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The understanding of mechanical performances and microscopic failure mechanisms of cortical bones under service condition is necessary prerequisite of fracture prevention, which would support the development of bone tissue engineering and design of bionic bones. By using miniaturized horizontal in situ compression tester, the effects of both temperature and sampling orientations on the compressive strengths and fracture morphologies were investigated. The significant difference between fracture strengths and compressive strains at various temperatures indicated that the cortical bone was sensitive to temperature. Direct experimental evidences revealed the gradually fibrotic trend of fracture surfaces as a function of sampling orientation. Through the Haversian canals distribution analysis, the relationship between the distribution of Haversian canals and fracture path was established. Essentially, the competition between high density Haversian canals and stress concentration factor determines the initiation and propagation of cracks.

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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.

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