Abstract

Shock wave fracturing is a novel technology in coalbed methane (CBM) exploitation. Understanding dynamic damage and failure mechanisms of rock with different grain sizes is of vital importance to shock wave fracturing. In this study, we investigate the influence of grain size on the dynamic compressive strength and failure modes of sandstones under strain rates ranging from 20 to 100 s⁻¹ and the corresponding mesoscopic responses. The Split Hopkinson Pressure Bar (SHPB) results show that finer-grained samples are mildly fragmented with a moderate strength increase (longer microcracks with lower density) at lower strain rates (\(\dot{\varepsilon }\) = 20–60 s⁻¹) but intensively fragmented with a steeper strength increase (shorter microcracks with higher density) at higher strain rates (\(\dot{\varepsilon }\) = 60–100 s⁻¹). However, the trend for coarse-grained samples is the opposite. Microcracking characteristics of the tested samples were then obtained using optical microscopy. The microcrack density and the microcrack length may be used to explain the strain-rate dependence of the dynamic compressive strength and fracturing modes of the samples, respectively. These results may be helpful to build a multiscale framework to analyze the natured mechanism of crack production in CBM exploitation.

Article Highlights

• The influence of grain size on the dynamic compressive strength and failure modes of sandstones were discussed under medium strain rates and the corresponding mesoscopic responses;

• Finer-grained samples are mildly fragmented with a moderate strength increase at lower strain rates but intensively fragmented with a steeper strength increase at higher strain rates compared with coarser-grained ones;

• The microcrack density and the microcrack length may be used to explain the strain-rate dependence of the dynamic compressive strength and fracturing modes of the samples, respectively.

中文翻译：

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粒度对砂岩应变率相关动力响应的影响

摘要

冲击波压裂是煤层气开采的一项新技术。了解不同粒度岩石的动态损伤和破坏机制对于冲击波压裂至关重要。在这项研究中，我们研究了在应变速率范围为 20 到 100 s ^-1以及相应的细观响应下，晶粒尺寸对砂岩的动态抗压强度和破坏模式的影响。Split Hopkinson Pressure Bar (SHPB) 结果表明，在较低应变率 ( \(\dot{\varepsilon }\)  = 20–60 s ⁻¹) 但在更高的应变率 ( \(\dot{\varepsilon }\)  = 60–100 s ^-1 ) 下，强度增加更陡峭（更短的微裂纹，更高的密度）被密集破碎。然而，粗粒度样本的趋势相反。然后使用光学显微镜获得测试样品的微裂纹特性。微裂纹密度和微裂纹长度可分别用于解释样品的动态抗压强度和断裂模式的应变率依赖性。这些结果可能有助于建立一个多尺度框架来分析煤层气开采中裂缝产生的本质机制。

文章亮点

• 讨论了中等应变率下粒度对砂岩动态抗压强度和破坏模式的影响及相应的细观响应；

• 与粗粒样品相比，细粒样品在较低应变速率下轻度破碎，强度增加中等，但在较高应变速率下强度增加更剧烈；

• 微裂纹密度和微裂纹长度可分别用于解释样品的动态抗压强度和断裂模式的应变率依赖性。