Crack tip opening displacement (CTOD) and fracture energy are determined from crack geometry and material properties for very slowly propagating cracks, less than 50 $$\upmu \mathrm {m/s}$$ μ m / s , in thin brittle hydrogels on the sub-millimeter scale. 2D fluorescent speckle images are captured using confocal microscopy during propagation, and 3D volumetric images are captured both before propagation begins and after the crack arrests. Fracture energy builds up until a critical value is reached and then remains constant as the crack propagates and eventually arrests when the energy is no longer sufficient for propagation. Once a crack arrests, more energy is needed for renucleation, suggesting that local toughening effects are at play. Based on observations of renucleation events and analysis of 3D crack shapes, this local toughening points to a mechanism for fracture surface roughening observed in the literature for slowly propagating cracks. Additionally, through-thickness variation in fracture energy, while expected from linear elastic fracture mechanics (LEFM) theory, suggests local toughening in the process zone which contributes to this roughening of crack surfaces.

中文翻译：

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软脆水凝胶中的裂纹扩展和再成核

裂纹尖端张开位移 (CTOD) 和断裂能由裂纹几何形状和材料特性确定，用于非常缓慢传播的裂纹，小于 50 $$\upmu \mathrm {m/s}$$ μ m/s，在薄脆性水凝胶中亚毫米级。2D 荧光散斑图像是在传播过程中使用共聚焦显微镜捕获的，3D 体积图像是在传播开始之前和裂纹停止之后捕获的。裂纹能量累积直到达到临界值，然后随着裂纹扩展保持恒定，并最终在能量不再足以扩展时停止。一旦裂纹停止，再成核需要更多的能量，这表明局部增韧效应在起作用。基于对再成核事件的观察和 3D 裂纹形状的分析，这种局部增韧表明了在文献中观察到的裂纹扩展缓慢的断裂表面粗糙化机制。此外，虽然从线性弹性断裂力学 (LEFM) 理论预计，断裂能的全厚度变化表明工艺区域中的局部增韧有助于裂纹表面的这种粗糙化。