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

Objectives. To 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.
Methods. Compact 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.
Results. For the RAR test-piece the plastic zones were measured to be 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 400m behind the crack front in the dry case. This could only be seen 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.
Significance. For crack growth parallel to the tubules the 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 active.