Strain Rate Dependent Compressive Response of Open Cell Polyurethane Foam

Experimental Mechanics volume 59, pages1087–1103(2019)Cite this article

Abstract

Polymeric foams are used for impact protection due to their ability to absorb large amounts of strain energy. In this work, the compressive response of an open cell polyurethane foam currently used as liner in the advanced combat helmet is examined across strain rates. A traditional load frame is used to investigate the quasi-static behavior, and two different modifications of a conventional Kolsky (split-Hopkinson) bar configuration are used to probe the dynamic response. A unique, independent method not relying on strain gage signals is presented that leverages high-speed full-field imaging to track the velocity on each side of the sample-bar interface and used to extract the dynamic stress-strain response; the results are compared against traditional strain gage measurements. X-ray tomography is used to examine the global morphological characteristics of the foam. The foam is found to be strongly rate dependent, where the characteristic properties vary logarithmically with strain rate. An analytical expression is presented to describe the rate dependency that collapses all stress-strain curves on a master curve. Full-field kinematic data from digital image correlation taken during loading is used to extract a nonlinear Poisson’s ratio as a function of strain, which is found to be strain rate insensitive. A tangent Poisson function is used to explore the foam’s auxetic behavior. These findings provide insight on physically-based constitutive modeling of foams, crucial to predictive brain injury simulations, as well as motivate the need to probe local heterogenous behavior across strain rates moving forward.

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Acknowledgements

This research is performed under grant N00014-18-1-2494 from the Office of Naval Research. The authors would like to thank Dr. Antonios Zavaliangos and his research group of the Department of Material Science and Engineering at Drexel University for allowing us to use the x-ray microtomograph. Special thanks for the fruitful discussions with the research teams of Dr. Christian Franck of the University of Wisconsin-Madison, Dr. Haneesh Kesari, Dr. Diane Hoffmann-Kim, and Dr. David Henann of Brown University, Mr. Ron Szalkowski of TeamWendy, Dr. Chad Hovey of Sandia National Laboratories. Additional thanks to Dr. Bo Song of Sandia National Laboratories for his discussion on properly utilizing the Kolsky method and Dr. Fabrice Pierron for his discussion on Poisson’s ratio quantification. Lastly, thank you to the Dynamic Multifunctional Materials Laboratory members at Drexel University for the direct and indirect support on this project.

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  1. Mechanical Engineering and Mechanics Department, Drexel University, 3141 Chestnut Street, Philadelphia, PA, 19104, USA

    S. Koumlis & L. Lamberson

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  1. S. Koumlis
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Correspondence to S. Koumlis.

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The authors gratefully acknowledge the support from the Office of Naval Research (Dr. Timothy Bentley) under grant N00014-18- 1-2494.

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Koumlis, S., Lamberson, L. Strain Rate Dependent Compressive Response of Open Cell Polyurethane Foam. Exp Mech 59, 1087–1103 (2019). https://doi.org/10.1007/s11340-019-00521-3

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Keywords

  • Polymeric foams
  • Compression
  • Global cell morphology
  • Rate sensitivity
  • Strain energy
  • Brain injury protective equipment