Taylor D, O'Reilly P, Vallet L, Lee T C
Department of Mechanical & Manufacturing Engineering, Trinity College, Dublin, Ireland.
J Biomech. 2003 Aug;36(8):1103-9. doi: 10.1016/s0021-9290(03)00104-0.
We have conducted a series of fatigue tests on samples of bovine compact bone loaded in cyclic torsion. The fatigue strength (i.e. the range of stress needed to cause failure in a given number of cycles) was found to be lower than the fatigue strength of the same material in compression by more than a factor of two. We also tested intact chicken metatarsals and found a similar reduction in strength compared to compression testing of chicken tibiae. These results were predicted using a theoretical model in which fatigue failure was assumed to be dependent on the growth of microcracks, oriented approximately parallel to the bone's longitudinal axis but having misorientation angles of up to 30 degrees. An effective stress range was derived which is a function of the normal and shear stresses, and thus of the Mode I and Mode II stress intensities experienced by the crack. These results may have important consequences for the understanding of fatigue in bone in vivo; relatively small amounts of longitudinal shear stress, which are often ignored in analysis, may contribute significantly to fatigue failures. This may shed light on the phenomenon of stress fractures and on the need for repair and adaptation in living bone.
我们对牛密质骨样本进行了一系列循环扭转加载的疲劳试验。结果发现，其疲劳强度（即在给定循环次数下导致失效所需的应力范围）比相同材料在压缩时的疲劳强度低两倍多。我们还测试了完整的鸡跖骨，与鸡胫骨的压缩测试相比，发现强度有类似程度的降低。这些结果是使用一个理论模型预测得出的，该模型假定疲劳失效取决于微裂纹的扩展，这些微裂纹大致平行于骨骼的纵轴方向，但取向角可达30度。由此推导出一个有效应力范围，它是法向应力和剪应力的函数，因此也是裂纹所经历的I型和II型应力强度的函数。这些结果可能对理解体内骨骼的疲劳具有重要意义；在分析中常常被忽略的相对少量的纵向剪应力，可能对疲劳失效有显著影响。这可能有助于阐明应力性骨折现象以及活体骨修复和适应性的必要性。
