Abstract In this paper, the mode I notch fracture toughness of graphite specimens weakened by notches of different shapes is investigated experimentally and theoretically. For this purpose, first, by conducting experiments, the apparent fracture toughness values of cracked semi-circular bending (SCB) graphite specimens with four various radii are determined. Next, numerous fracture experiments are performed on the notched SCB graphite specimens in four various scales. Then, the critical radius of the fracture process zone (FPZ) for the notched graphite specimens is obtained using two various equations already proposed in literature for cracked bodies. A well-known stress-based failure model, namely the point stress (PS) criterion, is employed for estimating the notch fracture toughness values of the specimens tested at different scales, taking advantages of two different critical distance formulas. The results show that the fracture toughness values of various notches obtained from PS criterion are in good agreement with the experimental results. Also, found in this research that the notch fracture toughness is considerably dependent on the nominal dimension (radius) of the specimen, such that, as the specimen radius increases, the mode I notch fracture toughness increases. Furthermore, it is revealed that for the present specimens tested, both the critical distance formulas provide almost identical predictions, suggesting that each of them can be arbitrarily selected for prediction.

中文翻译：

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缩放对 I 型加载下石墨试样缺口断裂韧性的影响

摘要 本文通过实验和理论研究了不同形状缺口削弱石墨试样的I型缺口断裂韧性。为此，首先，通过进行实验，确定了具有四种不同半径的裂纹半圆弯曲 (SCB) 石墨试样的表观断裂韧性值。接下来，在四个不同尺度的缺口 SCB 石墨试样上进行了大量断裂实验。然后，使用已在文献中针对裂纹体提出的两个不同方程，获得缺口石墨试样的断裂过程区 (FPZ) 的临界半径。一个众所周知的基于应力的失效模型，即点应力 (PS) 准则，用于估计在不同尺度下测试的试样的缺口断裂韧性值，利用两种不同的临界距离公式。结果表明，PS准则得到的各种缺口的断裂韧度值与实验结果吻合较好。此外，在这项研究中发现，缺口断裂韧性在很大程度上取决于试样的标称尺寸（半径），因此，随着试样半径的增加，I 型缺口断裂韧性增加。此外，揭示了对于目前测试的样本，两个临界距离公式提供几乎相同的预测，表明可以任意选择它们中的每一个进行预测。结果表明，PS准则得到的各种缺口的断裂韧度值与实验结果吻合较好。此外，在这项研究中发现，缺口断裂韧性在很大程度上取决于试样的标称尺寸（半径），因此，随着试样半径的增加，I 型缺口断裂韧性增加。此外，揭示了对于目前测试的样本，两个临界距离公式提供几乎相同的预测，表明可以任意选择它们中的每一个进行预测。结果表明，PS准则得到的各种缺口的断裂韧度值与实验结果吻合较好。此外，在这项研究中发现，缺口断裂韧性在很大程度上取决于试样的标称尺寸（半径），因此，随着试样半径的增加，I 型缺口断裂韧性增加。此外，揭示了对于目前测试的样本，两个临界距离公式提供几乎相同的预测，表明可以任意选择它们中的每一个进行预测。