Viscoelastic and poroelastic mechanical characterization of hydrated gels
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Measurement of the mechanical behavior of hydrated gels is challenging due to a relatively small elastic modulus and dominant time-dependence compared with traditional engineering materials. Here polyacrylamide gel materials are examined using different techniques (indentation, unconfined compression, dynamic mechanical analysis) at different length-scales and considering both viscoelastic and poroelastic mechanical frameworks. Elastic modulus values were similar for nanoindentation and microindentation, but both indentation techniques overestimated elastic modulus values compared to homogeneous loading techniques. Hydraulic and intrinsic permeability values from microindentation tests, deconvoluted using a poroelastic finite element model, were consistent with literature values for gels of the same composition. Although elastic modulus values were comparable for viscoelastic and poroelastic analyses, timedependent behavior was length-scale dependent, supporting the use of a poroelastic, instead of a viscoelastic, framework for future studies of gel mechanical behavior under indentation.
I. INTRODUCTION
Nanoindentation has become a common technique for characterizing the mechanical properties of a wide range of materials, including materials with time-dependent mechanical behavior such as polymers1 and biological materials.2 Until recently, the majority of nanoindentation studies concerning biological tissues were on stiff, hard mineralized tissues, and most samples were dehydrated for testing. An increasing interest in compliant, soft, and hydrated materials has driven the development of new techniques for expanding nanoindentation to new experimentally challenging materials sets. Both hydrated soft tissues and hydrated gel materials have been the subjects of recent nanoindentation investigations. Time-dependent mechanical behavior is a critical aspect of the mechanical response in many compliant and hydrated materials. Early nanoindentation studies emphasized time-independent, elastic-plastic material responses but many recent studies have incorporated viscoelastic3–7 or viscous-elastic-plastic8,9 material models. Linearly viscoelastic material characterization is now well developed for nanoindentation studies on a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs.org/jmr_policy DOI: 10.1557/JMR.2009.0129 J. Mater. Res., Vol. 24, No. 3, Mar 2009
glassy and rubbery polymers, and such analysis has been used (with caveats) for characterizing time-dependent nanoindentation creep responses in hydrated bone10,11 and indentation load-relaxation responses in soft biological tissues.12,13 A similar analysis based on linear viscoelasticity has also been used recently for nanoindentation load-relaxation in hydrated gel materials.14 Several open questions remain in the viscoelastic literature that are relevant
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