Abstract
Understanding 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.
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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