[论文解读] Tensile material properties of human rib cortical bone under quasi-static and dynamic failure loading and influence of the bone microstucture on failure characteristics

Damien Subit, Eduardo del Pozo de Dios+3 more|arXiv (Cornell University)|Aug 1, 2011

Automotive and Human Injury Biomechanics被引用 8

一句话总结

本研究采用一种新型夹持技术，最大限度减少约束并确保纯拉伸载荷，对人肋骨皮质骨在准静态（0.07%/s）和动态（170%/s）加载条件下下的拉伸性能进行了表征。主要发现显示其表现出各向同性的弹性行为且无塑性变形，但断裂常发生在标距段之外，这是由于微观结构的非均质性所致，挑战了材料均匀性的假设，并凸显了在胸腔有限元模拟中引入微观结构信息模型的必要性。

ABSTRACT

Finite element models of the thorax are under development to assist vehicle safety researchers with the design of countermeasures such as advanced restrain systems. Computational models have become more refined with increasing geometrical complexity as element size decreases. These finite element models can now capture small geometrical features with an attempt to predict fracture. However, the bone material properties currently available, and in particular the rate sensitivity, have been mainly determined from compression tests or tests on long bones. There is a need for a new set of material properties for the human rib cortical bone. With this objective, a new clamping technique was developed to test small bone coupons under tensile loading. Ten coupons were harvested from the cortical shell of the sixth and seventh left ribs from three cadavers. The coupons were tested to fracture under quasi-static (target strain rate of 0.07 %/s) and dynamic loading (target strain rate of 170 %/s). Prior to testing, each coupon was imaged with a computed micro-tomograph to document the bone microstructure. An optical method was used to determine the strain field in the coupon for the quasi-static tests. The rib bone coupons were found to be elastic, with brittle fracture. No plastic behavior was observed in this test series. The bone coupons were assumed isotropic, homogeneous and elastic linear, and the average Young's modulus for the quasi-static tests (13.5 GPa) and the failure stresses (quasi-static: 112 MPa, dynamic: 124.6 MPa) were in line with published data. Fracture however did not always occur in the gage area where the cross-sectional area was the smallest, which contradicted the assumption of isotropy and homogeneity. The comparison with material properties obtained for long bones suggests that the effective cross-section has an effect on the calculated material properties.

研究动机与目标

• 确定人肋骨皮质骨在准静态与动态加载条件下的拉伸材料性能。
• 开发并验证一种新型夹持技术，以最大限度减少约束并确保小尺寸骨试样在纯拉伸载荷下的受力。
• 研究骨微观结构（尤其是孔隙率和横截面变化）对失效特性及断裂位置的影响。
• 评估在肋骨皮质骨有限元模型中假设各向同性和均匀性的有效性。
• 为改进车辆安全与损伤预测用胸腔计算模型，提供更精确的材料性能数据。

提出的方法

• 从三具人尸的第6和第7根左侧肋骨中获取10个皮质骨试样。
• 采用定制设计的夹持系统，施加无预载荷，确保单轴拉伸，从而减少夹持引起的应力集中。
• 利用万能试验机对试样进行准静态（0.07%/s）和动态（170%/s）拉伸试验直至断裂。
• 在试验前利用显微计算机断层扫描（micro-CT）对每个试样的内部微观结构进行成像。
• 采用光学数字图像相关法（DIC）测量准静态加载过程中全场应变分布。
• 利用micro-CT分析试样长度方向上的有效横截面面积，以评估几何与微观结构效应对表观材料性能的影响。

实验结果

研究问题

• RQ1 人肋骨皮质骨在准静态与动态加载条件下的拉伸材料性能有何差异？
• RQ2 微观结构非均质性（如孔隙率和变化的皮质厚度）在多大程度上影响断裂起始与断裂位置？
• RQ3 在肋骨皮质骨有限元模型中假设各向同性和均匀性，是否能准确反映其机械行为？
• RQ4 基于micro-CT获得的有效横截面面积与实测材料性能及断裂行为之间存在何种相关性？
• RQ5 能否将孔隙等微观结构特征与导致断裂的局部应力集中联系起来？

主要发现

• 肋骨皮质骨在两种加载速率下均表现出纯弹性、脆性行为，无明显塑性变形。
• 准静态加载下的平均弹性模量为13.5 GPa，动态加载下也为13.5 GPa，与已有文献数据一致。
• 准静态加载下的断裂应力为112 MPa，动态加载下为124.6 MPa，表明强度具有速率相关性。
• 断裂并未始终发生在标距段（最小横截面），与均匀、各向同性行为的假设相矛盾。
• micro-CT成像显示皮质壳层存在显著孔隙率与结构非均质性，孔隙与厚度变化影响载荷传递。
• 预测（均匀）与实际观测（非均质）断裂位置之间的差异表明，微观结构对失效机制的影响远超简单几何效应。