Quasi-static and Dynamic Nanoindentation Testing of Lamellar and Inter-lamellar Trabecular Bone
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Quasi-static and dynamic nanoindentation testing of lamellar and inter-lamellar trabecular bone Thorsten Staedler*, Eve Donnelly**, Marjolein C. H. van der Meulen**, and Shefford P. Baker* * Department of Materials Science and Engineering, Cornell University, Ithaca, NY ** Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY ABSTRACT Bone is a complex natural composite with a lamellar structure on the micro scale. In this study, scanning nanoindentation was used to assess the mechanical properties of individual lamellar and inter-lamellar components of trabecular bone under static and dynamic loading conditions. The combination of careful sample preparation, good topographical characterization of the sample surface, and a scanning nanoindenter provided insight into the mechanical properties of the individual lamellar structures. The results of quasi-static, as well as dynamic nanoindentation tests of bone are reported and discussed. A surface roughness of only 18-20 nm RMS was achieved. Nonetheless, indentation depths in the range of 100 nm and above were required in order to minimize the effect of topography on the measurement. This requirement conflicts with the need to keep the probed volume small enough to sample only one component at a time. A simple volume-averaging model was developed and used as a tool to determine when individual lamellar bone structures were being probed and to help determine the properties of the lamellar and inter-lamellar materials. INTRODUCTION Bone is a complex natural composite material. The predominant form of bone in the normal skeleton after birth [1] is lamellar bone. Lamellar bone is characterized by its plywood-like microstructure, which contains layers of aligned, mineralized collagen fibrils whose orientations change between successive lamellae. On the micro-scale, many investigators have observed a peak and valley structure associated with individual lamellae in polished cross sections of lamellar bone [2, 3, and 4]. Although some previous nanomechanical studies of lamellar bone have adopted the terminology of thick and thin lamellae [3 and 5] to describe this topography, the microstructure of the specimens in the current study is best described by the terminology of Boyde and Hobdell [6], in which the peaks and valleys are termed lamellar and inter-lamellar bone, respectively. The details of structural differences between these two bone components seem to depend on species, anatomic location, and age, and are still under discussion [7, 8, and 9], however, it is obvious from all these studies that bone can be viewed as a micro-scale composite material, for a schematic see Figure 1 (a). To achieve a true understanding of the mechanical properties of bone, these properties must be studied on the length scale of the individual microstructural components, i.e., lamellae. Scanning nanoindentation offers this opportunity, as it allows one to probe small volumes with excellent indenter tip positioning accuracy relative to the size of struct
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