Influence Of Dynamic Stress Waveform On Rock Crack Propagation Using Numerical Simulation

At present,the rock breaking methods in rock engineering are mainly divided into two types—explosive blasting and mechanical rock breaking.The blasting process will form explosion stress waves with different waveforms due to the different types of explosives selected?the ratio of the explosives and the charging scheme.Rock-breaking machinery,such as rock drill hammers,will produce shock stress waves with different waveforms due to different impact pistons.At the same time,the heterogeneity,anisotropy and discontinuity of rock will cause the stress wave propagation process to be extremely complicated.Different waveforms of the stress wave will produce different crushing effects,especially when the rock contains holes?cracks and other defects.Under the action of dynamic loads,new cracks are easily generated at the defects and continue to expand and eventually lead to overall destruction.Therefore,it is of great practical significance to study the influence of stress waves with different waveforms on the crack propagation behavior of rock with defects.In this paper,the dynamic analysis software RFPA3D-Blasting/Dynamic was used,and the influence of rock non-uniformity on damage was considered based on the characteristics of the software.Based on the existence of defects in rocks,this paper focused on the response characteristics of rocks with crack and hole defects to dynamic stress wave parameters,and studied the influence mechanism of stress wave waveform on crack propagation in rock with defects.The specific content is divided into the following two aspects:(1)Explosion stress wave action: By applying 25 different kinds of explosion stress waves to the blast hole with prefabricated crack model,the influence of stress waveform parameters(stress wave peak value,rise/fall rate,energy)on rock crack propagation was comprehensively compared.The results showed that the rock crack growth form was controlled by the stress wave rise rate.And the faster the stress wave rose,the more broken the hole edge was,but the shorter the crack grew.The stress wave energy affected the rock crack propagation length,the greater the energy was,the longer the crack propagation was.When the stress wave energy was similar,the crack propagation was slightly longer under the low rise rate load.With the increase of rise rate and energy of stress wave rise edge,the radius of blast hole crushing zone increased.Combined with the analysis results,in the blasting operation,it is suggested to extend the action time by using water cannon mud seal or air column interval charge structure to expand the impact range of blasting.In addition,the appropriate type and proportion of explosives are also selected to increase the stress wave rise rate to improve the effect of hole edge crushing.(2)Effect of shock stress wave: By imposing the dynamic impact loads on the boundary of the numerical model,the dynamic failure process of single defect(prefabricated hole,prefabricated crack)and combined defect(crack-hole,hole-crack,double hole and double crack)models under the impact of multiple stress waves is simulated and studied.The influence of stress waveform parameters(stress wave rise/fall rate,peak value and wavelength)on rock crack initiation,propagation form,propagation angle and formation of interconnected cracks between defects was comparatively analyzed.It is found that the growth angle and average growth speed of X-shaped cracks formed at defects under impact loads decreased with the decrease of stress wave rising rate.And there was no obvious relationship with the stress wave fall rate and wavelength.When the action time and rise edge time of stress wave were short,the right side of the hole defect evolved to form a horizontal V-shaped crack,and the X-shaped crack formed at the pre-crack sagged inward at the boundary.Under the same geometric parameters and positions,the lower the rise rate was,the better the defect connected.Therefore,it is better to use pistons with high impact velocity and short length for rocks with few and large distance defects such as holes and structural planes,while pistons with small diameter and long length should be used for rocks with densely distributed and a large number of defects.