Laboratory investigation of penetration properties of the complete coal series

Int. J. Rock Mech, Min. Sci. & Geomech. Abstr. Vol. 12, pp. 213-217. Pergamon Press 1975. Printed in Great Britain

Laboratory Investigation of Penetration Properties of the Complete Coal Series B. DAS* V. HUCKA*

Penetration tests have been carried out in laboratory on coal specimens collected from the complete rank (deoree of coalification) of the coal series: lionites, brown coals, bituminous coals and anthracites. The results have been plotted against coal rank. They are also compared with Vicker's hardness in view of the essential similarity between Vicker's hardness and penetration strenoth. Some other penetration parameters have been defined and determined. The results are presented in 9raphs and tables. They seem to be of great use in minin9 practice.

INTRODUCTION

compressive strength, the body under test has free surfaces around the outer verge of loading points (surfaces The number of attributes added to the criterion of AC and BD in Fig. la), whereas these surfaces in case strength is steadily increasing with an increase in our of penetration strength are not free but surrounded knowledge of the behaviour of rocks. The various attriin all directions by the material itself. The resistance butes to strength may be listed as follows: to deformation cannot thus be identical in both the (1) Depending on the direction and type of deforma- cases. The surfaces EG and FH in Fig. l(b) are not tion-tensile, compressive, shear, twisting, bending, etc. free unlike the surfaces AC and BD in Fig. l(a). It (2) Depending on the number of dimensions of seems as if the body EGHF is under a triaxial state deforming force--uniaxial, triaxial, etc. of stress due to Poisson's effect. It is obvious, therefore, (3) Depending on the nature of loading--statical, that penetration strength will be higher than uniaxial dynamical, etc. compressive strength. Avoiding the complexity of the (4) Depending on the rate of loading--low rate, high problem the whole effect may roughly be considered rate, etc. as due to the effect of confinement. A similar view (5) Depending on the duration of load--elastic, in relation to indentation by Vicker's pyramid is creeping, etc. expressed by Brace[2]. Penetration strength, in (6) Depending on the environments--laboratory, in essence, is similar to indentation hardness [3]. Leaving situ. Here the effects of the volume of the sample, crack, aside the technical particulars and the mathematical temperature, moisture, confining pressure, etc., are exactness the concept behind the indentation hardness taken into account. and the penetration strength is the same. It is impossible to express strength as such in an explicit manner because different strength values follow different trends like the quantity elasticity Ill. The above list of attributes can in no way be considered as exhaustive and even with a particular type of strength, e.g. compressive strength, it is essential to fix a standardized set of test conditions to obtain comparable values. One of such attributes is the penetration strength. It may be defined as the strength of a body against failure due to a penetrating load. Penetration strength can be distinguished from uniaxial compressive strength in that in the latter case the load during tests is supposed to act on the total top surface (Fig. la), whereas in case of penetration strength the same is on a much smaller surface (Fig. lb}. In case of uniaxial * Department of Mining and Metallurgy, Laval University, Quebec, Canada.

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Fig. 1. (a) In uniaxial compressive strength test the load acts on the total cross-sectional area of the sample---the loading boundary surface is not confined. (b) In penetration strength test the load acts on a part of the cross-sectional area of the sample--the loading boundary surface is confined (dotted lines EG, FH).

213

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2. c > R/a > 1 3. R/a > c

b~

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Radius of sample

(R=DI2)

Fig. 2. The graph showing the course of change from uniaxial compressive strength (a.) for the radius of the sample equal to the radius of the bit (a = d/2) to penetration strength (ap) for the radius of the sample R >/Rp.

This paper describes the penetration tests on the complete range of the coal series, i.e. lignites, brown coals, bituminous coals, semi-anthracites and anthracites, describing the procedures of sample preparation, proximate analysis of the coal samples, the tests proper and the analysis of the test result.

The size (diameter = d = 2a) of the penetrating bit in relation to the size (diameter = D = 2R) of the test specimen (Fig. lb) influences the value of the penetration strength. When R/a = 1 the penetration strength is identified with uniaxial compressive strength. With increase in the value of the ratio R/a penetration strength increases progressively attaining almost a constant value at and beyond the radius of the specimen = Rp (Fig. 2). Let Rp/a = c where c is defined as the critical size ratio for penetration strength. The following relations will hold true in relation to strength for different values of R/a in actual tests.

TABLE 1. THE

Sample I 2 3 4 5 6 7 8 9 10 1l 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

33 34

no.

EXPERIMENTAL PROCEDURE For the purpose of investigating the penetration strength properties samples were collected from all ranks of coals, i.e. lignites, brown coals, bituminous coals, semi-anthracites, and anthracites. The samples were prepared in cylindrical form having diameter 6 cm and height 3 cm. In view of the brittle nature of coal and the difficulty to prepare regular samples, the irregular samples of approximate size were housed in cylindrical frames of the above dimensions and the gaps between the cylinder wall and the sample (if existed) were filled with some synthetic setting materials. The

RESULTS OF THE PENETRATION STRENGTH TESTS ON COAL

Carbon % of the coal substance

Volatile % of the coal substance

Penetration strength ( a p )

Penetration brittleness

Penetration modulus (E~)

Vicker's micro hardness

(d.m.f.)

(d.m.f.)

(kg/mm z)

(By) %

(kg/mmZ/mm)

(H) (kg/mm ~)

61.23 64.72 66.46 68.23 71.13 72.05 74.12 76.88 76.98 78.50 80.62 81.28 81'58 82-03 82" 12 82"34 82"95 83"12 83' 15 83.23 84-51 86"06 86.33 86.44 86-96 87'92 88'39 89"17 89'58 90.46 90.61 91.74 92"47 94-22

62.14 64.24 59.99 56-39 55.42 50.80 51.95 49.99 36.48 40.52 37.09 35.83 36.28 36"52 37'30 36-53 36-67 34-88 36"91 32"57 32'80 32"08 31.24 23'68 27.65 30-57 23-13 17-12 18'54 15.54 14-84 5'52 12'38 5"21

17.00 19-25 20.78 20-52 2164 25.00 26,37 2793 28.34 29.00 20.86 31.53 33.26 34.77 34-09 34-85 34"13 32'09 35"69 32'42 33"33 31-28 31.74 22"45 26.24 24.00 22-I 5 21"24 20-73 22"52 22-79 37-00 41" 18 63"38

10,2i 10.56 10.68 13.53 14.75 18-23 18.22 21-57 24.10 2622 41-97 30.23 32.20 32.29 33-77 34" 19 35-00 35-62 35"42 35'73 39.72 41 "33 41 '93 48"44 40.00 45"21 48"33 49"95 49.82 51.08 52.73 52"67 54"03 53-26

57.2 628 68-7 80-0 77.4 883 97.3 118.6 122'5 130-2 110-1 142.3 148.6 152.3 157"3 163-4 165"7 158-6 163.2 160'0 160'0 142.7 144.2 72'5 133"3 99-3 83"2 79"8 74'5 83'7 89-2 201'3 190"0 292"4

18,21 19,32 1%78 22,52 25,09 26,24 31,06 35,10 37q 7 38%2 25,35 41,97 40.25 42.03 39-24 38.07 35-02 39.11 35"23 35.62 35-60 31-52 26-87 24-42 26.16 24.55 23.80 23.22 24-41 26"75 27.72 No impression

Penetration Properties of the Complete Coal Series nature of the synthetic material did not affect the results of the experiments as the size of the samples (~ 6 cm) was quite large compared to the diameter of the indenter ( ~ 2 - 5 m m ) to qualify the tests as penetration tests. Trial probes on very large samples showed that specimens with radius greater than 3-4 times the radius of the indenter were good enough for penetration strength proper. In practice, the samples were put inside longer cylindrical casing and then cut into pieces to give shorter pieces of required size. It enables tests on different parts of the same samples. The ends of the samples in the cylinders were then ground to give smooth and parallel surfaces with standard tolerance. The coal substance from each sample was subjected to chemical analysis to determine the following: moisture, ash (mineral matter), volatile and carbon contents. From these results the values of volatile matter and carbon content were recalculated for pure coal substance deducting the moisture and ash. The procedure for this was as in standard coal analysis [4]. The results are presented in Table 1. Vicker's microhardness tests were also performed on these samples but specially prepared and polished for this purpose [4]. The results are presented in Table 1. The equipment for penetration tests is a force~teformation recorder (Fig. 3). The cylindrical sample is put on a horizontal platform and the indenter is forced in a vertical direction. The indenter is of cylindrical

215

shape 2-5 mm dia. The indenter is made of hard steel. The load is applied to the bit by means of a lever system. Both the load and deformations are recorded for the calculation of different parameters. THE INVESTIGATED PROPERTIES The following parameters, as may be clear with reference to Fig. 4 have been considered and determined: (1) Penetration strength (~p kg/mm 2) Pr _ 4Pp o'p- A red2 where: Pr--applied load at failure (kg); A--cross-section of the penetrating bit (mm2); and d--diameter of the bit (ram). (2) Penetration brittleness (Br%) Bp = Reversible strain energy just before failure/Total energy supplied just before failure = Area PCN/Area OACN × 100% = Area OBM/Area OACN × 100%. (3) Penetration modulus (Ep kg/mm2/mm).

Ep

da dP 1 l i m ~ = l i m ~ × ~-.

:

at

tTp Pp ~-~or P - ~ where: a--applied stress (kg/mm2); and (--deformation (ram). DISCUSSION OF THE RESULTS The results of the tests are presented in Table 1 and illustrated graphically in Figs. 5-9 and the important points are described below. (1) Penetration strength of coal is found to bear a definite relation with the rank of coal (Fig. 5). In lignite stage it is as low as 17 kg/mm 2, gradually rising with increase in rank through the stage of brown coal and sub-bituminous coal reaching a peak at about

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Fig. 3. A photographic view of the penetration testing device,

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Fig. 4. An idealized load-deformation curve as recorded in penetration strength test (A--elastic limit, B--rupture point).

216

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Fig. 5. Penetration strength (ap) of coal as a function of the rank of coal (carbon %): A--lignite, B--brown coal, C--bituminous coal, D--semi-anthracite, E -anthracite.

35 kg/mm ~ in low bituminous range (~ 80% C). Afterwards it starts decreasing through the complete bituminous stage reaching a minimum of about 20 kg/mm 2 in high rank bituminous stage (~20% C). It then assumes a quick rising trend in the semi-anthracite and anthracite zone. In anthracite penetration strength attains a value more than 65 kg/mm 2 and often much higher. (2) Penetration strength of coal is found to be about 7-13 times that of the uniaxial compressive strength. The uniaxial compressive strength of coal is found to vary between 200 and 450 kg/cm 2 depending on rank and type of coal whereas the penetration strength as

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Fig. 6. The penetration brittleness (B~,) and penetration modulus (E~) of coal as a function of the rank of coat ( c a r b o n '!i): A lignite,

B-brown

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bituminous coal. E anthracite.

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stated above varies approximately between 15 and 60kg/mm 2. This fact is of great significance both in theory as well as in practice. It indicates that in many cases in industrial processes it would be necessary to count with a very high value of strength compared to uniaxial compressive strength. (3) Penetration brittleness is found to increase steadily (Fig. 6) from lignite (about 10 per cent) to anthracite ( > 5 5 per cent). It does not show any reversal in the bituminous range as found for penetration strength--c~p (Fig. 5) and penetration m o d u l u ~ E r (Fig. 6). It indicates that if the coal substance is subjected to crushing stress the dust forming tendency will increase steadily from lignite to anthracite. (4) Penetration modulu~-Ep--bears a similar trend with the rank of coal, as that of the curve of penetration- C~pvs coal rank showing the typical wave form (Fig. 6). It appears that there is something in the structure of coal which changes with rank in a way exhibiting such behaviour. It may be of interest at this stage to mention that Hirsch's model of coal [4] explains satisfactorily the above wavy trend of the above mechanical properties with rank. (5) Penetration brittleness when plotted against penetration strength gives the wavy curve (Fig. 7) which is but natural in view of steady rise of penetration

200 E

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Fig. 7. The penetration brittleness (Bp) of coal as a function of penetration strength (%) of coal: A--lignite, B--brown coal, C---bitum i n o u s coal, D--semi-anthracite, E--anthracite.

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z

Fig. 8. The p e n e t r a t i o n m o d u l u s (Ep) of coal as a function of the p e n e t r a t i o n strength (crp) ol" coal.

Penetration Properties of the Complete Coal Series

general compared to its penetration strength might partly change the relation. (7) The mean curve of Vicker's microhardness--H of coal against that of penetration strength is found to be a straight line (Fig. 9). The equation connecting the above two quantities as determined from the graph may be written as follows:

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H = 1.27 ap - 3-3 where H and ap are both expressed in kg/mm 2.

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Fig. 9. Vicker's microhardness (H) of coal vs the penetration strength (ap) of coal.

The above equation indicates that there is something common between penetration strength and Vicker's hardness. As stated earlier that leaving aside the mathematical formulation, the above two quantities represent resistance to deformation under confined state. This equation is simply the quantitative correlation, CONCLUSION

brittleness--Bp with rank of coal but oscillating change of penetration strength--~rp with rank of coal. (6) Penetration strength--trp when plotted against penetration modulus--Ep (Fig. 8) gives a trend the mean path of which may be considered as a straight line. The equation of this straight line as determined from the graph is as follows: Ep = 7a r - 75 where Ep and trp are both expressed in units as stated before. It may be pointed out here that Herz [5] established an equation for a spherical identer where Young's modulus appears as a variable:

In view of the fact that penetration is a common matter in mining and other industries, it is necessary to investigate the mechanical behaviour of rock~ in penetration. In a simplified form the concept of triaxial state of stress as a result of simple confinement may account for the higher strength value in penetration as compared to uniaxial compressive strength, The mechanical behaviour of rocks in penetration is of vital importance in cutting, drilling, and other processes. The results of tests performed on coal gave us an opportunity to make a thorough investigation on penetration characteristics and the empirical relation deduced therefrom are found to have a good agreement with theories and practice.

a = 1"1091_ 2 \E~ + E~where a--radius of circular region of contact between the identer and the material; r--radius of the spherical identer; W--applied load; E1--Young's modulus of the identer; Ez--Young's modulus of the material. Making use of the above relation Honda and Sanada [7] deduced an equation relating Vicker's hardness (H) of coal with its Young's modulus (E) as follows: E = 2.71 H This equation is similar to that of the relation between Ep and ep. The difference in value of the multiplier may be attributed mainly to the fact that in one case simple Young's modulus is used whereas in the other case the penetration modulus is used. Moreover, the higher value of Vicker's microhardness of coal in

Received 20 November 1974.

REFERENCES 1. Das B. On the principle of measurement of elasticity and plasticity. Trans. Inst. Min. Met. Ostrava, Mining & Geol. Set. 17(3) 209 218 (1971). 2. Brace W. F. Behaviour of rock salt, limestone and anhydrite during indentation. J. Geophys. Res. 65 (5), 1773-1788 (1960). 3. Das B. Microhardness study of coal with special reference to the mechanism of dust genesis, Ph.D. Thesis, pp. 18-22. The Tech. University of Mines, Ostrava (1968). 4. Francis W. Coal--Its formation and composition, pp. 754--779. Edward Arnold, London (1961). 5. Hirsch P. B. Theoretical interpretation of the X-ray scattering curves of coal. Proc. R. Soc. A 226, p. 143. (1954). 6. Herz H. J. reine anyew. Math. 192, 156 (1881). 7. Honda H. and Sanada Y. Hardness of coal, Fuel, Lond, 35, 451 (1956).

Geomechanics Abstracts

CONTENTS

General Organisation, history and development of geomechanics Education Contracts and specifications Bibliographies Conferences Professional ethics and legal requirements

87A 87A 87A 87A 87A 88A

Properties of Rocks and Soils Texture, structure, composition and density Fracture processes in rocks Strength characteristics Shear deformation characteristics Friction in rocks Time-dependent behaviour Physico-chemical properties Compressibility, swelling and consolidation Experimental and numerical techniques Classification and identification

88A 88A 89A 89A 89A 90A 90A 90A 90A 90A 91A

Geology Tectonic processes Tectonic stress and strain Environmental effects, weathering and soil formation Earthquake mechanisms and effects Frost action, permafrost and frozen ground

91A 92A 92A

Hydrogeology Groundwater Chemical and physical changes due to water Measurement of water pressure and its effects

93A 93A 93A 93A

Underground Excavations Mines Power plants In-situ stresses in ground and stress around underground openings Surface subsidence and caving Temporary and permanent supports

93A 94A 94A

Site Investigation and Field Observation 102A Planning, geotechnical and structural mapping 102A Photographic techniques 103A Geophysical techniques 103A

94A 95A 95A

Subjects Peripheral to Rock Mechanics General geology Snow and ice mechanics

87A

92A 92A 92A

Geological factors of importance in underground excavations Construction methods Experimental and numerical techniques

Surface Structures Embankments and embankment dams Dams other than embankment dams Foundations Slopes Harbours, canals and coast protection works Earth retaining structures Base courses and pavements of roads, railways and airfields Geological factors of importance in surface structures Construction methods Influence of dynamic loads due to explosions or earthquakes Experimental and numerical techniques

95A 95A 96A 96A 96A 96A 96A 99A 99A 99A 100A 100A 100A 100A 100A

Comminution of Rocks Drilling Blasting Crushing and grinding Cutting

101A 101A 101A 102A 102A

Rock and Soil Improvement Techniques Bolts and anchors Grouting and freezing Soil stabilisation

102A 102A 102A 102A

103A 103A 103A

Explanation of Abstract Format The inJbrmation contained in the abstract entries themselves is described in the.foUowing example ,4bstract number

,422

Author

,HUDSON, JA CROUCH, SL FAIRHURST, C

TRANSP. ROAD RES. LAB. CROWTHORNE, BERKS, G.B DEPT. CIV. MINER. ENG. UNIV. MINNESOTA, U.S.A DEPT. CIV. MINER. ENG. UNIV. MINNESOTA, U.S.A

l Title

Affiliation

, Soft, s t i f f s e r v o - c o n t r o l l e d t e s t i n g m a c h i n e . A r e v i e w w i t h r e f e r e n c e to r o c k f a i l u r e . 2 3 F , I T , 54R.

l

N umber of references - - Number of tables Number (~[figures

Source

)ENGNG.

GEOLOGY,

V6, N 3 ,

[

1972, P155 189.

t

Page Number Volume Title of journal

Abstract

,This review contains a brief history of testing machines and a d e t a i l e d d i s c u s s i o n o f t h e p r i n c i p l e s i n v o l v e d in r o c k f a i l u r e . . .

F o r t h i s a n d f u t u r e i s s u e s , t h e r e a r e t h r e e f o r m s o f p r e s e n t a t i o n f o r t h e r e f e r e n c e s . T h e y a r e a s f o l l o w s : (1) R e f e r e n c e c o n t a i n i n g a f f i l i a t i o n w h e n k n o w n , title, a n d s o u r c e o f p u b l i c a t i o n ; (2) A s f o r (1), b u t w i t h a n e x t e n d e d t i t l e ; (3) A s f o r (1 }, b u t w i t h a n a b s t r a c t .

author.

Subiect and Author Index Cumulative

yearly subject and author

indexes are being issued at the end of each year.

Codes for Countries A ADN AL AND AUS

Austria Aden Albania Andorra Australia, Norfolk Islands

B BDS B(J BH BL BP BR BRG BRN BRU BS BUR

Belgium Barbados Bulgaria British Honduras Basutoland Bechuanaland Brazil British Guiana Bahrain Brunei Bahamas Burma

C CB CDN CH CL CNB

Cuba Belgian (ongo Canada Switzerhmd Ceylon British North Borneo, Labuan Colombia Costa Rica ('zechoslovakia Cyprus

(O (R ('S ('Y

D Germany DK Denmark DOM Dominican Republic

E

EAK EAT EAU FAZ EIR EQ ET

Spain, Balearic Islands, Canary Islands, Spanish. Guinea, Spanish Sahara Kenya Tanganyika Uganda Zanzibar, Pemba Republic of Ireland Ecuadm Egypt

F FL

France Liechtenstein

GB

Great Britain and Northern Ireland Alderney Guernsey Jersey Isle of Man Malta, Gozo Gibraltar Guatemala Ghana Greece, Crete, Dodecanese Islands

GBA GBG GBJ GBM GBY GBZ GCA GH GR

H HK

Hungary Hong Kong

I IL IND IR IRQ

Italy. Sardinia. S~cily Israel India Iran (Pcrsia~ Iraq

IS

Iceland

J JA JOR

Japan Jamaica Jordan

K ('atnbodta KWT Kuwait L Luxembourg LAO Laos

RCH RH RI RL RNR RNY RSM RSR RU

Chile Haiti Indonesia Lebanon North Rhodesia Nyasaland San Marino Southern Rhodesia Ruanda Urundi

S SD SF SCP SK SME SP SU

Sweden Switzerland Finland Singapore Sarawa k Surinam British Somaliland Union of Soviet Socialist Republics SWA South West Africa SY Seychelles SYR Syria

MA MC MEX MS

Morocco Monaco Mexico Mauritius

N NA NGN NIC NL NZ

Norway Netherlands Antilles Netherlands New Guinea Nicaragua Netherlands(Holland) New Zealand

p PA PAK PE PI PL PTM PY

Portugal Panama Pakistan Peru Philippine Islands Poland Malaya Paraguay

T TD TN TR

VN

Viet-Nam

R RA RC

Rumania Argentina Formosa

yI.

Yugoslavia

ZA

Union of South Afiica

Thailand Trinidad and Tobago Tunisia Turkey

lISA United glatesorAmcrica