Abstract
The size effects of mechanical properties influence the microdeformation behaviors and failure mechanisms of hierarchical lamellar bones. Investigations of the continuous deformation behaviors and structure–behavior–property relationships of nanoscale lamellar bones provide essential data for reducing the risk of fracture. Here, five pillars with diameters ranging from 640 to 4971 nm 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 was confirmed. The “analogous to serrated flow” stress fluctuation behaviors at the nanoscale exhibited a significant size effect, with coincident fluctuation cycles independent of diameter, and each cycle of the fluctuation manifested as a slow stress increase and a rapid stress release. The discontinuous fracture of collagen fibrils, embedded enhancement of hydroxyapatite crystals, and layered dislocation movement on the basis of strain gradient plasticity theory were expected to induce cyclical stress fluctuations with different amplitudes.
Highlights
As important carriers of the mechanical support and mineral metabolism of vertebrates, including humans, cortical bones utilize hierarchical and ordered mineralized collagen fibrils and embedded hydroxyapatite (HA) crystals to create resistance to fracture at both macroscopic and microscopic scales[1,2]
In situ mechanical testing is frequently used for the realtime investigation of the structural evolution of lamellar bones combined with the stress−strain relationship[28] and can quantitatively discover the morphological anisotropy[29], crack nucleation and propagation[30], discontinuous splitting[31], interfacial shearing behaviors[13] and cooperative deformation of mineralized composition and collagen fibrils[32]
Through in situ scanning electron microscopy (SEM) compression of pillars with diameters ranging from 640 to 4971 nm, this paper focuses on the mechanical size effects of nanoscale lamellar bones, including the enhanced strength, ductility, and stress fluctuation amplitude of micro/nanoscale lamellar bones
Summary
As important carriers of the mechanical support and mineral metabolism of vertebrates, including humans, cortical bones utilize hierarchical and ordered mineralized collagen fibrils and embedded hydroxyapatite (HA) crystals to create resistance to fracture at both macroscopic and microscopic scales[1,2]. Among the diversified in situ biomechanical testing technologies, in situ scanning electron microscopy (SEM) micropillar compression exhibits unique advantages, including controllable and accurate preparation of pillars and continuous loading with high resolution and observation of the entire pillar surface with a wide field of view. It is available for the investigation of significant variations in the mechanical properties of pillars with various diameters or length–diameter ratios. When carrying out AFM-based in situ nanoindentation, the structural interference between the indenter tip and surface can cause a blind zone underneath the indenter
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