Evonik ROHACELL® HERO Foam Core Used in Manufacture of Aircraft Nose Landing Gear Doors

This article describes the cost and weight evaluation of fully tested Dornier 728 nose landing gear doors reengineered using ROHACELL® HERO polymethacrylimide (PMI) closed cell rigid structural foam core material from German-based Evonik, one of the world’s leading specialty chemicals manufacturer.

The experiment involved comparison of a foam/infusion gear door model with a honeycomb/prepreg gear door model. The results showed that the overall manufacturing costs of an out of autoclave (OoA) finished part made using foam core material, are 25% lower than that of its counterpart, in addition to a overall weight saving of 19%. The significance of both cost and weight savings resulting from foam core holds promise in the development of next-generation composite aircraft models with complex geometrical shapes.

Aircraft Nose Landing Gear Doors Redesigning Project

The redesigning project was solely executed by the Composite Technology Centre (CTC), a subsidiary of Airbus and a specialist in composite processing technologies. CTC is focused on developing cost-effective production technologies of lightweight, carbon fiber reinforced plastic (CFRP) aircraft components. It has also carried out cost evaluation projects of producing lightweight CFRP sandwich parts for class A and B aircraft components.

CTC evaluated the performance of wear-resistant ROHACELL® HERO foam core samples from Evonik to the typical honeycomb material, and concluded that the key benefit of the foam core is its ability to fabricate without autoclaving. Out of all the available grades of foam core PMI material, the HERO range offers minimized resin absorption, visual detectability, better resistance to impact damage, and high elongation rate of 9 to 10%, making it ideal for exterior aircraft application.

CTC examined three different aircraft carbon fiber designs to provide relevant comparative analysis data: infused HERO core/ liquid epoxy resin, autoclaved HERO core/ epoxy prepreg and autoclaved honeycomb core (64 kg/m3, 4.8mm)/ epoxy prepreg resin system. The results proved that both foam core materials can be fabricated in less time and at low cost. When compared to the honeycomb design, the autoclaved HERO/ epoxy prepreg design is cheaper by 21%. The HERO core/ epoxy infusion resin OoA design, however, turned out to be the best option with light design, low overall production cost and low fabrication time consumption.

The figures 1 and 2 show the weight and cost comparison graphs of three carbon fiber designs of Dornier 728 NLG door.

Figure 1. Graph showing the weight comparison data of three carbon fiber designs of Dornier 728 NLG door.

Figure 2. Graph showing the cost comparison data of three carbon fiber designs of Dornier 728 NLG door.

Comparison of Time and Costs of Production

When compared to 42h of honeycomb/prepreg design production, the infused ‘single shot’ HERO foam core design was fabricated in 24h owing to the exclusion of two major steps of the ‘two shot’ curing process. With an infused closed cell foam core design, the preparation time of prepreg core and the process of curing, demoulding, and the integration of carbon fiber reinforced outer skin with the honeycomb are eliminated. As a result, the moulding ability of the foam core laminate design was doubled. CTC pointed out that these cost and time savings have been achieved for samples with simple geometry. The figure 3 shows the graph illustrating the time data comparison of three carbon fibers.

Figure 3. Graphical representation of time data comparison of three different carbon fibers

Fabrication of Infused Foam Core Design

German manufacturer INVENT produced the HERO core/ epoxy infusion resin nose landing gear doors. Earlier experiments have suggested that the resin infusion process is comparatively fast and cost-effective than autoclaving or prepreg. The pre-shaped foam core material was provided as an insert piece and the NC machined at appropriate dimensions with respect to the 71HERO core foam sheet.

All ROHACELL PMI foams, like a honeycomb core does not possess a closed cell structure. This helps in eliminating the need for a core filling paste to seal the edges or an adhesive film to bind the skins. CTC comparative analysis suggested the elimination of preparation time of 4h in addition to reducing the cost of using both adhesive and potting material.

MRO performance of ROHACELL® HERO foam core

Lower maintenance, repair and operations (MRO) cost are an added advantage of the ROHACELL® HERO foam core. By contrast, the honeycomb sandwich part is subjected to the cracking of potting material or epoxy resin resulting in the formation of water ingress leakage paths. However, with the closed cell core sandwich structure, the chances of skin debonding problems are eliminated.

Damage tolerance, visual damage detectability and structural integrity are the critical factors considered by CTC for selecting a core material. ROHACELL® HERO foam core delivers an overall high performance, satisfying the requirement of CTC. Figure 4 shows the infused ROHACELL® HERO Dornier 728 NLG door. Figure 4a and 4b show the low temperature Fraunhofer IWM’ NDT impact tests and comparative impact depth after 48hrs, respectively.

Figure 4a. Fraunhofer IWM’ NDT impact tests at low temperature (-55°C)

Figure 4b. Comparitive Impact Depth after 48hrs of tests

Future Work

Based on the evaluation results of time, weight and overall production cost savings, the experimental findings suggest that the infused foam core is ideal for OEM applications such as Airbus and for reengineering of certain complex aircraft components.

The Evonik team is further working on the use of different grades of ROHACELL® HERO foam core in various structural application projects with Airbus. The physical properties of ROHACELL® HERO PMI closed cell rigid foam core is shown in the Figure 5.

Figure 5. Physical properties data table - ROHACELL® HERO PMI closed cell rigid foam core

About Evonik Industries AG

Evonik is one of the world's leading specialty chemicals companies. Profitable growth and a sustained increase in the value of the company form the heart of our strategy, which is supported by our owners, RAG-Stiftung (74.99 percent) and funds managed by CVC Capital Partners (25.01 percent). Our specialty chemicals activities focus on high- growth megatrends—especially health, nutrition, resource efficiency, and globalization—and our goal is to enter attractive future-oriented markets.

This information has been sourced, reviewed and adapted from materials provided by Evonik Industries AG.

For more information on this source, please visit Evonik Industries AG.

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