Failure of bone at the sub-lamellar level using in situ AFM-SEM investigations
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Failure of bone at the sub-lamellar level using in situ AFM-SEM investigations Ines Jimenez-Palomar and Asa H. Barber Department of Materials, School of Engineering and Materials Science Queen Mary University of London Mile End Road, London E1 4NS, United Kingdom. ABSTRACT In this paper we examine the mechanical properties of individual lamellae from bone material using novel atomic force microscopy (AFM)-scanning electron microscopy (SEM) techniques. Individual lamellar beams were selected from bone using focussed ion beam (FIB) microscopy and mechanically deformed with the AFM while observing failure modes using SEM. Both the elastic and fracture behavior of the bone lamellae were determined using these techniques. INTRODUCTION Bone is a fibrous biological nanocomposite material, which is optimized to avoid catastrophic failure [1, 2]. The fracture behavior of bone is expected to be controlled by the various structural features present across the many existing hierarchical length scales [3]. However, micron sized lamellae in bone present the simplest composite unit in bone consisting of mineralized collagen fibrils within a protein matrix, with some work suggesting that this length scale dominates the fracture of whole bone [2]. The fracture process of bone can be described by considering the range of energy absorbing mechanisms such as the plasticity of the collagen phase, crack deflection along bone cement lines, diffuse micro-cracking and the bridging of cracks by ductile phases [4]. Peterlik et al. examined osteonal lamellar bone by controlled crack extension experiments of test specimens with millimeter dimensions and concluded that the micron sized lamellae dominated the fracture of whole bone [2]. The determination of the elastic properties of bone lamellae has been investigated using nanoindentation [5-7] and provides little information on fracture behavior. This paper therefore extends the understanding of bone mechanics by evaluating the deformation behavior of bone to failure at the micron length scales that correspond to the lamellar to sub-lamellar level of bone. EXPERIMENTS Femurs of 8-month-old sprague dawley rats were used as a source of bone material. Sublamellar bone micro-cantilever beams from these rat femurs were isolated using a dual beam system. The dual beam system is composed of a scanning electron microscope (SEM) and focused ion beam (FIB) working simultaneously. The FIB allows bone material to be milled out in order to pattern fine cantilever beams of bone for subsequent mechanical testing [8]. Micro cantilevers of bone with dimensions 10 x 2 x 2 μm were produced using FIB as described in previous work [8]. These dimensions ensured that the shear stress contribution to the bending stress was minimal (
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