Success of a hydraulic fracturing operation depends on the height and width of the induced fractures. One of the critical components controlling fracture size is fracture toughness of the formation. In this work, mode I fracture toughness of Berea Sandstone and Mancos Shale is measured by combining semi-circular bend test (SCB) and digital image correlation (DIC). Experiments were carried out in different notch orientations with respect to bedding. DIC is used to measure full-field displacements and to visualize and quantify fracture process zone (FPZ). Full-field displacements from DIC are utilized in Williams’ series solution to extract critical stress intensity factor, or fracture toughness. Accuracy of measuring fracture toughness using DIC displacements depends on area of interest (AOI), field of view (FOV), and the number of terms of solution ( N ). A parametric study is conducted, allowing to choose an optimal set of these parameters for evaluation of fracture toughness in rock specimens. It is known that fracture toughness values obtained directly from the SCB test, using conventional maximum load method, are underestimated due to the effect of nonlinear behavior caused by the fracture process zone. FPZ length is considered as an increase in the effective crack length. Irwin’s correction for effective crack length is utilized to measure fracture toughness values obtained directly from the SCB test that account for the fracture process zone. Fracture toughness values measured using DIC method and Irwin’s correction method are in a good match for both Berea Sandstone and Mancos Shale. Both methods show higher fracture toughness for samples in arrester orientation. In addition, the results show that FPZ length in Berea Sandstone is much larger than in Mancos Shale.

中文翻译：

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用数字图像相关评价砂岩和页岩的断裂韧性

水力压裂作业的成功取决于诱导裂缝的高度和宽度。控制裂缝尺寸的关键因素之一是地层的断裂韧性。在这项工作中，结合半圆弯曲试验（SCB）和数字图像相关（DIC）测量了Berea砂岩和Mancos页岩的I型断裂韧性。实验是在不同的槽口方向上进行的。DIC 用于测量全场位移以及可视化和量化断裂过程区 (FPZ)。在 Williams 的系列解决方案中利用 DIC 的全场位移来提取临界应力强度因子或断裂韧性​​。使用 DIC 位移测量断裂韧性的准确性取决于感兴趣区域 (AOI)、视场 (FOV)、和解的项数 (N)。进行参数研究，允许选择一组最佳参数来评估岩石试样的断裂韧性。众所周知，由于断裂过程区引起的非线性行为的影响，使用传统的最大载荷方法直接从 SCB 试验获得的断裂韧性值被低估了。FPZ 长度被认为是有效裂纹长度的增加。Irwin 对有效裂纹长度的修正用于测量直接从 SCB 测试获得的断裂韧性值，该值考虑了断裂过程区。使用 DIC 方法和 Irwin 校正方法测量的断裂韧度值与 Berea 砂岩和 Mancos 页岩非常匹配。对于避雷器方向的样品，这两种方法都显示出更高的断裂韧性。此外，结果表明，Berea 砂岩中的 FPZ 长度远大于 Mancos 页岩中的 FPZ 长度。