Peng Xu's research while affiliated with University of Science and Technology Beijing and other places

To investigate the effects of the impact velocity and joint dip angle on stress wave propagation and fracture evolution of rock masses, dynamic impact experiments of the gray sandstone samples with gypsum-filled joints were carried out by split Hopkinson pressure bar (SHPB), and the failure process was captured by high-speed photography. The numerical simulation was then presented. HYPERMESH software was used to establish the numerical model, which was then imported into LS-DYNA for calculation. The results show that the joint significantly changes stress wave propagation and particle vibration velocity and leads to a certain difference of stress distribution, which is the fundamental cause of the different failure modes. Not only P waves but also shear waves perpendicular to the particle vibration direction are generated in the joint, and the particle vibration velocity in the matrix at the non-loading end is obviously weaker than that at the loading end. In addition, the initial damage and the dynamic compressive strength of the rock masses increase with increasing impact velocity. The fracture is mainly caused by transverse tension in the axial direction, and the impact velocity influences the damage degree but does not change the failure mode. The peak compressive strength of the jointed gray sandstone decreases first and then increases with the joint dip angle, and the failure mode mainly presents as shear-tensile failure and separation failure of the joint and gray sandstone. Jointed gray sandstones are always under dominant tensile action, especially with a joint dip angle of 45°.

Dynamic mechanical properties of jointed rock mass have an important influence on the stability of geotechnical engineering. In this study, dynamic impacting experiment of gray sandstone samples with gypsum-filled joints was carried out by split Hopkinson pressure bar (SHPB) to study the influence of impact velocity and joint dip angle on wave propagation characteristics and fracture evolution of rock mass, and the failure process were captured by high-speed photography. To supplement and further study, numerical simulation was then presented. Hypermesh software was used to establish the numerical model, and then imported into LS-DYNA for calculation. The results show that the joint changes the stress wave propagation and particle vibration velocity significantly, and leads to a certain difference of stress distribution, which is the fundamental cause of different failure modes. Not only P waves but also shear waves perpendicular to the particle vibration direction are generated in the joint, and the particle vibration velocity in matrix at the non-loading end is obviously weaker than that at loading end. In addition, the initial damage and the dynamic compressive strength of rock mass increase with increasing impact velocity. The fracture is mainly caused by transverse tension in the axial direction and impact velocity influences the damage degree but cannot change the failure mode. The peak compressive strength of jointed gray sandstone decreases first and then increases with the joint dip angle, and the failure mode mainly presents as shear-tensile failure and the separation failure of joint and gray sandstone. Jointed gray sandstones are always under the dominant tensile action, especially with the joint dip angle of 45°.

The interaction mechanism of oppositely propagating cracks is an important basis for the study of rock fracture under dynamic loading. In this article, the stress field superposition and crack tip stress evolution are deeply investigated during the interaction of oppositely propagating cracks by using dynamic photoelastic experiment method and numerical simulation analysis. Obtained results indicate that the cracks interaction process is accompanied by the deflection of the crack, and the deflection of the cracks is mainly affected by the shear stress field at the crack tip. According to the difference of the crack deflection characteristics, the interaction process of the oppositely propagating cracks can be divided into two stages. The Stage I: the non-interlaced stage and Stage II: the interaction stage after the mutual staggered. It includes the period of Stage I that the oppositely propagating cracks are not interlaced with each other; the Stage II is the interaction period after the oppositely propagating cracks are interlaced with each other. The effects of stress intensity factor KI, stress intensity factor KII and far-field T stress on crack propagation process should be comprehensively considered when predicting the crack growth trajectory. The interaction of the stress field between cracks is the main factor determining the crack propagation trajectory. In addition, the vertical spacing between cracks also has a significant effect on the fracture behavior of oppositely propagating cracks. When the vertical distance between the two cracks is large, the local stress fields at the crack tip cannot overlap and interfere with each other, and the oppositely propagating cracks will expand independently of each other.

Investigating rock fragmentation mechanisms under blasting and developing new blasting technologies are important and challenging directions for blast engineering. Recently, with the development of experimental techniques, the fundamental theory of rock blasting has been extensively studied in the past few decades and has made important achievements in the full understanding of the rock fracturing process under blast loading. It is thus imperative to systematically review the progress in this direction. This paper mainly focuses on the experimental study of rock blasting, including the distribution characteristic of blast energy, evolution of the blast stress field, propagation mechanism of cracks, interaction mechanism between blast waves and cracks, and influence of geostatic stress on rock fragmentation. In addition, some newly developed blasting technologies and their applications are briefly presented. This review could provide comprehensive insights to guide the study on the rock fracturing mechanism under blasting and further provide meaningful guidance for optimizing blast parameters in engineering.

In this paper, the interaction between the oblique incident blast stress wave and the prefabricated crack is studied using the dynamic photoelastic method and numerical simulation method. First, blast loading dynamic photoelastic experiment system is established. Second, blast stress wave and crack interaction experiments are carried out. Epoxy resin plate is used as specimen material, while lead azide explosive is used to apply blasting load. In the experiment, the evolution of stress field at crack wall and crack tip are observed by a high-speed camera. The experimental results show that incident angle of blast stress wave has a great influence on the way the stress wave interacts with the crack and the field at the crack tip. Finally, based on the experimental results, two numerical models are conducted using FEM software. The numerical results of stress field are in good agreement with photoelastic experiment, which verifies the experimental analysis.

Overbreak and underbreak predictions can be used to effectively control blasting formation of rock roadways and to maintain roadway stability. In this paper, the profile of the roadway after blasting is simplified using fractal theory and quantitative relationships between the overbreak and underbreak block sizes and fractal dimensions are derived. A model for predicting the fractal dimensions of the roadway profile after blasting is proposed. The model demonstrated good accuracy and applicability in field tests. After blasting, rock roadways of coal mines are generally dominated by overbreak. This phenomenon is particularly prominent in soft rock roadways. Contours of the surrounding rock after blasting can be approximated as two fractal structures. Decreasing quality of formed roadways can lead to a highly complex surrounding rock profile, that is, a larger fractal dimension. In addition, the distribution of the volume of overbreak or underbreak approximately follows the Weibull distribution and the surrounding rock quality greatly influences the fractal dimension distribution. Poor rock quality results in a wider distribution of the volume of overbreak or underbreak. The fractal model for roadways with a typical lithology can be used to predict the distribution of overbreak and underbreak block sizes of roadways varying surrounding rock conditions, which can guide roadway construction in the field and ensure good blasting effects. The model showed that directional control blasting technology can reduce the size and discreteness of the roadway overbreak and underbreak in a test roadway and flatness of the roadway was significantly improved. This prediction model is suitable for establishing an overbreak (underbreak) warning and preventing system and it will be utilized as a foundation reference for a practical drift blasting reconciliation at mines for operation improvements.

To investigate the characteristics of the explosion damage from an eccentric decoupled charge, the rock-breaking mechanism of an eccentric decoupled charge is revealed from the perspectives of the explosion wave field and crack field through theoretical and experimental analyses. The ratio of the maximum to minimum pressure on an eccentric decoupled hole wall increases exponentially with an increase in the decoupling coefficient, but it does not change with a change in the explosive density or explosive detonation velocity. The explosive energy on the eccentric charge side has a certain accumulation effect, the velocity of the reflected shock wave at the blasthole wall is greater than that of the incident shock wave, and the incident velocity of the explosion wave on the eccentric side is greater than that on the non-eccentric side. The expansion range of the explosion gas is significantly better than that on the non-eccentric side, indicating that the eccentric decoupled charge can strengthen the action of the explosion gas product on the eccentric side. In addition, the overpressure of the explosion wave on the eccentric side is greater than that on the non-eccentric side. The fractal dimension and damage degree of the explosion crack on the eccentric side are larger than those on the non-eccentric side. The explosion cracks can be divided into intensive, transition, and sparse areas, and the fractal dimension and damage degree decrease successively in the three areas. The explosion cracks formed on the non-eccentric side outside the range of three times the blasthole diameter concentrate in the sparse area, and the destructive effect of the explosion on the non-eccentric side medium is effectively reduced.

The stress waves have a significant effect on crack propagation during blasting. In this paper, a microsecond controlled blasting case involved with the interactions between blast waves and an opposite propagating crack is studied using dynamic photoelasticity method. The results show that the stress waves influence the local stress field of dynamic crack tip, and alter the crack propagation behavior. Moreover, during crack-wave interactions, both the singular stresses and the nonsingular stresses around the crack tip changed apparently. With the incidence of dilatational wave, the non-singular stresses σox and σoy around the tip of opposite propagating crack increase remarkably, inducing an increase in the crack extension resistance, and decreasing both the crack velocity and the dynamic stress intensity factor KId. Whereas, when the shear wave impinges onto the opposite propagating crack, the crack tends to increase the crack velocity. In addition, the fractal dimension of the crack velocity decreases during the incidence of dilatational wave, and increases when the shear wave impinges upon the opposite propagating crack.

Crack defects make it difficult to predict the dynamic fracture of tunnel specimens under an impact load. To study the impact of the velocity and crack location on a roadway under dynamic load, specimens with tunnel-type voids were made using polymethyl methacrylate. The split-Hopkinson bar was used as the loading method, and a digital laser dynamic caustics system was used to observe the fracture process of the specimens. The dynamic fracture process was evaluated by the crack propagation velocity, displacement, and dynamic stress intensity factor. To predict and verify the test results, ABAQUS was used to simulate the test process. It was found that the results of the simulated combinations of the crack propagation path and initial fracture toughness change law are consistent with the test results. The initial fracture toughness and the peak value of the crack propagation velocity increased with the increase of the impact velocity. The crack propagation law and trajectory were affected by the location of the prefabricated cracks.