Anisotropic Crack Propagation and Deformation in Dentin Observed by Four-Dimensional X-ray Nano-Computed Tomography

Abstract

a b s t r a c tUnderstanding the cracking behaviour of biological composite materials is of practical importance. This paper presents the first study to track the interplay between crack initiation, microfracture and plastic deformation in three dimensions (3D) as a function of tubule and collagen fibril arrangement in elephant dentin using in situ X-ray nano-computed tomography (nano-CT). A nano-indenter with a conical tip has been used to incrementally indent three test-pieces oriented at 0°, 45°and 70°to the long axis of the tubules (i.e. radial to the tusk). For the 0°sample two significant cracks formed, one of which linked up with microcracks in the axial-radial plane of the tusk originating from the tubules and the other one occurred as a consequence of shear deformation at the tubules. The 70°test-piece was able to bear the greatest loads despite many small cracks forming around the indenter. These were diverted by the microstructure and did not propagate significantly. The 45°test-piece showed intermediate behaviour.In all cases strains obtained by digital volume correlation were well in excess of the yield strain (0.9%), indeed some plastic deformation could even be seen through bending of the tubules. The hoop strains around the conical indenter were anisotropic with the smallest strains correlating with the primary collagen orientation (axial to the tusk) and the largest strains aligned with the hoop direction of the tusk. Statement of SignificanceThis paper presents the first comprehensive study of the anisotropic nature of microfracture, crack propagation and deformation in elephant dentin using time-lapse X-ray nano-computed tomography. To unravel the interplay of collagen fibrils and local deformation, digital volume correlation (DVC) has been applied to map the local strain field while the crack initiation and propagation is tracked in real time. Our results highlight the intrinsic and extrinsic shielding mechanisms and correlate the crack growth behavior in nature to the service requirement of dentin to resist catastrophic fracture. This is of wide interest not just in terms of understanding dentin fracture but also can extend beyond dentin to other anisotropic structural composite biomaterials such as bone, antler and chitin.

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Micromechanical evaluation of cortical bone using in situ XCT indentation and digital volume correlation

“…In situ XCT Indentation has been previously employed for the evaluation of plastic deformation in elephant tusk (Lu et al, 2019). In that study the elephant dentin tubules were running parallel to the length of the tusk and three different tubules orientation with respect to the indentation axis were examined.…”

“…Vickers (Fischer-Cripps, 2006)) and is sustained by compressive and tensile strains in the whole volume, rather than just the area underneath the indenter (Fischer-Cripps, 1999). In that sense, in situ X-ray nanoCT indentation has been recently carried out on an elephant tusk to investigate the relation between plastic deformation and microstructure (Lu et al, 2019). Elephant dentin is an anisotropic material with hierarchical microstructure.…”

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