184 Hope Street, Providence, RI, 02912, USA.
184 Hope Street, Providence, RI, 02912, USA.
J Mech Behav Biomed Mater. 2021 Jul;119:104362. doi: 10.1016/j.jmbbm.2021.104362. Epub 2021 Mar 5.
Layered architectures are prevalent in tough biological composites, such as nacre and bone. Another example of a biological composite with layered architecture is the skeletal elements-called spicules-from the sponge Euplectella aspergillum. Based on the similarities between the architectures, it has been speculated that the spicules are also tough. Such speculation is in part supported by a sequence of sudden force drops (sawtooth patterns) that are observed in the spicules' force-displacement curves from flexural tests, which are thought to reflect the operation of fracture toughness enhancing mechanisms. In this study, we performed three-point bending tests on the spicules, which also yielded the aforementioned sawtooth patterns. However, based on the analysis of the micrographs obtained during the tests, we found that the sawtooth patterns were in fact a consequence of slip events in the flexural tests. This is put into perspective by our recent study, in which we showed that the spicules' layered architecture contributes minimally to their toughness, and that the toughness enhancement in them is meager in comparison to what is observed in bone and nacre [Monn MA, Vijaykumar K, Kochiyama S, Kesari H (2020): Nat Commun 11:373]. Our past and current results underline the importance of inferring a material's fracture toughness through direct measurements, rather than relying on visual similarities in architectures or force-displacement curve patterns. Our results also suggest that since the spicules do not possess remarkable toughness, re-examining the mechanical function of the spicule's intricate architecture could lead to the discovery of new engineering design principles.
分层结构在坚韧的生物复合材料中很常见，如珍珠层和骨骼。另一个具有分层结构的生物复合材料的例子是海绵 Euplectella aspergillum 的骨骼元素，称为骨针。基于结构的相似性，人们推测骨针也具有韧性。这种推测部分得到了从弯曲试验中观察到的骨针力-位移曲线中的一系列突然力下降（锯齿图案）的支持，这些图案被认为反映了断裂韧性增强机制的作用。在这项研究中，我们对骨针进行了三点弯曲测试，也得到了上述锯齿图案。然而，基于测试过程中获得的微观照片的分析，我们发现锯齿图案实际上是弯曲测试中滑移事件的结果。这与我们最近的研究结果相一致，我们在该研究中表明，骨针的分层结构对其韧性的贡献极小，与在骨骼和珍珠层中观察到的韧性增强相比，其韧性增强微不足道[Monn MA、Vijaykumar K、Kochiyama S、Kesari H（2020）：Nat Commun 11:373]。我们过去和现在的结果强调了通过直接测量推断材料断裂韧性的重要性，而不是依赖于结构或力-位移曲线模式的视觉相似性。我们的结果还表明，由于骨针没有显著的韧性，重新检查骨针复杂结构的机械功能可能会发现新的工程设计原则。
