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The true toughness of human cortical bone measured with realistically short cracks

Abstract

Bone is more difficult to break than to split. Although this is well known, and many studies exist on the behaviour of long cracks in bone, there is a need for data on the orientation-dependent crack-growth resistance behaviour of human cortical bone that accurately assesses its toughness at appropriate size scales. Here, we use in situ mechanical testing to examine how physiologically pertinent short (<600 μm) cracks propagate in both the transverse and longitudinal orientations in cortical bone, using both crack-deflection/twist mechanics and nonlinear-elastic fracture mechanics to determine crack-resistance curves. We find that after only 500 μm of cracking, the driving force for crack propagation was more than five times higher in the transverse (breaking) direction than in the longitudinal (splitting) direction owing to major crack deflections/twists, principally at cement sheaths. Indeed, our results show that the true transverse toughness of cortical bone is far higher than previously reported. However, the toughness in the longitudinal orientation, where cracks tend to follow the cement lines, is quite low at these small crack sizes; it is only when cracks become several millimetres in length that bridging mechanisms can fully develop leading to the (larger-crack) toughnesses generally quoted for bone.

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Figure 1: Crack-resistance curves (R-curves) showing the orientation and crack-size-dependent fracture resistance for human cortical bone measured at different strain rates and hydration levels.
Figure 2: Crack profiles, schematic diagrams and fractography of the different extrinsic toughening mechanisms in the transverse and longitudinal orientations.
Figure 3: Synchrotron X-ray computed tomography images showing the dominant mechanisms of crack deflection and twisting in the transverse orientation.

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Acknowledgements

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the US Department of Energy under contract no. DE-AC02-05CH11231, and by the Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory (LBNL). Computed X-ray tomography was carried out at LBNL’s Advanced Light Source, which is supported under the same contract. The authors thank H. Barth for help in preparing the X-ray computed tomographs.

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R.O.R. had full access to the experimental data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Data acquisition was carried out by K.J.K. Study design, interpretation and analysis of data and preparation of the manuscript were performed jointly by K.J.K., J.W.A. and R.O.R.

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Correspondence to R. O. Ritchie.

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Koester, K., Ager, J. & Ritchie, R. The true toughness of human cortical bone measured with realistically short cracks. Nature Mater 7, 672–677 (2008). https://doi.org/10.1038/nmat2221

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