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Study of rock–pin interaction in cerchar abrasivity test
International Journal of Rock Mechanics & Mining Sciences 72 (2014) 100–108
Contents lists available at ScienceDirect
International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
Technical Note
Study of rock–pin interaction in cerchar abrasivity test Mohammad-Taghi Hamzaban a,n, Hossein Memarian a, Jamal Rostami b, Hamidreza Ghasemi-Monfared c a
School of Mining Engineering, University of Tehran, Tehran, Iran Pennsylvania State University, University Park, PA, United States c School of Metallurgical and Materials Engineering, University of Tehran, Tehran, Iran b
art ic l e i nf o Article history: Received 4 November 2012 Received in revised form 31 August 2014 Accepted 6 September 2014
1. Introduction Excavating of rock requires drilling in conventional drilling and blasting methods or utilization of mechanized excavators such as roadheaders, TBMs, surface miners, and various other machines used in mining and civil applications. Various cutting tools are commonly used to penetrate the rock by the application of forces to crush and ultimately break the rock. These tools are subject to wear as the excavation process proceeds. As such, a major part of excavation costs are related to repair or replacement of the cutting tools. Replacement of worn bits, disks, and picks is one of the main activities during any rock excavation process which must be considered in the planning stage and minimized as much as possible. This is because, in addition to the cost of the tools, the cutter change will reduce machine utilization and hence productivity of the units. The life and performance of the rock cutting tools are a function of tool material, shape, and rock characteristics, in particular rock strength and abrasivity. A common procedure to assess abrasivity of rocks is laboratory testing. Many tests have been developed and used for this purpose. However, the basic problem with many of the tests is that the results are highly dependent on test mechanisms and its conditions [1]. Therefore, testing mechanism and its similarity with the real mechanism of wear is one of the most important parameters that should be considered in design and selection of methods to assess rock abrasivity characteristics. Among the rock abrasivity tests that have been proposed and are in use, the Cerchar abrasivity index test is a simple and fast method that is most commonly used for this purpose. The results of Cerchar testing are widely used in classification of rock
n
Corresponding author. Tel.: þ 98 21 8208 4230; fax: þ 98 21 8800 8838. E-mail address: [email protected] (M.-T. Hamzaban).
http://dx.doi.org/10.1016/j.ijrmms.2014.09.007 1365-1609/& 2014 Elsevier Ltd. All rights reserved.
abrasivity and estimation of wear life of the rock cutting tools and estimation of the tool consumption. The test was originally introduced by the CERCHAR Institute (Laboratoire du Centre d’ Etudes et Recherches des Charbonnages de France) in France in the 1970s [2]. In the early 1980s, publications by different authors evaluated the application and the merits of this test [3,4,5]. The Colorado School of Mines (CSM) was the first to use this test in the United States in 1980s [6]. Other countries and research and testing laboratories gradually adopted this system and in the process, a sizable database of test results was created. Many tunneling projects also used this test to describe rock abrasivity for contractual purposes. The original description of the testing procedure was offered in French Standard NF P 94–430–1 [7]. A standard for CAI testing was offered by the ASTM in 2010 [8]. Recently, an ISRM suggested method for CAI testing is published by Alber et al. [9]. There are various configurations for testing machine including three different generations of Cerchar testing devices. The general description and pros and cons of using each configuration were discussed by Rostami et al. [10]. Many researchers have investigated the effect of various factors on the results of the Cerchar abrasivity test. These factors can be divided into two categories of geological and geotechnical factors which are related to rock samples, equipment, and testing procedures. Table 1 summarizes the impact of these factors on testing results. Despite its popularity, there is a lack of understanding about the pin–rock interaction in the Cerchar test. Another deficiency in this testing is the lack of clear understanding of the effect of different parameters on the nature of the wear process. CAI test results are reported based on the wear of the steel stylus tip used in the testing and no attention is paid to what happens to the pin and the rock surface during the test. Wear indeed, is an interactive phenomenon. In the process of rock excavation, the cutting tool
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Table 1 Factors affecting the Cerchar test results. Factor
Correlation with CAI
Geological and geotechnical factors Hardness of minerals Direct Grains size Direct Grains angularity Direct Uniaxial Compressive Strength Direct (UCS) Modulus of Elasticity (E) P-wave velocity Direct Confining stress Direct Technical factors and testing procedures Pins hardness Indirect Roughness of testing surface Direct Testing length Direct Testing velocity Applied load Direct Measuring procedure
Comments
References
Hardness of minerals is not necessarily equivalent to the abrasivity. The effect of grain angularity is reduced when the rock grains are cut by the pin. The relationship is seems to be linear.
[3,5,11–13] [14–16] [14,16] [12,13,17,18]
CAI cannot use to predict modulus of elasticity. There is a linear correlation between P-wave velocity and CAI for all rock types. On more porous rocks confining pressure has more effect on the increase of CAI.
[17] [19] [20]
Harder pins result less scattered data. CAI has more values on natural fractured surfaces than sawn surfaces. A major part of the pin tip wear occurs in the initial stages of slip. The speed of the test does not affect the results significantly. A linear relationship is observed. Dispersion of data is decreased significantly when measuring the diameter of wear surface from the side view.
[6,11,21–24] [6,11,12,24] [11,12,24] [6,24] [24] [24]
Fig. 1. Schematic view of the testing device and its components [25].
wear occurs when it penetrates into the rock under the applied load and as it moves relative to the rock surface. So, in studying the abrasivity characteristics of various rocks, the cutting tool penetration into the rock must be considered as well. Thus, developing a new testing apparatus that could monitor the process of wear during the test could offer a better insight to the wear mechanism and possibly offer solutions to reduce operator sensitivity of the test by more accurate control of testing parameters such as testing length and speed. For this purpose a new device was designed to include an electric motor and several sensors to measure and control testing parameters such as velocity and distance of sliding. This device is able to monitor and record horizontal force and vertical displacements of pin continuously during the test. Detailed discussion of the new Cerchar testing device and the preliminary test results are offered by Hamzaban et al. [25]. Based on the results of the preliminary measurements and subsequent analysis of forces and displacements, a new approach is introduced to assess the abrasivity of rock samples.
2. New Cerchar testing device In order to control the parameters affecting the results of Cerchar test and monitor the interaction of the rock–pin during the testing process, a new version of the Cerchar testing device (Fig. 1) was designed and fabricated [25]. The device is equipped with an electric motor with variable and controllable speed of 0–1.7 cm/s to move the pin on the rock surface. A horizontal displacement sensor measures the relative sliding of pin–rock at any moment. A computer program monitors pin displacement and controls the sliding distance and speed with a high degree of precision. The sliding distance, speed, horizontal forces, and vertical movement of the pin are recorded in a data file at the end of the test.
3. Testing program To evaluate the pin–rock interaction in Cerchar test a testing plan was developed. All tests were carried out according to ISRM
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Table 2 Summary of the CAI test results.
Fig. 2. Average chart of (a) applied horizontal force on pin, T(x), and determination of Tult, and (b) pin vertical displacement, A(x), versus sliding distance obtained on an andesite by pins of 55 and 43 HRC.
and CERCHAR standards on sawn surfaces of rock samples. Hardness of pins used in the testing program was 43 and 55 in Rockwell Hardness scale (HRC). For each sample, CAI was determined for both hardnesses. Five pins were used for each CAI test and the mean values were used to determine CAI. In addition, the applied horizontal force and vertical displacements of the pins during the tests were recorded. To simplify the subsequent analysis, the force–displacement and pin vertical movement curves obtained for testing of each pin were combined in one curve, by averaging the measured values. The average curve was obtained by averaging the values of five measured curves at the same sliding distance (x). Results of averaging the curves for an andesite sample are shown in Fig. 2. Sliding distance of the pins on the rock sample is shown on the x axis. Applied horizontal shear force to scratch the pin on the rock surface is shown by T(x) and the pin's vertical displacement is illustrated by A(x) [25]. This Figure indicates that there could be a relationship between graphs of T(x) and A(x). The applied shear force T(x) on the pin seems to increase proportionally to penetration of the pin into the rock surface A(x). For each sample, CAI and charts of T(x) and A(x) were obtained by pins of 55HRC and 43HRC according to described procedure. The ultimate shear force on the pin was determined by a horizontal asymptotic line on the second half of the T(x) curves (Fig. 2a). This parameter is named as Tult. Values of CAI and Tult for pins of 43 and 55 HRC are reported in Table 2.
4. Calculation of pin penetration into the rock surface The value of the pin tip wear at the end point of the test is determined by CAI. At this point the pin also penetrates into the
Sample Rock type
CAI43 CAI55 Tult
AR AR-02 AR-04 AR-07 AR-08 AR-10 AR-12 AR-16 AR-17 AR-20 AR-22 AR-26 AR-30 AR-31 HL MB QZ SL-01 SL-02 SL-03 SL-04 SL-05 SL-06 SP-01 SP-02 SP-03 SP-04 SP-05 SP-06 SP-07 UT-01 UT-02 UT-03 UT-04 UT-05 UT-06 UT-07 UT-08 UT-09 UT-11 UT-13 UT-15
1.46 4.52 4.00 1.45 2.71 1.44 1.88 3.35 1.35 1.55 1.59 2.89 2.96 2.56 0.09 1.46 4.80 0.26 1.40 1.51 1.23 2.45 0.57 4.74 4.78 3.73 3.16 1.13 2.95 1.57 1.24 2.30 0.75 4.27 2.23 1.50 1.50 0.15 1.07 1.57 0.91 1.51
Marble Fine crystalline granite Fine crystalline granite Andesite Diorite Microcrystalline limestone Syenite Diorite Calcareous sandstone Clayey limestone Microcrystalline limestone Quartzitic sandstone Slate Quartz latite Halite Marble Quartzite Clayey siltstone Sandy limestone Sandy limestone Calcareous sandstone Compacted tuff Marl Granite Granite Coarse crystalline granite Schist Limestone Basalt Sandy dolomite Barite Amphibolite Tuff Anorthosite Dolomite Marble Travertine Halite Anhydrite Limestone Anhydrite Microcrystalline limestone
1.06 3.49 3.64 1.26 2.07 0.98 1.47 2.28 0.84 0.92 1.04 2.63 2.43 2.05 0.01 1.13 3.88 0.20 0.80 0.97 0.96 1.61 0.33 4.13 3.96 3.19 2.89 0.85 2.16 0.89 0.75 1.69 0.45 3.30 1.80 0.97 0.84 0.16 0.35 0.94 0.68 0.98
43
4.04 4.15 3.25 4.68 3.93 4.11 4.23 3.28 4.59 3.80 3.65 2.31 2.58 2.33 4.87 4.11 4.29 4.17 4.84 3.36 5.23 5.05 6.52 3.51 3.55 2.54 2.09 4.51 4.07 3.33 3.92 4.92 5.80 2.55 4.18 4.74 4.69 4.78 4.38 4.52 4.88 2.96
(Kgf) Tult55 (Kgf) 4.33 3.95 3.59 4.14 3.53 4.76 4.69 3.93 5.30 5.17 3.95 2.65 1.90 2.96 4.97 4.05 3.37 4.39 6.51 3.18 5.04 5.01 5.90 3.41 4.06 2.66 2.98 5.38 4.09 4.39 4.21 4.85 6.70 2.01 3.19 3.89 5.40 4.78 4.50 4.02 5.89 2.60
rock depending on the hardness of rock samples. This penetration at the end point of the test is ‘Pf’. The geometry of pin tip and its penetration into the rock surface at the end point of the test (x ¼10 mm) is shown in Fig. 3. According to this figure, at the end point of the test, the relationship between the value of measured vertical pin displacement (Af), the pin tip penetration into the rock surface (Pf) and CAIf can be expressed by the following equation: Af ¼ P f þ0:05CAIf
ð1Þ
where Af is the value of A(x) curve at the end point of the test, CAIf is the Cerchar abrasive index at the pin tip measured by the side view image of the pin under the microscope, and Pf is the amount of pin penetration into the rock surface. In this equation the units of Af and Pf are millimeters. Also, the unit of CAI is changed to millimeter by multiplying the measured CAI value by 0.1. The applied horizontal force at the end point of the test (Tult) can also be obtained from the graphs of T(x). So, the graphs of applied horizontal force versus the pin tip wear and its penetration into the rock can be drawn as shown in Fig. 4. According to Fig. 4, there is almost no relationship between the applied horizontal force and the pin tip wear. But, the applied horizontal force shows some correlation with pin penetration into the rock surface. The correlations in Fig. 4 are shown for the end point of the tests, but they can be easily extended to the entire length of the scratch. Therefore, it can be concluded that the work or
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Fig. 5. Description of MCAI and its relation to rock abrasivity. Fig. 3. Schematic view of the geometrical relation between pin tip wear (CAIf), pin tip penetration into the rock (Pf) and measured value of vertical displacement at the end point of the test (Af) (not to scale, dimensions are in millimeters).
Fig. 6. Relationship between CAI and MCAI for 55HRC pins.
Table 3 Rock abrasivity classification scheme based on CAI55 [26] and MCAI55 (Fig. 6).
Fig. 4. Applied force on the pin (Tult) versus to (a) pin tip wear (CAI55) and (b) pin tip penetration into the rock (Pf) in the end point of tests.
consumed energy during the test, which is a function of the horizontal forces, relates more to penetration of the pin tip into the rock rather than its wear. In other words, the energy used in the testing for most of the samples is primarily related to the work of plowing through the rock sample. CAI represents rock abrasivity. However, the applied shear force is proportional to the pin tip penetration into the rock and increases by the increase of the penetration. This means that, the more penetration of the pin tip into the rock, the higher the shear force required scratching the pin on the rock surface. Thus, as shown in Fig. 5, CAI/Tult can be considered as the parameter that simultaneously includes the wear of pin tip and its penetration into the rock. This normalized parameter is called ‘Modified Cerchar Abrasivity Index (MCAI)’. Low values of the MCAI, show
Classification
CAI55 (0.1 mm)
Classification
MCAI55 (0.1 mm/Kgf)
Non-abrasive Not very abrasive Slightly abrasive Medium abrasiveness to abrasive Very abrasive Extremely abrasive
0–0.3 0.3–0.5 0.5–1.0 1.0–2.0
Non-abrasive Slightly abrasive Abrasive Very abrasive
0–0.05 0.05–0.15 0.15–0.5 0.5–0.8
2.0–4.0 4.0–6.0
Extremely abrasive
40.8
small pin tip wear and considerable penetration. In contrast, high values of the MCAI represent high pin tip wear and a small penetration into the rock. Fig. 6 shows the test results using the new parameter MCAI (CAI/Tult). According to Fig. 6, a different rock abrasivity classification can be offered based on the results of MCAI. The proposed classification is summarized in Table 3. The Cerchar classification of rock abrasivity is also represented in Table 3. Table 4 shows the categorization of testing samples according to the suggested ranges based on CAI55 and based on MCAI55. Samples SL-02, UT-03 and UT-13 (sandy limestone, tuff and anhydrite, respectively) are classified as abrasive based on CAI55 but they are categorized as slightly abrasive based on MCAI55. Similarly, samples AR-12, SL-05 and UT-02 (syenite, compacted tuff and amphibolites, respectively) are classified as very abrasive based on CAI55, but based on MCAI55 they are categorized as abrasive. Finally, samples AR-26, AR-30, SP-03, SP-04 and UT-04 (quartzitic sandstone, slate, coarse crystalline granite, schist and anorthosite, respectively) are classified as very abrasive based on CAI55, but they are categorized as extremely abrasive based on MCAI55.
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Table 4 Classification of samples according to CAI55 and MCAI55. Classification
According to CAI55
According to MCAI55
Sample
Rock type
CAI55
Sample
Rock type
MCAI55
Non-abrasive
HL SL-01 UT-08
Halite Clayey siltstone Halite
0.01 0.20 0.16
HL SL-01 UT-08
Halite Clayey siltstone Halite
0.00 0.05 0.03
Slightly abrasive
SL-06 UT-09
Marl Anhydrite
0.33 0.35
SL-02 SL-06 UT-03 UT-09 UT-13
Sandy limestone Marl Tuff Anhydrite Anhydrite
0.12 0.06 0.07 0.08 0.12
Abrasive
AR AR-07 AR-10 AR-17 AR-20 AR-22 MB SL-02 SL-03 SL-04 SP-05 SP-07 UT-01 UT-03 UT-06 UT-07 UT-11 UT-13 UT-15
Marble Andesite Microcrystalline limestone Calcareous sandstone Clayey limestone Microcrystalline limestone Marble Sandy limestone Sandy limestone Calcareous sandstone Limestone Sandy dolomite Barite Tuff Marble Travertine Limestone Anhydrite Microcrystalline limestone
1.06 1.26 0.98 0.84 0.92 1.04 1.13 0.80 0.97 0.96 0.85 0.89 0.75 0.45 0.97 0.84 0.94 0.68 0.98
AR AR-07 AR-10 AR-12 AR-17 AR-20 AR-22 MB SL-03 SL-04 SL-05 SP-05 SP-07 UT-01 UT-02 UT-06 UT-07 UT-11 UT-15
Marble Andesite Microcrystalline limestone Syenite Calcareous sandstone Clayey limestone Microcrystalline limestone Marble Sandy limestone Calcareous sandstone Compacted tuff Limestone Sandy dolomite Barite Amphibolite Marble Travertine Limestone Microcrystalline limestone
0.24 0.30 0.21 0.31 0.16 0.18 0.26 0.28 0.30 0.19 0.32 0.16 0.20 0.18 0.35 0.25 0.16 0.23 0.38
Very abrasive
AR-08 AR-12 AR-16 AR-26 AR-30 AR-31 SL-05 SP-03 SP-04 SP-06 UT-02 UT-04 UT-05
Diorite Syenite Diorite Quartzitic sandstone Slate Quartz latite Compacted tuff Coarse crystalline granite Schist Basalt Amphibolite Anorthosite Dolomite
2.07 1.47 2.28 2.63 2.43 2.05 1.61 3.19 2.89 2.16 1.69 3.30 1.80
AR-08 AR-16 AR-31 SP-06 UT-05
Diorite Diorite Quartz latite Basalt Dolomite
0.58 0.58 0.69 0.53 0.56
Extremely abrasive
AR02 AR04 QZ SP-01 SP-02
Fine crystalline granite Fine crystalline granite Quartzite Granite Granite
3.49 3.64 3.88 4.13 3.96
AR-02 AR-04 AR-26 AR-30 QZ SP-01 SP-02 SP-03 SP-04 UT-04
Fine crystalline granite Fine crystalline granite Quartzitic sandstone Slate Quartzite Granite Granite Coarse crystalline granite Schist Anorthosite
0.88 1.01 0.99 1.27 1.15 1.21 0.97 1.20 0.97
Fig. 7. Picture of the sample surface after the test on non-abrasive samples.
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Fig. 8. Picture of the sample surface after the test on slightly abrasive samples.
Figs. 7–11 show the images of the scratches on rock samples classified in Table 3 (according to MCAI55). A perfect V shape groove seems to form in non-abrasive samples due to significant penetration and insignificant abrasivity of rock samples (Fig. 7). In Fig. 8, a narrow line is seen in the middle of the groove which has been caused by pin tip and indeed shows the floor of the groove. Bright bands on both sides of this line are side walls of the groove which the connect groove floor to the sample surface. The width of the central area is related to the width of the worn pin tip and the width of the side (brightly colored) bands represents the depth of the groove. In Fig. 9 the width of the central band increases which means the wider wear flat at the pin tip. The width of the upper and lower band is reduced which is synonymous with a lower depth of groove. On the edges overlooking the groove slopes, dark spots are seen which have been created by chipping of rock in these areas. The central area of the groove in the pictures of sample surfaces after testing shown in Fig.10 is still wider. Bright bands are very narrow and sometimes fading. Also, the number and extent of dark spots have been reduced considerably. In Fig. 11, almost no trace of grooves on the specimens can be observed. Bright bands of groove slopes and dark spots on the edges have completely disappeared. In contrast, the central strip is quite wide, which represents a severe wear on the pin tip. By careful reviewing of images, it is obvious that the grooves have been formed on samples of each class are very similar. So, it can be concluded that the proposed classification, based on MCAI55, is physically meaningful. Also, comparison of Figs. 7–11 show that classifying samples SL-02, UT-03 and UT-13 (sandy limestone, tuff and anhydrite, respectively) in the non-abrasive group, AR-12, SL-05 and UT-02 (syenite, compacted tuff and amphibolites, respectively) in the abrasive group and finally AR26, AR-30, SP-03, SP-04 and UT-04 (quartzitic sandstone, slate, coarse crystalline granite, schist and anorthosite, respectively) along with extremely abrasive samples is more reasonable than classifying them in groups indicated by CAI55. The rock–pin interaction, when using the pins of various hardnesses, can be compared by MCAI ratio. Fig. 12 shows a graph
of MCAI55 versus MCAI43. Also, the 1:1 line of MCAI55 and MCAI43 is drawn in this graph. Most of the data points and the trend line are located above this line. This means that MCAI43 is greater than MCAI55. In other words, higher pin tip wear and the lower penetration into the rock were recorded when using 43HRC pins as compared to 55HRC pins. According to Figs. 7–11, the higher values of CAI are obtained with the lower pin tip penetration into the rock and vice versa. High values of MCAI can be correlated with low penetration into rock and high wear of the cutting tool. The opposite is true in rocks with low MCAI values where high penetration with lower wear can be anticipated. Another important point is that, the trend line fitted on data crosses the bisector line in low and high values of MCAI. In other words, at low and high values of MCAI, the rock–pin interaction is similar when using 55HRC or 43HRC pins. The practical implication of this observation is that in softer non-abrasive rocks the anticipated performance of the cutting tools will be very good. This includes high penetration into the rock against negligible wear, indicating lower impact of the tool hardness. The reverse is true in rocks with low drillablity (stronger and more abrasive) where low penetration with very high wear rate should be expected with less sensitivity to the tool hardness.
5. Conclusion In this study, a new device is introduced for conducting the Cerchar abrasivity test on rock with additional features on the device that allows for better monitoring of rock–pin interaction. This includes accurate and automatic control of the test variables such as sliding length and its velocity, and continuous measurement horizontal force for scratching pin on rock surfaces and its vertical displacements. The analysis of the test results using the new Cerchar testing device has led to the following conclusions. The force required for scratching pin on rock is mostly related to pin penetration into the specimen. Accordingly, the area under the force–displacement curve of which represents the energy
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Fig. 9. Picture of the sample surface after the test on some of abrasive samples.
consumed in process, primarily reflects the energy spent by making a groove in the rock surface. A Modified Cerchar Abrasivity Index (MCAI) is introduced in this study that combines pin tip wear which is currently measured and reported as CAI, as well as pin penetration into the rock surface, as represented by the horizontal forces used in sliding the pin. Experiments showed that classification of rocks based on the MCAI has the potential to express the rock abrasivity characteristics in a more realistic and practical manner. More testing and analysis of the rock abrasivity using the proposed new device is underway to examine the nature of the pin–sample surface interactions on single mineral specimens. The additional analysis and examination of the applicability of MCAI in the prediction of cutter wear on various rock drilling and
excavation machines is also underway and could examine the merits of the proposed new abrasivity index for use in practical applications.
Acknowledgments The authors would like to express their gratitude to Mr. Reza Sanajyan and Mrs. Mahdieh Nasiri in Folowrd Industrial Plans Co. for their technical support in designing and manufacturing of the testing device. Also, the authors wish to appreciate Iran Water & Power Resources Development Co., Mr. Banihashemi of Mahab Ghods Consultant Engineers Co. and Mr. Mahinraad of Moshanir Consultant Engineers Co. for providing the samples used in testing
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Fig. 10. Picture of the sample surface after the test on very abrasive samples.
Fig. 11. Picture of the sample surface after the test on extremely abrasive samples.
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Fig. 12. relationship between MCAI43 and MCAI55.
and additional information necessary to conduct experiments and subsequent analysis. References [1] P.N.W. Verhoef. Wear of rock cutting tools. Rotterdam: Balkema; 1997. [2] CERCHAR. Cerchar tests for the measurement of hardness and abrasivity of rocks. French coal industry: centre study research; 1973. [3] Suana M, Peters T. The Cerchar abrasivity index and its relation to rock mineralogy and petrography. Rock Mech Rock Eng 1982;15:1–7. [4] Hughes HM. The relative cuttability of coal-measures stone. Min Sci Tech 1986;3:95–109. [5] West G. Rock abrasiveness testing for tunneling. Int J Rock Mech Min Sci 1989;26:151–60. [6] Rostami J. CAI testing and its implications. Tunn Tunn Int 2005;37:43–5. [7] AFNOR. De´termination du pouvoir abrasive d’une roche – Partie 1: Essai de rayure avec une pointe (NF P 94-430-1). Paris: AFNOR; 2000. [8] ASTM D7625-10. Standard test method for laboratory determination of abrasiveness of rock using the CERCHAR method. ASTM International; 2010.
[9] Alber M, Yarali O, Dahl F, Bruland A, Kasling H, Michalakopoulos TN, et al. ISRM suggested method for determining the abrasivity of rock by the CERCHAR abrasivity test. Rock Mech Rock Eng 2014;47:261–6. [10] Rostami J, Ozdemir L, Bruland A, Dahl F. Review of issues related to Cerchar abrasivity testing and their implications on geotechnical investigations and cutter cost estimates. In: Proceedings of rapid excavation and tunneling conference (RETC), Seattle; 27–29 June 2005. pp. 738–751. [11] Al-Ameen SI, Waller MD. The influence of rock strength and abrasive mineral content on the cerchar abrasive index. Eng Geol 1994;36:293–301. [12] Plinninger RJ, Kasling H, Thuro K, Spaun G. Testing conditions and geomechanical properties influencing the cerchar abrasivity index (CAI) value. Int J Rock Mech Min Sci 2003;40:259–63. [13] Yarali O, Yasar E, Bacak G, Ranjith PG. A study of rock abrasivity and tool wear in coal measures rocks. Int J Coal Geol 2008;74:53–66. [14] Jager W. An investigation into the abrasive capacity of rocks. Memoirs of the Centre of Engineering Geology in the Netherlands, 52. Delft University of Technology; 1988; 99. [15] Reinking MW. Linear cutting tests in artificial sand-clay mixtures to study the influence of brittleness on abrasive wear of cutting tools. Memoirs of the Centre of Engineering Geology in the Netherlands, 75. Delft University of Technology; 1989; 95. [16] Deketh HJR. Wear of the cutting tools of rock excavation machines. In: Proceedings of the 35th US symposium of rock mechanics. Reno; 5–7 June 1995. pp. 342–347. [17] Kahraman S, Alber M, Fener M, Gunaydin O. The usability of Cerchar abrasivity index for the prediction of UCS and E of Misis Fault Breccia: regression and artificial neural network analysis. Expert Syst Appl 2010;37:8750–6. [18] Deliormanli AH. Cerchar abrasivity index (CAI) and its relation to strength and abrasion test methods for marble stones. Constr Build Mater 2012;30:16–21. [19] Khandelwal M, Ranjith PG. Correlating index properties of rocks with P-wave measurements. J Appl Geophys 2010;71:1–5. [20] Alber M. Stress dependency of the Cerchar abrasivity index (CAI) and its effects on wear of selected rock cutting tools. Tunn Und Space Tech 2008;23:351–9. [21] Buchi E, Mathier JF, Wyss CH. Rock abrasivity – a significant cost factor for mechanical tunnelling in loose and hard rock. Tunnel 1995;95:38–44. [22] Plinninger RJ, Kasling H, Thuro K. Wear prediction in hard rock excavation using the Cerchar abrasiveness index (CAI). In: Proceedings of the ISRM regional symposium EUROCK 2004 and 53rd geomechanics colloquy. Salzburg; 6–8 October 2004. pp. 599–604. [23] Michalakopolous TN, Anagnostou VG, Bassanou ME, Panagiotou GN. The influence of styli hardness on the Cerchar abrasiveness index value. Int J Rock Mech Min Sci 2006;43:321–7. [24] Rostami J, Ghasemi A, Alavi E, Dogruoz C, Dahl F. Study of dominant factors affecting Cerchar abrasivity index. Rock Mech Rock Eng 2013;47(5):1905–19. http://dx.doi.org/10.1007/s00603-013-0487-3 (Available online in October). [25] Hamzaban MT, Memarian H, Rostami J. Continuous monitoring of pin tip wear and penetration into rock surface using a new Cerchar abrasivity testing device. Rock Mech Rock Eng 2014;47:689–701. http://dx.doi.org/10.1007/ s00603-013-0397-4. [26] CERCHAR — Centre d’ Etudes et Recherches de Charbonnages de France. The CERCHAR abrasiveness index, 12 S., Verneuil, 1986.
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International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
Technical Note
Study of rock–pin interaction in cerchar abrasivity test Mohammad-Taghi Hamzaban a,n, Hossein Memarian a, Jamal Rostami b, Hamidreza Ghasemi-Monfared c a
School of Mining Engineering, University of Tehran, Tehran, Iran Pennsylvania State University, University Park, PA, United States c School of Metallurgical and Materials Engineering, University of Tehran, Tehran, Iran b
art ic l e i nf o Article history: Received 4 November 2012 Received in revised form 31 August 2014 Accepted 6 September 2014
1. Introduction Excavating of rock requires drilling in conventional drilling and blasting methods or utilization of mechanized excavators such as roadheaders, TBMs, surface miners, and various other machines used in mining and civil applications. Various cutting tools are commonly used to penetrate the rock by the application of forces to crush and ultimately break the rock. These tools are subject to wear as the excavation process proceeds. As such, a major part of excavation costs are related to repair or replacement of the cutting tools. Replacement of worn bits, disks, and picks is one of the main activities during any rock excavation process which must be considered in the planning stage and minimized as much as possible. This is because, in addition to the cost of the tools, the cutter change will reduce machine utilization and hence productivity of the units. The life and performance of the rock cutting tools are a function of tool material, shape, and rock characteristics, in particular rock strength and abrasivity. A common procedure to assess abrasivity of rocks is laboratory testing. Many tests have been developed and used for this purpose. However, the basic problem with many of the tests is that the results are highly dependent on test mechanisms and its conditions [1]. Therefore, testing mechanism and its similarity with the real mechanism of wear is one of the most important parameters that should be considered in design and selection of methods to assess rock abrasivity characteristics. Among the rock abrasivity tests that have been proposed and are in use, the Cerchar abrasivity index test is a simple and fast method that is most commonly used for this purpose. The results of Cerchar testing are widely used in classification of rock
n
Corresponding author. Tel.: þ 98 21 8208 4230; fax: þ 98 21 8800 8838. E-mail address: [email protected] (M.-T. Hamzaban).
http://dx.doi.org/10.1016/j.ijrmms.2014.09.007 1365-1609/& 2014 Elsevier Ltd. All rights reserved.
abrasivity and estimation of wear life of the rock cutting tools and estimation of the tool consumption. The test was originally introduced by the CERCHAR Institute (Laboratoire du Centre d’ Etudes et Recherches des Charbonnages de France) in France in the 1970s [2]. In the early 1980s, publications by different authors evaluated the application and the merits of this test [3,4,5]. The Colorado School of Mines (CSM) was the first to use this test in the United States in 1980s [6]. Other countries and research and testing laboratories gradually adopted this system and in the process, a sizable database of test results was created. Many tunneling projects also used this test to describe rock abrasivity for contractual purposes. The original description of the testing procedure was offered in French Standard NF P 94–430–1 [7]. A standard for CAI testing was offered by the ASTM in 2010 [8]. Recently, an ISRM suggested method for CAI testing is published by Alber et al. [9]. There are various configurations for testing machine including three different generations of Cerchar testing devices. The general description and pros and cons of using each configuration were discussed by Rostami et al. [10]. Many researchers have investigated the effect of various factors on the results of the Cerchar abrasivity test. These factors can be divided into two categories of geological and geotechnical factors which are related to rock samples, equipment, and testing procedures. Table 1 summarizes the impact of these factors on testing results. Despite its popularity, there is a lack of understanding about the pin–rock interaction in the Cerchar test. Another deficiency in this testing is the lack of clear understanding of the effect of different parameters on the nature of the wear process. CAI test results are reported based on the wear of the steel stylus tip used in the testing and no attention is paid to what happens to the pin and the rock surface during the test. Wear indeed, is an interactive phenomenon. In the process of rock excavation, the cutting tool
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Table 1 Factors affecting the Cerchar test results. Factor
Correlation with CAI
Geological and geotechnical factors Hardness of minerals Direct Grains size Direct Grains angularity Direct Uniaxial Compressive Strength Direct (UCS) Modulus of Elasticity (E) P-wave velocity Direct Confining stress Direct Technical factors and testing procedures Pins hardness Indirect Roughness of testing surface Direct Testing length Direct Testing velocity Applied load Direct Measuring procedure
Comments
References
Hardness of minerals is not necessarily equivalent to the abrasivity. The effect of grain angularity is reduced when the rock grains are cut by the pin. The relationship is seems to be linear.
[3,5,11–13] [14–16] [14,16] [12,13,17,18]
CAI cannot use to predict modulus of elasticity. There is a linear correlation between P-wave velocity and CAI for all rock types. On more porous rocks confining pressure has more effect on the increase of CAI.
[17] [19] [20]
Harder pins result less scattered data. CAI has more values on natural fractured surfaces than sawn surfaces. A major part of the pin tip wear occurs in the initial stages of slip. The speed of the test does not affect the results significantly. A linear relationship is observed. Dispersion of data is decreased significantly when measuring the diameter of wear surface from the side view.
[6,11,21–24] [6,11,12,24] [11,12,24] [6,24] [24] [24]
Fig. 1. Schematic view of the testing device and its components [25].
wear occurs when it penetrates into the rock under the applied load and as it moves relative to the rock surface. So, in studying the abrasivity characteristics of various rocks, the cutting tool penetration into the rock must be considered as well. Thus, developing a new testing apparatus that could monitor the process of wear during the test could offer a better insight to the wear mechanism and possibly offer solutions to reduce operator sensitivity of the test by more accurate control of testing parameters such as testing length and speed. For this purpose a new device was designed to include an electric motor and several sensors to measure and control testing parameters such as velocity and distance of sliding. This device is able to monitor and record horizontal force and vertical displacements of pin continuously during the test. Detailed discussion of the new Cerchar testing device and the preliminary test results are offered by Hamzaban et al. [25]. Based on the results of the preliminary measurements and subsequent analysis of forces and displacements, a new approach is introduced to assess the abrasivity of rock samples.
2. New Cerchar testing device In order to control the parameters affecting the results of Cerchar test and monitor the interaction of the rock–pin during the testing process, a new version of the Cerchar testing device (Fig. 1) was designed and fabricated [25]. The device is equipped with an electric motor with variable and controllable speed of 0–1.7 cm/s to move the pin on the rock surface. A horizontal displacement sensor measures the relative sliding of pin–rock at any moment. A computer program monitors pin displacement and controls the sliding distance and speed with a high degree of precision. The sliding distance, speed, horizontal forces, and vertical movement of the pin are recorded in a data file at the end of the test.
3. Testing program To evaluate the pin–rock interaction in Cerchar test a testing plan was developed. All tests were carried out according to ISRM
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Table 2 Summary of the CAI test results.
Fig. 2. Average chart of (a) applied horizontal force on pin, T(x), and determination of Tult, and (b) pin vertical displacement, A(x), versus sliding distance obtained on an andesite by pins of 55 and 43 HRC.
and CERCHAR standards on sawn surfaces of rock samples. Hardness of pins used in the testing program was 43 and 55 in Rockwell Hardness scale (HRC). For each sample, CAI was determined for both hardnesses. Five pins were used for each CAI test and the mean values were used to determine CAI. In addition, the applied horizontal force and vertical displacements of the pins during the tests were recorded. To simplify the subsequent analysis, the force–displacement and pin vertical movement curves obtained for testing of each pin were combined in one curve, by averaging the measured values. The average curve was obtained by averaging the values of five measured curves at the same sliding distance (x). Results of averaging the curves for an andesite sample are shown in Fig. 2. Sliding distance of the pins on the rock sample is shown on the x axis. Applied horizontal shear force to scratch the pin on the rock surface is shown by T(x) and the pin's vertical displacement is illustrated by A(x) [25]. This Figure indicates that there could be a relationship between graphs of T(x) and A(x). The applied shear force T(x) on the pin seems to increase proportionally to penetration of the pin into the rock surface A(x). For each sample, CAI and charts of T(x) and A(x) were obtained by pins of 55HRC and 43HRC according to described procedure. The ultimate shear force on the pin was determined by a horizontal asymptotic line on the second half of the T(x) curves (Fig. 2a). This parameter is named as Tult. Values of CAI and Tult for pins of 43 and 55 HRC are reported in Table 2.
4. Calculation of pin penetration into the rock surface The value of the pin tip wear at the end point of the test is determined by CAI. At this point the pin also penetrates into the
Sample Rock type
CAI43 CAI55 Tult
AR AR-02 AR-04 AR-07 AR-08 AR-10 AR-12 AR-16 AR-17 AR-20 AR-22 AR-26 AR-30 AR-31 HL MB QZ SL-01 SL-02 SL-03 SL-04 SL-05 SL-06 SP-01 SP-02 SP-03 SP-04 SP-05 SP-06 SP-07 UT-01 UT-02 UT-03 UT-04 UT-05 UT-06 UT-07 UT-08 UT-09 UT-11 UT-13 UT-15
1.46 4.52 4.00 1.45 2.71 1.44 1.88 3.35 1.35 1.55 1.59 2.89 2.96 2.56 0.09 1.46 4.80 0.26 1.40 1.51 1.23 2.45 0.57 4.74 4.78 3.73 3.16 1.13 2.95 1.57 1.24 2.30 0.75 4.27 2.23 1.50 1.50 0.15 1.07 1.57 0.91 1.51
Marble Fine crystalline granite Fine crystalline granite Andesite Diorite Microcrystalline limestone Syenite Diorite Calcareous sandstone Clayey limestone Microcrystalline limestone Quartzitic sandstone Slate Quartz latite Halite Marble Quartzite Clayey siltstone Sandy limestone Sandy limestone Calcareous sandstone Compacted tuff Marl Granite Granite Coarse crystalline granite Schist Limestone Basalt Sandy dolomite Barite Amphibolite Tuff Anorthosite Dolomite Marble Travertine Halite Anhydrite Limestone Anhydrite Microcrystalline limestone
1.06 3.49 3.64 1.26 2.07 0.98 1.47 2.28 0.84 0.92 1.04 2.63 2.43 2.05 0.01 1.13 3.88 0.20 0.80 0.97 0.96 1.61 0.33 4.13 3.96 3.19 2.89 0.85 2.16 0.89 0.75 1.69 0.45 3.30 1.80 0.97 0.84 0.16 0.35 0.94 0.68 0.98
43
4.04 4.15 3.25 4.68 3.93 4.11 4.23 3.28 4.59 3.80 3.65 2.31 2.58 2.33 4.87 4.11 4.29 4.17 4.84 3.36 5.23 5.05 6.52 3.51 3.55 2.54 2.09 4.51 4.07 3.33 3.92 4.92 5.80 2.55 4.18 4.74 4.69 4.78 4.38 4.52 4.88 2.96
(Kgf) Tult55 (Kgf) 4.33 3.95 3.59 4.14 3.53 4.76 4.69 3.93 5.30 5.17 3.95 2.65 1.90 2.96 4.97 4.05 3.37 4.39 6.51 3.18 5.04 5.01 5.90 3.41 4.06 2.66 2.98 5.38 4.09 4.39 4.21 4.85 6.70 2.01 3.19 3.89 5.40 4.78 4.50 4.02 5.89 2.60
rock depending on the hardness of rock samples. This penetration at the end point of the test is ‘Pf’. The geometry of pin tip and its penetration into the rock surface at the end point of the test (x ¼10 mm) is shown in Fig. 3. According to this figure, at the end point of the test, the relationship between the value of measured vertical pin displacement (Af), the pin tip penetration into the rock surface (Pf) and CAIf can be expressed by the following equation: Af ¼ P f þ0:05CAIf
ð1Þ
where Af is the value of A(x) curve at the end point of the test, CAIf is the Cerchar abrasive index at the pin tip measured by the side view image of the pin under the microscope, and Pf is the amount of pin penetration into the rock surface. In this equation the units of Af and Pf are millimeters. Also, the unit of CAI is changed to millimeter by multiplying the measured CAI value by 0.1. The applied horizontal force at the end point of the test (Tult) can also be obtained from the graphs of T(x). So, the graphs of applied horizontal force versus the pin tip wear and its penetration into the rock can be drawn as shown in Fig. 4. According to Fig. 4, there is almost no relationship between the applied horizontal force and the pin tip wear. But, the applied horizontal force shows some correlation with pin penetration into the rock surface. The correlations in Fig. 4 are shown for the end point of the tests, but they can be easily extended to the entire length of the scratch. Therefore, it can be concluded that the work or
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Fig. 5. Description of MCAI and its relation to rock abrasivity. Fig. 3. Schematic view of the geometrical relation between pin tip wear (CAIf), pin tip penetration into the rock (Pf) and measured value of vertical displacement at the end point of the test (Af) (not to scale, dimensions are in millimeters).
Fig. 6. Relationship between CAI and MCAI for 55HRC pins.
Table 3 Rock abrasivity classification scheme based on CAI55 [26] and MCAI55 (Fig. 6).
Fig. 4. Applied force on the pin (Tult) versus to (a) pin tip wear (CAI55) and (b) pin tip penetration into the rock (Pf) in the end point of tests.
consumed energy during the test, which is a function of the horizontal forces, relates more to penetration of the pin tip into the rock rather than its wear. In other words, the energy used in the testing for most of the samples is primarily related to the work of plowing through the rock sample. CAI represents rock abrasivity. However, the applied shear force is proportional to the pin tip penetration into the rock and increases by the increase of the penetration. This means that, the more penetration of the pin tip into the rock, the higher the shear force required scratching the pin on the rock surface. Thus, as shown in Fig. 5, CAI/Tult can be considered as the parameter that simultaneously includes the wear of pin tip and its penetration into the rock. This normalized parameter is called ‘Modified Cerchar Abrasivity Index (MCAI)’. Low values of the MCAI, show
Classification
CAI55 (0.1 mm)
Classification
MCAI55 (0.1 mm/Kgf)
Non-abrasive Not very abrasive Slightly abrasive Medium abrasiveness to abrasive Very abrasive Extremely abrasive
0–0.3 0.3–0.5 0.5–1.0 1.0–2.0
Non-abrasive Slightly abrasive Abrasive Very abrasive
0–0.05 0.05–0.15 0.15–0.5 0.5–0.8
2.0–4.0 4.0–6.0
Extremely abrasive
40.8
small pin tip wear and considerable penetration. In contrast, high values of the MCAI represent high pin tip wear and a small penetration into the rock. Fig. 6 shows the test results using the new parameter MCAI (CAI/Tult). According to Fig. 6, a different rock abrasivity classification can be offered based on the results of MCAI. The proposed classification is summarized in Table 3. The Cerchar classification of rock abrasivity is also represented in Table 3. Table 4 shows the categorization of testing samples according to the suggested ranges based on CAI55 and based on MCAI55. Samples SL-02, UT-03 and UT-13 (sandy limestone, tuff and anhydrite, respectively) are classified as abrasive based on CAI55 but they are categorized as slightly abrasive based on MCAI55. Similarly, samples AR-12, SL-05 and UT-02 (syenite, compacted tuff and amphibolites, respectively) are classified as very abrasive based on CAI55, but based on MCAI55 they are categorized as abrasive. Finally, samples AR-26, AR-30, SP-03, SP-04 and UT-04 (quartzitic sandstone, slate, coarse crystalline granite, schist and anorthosite, respectively) are classified as very abrasive based on CAI55, but they are categorized as extremely abrasive based on MCAI55.
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Table 4 Classification of samples according to CAI55 and MCAI55. Classification
According to CAI55
According to MCAI55
Sample
Rock type
CAI55
Sample
Rock type
MCAI55
Non-abrasive
HL SL-01 UT-08
Halite Clayey siltstone Halite
0.01 0.20 0.16
HL SL-01 UT-08
Halite Clayey siltstone Halite
0.00 0.05 0.03
Slightly abrasive
SL-06 UT-09
Marl Anhydrite
0.33 0.35
SL-02 SL-06 UT-03 UT-09 UT-13
Sandy limestone Marl Tuff Anhydrite Anhydrite
0.12 0.06 0.07 0.08 0.12
Abrasive
AR AR-07 AR-10 AR-17 AR-20 AR-22 MB SL-02 SL-03 SL-04 SP-05 SP-07 UT-01 UT-03 UT-06 UT-07 UT-11 UT-13 UT-15
Marble Andesite Microcrystalline limestone Calcareous sandstone Clayey limestone Microcrystalline limestone Marble Sandy limestone Sandy limestone Calcareous sandstone Limestone Sandy dolomite Barite Tuff Marble Travertine Limestone Anhydrite Microcrystalline limestone
1.06 1.26 0.98 0.84 0.92 1.04 1.13 0.80 0.97 0.96 0.85 0.89 0.75 0.45 0.97 0.84 0.94 0.68 0.98
AR AR-07 AR-10 AR-12 AR-17 AR-20 AR-22 MB SL-03 SL-04 SL-05 SP-05 SP-07 UT-01 UT-02 UT-06 UT-07 UT-11 UT-15
Marble Andesite Microcrystalline limestone Syenite Calcareous sandstone Clayey limestone Microcrystalline limestone Marble Sandy limestone Calcareous sandstone Compacted tuff Limestone Sandy dolomite Barite Amphibolite Marble Travertine Limestone Microcrystalline limestone
0.24 0.30 0.21 0.31 0.16 0.18 0.26 0.28 0.30 0.19 0.32 0.16 0.20 0.18 0.35 0.25 0.16 0.23 0.38
Very abrasive
AR-08 AR-12 AR-16 AR-26 AR-30 AR-31 SL-05 SP-03 SP-04 SP-06 UT-02 UT-04 UT-05
Diorite Syenite Diorite Quartzitic sandstone Slate Quartz latite Compacted tuff Coarse crystalline granite Schist Basalt Amphibolite Anorthosite Dolomite
2.07 1.47 2.28 2.63 2.43 2.05 1.61 3.19 2.89 2.16 1.69 3.30 1.80
AR-08 AR-16 AR-31 SP-06 UT-05
Diorite Diorite Quartz latite Basalt Dolomite
0.58 0.58 0.69 0.53 0.56
Extremely abrasive
AR02 AR04 QZ SP-01 SP-02
Fine crystalline granite Fine crystalline granite Quartzite Granite Granite
3.49 3.64 3.88 4.13 3.96
AR-02 AR-04 AR-26 AR-30 QZ SP-01 SP-02 SP-03 SP-04 UT-04
Fine crystalline granite Fine crystalline granite Quartzitic sandstone Slate Quartzite Granite Granite Coarse crystalline granite Schist Anorthosite
0.88 1.01 0.99 1.27 1.15 1.21 0.97 1.20 0.97
Fig. 7. Picture of the sample surface after the test on non-abrasive samples.
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Fig. 8. Picture of the sample surface after the test on slightly abrasive samples.
Figs. 7–11 show the images of the scratches on rock samples classified in Table 3 (according to MCAI55). A perfect V shape groove seems to form in non-abrasive samples due to significant penetration and insignificant abrasivity of rock samples (Fig. 7). In Fig. 8, a narrow line is seen in the middle of the groove which has been caused by pin tip and indeed shows the floor of the groove. Bright bands on both sides of this line are side walls of the groove which the connect groove floor to the sample surface. The width of the central area is related to the width of the worn pin tip and the width of the side (brightly colored) bands represents the depth of the groove. In Fig. 9 the width of the central band increases which means the wider wear flat at the pin tip. The width of the upper and lower band is reduced which is synonymous with a lower depth of groove. On the edges overlooking the groove slopes, dark spots are seen which have been created by chipping of rock in these areas. The central area of the groove in the pictures of sample surfaces after testing shown in Fig.10 is still wider. Bright bands are very narrow and sometimes fading. Also, the number and extent of dark spots have been reduced considerably. In Fig. 11, almost no trace of grooves on the specimens can be observed. Bright bands of groove slopes and dark spots on the edges have completely disappeared. In contrast, the central strip is quite wide, which represents a severe wear on the pin tip. By careful reviewing of images, it is obvious that the grooves have been formed on samples of each class are very similar. So, it can be concluded that the proposed classification, based on MCAI55, is physically meaningful. Also, comparison of Figs. 7–11 show that classifying samples SL-02, UT-03 and UT-13 (sandy limestone, tuff and anhydrite, respectively) in the non-abrasive group, AR-12, SL-05 and UT-02 (syenite, compacted tuff and amphibolites, respectively) in the abrasive group and finally AR26, AR-30, SP-03, SP-04 and UT-04 (quartzitic sandstone, slate, coarse crystalline granite, schist and anorthosite, respectively) along with extremely abrasive samples is more reasonable than classifying them in groups indicated by CAI55. The rock–pin interaction, when using the pins of various hardnesses, can be compared by MCAI ratio. Fig. 12 shows a graph
of MCAI55 versus MCAI43. Also, the 1:1 line of MCAI55 and MCAI43 is drawn in this graph. Most of the data points and the trend line are located above this line. This means that MCAI43 is greater than MCAI55. In other words, higher pin tip wear and the lower penetration into the rock were recorded when using 43HRC pins as compared to 55HRC pins. According to Figs. 7–11, the higher values of CAI are obtained with the lower pin tip penetration into the rock and vice versa. High values of MCAI can be correlated with low penetration into rock and high wear of the cutting tool. The opposite is true in rocks with low MCAI values where high penetration with lower wear can be anticipated. Another important point is that, the trend line fitted on data crosses the bisector line in low and high values of MCAI. In other words, at low and high values of MCAI, the rock–pin interaction is similar when using 55HRC or 43HRC pins. The practical implication of this observation is that in softer non-abrasive rocks the anticipated performance of the cutting tools will be very good. This includes high penetration into the rock against negligible wear, indicating lower impact of the tool hardness. The reverse is true in rocks with low drillablity (stronger and more abrasive) where low penetration with very high wear rate should be expected with less sensitivity to the tool hardness.
5. Conclusion In this study, a new device is introduced for conducting the Cerchar abrasivity test on rock with additional features on the device that allows for better monitoring of rock–pin interaction. This includes accurate and automatic control of the test variables such as sliding length and its velocity, and continuous measurement horizontal force for scratching pin on rock surfaces and its vertical displacements. The analysis of the test results using the new Cerchar testing device has led to the following conclusions. The force required for scratching pin on rock is mostly related to pin penetration into the specimen. Accordingly, the area under the force–displacement curve of which represents the energy
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Fig. 9. Picture of the sample surface after the test on some of abrasive samples.
consumed in process, primarily reflects the energy spent by making a groove in the rock surface. A Modified Cerchar Abrasivity Index (MCAI) is introduced in this study that combines pin tip wear which is currently measured and reported as CAI, as well as pin penetration into the rock surface, as represented by the horizontal forces used in sliding the pin. Experiments showed that classification of rocks based on the MCAI has the potential to express the rock abrasivity characteristics in a more realistic and practical manner. More testing and analysis of the rock abrasivity using the proposed new device is underway to examine the nature of the pin–sample surface interactions on single mineral specimens. The additional analysis and examination of the applicability of MCAI in the prediction of cutter wear on various rock drilling and
excavation machines is also underway and could examine the merits of the proposed new abrasivity index for use in practical applications.
Acknowledgments The authors would like to express their gratitude to Mr. Reza Sanajyan and Mrs. Mahdieh Nasiri in Folowrd Industrial Plans Co. for their technical support in designing and manufacturing of the testing device. Also, the authors wish to appreciate Iran Water & Power Resources Development Co., Mr. Banihashemi of Mahab Ghods Consultant Engineers Co. and Mr. Mahinraad of Moshanir Consultant Engineers Co. for providing the samples used in testing
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Fig. 10. Picture of the sample surface after the test on very abrasive samples.
Fig. 11. Picture of the sample surface after the test on extremely abrasive samples.
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Fig. 12. relationship between MCAI43 and MCAI55.
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