Abstract Understanding and quantitatively characterizing the hidden distribution and evolution of the whole-field stress in fractured coal is crucial for accurately predicting crack propagation and connection that affect the transport capability of coalbed methane in reservoirs during fracture stimulation. However, it is challenging to directly reveal and quantify the hidden stress field using traditional methods because of the difficulties associated with extracting and identifying the stress distribution around randomly distributed, discontinuous, irregular-geometry cracks as well as with fabricating a model with embedded cracks. This study presents an optical method to reveal and quantify the continuous evolutions of the whole-field principal stress difference and shear stress in fractured coal by combining three-dimensional (3D) printing technology and digital photoelasticity. 3D printing technology is used to replicate the transparent model of fractured coal. The ten-step phase-shifting method and quality-guided phase unwrapping algorithm in photoelasticity are used to estimate the whole field photoelastic parameters (isoclinic and isochromatic) in the fractured coal model. The comparison between experimental and numerical results is provided to further prove the efficacy of this method in handling the natural fractured coal structure. The effects of crack irregularity and discontinuity on the stress evolution in fractured coal are evaluated as well.

中文翻译：

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定量分析单轴压缩载荷作用下裂隙煤中全场应力演化的光学方法

摘要 理解和定量表征裂缝煤中全场应力的隐藏分布和演化对于准确预测裂缝扩展和连接影响裂缝改造过程中储层煤层气输送能力至关重要。然而，由于与提取和识别随机分布、不连续、不规则几何裂纹周围的应力分布以及制造带有嵌入裂纹的模型相关的困难，使用传统方法直接揭示和量化隐藏应力场具有挑战性。本研究提出了一种结合三维 (3D) 打印技术和数字光弹性的光学方法来揭示和量化裂隙煤中全场主应力差和剪应力的连续演变。3D打印技术用于复制破碎煤的透明模型。使用光弹性中的十步相移方法和质量引导的相位展开算法来估计裂缝煤模型中的全场光弹性参数（等斜和等色）。实验结果与数值结果的对比进一步证明了该方法在处理天然裂隙煤结构中的有效性。裂纹不规则和不连续性对破碎煤中应力演化的影响也进行了评估。