Investigation of dynamic fracture characteristics of frozen red sandstone using notched semi-circular bend method
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摘要: 采用紅砂巖制作中心直裂紋半圓盤彎曲試樣（Notched semi-circular bend, NSCB），設置不同的負溫溫度對巖石試樣預處理，隨后利用改進后的分離式霍普金森桿（SHPB）實驗系統開展動態試驗。結果表明：巖石的斷裂韌度存在明顯的加載率效應，斷裂韌度試驗值隨加載率的增加近似呈指數型增大；當加載率一定時，巖石斷裂韌度由常溫進入負溫后先緩慢后快速增加，在–20 ℃時達到最大值，隨著溫度進一步降低，巖石斷裂韌度快速減小。進一步對巖石破裂過程分析發現，不同溫度下巖石的斷裂過程基本一致，且裂紋擴展速度受溫度影響較小。基于巖石斷面的掃描電子顯微鏡結果分析巖石斷裂模式為：負溫下紅砂巖的斷裂以沿晶破裂和膠結物的撕裂為主，伴有少量的穿晶破裂現象，同時當溫度降低至–25 ℃時，巖石內部微裂隙數量明顯增多，說明負溫對巖石具有劣化作用。最后探討了溫度對巖石內部結構的影響機制，對分析巖石斷裂特性的低溫效應具有一定參考意義。Abstract: Considering that fluctuations in temperature can cause variations in both the internal structure as well as the mineral composition of rocks, their fracture characteristics must be impacted accordingly. With the exponential development of geotechnical engineering in cold regions, it is urgent to study the influence of the sub-zero temperature environment on the mechanical properties and dynamic properties of rocks. In order to investigate the influence of sub-zero temperature gradient on the dynamic fracture characteristics of rocks, red sandstone was used for the preparation of notched semi-circular bend specimens. First, a water-saturated machine and a sub-zero temperature incubator were utilized to pretreat the rock for 48 h, conducive for both satiation and freezing processes. Subsequently, the dynamic tests were carried out utilizing an improved split Hopkinson bar experimental system with a cryogenic sub-system. Concurrently, the striker velocity was modulated by setting distinctive air pressures, following which the rock was loaded at various loading rates. The test results demonstrate that the fracture toughness of the rock has an evident loading rate effect, and the fracture toughness proliferates exponentially with the increase in the loading rate. In the event that the loading rate is certain, the fracture toughness of the rock primarily increases gradually and then expeditiously over the course of advancement from room temperature to ?20 ℃. Contradictorily, the rock fracture toughness diminishes abruptly with plummeting temperature. Analysis of the rock fracture process, accommodated by a high-speed camera, revealed that the fracture process of the rock at distinctive temperatures is fundamentally equivalent, and the crack propagation speed is negligibly influenced by the temperature. Furthermore, the rock fracture mode was analyzed by employing a scanning electron microscope (SEM) system. The SEM images of the rock depicted that the fracture of red sandstone at sub-zero temperature is predominantly intergranular fracture and cement tearing, accompanied by a trace of transgranular fracture. Meanwhile, the experimentation also revealed that the number of micro-cracks in the rock significantly multiplied when the temperature declined to ?25 ℃, illustrating that sub-zero temperature has a deteriorating effect on the rock. Conclusively, the influence mechanism of temperature on the internal structure of the rock is discussed, and it is assumed that the change in the internal structure of the rock is the collaborative effect of thermal expansion-cold contraction and ice-water phase transition. The interpretation of this study has substantial reference significance for the further consequential analysis of frigidity on the fracture properties of the rock.
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Key words:
- NSCB /
- frozen /
- loading rate /
- fracture toughness /
- fracture mode
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圖 6 ?15 ℃時巖石應力強度因子時程曲線
Figure 6. History curves of rock fracture stress intensity factor at ?15 ℃
圖 7 不同溫度下巖石的加載率及動態斷裂韌度
Figure 7. Loading rate and dynamic fracture toughness of rock at different temperatures
圖 8 動態斷裂韌度隨溫度的變化趨勢. (a) 三維擬合圖；(b) 斷面提取曲線；(c) 實測數據
Figure 8. Trends of dynamic fracture toughness with temperature: (a) 3D fitting surface; (b) sectional maps extracted from fitting surface; (c) measured curves
圖 9 不同溫度下巖石試樣的漸進破裂過程
Figure 9. Dynamic failure processes of NSCB specimen at different temperature
圖 11 典型試樣裂紋擴展長度和速度
Figure 11. Crack length and crack propagation velocity of the typical specimen
圖 12 不同溫度下巖石裂紋擴展速度
Figure 12. Rock crack propagation velocity at different temperature
圖 13 典型試樣測試后破壞形態. (a) 25 ℃；(b) –5 ℃；(c) –10 ℃；(d) –15 ℃；(e) –20 ℃；(f) –25 ℃
Figure 13. Typical tested NSCB specimens: (a) 25 ℃; (b) –5 ℃; (c) –10 ℃; (d) –15 ℃; (e) –20 ℃; (f) –25 ℃
圖 14 常溫時典型破壞試樣的SEM圖. （a）裂紋擴展方向；（b）少量穿晶破壞；（c）顆粒剝離
Figure 14. SEM images of the typical failure sample at room temperature (The arrow indicates the direction of crack propagation): (a) crack extension direction; (b) small amount of TG damage; (c) particle peeling
圖 15 低倍率下典型破壞試樣的SEM圖. (a) –5 ℃; (b) –10 ℃; (c) –15 ℃; (d) –20 ℃; (e) –25 ℃
Figure 15. SEM images of typical failure specimens at low magnification: (a) –5 ℃; (b) –10 ℃; (c) –15 ℃; (d) –20 ℃; (e) –25 ℃
圖 16 高倍率下典型破壞試樣的SEM圖. (a) –5 ℃; (b) –10 ℃; (c) –15 ℃; (d) –20 ℃; (e) –25 ℃
Figure 16. SEM images of typical failure specimens at high magnification: (a) –5 ℃; (b) –10 ℃; (c) –15 ℃; (d) –20 ℃; (e) –25 ℃
圖 17 基于孔隙連通性的孔隙分類[49]
Figure 17. Pore space classification in accordance with connectivity
圖 18 端閉孔內凍脹力導致巖石破壞示意圖
Figure 18. Schematic of rock damage due to frost heaving pressure at the end closure hole
表 1 加載率與動態斷裂韌度擬合曲線參數
Table 1. Loading rate and dynamic fracture toughness fitting curve parameters
Temperature /℃ a b c R2 25 13.54 –8.7 412.74 0.8891 –5 10.97 –7.58 178.82 0.9504 –10 11.83 –8.52 192.34 0.9905 –15 12.11 –10.39 135.68 0.9582 –20 16.98 –13.92 261.68 0.9533 –25 13.26 –10.01 192.09 0.9461 Notes: R2 represents correction of fitting. 
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