Giornata Seminariale ProgettoGiornata Seminariale Progetto RERE--FRESCOSFRESCOSGiovedGiovedìì 1 1 LuglioLuglio, 2010, , 2010, AulaAula Albenga (DISTR)Albenga (DISTR)

Emissioni acusticheEmissioni acustiche ed ed elettromagnetiche elettromagnetiche durantedurante la la propagazione della fratturapropagazione della frattura

A. Carpinteri, G. Lacidogna, A. ManuelloA. Carpinteri, G. Lacidogna, A. Manuello

Department of Structural Engineering and Geotechnics, PolitecnicDepartment of Structural Engineering and Geotechnics, Politecnico di Torino, Italyo di Torino, Italy

G. Niccolini, A. Schiavi, A. AgostoG. Niccolini, A. Schiavi, A. Agosto

National Research InstituteNational Research Institute ofof MetrologyMetrology –– INRiMINRiM, Italy, Italy

In this work we investigate the mechanical behaviour of concreteIn this work we investigate the mechanical behaviour of concrete and rocks and rocks specimens loaded up to failure by the analysis of Acoustic Emissspecimens loaded up to failure by the analysis of Acoustic Emission (AE) and ion (AE) and Electromagnetic Emission (EME). Electromagnetic Emission (EME).

All specimens were tested in compression at a constant displacemAll specimens were tested in compression at a constant displacement rate and ent rate and monitored by piezoelectric (PZT) transducers for AE data acquisimonitored by piezoelectric (PZT) transducers for AE data acquisition. tion.

Simultaneous investigation of magnetic activity was performed bySimultaneous investigation of magnetic activity was performed by a a measuring device calibrated according to metrological requiremenmeasuring device calibrated according to metrological requirements.ts.

In all the considered cases, the presence of AE events has been In all the considered cases, the presence of AE events has been always always observed during the damage process. observed during the damage process.

In the same tests EME signals were generally observed only in coIn the same tests EME signals were generally observed only in correspondence rrespondence of the final collapse or sharp stress drops.of the final collapse or sharp stress drops.

The Acoustic Emission TechniqueThe Acoustic Emission Technique

S1

S2

S

νPeak amplitude

Time A

B

10 µs

Sign

al v

olta

ge

Cracking is accompanied by the emission of elastic waves which pCracking is accompanied by the emission of elastic waves which propagate withinropagate within the bulk of the material. the bulk of the material.

These waves can be received by transducers applied to the surfacThese waves can be received by transducers applied to the surface of the structurae of the structuraelements.elements.

The adopted sensors are PZT transducers (50The adopted sensors are PZT transducers (50--500 kHz), 500 kHz), and PZT and PZT accelerometricaccelerometric transducers (1transducers (1--10kHz).10kHz).

AEAE--HF signals due to particle vibration HF signals due to particle vibration around their equilibrium position.around their equilibrium position.

AEAE--HF signals:HF signals:

-- duration duration ~~ 0.025 ms0.025 ms-- frequency frequency ~ 200 kHz~ 200 kHz-- wavelength ~ 0.02 mwavelength ~ 0.02 m

AEAE--LF signals due to relevant LF signals due to relevant oscillations of the entire specimen.oscillations of the entire specimen.

AEAE--LF signals:LF signals:

-- duration duration ~~ 3.5 ms3.5 ms-- frequency frequency ~ 6 kHz~ 6 kHz-- wavelength ~ 0.66 mwavelength ~ 0.66 m

Typical AE signals detected during compression testsTypical AE signals detected during compression tests

AE, high-frequency waves ELE, low-frequency wavesAE

Typical EM signal detected during compression testsTypical EM signal detected during compression tests

The analysed time duration of the EME is almost similar to the AThe analysed time duration of the EME is almost similar to the AE duration. E duration.

The main frequency is close to 160 kHz according to the working The main frequency is close to 160 kHz according to the working frequency frequency range (10 Hz and 400 kHz) of the adopted device. range (10 Hz and 400 kHz) of the adopted device.

•• PZT transducers are set on a frequency range between 50 kHz and PZT transducers are set on a frequency range between 50 kHz and 500 kHz.500 kHz.

•• Data acquisition system consisting in: 6 PreData acquisition system consisting in: 6 Pre--Amplified sensors, 6 Data storage Amplified sensors, 6 Data storage provided trigger, a central unit for the synchronization phase, provided trigger, a central unit for the synchronization phase, a threshold a threshold measurer.measurer.

•• From this elaborationFrom this elaboration microcracksmicrocracks localisationlocalisation isis performedperformed and theand the condition condition of theof the monitoredmonitored specimen canspecimen can be determinedbe determined..

AE Data acquisition system AE Data acquisition system

AE counting methodAE counting method

•• The event intensity is measured by the oscillation number The event intensity is measured by the oscillation number NNTT..

•• The oscillation number The oscillation number NNTT increases with the signal amplitude.increases with the signal amplitude.

As a first approximation, the number of counts As a first approximation, the number of counts NN can be compared with the can be compared with the quantity of energy released during the loading process, and we mquantity of energy released during the loading process, and we may assume ay assume that the corresponding increments grow proportionally to the widthat the corresponding increments grow proportionally to the widening of ening of the crack faces.the crack faces.

1 2 3

Sign

al v

olta

ge

Time Events

Envelope curve

Threshold level

Oscillations beyond threshold

Oscillation counting

1

Events counting

Time

(a) (b)

Sign

al v

olta

ge

Oscillation number NT=3

NT=3

AE behaviour of a quasiAE behaviour of a quasi--brittle material in compressionbrittle material in compression

The compression test were performed in displacement control, by The compression test were performed in displacement control, by imposing a imposing a constant rate of displacement of the upper loading platen.constant rate of displacement of the upper loading platen.

Compressive load vs. time, cumulated event number, and event ratCompressive load vs. time, cumulated event number, and event rate (for each e (for each second of the testing period) are depicted in this figure. second of the testing period) are depicted in this figure.

AE sensor

Steel platen

h/d = 2

PP

PP

AE sensor

Steel platen

h/d = 2

PP

PP

)(vN Log

v Log

b

1

0c Log

AE AE signalssignals amplitude distribution amplitude distribution (I)(I) (II)(II)

∫∞

= v pp dv)v(n)v(N

bovc)v(N −=

vbLogcLog)v(NLog o −=

Cumulative Cumulative signalssignals distributiondistribution

[ ]∫∞

=

v pp

2P s/Jdv)v(nv)t(E

Energy calculationEnergy calculation

(I)(I) Carpinteri, A., Carpinteri, A., ““Scaling laws and renormalization groups for strength and toughneScaling laws and renormalization groups for strength and toughness ss of disordered materialsof disordered materials””. . Int. Journal of Solids and StructuresInt. Journal of Solids and Structures, 31, 291, 31, 291--302 (1994).302 (1994).

(II)(II) Carpinteri, A., Lacidogna, G., Niccolini, G., Carpinteri, A., Lacidogna, G., Niccolini, G., ““Fractal analysis of damage detected in Fractal analysis of damage detected in concrete structural elements under loadingconcrete structural elements under loading””, , Chaos, Solitons & FractalsChaos, Solitons & Fractals, 42, 2047, 42, 2047--2056 2056 (2009).(2009).

•• The adopted device The adopted device −−NardaNarda ELT 400 exposure level ELT 400 exposure level testertester−− works in the frequency range between 10 Hz works in the frequency range between 10 Hz and 400 kHz.and 400 kHz.

•• The measurement rangeThe measurement range is between 1 is between 1 nTnT to 80 to 80 mTmT..

•• The triThe tri--axial measurement system has axial measurement system has a 1a 100 cm00 cm22

magnetic field sensor for each axis. magnetic field sensor for each axis.

•• This device was placed 1 m away from the specimensThis device was placed 1 m away from the specimens..

EM emission acquisition system EM emission acquisition system

Data acquisition of the EME signals was triggered when the magneData acquisition of the EME signals was triggered when the magnetic field tic field exceeded the exceeded the threshold fixed at 0.2 threshold fixed at 0.2 µµTT after preliminary measurements to after preliminary measurements to filter out the magnetic noise in the laboratory. filter out the magnetic noise in the laboratory.

The outputs of the AE transducer and the EM tester were connecteThe outputs of the AE transducer and the EM tester were connected to d to an oscilloscope in order to acquire simultaneously AE and EME sian oscilloscope in order to acquire simultaneously AE and EME signals gnals associated with the same fracture event (10 M Samples sassociated with the same fracture event (10 M Samples s––11). ).

100 cm100 cm

AEAESensorSensor

EMEMTesterTester

FridFrid et al. et al. (III)(III) and and RabinovitchRabinovitch et al. et al. (IV)(IV) recently proposed a general mechanism recently proposed a general mechanism to explain the EME origin, to explain the EME origin, which does not require any specific properties (such which does not require any specific properties (such as piezoelectricity) to take place.as piezoelectricity) to take place.

In this model mechanical and electrical equilibrium are broken aIn this model mechanical and electrical equilibrium are broken at the fracture t the fracture surfaces with creation of ions moving collectively.surfaces with creation of ions moving collectively.

Lines of positive ions on both newly created faces oscillate colLines of positive ions on both newly created faces oscillate collectively around lectively around their equilibrium positions in opposite phase to the negative ontheir equilibrium positions in opposite phase to the negative ones. es.

The resulting oscillating dipoles created on both faces of the pThe resulting oscillating dipoles created on both faces of the propagating ropagating fracture act as the source of EME. fracture act as the source of EME.

A model for EME origin A model for EME origin

(III)(III) FridFrid ,V., ,V., RabinovitchRabinovitch, A. and , A. and BahatBahat, D. , D. ““Fracture induced electromagnetic radiationFracture induced electromagnetic radiation””, , J. Phys. D.J. Phys. D., , 36, 162036, 1620--1628 (2003).1628 (2003).

(IV)(IV) RabinovitchRabinovitch, A., , A., FridFrid, V. and , V. and BahatBahat, D. , D. ““Surface oscillationsSurface oscillations. A . A possible sourcepossible source of of fracture induced fracture induced electromagnetic oscillationselectromagnetic oscillations””, , TectonophysicsTectonophysics, 431, 15, 431, 15--21 (2007).21 (2007).

This model correctly accounts for the occurrence of EME associatThis model correctly accounts for the occurrence of EME associated with ed with abrupt stress dropsabrupt stress drops(V)(V) in loadin load−−time diagrams which accompany time diagrams which accompany discontinuous crack propagation.discontinuous crack propagation.

stable

O

peak stress

Axial Strain, ε

Axi

al S

tress

, σ

A

E D

B C

unstable

Post-peak regime

Released Energy

stable

O

peak stress

Axial Strain, ε

Axi

al S

tress

, σ

A

E D

B C

unstable

Post-peak regime

Released Energy

X

Y

X

YZ

Direction of crack propagation

Hypothetical crack width

Layer of oscillating atoms

X

Y

X

YZ

Direction of crack propagation

Hypothetical crack width

Layer of oscillating atoms

X

Y

X

YZ

Direction of crack propagation

Hypothetical crack width

Layer of oscillating atoms

X

Y

X

YZ

Direction of crack propagation

Hypothetical crack width

Layer of oscillating atoms

““SnapSnap--backback”” instabilityinstabilityPropagating fracture as EME source Propagating fracture as EME source

(V)(V) Carpinteri, A., Carpinteri, A., ““Cusp catastrophe interpretation of fracture instabilityCusp catastrophe interpretation of fracture instability””, , J. of Mechanics and J. of Mechanics and Physics of SolidsPhysics of Solids, 37 (5), 567, 37 (5), 567--582 (1989).582 (1989).

(a)(a) (b)(b) (c)(c) (d)(d)

Views of the test specimensViews of the test specimens

CConcreteoncrete CarraraCarraraMarbleMarble

Luserna Luserna GraniteGranite

Syracuse Syracuse LimestoneLimestone

ExperimentalExperimental setset--upup

1.01.0××1010––66ππ××2.52.522××1010CylindricalCylindricalSyracuse LimestoneSyracuse LimestoneP4P4

2.02.0××1010––6666××66××10 10 PrismaticPrismaticGreen Luserna GraniteGreen Luserna GraniteP3P3

0.50.5××1010––6666××66××1010PrismaticPrismaticCarraraCarrara MarbleMarbleP2P2

0.50.5××1010––661010××1010××1010CubicCubicConcreteConcreteP1P1

PISTON VELOCITYPISTON VELOCITY[[ m sm s––11]]

VOLUME VOLUME [[cmcm33]]

SHAPESHAPEMATERIALMATERIALSPECIMENSPECIMEN

MaterialsMaterials,, shapesshapes,, sizessizes of the of the tested specimenstested specimens andand piston velocitiespiston velocities

The The dasheddashed line line representrepresent thethe cumulated numbercumulated number of AE (highof AE (high--frequency) frequency) whilewhile the the dotteddotted line the ELE (line the ELE (lowlow--frequency).frequency).

The star on theThe star on the graph showsgraph shows the moment of EMEthe moment of EME event with magnetic event with magnetic componentcomponent of 2.0of 2.0 µµTT..

Time (s)

0

100

200

300

400

500

0 500 1000 1500 2000 2500 3000 3500 40000

200

400

600

800

1000

Cumulated ELE

Load

Piston velocity 0.5×10−6 m s−1

Concrete specimen 100x100x100 mm3 EME (2.0 µT)

Cumulated AE

Cum

ulat

ed A

E ev

ents

Load

(kN

) Concrete Specimen P1 Concrete Specimen P1 -- LoadLoad vs. Time Curve vs. Time Curve

TheThe dasheddashed lineline representsrepresents thethe cumulated numbercumulated number of AEof AE--HF.HF.

The star on the graph shows the moment of EME event with magnetiThe star on the graph shows the moment of EME event with magnetic c component of 1.8 component of 1.8 µµT.T.

CarraraCarrara Marble Specimen P2 Marble Specimen P2 -- Load vs. Time CurveLoad vs. Time Curve

0

50

100

150

200

0 200 400 600 800 1000 1200 1400 16000

200

400

600

800

1000

Load

Cumulated AE

Cum

ulat

ed A

E ev

ents

Load

(kN

) EME (1.8 µT)

Carrara Marble specimen 60x60x100 mm3 Piston velocity 0.5×10−6 m s−1

Time (s)

TheThe dasheddashed lineline representsrepresents thethe cumulated numbercumulated number of AEof AE--HF.HF.

The star on the graph shows the moment of EME event with magnetiThe star on the graph shows the moment of EME event with magnetic c component of 1.9 component of 1.9 µµT.T.

Luserna GraniteLuserna Granite Specimen P3 Specimen P3 -- LoadLoad vs. Time Curve vs. Time Curve

0

100

200

300

400

500

0 500 1000 1500 20000

500

1000

1500

2000

2500

Load

Cumulated AE

Cum

ulat

ed A

E ev

ents

Load

(kN

)

EME (1.9 µT) Green Luserna Granite specimen 60x60x100 mm3 Piston velocity 2.0×10−6 m s−1

Time (s)

TheThe dasheddashed lineline representsrepresents thethe cumulated numbercumulated number of AEof AE--HF.HF.

The stars on the graph show the moments of EME events with magneThe stars on the graph show the moments of EME events with magnetic tic component comprised between 1.4 and 1.8 component comprised between 1.4 and 1.8 µµT.T.

SyracuseSyracuse LimestoneLimestone Specimen P4 Specimen P4 -- LoadLoad vs. Time Curvevs. Time Curve

0

20

40

60

80

100

0 1000 2000 3000 4000 5000 60000

500

1000

1500

2000

2500

3000

Load

(kN

)

Syracuse Limestone specimen π × 2.52 ×10 cm3

Load

EME 1 (1.4 µT)

EME 2 (1.5 µT)

EME 3 (1.8 µT)

Time (s)

Cum

ulat

ed A

E ev

ents

Piston velocity 1.0× 10-6 m s-1

Cumulated AE

TheThe dasheddashed lineline representsrepresents thethe cumulated numbercumulated number of AEof AE--HF, theHF, the histograms histograms representrepresent the AEthe AE--HF HF countcount rate.rate.

After the stressAfter the stress dropsdrops,, acoustic emissionsacoustic emissions areare very low duringvery low during thethe reloadingreloading of of the materialthe material untiluntil the stressthe stress exceedsexceeds thethe previous reached valuesprevious reached values..

SyracuseSyracuse LimestoneLimestone Specimen P4 Specimen P4 -- LoadLoad vs. Time Curvevs. Time CurveLo

ad (k

N)

300

250

200

150

100

50

0

3000

2500

2000

1500

1000

500

0

120

100

80

60

40

20

0

Syracuse Limestone specimen π × 2.52 ×10 cm3

Load

EME 1 (1.4 µT)

EME 2 (1.5 µT)

EME 3 (1.8 µT)

Time (s)

Cum

ulat

ed A

E

Piston velocity 1.0× 10-6 m s-1

Cumulated AE

0 1000 2000 3000 4000 5000 6000

AE

coun

t rat

e

AE count rate

Syracuse Limestone specimen π × 2.52 ×10 cm3 Piston velocity 1.0× 10-6 m s-1

0.0003 0.00035 0.0004 0.00045 0.0005

Time (s)

0.0

1.0

2.0

3.0

4.0

−1.0

EM fi

eld

inte

nsity

(µT)

AE

(arb

itrar

y un

its)

SyracuseSyracuse LimestoneLimestone Specimen P4 Specimen P4 Displacement rate:Displacement rate: 1.0 1.0 ×× 1010––6 6 ss––11

A pair of AE and EME signals associated with the same fracture eA pair of AE and EME signals associated with the same fracture event.vent.

EME 1 (1.4 µT)

AE

Load vs. time diagram represented with cumulative Load vs. time diagram represented with cumulative AEAE--HF and AEHF and AE--LF counts for each minute of the testing time.LF counts for each minute of the testing time.

DetailedDetailed AE dataAE data analysisanalysis on the Concrete Specimen P1on the Concrete Specimen P1

AEAE--HF and AEHF and AE--LF time history on Concrete Specimen P1. LF time history on Concrete Specimen P1.

Peak load Peak loadAE-HF rate AE-LF rate

Peak load

2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10 11

120

110

100

80

70

60

50

40

30

90

(a)

Frequency (kHz)

Am

plitu

de (d

B)

Signal Time: 3424 s Total level: 115.0 dB Measured acceleration: 0.56 m/s2 ELE signal energy: ~3⋅10-9 J

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10 11

120

110

100

80

70

60

50

40

30

90

(a)

Frequency (kHz)

Am

plitu

de (d

B)

Signal Time: 3424 s Total level: 115.0 dB Measured acceleration: 0.56 m/s2 ELE signal energy: ~3⋅10-9 J

1.0

3524

AE AE lowlow--frequency frequency signals spectral contentssignals spectral contents

Peak Peak frequenciesfrequencies time time historyhistoryBefore Before the the peak peak load load

After theAfter thepeak eak load load

Frequency (kHz)

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10 11

120

110

100

90

80

70

60

50

40

30

Signal Time: 2757 s Total level: 77.1 dB Measured acceleration: 0.007 m/s2 ELE signal energy: ~10-13 J

Am

plitu

de (d

B)

AE FrequencyAE Frequency--Magnitude StatisticsMagnitude Statistics

The The magnitudemagnitude in terms of AE technique is defined as follows:in terms of AE technique is defined as follows:

(2)(2)

AAmaxmax : signal amplitude, measured in microvolt; : signal amplitude, measured in microvolt;

ff((rr)) : correction taking in: correction taking intoto account that the amplitude is a decreasing account that the amplitude is a decreasing

function of the distance function of the distance rr between the source and the sensorbetween the source and the sensor..

),(max10 rfALogm +=

The GR relationship has been tested successfully in the acousticThe GR relationship has been tested successfully in the acoustic emission field emission field to study the scaling of the to study the scaling of the ‘‘‘‘amplitude distributionamplitude distribution’’’’ inin AE wavesAE waves::

(1)(1),10)(or,)(10bmamNbmamNLog −=≥−=≥

NN : : cumulative number of AE events with magnitude cumulative number of AE events with magnitude ≥≥ mm..

The The bb--value changes systematically with the different stages of fractuvalue changes systematically with the different stages of fracture growth re growth and hence it can be used to estimate the development of fractureand hence it can be used to estimate the development of fracture process.process.

In particular, recent resultsIn particular, recent results(VI)(VII(VI)(VII)) from AE laboratory tests on different types offrom AE laboratory tests on different types of specimens, as well as specimens, as well as in situin situ AE investigations, show that at AE investigations, show that at the condition of the condition of criticalitycriticality, when the external load equals the peak load:, when the external load equals the peak load:

bb--value value ≅≅ 1.5.1.5.

In the later stages of damage evolution, when theIn the later stages of damage evolution, when the fifinal nal failure is imminentfailure is imminent::

bb--value value →→ 11..

(VI)(VI) Carpinteri, A., Lacidogna, G., Carpinteri, A., Lacidogna, G., PuzziPuzzi, S., , S., ““From criticality to final collapse: evolution of From criticality to final collapse: evolution of the the ““bb--valuevalue”” from 1.5 to 1.0from 1.5 to 1.0””, , Chaos, Solitons & FractalsChaos, Solitons & Fractals, 41, 843, 41, 843--853 (2009). 853 (2009).

(VII)(VII) Carpinteri, A., Lacidogna, G., Niccolini, G., Carpinteri, A., Lacidogna, G., Niccolini, G., PuzziPuzzi, S., , S., ““Critical defect size distributions Critical defect size distributions in concrete structures detected by the acoustic emission techniqin concrete structures detected by the acoustic emission techniqueue””, , MeccanicaMeccanica, 43, 349, 43, 349--363 (2008). 363 (2008).

TrendsTrends of the of the bb--value computed forvalue computed for AE and ELE AE and ELE signalssignals..

0.0

0.4

0.8

1.2

1.6

2.0

0 500 1000 1500 2000 2500 3000 3500 4000

b-value = 1.0

b-va

lue

Time (s)

AE ELE

b-value = 1.5 Peak Load Peak load

AE, high-frequency wavesAE, low-frequency waves

Load vs. time curveLoad vs. time curve and and EME EME events with their magnetic events with their magnetic component.component.

Relation between EME and stress dropsRelation between EME and stress dropsCylindricalCylindrical Luserna Granite specimen, Volume: Luserna Granite specimen, Volume: ππ××1.41.422××2.8 cm2.8 cm33

Load vs. time curveLoad vs. time curve and time and time derivative of derivative of loadload up to the final up to the final failurefailure..

VIII)VIII) WanWan, G. , Li, X. and Hong, L. , G. , Li, X. and Hong, L. ““Piezoelectric responsesPiezoelectric responses ofof brittlebrittle rock massrock mass containing quartzcontaining quartz to to statstatistress andstress and explodingexploding stressstress wave respectivelywave respectively””, , J. Cent.J. Cent. South UnivSouth Univ.. TechnolTechnol.,., 15, 34415, 344--349 (2008).349 (2008).

IX)IX) FukuiFukui, K., , K., OkuboOkubo ,S., and ,S., and TerashimaTerashima T. T. ““Electromagnetic radiation fromElectromagnetic radiation from rockrock during uniaxial during uniaxial compression testingcompression testing””, , RockRock MechMech. Rock . Rock EngngEngng., 38, 411., 38, 411--423 (2005).423 (2005).

Recently proposed relationsRecently proposed relations(VIII) (IX)(VIII) (IX) betweenbetween stress and EME stress and EME eventsevents generallygenerally showshowthat increasing that increasing the stress the stress levellevel, the EME, the EME intensityintensity growsgrows. .

From our experiments From our experiments the relation can be well approximated by: the relation can be well approximated by: E E ∝∝ ∆∆σ σ 22

Relation between EME and compression stress

02468

101214161820

0 10 20 30 40 50 60

Stress Drop (MPa)EM

E ev

ent (

µT) Best Fit Function

Experimantal Data

∆σ

Axial Strain, ε

Axia

l Stre

ss, σ

Conclusions•• The experimental observations confirm the occurrence of AE and EThe experimental observations confirm the occurrence of AE and EME signals ME signals

as failure precursors in materials like rocks and concrete. as failure precursors in materials like rocks and concrete.

•• The simultaneous detection of AE and EME signals is an evidence The simultaneous detection of AE and EME signals is an evidence that that cracking generates temporally varying EM fields. cracking generates temporally varying EM fields.

•• In all tested materials the EME events are associated to abrupt In all tested materials the EME events are associated to abrupt stress drops stress drops suggesting that EME originates from crack propagation.suggesting that EME originates from crack propagation.

•• Mechanisms such as electrification due to friction effects seem Mechanisms such as electrification due to friction effects seem to be excluded to be excluded by the experimental observations. by the experimental observations.

•• In fact, we donIn fact, we don’’t detect EME during postt detect EME during post--failure stages, manly characterized failure stages, manly characterized by friction due to sliding of the existing crack faces.by friction due to sliding of the existing crack faces.

•• As AE and EME have been observed As AE and EME have been observed before the onset of earthquakesbefore the onset of earthquakes, , detection detection of these phenomena during laboratory experiments looks promisingof these phenomena during laboratory experiments looks promising for for effective applications at structural and geological scales.effective applications at structural and geological scales.