Mechanistic Aspects of Fracture of Human Cortical Bone
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Mechanistic Aspects of Fracture of Human Cortical Bone Ravi K. Nalla1, Jamie J. Kruzic1, John H. Kinney2 and R. O. Ritchie1 Materials Sciences Division, Lawrence Berkeley National Laboratory, and Materials Science & Engineering Department, University of California, Berkeley, CA 94720 2 Lawrence Livermore Nat. Laboratory, and University of California, San Francisco, CA 94143 1
ABSTRACT There has been growing interest of late in the fracture properties of human bone. As understanding such properties in the context of the hierarchical microstructure of bone is of obvious importance, this study addresses the evolution of the in vitro fracture toughness with crack extension (Resistance-curve behavior) in terms of the salient mechanisms involved. Fracture-mechanics based measurements were performed on compact-tension specimens hydrated in Hanks’ Balanced Salt Solution using cortical bone from mid-diaphyses of 34-41 year-old human humeri. Post-test observations of the crack path were made by optical microscopy and three-dimensional X-ray computed tomography. The fracture toughness was found to rise linearly with crack extension with a mean crack-initiation toughness of Ko ~ 2.0 MPa√m for crack growth in the proximal-distal direction. The increasing cracking resistance had its origins in several toughening mechanisms, most notably crack bridging by uncracked ligaments. Uncracked-ligament bridging, which was observed by tomography in the wake of the crack, was identified as the dominant toughening mechanism responsible for the observed Rcurve behavior through compliance-based experiments. The extent and nature of the bridging zone was examined quantitatively using multi-cutting compliance experiments in order to assess the bridging stress distribution. The results obtained in this study provide an improved understanding of the mechanisms associated with the failure of cortical bone, and as such are of importance from the perspective of developing a realistic framework for fracture risk assessment, and for determining how the increasing propensity for fracture with age can be prevented. INTRODUCTION The fracture resistance of human bone, and its deterioration with aging, represents an issue of marked clinical significance. While traditional thinking of the role of aging has concentrated on a loss in bone mineral density as a determining factor, there is increasing evidence that this alone cannot explain the therapeutic benefits of anti-resorptive agents in treating conditions such as osteoporosis [1]. Consequently, there is a necessity to understand how other factors control bone fracture and this has led to an increased focus on mechanical properties (e.g., elastic modulus, strength, and toughness) that might affect fracture propensity. Although there have been many studies on the fracture properties of bone, most have reported “single-value” fracture toughness behavior, using such parameters as the critical stress-intensity factor, Kc, or the critical strain-energy release rate, Gc (e.g., [2, 3]). While the use of
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