At least two reasons stipulate a constantly increasing interest to the study of rocks strength
properties. On the one hand, such kind of research improves the understanding of the
nature ability to create a lasting materials resistant to super-high pressures, sharp
temperature changes, etc. On the other hand, the steady tendency to the industrial use of
different rocks in combination with the various materials for the technical and civilian
buildings, roads, sanitary equipment and many other applications is observed in the last
decade (1).
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:
- detection of the sample microstructure, size and distribution of inhomogeneities using
optical microscopy;
- evaluation of mechanical characteristics such as microhardness and microbrittleness.estimated by microindentation method in combination with registration of acoustic
emission (AE) signals.
A further development of this complex method for the minerals and rocks is carried out in
the present work. These methods can be used not only for rock study but also for other
non-homogenous structures and materials such as various alloys, ceramics, composite
materials, glasses and plenty of multicomponent solids.
A series of ten specimens of rocks taken from different places of the Carpathian Mountains
(Romania) was used for this investigation. With the aid of the microstructural
metallography it has been shown, that specimens contain many phases with dimensions
from some millimeters to some micrometers (figure 1). That is why, the mentioned
microindentation methods prove to be suitable for the determination of the mechanical
properties of components.
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 Ø• (20-30) ì m), diamond paste (Ø• 10 ì m), and
Cr2O3 powder (Ø• 2 ì 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):
Hv = 1854 P/d2 , (1)
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:
- if the studied material has a homogenous structure.
- if the hardness is estimated on the mineral plane with the known (certain)
orientation;
- for heterogeneous materials, when it is sufficiently to estimate an average value of
hardness, not taking into consideration their multiphase composition;
But in the case when mineral possesses any structural heterogeneity for the correct
hardness estimation it is necessary to use the second method (3), and namely, the
evaluation of the most probable hardness (Hpv). This method have been used in this work
along with the first one.
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:
- the hardness amplitude variation R = Xmax - Xmin ;
- the limitation of the group interval h = R / L, where L is the number of intervals;
- the frequency of certain H value appearance (m) and the frequency accumulation
expressed in per cents (p%).
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 selected specimens were subjected to the multilateral and detailed investigations in
order to detect the mineral and rock components and to characterize the peculiarities of the
mechanical properties of these minerals. The obtained results have been compared with
data from the literature (3,5,6). It was established that rocks under investigation consisted
of different minerals of various crystallographic categories and hardness groups (pyrites,
quartz, feldspar, magnetite, pyrotine, hematite, etc.) (see tables 2 and 3).
* 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 (ÓN) (see table 4). The following tendency can be noted: the harder is the
mineral the greater amount of ÓN is registered. This fact gives evidence that hard rocks are
more fragile in comparison with the soft ones.
a | 
b
| 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
CoKá, 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.
Fig 1: Optical photographs of the rock microstructure.