Aircraft sandwich composite reinforcing and repairs

Paul Boughton

The most significant challenges involved in reinforcing honeycomb composites using traditional syntactic techniques are long manufacturing times, technical performance issues and overall finished component quality.  

With its latest range of epoxy syntactic systems, Huntsman is well placed to provide the ultimate aircraft reinforcing or repairs solution for sandwich structures.

Huntsman Epocast epoxy syntactics and Eposert syntactic based materials provide solutions for edge sealing, forming and bonding of honeycomb, metallic and plastic insert potting applications and for honeycomb reinforcement and repairs.

Syntactics are formulated by mixing either hollow or plain spheres into epoxy systems. Hollow spheres are used when a lightweight is needed, whereas plain spheres are employed when the load of the application requires high density and high compressive strength. The spheres can be made from various materials for example, glass, plastic or phenolic, combined with other additives to form a moldable mass. Huntsman’s ultra-low to high density syntactics range covers a range of applications for both interior and exterior applications and for structural and non-structural needs.[Page Break]

Apart from their density and compressive strength characteristics, Huntsman’s syntactics are differentiated by their processing properties, offering availability as either one or two component solutions.  They are also differentiated in application as several products satisfy interior FAA regulations (flame retardancy).

To increase strength and moisture resistance of honeycomb composite sandwich panels, Huntsman manufactures a full range of epoxy syntactics designed specifically for edge sealing. Aircraft manufacturers and repair stations use these materials to fabricate and refurbish cabin components such as overhead baggage bins, floor panels and lavatory cabinets as well as flight control surfaces, nacelles and landing gear doors. Huntsman edge sealing syntactics are produced in a range of densities to meet the handling and performance requirements of customers.

Suitable products for edge sealing include: Araldite 1644-A/B ultra-low density syntactic, Epocast1617-A/B and 1618-B/D low-density syntactics and Epocast 89537-A/B and 1652-A/B medium density syntactics. Huntsman also supplies a one-component epoxy designated Epocast 1610-A1 ultra-low density syntactic.The majority of Huntsman edge sealing materials are self-extinguishing and feature easy to apply viscosities, sag-resistance for use on vertical surfaces and high strength.[Page Break]

Medium and low-density grades of Epocast epoxy syntactic and Uralane 5774-A/C polyurethane adhesive comprise the range of products for insert potting, providing a dependable reinforcement for honeycomb composite panels before inserting fasteners. Typical applications include composite floor panels, galley walls, bulkheads or lavatory cabinets.

Today, interior aircraft components designs frequently stipulate  the use of metallic or thermoplastic inserts; The most conventional type of insert are metallic ones and in this case several Epocast® syntactics can be used such as the fast curing and self-extinguishing Epocast® 1618- B/D.

The latest designs often include thermoplastic inserts (such as polyamide-imide based ones); in this case standard epoxy syntactics cannot be used.[Page Break]

In response to this, Huntsman has developed Uralane 5774-A/C, a two component polyurethane adhesive.  The polymer matrix combined with a number of additives and fillers produces an adhesive with excellent performance characteristics.

Uralane 5774-A/C demonstrates good FST properties and is formulated as a fast-setting adhesive with a work-life of 15-25 minutes.  In order to meet requirements within the aerospace repair and maintenance sector, it can be handled after only 4 hours. On application, the adhesive has a paste-like consistency and demonstrates good slump resistance on vertical surfaces.  Uralane 5774-A/C also provides good surface wetting characteristics and can be applied to most substrates with minimal substrate surface preparation.

The cured adhesive produces tough, impact-resistant bonds on both thermoplastic and metallic substrates.  

In the past, polyurethane adhesives have been scrutinised because of their proneness to hydrolytic degradation under extreme environmental conditions. Table 2 compares the tensile lap shear performance of non-aged bonded samples against aged samples on a variety of substrates after 14 days exposure at 48°C, 95 per cent relative humidity.

As the results show, Uralane 5774-A/C performs well in this testing environment.  In many cases, the tensile lap shear strength improved on ageing, indicating that this polyurethane adhesive will remain stable in the most aggressive environments that an aircraft interior is likely to offer.[Page Break]

Huntsman’s insert potting materials, including products within the low to medium-density grade Epocast range and Uralane 5774-A/C polyurethane adhesive are now frequently selected to reliably reinforce honeycomb composite panels before fasteners are inserted.

Offering easy to handle viscosities and high strength, these epoxy syntactic and adhesive systems deliver enhanced technical performance, helping to overcome the challenges associated with the use of conventional methods.  

Reinforcement of sandwich composites where high loading is required can be made by two means. Epocast products can be applied directly into the honeycomb or pre-cured and molded to the desired insert dimension. [Page Break]

Huntsman also offers a unique range of pre-formed and cured inserts available under the Eposert brand that can be installed rapidly in a honeycomb core before fasteners are added.  These low-density inserts are well suited to aircraft manufacturing techniques and repair applications for reinforcing composite floor panels, galley walls, bulkheads and lavatory cabinets.

Epocast solutions meet stringent requirements of numerous aircraft specifications. The table below shows the main products used in sandwich structures reinforcement.

With its Eposert range of pre-formed epoxy inserts, Huntsman has developed an advanced concept in syntactic compounds featuring a number of performance and handling benefits that can significantly increase productivity and finished part quality.  

Formed with minimal void content which can often be a result of mixing a resin and hardener component together, these epoxy inserts produce parts with improved strength, optimised compressive properties, greater reinforced area and part to part consistency.  [Page Break]

Operator exposure to reactive materials is also minimised as the inserts are pre-cured.  As reactive materials are eliminated, these inserts are placed under different transportation classifications to conventional syntactic paste materials.  Material waste is also reduced during repairs both in terms of the amount of material used and the virtually unlimited shelf life of these inserts.

Eposert insert technology can reduce the time required to repair a damaged section of honeycomb by up to 95 per cent when compared to traditional syntactic repair techniques.  This is because the ready-to-use inserts eliminate time consuming steps such as weighing, mixing, gelling and curing cycles, which are otherwise required.

These syntactic inserts can be quickly installed in a honeycomb core before fasteners are added.  One of two methods can be used to bond the preformed insert into the honeycomb.  With the first, a piece of expanding core splice tape adhesive is cut to the required size and wrapped around the Eposert® syntactic.  After the release film is removed, the insert can be installed in the honeycomb cavity.

Alternatively, the honeycomb core can be reinforced by applying a foaming adhesive around the insert until the surrounding honeycomb cavities are completely filled. When all the areas of the honeycomb have been reinforced, fabrication of the composite structure can be completed.

The adhesive materials used around the Eposert syntactic will co-cure with the honeycomb composite part. Secondary drilling and boring operations can then be performed.

Huntsman’s pre-formed syntactics significantly increase productivity, optimise compressive properties and improve the strength of reinforced honeycomb composite parts, while offering unlimited shelf life.

Generally, damage to honeycomb structures occurs as a result of delamination between the laminate and honeycomb structure or damage to the basic structure due to impact.  Depending on the type of damage, a number of conventional repair methods can be used.  Firstly, the damaged area can either be filled with a foam or syntactic type material.  [Page Break]

Secondly, the sandwich skin and honeycomb can be removed from the damaged area.  The hole can then be filled with syntactic foam and a new skin constructed over the top of the area to complete the repair.  Finally, the damaged skin and honeycomb is removed, a new piece of honeycomb is cut to replace the damaged honeycomb.  This is then bonded into place with a fast setting paste adhesive or syntactic, although a core splicing adhesive is more common.  Again, a new skin is constructed over the top of the area and the core splicing adhesive is either cured before or co-cured with the composite structure.  This type of repair often allows the original compressive properties of the structure to be recovered without significantly increasing the structure’s weight.

In the drive to implement more efficient manufacturing methods and improve quality for excellent part consistency, many leading aircraft manufacturers and repair facilities are relying on epoxy syntactic systems from Huntsman Advanced Materials.

Fulfilling a number of aerospace specifications, Huntsman’s Epocast, Eposert and Uralane are used in numerous airborne designs across the entire life cycle - from designers, formulators and prepreggers through to the manufacturers and repair facilities.

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