Recently, the research paper “Aggravated stress fluctuation and mechanical size effects of nanoscale lamellar bone pillars” was published online inNPG Asia Materials(IF=10.481, 2020), an academic journal of Nature Publishing Group (https://doi.org/10.1038/s41427-021-00328-6). It was co-authored by SMAE Prof. Zhao Hongwei’s team, Department of Precision Instrument of Tsinghua University, the State Key Laboratory for Mechanical Behavior of Materials of Xi’an Jiaotong University, and Royal Melbourne Institute of Technology, Australia, with Prof. Ma Zhichao as the first author.
This paper focuses on the microdeformation behaviors and the size effect of failure mechanisms of bone structure. The size effect of mechanical properties influences the microdeformation behaviors and failure mechanisms of hierarchical lamellar bones. Investigations into the continuous deformation behaviors and structure-behavior-property relationships of nanoscale lamellar bones provide essential data for reducing the risk of fracture. This paper mentioned that five micropillars with diameters ranging from 640 to 4,971nm inside a single lamella were fabricated. In situ pillar compressive tests inside a scanning electron microscope directly revealed the diameter-dependent enhanced strength, ductility, and stress fluctuation amplitude. Real-time observations also revealed the segmented deformation and morphological anisotropy of pillars with smaller diameters and the slight elastic recovery of pillars with larger diameters. The critical diameter leading to the brittle-to-ductile transition at the microscale was confirmed. The “analogous to serrated flow” stress fluctuation behaviors at the nanoscale exhibited a significant size effect, and each cycle of the fluctuation manifested as a slow stress increase and a rapid stress release.
In this paper, a discontinuous fracture theory is proposed to describe the macroscopic stress fluctuation behavior of collagen fibrils. The slow bending and instantaneous fracture of collagen fibrils with a relatively larger length-diameter ratio facilitate the stress increment and decrement, respectively. Based on the strain gradient plasticity theory, the microscopic stress fluctuation behaviors are elaborated through a layered dislocation movement theory. The dislocation movement inside the hydroxyapatite crystals and dislocation pile-up at the fibrillar matrix interface induce a slow stress increase, and the overflow of dislocations at the free surface induces a rapid stress decrease. Through the in-situ test of microscale bone deformation behaviors, the interface failure essence is revealed, which provides test support for the research and development of multiphase biomimetic bone materials and the strengthening and ductility design.
The research was supported by the National Natural Science Fund (51875241), the National Key R&D Program of China (2018YFF010124), and the Jilin Province Science and Technology Development Plan Project (20190302078GX, YDZJ202101ZYTS129).
Link to the paper:https://www.nature.com/articles/s41427-021-00328-6
