Composite repair

Last updated August 07, 2023

Composite repairs are performed on damaged laminate structures, fibre reinforced composites and other composite materials. The bonded composite repair reduces stresses in the damaged region and prevents cracks from opening or growing. Composite materials are used in a wide range of applications in aerospace, marine, automotive, surface transport and sports equipment markets. Damage to composite components is not always visible to the naked eye and the extent of damage is best determined for structural components by suitable Non Destructive Test (NDT) methods.

Composite structures of interest

Figure 1: Composite structures (a) laminated and (b) sandwich

The composite structures of interest are mainly components composed of laminated plies or sandwich structures as shown in Figure 1. Laminate structures are assembled so that the fibre orientation provides most of the desired mechanical properties and the matrix largely determines the environmental performance. In sandwich structures thin, high strength skins are separated by, and bonded to, lightweight honeycomb cores; the thicker the core the stiffer the panel with minimal weight increase.^[1]

Typical damage of composite structures

Figure 2: Typical damage cases of laminates and sandwich structures Figure-2-typical-damage-cases-of-laminates-and-sandwich-structures.jpg — Figure 2: Typical damage cases of laminates and sandwich structures

The most important damage to fibre reinforced composites is the result of impact incidents. Low velocity and high velocity impact can result in significantly different damage patterns for a given composite configuration. In metals the impact energy is dissipated through elastic and plastic deformations and still the structure retains a good margin of structural integrity. In fibre reinforced composite materials however, the damage is usually more extensive than that seen on the surface. Some typical damage cases for composite structures are shown in Figure 2.

In monolithic laminates the underlying damage can extend to a much greater extent than the barely visible evidence on its surface. Other type of damage is laminate splitting. Here the damage does not extend through the full length of the part. The effects on the mechanical performance depend on the length of split relative to the component thickness.

In sandwich structures, impact results in dents of various sizes and depending on the energy levels, puncture damage is not unusual. In this case both skins may be damaged. Other common damage types include heat damage and bolt hole damage. The heat damage is caused by the exposure at high temperature which causes a local fracture with separation of surface plies. The bolt hole damage is cause by the bearing stresses at the contact surfaces of the composite structure with a bolt or rivet used for joining purposes. This could result in elongation of the hole causing laminate splitting, or damage to the upper plies. In any case, the effect on the mechanical performance depends on the thickness of the damaged part.

Further implications, apart from the initial impact damage, come from the exposure of the damaged area to moisture and other degrading factors like chemicals, lubricants, fuel, hydraulic fluids, etc. The presence of such environments may result in further deterioration of the mechanical performance.

Repair flow chart

When performing a composite repair there are certain steps that should be followed. In Figure 3, a typical composite repair flowchart is given.

The first step in the procedure should be a careful damage assessment. Some damage to composites is obvious and easily assessed but in many cases the damage may first appear quite small, although the real damage is very much greater. Impact damage to a fibre can appear as a small dent on the reinforced composite surface but the underlying damage can be much more extensive. The decision to repair or scrap is determined by considering the extent of repair needed to replace the original structural performance of the composite. Other considerations are the repair costs, the position and accessibility of the damage and the availability of suitable repair materials.

The initial assessment will determine the repair type to be performed. Easy repairs are usually small or do not affect the structural integrity of the component. These repairs are made by following the simple guidelines indicated for laminate or sandwich panels. Complex repairs are needed when the damage is extensive and needs to replace the structural performance of the component. The best choice of materials would be to use the original fibres, fabrics and matrix resin. Any alternative would need careful consideration of the service environment of the repaired composite, i.e. hot, wet and mechanical performance. The proposed repair scheme should meet all the original design requirements for the structure.

Some repairs need the specialist equipment of the workshop and some form of improvised repair is needed to return the component to a suitable repair workshop. A temporary repair, usually in the form of a patch, can be fixed to the component to ensure safety until the component can be repaired at a later date. For a permanent repair all the approved general guidelines for laminate and sandwich repairs should be followed. These repair operations should be carried out in controlled workshop areas to ensure high quality. Operating in a controlled environment and attention to the detail will ensure success.

Before returning to service the quality check is always required. For comprehensive inspection of repaired parts, a number of Nondestructive Tests (NDT) can be used. Special attention should be given to the quality of the repaired area and more specifically to the interface between the original part and the repaired area. Usual inspection methods include some form of ultrasonic or X-ray inspection.

Typical composite repairs

A typical composite repair usually starts after damage detection either by unaided eye or various other NDT-techniques. After evaluating the extent of the damage, the damage zone is prepared for reparation. This is done by removing the composite material around the damage zone 1. Three techniques are known to be utilized, being slightly different depending on the nature of the composite, as shown in Figure 4.

Figure 4: Typical laminate and sandwich repairs Figure-4-typical-laminate-and-sandwich-repairs.jpg — Figure 4: Typical laminate and sandwich repairs

When a composite repair is applied the proper surface treatment is essential for a successful result. The above-mentioned repairs can be time consuming and often require high skills and experience to be performed. This is why current developments tend to focus on automating this process either by advanced mechanical milling or alternate technologies like nanosecond-pulsed lasers 2-3. After the damage zone has been completely excavated, surfaces are cleaned and further prepared for the final repair by patches. This can be done by plasma burning of surface contaminants, exposing fibres by removing matrix material through laser radiation or improving the surface wettability for adhesives by photochemical reactions induced by UV-laser light 4.

In a typical repair, the patch is applied under vacuum and at temperatures high enough for the curing of the adhesive. For these purposes a portable hot bonder device may be used for in the field repairs. For more complex and higher quality repairs an autoclave should be used. A hot bonder unit is shown in Figure 5.

Figure 5: Typical hot bonder portable unit. It can apply vacuum, heat up the repair area and constantly monitor and regulate temperature Figure-5-typical-hot-bonder-portable-unit-it-can-apply-vacuum-heat-up-the-repair-area-and-constantly-monitor-and-regulate-temperature.jpg — Figure 5: Typical hot bonder portable unit. It can apply vacuum, heat up the repair area and constantly monitor and regulate temperature

In any case, the application of a vacuum bag is a necessary step for high quality repairs. Vacuum bag processing is suited to components with thin sections and large sandwich structures. The vacuum bag technique involves the placing and sealing of a flexible bag over a composite lay-up and evacuating all the air from under the bag as schematically shown in Figure 6.

Figure 6: Left: Details of vacuum bag lay-up. Right: Vacuum bag sealing and vacuum application results in atmospheric pressure on the repair region (prepreg)

The removal of air forces the bag down onto the lay-up with consolidation pressure of 1 atmosphere (1 bar). The completed assembly, with vacuum still applied, is heated up to the desired temperature for curing. This can be achieved by using heating mat or by placing the assembly inside an oven with good air circulation. For thicker sections and high quality bonding, the use of an autoclave with regulated temperature and additional overpressure should be sought.

The most important technical challenges in the implementation of a successful composite patch repair are: (a) proper design of the repair patch and the procedures that will be followed, (b) selection of the most suitable materials and tools for the application, (c) careful surface preparation, (d) implementation of the composite patch repair and careful application of the cure cycle, (e) non-destructive evaluation of the repair by suitable methodology and, (f) monitoring of the structural integrity of the repair either at specific time intervals or continuously.

Related Research Articles

A composite material is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites from mixtures and solid solutions.

Fiberglass or fibreglass is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cloth. The plastic matrix may be a thermoset polymer matrix—most often based on thermosetting polymers such as epoxy, polyester resin, or vinyl ester resin—or a thermoplastic.

In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure, or mixing with a catalyst. Heat is not necessarily applied externally, but is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

Fibre-reinforced plastic is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as paper, wood, boron, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.

Acoustic emission (AE) is the phenomenon of radiation of acoustic (elastic) waves in solids that occurs when a material undergoes irreversible changes in its internal structure, for example as a result of crack formation or plastic deformation due to aging, temperature gradients, or external mechanical forces.

Nonwoven fabric is a fabric-like material made from staple fibre (short) and long fibres, bonded together by chemical, mechanical, heat or solvent treatment. The term is used in the textile manufacturing industry to denote fabrics, such as felt, which are neither woven nor knitted. Some non-woven materials lack sufficient strength unless densified or reinforced by a backing. In recent years, non-wovens have become an alternative to polyurethane foam.

In materials science, a sandwich-structured composite is a special class of composite materials that is fabricated by attaching two thin-but-stiff skins to a lightweight but thick core. The core material is normally low strength, but its higher thickness provides the sandwich composite with high bending stiffness with overall low density.

Glass fibre reinforced concrete (GFRC) is a type of fibre-reinforced concrete. The product is also known as glassfibre reinforced concrete or GRC in British English. Glass fibre concretes are mainly used in exterior building façade panels and as architectural precast concrete. Somewhat similar materials are fibre cement siding and cement boards.

Spray-Up also known as chop method of creating fiberglass objects by spraying short strands of glass out of a pneumatic gun. This method is used often when one side of the finished product is not seen, or when large quantities of a product must be made cheaply and quickly with moderate strength requirements. Corvette fenders and boat dinghies are commonly manufactured this way.

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Carbon fiber-reinforced polymers, carbon-fibre-reinforced polymers, carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic, also known as carbon fiber, carbon composite, or just carbon, are extremely strong and light fiber-reinforced plastics that contain carbon fibers. CFRPs can be expensive to produce, but are commonly used wherever high strength-to-weight ratio and stiffness (rigidity) are required, such as aerospace, superstructures of ships, automotive, civil engineering, sports equipment, and an increasing number of consumer and technical applications.

Z-pinning is a technique to insert reinforcing fibres along the Z-direction of continuous fibre-reinforced plastics. Z-pins can be made of metal or precured unidirectional composite fibres. It is designed for use within pre-preg technology; there is extensive experimental evidence that Z-pinning dramatically improves the resistance of the composite structure to delamination. The figure on the right shows a Z-pin inserted in between the fibres of the material. The pin spreads the fibres and creates an oval shaped gap that is filled with resin. The Z-pin prevents the composite from delamination. When a load is applied the cracks will typically form along the line of the opening.

A crack arrestor is a structural engineering device. Being typically shaped into ring or strip, and composed of a strong material, it serves to contain stress corrosion cracking or fatigue cracking, helping to prevent the catastrophic failure of a device.

Out of autoclave composite manufacturing is an alternative to the traditional high pressure autoclave (industrial) curing process commonly used by the aerospace manufacturers for manufacturing composite material. Out of autoclave (OOA) is a process that achieves the same quality as an autoclave but through a different process. OOA curing achieves the desired fiber content and elimination of voids by placing the layup within a closed mold and applying vacuum, pressure, and heat by means other than an autoclave. An RTM press is the typical method of applying heat and pressure to the closed mold. There are several out of autoclave technologies in current use including resin transfer molding (RTM), Same Qualified Resin Transfer Molding (SQRTM), vacuum-assisted resin transfer molding (VARTM), and balanced pressure fluid molding. The most advanced of these processes can produce high-tech net shape aircraft components.

The term "smart structures" is commonly used for structures which have the ability to adapt to environmental conditions according to the design requirements. As a rule, the adjustments are designed and performed in order to increase the efficiency or safety of the structure. Combining "smart structures" with the "sophistication" achieved in materials science, information technology, measurement science, sensors, actuators, signal processing, nanotechnology, cybernetics, artificial intelligence, and biomimetics, one can talk about Smart Intelligent Structures. In other words, structures which are able to sense their environment, self-diagnose their condition and adapt in such a way so as to make the design more useful and efficient.

Microwave imaging is a science which has been evolved from older detecting/locating techniques in order to evaluate hidden or embedded objects in a structure using electromagnetic (EM) waves in microwave regime. Engineering and application oriented microwave imaging for non-destructive testing is called microwave testing, see below.

Carbon fiber testing is a set of various different tests that researchers use to characterize the properties of carbon fiber. The results for the testing are used to aid the manufacturer and developers decisions selecting and designing material composites, manufacturing processes and for ensured safety and integrity. Safety-critical carbon fiber components, such as structural parts in machines, vehicles, aircraft or architectural elements are subject to testing.

Welding of advanced thermoplastic composites is a beneficial method of joining these materials compared to mechanical fastening and adhesive bonding. Mechanical fastening requires intense labor, and creates stress concentrations, while adhesive bonding requires extensive surface preparation, and long curing cycles. Welding these materials is a cost-effective method of joining concerning preparation and execution, and these materials retain their properties upon cooling, so no post processing is necessary. These materials are widely used in the aerospace industry to reduce weight of a part while keeping strength.

Advanced thermoplastic composites (ACM) have a high strength fibres held together by a thermoplastic matrix. Advanced thermoplastic composites are becoming more widely used in the aerospace, marine, automotive and energy industry. This is due to the decreasing cost and superior strength to weight ratios, over metallic parts. Advance thermoplastic composite have excellent damage tolerance, corrosion resistant, high fracture toughness, high impact resistance, good fatigue resistance, low storage cost, and infinite shelf life. Thermoplastic composites also have the ability to be formed and reformed, repaired and fusion welded.

A Lay-Up process is a moulding process for composite materials, in which the final product is obtained by overlapping a specific number of different layers, usually made of continuous polymeric or ceramic fibres and a thermoset polymeric liquid matrix. It can be divided into Dry Lay-up and Wet Lay-Up, depending on whether the layers are pre-impregnated or not. Dry Lay-up is a common process in the aerospace industry, due to the possibility of obtaining complex shapes with good mechanical properties, characteristics required in this field. On the contrary, as Wet Lay-Up does not allow uni-directional fabrics, which have better mechanical properties, it is mainly adopted for all other areas, which in general have lower requirements in terms of performance.

The main stages of the Lay-Up process are cutting, lamination and polymerization. Even though some of the production steps can be automated, this process is mainly manual, leading to laminates with high production costs and low production rates with respect to other techniques. Hence, nowadays, it is mainly suitable for small series production runs of 10 to 1000 parts.

References

↑ Baker, A. A.; Rose, L. R. F.; Jones, Rhys (2003-01-23). Advances in the Bonded Composite Repair of Metallic Aircraft Structure. Elsevier. ISBN 978-0-08-052295-1.

Völkermeyer, F.; Fischer, F.; Stute, U.; Kracht, D.: Laser-based Approach for Bonded Repair of Carbon Fiber Reinforced Plastics, Physics Procedia 12, 2011, S. 537-542 doi : 10.1016/j.phpro.2011.03.066
Fischer, F.; Romoli, ; Kling, R.; Kracht, D.: Laser-based repair for carbon fibre reinforced composites, In Hocheng, H., Machining Technology for Composite Materials: Principles and Practice (pp. 309–330), Cambridge, UK: Woodhead Publishing Limited, 2011 at Google Books
Dittmar, H.; Bluemel, S.; Jaeschke, P.; Stute, U.; Kracht, D.: Advantages and Challenges of CFRP Laser Machining with ns-Pulses, Proceedings of 31st International Congress on Applications of Lasers & Electro-Optics, 2012, Anaheim, USA
Völkermeyer, F.; Jaeschke, P.; Stute, U.; Kracht, D.: Laser-based modification of wettability for carbon fiber reinforced plastics, Applied Physics A, 09/2012, doi : 10.1007/s00339-012-7237-3

External links

Composite research projects

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[1] Baker, A. A.; Rose, L. R. F.; Jones, Rhys (2003-01-23). Advances in the Bonded Composite Repair of Metallic Aircraft Structure. Elsevier. ISBN 978-0-08-052295-1.

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