Experimental Study On Explosive Process And Dynamic Fracture Mechanics Properties With Linear Charge Blasting

For rock breaking mechanism of blasting,concentration charge was used to analyze explosion stress field distribution and dynamic mechanical response of the blasting medium.At present,people have studied the mechanism of blasting rock breaking for concentration charge and received many research achivements.However,linear charge was widely used in blasting engineering,the blasting stress wave propagation,attenuation law and crack propagation patterns differ greatly from concentration charge.The study of rock blasting mechanism based on the propagation law of stress wave and the mechanism of rock breaking.Therefore,the explosive stress wave propagation and attenuation law of linear charge,the influence of detonation position,the length-diameter ratio and static pressure to linear charge explosive stress field,and dynamic fracture characteristics of three-dimensional blast-induced cracks were studied by theoretical analysis,experimental research and numerical simulation.The research results provide theoretical basis for deepening the blasting theory and improving the understanding of the process of linear charge blasting.?1?Using dynamic caustic method,the semi-circular specimen containing pre-cracks with different angles was subjected to the drop hammer impact test,the fracture toughness of mode I fracture and mixed mode I-II fracture of Polymethylmethacrylate?PMMA?were achieved.The dynamic fracture characteristics of PMMA with holes and open joints under dynamic loading were analyzed.Under low drop impact loading rate,fracture toughness K_IC was 2.04 MN/m^3/2 for mode I fracture,and fracture toughness K_IC gradually decreased for mixed mode I-II fracture,but fracture toughness K_IIC gradually increased.Under the same impact loading,during the crack transformation from pure mode I to mixed mode I-II for PMMA,and the time-interval before crack initiation increased,but the time-interval decreased from crack initiation to crack arrest,the average velocity of crack propagation increased,indicating that mixed mode I-II fracture need more energy than pure mode I fracture.When dynamic crack extends to empty holes,empty holes have an inhibitory effect on the velocity of the dynamic crack and the stress intensity factor at the crack tip.When dynamic cracks run through empty hole,due to the influence of the holes,the crack tip was"passivated",dynamic crack arrest,the energy accumulated at the top of the hole.When the dynamic crack propagated again above the hole,the fracture toughness increased by more than 60%compared with the stress intensity factor before penetrating the hole.When dynamic cracks propagated to defects,defects changed the fracture behavior of dynamic cracks.The initiation at defects need less time,when it was collinear defect,and propagated as mode I fracture;the initiation at defect need more time,when it was vertical defect,and propagated from mode I-II fracture to mode I fracture.?2?Ultra-dynamic strain test method,dynamic caustic method and ABAQUS numerical simulation were used to study the factors influencing the stress field distribution in linear charge explosion,such as detonation position,the length-diameter ratios of the borehole.When the linear charge detonated at the middle-point,the strain differences were significant around borehole.Strain in mid-perpendicular direction>strain in vertical direction at the end of borehole>strain along axial direction.The maximum difference appeared at the nearest measurement point of the borehole.The compressive strain peak value of the concentration charge was 20728??,the compressive strain peak in mid-perpendicular direction and in vertical direction at the end of borehole and in the direction along axis were 13920??,10380??and 6790??respectively compared with concentration charge,which reduced by 32.8%,49.9%and 67.2%.The radial strain and tangential strain of linear charge were different for different length-diameter ratios.With the increasement of length-diameter ratios,the density of the charge reduced,the strength of the explosive stress field of the blasting charge weakened,but the time of compressive strain gradually increased.With the length-diameter ratio increasing,the zoning feature of the stress field around the blast hole were significantly increased.When length-diameter ratio was less than 10,the stress difference between the side and the end of linear charge was small,and the Mises stress around the linear borehole was similar to that of the concentrated charge.When length-diameter ratio was between 10 and 20,the Mises stress was uniform on the side of linear charge.When length-diameter ratio was greater than 20,the Mises stress on the side of linear charge performed"the middle was large and the two ends were small",and the stress in the mid-vertical direction of the borehole was the strongest,but gradually weakened on both ends of the borehole.Detonation position has a significant effect on the propagation of explosive stress waves.When one end of the borehole was detonated,the explosion stress wave propagated in“taper”,and the stress waveform was asymmetric.During the process of explosion wave propagating from the detonation end to the non-detonating end,the detonation end was in low stress region,and decreased quickly.The stress value around the detonation point gradually increased and decreased slowely.The stress at the other end was the highest and decreased slowest.After detonation at the mid-point of linear charge,the explosion stress wave symmetrically distributed,and its waveform gradually performed as"ellipse”.With the stress wave propagating,the stress field attenuation has zoning feature.When the two ends of the blasting detonated at the same time,the stress waveform of the explosive stress turned to be from"oval"to“dumbbell”.The peak stress on the midperpendicular of linear charge was the highest,and decreased slowest,the stress value in the axial direction of the both ends of the hole was low and decreased fastest.?3?The dynamic mechanical behavior of the blast-induced cracks at the end of the linear charge was studied by using dynamic caustic method and numerical simulation by considering the parameters of length-diameter ratio,the static pressure of the model and the number of linear charges.For linear charge blasting,the distribution of compression region around the borehole was more significant along the axis of borehole,and compression region was"elliptical"as a whole.Compared with compressive damage region,the tensile damage region formed around the borehole,which mainly distributed around the end of the borehole,and gradually propagated in the form of a"bundle"at approximately 45°.Finally,the radius of the tensile damage region formed around the borehole was about five times the radius of the compression region.The crack region at the end of borehole decreased with length-diameter ratios increasing.When length-diameter ratio was 20,the further increasement of length-diameter ratios could not affect the crack region.Both crack propagating velocity and stress intensity factor at crack tip increased significantly,but not always increased.When length-diameter ratio was 20,the crack propagating velocity shows the maximum value as a whole.Then the crack propagating velocity tended to decrease with length-diameter ratios increasing.The static pressure has a significant effect on blast-induced cracks.With the increasement of the vertical static pressure,the stress intensity factor at the crack tip decreased,the crack propagation velocity decreased quickly and the crack propagation length reduced.When multi-linear charges detonated at the same time,the stress between two adjacent boreholes increased and promoted the initiation and propagation of the cracks between the adjacent boreholes.The stress wave superimposed outside of the boreholes,and the stress field outside of the boreholes was in the process of alternating tension and compression,which increased the fluctuation of the crack propagation.?4?The dynamic fracture characteristics of the material containing defects was studied by dynamic caustic method.Under blasting stress wave of linear charge,the wing crack and anti-wing crack initiated at the end of the pre-crack on the side of the linear charge.In the process of the interaction between blasting stress wave and the open pre-cracks,the open pre-crack underwent an"open-closed-open"process.The explosive stress wave gradually transmitted to the back side of the crack surface,the wing crack initiated by shear stress at the end of the pre-crack,the stress of the closed pre-crack surface gradually decreased,and the elastic deformation energy on the crack surface gradually released with the attenuation of the explosive stress wave.The crack surface was"opened"from"closed"and the stress concentration formed again at the tip of the pre-crack to produce the caustic spots,which formed anti-wing cracks with approximately horizontal propagation at the pre-crack tip.After the explosion of the linear charge,the blasting stress wave propagated to the pre-cracks,reflection stress wave produced from the pre-crack surface and superposed with the blasting stress wave.The stress field enhanced between borehole and pre-cracks,the blast-induced cracks initiated and propagated to the pre-cracks.Meanwhile,the blast stress wave diffracted at the ends of pre-crack,and the stress concentration formed,thus the wing cracks initiated.After blast-induced cracks went through pre-cracks,the explosion gas passed from main cracks to pre-cracks,and the explosive gas"wedged",the anti-wing cracks produced at the end of the pre-cracks.Vertical static pressure promoted the wing cracks initiation at the end of the pre-cracks on the side of linear charge and suppressed the initiation and propagation of the anti-wing cracks at the same position.For the pre-cracks on the ends of the linear charges,the extension length of the blast-induced wing cracks increased and the extension length of the anti-wing cracks decreased with the vertical static pressure increasing.?5?Using three dimensional experimental model materials,high-speed photography method was used to study the propagation process of cracks.Based on the phenomenological analysis and the multifractal theory,the temporal and spatial distribution characteristics of the radial blasted-induced cracks during linear charge explosion were studied.The numerical simulation method was used to analyze the damage degree and the internal mechanism of blasting damage caused by different detonation methods.From the extension form of blast-induced cracks,three dimensional radial blast-induced cracks can be divided into two types,one was extended to the free surface of the model"long knife"explicit blast-induced cracks,the other one was only in the model of internal expansion"fan shape"implicit blast-induced crack.In the"explicit"cracks,the crack propagation velocity was the largest on the surface of the specimen,and the crack propagation velocity in the middle of the cracks propagation surface was the largest in the"implicit"cracks.The propagation velocity of the"explicit"blast-induced crack appeared nonlinear attenuation,but the propagation velocity of the"implicit"blast-induced crack appeared linear attenuation.In the process of crack propagation,the“implicit”crack surface underwent"dense ripple?smooth flattened?radial needles".Detonation position has an important effect on the distribution of blast-induced cracks of linear charge.In the case of positive detonation,the blast-induced crack mainly produced on the side of the borehole and propagated along radial direction.In the reverse detonation,in addition to the radial cracks,there was a"taper"crack extending obliquely downward at the bottom of the borehole,and the presence of the"taper"crack cut off blast-induced cracks propagating downward,and it was conducive to fragmentation and throwing of the side of linear charge.Further study shows that reverse detonation should be used in the cutting blasting in order to improve the rock throwing effect;and positive detonation should be used in the loose blasting in order to increase the length of the radial cracks.The variation law of Mises stress and the spatiotemporal evolution of damage zone were analyzed by numerical simulation.For positive detonation,the formation of the blasting damage area mainly concentrated in the upper part of the blasting hole;the damage at the bottom of borehole was small.For middle detonation,the blasting damage area uniformly distributed along the borehole axis.For reverse detonation,the damage areas gradually appeared from the bottom to the top of the borehole.