مطالعه رفتار شكست دینامیک ترک مدل I تحت بارهای انفجاری
Abstract
1- Introduction
2- Experimental study
3- Numerical simulation
4- Critical dynamic SIFs
5- Conclusions
References
Abstract
Dynamic fracture behavior under impacting loads has been well studied, but for that under blasting loads, less attention has been paid. In order to investigate mode I crack propagation behavior of brittle materials under blasting, a new configuration specimen, i.e. single internal crack circular disc (SICCD) specimen was proposed in this paper, and it was applied in the blasting experiments. Crack propagation gauges (CPGs) were stuck along crack propagation paths to measure crack initiation and propagation time and crack propagation speeds. Green sandstone and PMMA were selected to make the SICCD specimens. Finite difference models were established by using AUTODYN code according to the SICCD specimen dimension and the loading curve measured near the borehole. Generally, the simulation results of crack propagation paths agree with the test results. Finite element code ABAQUS was applied in the calculation of dynamic stress intensity factors (SIFs), and the curves of dynamic SIFs versus time were obtained. By using these curves and the breaking time of the CPG wires, the mode I critical dynamic SIFs in initiation and in propagation were obtained. The results show that the measuring method of the critical dynamic SIFs of brittle materials under blasting presented in this paper is feasible and applicable. During crack propagations, the crack speed is not a constant, and the critical dynamic SIFs in propagation decreases with the increase of crack propagation speeds.
Introduction
As a traditional rock breaking method, fragmentation by explosive has the property of low cost and easy operation [1], and therefore, it is still a widely applied rock excavation method in mining, quarrying and tunneling. With increasing scale of such operations, proper designs of blasts and precise predictions of blasting results have become imperative in most operations. However, our understanding of the blasting process and the mechanism of blast-induced rock failure is only in the preliminary stage, as both the commercial explosives and the target rock are complex materials. The energy release characteristics in the former are highly variable, depending on the prevailing field parameters such as borehole diameter, density gradient, and sympathetic pressures from the detonation of neighbouring holes [2]. Similarly, the response of the target rock to high dynamic loading, which may last only for a few milliseconds, remains largely unknown. Cracks exist widely in brittle materials, and under nearby blasting, the cracks may initiate and propagate [3], which may help to enhance rock fragmentation efficiency, but on the other hand, it may induce large geotechnical disasters, such as rockburst. Therefore, it is essential to implement the corresponding experimental and numerical study on crack propagation behavior under blasting, and the measuring method of the critical dynamic SIFs in initiation and in propagation of rock under blasting is one imperative task. This is because the critical dynamic SIFs is a threshold value which can be used to predict crack dynamic behavior so to predict cracked rock structure stability. Currently, the study on rock dynamic fracture mainly focuses on impact loads, such as split Hopkinson pressure bar (SHPB) impacts and drop weight impacts, and many significant results have been achieved. With the development and improvement, the SHPB test system has been widely applied in the study of rock dynamic fracture [4–10]. However, the SHPB test system also has a disadvantage, i.e. the diameter of the pressure bar is not large enough, and therefore, for large size specimens, it is not suitable [11,12]. For small size specimens, the reflected tensile stress waves may reach crack tip to affect crack dynamic behavior during crack propagation [13]. Therefore, in dynamic experiments, the specimen size must be large enough so that as the reflected tensile stress wave from specimen boundary reaches crack tips, the crack dynamic activity have completed already.