Diyuan Li

Dynamic Strength and Fracturing Behavior of Single-Flawed Prismatic Marble Specimens Under Impact Loading with a Split-Hopkinson Pressure Bar

Dynamic impact tests are performed on prismatic marble specimens containing a single flaw using a modified split-Hopkinson pressure bar device. The effects of pre-existing flaws with different flaw angles and lengths on the dynamic mechanical properties are analyzed. The results demonstrate that the dynamic strength of marble is influenced by the flaw geometry. The dynamic fracturing process of flawed specimens is monitored and characterized with the aid of a high-speed camera. Cracking of marble specimens with a single pre-existing flaw under impact loading is analyzed based on experimental investigations. Cracking involves two major stages: formation of white patches and development of macrocracks. Six typical crack types are identified on the basis of their trajectories and initiation mechanisms. The presence of an artificial flaw may change the failure mode of marble from splitting-dominated for an intact specimen to shear-dominated for a flawed specimen under dynamic loading. Nevertheless, the geometry of the flaws appears to have a slight influence on the failure modes of flawed specimens under impact loading.

Experimental and Numerical Investigations on Feasibility and Validity of Prismatic Rock Specimen in SHPB

The paper presents experimental and numerical studies on the feasibility and validity of using prismatic rock specimens in split Hopkinson pressure bar (SHPB) test. Firstly, the experimental tests are conducted to evaluate the stress and strain uniformity in the prismatic specimens during impact loading. The stress analysis at the ends of the specimen shows that stress equilibrium can be achieved after about three wave reflections in the specimen, and the balance can be well maintained for a certain time after peak stress. The strain analysis reveals that the prismatic specimen deforms uniformly during the dynamic loading period. Secondly, numerical simulation is performed to further verify the stress and strain uniformity in the prismatic specimen in SHPB test. It indicates that the stress equilibrium can be achieved in prismatic specimen despite a certain degree of stress concentration at the corners. The comparative experiments demonstrate that the change of specimen shape has no significant effect on dynamic responses and failure patterns of the specimen. Finally, a dynamic crack propagation test is presented to show the application of the present work in studying fracturing mechanisms under dynamic loading.

Failure Characteristics of Granite Influenced by Sample Height-to-Width Ratios and Intermediate Principal Stress Under True-Triaxial Unloading Conditions

The failure modes and peak unloading strength of a typical hard rock, Miluo granite, with particular attention to the sample height-to-width ratio (between 2 and 0.5), and the intermediate principal stress was investigated using a true-triaxial test system. The experimental results indicate that both sample height-to-width ratios and intermediate principal stress have an impact on the failure modes, peak strength and severity of rockburst in hard rock under true-triaxial unloading conditions. For longer rectangular specimens, the transition of failure mode from shear to slabbing requires higher intermediate principal stress. With the decrease in sample height-to-width ratios, slabbing failure is more likely to occur under the condition of lower intermediate principal stress. For same intermediate principal stress, the peak unloading strength monotonically increases with the decrease in sample height-to-width. However, the peak unloading strength as functions of intermediate principal stress for different types of rock samples (with sample height-to-width ratio of 2, 1 and 0.5) all present the pattern of initial increase, followed by a subsequent decrease. The curves fitted to octahedral shear stress as a function of mean effective stress also validate the applicability of the Mogi–Coulomb failure criterion for all considered rock sizes under true-triaxial unloading conditions, and the corresponding cohesion C and internal friction angle φ are calculated. The severity of strainburst of granite depends on the sample height-to-width ratios and intermediate principal stress. Therefore, different supporting strategies are recommended in deep tunneling projects and mining activities. Moreover, the comparison of test results of different σ2/σ3 also reveals the little influence of minimum principal stress on failure characteristics of granite during the true-triaxial unloading process.

Reply to Comment by Saffet Yagiz on “Point Load Test on Meta-Sedimentary Rocks and Correlations to UCS and BTS” by Diyuan Li and Louis Ngai Yuen Wong, Rock Mechanics and Rock Engineering, doi:10.1007/s00603-012-0299-x

Uniaxial compressive strength (UCS) tests were conducted also at the Nanyang Technological University (NTU) on rock blocks collected from the same site under oven-dried and water-saturated conditions (Ma and Wu 2010). In our study, the point load tests were only conducted on dry specimens. Therefore, only the UCS values under dry condition were considered in the correlation study. Brazilian tensile strength (BTS) tests were conducted by another researcher under the supervision of the second author at NTU. The results have not been published yet. Table 1 summarizes the test results of mean BTS values and corresponding standard deviations, where the influence of anisotropy of the meta-sedimentary rocks was taken into consideration. In the first manuscript submission, we used the average values of BTS tested under dry condition in both longitudinal and transverse bedding orientations, which were 8.99 and 7.94 MPa for meta-siltstone and meta-sandstone, respectively, as shown in the main text of the paper. When we revised the manuscript after receiving the reviewers’ comments, we replaced the above values by the dry BTS values tested in transverse bedding orientation since the point load tests were mostly conducted in transverse bedding orientation. We revised the corresponding BTS values in Table 3 of the published paper, which were 8.41 and 8.88 for meta-siltstone and meta-sandstone, respectively. We mistakenly forgot to revise the corresponding values in the text of the paper, hence leading to confusion.