In situ X-ray microtomography of the compression behaviour of eTPU bead foams with a unique graded structure
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In situ X-ray microtomography of the compression behaviour of eTPU bead foams with a unique graded structure Arun Sundar S. Singaravelu1, Jason J. Williams1, Jasmin Ruppert2, Mark Henderson3, Chris Holmes2, and Nikhilesh Chawla1,4,* 1
Center for 4D Materials Science, Arizona State University, Tempe, AZ 85287-6106, USA Future Team, Adidas AG, 91074 Herzogenaurach, Germany 3 Future Team, Adidas AG, Portland, OR 97217, USA 4 School of Materials Engineering, Neil Armstrong Hall of Engineering, Purdue University, 701 West Stadium Avenue, West Lafayette, IN 47907-2045, USA 2
Received: 2 September 2020
ABSTRACT
Accepted: 24 November 2020
In situ X-ray microtomography was used to characterize the changes in internal structure of an expanded thermoplastic polyurethane (eTPU) bead foam used in footwear midsole during compression. Quantitative data on cell size and volume changes based on conventional segmentation were computed. Image correlation on the 4D datasets, i.e. a 3D tomogram collected at various points during deformation, was performed to map the strain fields to aid in analysis of observed deformation. Morphological and structural characterization revealed the gradient porosity and ligament thickness and its influence on the global mechanical behaviour of individual bead foam. The correlation of changes in void size and shape, identified areas of potentially weak regions. Strain maps derived by using voids as markers were adequate to capture the instabilities in local cell collapse. The smaller cells at the surface densified more readily providing more resistance to deformation. In addition, the thicker ligaments in the centre of the bead also aided resistance to deformation. Finally, our results showed the initiation of failure in cell structure was influenced by the heterogeneities around the immediate neighbours in a cluster of voids.
Springer Science+Business
Media, LLC, part of Springer Nature 2020
Handling Editor: M. Grant Norton.
Address correspondence to E-mail: [email protected]
https://doi.org/10.1007/s10853-020-05621-3
J Mater Sci
Introduction Foams are ubiquitous in our modern world, ranging from novel sandwich foam cores in aerospace to simple packaging materials [1–3]. In recent times, bead foams have received a lot of attention due to their ease in filling intricate shapes compared to conventional foams. Several papers have focused on understanding the morphological development, processing techniques, and on techniques for developing polymers suitable for bead foaming [4]. In particular, thermoplastic polyurethane [5] has been used to make beads that are used extensively as the midsole of adidas ‘Energy BOOST’ running shoes. The function of the midsole is to absorb the energy generated as the foot lands while keeping the peak force below a certain threshold, minimizing the chances of injury [6]. These characteristics come from the peculiar stress–strain behaviour displayed by these materials which can be altered by changing the deformability of the cellular structure. The mechanical pro
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