ISSN 1069�3513, Izvestiya, Physics of the Solid Earth, 2012, Vol. 48, No. 5, pp. 415–420. © Pleiades Publishing, Ltd., 2012.Original Russian Text © V.I. Vettegren, V.S. Kuksenko, R.I. Mamalimov, I.P. Shcherbakov, 2012, published in Fizika Zemli, 2012, No. 5, pp. 58–63.

415

INTRODUCTION

The methods of electromagnetic emission (Vol�arovich, Sobolev, and Parkhomenko, 1962;Parkhomenko, 1968; 1978; Sobolev and Demin, 1980;Sobolev, 1987; 1993) and acoustic emission (Zhurkov,1977; Kuksenko et al., 1996) are widely used for thestudy of rock deformation and cracking. Recently, thefractoluminescence technique, which allows investi�gating the formation of microcracks, was suggested forthese studies (Silin’ and Trukhin, 1985; Streletskii,Pakovich, and Butyagin, 1986; Kawaguchi, 1995;1996; Vettegren et al., 2008; 2011).

However, some questions still remain unclear:

(i) What are the dynamics of microcrack forma�tion?

(ii) How does the process of microcrack initiationmanifest itself in EME and AE?

(iii) What are the sizes of cracks detectable by theFL, EME, and AE methods?

(iv) Is FL caused by the emission of the atoms of airnitrogen excited by the electric discharge between thecrack edges, or is it caused by the relaxation of excita�tion of the broken chemical bonds on the edges of thecrack?

In this work, in order to answer these questions, weanalyze the time dependences of AE, EME, and FL inthe granite sample after the impact against its surface.

EXPERIMENTAL TECHNIQUE

Cubes of Ladogian granite with edges of 20 mm inlength were used as the samples.

The construction of the setup, used for studyingFL, EME, and AE upon impact, is described in(Vettegren et al., 2008). The emission of light, sound,and electromagnetic waves was excited by an impact ofa load of 100 g falling from a height of ≈10 cm onto asteel striker placed on the surface of the sample in itscenter. The quartz lens focused the induced radiationon the cathode surface of the photoelectric multiplierPEM�136. A Hertz dipole with a length of ≈50 cm wasplaced at a distance of 5 cm from the sample to studythe electromagnetic radiation. In order to measure theprofile and velocity (which turned out to be ≈1 km/s)of the strain wave induced in the sample, a squareCTS�19 piezoceramic plate with 4�mm sides wasmounted on the sample surface at a distance of 3 mmfrom the striker.

The intensity of FL is insufficient for obtaining itsspectrum and ascertaining the nature of the emissionsources. The spectra were recorded using the setupintended for studying luminescence at friction on asteel roller, similar to that described in (Vettegren,2011). The obtained spectrum was corrected with thespectral sensitivity of PEM�136 taken into account.

The output voltage from PEM�136, the CTS�19plate, and the Hertz dipole was fed to the inputs of theASK�3106 analog�to�digital convertor (ADC) (man�ufactured by AKTAKOM) with a band pass of100 MHz. The system was started at the instant when

Dynamics of Fractoluminescence, Electromagneticand Acoustic Emissions upon Impact on a Granite Surface

V. I. Vettegren, V. S. Kuksenko, R. I. Mamalimov, and I. P. ShcherbakovIoffe Physical Technical Institute, Russian Academy of Sciences, ul. Politekhnicheskaya 26, St. Petersburg, 194021, Russia

Received April 17, 2011; in final form, May 3, 2011

Abstract—The dynamics of fractoluminescence (FL), electromagnetic (EME) and acoustic emissions (AE)induced in a granite sample by a striker blow was studied. The impact causes strong irregular vibrations in thestriker–sample system. About 900 µs after the impact, the amplitude of these vibrations decreases approxi�mately by a factor of 100. As granite possesses piezoelectric properties, the mechanical vibrations induceequally strong electromagnetic oscillations. FL arises about 60 µs after the impact. The FL spectra containthe bands of ≈1.9 and 2.6 eV, which means that under the impact, the SiOSi bonds are broken and SiO• freeradicals and Si: centers are formed. FL appears as a series of flashes with a duration of ≈10 ns. Each flash isassumed to correspond to the initiation of a microcrack. The linear sizes of the microcracks estimated fromthe intensity of the flashes and the velocity of elastic waves range from ≈7 µm to ≈40 µm, with an average of≈10 µm. The microcracks are grouped in microseries with a duration of ≈4µs, which causes modulation ofstrong AE oscillations by weak oscillations with a period of ≈8 µs. The formation of microcracks with a linearsize exceeding ≈30 µm causes discontinuities in the time dependence of EME.

DOI: 10.1134/S1069351312040118

416

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 48 No. 5 2012

VETTEGREN et al.

the load contacted the striker. The output voltage ofADC was sampled every 10 ns and recorded in thecomputer memory.

RESULTS

The impact upon the striker induces an elastic wavethat reaches the sample in ≈12 µs and causes strongirregular vibrations in the striker–sample–base system(Fig. 1). The vibration amplitudes decrease by a factorof ≈102 in ≈800–900 µs.

Due to the piezoelectric properties of granite, thesevibrations induce electromagnetic waves (Fig. 2).

The FL spectrum of granite is shown in Fig. 3. Twobroad maxima (1.9 and 2.6 eV) and a series of weaknarrow bands (3.06, 3.46, and 3.78 eV) are seen. Thebands of 1.9 and 2.6 eV were previously observed in thetribo� and fractoluminescence spectra of quartz by(Chapman, 1983; Silin’ and Trukhin, 1985; Stre�letskii, Pakovich, and Butyagin, 1986; Kawaguchi,1995; Vettegren et al., 2008; Vettegren, Kuksenko,and Shcherbakov, 2011). These bands were assignedto the excited free radicals ≡Si–O• and centers ≡Si,which arise when the bonds SiOSi break. The weaknarrow bands of 3.06, 3.46, and 3.78 eV correspond tothe air nitrogen ions (El’yashevich, 2001) excited bythe discharge upon breakdown. Their intensities aresignificantly lower than that of the 1.9 and 2.6 eVbands. This means that the intensity of FL is mainlycontributed by the 1.9 and 2.6 eV bands that arise dueto the bond rupture.

Figures 1 and 2 show that the time dependence ofFL consists of several thousand flashes with differentintensity. Apparently, the free radicals ≡Si–O• andthe centers ≡Si: are located at the microcrack edges insuch a brittle solid as granite. Optical microscopy of

the granite surface indicates that after the impact itindeed contains cracks with linear sizes from a few µmto 100 µm. Thus, the flashes is the initiation micro�cracks formation, and the area of microcracks is pro�portional to the intensity of flashes. By thoroughlyinspecting the shapes of the flashes, we found that,irrespective of intensity, all flashes follow a commonpattern (Fig. 4): the intensity attains the peak value in≈10 ns and then exponentially decreases with time,with the average decay time τ = 30 ns. The fact thatthese values are constant is associated with the con�struction features of the device, since the minimumoperation time of ADC is ≈10 ns, while the averagesignal decay time of PEM�136 after termination ofluminescence (which is determined by the straycapacitance and load resistance) is 30 ns. Thus, oursetup only detects the occurrences of flashes; however,the time resolution of the setup is insufficient fordetermining the actual time dependences of the flashintensities.

The maximum intensity of a flash, Im, is propor�tional to the number of the broken bonds and, corre�spondingly, to the area of the crack S: Im ≈ qS, where qis the scaling factor. The measurements show that theaverage Im in our setup is about 0.035 V. The velocity ofthe elastic strain wave is about 1 km/s (see above);therefore, during an interval of ≈10 ns, the cracks growby ≈10 µm, and their surface area is ≈1 × 102 µm2.Hence, q ≈ 3.5 × 10–4 µm–2, and the linear size ofmicrocracks is estimated as L ≈ 53(Im)1/2. The maxi�mum and the minimum flash amplitudes throughoutall granite samples are ≈0.6 and ≈0.017 V, respectively.Therefore, we may assess the size of the largest and thesmallest cracks as ≈40 and ≈7 µm, respectively.

The initial fragment of the time dependence of FLis shown in Fig. 5. During the first 50–60 µs, there are

0.6

0.5

0.4

0.3

0.2

0.1

0 800600400200

FL

inte

nsi

ty, V

AE

inte

nsi

ty,

V

0.4

0.2

0

–0.2

–0.4

–0.6

–0.8

2

1

1000 1200t, µs

–1.0

Fig. 1. The time dependences of the intensities of (1) FL and (2) AE after an impact.

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 48 No. 5 2012

DYNAMICS OF FRACTOLUMINESCENCE, ELECTROMAGNETIC 417

several (typically, not more than ten) flashes with lowintensity. Then, the intensity of the flashes sharplyincreases (Figs. 1 and 2). The flashes form two, some�times three series, each lasting for ≈200 µs and includ�ing about ≈103 flashes. The intensity of flashes withina series varied severalfold. The series are separated bythe intervals with a duration of 50–100 µs, duringwhich the intensity and the number of flashes aremuch lower. After ≈900 µs, the series of strong flashesdisappear, and then the time dependence only con�tains weak events.

Let us again return to Fig. 5, which shows that thepiezoelectric element voltage starts to increase ≈30 µsafter the blow on the striker. Twelve µs of this timeinterval is the travel time of the deformation wave tothe granite surface and another 3 µs is the travel timeto the piezoelectric element. The rest of the time,≈30 – 15 ≈ 15 µs, is spent, apparently, in the penetra�tion of the striker into granite and the energy transferto the base. The increase in the piezoelectric elementvoltage reflects a bulging of the granite surface andemergence of extensional strains on it. The FL starts to

0.8

0.5

0.4

0.3

0.2

0.1

0 800600400200

FL

inte

nsi

ty,

V

EM

E in

ten

sity

, V

0.4

0.2

0

–0.2

–0.4

–0.6

–1.0

–0.8

2

1

1000 1200

0.6

0.7

t, µs

Fig. 2. The time dependences of the intensities of (1) FL and (2) EME after an impact.

6

5

4

3

2

1

03.53.02.52.01.5

Intensity, arb. units

4.0E, eV

Fig. 3. The FL spectrum of granite.

0.10

0.05

66.4066.3566.30

Intensity, V

t, µs

Fig. 4. A single FL flash.

418

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 48 No. 5 2012

VETTEGREN et al.

increase at ≈45 µs, and this increase somewhat lagsbehind the emergence of the extensional strains.Apparently, these strains attain values sufficient toinduce the thermofluctuational rupturing of SiOSibonds not less than 45 µs.

The amplitude of extensional strains starts todecrease at ≈80 µs. It reaches zero at ≈130 µs (Fig. 1)and passes it, and the surface contraction attains itsmaximum at ≈270 µs. At this time, the intensity andthe number of the FL flashes sharply decreases; how�ever, they continue to accumulate and, hence, the pro�cess of microcrack accumulation progresses.

The subsequent, irregular oscillations of the con�traction and extension of the surface are accompaniedby intense accumulation of microcracks in the granitesurface layer. The accumulation process starts todecrease only when the amplitude of the elastic oscil�lations of the surface decreases by a factor of ≈102.However, cracks continue to accumulate up to the endof measurements (1.3 ms).

Figure 5 shows that ≈70 µs after the impact, weakoscillations with a period of about 8 µs develop againstthe strong background AE oscillations. It is importantthat the onset of these weak oscillations coincides withthe start of the intensive formation of microcracks.Two hundred µs after the impact, when the surfacetension is close to zero, weak oscillations die out(Fig. 6); simultaneously, the intensity and the numberof FL flashes abruptly decrease. Starting from ≈310 µs,the microcrack formation again sharply intensifies,and AE oscillations with small amplitudes appear.Finally, at times larger than 900 µs, such oscillationspractically disappear and the intensity of microcrackformation decreases. Thus, the periods of intensemicrocrack accumulation are marked by low�intensity

AE oscillations, which practically vanish when theintensity of crack accumulation decreases.

Thorough examination of the flash accumulationprocess has shown that it is nonuniform. The flashesare grouped in microseries with a length of ≈4 µs(Fig. 6), which is close to half the period of low�inten�sity AE oscillations. Within each series, there are twoor three rather strong and about 40 weak flashes. Whenemerging, the microcracks change the granite volume.Quasi�periodic decreasees and increases in the micro�crack concentration cause elastic strain oscillations ofgranite lattice, which is reflected in the oscillations inthe AE intensity.

Let us estimate the microcrack concentrationaccumulated over ≈4 µs. Let the thickness of the light�emitting surface layer be ≈10 µm (corresponding tothe average crack size l). Then, the volume of this layerV ≈ 4 × 105 µm3, and the crack concentration in it Ccr ≈

40/4 × 105 = 1 × 10–4 µm–3. The average distance L

between the cracks is ≈ ≈ 200 µm, and the ratioL/l ≈ 2. As is known (Kuksenko et al., 1996; Tomilin,Damaskinskaya, and Kuksenko, 1994; Petrov, Bash�karev, and Vettegren’, 1993), when the ratio L/l attains≈3 in different materials, the microcracks coalesceforming larger cracks. A sharp increase in the flashintensity in the middle of each series agrees with thisinterpretation. Apparently, after a large microcrackwith a linear dimension of ≈30–40 µm is formed bycoalescence of the smaller microcracks with a lineardimension of ≈10 µs, the tension in its vicinitydecreases and so does the rate of crack accumulation.Then, the process is repeated again.

The correlation between the series of FL flashesand low�intensity AE oscillations shows that thismethod provides detection of the emergence of a series

cr31 C

0.6

0.3

0 80604020

FL

inte

nsi

ty,

V

AE

inte

nsi

ty,

V

0

–0.6

2

1

100 120 140t, µs

Fig. 5. The fragments of time dependences of (1) FL and (2) AE 0–140 µs after the impact (at the instants of granite surfaceswelling).

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 48 No. 5 2012

DYNAMICS OF FRACTOLUMINESCENCE, ELECTROMAGNETIC 419

of ≈40 microcracks with a linear dimensions of≈10 µm during ≈4 µs.

Obviously, not only the formation of microseries ofcracks but also the emergence of separate crackschanges the slope of the time dependences of AE.However, these changes are unrecognizable as theysink in the ADC noise. Their existence is evident fromthe time dependences of EME. Indeed, the EMEintensity is proportional to the second derivative of thestrain; therefore, even weak variations in the slope ofthe time dependence of AE should cause sharp,“breaking” changes in the time dependence of EME.Scrupulous investigation of EME has shown that thistechnique indeed is capable of detecting the emer�gence of single cracks. To demonstrate this, we com�pare the fragments of time dependences of EME andFL. It can be seen that the occurrences of the strongestFL flashes correspond to the discontinuities in thetime dependence of EME (Fig. 7). However, due tothe ADC noise, we only succeeded in detecting thediscontinuities during the emergence of rather largemicrocracks exceeding ≈30 µm in linear size.

Since the granite sample is opaque, the FL tech�nique allows studying the microcrack initiation in thesurface layer only. In contrast, EME carries informa�tion about the entire volume of the sample. The syn�chronous occurrence of the FL flashes and the discon�tinuities in the time dependence of EME indicatesthat the largest microcracks (with linear sizes in excessof 30 µm) are largely formed in the surface layer of thesample.

The results discussed above allow us to answer thequestions formulated at the beginning of the paper.

It turned out that the dynamics of microcracks isdetermined by propagation of elastic strain wavesinduced by the impact against the surface of the sam�ple and by the interaction between the microcracks.

According to the study, the rate of microcrack forma�tion is maximal when the surface layer of the samplestretches due to the action of the strain wave; con�versely, the rate of microcrack formation is minimal(although nonzero) when the surface layer contracts.Interaction between the cracks results in the formationof a larger crack when the ratio of the mean distancebetween the cracks to their linear size is close to 2–3.As a result, tension in the vicinity of the large crackdecreases and, hence, so does the rate of microcrackformation.

0.6

0.5

0.4

0.3

0.2

0.1

0280260240220200

FL

inte

nsi

ty,

V

AE

inte

nsi

ty,

V

0.4

0.2

0

–0.2

–0.4

–0.6

–0.8

–1.0

2

1

300 320t, µs

340

Fig. 6. The fragments of time dependences of (1) FL and (2) AE 200–340 µs after the impact (at instants of granite surface con�traction).

0.6

0.4

908575

1

t, µs

2

70 80

0.8Intensity, V

Fig. 7. The fragment of time dependences of (1) EMEshowing the discontinuities in EME at the instants of for�mation of microcracks larger than 30 µm. The verticalquasi�lines show strong(2) FL flashes.

420

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 48 No. 5 2012

VETTEGREN et al.

The comparison of the FL and AE data showed thatthe AE method does not allow monitoring the forma�tion of individual microcracks with a linear sizesmaller than ≈40 µm. This technique only detectsmicroseries of 40 microcracks during 4 µs. In contrastto the AE technique, the EME technique can detectthe formation of individual microcracks, whichappear as sharp changes in the time dependence of theelectromagnetic emission. Using this technique, wehave only detected the microcracks larger than 30 µmdue to the noise of our setup.

The study of the FL spectrum has shown that theintensity of the band corresponding to the products ofrupturing of SiOSi chemical bonds in quartz (SiO•and ≡Si:) is larger by several orders of magnitude thanthe intensity of the bands corresponding to the airnitrogen ions excited by the discharge between thecrack edges. This indicates that the FL intensity islargely controlled by the concentration of the productsof the rupturing of chemical bonds of SiOSi.

CONCLUSIONS

The blow of a steel striker against granite surfaceinduces strain waves and flashes of fractoluminescencecaused by energy relaxation in the free excited SiO•radicals and the centers Si: which are formed afterrupturing of the SiOSi bonds. It is assumed that thebroken bonds are largely located on the edges of themicrocracks. This allows one to assess the linear sizesof the microcracks, which were found to vary from ≈7to ≈40 µm. According to the analysis of the timedependences of FL, microcracks with an average sizeof ≈10 µm are formed in microseries (in ≈40 units dur�ing ≈4 µs), each microseries terminating in the forma�tion of a 3–4�fold larger crack.

It turned out that the acoustic emission methoddoes not allow identifying individual microcracks, butonly the series of ≈40 such cracks. In contract to thistechnique, the electromagnetic emission methodenables detecting the emergence of individual micro�cracks from the discontinuities in the time depen�dence of EME. Due to the instrumental noise, usingthis method, we were able to detected rather largemicrocracks with a linear size of at least 30 µm.

ACKNOWLEDGMENTS

The work was supported by the Russian Founda�tion for Basic Research (grant no. 10�05�00256�a).

REFERENCES

El’yashevich, M.A., Atomnaya i molekulyarnaya spektrosk�opiya. Izd. 2 (Atomic and Molecular Spectroscopy), Mos�cow: Editorial URSS, 2001, 2nd edition.

Kawaguchi, Y., Time�Resolved Fractoluminescence Spec�tra of Silica Glass in Vacuum and Nitrogen Atmosphere,Phys. Rev. B:, 1995, vol. 52, p. 9224.

Kuksenko, V., Tomilin, N., Damaskinskaja, E., and Lock�ner, D., Two�Stage Model of Fracture of Rock, Pure Appl.Geophys, 1996, vol. 146, no. 1, pp. 253–263.

Parkhomenko, E.I., Yavleniya elektrizatsii v gornykhporodakh (Electrization Phenomena in Rocks), Moscow:Nauka, 1968.

Parkhomenko, E.I., Electrical Properties of Minerals andRocks, in Spravochnik po fizicheskim svoistvam mineralov igornykh porod pri vysokikh termodinamicheskikh parame�trakh (Reference Book on Physical Properties of Mineralsand Rocks at High Thermodynamical Parameters), Vol�arovich, M.P., Ed., Moscow: Nedra, 1978.

Petrov, V.A., Bashkarev, A.Ya., and Vettegren’, V.I., Fiz�icheskie osnovy prognozirovaniya dolgovechnosti konstrukt�sionnykh materialov (Physics of Forecasting Lifetime ofConstructional Materials), St. Petersburg: Politekhnika,1993.

Radtsig, A.A. and Smirnov, B.M., Spravochnik po atomnoi imolekulyarnoi fizike (Reference Book on Atomic andMolecular Physics), Moscow: Atomizdat, 1980.

Silin’, A.R. and Trukhin, A.N., Tochechnye defekty i ele�mentarnye vozbuzhdeniya v kristallicheskom i steklo�obraznom SiO2 (Point Defects and Elementary Excitationsin Crystalline and Glass SiO2), Riga: Zinatne, 1985.

Sobolev, G.A. and Demin, V.M., Mekhanoelektricheskieyavleniya v Zemle (Mechanoelectrical Phenomena in theEarth), Moscow: Nauka, 1980.

Sobolev, G.A., Fizicheskie osnovy prognozirovaniya razrush�eniya gornykh porod pri zemletryaseniyakh (Physics of Fore�casting Rock Fracturing during Earthquakes), Moscow,1987.

Sobolev, G.A., Osnovy prognoza zemletryasenii (Basics ofForecasting the Earthquakes), Moscow: Nauka, 1993.

Streletskii, A.N., Pakovich, A.B., and Butyagin, P.Yu.,Structural Defects and Triboluminescence Excitation inAmorphous Silicon Dioxide, Izv. Akad. Nauk SSSR, Ser.Fiz., 1986, vol. 50, no. 3, pp. 477–482.

Tomilin, N.G., Damaskinskaya, E.E., and Kuksenko, V.S.,Formation of Fracture Nucleus during Deformation ofHeterogenic Materials (Granite), Fiz. Tverd. Tela, 1994,vol. 36, no. 10, pp. 3101–3112.

Vettegren, V.I., Bashkarev, A.Ya., Mamalimov, R.I., andShcherbakov, I.P., Fractoluminescence of CrystallineQuartz upon an Impact, Phys. Solid State, 2008, vol. 50,no. 1, pp. 28–31.

Vettegren’, V.I., Lyashkov, A.I., and Shcherbakov, I.P.,Effect of the Fiber Orientation on the Microcrack Forma�tion in a Fibrous Polymer Composite during Friction, Tech.Phys., 2010, vol. 55, no. 12, pp. 1821–1824.

Vettegren’, V.I., Kuksenko, V.S., and Shcherbakov, I.P.,Emission Kinetics of Light, Sound, and Radio Waves fromSingle�Crystalline Quartz after Impact on Its Surface, Tech.Phys, 2011, vol. 56, no. 4, pp. 577–580.

Volarovich, M.P., Sobolev, G.A., and Parkhomenko, E.I.,Piezoelectric Effect of Pegmatitic and Quartz Veins, Izv.Akad. Nauk SSSR, Ser. Geofiz., 1962, no. 2, pp. 145–152.