人字形切槽巴西圆盘岩石试样复合型断裂渐进过程数值模拟研究

Numerical analysis of mixed mode progressive rock fracture mechanism of cracked chevron notched Brazilian disc specimens

人字形切槽巴西圆盘(CCNBD)岩石试样由于诸多优点,被国际岩石力学学会确定为岩石I型断裂韧度测试建议方法,并被众多学者应用于复合型(包括纯Ⅱ型)断裂试验研究。然而,CCNBD在复合型荷载下的渐进断裂机理尚未完全获悉,复合型断裂韧度测试基于的穿透直裂纹扩展假设并未严格评估,用于复合型断裂研究的合理性还未得到有效验证。采用细观损伤力学软件首次模拟得到CCNBD在复合型荷载下的渐进破坏过程。结果显示:裂纹不仅萌生于人字形韧带尖端,且易产生于切槽边缘,造成真实裂纹前缘为曲线形,与人字形切槽试样断裂韧度测试所基于的穿透直裂纹假设不符;裂纹并非沿着预制人字形韧带平面扩展到其根部,切槽边缘的破裂方向均朝向加载端,形成扭曲的三维翼形裂纹。数值模拟结果同物理实验对比,十分吻合。CCNBD的裂纹形态和渐进扩展规律不符合当前普遍采用的复合型测试原理,因此采用CCNBD试样进行岩石复合型(包括纯Ⅱ型)断裂韧度测试值得商榷。

The cracked chevron notched Brazilian disc（CCNBD） method, due to its distinct advantages, has been suggested by ISRM for mode I fracture toughness determination, and later applied extensively to mixed mode fracture researches（pure mode II included）. When the CCNBD is applied to mixed mode fracture researches, however, the progressive fracture mechanism and relevant hypotheses have not been thoroughly assessed, and the rationality of mixed mode fracture tests using CCNBD is under suspect. In this paper, the progressive rock fracture mechanism of CCNBD rock samples under mixed mode conditions is first simulated using the meso-scale damage mechanics analysis code. The results show that the fracture initiates not only from the notch tip but also from the saw-cut chevron notch edges, as a result, the crack front is significantly curved, inconsistent with the straight through crack assumption. Furthermore, cracks fail to grow stably to the base of the notch, but depart from the pre-cracked notch plane and develop towards the loading direction, forming a couple of twisty 3D-wing cracks. The results from simulation correspond very well with those from our experiments. Both the fracture pattern and the growing trend of CCNBD under mixed mode conditions are inconsistent with the present hypotheses, which suggests that the application of the CCNBD specimens to the mixed mode（including pure mode Ⅱ） fracture tests is questionable.