Technology Development for the Sound-Absorbing Structure with Multiple-Height Filler from Polymer Composite Materials Manufacturing

Technological Issues of the Flying Vehicles Composite Structures Manufacturing


Аuthors

Akhunzianova K. A.*, Pisarev P. V.**

Perm National Research Polytechnic University, PNRPU, 29, Komsomolsky Prospekt, Perm, 614990, Russia

*e-mail: karina-maksimova0402@yandex.ru
**e-mail: pisarev85@live.ru

Abstract

Ensuring acoustic comfort and reducing aircraft engine noise is a priority task in modern engine manufacturing. One of the most effective methods for the noise combating is the sound-absorbing structures (SAS) application in the aircraft engine nacelles. Conventional technologies for the SAS manufacturing from polymer composite materials (PCM), such as vacuum infusion and autoclave molding, impose several limitations, namely low geometric accuracy during infusion and high labor intensity during autoclave molding. Despite their design freedom, additive technologies are unfitted for operating under the high-temperature conditions of aircraft engines. The main technological challenge consists in the lack of the proven and reproducible methods for prototypes manufacturing of the advanced SAS (especially with multiple-height fillers) maximally approximated to actual operating conditions with off-the-shelf aviation materials. The objective of this study consists in try-out of the technology for manufacturing the single-layer, wide-strip, fiberglass composite core samples with a multiple-height polycarbonate honeycomb core, ensuring satisfactory geometric accuracy and the required acoustic characteristics.

This study examines the autoclave molding technology of the carbon fiber prepreg skins (shells) followed by bonding them to a multi-height, honeycomb core by both cold- and hot-curing adhesive films. To create the difference in height (cell depth variation), a paste was employed to fill the cells of a commercially produced fiberglass honeycomb with hexagonal geometry. A filling scheme that ensured the paste height varying from 3 to 18 mm in the 3 mm increments was developed. Auxiliary tooling, namely limiters (matrices) with protective polyimide film for the precise paste dosing were fabricated by the 3D-printing method. This fabrication process included the billets cutting, dedusting, degreasing, limiters mounting, the paste preparing and curing (at 20 ±3°C during 36 hours), the surface polishing, forming adhesive fillets by the heating unit (80°, 100–150 m distance) and final autoclave forming (180C°, 4 hours). Semi-automated perforation of the 2 mm diameter holes (1.59% perforation percentage) in the upper shell was performed according to a preset control program.

To confirm the acoustic efficiency of the manufactured samples, interferometer tests were conducted with normal sound wave incidence at the sound pressure levels of 100, 130, and 150 dB. The dependences of the sound absorption coefficient (α), and both real (ReZ) and imaginary (ImZ) parts of the impedance on frequency were obtained. Analysis of the graphs revealed the presence of several sound absorption peaks. Maximum sound absorption coefficient was 0.75 at the natural frequency of 1028 Hz (at 100 dB). The broadbandness of the developed SAS for the sound absorption coefficient value of 0.5 achieved 980 Hz (the 792–1772 Hz range). It was demonstrated that at a low perforation rate (1.59%), one or two resonant frequencies dominate, limiting the full-scale utilization of the cells volume.

As part of this study, the technology was successfully developed and unique prototypes of the single-layer broadband SAS with a PCM multiple-height filler were manufactured. It was experimentally confirmed that the manufactured structures are acoustically efficient and broadband. The necessity of further tests with the perforation percent closer to those utilized in the aviation engines (closer to 15%) for the full-fledged involvement of all volumes of the cells was revealed. The developed technological solutions are aimed at automating assembly, implementing co-curing methods and hybridizing technologies, which will reduce labor intensity and improve acoustic characteristics of the promising SAS for the aircraft engine manufacturing.

Keywords:

multiple-height honeycomb filler, sound-absorbing structure, polymer composite materials, autoclave molding, carbon fiber prepreg, perforation, assembly technology, aviation engine building

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