Effect of hydration and crack orientation on crack-tip strain, crack opening displacement and crack-tip shielding in elephant dentin

Xuekun Lu,Shelley D Rawson,Philip J Withers

doi:10.1016/j.dental.2018.04.002

Xuekun Lu, Shelley D Rawson + Show 1 more

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https://doi.org/10.1016/j.dental.2018.04.002

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Abstract

ObjectivesTo quantify the extent of crack-tip plasticity, crack opening displacement (COD) and crack bridging for crack growth perpendicular (HAH) and parallel (RAR) to the tubules in elephant dentin under both hydrated and dry conditions to better understand their influence on intrinsic and extrinsic toughening during crack growth. MethodsCompact tension test-pieces were prepared from a tusk of African elephant ivory. Crack-tip strain mapping and COD measurements by digital image correlation (DIC) technique were made under incremental loading and unloading of cracks for hydrated and dry dentin of different orientations. ResultsFor the RAR test-piece the plastic zones were significantly larger in the hydrated condition compared to when dry. By contrast, the plastic strains in the HAH test-piece were negligible in both wet and dry conditions. In the RAR condition the crack front was broken up into overlapping longitudinal ‘fingers’ with crack bridging regions in between, the ligaments extending 400μm behind the crack front in the dry case. This could only be seen in 3D by X-ray CT. Extrinsic shielding reduces the crack-tip stresses by 52% and 40% for hydrated and dry RAR test-pieces respectively. No significant bridging was found in the HAH case. SignificanceFor crack growth parallel to the tubules, collagen plasticity determines the intrinsic toughening, whereas microcracking from the tubules governs extrinsic shielding via ligament bridging, which is maintained further behind the crack in the hydrated case. For cracks grown perpendicular to the tubules, neither toughening mechanisms are significant.

Highlights

The crack-resistance of dentin, as the major constituent of teeth and tusk, is a subject of considerable biomechanical interest
This paper aims to characterize and compare the extent of crack-tip plasticity and bridging of elephant dentin under hydrated and dry conditions to reveal the importance of the microstructure in inhibiting crack propagation
The elastic strain will give rise to a residual stress arising from the plastic misfit generated by the crack-tip stress field

Summary

Introduction

The crack-resistance of dentin, as the major constituent of teeth and tusk, is a subject of considerable biomechanical interest. While human dentin has tubules of approximately circular cross-section, the tubules in elephant dentin are elliptical with the major axis parallel to the length of the tusk (see Fig. 1c and f). This ellipticity is often exaggerated further in micrographs because the tubules have a periodic wavy trajectory as they emanate radially from the central pulp cavity of the tusk to the cementum layer [10]. Elephant dentin has no highly mineralized peritubular cuff Another critical difference is that in human dentin the collagen fibrils are arranged in a planar random mat, perpendicular to the long axis of the tubules [11]. By contrast recent work by Alberic [13] and Lu [14] suggest that for elephant dentin the collagen fibrils are approximately aligned (+/−10◦) to the semi major axis of the tubules, which coincides with the axial direction of the tusk (see Fig. 1e and f)

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