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 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 μm 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.

Significance

For 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.

中文翻译：

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水化和裂纹取向对大象牙本质裂纹尖端应变，裂纹开口位移和裂纹尖端屏蔽的影响

目标

为了量化在水合和干燥条件下大象牙本质中与小管垂直（HAH）和平行（RAR）的裂纹扩展的裂纹尖端可塑性，裂纹开口位移（COD）和裂纹桥接的程度，以更好地了解它们对内在性的影响裂纹扩展过程中的外在增韧。

方法

紧凑的张力测试件是用非洲象牙的象牙制成的。在不同方向的水合和干燥牙本质的裂纹的增量加载和卸载下，通过数字图像相关（DIC）技术进行了裂纹尖端应变映射和COD测量。

结果

对于RAR试件，在水合条件下，与干燥时相比，塑料区域明显更大。相比之下，HAH测试件中的塑性应变在潮湿和干燥条件下均可以忽略不计。在RAR条件下，裂纹前沿被分解成重叠的纵向“指状”，中间有裂纹桥接区域，在干燥情况下，韧带在裂纹前沿后面延伸了400μm。这只能通过X射线CT在3D模式下看到。外加屏蔽可将水合和干燥RAR测试件的裂纹尖端应力分别降低52％和40％。在HAH病例中未发现明显的桥接。

意义

对于与小管平行的裂纹扩展，胶原蛋白的可塑性决定了其固有的增韧性，而从小管产生的微裂纹则通过韧带桥接来控制外在的屏蔽，在水合情况下，这种保护作用可保持在裂纹的后面。对于垂直于小管生长的裂纹，增韧机理都不重要。