Effects of freeze-thaw cycles on dynamic fracture initiation characteristics of surrounding rock with pure Ⅰ type fracture under impact loads
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摘要: 以寒区隧道为工程背景研究在冻融循环作用下围岩内Ⅰ型裂纹的动态起裂特性演化规律,采用隧道模型试件作为研究对象,开展冻融循环试验与大尺度落锤冲击试验,得到岩石试件经历不同冻融循环次数后的相关力学参数,并在裂纹尖端粘贴裂纹扩展计(crack propagation gauge, CPG)测量预制裂纹的动态起裂时间。采用有限元软件建立相应的数值模型计算裂纹尖端的动态应力强度因子,采用试验-数值法计算动态起裂韧度,随后采用电镜对冻融循环后的试样进行扫描,研究冻融循环对岩石材料的细观损伤机制。研究结果表明:随着冻融循环次数的增加,岩石材料的纵波、横波波速与弹性模量逐渐减小,而泊松比逐渐增大;砂岩材料的动态起裂韧度随着冻融循环次数的增加逐渐减小,表征线性反比例的特性;材料内部的胶结物质会由于冻融循环的影响而流失,材料的孔隙和裂纹也随着冻融循环次数的增加而变多变大。Abstract: In order to investigate the dynamic initiation and evolution of mode Ⅰ crack in surrounding rock under the action of freeze-thaw cycle, taking a cold area tunnel as the engineering background, the freeze-thaw cycle test and large-scale drop weight test were carried out by using tunnel model specimens that were made of green sandstone in Sichuan province. The dynamic mechanical characteristics of specimens after different freeze-thaw cycles were measured and discussed. The elastic modulus and Poisson’s ratio of the specimens were calculated by longitudinal wave velocity, shear wave velocity. The dynamic strain gauges were glued at the incident plate and transmitted plate to collect voltage signals. The voltage signal was applied to calculate the curves of dynamic loading versus time recorded from the incident plate and transmission plate. Crack initiation time was determined by using a crack propagation gauge (CPG) measuring system. A traditional finite element method code was applied to establish some numerical models to calculate the curves of dynamic stress intensity factor under impact loads. The experimental-numerical method was used to determine dynamic fracture initiation toughness according to crack initiation time. A scanning electron microscope (SEM) was applied to analyze the micro-structure of sandstone material after different freeze-thaw cycles, and the mesoscopic damage mechanism of rock materials was obtained. The test results show that the longitudinal wave velocity, shear wave velocity and elastic modulus of sandstone gradually decrease with the number of freeze-thaw cycles, while Poisson’s ratio increases with the number of freeze-thaw cycles. The crack initiation time and dynamic initiation toughness of rock material decrease with the number of the freeze-thaw cycles. The cement material inside the rock will loss due to the effect of freeze-thaw cycles, and the pores and micro-cracks of the sandstone also increase with the number of freeze-thaw cycles.
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表 1 5组试件材料的力学参数
Table 1. Mechanical parameters for five groups of specimen
冻融次数 孔隙率/% 泊松比 弹性模量/GPa 纵波波速/(m·s−1) 横波波速/(m·s−1) 纵波波速降/% 横波波速降/% 0 12.52 0.262 12.56 2 562 1 455 0 0 10 12.96 0.270 10.50 2 378 1 334 7.2 8.3 20 13.35 0.276 9.14 2 254 1 253 12.0 13.9 30 13.87 0.283 8.10 2 153 1 185 16.0 18.5 40 14.51 0.286 7.31 2 085 1 141 18.6 21.6 表 2 冲击试验结果
Table 2. Impact test results
试件 冲击速度/(m·s−1) 起裂时间/μs 动态起裂韧度/(MPa·m1/2) 动态加载率/(MPa·m1/2·s−1) 0-1 6.56 362 3.09 10 404 0-2 6.59 374 3.29 10 716 0-3 6.61 367 3.18 10 603 10-1 6.48 375 2.87 9 410 10-2 6.53 371 2.77 9 082 10-3 6.48 366 2.66 8 866 20-1 6.57 381 2.54 8 274 20-2 6.51 389 2.75 8 730 20-3 6.56 386 2.66 8 445 30-1 6.52 393 2.41 7 670 30-2 6.56 397 2.51 7 952 30-3 6.59 390 2.31 7 549 40-1 6.56 395 2.11 6 762 40-2 6.50 392 2.04 6 623 40-3 6.47 399 2.21 7 015 -
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