymer adhesive during exposure at higher temperatures [ 95 ]. A small number thermal cycles would be advantageous to the joint as an occurrence of post-cure. Residual thermal stresses are induced at higher temperature of the joint due to the CTE mismatch between the adhesive and the adherends [ ]. The higher temperatures facilitate polymer chain mobility and lead to some degree of relaxation of these stresses. However, when cooling the joint, the stress relaxation is reflected in an increased interfacial stress between the substrate and adhesive layer.
Fracture toughness of the composite laminate bonded joints is widely used to predict the performance of composite bonded joints under different temperature condition. It has generally been found that there is an increase in fracture toughness, G IC , with increasing temperature while at lower temperature decreases with respect to the room temperature under mode I tensile loading DCB specimens as shown in Fig. An increase of the matrix ductility, an increment in the amount of fiber bridging and fiber breakage are the most common explanation for the improvement in fracture toughness [ , , , , , ].
On the other hand, if test temperature is above the T g , the fracture toughness decreases due to the loss of adhesion between the fibers and the matrix Rubbery state , but below the T g , it was observed an increase in G IC due to strongest bond strength between the fiber and the matrix [ ]. While, Russell et al. In pure mode II tests, however, do not exhibit the same trend, some of the authors observed a decrease G Ic , while some found increased values with respect to the test temperature [ 5 , , , ]. A small number of results for the mixed mode I—II behaviour have been published, but still there is not a clear consensus about the trend of fracture toughness value with the effect of temperature under mixed loading [ 7 , , ].
Fracture toughness with respect to the temperature [ 7 , , , , , , ]. Activation of each mechanism depends on different parameters such as adhesive- adherend material properties, test temperature, curing temperature, glass transition temperature, moisture content, etc. Mixed trend of fracture toughness was observed over the temperature range under different loading condition. Hence, it is important to consider the glass transition temperature and the expected maximum temperature that can be reached by the composite structure during service period, while selection of adhesive and composite patch for repair. Moisture primarily affects the resins and adhesives in FRP composites and bonded assemblies structures.
Generally, the adhesive absorbs more moisture content than the composite laminate, matrix and interface in any composite structure. Each adhesive type absorbs moisture up to certain extent and its absorption rate and saturation limits are dependent on a number of factors such as exposure condition, exposure time, temperature and humidity level, etc. So, the moisture absorption data of each adhesive in different environmental condition for long duration are almost difficult to have. Instead of it, the worst possible, attack by the moisture on the adhesive is considered for the design purpose to maintain the safe design. In general moisture can change adhesive properties through plasticization, swelling, cracking and hydrolysis phenomenon. Figure 10 shows the representative moisture curve of sorption, desorption and resorption level of particular adhesive [ ].
Faster resorption and higher saturation limit in a subsequent cycle compared to previous one indicate a change in physical and chemical properties after a cycle of sorption and desorption. It has been reported that the penetration of the moisture into the polymer will increase the free volume by the swelling effect and cracking during moisture absorption [ , ].
A subsequent step, in resorption cycles, this free volume occupies the moisture in the resorption process which adds more moisture content and faster than the previous cycle. Desorption from highly moisture saturation tended to leave small residual moisture content, which could only be removed by heating at high enough temperature but blistering may occur [ , ]. So, for composite repair structure the knowledge of desorption, resorption and a saturation limit should be needed as the structure face higher impact than previous as proved by earlier studies. Moisture curve of sorption, desorption and resorption level of adhesive [ ].
The effects of the moisture on the mechanical behavior of the epoxy system have been studied by many researchers [ , , , , ]. The detailed trend of tensile strength and elastic modulus is shown in Fig. The possible reasons for the degradation of the strength are plasticization, decreasing the values of the glass transition temperature, stress generation due to the swelling of the system and a possible chemical degradation [ , , , , ]. It was noted that the glass transition temperature, T g value decreased with moisture plasticise the epoxy and softening of the adhesive [ 73 , , , , , ].
Therefore, it is important to have the moisture absorption history of the adhesive, which is going to use for repair and its mechanical behaviour and also consider moisture accumulation by the adhesive during the storage in freezer. Normalised tensile strength at various levels of moisture uptake [ 50 , 73 , , , , , , ]. The moisture uptake by the composites is consist of polymer matrix, interface of matrix-fiber and very negligible by the fiber. Moisture absorption by the composite is mainly conducted by the diffusion mechanism. These two damage-dependent mechanisms are increasing both the rate and the maximum capacity of moisture absorption in an auto-accelerative manner [ 51 , , , ].
The degree of absorption depends on both matrix and fiber properties, matrix-fiber interface, fiber volume fraction, composite void content and epoxy resin curing agent ratio, etc. Moisture absorption by composite laminate during repair time is not only the concern but also the moisture that might be absorbed by uncured composites prepreg during storage. Finally, repair materials that are left uncovered during the multi-step process of bonded repairs may also absorb and trap atmospheric moisture. Figure 12 shows the moisture uptake during the first four absorption cycles.
The moisture level and diffusivity of composites increases during each subsequent reabsorption cycles [ 51 ]. This behavior has been associated with the penetrant molecules can rearrange the polymer network, causing swelling of the material and micro-cracking occurring in the matrix during each sorption. As already mentioned, moisture absorption may induce irreversible changes to polymers and composites, such as chemical degradation, cracking and debonding [ , , , ]. This data would help for proper selection of composite patch material in the composite repair. The influence of moisture absorption on mechanical properties of FRP composites is well documented in literature, regarding the tensile, interlaminar shear and flexural properties [ , , , , , , , ].
The absorbed moisture results in more detrimental effects on the mechanical properties of composite materials since the water not only interacts with polymer matrices, physically, i. Akay et al. This has been supported by an increase of bare fibres on SEM inspections of fractured surfaces, which indicate a weak adhesion between matrix and fiber. Also strong mismatch in swelling behaviour between the matrix and the fibre was observed, which may introduce weak adhesion integrity [ ]. The drying of FRP composites is compulsory but complete drying also lead to damage the FRP composite by introducing the micro-cracking during desorption, so it is important to consider the drying temperature and time, in order to avoid any damage caused by drying [ ].
Presence of moisture in composites may affect the properties of the repair as it can cause an increase in bond line porosity and a decrease in joint strength. The majority of the research papers recommend drying the composite substrates before bonding to prevent the diffusion of moisture from the substrate into the joint during repair cure cycle. For increased durability of composite materials, their capacity for sustained performance under harsh and changing environmental conditions must be quantified. Long term durability of joints in severe environments has been recognized as one of the obstacles to the widespread application of adhesive, specially for aerospace, marine, and offshore structure which exposed to severe environmental conditions.
Composite structure absorbs more moisture through the atmospheric condition during its service period, but we could not neglect the moisture absorbed by an individual components such as composite laminate, adhesive before bonding process. The moisture absorbs before bonding called pre-bond moisture and after the bonding term as post-bond moisture. In subsequent sections, both pre-bond and post-bond moisture are described individually in details. Pre-bond moisture issue is very important for joints formed between polymeric-composite substrates as it directly influences on the performance of adhesive joints. There are several potential sources of pre-bond moisture in composite substrates such as: during the manufacturing process, CFRP panel undergoes several treatment procedures like wet abrasion, water break test, transportation of CFRP panel from one place to another, storing the laminate for longer periods in freezer and exposing to environmental conditions during composite repair in field etc.
There are limited studies [ 4 , 34 , 41 , 44 , 47 ] reported on the pre-bond moisture effect on the mechanical properties of the bonded joints and most of them found the decrease in strength when the moisture is present in the composite. Parker et al. Voiding, plasticization of the adhesive, and a reduction in interfacial adhesion are the possible causes for the reduction in strength [ 41 , , ]. An increase in the pre-bond moisture of the composite substrate yielded an increase in void content of the joint further support for higher degradation [ 41 ].
Drying the composite substrates and curing the bonded joints under isostatic pressure was found to prevent the occurrence of voids [ 44 , 47 , ]. Most of the entrapped air could be evacuated prior to cure for the method using a textured adhesive film. These air evacuation strategies reduced the bondline void and exhibited a higher strength of repair bonded joints [ 39 , 63 ]. Previous work on BMI adhesives suggested that void reduction was also possible if vacuum was removed at the adhesive flow temperature and small positive pressure was maintained during cure [ ]. A small amount of pre-bond moisture below 0. However, pre-bond moisture of about 1. There was a slight decline in strength with moisture content up to a moisture content of around 1.
It is clear that if the composite content small percentage of moisture 0. Generally, the moisture levels usually found in composite components in service are typically 0. Research is needed to establish whether the effect of pre-bond moisture is always detrimental or whether a small percentage of moisture in the composite is acceptable. Extending the drying time of the substrate cause an improvement on the composite bonded strength and fracture toughness of the joint although not fully recovered [ 44 , ].
Thus, this is one of the areas that need a further attention. In addition to that fatigue behavior is still not well developed under the effect of pre-bond moisture. Thus, these are the areas that need a further attention. It is needed to elaborate this results in more fraction of moisture and provide a significant explanation for this. Nevertheless, it is necessary to have more experimental studies in order to justify or set the proper process parameter which link up the relationship between pre-bond moisture and bonded joint strength.
The moisture absorption of the composite structures are mainly depends on the exposure condition such as: humidity, temperature, wind, UV radiation, thermal cycling, water and the exposure time. Moisture can ingress into the joint through diffusion into the bulk adhesive, composite laminate and wicking along the interface or capillary action into cracks and voids. Several researchers [ 41 , 46 , , , , ] reported the reduction of bonded joint strength with effect of post-bond moisture. Weakening of bonding between the fiber and matrix and softening of matrix are the possible causes for the reduction in strength [ , , , ]. The strength reduction rate depends on the exposure time, exposure condition, type of adhesive material and adhered, which ultimately lead to final moisture content in the bonded joints.
Jeoung et al. However, when the moisture content increased to 2. It is believed that the composite joint strength increased at low moisture content due to the prevention of delamination by the compressive stress created between the plies of the adherend. Drying is the best suited treatment to recover the strength, However the full recovery was not achieved [ 34 ]. Drying at high temperature could improve the strength up to certain extent but blistering and some crack on the composite surface occur. Therefore, behavior of specific bonded systems exposed to various environments should be taken into account in durability design. In addition to that interface between adhesive-adherend and bonding manufacturing process such as co-curing, co-bonding, etc.
So, complete performance data of the adhesive and the composite laminate should be in hand in the presence of moisture before selection for composite bonded joints. The combined effect of moisture and temperature is more severe than the adverse effect of each individual condition temperature and moisture. Generally, moisture sensitivity moisture absorption, desorption, and saturation is more effective when the structure is exposed to elevated temperature.
Composite usually absorbs more water at high temperature and this is a common way to accelerate water absorption. It is clear from the Fig. The glass transition temperature under dry and saturated conditions is a critical property for composites as the maximum service temperature depends on it. The mechanical performance of composite material is influenced by the hygrothermal effect. Adhesive epoxy resin usually absorbs more moisture as compared to the composite laminate and composite structure, but it absorbs even more at a particular temperature elevated. The absorbed water molecules in an epoxy can exist in either the free or bound states [ , , ]. Free water molecules act as a plasticizer, strongly reducing T g and the modulus of elasticity [ ].
Usually, when the material is exposed in a hygrothermal environment the T g decreases and, therefore, the service temperature of the material changes. Not only temperature and moisture but also exposure time and exposing temperature also decide the variation of glass transition temperature of epoxy resin [ , ]. Hence selection of adhesive material and its glass transition temperature should be notified before the application [ ]. Limited research was carried out on hygrothermal effect on the bonded joints strength. Most of the researchers [ 41 , , , ] reported the reduction in strength of ageing specimen joints at elevated temperature. However the strength of the pre-saturated joint up to 1.
A decrease in strength was observed in the case of higher temperature and longer exposure time to humidity. The tensile strength and ILSS decrease when the material has been exposed to moisture and tested at elevated temperature. But, no significant difference was reported for strength in between autoclave and vacuum-cured materials. This result supports the feasibility of scarf joint repairs with pre-cured or cocured patches under vacuum curing conditions in field-level facilities. Therefore, repairs with vacuum pre-cured or vacuum co-cured patches requiring less equipments seem to be a serious potential alternative to the composite patch repair requiring autoclave conditions which might be only available at depot level maintenance centers [ 96 ].
To summarize, the individual effect of moisture and temperature on the mechanical properties of adhesive material and joints is well understood, but there is a still a lack if systematic ageing conditions to clearly identify the combined effect of each environmental parameter. An increasing complexity in geometry and material non-linearity of composite repair bonded joints makes difficult to obtain an overall governing equation. In addition to that, incorporation of the environmental parameters i. However, the experiments are often time consuming and costly. Therefore, the finite element analysis can be employed to overcome the limitations of the analytical methods.
Several researchers [ 6 , 10 , 11 , 13 , , , , ] successfully used finite element and analytical tools to perform a broad geometric and material parametric studies to optimise the parameters for maximum repair joint performance. Figure 14 shows the main geometrical parameters such as scarf angle, number of steps, patch thickness, adhesive thickness, overlap length, doubler plate, stacking sequence etc. Selection of failure criterion is an important parameter for finite element analysis of the composite bonded joints. For linear elastic analysis, peal stress and shear stress value were considered as a failure criterion performance quality indicator , where the adhesive behavior assumed to be elastic.
When the adhesive behaviour became non-linear, the maximum shear strain of the adhesive layers was used to assess the joint strength. One problem with the allowable stress or allowable strain criterion is the mesh dependent singularity at the tip of the crack geometric singularity , as well as the singularity at the intersection of each ply and the adhesive stiffness mismatch [ 17 , ]. Recently, a cohesive zone model CZM modelling methodology has been shown to be a versatile approach to predict the durability of adhesively bonded joints exposed to humid environments [ 68 , , , , , ].
The accurate prediction of failure behaviour should be correctly implemented using a traction—separation law which includes triangular, trapezoidal and exponential shape [ , ]. The parameters that principally define the traction—separation response are the cohesive fracture energy and the critical traction of the adhesive in each fracture mode. A proper selection of traction—separation law behavior is important. For example, a trapezoidal law predict more accurate for temperature variation in the joint [ 68 , ]. As the moisture concentration adversely influences the cohesive properties, moisture-dependent cohesive properties are required to accurately predict the failure behavior of a saturated or unsaturated adhesively bonded joint using the cohesive zone approach.
Incorporation of fracture data from the ageing test into a fracture prediction methodology to enable the prediction of real closed joints is a real issue and time taking also, hence it is essential to use testing techniques that accelerate the ageing. To accelerate the ageing the open-faced method has shown a great promise in significantly reducing the time and cost of fracture tests. However, the challenge is how to incorporate the fracture data from accelerated ageing test into a fracture prediction methodology to enable the prediction of real closed joints. Ameli et al. A framework for the assessment of the applicability of the open faced technique to the prediction of the durability of closed DCB CDCB joints is shown in Fig. The significance of this framework is the ability to remarkably reduce the exposure time by using the open-faced technique and to incorporate the spatial variation of degradation in the closed joint with the aid of the 3D finite element model [ ].
Framework for the FE prediction and validation of fracture toughness in environmentally degraded closed adhesive joints [ ]. The lifetime of bonded joints is difficult to model accurately and their long term performance cannot easily and reliably be predicted, especially under the combined effect of an aggressive environment and mechanical loading. In addition to that, incorporation of manufacturing process of the bonded repair on its stress state in cohesive zone model is needed, as this parameters shows positive response on bonded repair performance. The problem of durability of adhesive joints to hostile environments has become the main challenge for researchers in this area.
This mechanism can however be included by defining the delamination strength for the composite with a mode dependent CZM parameters. Important concerns are critically expressed here regarding the environmental variants moisture, temperature, humidity etc. In recent years, many developments have been made by researchers to improve the environmental resistance of the composite structures such as new advanced composite and adhesive material, curing method, manufacturing bonded joints method, etc. Hence, there is strong need for improving the current composite repair subjected to environmental issues such as moisture, temperature etc.
In this review, several scientific challenges and opportunities have been identified in order to develop more durable and cost-effective composite bonded repair technologies with short repair cycle:. There is no generalised trend with respect to the effect of moisture and temperature on the bonded joint as it depends on a number of factors: such as curing temperature, curing method, adhesive and composite laminate material. Hence, an urgent need to assess and evaluate the behavior of advanced composite laminate and adhesive material under high and low temperature as well under different moisture conditions in order to utilize the full capability of the material for bonded repair joints.
Advanced structural adhesives and composite material, could offer opportunities to enhance strength and long-term durability of bonded repairs. Time required to fabricate bonded repair mainly depends on drying of composite before repair bonding and curing the same repair joints, could significantly influence the associated economical aspects. The material system that can cured at low temperature with short cycle but should have higher glass transition temperature could be a good for bonded repairs. A complete drying of composite is in current practice of composite repair, but it is not necessary always as deduced by researchers.
So curing at low temperature and not complete drying, both help to reduce the repair time which impact on huge economical aspect. Performance of composite repair can be improved by implementing new methods: such as curing by vacuum method which produce a good quality repair with low bondline void as similar to the autoclave curing method, manufacturing by co-bonded method joints, which absorb less moisture compare to the co-cured bonded joints method.
There is need to work on this aspect and plan for more tests and confirm assurance for the better composite repair bonded joints. The available studies focusing on the effect of moisture and temperature on the mechanical behavior of adhesively bonded joints still have considerable differences in terms of the adherend and adhesive material properties, the material processing methods, adhesive curing temperature and specimen configurations. Thus it is important to have the pre-knowledge such as curing temperature, glass transition temperature, moisture absorption—desorption limit, swelling, thermal expansion, etc. Limited studies were carried out on the effect of hygrothermal moisture and temperature on the composite bonded joints and it is highly demand for this study as the combined effect is more sever than individual condition.
Aerospace industry demand for lower frequency of repair and maintenance of the composite structure and this can be possible by introducing the self healing materials which can help to improve the durability of the structure. Also composite bonded structure should easily disbond without damaging the structure, then it can be used for reuse and recycle. Both these aspect should be implemented in the current scenario in order to reduce the frequency of the maintenance and easily separate without damaging the parent structure at the time of repair.
Finite element method is well developed numerical tool and used to optimise the geometry and material parameter of repair joint for better performance of the structure. Failure criterion of the composite bonded joints and incorporation of moisture and temperature parameter is the main constraint in scarf and stepped lap joints. Cohesive zone model successfully incorporate the environmental issue on the bonded joints and analyse the joints under the influence of moisture and temperature.
Still long term durability is a major concern, as it is difficult to predict accurate environmental behavior of the joints. Open face specimen technique introduced accelerated ageing which help to reduce the exposure time. Open face technique offer opportunities for developing accurate prediction of joint behavior for long term using cohesive zone modeling to any adhesive system that exhibits nonuniform degradation. Compressive strength of damaged and repaired composite plates. J Compos Mater. Google Scholar. Repair of advanced composite structures. J Aircraft. Article Google Scholar. The effects of manufacturing-induced and in-service related bonding quality reduction on the mode-I fracture toughness of composite bonded joints for aeronautical use.
Compos Part B Eng. The bonded repair of fibre composites: effect of composite moisture content. Compos Sci Technol. Tensile behaviour of patch-repaired CFRP laminates. Compos Struct. Hu FZ, Soutis C. Strength prediction of patch-repaired CFRP laminates loaded in compression. Asp L. J Asian Archit Build Eng. Optimisation study of tapered scarf and stepped-lap joints in composite repair patches. Optimization of a composite scarf repair patch under tensile loading. Compos Part A Appl S. Effects of external patch configuration on repaired composite laminates subjected to multi-impacts.
Design and optimization of bonded patch repairs of laminated composite structures. Jefferson Andrew J, Arumugam V. Int J Adhes Adhes. Experiment and design methods of composite scarf repair for primary-load bearing structures. On the design methodology of scarf repairs to composite laminates. Compos Sci and Technol. Abdel Wahab MM. Fatigue in adhesively bonded joints: a review. Adhesively bonded joints in composite materials: an overview. Proc Inst Mech Eng L. An updated review of adhesively bonded joints in composite materials. Fracture mechanics tests in adhesively bonded joints: a literature review.
J Adhes. The effect of adhesive thickness on the mechanical behavior of a structural polyurethane adhesive. Effect of material, geometry, surface treatment and environment on the shear strength of single lap joints. Influence of the interface ply orientation on the fatigue behaviour of bonded joints in composite materials. Int J Fatigue. Effects of composite adherend properties on stresses in double lap bonded joints. Mater Des. Strength prediction of bonded assemblies using a coupled criterion under elastic assumptions: effect of material and geometrical parameters. Effect of manufacturing methods on the shear strength of composite single-lap bonded joints.
A review on the temperature and moisture degradation of adhesive joints. Smart Adhesive joints: an overview of recent developments. Kumar S, Sridhar I. J Adhes Sci Technol. Pimenta S, Pinho ST. Recycling carbon fibre reinforced polymers for structural applications: technology review and market outlook. Waste Manage. Special issue on primary bonded repairs in composite structures.
Armstrong K. Effects of absorbed water in CFRP composites on adhesive bonding. Kohli DK. Selection of patch and adhesive materials for helicopter battle damage repair applications. Damage tolerance investigation of high-performance scarf joints with bondline flaws under various environmental, geometrical and support conditions. Processing of co-bonded scarf repairs: void reduction strategies and influence on strength recovery. Out-of-autoclave scarf repair of interlayer toughened carbon fibre composites using double vacuum debulking of patch. Parker B. Some effects of absorbed water in CFRP composites on adhesive bonding.
Repair of composites: design choices leading to lower life-cycle cost. Appl Compos Mater. Care and repair of advanced composites. Note that the repair chemical goes to all layers of the composite. Far right is a large airplane wing piece showing a continuous array of repair vessels. Self-repair of composites is a concept that Dr. Carolyn Dry invented. There are seven patents issued. The work, under the Air Force Small Business Innovative Research SBIR program, on self-repair in impacted composite laminates processed by lay-up and autoclaving at F for fiberglass and at F for graphite laminates, was successful.
Further the chemical penetrated to all layers in the fiberglass and graphite composite laminates.. Recently, very fast adhesive systems for repair have been developed. Photos taken from a video of the dynamic system of impact and repair shows very fast delamination and fiber break, chemical release, flow into damage site and repair chemical cure, all in less than a minute. Far left is a photo of dyed pink repair chemical that vacated the tubes and filled the delamination and crack caused by an impact. Next is an outside view of the laminate 1 second after impact the repair chemical has already filled the damaged area and next is the same repaired area, backlit, at 5 seconds after impact. Note that the repair chemical flows into delaminations and cracks, as well as broken fibers as seen on the right.
Far right is a sample that was impacted twice. The repair chemical is dyed red. The light color in the release vessels reveals that half of the repair chemical had been used. Self-repairing, fiber reinforced matrix materials consist of a matrix material, which includes inorganic as well as organic matrices, as well as reinforcements disposed within the matrix, hollow fibers having a selectively releasable modifying agent contained therein. The hollow fibers may be inorganic or organic and of any desired length, wall thickness or cross-sectional configuration. The modifying agent is selected from materials capable of beneficially modifying the matrix fiber composite after curing. The modifying agents are selectively released into the surrounding matrix in response to a predetermined stimulus, be it internal or externally applied.
The hollow fibers may be closed off or even coated to provide a way to keep the modifying agent in the fibers until the appropriate time for selective release occurs. This application is a continuation of application Ser. In the last decade the use of polymers has grown so much that polymer bridges exist and use of polymers in airplanes had doubled. This, despite the drawbacks of polymers in reliability and consistency, is because polymer composites have so many advantages over steel or concrete. Polymer composites are three times stronger than steel and five times lighter. Composite materials have applications in rehabilitation of existing bridges as complete structural replacement or new construction [5, 6]. Figure 2. Fifty percent of the flight surface is carbon fiber .
Advantages include savings and weight reduction. These savings range from lives spared to profits saved to taxpayer savings. Also, the weight of the planes could be reduced by use of less over designed, thick laminates, while the expense is reduced by use of cheaper, less toughened prepregs. Damage, which is internal, is very common in composites. Repair of this damage is important in order that failures do not progress to catastrophic failure. However, delaminations are hard to detect. Damage is usually repaired in the field by hand, but not all of the original strength is restored.
Relevant References Written by Dr. Dry on Self Repair of Composites. Patent Number Title 1 6,, Self-repairing, reinforced matrix materials 2 6,, Self-repairing, reinforced matrix materials 3 5,, Self-repairing, reinforced matrix materials 4 5,, Smart-fiber-reinforced matrix composites 5 5,, Self-repairing, reinforced matrix materials 6 5,, Cementitious materials 7 5,, Self-repairing, reinforced matrix materials 8 5,, Natural Process Design Inc.In Summary: Composite Repair Design first case, there are no efficiencies realized Summary: Composite Repair Design avoiding multiple set-ups for example where the same step for Summary: Composite Repair Design parts can be carried out with one set-up. Carbon Summary: Composite Repair Design sheets are costly, Chasing Ice Film Review rigid, and difficult to cut, design and apply in comparison with glass fiber reinforcement. Drying at high temperature could improve the strength up to certain extent Summary: Composite Repair Design blistering and some crack on the composite surface Battle Royal Ralph Ellison Analysis. Nor do they Summary: Composite Repair Design to the fiber volume fraction as a significant material parameter in terms Summary: Composite Repair Design documentation and quality control, Summary: Composite Repair Design the thickness measured Summary: Composite Repair Design help to Summary: Composite Repair Design and compare in general. The first, and most common method, is to apply it as a wrap. Google Scholar.