Abstract Brittle fracture in carbon steel has a strong impact on the safety of the structures. Especially, the arresting technology of the running crack is the measure of last resort for ensuring structural integrity. Due to such high importance, many experimental and theoretical studies of brittle crack propagation have been conducted from both mechanical and microstructural viewpoints. It is thought that the elementary step of the brittle fracture of polycrystalline steel is the cleavage in each crystal grain and their connection process. However, the detailed mechanisms of brittle fracture have not been fully understood; for example, it is still unclear why the propagation rate under a large driving force is not increased up to the Rayleigh wave speed. Several difficulties hinder the achievement of a detailed understanding so far: 1) crack propagation is quite rapid, 2) the crystal grain is usually too small (10–100 μm) to collect sufficient information, 3) the microstructure is very complicated in most case, i.e., containing different phases, microstructures, precipitates and inclusions. In this study, to eliminate such difficulties, 3% silicon steel in which microstructure is the ferrite single phase and the grain size is increased to 4–5 mm is used. This steel can be fractured in a brittle manner, even under ambient temperature. For this steel, by using a high speed camera and strain gauge data with a high sampling rate, the elementary process of brittle crack propagation is elucidated. As a result, it is revealed that the brittle crack propagation rate even in a single-crystal grain is much slower than the Rayleigh wave speed. This seems to be due to the presence of twin deformations and twin boundary cracks in crystal grain as observed on the fracture surface. Using the analysis of electron back scatter diffraction (EBSD) data, the mechanism of twin deformation and twin boundary crack is revealed. Additionally, it is shown that the brittle crack propagation rate where the path includes crystal grain boundaries is much slower. This delay seems to be related to the misorientation angle at the GB. By applying our simplified model, the delay effect at the grain boundary can be successfully explained.

中文翻译：

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3% Si-Fe合金晶粒内部和大角度晶界处的脆性裂纹扩展阻力

摘要 碳钢脆性断裂对结构的安全性有很大影响。尤其是止裂止裂技术是保证结构完整性的最后手段。由于如此重要​​，许多脆性裂纹扩展的实验和理论研究已经从机械和微观结构的角度进行了。认为多晶钢脆性断裂的基本步骤是各晶粒内的解理及其连接过程。然而，脆性断裂的详细机制尚未完全了解。例如，目前还不清楚为什么在大驱动力下的传播速度没有增加到瑞利波速度。到目前为止，有几个困难阻碍了详细了解：1) 裂纹扩展非常快， 2) 晶粒通常太小 (10-100 μm)，无法收集足够的信息，3) 在大多数情况下，微观结构非常复杂，即包含不同的相、微观结构、析出物和夹杂物. 在本研究中，为了消除这些困难，使用了显微组织为铁素体单相且晶粒尺寸增加到 4-5 mm 的 3% 硅钢。即使在环境温度下，这种钢也会以脆性方式断裂。对于这种钢，通过使用高速相机和高采样率的应变仪数据，阐明了脆性裂纹扩展的基本过程。结果表明，即使在单晶粒中脆性裂纹扩展速度也比瑞利波速度慢得多。这似乎是由于在断裂表面上观察到的晶粒中存在孪晶变形和孪晶界裂纹。通过对电子背散射衍射（EBSD）数据的分析，揭示了孪晶变形和孪晶边界裂纹的机理。此外，表明路径包括晶界的脆性裂纹扩展速率要慢得多。这种延迟似乎与 GB 的错误定向角有关。通过应用我们的简化模型，可以成功解释晶界处的延迟效应。结果表明，路径包括晶界的脆性裂纹扩展速率要慢得多。这种延迟似乎与 GB 的错误定向角有关。通过应用我们的简化模型，可以成功解释晶界处的延迟效应。结果表明，路径包括晶界的脆性裂纹扩展速率要慢得多。这种延迟似乎与 GB 的错误定向角有关。通过应用我们的简化模型，可以成功解释晶界处的延迟效应。