Dynamic mechanical properties and fracture processes of rocks is an important consideration in geophysical processes and engineering applications such as civil works and drilling operations. Therefore, to prevent unsafe working conditions and the occurrence of hazards, efficient methods for the determination of mechanical properties and fracture processes in rock materials are highly desirable. Among the available measurement methods, dynamic compression tests, semi-circular bend tests, and Brazilian disk tests are commonly used in various experiments involving Split Hopkinson Pressure Bar loading. This can be attributed to its reliability in investigating the dynamic relationship of rocks at high strains. Additionally, recent studies have shown that rock fracture is majorly caused by compression strength and thus should also be considered during the investigation of the dynamic properties of rocks for effective disaster prevention mechanisms.
Tensile strength testing of rocks can be grouped into direct tensile strength and indirect tensile strength. However, the latter is commonly used in analyzing dynamic mechanical properties due to the limitations of the direct method. Unfortunately, indirect measurement methods produce stress-strain curves that do not accurately represent the impact failure process due to the influence of crack initiation, evolution, and propagation on the dynamic behavior of rocks. Alternatively, numerical simulation methods have been adopted to verify various experimental results under different loading conditions.
Recently, a group of researchers at the China University of Mining and Technology (Beijing): Dr. Dihao AI, Yuechao Zhao, Dr. Qifei Wang, and Prof. Chengwu LI investigated and analyzed the dynamic properties, crack propagation and their relationship and influence on the dynamic properties of rock materials under different impact loading velocities. The study was based on laboratory experiments and simulation approach under the Split Hopkinson Pressure Bar. The work is currently published in International Journal of Impact Engineering.
Briefly, the study entailed simultaneous capturing of the stress wave signals and fracture process videos using dynamic strain gauge and high frame rate camera respectively. Secondly, the relationship between the crack propagation law and dynamic mechanical properties was analyzed at high strain rate using the wave theory and image processing technique. Eventually, they analyzed the stress-strain characteristics and crack propagation features.
The authors observed that the increase in the impact velocity resulted in the rise of crack propagation rate along the X-Y direction due to the larger strain along the horizontal axis than the vertical axis. Additionally, the two parameters i.e. vertical and horizontal axis strains describing the propagation rate of the central crack satisfied the linear growth function. For particular loading conditions, the quantitative relationship was exhibited between the stress-strain state and the crack area on the rock surface.
In summary, the scientists performed an experimental and numerical investigation of the dynamic properties and crack propagation using indirect tension tests. The results were validated using a Peridynamic theory-based numerical model through Brazilian disk simulation in an indirect tension test. Interestingly, the model accurately simulated the crack initiation, propagation and penetration morphology. Based on the obtained results, the study will advance investigation of the fracture behavior and mechanism of rocks materials under high strain impact loading.
Ai, D., Zhao, Y., Wang, Q., & Li, C. (2019). Experimental and numerical investigation of crack propagation and dynamic properties of rock in SHPB indirect tension test. International Journal of Impact Engineering, 126, 135-146.Go To International Journal of Impact Engineering