Abstract
As a composite material, the mechanical properties of bone are highly dependent on its hierarchical organisation, thus, macroscopic mechanical properties are dictated by local phenomena, such as microdamage resulting from repetitive cyclic loading of daily activities. Such microdamage is associated with plastic deformation and appears as a gradual accumulation of residual strains. The aim of this study is to investigate local residual strains in cortical bone tissue following compressive cyclic loading, using in situ X-ray computed tomography (XCT) and digital volume correlation (DVC) to provide a deeper insight on the three-dimensional (3D) relationship between residual strain accumulation, cortical bone microstructure and failure patterns. Through a progressive in situ XCT loading–unloading scheme, localisation of local residual strains was observed in highly compressed regions. In addition, a multi-scale in situ XCT cyclic test highlighted the differences on residual strain distribution at the microscale and tissue level, where high strains were observed in regions with the thinnest vascular canals and predicted the failure location following overloading. Finally, through a continuous in situ XCT compression test of cycled specimens, the full-field strain evolution and failure pattern indicated the reduced ability of bone to plastically deform after damage accumulation due to high number of cyclic loads. Altogether, the novel experimental methods employed in this study, combining high-resolution in situ XCT mechanics and DVC, showed a great potential to investigate 3D full-field residual strain development under repetitive loading and its complex interaction with bone microstructure, microdamage and fracture.
Highlights
Cortical bone is a complex composite material whose structure is hierarchically organized from the nano- to the macroscale to withstand physiological loads and resist fracture (Wolfram and Schwiedrzik, 2016)
The accumulation of microdamage during cyclic loading plays a key role in weakening cortical bone and leading to complete fracture as a result of the degradation in its mechanical properties, which is evidenced by the development of residual strains upon unloading
The main goal of this study was to explore the capability of digital volume correlation (DVC) to assess local residual strains in cortical bone tissue following in situ X-ray computed tomography (XCT) cyclic loading in order to gain a greater understanding of the 3D relationships between residual strain accumulation, cortical bone microstructure and failure pattern
Summary
Cortical bone is a complex composite material whose structure is hierarchically organized from the nano- to the macroscale to withstand physiological loads and resist fracture (Wolfram and Schwiedrzik, 2016). The mechanical competence of bone is often impaired by the accumulation of microdamage due to isolated overloading events (Gauthier et al, 2019; Morgan et al, 2005) or after suffering fatigue from a large number of loading cycles (Burr et al, 1997; Diab et al, 2006; Schaffler et al, 1995; Zioupos and Currey, 1998). The formation of microcracks is dependent on the loading mode, (Mirzaali et al, 2015; Reilly and Currey, 1999) and they are influenced by the morphological complexity and porosity of cortical bone (Loundagin et al, 2020; Turnbull et al, 2014). Microcracks can grow and cause fractures in bone, clinically known as stress fracture (Zioupos et al, 1996), there is considerable interest in understanding the failure mechanism of cortical bone following cyclic loading
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