Department of Materials Science and Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA.
J Mech Behav Biomed Mater. 2011 Jul;4(5):744-55. doi: 10.1016/j.jmbbm.2011.02.013. Epub 2011 Mar 1.
Nature, to a greater extent than engineering, takes advantage of hierarchical structures. These allow for optimization at each structural level to achieve mechanical efficiency, meaning mechanical performance per unit mass. Palms and bamboos do this exceptionally well; both are fibre-reinforced cellular materials in which the fibres are aligned parallel to the stem or culm, respectively. The distribution of these fibres is, however, not uniform: there is a density and modulus gradient across the section. This property gradient increases the flexural rigidity of the plants per unit mass, mass being a measure of metabolic investment made into an organism's construction. An analytical model is presented with which a 'gradient shape factor' can be calculated that describes by how much a plant's bending efficiency is increased through gradient structures. Combining the 'gradient shape factor' with a 'microstructural shape factor' that captures the efficiency gained through the cellular nature of the fibre composite's matrix, and a 'macroscopical shape factor' with which the tubular shape of bamboo can be described, for example, it is possible to explore how much each of these three structural levels of the hierarchy contributes to the overall bending performance of the stem or culm. In analogy, the bending efficiency of the commonly used wood-based composite medium-density fibreboard can be analysed; its property gradient is due to its manufacture by hot pressing. A few other engineered materials exist that emulate property gradients; new manufacturing routes to prepare them are currently being explored. It appears worthwhile to pursue these further.
自然在更大程度上利用了层次结构，这使得在每个结构层次上都可以进行优化，以实现机械效率，即单位质量的机械性能。棕榈树和竹子在这方面表现得尤为出色；它们都是纤维增强的多孔材料，其中纤维分别与茎或秆平行排列。然而，这些纤维的分布并不均匀：在横截面上存在密度和模量梯度。这种性能梯度增加了植物每单位质量的弯曲刚度，质量是衡量生物体结构代谢投资的一个指标。本文提出了一个分析模型，可以计算出“梯度形状因子”，该因子描述了通过梯度结构可以使植物的弯曲效率提高多少。通过将“梯度形状因子”与描述纤维复合材料基质的细胞性质所带来的效率增益的“微观结构形状因子”以及例如可以描述竹子管状形状的“宏观形状因子”相结合，可以探索层次结构的这三个结构水平中的每一个对茎或秆的整体弯曲性能的贡献程度。类似地，可以分析常用的基于木材的复合材料中密度纤维板的弯曲效率；其性能梯度是由于其通过热压制造而产生的。还有其他一些工程材料具有模拟性能梯度的特性；目前正在探索新的制造方法来制备它们。进一步研究这些材料似乎是值得的。
