| The rock blasting theory includes two parts. One is the dynamic action of explosive stress wave, the other is the quasi static effect of explosive gas. Presently, the explosive rock burst is the confluence of both of them, which only differs in their respective role under the condition of blasting parameters and loading. So, people must consider their contribution to the rock crushing and improve when studying the rock blasting theory and improve the blasting effect and deepen the blasting theory. Based on the above considerations, this paper respectively conducts test research on the dynamic action of explosive stress wave and the quasi static effect of explosive gas, in the meanwhile, this paper also combines with the numerical simulation of DLSM and analyzes the propagation process of dynamic crack. The research result of this subject will offer theoretical basis to the directional fracture controlled blasting.The research contents of this paper includes the following aspects:1. Based on the test method of NSCB, the author uses Hopkinson bar test system and combines with the technical methods like high speed photography, DLSM numerical analysis, SEM scanning electron microscopy and P wave velocity measurement to study the dynamic fracture toughness of several typical rock materials like sandstone under the conventional and special state and finds that:The dynamic fracture toughness of rock materials shows obvious loading rate dependence. With the increase of the loading rate, the dynamic fracture toughness of of rocks shows a gradually increasing trend. In the test, the author finds that under the same loading rate, the fracture toughness of granite is highest with that of coal lowest, the sand stone and the mud stone are relatively similar with each other and the fracture toughness of plexiglass is lower than three kinds of rocks but higher than coal.The numerical analysis of DLSM has gained the similar result, but the loading surface has important effects on test result. The ideal linear loading is not suitable for the test research on the dynamic fracture toughness of rock materials whereas the calculation result of free surface loading and 5mm surface loading matches the test well. In the meanwhile, the constraint condition of bottom support also has impacts on the test.In the test of fracture toughness of sandstone after high-temperature processing, the author finds that at the same heat treatment temperature, fracture toughness shows increasing linear trend with the change of loading rate. Particularly, when the loading rate is low, fracture toughness of every heat treatment temperature is relatively similar but when the loading rate is higher, fracture toughness differs much with each other and the slope of curve of fracture toughness becomes smaller as the temperature of heat treatment increases.The test of dynamic fracture toughness bedding containing coal finds that with the increase of joint angle, “dynamic fracture toughness” shows a decreasing trend but not shows the linear decreasing relationship. The natural bedding structure with different occurrence state and mineral compositions is unevenly distributed, which will influence the test result.2. The author uses the method of dynamic caustic test digital laser(DLDC) to make different charge structure of slit charge blasting test and reveals that slit charge decouples charge blasting gas static mechanism. In the meanwhile, the author also utilizes the explicit dynamic analysis program LS-DYNA to simulate the slit charge blasting and the formation of the initial crack and discusses the relationship between decoupling coefficient and the blasting damage.The decoupling coefficient has obvious effects on the burst crack.When α1 is1.67, the length of the main crack and the number of crack are best. The explosive stress wave and explosive gas has impacts on the propagation of the crack. In the test of taking clay as the medium, the stress intensity factor and the speed changes little. The medium of clay, as the buffer layer between the explosive product and the hole wall will increase the energy transfer, prolongs the action of stress wave and broaden the scope of explosion.The dynamic energy release rate of the crack tip is smaller than two main cracks. The energy along the slit direction of slit charge wall release first, which makes the radical crack in the direction of hole cutting seam expand rapidly by a strong tensile stress so as to inhibit the expansion of the crack in the non-tangential direction.The result of numerical simulation indicates that the air is not coupled with explosion and the high strength compressive stress produced in the solid medium exceeds its compressive strength, it will form a crushing area in the hole wall with small area but high energy consumption. In order to avoid the formation of crushing area and make the energy produces by explosion be more used for the crack propagation in the direction of crack. From the perspective of improving directional breaking blasting effect, the author concludes that the charge efficient of decoupling is about 1.67, which is in line with the result obtained from the experiment.3. The author proposes a simple dynamic and static separation device, makes wave characteristics analysis and frequency analysis based on energy on the strain wave obtained from the test of plug hole and open blasting cement mortar and finds that:In the process of rock breaking of slit charge action, at the measurement point with equidistance to the blast hole, the strain peak in the direction of the cut is largest and the direction of non-slit is second with the direction of 135° third and 45° the smallest. The changing trend of maximum strain rate is the same. With the increase of the ratio of distance in the cutting direction, the strain peak and maximum strain rate gradually decrease and the strain peak of plug hole blasting with same proportional distance is higher than that of open hole. When the general charge blasts, the peak of tensile strain and the peak of compressive strain tested in the plug hole test is two times as much as that of open hole.In the test of rock breaking of slit charge action, at the measurement point with equidistance to the explosion source, there emerges similar low frequency band concentrating at 292.8-448 Hz. The angles formed by measurement points and the cut are different, which to a degree, influences the propagation of stress wave and causes the uneven distribution of high frequency bands. In the comparative test of dynamic and static effect of slit charge, with the increase of proportional distance, the band gradually concentrates and the vibration frequency of measurement point is obviously higher than that of open hole. In the comparative test of dynamic and static effect of general charge blasting, the frequency bands of measurement point has similar distribution characteristics but has no abrupt low frequency and high frequency band, which indicates that the general charge blasting has little impact on the band distribution.4. Through using the caustic test and numerical analysis DLSM and studying the crack penetration mechanism two holes blasting of dynamic and static effect, the author finds that:When the two holes explode at the same time, the holes going through the crack tip does not meet directly but up and down and continue to expand and move toward the existing crack of different directions. The dynamic stress intensity factor KI rapidly becomes smaller from the initial value and after repeated oscillation, the value of KI gradually increase and reaches the second peak, then it begins to become decrease. In the process of crack propagation, KII is basically smaller than KI. The dynamic stress intensity factor of crack AI and BI is greater than that of outside crack AO and BO. The crack growth velocity and acceleration are presented with the fluctuation of wave and the tensile stress wave and the reflected wave reflected from the interface diffract and scatter at the crack tip, which produces impacts on the crack propagation. The peak values of velocity and acceleration occur alternately with the acceleration reaches the peak first and then the velocity reaches the peak. The dynamic energy release rate decreases rapidly from the maximum value and gets to the peak again after vibration and then gradually decreases.Cutting and charge structure will have impacts on the double cracks of two holes, which are mainly manifested in that compared with the cutting hole, the crack length is shorter than that of hole in the cutting hole, the velocity is lower and the stress intensity of crack tip is lower.After the test specimen containing the defected medium is exploded, the main crack is not at the intersection of the crack section without interaction with each other, which indicates that the defect has important impacts on the directional fracture controlled blasting effect.The numerical analysis DLSM shows that different loading rates produce different types of crushed zones and radical tensile fracture. When the peak stress wave exceeds the dynamic compressive strengthen of the medium, there emerge crushing area nearby the explosive hole. In the crushing area, due to higher energy dissipation rate, the particles are damaged. After the crushing area is formed, the radial crack will inform. The crushing areas formed b the same segment blasting with different blasting hole spacing are different.5. DLDC test of crack propagation of defect medium under the dynamic loading. In caustics test, the oscillation of dynamic stress intensity factors at the tip of main crack increases to the peak value, then decreases and then increases to the maximum. And at that moment, the main crack starts to fracture. Before the cracking of secondary crack, KI has oscillation and variation. After cracking, the secondary crack rapidly increases and after that the oscillation decreases. With the increasing of angle of inclination of prefabricated crack, the trajectory of main crack gets more and more deviated from the vertical direction and it becomes bent while the trajectory of secondary cracks that impenetrate the entire specimen becomes more and more flat. At that moment, the peak value of dynamic stress intensity at crack tip will be greater with the increase of angle of inclination. After cracking, the speed of main crack will quickly decrease. The propagation rate of secondary crack will firstly increase and then will be reduced with oscillation, which will keep consistent with the change trend of KI.DLSM simulation result and the experimental result are well coincident. The cracks in specimen firstly start from the front of gap at the bottom and then gradually expand to prefabricated defects. Afterwards, the cracks will fracture from one end of prefabricated defects, impenetrate the entire specimen and divide it into two and a half. The change of angle of inclination of prefabricated defects has little impact on the propagation of initial main crack. For the propagation rate of secondary crack, there are the change characteristics which show that this rate will decrease with the increase of angle of inclination of prefabricated defects. But the propagation rate of secondary crack under the condition of horizontal prefabricated defects is far higher than that under the condition of tilting defects. The existence of prefabricated defects has changed the propagation path of stress wave in specimen. The impact of support constraints on the extension rate of cracks under shock load is not so big. But just under the condition of "Two supports are entirely smooth in X direction and have restraint conditions in Y direction", the cracking time of secondary crack is relatively early and its extension rate is bigger. In heterogeneity simulation, the extension speed of main crack and secondary crack presents the oscillation change of "high- low- high". Moreover the greater the density of impurities is, the more obvious this kind of oscillation is. |
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