Research Papers

Toward Functionally Graded Cellular Microstructures

[+] Author and Article Information
Carmen Torres-Sanchez

DMEM, University of Strathclyde, James Weir Building, Glasgow G1 1XJ, UKcarmen.torres@strath.ac.uk

Jonathan R. Corney

DMEM, University of Strathclyde, James Weir Building, Glasgow G1 1XJ, UKjonathan.corney@strath.ac.uk

J. Mech. Des 131(9), 091011 (Aug 19, 2009) (7 pages) doi:10.1115/1.3158985 History: Received November 13, 2008; Revised April 20, 2009; Published August 19, 2009

The design of multifunctional materials offers great potential for numerous applications in areas ranging from biomaterial science to structural engineering. Functionally graded microstructures (e.g., polymeric foams) are those whose porosity (i.e., ratio of the void to the solid volume of a material) is engineered to meet specific requirements such as a superior mechanical, thermal, and acoustic behavior. The controlled distribution of pores within the matrix, as well as their size, wall thickness, and interconnectivity are directly linked to the porous materials properties. There are emerging design and analysis methods of cellular materials but their physical use is restricted by current manufacturing technologies. Although a huge variety of foams can be manufactured with homogeneous porosity, for heterogeneous foams there are no generic processes for controlling the distribution of porosity throughout the resulting matrix. This paper describes work to develop an innovative and flexible process for manufacturing engineered cellular structures. Ultrasound was applied during specific foaming stages of a polymeric (polyurethane) melt, and this affected both the cellular architecture and distribution of the pore size, resulting in a controlled distribution that can be designed for specific purposes, once the polymeric foam solidified. The experimental results demonstrate that porosity (i.e., volume fraction) varies in direct proportion to the acoustic pressure magnitude of the ultrasonic signal.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 4

Contour lines connecting points of equal density of the material in the cross section of the sonicated foam in Fig. 1

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Figure 5

(a) MATLAB ™ interface, (b) isoporosity contour lines, and ((c) and (d)) corresponding areas in image analysis and contour lines

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Figure 6

Procedure for analysis of foam irradiated at 25 kHz and 8.85 cm distance from the sonotrode while immersed in the water bath: (a) Isoporosity contours from topoporosity image analysis program applied to the cross-section of a foam sample; (b) vertical plane extracted from the COMSOL ™ simulation of the acoustic pressure distribution within the foam vessel immersed in the water bath; (c) comparison of experimental porosity values (inverse of density values) extracted along mid-line (AA') of ‘sonication plane’ aligned with sonotrode versus simulated sound pressure level distribution extracted along mid-line (BB') of the ‘sonication plane’ aligned with sonotrode for two acoustic impedances (Z=1.48 MRayl is water and Z=2.6 MRayl is bone) for the irradiated foam.

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Figure 7

Comparison of porosity and sound pressure distributions for foams irradiated at the following conditions: (a) 20 kHz and 18 kPa, (b) 25 kHz and 12 kPa, and (c) 30 kHz and 8.9 kPa

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Figure 8

Stages of acoustic cavitation exploited for the tailoring of polymeric foams

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Figure 1

Cross section of two foams sonicated at different positions in an ultrasonic standing wave (irradiating source was located on the left of these cross sections)

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Figure 2

Schematic of the experimental rig, lateral and plan views, showing variable positions of a single foam container

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Figure 3

Mapping of water bath, showing the attenuation and partial maxima at half-wavelengths on the sonication plane



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