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.
Highlights
It is well known that considerable toughness is achieved in many biological structural materials through the use of interfaces and a hierarchical architecture [7] as a means of hinder-⇑ Corresponding author.ing crack growth
For the 0° test-piece, the horizontal orthoslices (A1-3) in Fig. 5 show that cracks initiate from the indenter and propagate initially parallel to the length of the tusk linking up with microcracks emanating from the tubules parallel to their semi major axes and parallel to the plane of the collagen fibrils (i.e in the axial-radial plane of the tusk)
This study has examined crack initiation, microfracture and plastic deformation in three types of test-pieces with different orientations in ivory tusk to understand their interplay with the tubule and collagen fibril arrangement using in situ 3D X-ray nano-computed tomography
Summary
It is well known that considerable toughness is achieved in many biological structural materials (e.g. bone [1], dentin [2,3], beetle cuticle [4], lobster [5], and nacre [6]) through the use of interfaces and a hierarchical architecture [7] as a means of hinder-. The crack-resistance of dentin, as the major constituent of teeth and tusk, is a subject of considerable biomechanical interest.
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