The storage of liquefied natural gas (LNG) in underground coal mine tunnels offers numerous advantages, with the fracturing characteristics of surrounding rock under cryogenic conditions (−162 °C at the lowest) closely tied to the safety of the storage. However, a substantial knowledge gap persists within the entire temperature range, presenting a significant challenge in our understanding. The innovation of this work lies in the comprehensive study of fracture toughness and the revelation of damage mechanisms in both dry and saturated rock across a complete temperature range from −162 to 20 °C. Both dry and saturated cracked straight-through Brazilian disc (CSTBD) sandstone specimens underwent treatment at various temperatures (20, −40, −80, −120, and −162 °C) through liquid nitrogen cooling, with subsequent testing of fracture toughness. The results indicate that the fracture toughness of dry sandstone experiences a marginal decrease (∼5 %) with lowering temperatures. In contrast, the reduction in fracture toughness for saturated sandstone is more pronounced, reaching approximately 30 % from room temperature to a cryogenic critical temperature (above −40 °C). Interestingly, within the critical temperature range to −162 °C, the fracture toughness of saturated sandstone remains relatively stable. At ultralow temperatures, dry sandstone exhibits failure cracks that are more crooked due to shrinkage damage. On the other hand, saturated sandstone, subjected to ultralow temperature treatment, displays crooked failure cracks with branched cracks, attributed to frost heaving damage. Consequently, the damage mechanisms under cryogenic conditions are identified as shrinkage damage and frost heaving damage. For dry sandstone, shrinkage forces can induce some microcracks between mineral grains and cementation, but the number of cracks is limited. In contrast, both shrinkage and frost heaving forces can lead to more microcracks in saturated sandstone. Comparative analysis of experimental results suggests that frost heaving damage is the primary mechanism in saturated sandstone. It's important to note that for sandstones with different cementation types and porosity, the main damage mechanism may vary, requiring further research. These findings contribute to a more accurate stability evaluation of LNG underground storage.

中文翻译：

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低温条件下干饱和砂岩I型断裂韧性及损伤机制


在地下煤矿巷道中储存液化天然气 (LNG) 具有众多优势，低温条件下（最低 −162 °C）围岩的破裂特性与储存的安全性密切相关。然而，在整个温度范围内仍然存在巨大的知识差距，这对我们的理解提出了重大挑战。这项工作的创新之处在于综合研究了干燥和饱和岩石在-162至20°C整个温度范围内的断裂韧性和损伤机制。干燥和饱和裂纹直通式巴西圆盘 (CSTBD) 砂岩样本均通过液氮冷却在不同温度（20、-40、-80、-120 和 -162 °C）下进行处理，并随后测试断裂韧性。结果表明，随着温度的降低，干砂岩的断裂韧性略有下降（∼5%）。相比之下，饱和砂岩的断裂韧性降低更为明显，从室温到低温临界温度（-40°C 以上）达到约 30%。有趣的是，在-162°C的临界温度范围内，饱和砂岩的断裂韧性保持相对稳定。在超低温下，干砂岩会因收缩损坏而出现更加弯曲的破坏裂纹。另一方面，经过超低温处理的饱和砂岩会出现弯曲的破坏裂缝和分支裂缝，这是由于冻胀损坏造成的。因此，低温条件下的损伤机制被确定为收缩损伤和冻胀损伤。 对于干砂岩，收缩力会在矿物颗粒和胶结物之间诱发一些微裂纹，但裂纹的数量是有限的。相反，收缩力和冻胀力都会导致饱和砂岩中出现更多的微裂纹。实验结果对比分析表明，冻胀损伤是饱和砂岩的主要机制。需要注意的是，对于不同胶结类型和孔隙度的砂岩，其主要损伤机制可能有所不同，需要进一步研究。这些发现有助于更准确地评估液化天然气地下储存的稳定性。