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

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Document pages: 39 pages

Abstract: Understanding the cracking behaviour of biological compositematerials is of practical importance. This paper presents the first studyto track the interplay between crack initiation, microfracture andplastic deformation in three dimensions as a function of tubule andcollagen fibrils arrangement in elephant dentin using in situ X-ray nanocomputed tomography (nano-CT). A nano-indenter with a conical tip hasbeen 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). Forthe 0° sample two significant cracks formed, one of which linked up withmicrocracks in the axial-radial plane of the tusk originating from thetubules and the other one occurred as a consequence of shear deformationat the tubules. The 70° test-piece was able to bear the greatest loadsdespite many small cracks forming around the indenter. These werediverted by the microstructure and did not propagate significantly. The45° test-piece showed intermediate behaviour. In all cases strainsobtained by digital volume correlation were well in excess of the yieldstrain (0.9 ), indeed some plastic deformation could even be seen throughbending of the tubules. The hoop strains around the conical indenter wereanisotropic with the smallest strains correlating with the primarycollagen orientation (axial to the tusk) and the largest strains alignedwith the hoop direction of the tusk.

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