Multiscale mechanical characterization of biomimetic physically associating gels
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Peter L. Drzal PPG Industries, Inc., Allison Park, PA 15101
Tusit Weerasooriya, Paul Moy, and Mark R. VanLandinghamb) U.S. Army Research Laboratory, Weapons & Materials Research Directorate, Aberdeen Proving Ground, Maryland, 21005 (Received 6 January 2006; accepted 28 April 2006)
The mechanical response of living tissue is important to understanding the injury-risk associated with impact events. Often, ballistic gelatin or synthetic materials are developed to serve as tissue surrogates in mechanical testing. Unfortunately, current materials are not optimal and present several experimental challenges. Bulk measurement techniques, such as compression and shear testing geometries, do not fully represent the stress states and rate of loading experienced in an actual impact event. Indentation testing induces deviatoric stress states as well as strain rates not typically available to bulk measurement equipment. In this work, a ballistic gelatin and two styrene-isoprene triblock copolymer gels are tested and compared using both macroscale and microscale measurements. A methodology is presented to conduct instrumented indentation experiments on materials with a modulus far below 1 MPa. The synthetic triblock copolymer gels were much easier to test than the ballistic gelatin. Compared to ballistic gelatin, both copolymer gels were found to have a greater degree of thermal stability. All of the materials exhibit strain-rate dependence, although the magnitude of dependence was a function of the loading rate and testing method.
I. INTRODUCTION
Blunt trauma may have significant detrimental effects on thoracic organs, and understanding these effects is critical for the design of protective equipment used in automotive, body armor, and sports applications, among others. Improved understanding of energy dissipation and damage assessment is needed to model interactions between human soft tissue, personnel protective equipment, and the impact source. Experiments using cadavers, instrumented dummies, and synthetic biomimetic materials measure and model effects of extreme pressure and velocity on soft tissue. As a result, standard tests and surrogate materials have been developed to evaluate and rank protective equipment.1 a)
Present address: National Institute of Standards and Technology, Building and Fire Research Laboratory, 100 Bureau Drive, Gaithersburg, MD 20899. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0254 2084 J. Mater. Res., Vol. 21, No. 8, Aug 2006 http://journals.cambridge.org Downloaded: 17 Mar 2015
Recent work to better understand how impact forces are transmitted through soft tissue have led to the fabrication of instrumented surrogate torsos composed of modified ballistic gelatin (BG)2 or modified silicones.3 To an extent, these testing platforms mimic the mechanical response of soft tissue and are often instrumented to gather pressure and velocity data during impact. This data is further utilized to validate finite element models, which help to desc
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