| Due to the great difference between the filling materials and the rock properties on both sides of the joint,the propagation of stress wave in the filling joint is more complex.At present,the research on the defective rock mass mainly focuses on the fracture and interpenetration behavior of the defect,and ignores the influence of the diffraction wave at the end of the defect on the distribution of stress field.How the diffraction wave affects the crack propagation in the evolution process needs further analysis.In this paper,the super dynamic strain test method,dynamic caustics method and numerical simulation method are used to systematically study the propagation characteristics of explosion stress wave in joint filled medium and crack propagation behavior of defect medium,and the influence of stress field evolution characteristics under the effect of diffraction wave at defect end on crack propagation behavior is emphatically analyzed.It is of great significance to improve the utilization rate of explosion energy,improve blasting effect and control blasting vibration.The main contents and achievements are as follows:(1)Using polycarbonate plate to simulate rock material,the effects of joint thickness,angle and filling material on propagation velocity,attenuation and reflection of explosion stress wave on joint surface were obtained by super dynamic strain measurement system.The order of attenuation of explosive stress wave by filling material is from high to low is rubber,water,clay,cement mortar,epoxy resin.With the increase of joint thickness,the attenuation intensifies.Different filling materials have different contributions to the attenuation of explosion stress wave,and the sensitivity of natural attenuation to the variation of joint thickness is also different.The experimental results show that the attenuation degree of peak strain at different positions behind the joint is basically unchanged,and the wave impedance difference between the filling material and the media on both sides leads to the change of the reflected wave type at the joint interface.The relatively soft material generates the reflected tensile wave on the joint surface,and the hard material generates the reflected compressive wave,and the greater the wave impedance difference and the greater the joint thickness,the stronger the reflection intensity.As the joint is far away from the blasting source,the peak strain of the measuring point after the joint first increases and then decreases.By introducing the equivalent proportional distance,the relationship between the peak strain of the measuring point at any position and the joint position is fitted.The larger the incident angle of stress wave is,the more serious the attenuation of stress wave is,and the linear formula of normalized coefficient of strain peak and incident angle is obtained.(2)A numerical model for the propagation and attenuation of explosion stress wave in jointed media is established.The parameters of JWL equation of state of lead azide are obtained by using theγfitting method for the first time.It is discussed that the influence of the mesh size on the numerical results is not the absolute accuracy,but the relative accuracy.The established numerical model reproduces the electrical measurement results,and the attenuation law of strain waveform,propagation velocity and compression peak strain is basically consistent,which provides a numerical model basis for the analysis of explosion stress field in defective medium.Based on the verified model,the propagation law of diffracted and transmitted waves in small-scale joint model is studied.The results show that the smaller the joint length L is,the easier it is to cause the superposition of transmitted wave and diffracted wave in the propagation process,and finally the transmission intensity increases and the diffraction intensity decreases at a longer distance.The larger the L is,the weaker the diffraction effect is.The diffraction intensity first decreases,then increases,and then decreases with the distance.The shorter the joint is,the faster the attenuation of diffraction wave is.In the process of diffraction wave propagation,there will be a weak stress zone near the joint.The longer the L is,the larger the range is.(3)The dynamic caustics method is used to obtain the crack propagation behavior at the end of the defect in the model with two defects,and the influence mechanism of different charge amount,defect filling material and defect thickness on the crack propagation is clarified.The crack initiation toughness of the front defect end increases with the increase of charge,which show that the dynamic fracture toughness of high loading rate is different from the static problem,which depends not only on the properties of the material itself,but also on the loading rate and crack propagation rate.The peak value of K_Ⅰat the right end of the post defect is higher than that at the left end,which indicates that the right end absorbs more energy than the left end.With the decrease of charge,the crack initiation sequence at both ends of the post defect gradually changes from the first crack initiation at the left end to the simultaneous crack initiation at both ends,or even the first crack initiation at the right end.As the properties of the filler are closer to the matrix,the direction of the maximum tensile stress at the end of defect a is closer to the direction perpendicular to the propagation of explosion stress wave,and the energy accumulation ability and transmission strength at the end of defect A are weaker and stronger.The mechanical characteristics of crack propagation and the final propagation length are the result of diffraction wave,free surface condition,transmission wave and boundary reflection wave.However,the contribution of different types of waves in different filling media models is different,which leads to the difference of crack propagation velocity and stress intensity factor curve characteristics,but the difference of final crack propagation length is small.With the increase of the thickness of defect A,the crack propagation length decreases gradually.When the thickness is 6 mm,only one wing crack can be produced at the end of defect A,and the blasting main crack deflects to both ends of defect A to form a triangular crack area.The damage in this area is intensified.When the thickness is more than 4 mm,the peeling failure begins,which forms a good free surface for the crack propagation at the end of defect B and promotes the crack propagation.At the same time,the transmission wave intensity decreases with the increase of thickness.Under the interaction of the two aspect,the K_I mean value of the end crack of defect B first decreases and then increases with the increase of thickness.When the thickness is 2 mm,the crack propagation length at both ends of defect B is the smallest.(4)The caustics method is used to study the fracture behavior of the defect medium.Combined with the numerical simulation,the evolution law of the diffraction stress wave at the end of the defect with different length,angle and shape of the defect is analyzed,and the fracture mechanism of the crack at the end of the defect under the action of the diffraction wave is revealed.With the increase of defect A length,propagation length,crack initiation time and initial propagation angle of end crack increase,while the initial propagation velocity decreases,and the influence of crack propagation direction on the later propagation direction of defect B is weakened.Although defect A weakens the effect of transmission wave on defect B,the superposition effect of diffraction wave at the end of defect A with smaller length increases,which promotes the dynamic crack propagation behavior at the end of defect B.In addition,the existence of defect A changes the difficulty of crack initiation at both ends of defect B,making it easier to crack.The numerical simulation results show that with the increase of the connection angle between defect A and blast hole,the peak value of transmission intensity increases and the diffraction intensity decreases at the left end of defect B,and the opposite is true at the right end.The smaller the(?),the more favorable the order of diffracted wave action is to enhance the duration of stress field,thus increasing the overall stress level.Coupled with better free surface conditions,the dynamic crack propagaton behavior is enhanced.At the same time,the uneven distribution of stress field will lead to the serrated crack propagation at the end of defect B.The experimental results and numerical simulation show that the triangular tip is the best for diffraction propagation(the larger the tip angle is,the smaller the diffraction effect is),the semicircle is the second,the rectangle is the weakes.The crack initiation position of defect A remains unchanged,the rectangular end has the largest crack initiation toughness,and more energy accumulation enhances the dynamic propagation behavior of the end crack.The final crack propagaton length at the end of defect B is not completely dependent on the diffraction intensity,but also related to the change of crack initiation sequence caused by insufficient diffraction intensity. |
