Research: Aircraft interior panels made from fibre-reinforced geopolymer composites - Aircraft Interiors International

Nov 02, 2024

Air-transport statistics show that a significant number of fatalities resulting from aircraft accidents are linked to situations where fire is involved [1][2]. The application of inherently non-flammable, fibre-reinforced geopolymer-based composite materials in the aircraft structure is a promising way to achieve a higher level of safety in accidents.

This article describes the application of Fibre Reinforced Geopolymer Composite (FRGC) material in the aircraft cabin interior, and its test results in terms of flame resistance, fire/smoke/toxicity (FST) and mechanical properties. A full-scale demonstrator of an aircraft interior panel made completely of FST-safe FRGC, honeycomb and foam materials has been developed.

In the context of the Future Sky Safety project (an EU-funded transport joint research programme focused on aviation safety), novel, material solutions with high potential for mitigating the risks of fire, smoke and fumes in the cabin environment were studied and investigated. The Czech Aerospace Research Centre (VZLU) was involved in this project and has contributed to the research by developing FRGC materials applicable in aircraft cabins as well as in other types of transport interiors. The key idea behind the project was to replace existing widely used glass/phenolic interior structures (including new thermoplastic solutions), especially in terms of FST safety.

Other important criteria such as specific weight, cost, and mechanical endurance had to be maintained at a level competitive with glass/phenolic interior structures. Within the project, FRGC materials were subject of research and testing in terms of flame resistance, FST and mechanical properties. Particular attention was paid to the development of geopolymer-based structural foams and honeycombs as a replacement for conventional sandwich core materials. The project programme was concluded by manufacturing and testing full-scale demonstrators, representing the interior wall panel of a small commuter aircraft, made completely of FRGC materials.

Geopolymers are amorphous, inorganic aluminosilicate polymers that combine low temperature, standard thermoset-like processing with high-temperature (~1 000°C) stability. A combination of unique properties makes geopolymers an interesting alternative to existing polymeric or ceramic matrix materials, and offers high potential for the development of cost-efficient composites for applications involving heat and flame exposure.

For the project purposes, the Composites Department of VZLU developed a unique recipe of low-viscosity geopolymer resin. The low cost, water-based GPL30 resin system is suitable for most composite manufacturing techniques at no more cost than the existing materials used. The resin cures at room or slightly elevated (~80°C) temperatures, and is then ready for use. GPL30 does not require any high-temperature post-curing or costly sintering processes. When creating a composite material, most high-modulus fibres can be utilised as the reinforcing phase.

During this project, industrial-grade and recycled carbon, as well as para-aramid fibres, proved to be the optimal choice in terms of strength, thermal resistance and price. A standard wet-laminating method was applied for the manufacturing of all test specimens, panels and demonstrators. The common and cost-effective room temperature vacuum-bagging technique was typically used for the material processing.

An important fact had to be considered for the intended application of the geopolymer composite: it is a relatively new and as-yet unproven material. Hence a database of basic mechanical properties had to be created. The set of mechanical tests conducted in the VZLU test labs included tensile (ASTM D3039), compressive (ASTM D6641, ASTM D695), flexural (ISO 14125), shear (ASTM D3518), and inter-laminar strength (ASTM D2344) tests.

Due to the fact that environmental influence on FRGC mechanical properties has not been thoroughly studied before, increased attention was paid to this issue. All of the tests mentioned above were repeated after the material was subjected to long-term exposures in hot/wet conditions, salt mist, Jet A-1 fuel, engine and hydraulic oils per EN ISO 175:2000, ISO 9227:2017-NSS and ČSN EN 60068-2-78 standards. Selected test results (modules of elasticity) are presented in Figure 1.

Both monolithic and sandwich FRGC structures were extensively tested per CS25, App. F, Part III (flame penetration test) and Part I (vertical flame test) standards. Interior-qualified glass/phenolic GURIT PHG 600 prepreg laminate was applied as the reference material.

When evaluating the App. F, Part III test results, there was no evidence of flame penetration or visible smoke generation in any FRGC specimen. The panels showed excellent fire resistance and post-test structural integrity. On the contrary, the reference glass/phenol typically emitted dense smoke at the beginning of the tests, followed by flash out and a quick loss of mechanical integrity due to resin burnout [3].

Particular attention was focused to Heat Release parameters. These are significant characteristics of material fire safety and were tested per App. F, Part IV standard. GURIT glass/phenol was again assessed as the reference (Figure 2) [3].

Both FRGC and the reference glass-phenol composite materials were tested on CO and CO2 concentrations: specifically the optical density of the smoke, the accumulated value of the optical density of the smoke, the conventional toxicity index, and the fractional effective dose of toxins. In terms of the mentioned criteria, FRGC yielded significantly better results in comparison with the reference glass/phenol in all evaluated FST parameters. As an example, CO content in the combustion products and smoke emission parameters are shown on Figure 3 and Figure 4 [4].

The structural foam and honeycomb used in the geopolymer base were developed and tested as a potential replacement for conventional sandwich core materials. In order to achieve an extra fine and uniform geopolymer foam structure, a unique (patent pending) technique utilising thermally expanding microspheres was developed. The tests showed, as expected, excellent fire resistance and almost zero generation of smoke and combustion gases, even at long-term exposure at 1,350°C. On the other hand, the foam features a higher specific weight-to-strength ratio and natural brittleness, which has to be taken into account when designing the structure [3].

Concerning the honeycomb, the composite consisting of carbon nonwoven fabric and GPL30 resin was applied as a default material. The hexagonal cell honeycomb was manufactured using a technique known as a ‘corrugated process’, in specific weights ranging from 90 to 230 kg/m3 [5]. The honeycomb is also patent pending (see Figure 5).

Prior to manufacturing the full-scale FRGC demonstrator, critical functional parameters were validated on s simplified demonstrator – ‘Demonstrator No. 1’ – in the form of a 406 x 610mm flat panel. Applying para-aramid and recycled carbon fibres, its structure has been tuned to achieve competitive specific weight (Figure 6), impact resistance and production costs.

As a logical completion of the development process, a full-scale demonstrator – ‘Demonstrator No. 2’ – was designed and manufactured. The prototype represented the emergency exit door panel of a regional CS23-certified turboprop aircraft. This panel was identified as the optimal choice for the demonstrator due to its typical attributes featuring sandwich stiffening, a window cutout and a recess for the opening lever (Figure 7).

Such panels are generally made by putting aerospace-qualified glass/phenol prepreg in an autoclave. The lay-up architecture of Demonstrator No. 2 followed the lay-up pattern of the already proven and tested Demonstrator No. 1. Demonstrator No. 2 underwent a harsh and extended 15-minute flame-penetration test in burning Jet A-1 fuel per CS25, App. F test standard (Figure 8).

The pioneering work of the VZLU’s composites department as part of the Future Sky Safety project, was crowned by the resulting large material database and the set of demonstrators, which were used to prove the manufacturability, low weight and cost competitiveness of FRGC aircraft parts, as well as their exceptional fire / smoke / toxicity properties.

Extensive tests demonstrated that the FRGC material highly surpasses current ‘organic’ competitors in all FST parameters (heat release, flame penetration, vertical flame test, smoke density, toxicity). Mechanical tests showed the mechanical and environmental resistance of the material, including not insignificant weight savings.

The next, logically following step, will be to test the panel in real-world airline operating conditions.

[1] Chaturvedi & Sanders. Aircraft Fires, Smoke Toxicity, and Survival. Aviation Space and

Environmental Medicine. 1996, 1996(67(3), 275-8.

[2] Aviation Safety Network >. Aviation Safety Network > [online]. Copyright © 1996 [cit.

21.10.2019].

[3] Download – Future Sky Safety. Homepage – Future Sky Safety [online].

[4] Martaus, F. Fire Effluents and Smoke Optical Density of Fiber Reinforced Geopolymer

Composites. Research report R-6897/2018. Praha: Czech Aerospace Research Centre, 2018.33 p.

[5] Martaus, F. Technological Demonstrators of Geopolymer Based Honeycomb. Research report

R-6958/2018. Praha: Czech Aerospace Research Centre, 2018.42 p.