Estimation of Specific Energy in Rock Indentation Test

Transcription

1 Estimation of Specific Energy in Rock Indentation Test Balla Kalyan 1, Ph.D. Scholar Ch.S.N.Murthy 2, Professor R.P.Choudhary 3 Asst. Professor, Department of Mining Engineering., National Institute of Technology Karnataka, Surathkal, India ABSTRACT Static indentation tests were carried out in six types of rocks namely pink marble, limestone, basalt, steel gray granite, moon white granite and black galaxy granite using commercial drill bits (Cross bits) of 35, 38, 45, 48mm diametersat a loading rate of 0.1 kn/sec on Microcontroller compression testing machine (with fabrication work to hold the indenter).the tests were conducted on rocks at index angles of 10 0,20 0,30 0,40 0. The objective is to estimate the specific energy (energy necessary to excavate a unit volume of rock) during indentation and to study the influence of index angle on specific energy. From the experimental data, Force-Penetration (F-P) curves were plotted and specific energy values were calculated from F-P curves for each bit rock combinations. The optimum specific energy is at 30 index angle for the soft rocks (pink marble, limestone) whereas; the optimum specific energy is at 20 index angle for the rocks (basalt, steel gray granite, moon white granite and black galaxy granite). Keywords: Indentation, Specific energy, Index angle, Micro controller compressive testing machine 1. Introduction: In Rock Excavation Technology, the methods used to excavate the earth materials are 1) drill& Blast and 2) mechanical excavation. In drilling and mechanical excavation, breaking the rock by the penetration of indenter is the basic mode of action. The rock indentation represents the fundamental process and therefore it is necessary to investigate the basic deformation and failure mechanisms during the process of rock indentation (Kaharaman et al., 2012). It is very essential to know the process of indentation to assess the drill/cutting machine performance and also to know the strength of rocks for the suitability of drill/cutting picks for particular type of rocks (HaoZhang et al., 2012). Central to the scientific evaluation of the rock fragmentation process induced by an indenter is the indentation test and widely adopted as a standard indicator of material hardness. 33

2 The indentation test is a fracture process in which small-scale fractures initiate and propagate within highly localized stress fields (Lawn and Wilshaw, 1975). Number of researchers has performed indentation tests with various shapes and sizes of indenters on rocks and the Force-Penetration (F-P) behaviour has been observed and recorded. The formation of crater, fractures and depth of penetration under the indenter have been investigated (Lindqvist, 1982; Lindqvist et al., 1994, Cook et al., 1984; Pang and Goldsmith, 1990; Murthy, 1998). These experiments have been essential for understanding rock fragmentation phenomena during drilling and cutting and for establishing models for rock breakage by mechanical tools. An accurate prediction of penetration rate/drillability of rocks helps to make more efficient planning of the rock excavation process (Kaharaman et al., 2003). Similarly prediction of cuttability (resistance to cutting by mechanical tools) of rocks with different pick cutters and roller cutters helps selecting and designing rock cutting machines and predicting their performance, which is used for feasibility and planning purposes (Copur et al., 2003). The cuttability can be measured by full-scale linear cutting tests, and some index tests requiring core samples, such as small-scale cutting tests, indentation tests, uniaxial compressive strength tests, Brazilian tensile strength tests, point load tests, etc. (Fowell and Smith, 1976; McFeat-Smith and Fowell, 1977). Based on these tests, the specific energy, which is energy required to excavate (or cut/ drill) a unit volume of rock, optimum cutting/drilling geometry, and forces acting on drill bits/cutters are measured and/or predicted. Specific energy (SE), is anessential parameter in drilling, cutting, crushing, excavation and breaking using a particular breakage method. It can also be taken as an index of the mechanical efficiency of a rock working process, to indicate drill/cutter conditions and rock characteristics such as strength, hardness, abrasiveness and texture (Ersoy and Atici, 2007). However, it is highly dependent on the mode of rock breakage, and the size and type of the equipment used. There are many methods of determining specific energy but results are only comparable if the drill or apparatus is the same. Specific energy has also been used in relation to different excavation methods as a mode of evaluating efficiency (McFeat-Smith, 1977; Aleman, 1982; Rogers, 1991). The concept of specific energy was proposed by Teale (1965) as a quick means of assessing rock drillability. Teale defined SE as the energy required to remove a unit volume of rock (Teale, 1965). The concept of the Specific energy has been utilized for many decades to assist in assessing the efficiency of the drilling processes and excavating the rock masses. It is a parameter that can be determined in real time from the data regarding the performance of a drilling machine and tunnel boring machineetc.(acaroglu et al., 2008). The present investigation was carried out to assess specific energy in rock indentation at different index angles for each bit rock combinations used and is an important factor in rock drilling. In this study the trends in the specific energy of cross drill bits (used as indenters) on six types of rocks are obtained. This work aims to investigate at what optimum index angle at which the specific energy is minimum for different rocks. 2. Experimental set up: Static indentation tests were conducted on a micro controller compression testing machine (2000kN) capacity. An indenter (bit) holder was fabricated for compression testing machine as shown in the figure 2.1. At one end of the indenter (bit) holder, the different diameters of bits as 34

3 shown in figure2.3, were attached by clamp and screw arrangement. The rock blocks were confined inside a specimen box which was fabricated specially for this purpose as shown in the figure 2.2 and rigidly held on four sides with the help of nut and bolt arrangement. This specimen box was placed on the top of the bottom platen of the compression testing machine during the test. Similarly a digital dial gauge was attached to the magnetic holder and the assembly is mounted to the side plate of the testing machine. The experimental set up with all arrangements to carry out indentation test on rock samples is shown infigure 2.4. Figure 2.1 Indenter (bit) holder, Digital dial gauge, Magnetic holder and Stop watch used in test Figure2.2 Specimen box, Figure2.3 Indenters (bits) used in indentation test 35

4 Figure 2.4 Experimental Set up 3. Experimentalprocedure of static indentation test: Indentation tests were conducted on the rock types rocks namely pink marble, limestone, basalt, steel gray granite, moon white granite and black galaxy granite. These tests were carried out at four indexing angles viz10 0,20 0,30 0,40 0 for 35, 38, 45 and 48 mm diameters of cross bit. To carry out static tests, cubical blocks (0.127 m length m width m height) were prepared with the help of a rock cutting machine, from the rock blocks collected from various mines/quarries in India. They were polished to produce perfectly parallel and mutually perpendicular faces. All rock samples are marked with lines of desiredindex angles in the present study. Then rock sample with above markings was placed in the specimen box and thoroughly clamped by packing materials and with the help of nuts and bolts of the specimen box. The complete assembly of specimen box along with the rock sample was placed on the top of the bottom platen of the testing machine. The indenter (bit) holder assembly fabricated for this purpose, attached at the bottom of the upper platen, and the length adjusted for the indenter (bit) to rest on the surface of the rock sample. Then rate of loading was set to 0.1 kn/sec in the control unit of the machine. The static indentation tests were conducted at above rate of loading which was kept constant for all the experiments. This rate chosen was within the static loading range and as per ISRM guide line (Szwedzicki, 1998).During testing, a start button was pressed for applying the axial compressive load. The bottom platen was moved up by the pump unit mechanism of the machine, so that the indenter(bit) was pressed onto the rocksimulating a situation as prevailing in actual drilling. The sample was loaded continuously for 60 seconds for all the bit-rock combinations. After 60 seconds the loading is stopped. Afterwards unloading is done. During unloading also, at every 5 seconds, force (from control unit) and penetration (from dial gauge) readings were noted down. However, the unloading time varied depending upon the bit-rock combinations. The time during loading and unloading was carefully recorded with the help of a stop watch. 36

5 A digital dial gauge with a least count of 0.01 mm, was fixed, with help of a magnetic holder attached to the side frame of the compression testing machine and it was adjusted such that the tip of its needed touched the top surface of the rock sample, so as to measure the depth of penetration of the indenter (bit) into the rock during each cycle of loading and unloading. The procedure followed for every loading and unloading for each indentation, and rock chips were collected so formed during indentation. Then the specimen box containing the rock sample was rotated through the new marked line of indexing angle for new indentation. For each indexing angle, three indentations, each time on new rock sample, were made and the average of three indentation results were considered for calculating the energy under Force-penetration (F-P) curve. The volume of the crater was calculated by dividing the weight of the rock chips and its powder collected from the crater by the density of the rock. The volumes of very few craters were also measured using dental wax by pressing into the crater and the volume of the wax filled in the crater was determined using water displacement method. The volume calculated from the weight of the rock chips and its powder was found to be approximately 2 % higher than the one obtained by the dental wax method. Therefore, the crater volumes were calculated using the weight of the rock chips and powder generated during the static indentation tests for all the bit- rock combinations. Figure 3.1.Indentation of marble (20 0 index angle) angle) Figure 3.2 Indentation of limestone (20 0 index Figure3.2 Indentation of basalt (30 0 index angle) Figure3.2 Indentation of moon white granite (20 0 index angle) 37

6 Figure3.2 Indentation of black galaxy granite Figure. 3.2 Indentation of steel gray granite (30 0 index (30 0 index angle) angle) 4. Force-Penetration (F-P)curves and calculation of specific energy: Force-Penetration curves were plotted for all the bit-rock combinations from the experimental data obtained. The energy expended in each test was the area under the force-penetration curve. This area was measured using a planimeter. The penetration at the end of loading cycle (60 seconds) was the maximum penetration, whereas the penetration obtained at the end of unloading cycle was the actual penetration as shown figure 4.1 (a). After unloading, the rock was relaxed and this relaxation depends on elastic properties such as Young s modulus and Poisson s ratio of rocks. The hatched portion in figure 4.1 (a) is relaxation of rock after unloading cycle. Energy used, i.e., the difference of energy given (area of the F-P curve up to the maximum penetration) and the energy due to the elastic rebound (area of F-P curve within the maximum penetration and the actual penetration) were calculatedfrom the area of F-P curve as shown infigure 4.1 to 4.4. The ratio of the expended energy to the crater volume is the specific energy, which is the energy required to break a unit volume of rock, was determined for all the bit-rock combinations at all the index angles. Force Penetration (F-P) relationships in static indentation test for 20 0 index angle for all six types of rocks for 48, 45, 38 and 35mm only shown in figure 4.1 to 4.4. The trend of F-P relationship at other index angles was similar for all the bit rock combinations tested. 38

7 (a) Marble b) Limestone c) Basalt d) Steel gray granite e) Moon white granite f) Black galaxy granite Figure 4.1(a-f) Force Penetration (F-P) diagrams in static indentation test for 20 degree index angle of 48 mm diametercross bit in six types of rocks 39

8 a) Marble b) Limestone c) Basalt d) Steel gray granite e) Moon white granite f) Black galaxy granite Figure 4.2(a-f) Force Penetration (F-P) diagrams in static indentation test for 20 0 index angle of 45mm diameter cross bit in six types of rocks 40

9 a) Marble b) Limestone c) Basalt d) Steel gray granite e) Moon white granite f) Black galaxy granite Figure4.3(a-f) Force Penetration (F-P) diagrams in static indentation test for 20 0 index angle of 38 mm diameter cross bit in six types of rocks 41

10 a) Marble b) Limestone c) Basalt d) Steel gray granite e) Moon white granite f) Black galaxy granite Figure4.4(a-f) Force Penetration (F-P) diagrams in static indentation test for 20 0 index angle of 35 mm diameter cross bit in six types of rocks 42

11 , IJREAS VOLUME 5, ISSUE 8 (August, 2015) (ISSN ) 5. Influence of index angle on specific energy: For efficient drilling with single or multiple type cutting wedges, the bit must rotate between each blow. If the bit is not rotated, a groove is broken in the rock and chipping and penetration cease after a few blows. Rotation of the bit presents a new surface tothe bit for each blow, causing chipping, crushing and consequent penetration (Rao and Misra, 1998). The angle at which the bit rotates between the successive blows is called index angle and with properties of rock. In the present study, specific energies were found at 10 0, 20 0, 30 0, and 40 0 index angles andvarious graphs were plotted for the bit-rock combinations to study the influence of index angle (10 0, 20 0, 30 0, and 40 0 ) on specific energy for different bit rock combinations Specific Energy, Nm/m 3 x R² = R² = R² = R² = R² = R² = Marble Lime Stone Basalt Steel Gray Granite Moon White Granite Black Galxy Granite Index Angle, degrees Figure5.1 Influence of index angle on specific energy for six types of rocks for a cross bit of 48 mm diameter 43

12 Specific Energy, Nm/m 3 x R² = R² = R² = R² = 0.87 R² = R² = Marble Lime Stone Basalt Steel Gray Granite Moon White Granite Black Galxy Granite Index Angle, degrees Figure5.2 Influence of index angle on specific energy for six types of rocks for cross bit of 45 mm diameter Specific Energy, Nm/m 3 x R² = R² = R² = R² = R² = R² = Marble Lime Stone Basalt Steel Gray Granite Moon White Granite Black Galxy Granite Index Angle, degrees Figure5.3 Influence of index angle on specific energy for sixtypes of rocks for cross bit of 38 mm diameter 44

13 Specific Energy, Nm/m 3 x R² = R² = R² = R² = R² = R² = Marble Lime Stone Basalt Steel Gray Granite Moon White Granite Black Galaxy Granite Index Angle, degrees Figure5.4 Influence of index angle on specific energy for sixtypes of rocks for cross bit of 35 mm diameter 6. Results and discussion: The static indentation tests on softer rocks (such as limestone and pink marble) showed that the F-Pcurves are represented by initial large penetration at low forces followed steep rising curve. The steepness indicates that after reaching certain penetration, penetration was less against the rise in forces. Whereas in hard rocks (like basalt, steel gray granite, moon white granite and black galaxy granite), the F-P curves showed an increasing slope followed by a decreasing sloping. This event was repeated until a maximum force was reached. The combination of increasing and decreasing sloping (force drop) was observed which represent crushing the chipping phases respectively. The specific energy values at 20 0 index angle for harder group of rocks (like basalt, steel gray granite, moon white granite and black galaxy granite) were found to be less than other indexing angles and specific energy values at 30 0 index angle for softer group of rocks (such as limestone and pink marble) were found to be less than other indexing angles (figure 5.1 to 5.4). The harder group of rocks(like basalt, steel gray granite, moon white granite and black galaxy granite) offer more resistance to penetration. To get more penetration along with chip formation, more number of indentations/blows with indenter (drill bit) per revolution is required. This can be better achieved, if the indexing angle is smaller or in other wards the successive indentations/blows are required at closer interval (indexing). But soft rock possess less strength properties and offers less resistance to penetration, the chip formation is easy and is more due over break between successive indentations. With higher index angle in soft rocks, the rock breakage is effective between successive indentations so that with less number of indentations the hole is formed in case of percussive drilling. 45

14 7. Conclusions: The force at which penetration is about to begin varied for different rock types and the magnitude of this force depends on rock properties.therefore, specific energy values of different rocks vary with the properties of rocks. The optimum index angles for harder group of rocks (like basalt, steel gray granite, moon white granite and black galaxy granite) and for softer group of rocks (such as limestone and pink marble), are around 20 0 and 30 0 respectively. Therefore, the index angle during percussive drilling influences the specific energy values. References: [1] Acaroglu O., Ozdemir L. And Asbury B., 2008, A Fuzzy Logic Model to Predict Specific Energy Requirement for TBM Performance Prediction, Tunnelling and Underground Space Technology., 23, pp [2] Aleman V., 1982., Characterisation of Strata with Particular reference to Roadway Tunnelling Machines. Ph.D. thesis, Nottingham University, 445., 1982 [3] Cook N.G.W., Hood M., and Tsai F., 1984, Observation of Crack Growth in Hard Rock loaded by an Indenter. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 21, pp , [4] Copur H., Bilgin N., Tuncdemir H., and Balci C., 2003, A Set of Indices based on indentation tests for Assessment of Rock Cutting Performance and Rock Properties, The Journal of The South African Institute of Mining and Metallurgy, pp [5] Ersoy A., and Atici U., 2007 Correlation of P and S-waves with Cutting Specific Energy and Dominanat Properties of Volcanic and Carbonate Rocks, Rock Mechanics and Rock Engineering., 40(5), pp [6] Fowell R.J., and Smith I.M., 1976, Factors influencing the Cutting Performance of a Selective Tunnelling Machine, Int. Tunnelling 76 Symp., 1-5 Mar.,London, pp [7] HaoZhang.,Ganyun Huang., Haipeng Song and Yilan Kang., 2012, Experimental Investigation of Deformation and Failure Mechanisms in Rock under Indentation by Digital Image Correlation. Engineering Fracture Mechanics., 96, pp [8] Kahraman S., Bilgin N., and Feridunoglu C.,2003, Dominant Rock Properties affecting the penetration rate of Percussive Drills, International Journal of Rock Mechanics & Mining Sciences,40, pp [9] Kahraman, S., Fener, M. and Kozman, E., 2012, Predicting the Compressive and Tensile Strength of Rocks from Indentation Hardness Index, The Journal of the South African Institute of Mining and Metallurgy., 112, pp , [10] Lawn B., Wilshaw R., 1975, Review Indentation Fracture: Principles and Applications. Journal of Material Science;10, pp [11] Lindqvist P.A., Suarez del Rio L.M., Montoto M., Tan X., and Kou, S., 1994, Rock Indentation Database-testing Procedures, Results and main Conclusions, SKB Project Report, PR , 1994 [12] Lindqvist P.A., Rock Fragmentation by Indentation and Disc Cutting. Ph.D. Thesis, 20D, Lulea, University of Technology, Lulea, Sweden., [13] Mcfeat-Smith I., and Fowell R.J., 1977, Correlation of Rock Properties and the Cutting Performance of Tunnelling Machines. Conference on Rock Engineering, UK, organized jointly 46

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