Date of Award

8-2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Thompson, Lonny L

Committee Member

Li , Gang

Committee Member

Daqaq , Mohammed

Abstract

Honeycomb cellular materials are widely used in engineering applications due to their high strength to weight ratio and controllable effective mechanical properties. The effective properties are controlled by varying the geometry of the repetitive unit cells of honeycomb structure. Sandwich panels made of honeycomb cores are beneficial in many applications including vibration isolation and sound transmission reduction. Sandwich panels with standard honeycomb core configurations have previously been studied with regards to sound transmission behavior. It has been established that the auxetic honeycomb cores, having negative in-plane Poisson's ratio, exhibit higher sound transmission loss as compared to regular honeycomb cores. In this study, the vibration and sound transmission response of novel auxetic chiral honeycomb structures (both hexa-chiral and anti-tetra chiral), have been investigated in detail using finite element analysis with two-dimensional plane elasticity elements.
Chiral honeycomb structures are made up of a linear tessellation of periodic unit cell, which consists of circular nodes of radius ` r ' connected to each other by tangent ligaments of length ` L '. The distance between two adjacent circular nodes is ` R '. These geometric parameters are tailored to obtain the chiral structure with desired effective mechanical properties of in-plane Poisson's ratio, Young's modulus and shear modulus. Results show that, for both the hexa-chiral and anti-tetra-chiral configurations with same thickness, structures with smaller node radius `r' have higher in-plane negative Poisson's ratio, effective Young's modulus, and shear modulus. The Poisson's ratio of anti-tetra-chiral structure with small node radius and thickness is found to approach the limit of -1.
A steady state dynamic response of the chiral honeycomb sandwich panel subjected to uniform pressure load on the bottom face-sheet is also investigated over a frequency range of 1 Hz to 2000 Hz. It is observed that, by changing the node radius of the chiral structures, the frequency range for the global sandwich structure bending resonances and local intra-cell core resonances can be shifted. Within the bandwidth controlled by the intra-cell core resonances we observe higher surface velocity vibration amplitude and decrease in sound transmission loss. For the structure with bigger node radius, the bending resonances and intra-cell resonance are shifted to lower frequencies as compared to the structure with smaller node radius.
Finally, the sound transmission loss behavior of sandwich panels made of chiral honeycomb cores is investigated with plane pressure wave incident at normal as well as variable incidence angles. The results suggest that, in case of both the hexa-chiral and anti-tetra-chiral sandwich panels, the core structure with smallest node radius exhibits higher sound transmission loss as compared to the core structure with bigger node radius. Among all the different chiral honeycomb structures investigated in this study, the anti-tetra-chiral structure with smallest node radius exhibits the highest sound transmission loss. It is interesting to observe that this is also the structure with highest value of negative in-plane Poisson's ratio.

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