Center for Composite Materials - University of Delaware

Research Summary

Natural Cork Core Sandwich Structures: Noise Mitigation and Energy Absorption

Authors: Joseph Walsh, Dr. Hyung Ick Kim, Dr. Jonghwan Suhr

MOTIVATION

• Sandwich composites are popular in aerospace
• However, these strong lightweight sandwich structures propagate unwanted vibrations which shorten the structure’s lifespan and produce noise as a result.

OBJECTIVES

• Compare sandwich structures with common core materials to cork and expanded cork agglomerates
• Examine acoustic and damping properties
• Measure bending stiffness of structures

TERMINOLOGY

• Wave Number: spatial frequency of a vibratory wave
• Coincidence Frequency: point where structure passes into supersonic range and radiates noise efficiently
• Structural Loss Factor: Damping measure. Higher loss factor corresponds to increased damping

SANDWICH COMPOSITES

• Comprised of Two Face sheets and a core
• Similar to an I-beam in theory
• Generally boast a high strength/stiffness-to-weight ratio
• Commonly used in aerospace applications such as: airplane wings, fairings, cabin floors, etc.
• Poor acoustic and damping behavior

MATERIALS

• Carbon fiber face sheets used for their high strength
• Synthetic cellular foams often used as core in aerospace structures
• Cork and expanded cork agglomerates are natural renewable materials with a cellular

WAVENUMBER

• Rohacell core coincidence frequency close to 2 kHz
• Cork and preliminary data from expanded cork both diverge from the noise radiating region
• Cork sandwich composites are much quieter over the frequency range

DAMPING

• Better damping will increase lifespan of structure
• Cork and preliminary data from expanded cork both show higher loss factors up to 4 kHz
• Implies improved damping over that frequency range

BENDING

• Cork agglomerate has similar density to synthetic foams
• Decrease in shear modulus but equivalent bending stiffness
• No sacrifice in stiffness-to-weight ratio

CONCLUSIONS

• Cork agglomerate has improved acoustic and damping properties over conventional foam
• More testing is needed to confirm the same result for expanded cork

Acknowledgements

This work is supported by The University of Delaware. Also materials have been provided courtesy of M.C. Gill and Amorim Isolamentos, S.A. (Portugal)

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