Issue 49

C.Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 557-567; DOI: 10.3221/IGF-ESIS.49.52 560 Brittleness evaluation method based on hardness Hucka and Das proposed a rock brittleness evaluation method based on the difference between rock micro-hardness and macro-hardness. Lawn and Marshall [24] established the brittleness index B 18 for the ceramic engineering field. Quinn and Quninn [25] described an index B 19 to measure the rock brittleness based on the ratio of the deformation energy per unit volume to the fracture surface energy per unit area. This brittleness evaluation index considers too few factors and is only applicable to the ceramic field, so its accuracy and applicability should be further considered in applications. R OCK BRITTLENESS INDEX BASED ON STRESS - STRAIN DROP AND PEAK STRAIN n the brittleness evaluation in hydraulic fracturing and rockburst prediction, the existing brittleness evaluation methods consider only a few mechanical parameters. What is more, many of them are only applicable to uniaxial load conditions, and not suitable for high surrounding rock pressure in deep tunnel construction. The stress-strain curve, on the other hand, reflects the whole process of the rock from deformation failure to the ultimate loss of bearing capacity under external load, and is applicable to the state analysis of rock failure under surrounding rock pressure. Based on the stress-strain curve of the rock failure, quantitative brittleness parameters can be obtained. Therefore, the post-peak stress-strain shape obtained in the laboratory is the main method for researchers to qualitatively understand the rock brittleness. Based on the above, this paper proposes a brittleness evaluation method based on post-peak stress drop rate and pre-peak brittle failure. Figure 1 : Simplified stress-strain curve In Fig. 1, the polyline OABC is a simplified Class I stress-strain curve. Point A ( , ε ) corresponds to the peak point, and and ε are the peak intensity and the peak strain, respectively; point B ( , ε ) corresponds to the residual point, and and ε are the residual stress and the residual strain, respectively. It is obvious in Fig. 1 that the polyline OABC is divided by point A ( , ε ) and point B ( , ε ), so that the corresponding mechanical parameters can be quantitatively obtained. The drawbacks of the brittleness indices B 10 - B 11 are already discussed above. Based on these two methods, this paper proposes a new method L1 , which considers both the stress drop B 8 and the strain drop B 9 . At the same time, the faster the stress drop rate, the higher the brittleness, so the difference between the peak stress and the residual stress is proportional to the brittleness index and the difference between the peak strain and the residual strain is inversely proportional to the brittleness index. In addition, in order to emphasize the final increase of the post-peak strain, the residual strain is used to replace the peak strain in the denominator of the brittleness index B 9 . First, the post-peak brittleness index L1 is: L1 = σ p −σ r σ p ε r −ε p ε r (1) I