Damage Evolution of Onnagawa Shale by Postmortem Thresholding of X‐Ray Computed Tomography
Damage Evolution of Onnagawa Shale by Postmortem Thresholding of X‐Ray Computed Tomography
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DOI：
10.1029/2021jb022056
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发表时间：
2021-10
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影响因子：
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通讯作者：
E. Jayawickrama;J. Muto;O. Sasaki;H. Nagahama
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作者：
E. Jayawickrama;J. Muto;O. Sasaki;H. Nagahama
增大围压后，女川页岩试样经历了脆性-韧性转变。脆性变形的特征是在3%轴向应变下试样发生纵向分裂。在应变硬化过程中，塑性场的宏观变形表现为分布的共轭断裂网络。从脆性变形到延性变形的转变发生在轴向应变的4%至5%之间，并具有单一剪切面定义的破坏。在市售的X射线CT机上扫描变形样品，以研究裂缝网络对阈值选择的敏感性。将变形岩石的原始体素值进行反求，将其密度分布近似为正态分布，提取出构成裂缝网络（残差）的体素值密度分布。残差直方图的连续阈值化表明，生成的裂缝网络对阈值的选择高度敏感。残差直方图的峰后阈值单独生成裂缝体素体积，连续阈值表明，获得的裂缝体积段可以解释损伤区域内可能的成核、生长和合并。结果进一步显示了与先前通过声发射研究进行的四维层析应变局部化研究和损伤尺寸分布的相似性。因此，尽管研究的性质是事后分析，但新技术为研究在高围压和没有高能同步加速器设施的情况下裂缝性质的可能演变提供了可能性。
Onnagawa shale samples are deformed through the brittle‐ductile transition by increasing the confining pressure. Brittle deformation is characterized by longitudinal splitting of the sample at 3% axial strain. A distributed conjugate fracture network characterizes the macroscopic deformation in the ductile field with strain hardening. The onset of transition from brittle to ductile deformation is between 4% and 5% axial strain with a single shear plane defined failure. Deformed samples are scanned in a commercially available X‐ray CT machine to investigate the sensitivity of the fracture network to the choice of threshold voxel value. The primary voxel values of the deformed rock are reversed, and their density distribution is approximated by a normal distribution to extract the voxel value density distribution that fabricated the fracture network (residual). Successive thresholding of the residual histogram shows that the generated fracture network is highly sensitive to the choice of threshold. Post peak thresholding of the residual histogram generates voxel volumes of fractures alone and consecutive thresholding shows that the obtained volume segments of the fractures can interpret possible nucleation, growth, and coalescence within the damaged zone. Results further show similarity to previous 4D tomographic strain localization investigations and damage size distributions by acoustic emission studies. Therefore, despite the postmortem nature of the investigation, the new technique opens possibilities to investigate the possible evolution of fracture properties under elevated confining pressures and in the absence of high energy synchrotron facilities.