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Elastomers help aircraft noise insulation structures

By David Vink
Posted 7 November 2012

At the IKV institute of plastics processing colloquium in Aachen, Germany, in March, IKV researcher Arne Böttcher won the prize for studies in fibre reinforced plastics (FRP). Böttcher has developed acoustically optimised sandwich structures with integrated elastomer layers for large aircraft interior parts. The €1,000 prize, awarded for the third time this year, was initiated and is sponsored by Dr Peter Ehrentraut, honorary member of the AVK federation of reinforced plastics.

Böttcher and his colleague Tobias Preuβ said current passenger aircraft interior sandwich structures have cores made from glued-together hexagonal aramide fibre based paper in a honeycomb structure. The outer layers are 40% phenolic resin impregnated face sheets, which on the exterior have a coarse so-called "grid pattern" glass fibre (GF) mesh prepreg, and on the interior a finer structure GF prepreg fabric.

Such structures have poor vibration behaviour, requiring additional damping material, leading to increased weight. Preuβ said passengers are particularly affected by noise inside aircraft cabins during take-off and landing.

The alternative approach adopted by the researchers involves introduction of an EVA or EPDM elastomeric interlayer between the facing sheets, resulting in a sandwich structure which combines high stiffness and improved acoustic properties. Böttcher suggested there is potential to use carbon fibre reinforced plastic face sheets.

In the research project, 14 variations on present structures were prepared in a vacuum-bag prepreg autoclaving process at 1.1bar with 0.5bar vacuum build-up pressure. In this process, the elastomer interlayer was simultaneously cured along with the face sheet layers at 160°C for 30 minutes, followed by cooling at 2.2°C/min.

Samples measuring 200 x 180mm were cut from these structures, and then clamped and submitted to excitation by sound frequencies in a range of 1-5,000Hz. The maximum amplitudes and resonance frequencies of "structure-borne" noise were detected with a Polytec laser Doppler vibrometer sensor system.

Tests against conventional 9.8mm thick reference samples showed the elastomer/prepreg sandwich composite gave a 70% peel strength improvement (the separation force improved from 0.6N/mm to around 1.0N/mm). There was also a 39% improvement in impact penetration resistance (energy absorption increased to well above 1.5 J/mm).

However, performance against a bending force was 25% worse (down from 250N to slightly under 200N), and Böttcher said further optimisation is needed.

Gummiwerk Kraiburg supplied the 0.5mm thick EPDM and EVA foils used in the IKV trials and Preuβ said that there was little difference in mechanical loss factor (tan δ) at lower frequencies. But the loss with EVA became more significant as the frequency increased (at 5,000Hz, the result was tan δ 0.3 for EVA, and slightly above 0.1 for EPDM).

Due to autoclaving and vacuum build-up pressure differences, the honeycomb initially penetrated the elastomer interlayer, reducing its acoustic damping effect to some extent. But this was solved when the researchers introduced a 33g/m² dry non-woven GF fleece from Johns Manville as a decoupling layer between the honeycomb and the elastomer.

Current sandwich structures with honeycomb core and GF prepreg facings have an overall area weight of 1,163 g/m², the IKV researchers stated. This is made up of the honeycomb with 273 g/m², the interior face sheet 490 g/m2 and the exterior sheet 400 g/m².

In the new structure, the added weight is 515 g/m² for an EPDM interlayer or 780 g/m² for an EVA interlayer. Together with a 33 g/m² decoupling layer, this results in higher overall area weights of 1,795 g/m² for the EPDM version and 2,017 g/m² for EVA.

But the study did not allow for the area weight of additionally damped parts in current sandwich structures. If eliminated this may compensate for the IKV sandwich structure.

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