| The dynamic behaviors of rock under impact and blasting load include the mechanical response and fracture control mechanism of rock,as well as the crack propagation behavior and stress wave fluctuation characteristics in rocks.The accurate determination of fracture mechanical parameters of propagation crack in rock is of great significance for rock directional fracture,rock blasting optimization and surrounding rock stability research.Based on the theoretical analysis and laboratory tests,the strain gauge method is applied to calculate and analyze the dynamic fracture mechanics parameters of type-? crack under impact and blasting dynamic load.Some experiments are supplemented by dynamic caustics method.The main contents and conclusions of the research are as follows:(1)The strain gauge method was proposed to measure the dynamic fracture mechanics parameters of type-? crack in rock materials.Based on the linear elastic fracture mechanics theory,the stress field in the vicinity of the propagation type-?crack was characterized by the Westergaard complex function.When the orientation angle between strain gauge and the crack path was at specific acute and obtuse angles,the normalized strain around a propagation crack could be clearly described by a two-parameter formula,which was related to the velocity of propagation crack and the location of the strain gauge.Both the determining process of the corresponding coefficient,which was based on the characteristic time of the two-parameter formula in the vicinity of crack tip,and dynamic stress intensity factor(DSIF)formula were given.In order to provide a theoretical foundation for improving the calculation accuracy of type-? crack propagation velocity and DSIF in future experiments,the influence of the coefficient of the two-parameter formula,such as strain gauge distance,strain gauge orientation,crack propagation velocity and Poisson's ratio,on theoretical calculation curve was carefully analyzed.(2)Combining with the adaptability experiments of strain gauge method for three-point bending beam under impact load,the variation characteristics of dynamic fracture parameters of crack under different test conditions were obtained.The running speed obtained from both the acute and obtuse strain gauge method and DSIF calculated from the obtuse strain gauge method were close to results from the caustics experiments that conducted simultaneously.In addition,the difference between DSIF of the dynamic caustics method and the obtuse strain gauge method was small in the middle of the specimen,and was large in other positions.The correction coefficient of DSIF for obtuse angle strain gauge method was given.When the strain gauge was pasted within 0.6 times the thickness distance of the specimen near the crack tip,the DSIF was in good agreement with the dynamic caustics experiment results.With the increase of specimen thickness,the greater the distance between strain gauge and propagation crack,the lower the accuracy of DSIF.The calculation results of crack propagation velocity were not affected by specimen thickness.(3)For the first time,the strain gauge method was used to reveal the change law of dynamic propagation velocity and DSIF of type-1 crack in opaque rock materials under impact load.Combining with the digital high-speed camera technology and strain gauge method,it was found that the rock fracture was characterized by instantaneous release of strain energy at crack tip during damage propagation of white spot microcracks.The scanning electron microscopy(SEM)images of the crack fracture surface showed that the crack propagated in a straight line,and the tensile stress was dominant.The rock fracture section exhibited propagation fracture characteristics of type-? crack.The strain near the crack tip of rock specimens was measured to be between 364 and 536 ??,which was significantly lower than the strain at the crack tip of Polymethyl methacrylate(PMMA)(between 3870 and 5312 ??).The crack propagation velocity of granite rock was between 475 and 687 m/s,and the DSIF vwas between 4.26 and 6.12 MN/m3/2,which were larger than the dynamic fracture parameters of PMMA material.(4)The action of slotted cartridge blasting mainly contained two stages.In stage ?,when the detonation wave acted on the borehole wall,a large number of micro-cracks appeared in the wall.Due to the blasting energy concentrated by the directional charging effect of the slotted cartridge charge,the micro-crack in the direction of the slotted cartridge tended to generate further.In stage ?,under the action of the "gas wedge effect,of the explosive gas,the blasting induced crack propagated at a high velocity,and the crack propagation velocity fluctuated slightly.The crack continued to propagate at the original velocity after the action of reflected stress wave.Firstly,the strain gauge at the measuring point was subjected to blasting expansion wave,and the strain of rock was much smaller than that of PMMA.Then the propagation crack continued to propagate.The ultimate crack propagation velocity of rock reached 922 m/s,and the ultimate DSIF was much larger than the crack crack propagation velocity(457 m/s)and DSIF of PMMA specimen.(5)Aiming at the propagation behavior research of type-? crack at the tip of a single prefabricated crack under the slotted cartridge blasting load,the strain gauge method was applied to analyze the changing characteristic of crack propagation velocity and DSIF,and the dynamic caustics method was carried out simultaneously and comparatively.Under the action of the blasting stress wave,the strain measured by the specific orientation angle strain gauge was negative,indicating that the tested points exhibited compressive strain under the action of blasting wave.As the distance of the tested points increased,the strain peak decayed.After the main crack penetrated the defect,the type-? branch crack initiated from the tip of the defect.The fracture mechanics parameters of type-? branch crack measured by strain gauge method and dynamic caustics method were consistent.For the sum length of the main crack and the branch crack induced by single borehole,one side single horizontal prefabricated defect specimen>intermediate single vertical prefabricated defect specimen>intermediate double vertical prefabricated defects specimen. |
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