Zongxian Zhang

Effects of loading rate on rock fracture

Various types of rock fracture occur at different loading rates. For example, rock destruction by a boring machine, a jaw or cone crusher, and a grinding roll machine are within the extent of low loading rates, often called quasistatic loading condition. On the contrary, rock fracture in percussive drilling and blasting happens under high loading rates, usually named dynamic loading condition. This chapter presents loading rate effects on rock strengths, rock fracture toughness, rock fragmentation, energy partitioning, and energy efficiency. Finally, some of engineering applications of loading rate effects are discussed.

The flattened Brazilian disc specimen used for testing elastic modulus, tensile strength and fracture toughness of brittle rocks: analytical and numerical results

Abstract The flattened Brazilian disc specimen is proposed for determination of the elastic modulus E , tensile strength σ t and opening mode fracture toughness K IC for brittle rocks in just one test. This paper is concerned with the theoretical analysis as well as analytical and numerical results for the formulas. According to the results of stress analysis and Griffiths strength criteria, in order to guarantee crack initiation at the centre of the specimen, which is considered to be crucial for the test validity, the loading angle corresponding to the flat end width must be greater than a critical value (2 α ⩾20°). The analysis shows that, based on the recorded complete load–displacement curve of the specimen (the curve should include the ‘fluctuation’ section after the maximum load), E can be determined by the slope of the section before the maximum load, σ t by the maximum load, and K IC by the local minimum load immediately subsequent to the maximum load. The relevant formulas for the calculation of E , σ t , K IC are obtained, and the key coefficients in these formulas are calibrated by finite-element analysis. In addition, some approximate closed-form formulas based on elasticity are provided, and their accuracy is shown to be adequate by comparison with the finite-element results.

Effects of loading rate on rock fracture : fracture characteristics and energy partitioning

By means of the Scanning Electron Microscope (SEM), an examination was performed of the fracture surfaces (including their vertical sections) of both Fangshan gabbro and Fangshan marble specimens fractured at the loading rates _ ka 10 ˇ2 010 6 MPa m 1/2 s ˇ1 . The results showed that one or more branching cracks near the fracture surfaces of dynamic rock specimens were clear and the cracks increased with increasing loading rates. However, such branching cracks were rarely seen near the static fracture surfaces. In addition, with the aid of the Split Hopkinson Pressure Bar (SHPB) testing system and a high-speed framing camera, the energy partitioning in the dynamic fracture process of a short rod (SR) rock specimen was analysed quantitatively. The total energy WL absorbed by an SR specimen in the dynamic fracture process mainly consisted of the fracture and damage energy WFD and the kinetic energy WK of flying fragments. The energies WL and WK could be quantitatively calculated through stress wave measurement and high-speed photography in the SHPB testing system. Thus, the fracture and damage energy WFD could be obtained. The results showed that: (1) the energy WK increased with an increase in the impact speed of the striker bar or the loading rate; (2) the energy WFD for dynamic rock fracture was markedly greater than that for static rock fracture, and the WFD increased with an increase in the impact speed of the striker bar or the loading rate; and (3) the value WL/WB (WB is the energy input into the loading system) in the case of dynamic fracture is much lower than that in the case of static fracture. In addition, the ratio decreases with an increase in the loading rate or the impact speed of the striker bar. This means that the energy utilisation decreases when the loading rate or the impact speed of the striker bar rises. Finally, some application problems are discussed in the paper. 7 2000 Elsevier Science Ltd. All rights reserved.

Effects of high temperatures on dynamic rock fracture

Abstract The dynamic fracture toughness of Fangshan gabbro and Fangshan marble subjected to high temperature was measured by means of the split Hopkinson pressure bar (SHPB) system. The specimens for measuring the fracture toughness were manufactured according to the requirements for the Short Rod (SR) specimen suggested by ISRM. Two cases were investigated: (1) the SR specimens of the gabbro and marble were fractured at high temperature (100–330°C), and (2) the specimens of the rocks were first pre-heat-treated at 200°C for the marble and 600°C for the gabbro, and then fractured at room temperature. The experimental results showed that under dynamic loading the fracture toughness of both the gabbro and the marble tested in the above-mentioned cases increased with increasing loading rates. The relationship between the fracture toughness and the loading rates in the two cases is similar to that obtained in the room temperature environment, i.e., without high temperature. (This is defined as the third case.) It can be concluded that temperature variation affects the dynamic fracture toughness of the two rocks to a limited extent within the temperature ranges tested. This is different from the results obtained under the static loading condition. Furthermore, by means of the scanning electronic microscope (SEM), the vertical sections of the fracture surfaces for some gabbro specimens were examined. In addition, the fractal dimensions of the fracture surfaces of some specimens were measured by means of fractal geometry. The results showed that under dynamic loading: (1) macro-crack branching near the fracture surfaces was universal; (2) the fractal dimensions increased with increasing loading rates; (3) in the sections of the specimens tested at high temperature there were many micro-cracks that were probably induced by thermal cracking. On the basis of the above macro- and micro-experimental investigation, an energy analysis of the process of dynamic rock fracture was performed. The results showed that the energy utilisation in dynamic fracture was much lower than that in static fracture.

More accurate stress intensity factor derived by finite element analysis for the ISRM suggested rock fracture toughness specimen—CCNBD

More accurate stress intensity factor derived by finite element analysis for the ISRM suggested rock fracture toughness specimen-CCNBD

Increasing ore extraction by changing detonator positions in LKAB Malmberget mine

By studying the charging plan of a ring and analysing the propagation of the stress waves caused by blasting in sub-level caving, the effects of detonator positions on ore fragmentation and ore extraction were investigated. Two detonator positions were studied and compared with each other. (1) Detonators were placed at the lowest charged position in a blast hole. This position is used in daily production blasting in Malmberget mine, and a ring with the lowest detonator position is called ordinary ring. (2) Detonators were placed at the middle of a charged blast hole, and a ring with such a detonator position is called a test ring. The study on charging plan and the analysis on stress wave propagation showed that a test ring has the following advantages over an ordinary ring: (a) fewer not-detonated (or initiated) blast holes in a ring; (b) more detonation energy utilized; (c) less back breakage to “eye brow”; (d) better fragmentation; (e) more ore extraction. On the basis of the above analysis, a total of...

Reducing ground vibrations caused by underground blasts in LKAB Malmberget mine

Due to the large-scale sub-level caving in Malmberget mine and the short distance between the mine and Malmberget town, the ground vibrations in the town have reached a high level since the year 2001 when large scale caving mining started. In order to control and reduce the high vibrations, LKAB launched a research project on active reduction of vibrations in Malmberget by using the wave interference or wave superposition method with electronic detonators. By means of this method, the vertical vibrations were reduced by 10% and the total vibration time for a ring blast was reduced by 80% according to five ring tests in the mine. For a further reduction of the vibrations, a second method, named changing initiation sequence in ring blasts, was developed on the basis of stress wave theory and the geographic conditions of the town and the mine. The second method has so far been applied in all of the drifts near the town, and the vibrations measured at the town show that the vertical vibrations caused by produ...

A feasibility study on controlling ground vibrations caused by blasts in Malmberget underground mine

In order to control or reduce the ground vibrations caused by underground blasts in Malmberget mine, a number of blast tests were carried out during production blasts and a series of single shot wa ...

Borehole Instability in Malmberget Underground Mine

Borehole instability causes borehole failure, presenting a challenge to the drilling industry (Muller et al. 2009; Zhang 2013). The borehole stability of Marcellus shale wells in long wall mining areas in southeast Pennsylvania, West Virginia, and eastern Ohio has been evaluated by Wang et al. (2014). Here, the ground deformation caused by coal mining generates large ground movement and creates complex stress changes in subsurface rock. This, in turn, triggers interruptions in the operation of the borehole well causing safety and environmental concerns. Borehole walls may fail when the surrounding stress exceeds the tensile, the compressive, or the shear strengths of the rock formation, whichever is reached first (Zhang et al. 2003). Horizontal well borehole stability is analyzed by the new in situ stress prediction model in shale gas reservoirs (Yuan et al. 2013). Besides this, in hard rock underground mining, such as sublevel caving (SLC), uncharged and undetonated blast holes reduce specific charge, result in poor fragmentation and may even lower ore recovery (Zhang 2005). It is often found that boreholes are blocked by stones and the charge work has to stop in the field. In many SLC rings two or three boreholes in each ring are often broken or blocked by either stones or pieces of concrete according to the mine’s production archives that contain daily notes on various production problems met by the miners. In order to get detailed information on the borehole problems occurring in LKAB’s two mines, a preliminary investigation was carried out by Kangas (2007) with a mini-video camera, showing that typical borehole problems were borehole deformation and boreholes jammed by stones. Due to various reasons, the above investigation rested for a while. This study is a follow-up of the previous investigationmade by Kangas (2007) in more ore bodies and more boreholes. The main objective of this study is to identify various instability problems of boreholes by field filming in different ore bodies in the mine.