人字形切槽巴西圆盘Ⅰ型渐进断裂数值模拟研究

A NUMERICAL STUDY OF MODE-I PROGRESSIVE ROCK FRACTURE PROCESS OF CRACKED CHEVRON NOTCHED BRAZILIAN DISC

对国际岩石力学学会(ISRM)建议的I型断裂韧度测试人字形切槽巴西圆盘(CCNBD)试样进行细观损伤力学模拟,考虑不同切槽宽度、试样几何、力学参数与材料非均质性等,直观获悉其渐进断裂过程。通过模拟的应力和声发射分布,对CCNBD量纲一的应力强度因子标定基于的穿透直裂纹等假设进行严格评估。结果表明:无论岩石材料是否均质,人字形韧带中的裂纹前缘均为曲线形,并不符合传统方法基于的穿透直裂纹假设,而且人字形韧带越狭长的试样,裂纹前缘弯曲越严重;利用模拟得到的临界裂纹保守地标定临界量纲一的应力强度因子,发现传统方法所得结果比真实值偏小5%~10%;CCNBD易从人字形韧带尖端厚度边缘起裂,为了使真实断裂路径更符合测试原理,建议制作和选取较小切缝宽度的试样。数值模拟结果表明,应该重视测试试样的真实渐进断裂机制,量纲一的应力强度因子标定宜考虑更真实的断裂特征,以获取准确的断裂韧度值。

The cracked chevron notched Brazilian disc(CCNBD) method,which is suggested by the International Society for Rock Mechanics(ISRM) for measuring mode-I fracture toughness of rocks, was numerically investigated by a code based on microscopic damage mechanics. Varying notch width,sample geometries,material parameters and inhomogeneity are considered,and the progressive fracture process of the CCNBD specimen was intuitively presented. Through the simulated stress distributions and acoustic emission activities,the straightthrough crack assumption(STCA) was strictly assessed,which is used by traditional methods for calibrating the dimensionless stress intensity factor(SIF). Numerical results show that,crack fronts of CCNBD specimens are always curved regardless of rock heterogeneity,significantly violating the STCA. For the specimen with a more slender chevron ligament,the crack front is curved more severely. Traditional critical SIFs are found to be only 90 to 95 percent of the values that are calibrated by utilizing the simulated critical crack fronts. CCNBD samples always crack from two sides of the notch width of the chevron ligament tip,and thus the specimens with relatively smaller notch width are suggested,so that the realistic fracture route of CCNBD can align well with the ideal test principle. The numerical results call for more attention to the real fracture mechanism of the chevron notched samples. A better calibration of dimensionless SIFs should consider more realistic fracture characteristics for a better measure of fracture toughness.