Koester K J, Ager J W, Ritchie R O
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Nat Mater. 2008 Aug;7(8):672-7. doi: 10.1038/nmat2221. Epub 2008 Jun 29.
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 microm) 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 microm 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.
骨头断裂比劈裂更难。尽管这是众所周知的，并且有许多关于骨头中长裂纹行为的研究，但仍需要有关人类皮质骨方向依赖性裂纹扩展阻力行为的数据，以便在适当的尺寸尺度上准确评估其韧性。在这里，我们使用原位力学测试来研究生理相关的短（<600微米）裂纹如何在皮质骨的横向和纵向方向上扩展，使用裂纹偏转/扭转力学和非线性弹性断裂力学来确定裂纹阻力曲线。我们发现，在仅500微米的裂纹扩展后，由于主要在水泥鞘处的主要裂纹偏转/扭转，裂纹扩展的驱动力在横向（断裂）方向上比纵向（劈裂）方向上高五倍以上。实际上，我们的结果表明，皮质骨的真正横向韧性远高于先前报道的。然而，在纵向方向上，裂纹倾向于沿着水泥线扩展，在这些小裂纹尺寸下韧性相当低；只有当裂纹长度达到几毫米时，桥接机制才能充分发展，从而导致通常引用的（较大裂纹）骨头韧性。
