Indentation variability of natural nanocomposite materials
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Ching-Chang Ko Orthodontics, School of Dentistry, University of North Carolina, Chapel Hill, North Carolina 27599-7450 (Received 3 August 2007; accepted 6 November 2007)
Small-scale depth-sensing indentation (nanoindentation) is a popular technique for measuring the mechanical properties of a wide range of materials. Contact mechanics solutions used in data analysis are based on the indentation of a homogeneous half-space, but the experiments are frequently conducted on mineralized biological tissues—biocomposite materials with nanometer-scale features—such as bone and dentin. The current study examines the experimental indentation response of bone across orders of magnitude in contact dimension length-scale, from nanometers to micrometers. Scaling arguments are used to establish the need for nanoscale simulations of mineralized tissue indentation. A finite element model of an inhomogeneous contact problem is developed and used to interpret experimental indentation data on bone and dentin. Both experimental data and modeling results demonstrate a convergence in apparent elastic modulus at increasing contact length-scales. Models results are used to estimate a feature size associated with inhomogeneity of the indentation response; for experiments conducted here the characteristic feature size is found to be substantially larger for bone than for dentin, and in both cases larger than for individual nanometer-scale mineral platelets.
I. INTRODUCTION
Recent advances in instrumentation have led to the development of nanoindentation testing techniques, in which the mechanical response of a material can be probed at length-scales in the nanometer to micrometer range. The popularization in the use of nanoindentation testing techniques was necessarily preceded by the development of simple algorithms for the elastic1 and elastic– plastic2–4 contact mechanics problem for pyramidal (Berkovich three-sided or Vickers four-sided) indenters. The contact mechanics solutions utilized in routine nanoindentation experiments are based on the indentation of an elastic or elastic–plastic half-space, typically assumed to be homogeneous and elastically isotropic. Because of the capability for localized testing, nanoindentation testing is particularly well-suited to the mechanical analysis of biological materials, in which the a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this journal 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/ publications/JMR/policy.html DOI: 10.1557/JMR.2008.0103
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J. Mater. Res., Vol. 23, No. 3, Mar 2008
mechanical properties can vary substantially from point to point due to variations in local composition and structure.5–7 These property variations are frequently associated with the length-scales of extracellular matrix components (i.e., length scales of tens of nanometers to micrometers) and of cell activity (i.e., length-scales of micrometers to t
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