|NDT.net - April 2003, Vol. 8 No.4|
The composition, microstructure and mechanical properties (hardness, strength, fracture stability) are therefore very important characteristics of rocks. The knowledge of these parameters opens up a new possibilities for practical application. However, it is known that rocks are the heterogeneous media and this reason presents difficulties for the study of their mechanical properties. In consequence, only the macrocharacteristics of rocks, namely the average value of strength, hardness and brittleness, have been studied at present. At the same time the microstructural investigations of rocks show that in such materials the high strength regions alternate with the soft ones. The brittleness of these materials is also non-homogeneous. Therefore, it is essential to study the rock microproperties for the understanding of the material nature and forecasting their behaviour at various conditions. That's why it is necessary to elaborate new ways for the estimation of rock strength and brittleness characteristics.
The way proposed in the paper includes some methods completing each other reciprocally. They are:
Fig 1: Optical photographs of the rock microstructure.|
a - the surface pattern,
b – the indentations made on the different phases. a- x100; b- x120.
Mineral microstructure and morphology were studied by optical microscopy in the transmission and reflection regimes on both the natural surfaces of rocks and faces obtained after a fine mechanical polishing. The polishing was made using, successively,.the diamond spar (with grain diameter ״• (20-30) ל m), diamond paste (״• 10 ל m), and Cr2O3 powder (״• 2 ל m), as abrasive.
The microhardness tester (PMT-3) was used for the hardness estimation. Loads (P) applied to the Vickers indenter were 0.5; 1.0 and 2.0 N. Microhardness was calculated by the formulae (2):
were d is the indentation diagonal and P - the load applied to the indenter. At present the evaluation of mineral hardness is made by two methods: determination of the average hardness value (Hv) and estimation of the most probable hardness value (Hpv) (2,3). The former method is more simple and rapid and therefore it is used more often. Here, each hardness Hv value is a mean value calculated from 10-15 indentations made and distributed regularly on the studied mineral surface. However, this method is correct and can be used only in definite cases, namely:
In accordance with the second method , first of all, each plotted indentation is measured, then the hardness is calculated and included in the table. The maximum (Xmax) and minimum (Xmin) of the hardness value are found from the table 1.
|Table 1. Microhardness values distribution according to the grouping interval (specimen nr. 1)|
Then the next characteristics are calculated:
On the base of these parameters the variational curves are plotted (see figure 2). The analysis of these curves allows to reveal the most precise and probable hardness values of the studied material. The minerals corresponding these values are detected using the mineral hardness index (3,5).
Fig 2: Variational curve of specimen nr. 1 versus its microhardness
The method of acoustic emission (AE) signals allows to obtain the information about the material strength properties, to estimate the surface layer brittleness, to study the influence of various factors on the mechanical properties. The extensive application of this method is based on the concept that the deformation and fracture processes are the sources of the acoustic signals bearing the profound information about the internal structure transformation. In the case of quasi-static indentation the AE signals are registered under both indenter penetration into material (N1) and removal from it (N2=• N- N1), where • N is a summary account of AE signals accumulated under loading/unloading process (4).
Therefore, the proposed investigation complex allows to establish not only the average values of rock strength and brittleness but also to determine the microhardness and microbrittleness of separate zones and phases, which form these materials. Besides, comparing the microhardness of these separate phases with the typical microhardness of certain (known) minerals, one can identify the minerals forming the investigated rock and estimate their relative content. The data about hardness, brittleness and percent content of minerals which compose the rock will give the possibility to judge about the strength properties and stability of a given material.
* The limits of intervals are taken approximately|
Table 2. Rational classification of minerals according to hardness (3)
Table 3. The minerals detected in the investigated rocks (specimen nr. 1)
It has also been found that the largest variety of minerals is presented in sample number 2. Minerals from four groups (II, III, IV and V) can be detected here in accordance with the rational mineral classification (see table 2 and 3). The content of each mineral in this specimen of rock is approximately equal. On the other hand, the specimen number 5 has the most uniform composition. Only two minerals (magnetite in a large proportion and hematite in a small one) have been detected in it. Various minerals (plastic, with middle hardness and super-hard) have been revealed in specimen number 6, as well. The rest specimens are formed from 4-5 minerals belonging to materials with middle, high and super-high hardness.
The hardness values of two studied specimens are presented in figure 3. One can see that the specimen on the base of alluvium concentrates (see figure 3a) consists of more hard and brittle minerals than specimen of bulky rock (see figure 3b).
The obtained results also illustrate the correlation between the microhardness (H) and the AE signal sum (׃N) (see table 4). The following tendency can be noted: the harder is the mineral the greater amount of ׃N is registered. This fact gives evidence that hard rocks are more fragile in comparison with the soft ones.
|Figure 3. Hardness diagram for specimens on the base of alluvium
concentrates (a) and of the bulky rock (b).
|Number of specimen||Microhardness, Hv, MPa||Hardness on the Mohs scale||
Brittleness, number of
AE signals, N
||1 ||6400 ||5.4 ||2300
||2|| 7500 ||5.6|| 2100
||3 ||8500|| 6.2 ||2500
||4 ||6000|| 5.8|| 2000
||5 ||5450 ||5.0 ||1900
||6 ||5000|| 4.8 ||1060
||7 ||4800 ||4.7 ||1500
||8 ||3350 ||4.4 ||1100
||Table 4. The average mechanical parameters of some investigated specimens|
The X-ray difraction analysis of mineral phase composition has been also carried out to control the results obtained by hardness testing. The difractometer DRON-3 (radiation CoKב, filter Fe, scanning velocity of spectrum 2 grade/min and 4 grade/min) has been used for this experiment. The preliminary phase analysis was made using ASTM international card index. The performed difraction analysis confirmed that the microindentation method along with acoustic emission measurements are suitable for the microstructural analysis and detection of mechanical properties of hard and frail complex materials in particular of rocks and minerals.
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