In this study, the dynamic cracking processes in porcine cortical bone were visualized in real-time using the high-speed synchrotron X-ray phase-contrast imaging (PCI) technique in three osteon orientations: in-plane transverse, out-of-plane transverse and in-plane longitudinal. The dynamic flexural loading applied on the pre-notched bone specimens was introduced by a modified Kolsky compression bar. High-speed X-ray images of the entire loading events were documented with a high-speed camera. Three-dimensional X-ray micro-computed tomography was conducted to examine the intact microstructures and obtain the basic material properties of the bone material used for mechanical characterizations. The onset location, where crack initiated, and the subsequent direction, along which the incipient crack propagated, were measured quantitatively using the high-speed X-ray images and the latter was found dependent on the osteon direction significantly. The crack propagation velocities were dependent on crack extension over the entire crack path significantly for all the three directions while the initial velocity for in-plane longitudinal direction was lower than the other two directions. Straight-through crack paths were observed for in-plane longitudinal specimens while the cracks were deflected and twisted in the in-plane transverse direction. For out-of-plane transverse direction, the cracks follow paths with tortuosity fall in between the other two directions, showing a mixed mode of fractures of the former two extreme cases. The toughening mechanisms, visualized by the high-speed X-ray images, and the corresponding fracture toughness, evaluated in terms of fracture initiation toughness and crack growth resistance curve (R-curve), were also found significantly different among the three osteon directions, suggesting an overall transition from brittle to ductile-like fracture behaviors at the dynamic displacement rate (5.4 m/s) as the osteon orientation varies from in-plane longitudinal to out-of-plane transverse, and to in-plane transverse eventually.

在这项研究中，使用高速同步加速器X射线相衬成像（PCI）技术在三个骨骼方向上实时显示了猪皮质骨中的动态开裂过程：平面内横向，平面外横向和平面内纵向。通过改良的Kolsky压缩杆引入施加在预先开槽的骨骼样本上的动态弯曲载荷。整个装载过程的高速X射线图像通过高速摄像机记录下来。进行了三维X射线计算机断层扫描，以检查完整的微观结构并获得用于机械表征的骨材料的基本材料性能。裂纹产生的起始位置和裂纹扩展的后续方向，使用高速X射线图像进行定量测量，发现X射线显着取决于骨骼方向。在所有三个方向上，裂纹扩展速度都显着取决于整个裂纹路径上的裂纹扩展，而平面内纵向的初始速度低于其他两个方向。观察到平面内纵向试样的直通裂纹路径，而裂纹在平面内横向方向上发生偏转和扭曲。对于平面外横向，裂纹沿着曲折路径落在其他两个方向之间，这表明前两种极端情况的混合断裂模式。通过高速X射线图像显示的增韧机理以及相应的断裂韧性，R曲线）也被发现在三个骨骼方向之间存在显着差异，这表明随着骨骼方向从平面内纵向变化到垂直方向，在动态位移速率（5.4 m / s）下从脆性到韧性样断裂行为的整体过渡。平面外横向，最终到平面内横向。