mechanical and electromagnetic emissions related to … · 2012-09-13 · mechanical and...

T E C H N I C A L A R T I C L E

Mechanical and Electromagnetic Emissions Relatedto Stress-Induced CracksA. Carpinteri1, G. Lacidogna1, A. Manuello1, G. Niccolini2, A. Schiavi2, and A. Agosto2

1 Department of Structural Engineering and Geotechnics, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino, Italy

2 National Research Institute of Metrology—INRIM, Strada delle Cacce 91, Torino, Italy

KeywordsAcoustic Emission, Electromagnetic Emission,

Brittle Fracture, Stress Drop, Crack Growth

CorrespondenceG. Lacidogna,

Department of Structural Engineering &

Geotechnics, Politecnico di Torino, Corso

Duca degli Abruzzi 24, 10129 Torino, Italy

Email: [email protected]

Received: December 31, 2009; accepted:

September 23, 2010

doi:10.1111/j.1747-1567.2011.00709.x

Abstract

The present research focuses on acoustic emission (AE) and electromagneticemission (EME) detected during laboratory compression tests on concrete androcks specimens. We investigated their mechanical behavior up to failure bythe AE and EME due to micro- and macrocrack growth. Among the testedspecimens, a concrete sample was analyzed by applying to its surface bothpiezoelectric (PZT) transducers for detection of high-frequency AE waves,and PZT accelerometric transducers for detection of low-frequency AE (elasticemission or ELE). Besides the high-frequency AEs, the emergence of low-frequency ELE just before the failure describes the transition from diffusedmicrocracking to localized macrocracks which characterizes the failure in brittlematerials. For all the specimens, a simultaneous analysis of magnetic activitywas performed by a measuring device calibrated according to metrologicalrequirements. In all the considered specimens, the presence of AE events hasbeen always observed during the damage process, whereas it is very interestingto note that the EMEs were generally observed only in correspondence withsharp stress drops or the final collapse. The experimental evidence confirms AEand EME signals as collapse precursors in materials like concrete and rocks.

The present research focuses on acoustic emission(AE) and electromagnetic emission (EME) detectedduring laboratory compression tests on concrete androcks specimens. We investigated their mechani-cal behavior up to failure by the AE and EMEdue to micro- and macrocrack growth. Among thetested specimens, a concrete sample was analyzed byapplying to its surface both piezoelectric (PZT) trans-ducers for detection of high-frequency AE waves, andPZT accelerometric transducers for detection of low-frequency AE (elastic emission, or ELE).1 Besides thehigh-frequency AEs, the emergence of low-frequencyELEs just before the failure describes the transitionfrom diffused microcracking to localized macrocrackswhich characterizes the failure in brittle materials.

For all the specimens, a simultaneous analysis ofmagnetic activity was performed by a measuringdevice calibrated according to metrological require-ments. In all the considered specimens, the presence

of AE events has been always observed during thedamage process, whereas it is very interesting to notethat the EMEs, detected by a dedicated Narda ELT-400device, were generally observed only in correspon-dence with sharp stress drops or the final collapse.2

The experimental evidence confirms AE and EMEsignals as collapse precursors in materials like concreteand rocks. As AE and EME coincide when certaintypes of rocks fail, the influence of EM fields onthe AE transducers has been previously evaluatedin order to be minimized. Given a fracture process,the AE activity behaves as fracture precursor, since itprecedes EME events, which accompany stress dropsand related discontinuous fracture advancements.While the mechanism of AE is fully understood,being provided by transient elastic waves due to stressredistribution following fracture propagation,3–8 theorigin of EME from fracture is not completely clearand different attempts have been made to explain it.

Experimental Techniques 36 (2012) 53–64 © 2011, Society for Experimental Mechanics 53

Mechanical and Electromagnetic Emissions A. Carpinteri et al.

Elastic Wave Propagation in High- andLow-Frequency Ranges

During the damage of a brittle material in compres-sion, micro- and macrocracks generate mechanicalvibrations or elastic waves of frequency and wave-length related to the size of the cracks.

In the case of AE, the wave propagation is dueto the oscillations of material particles around theirequilibrium positions (Fig. 1(a and b)). As the collapseis approaching, macrocracks are created and somemacroscopic portions of the specimen are subjectedto sudden and appreciable changes of their spatiallocations. The impulse generated by a macrocrackinvolves a perturbation (ELE) able to ‘‘shake’’ thewhole specimen with relevant oscillation amplitudesand low oscillation frequencies (Fig. 1(c and d)).These oscillations, detected at the very last stagesof the test, are analogous to ‘‘shock waves’’ forwhich the medium moves from the equilibriumposition.9,10 Based on these considerations, it istherefore possible to distinguish the effects of thetwo different emissions, AE and ELE, and analyzethem separately.

In Fig. 1(a), a typical impulsive AE signal detectedduring a compression test is reported. In a similar way,in Fig. 1(c), a typical ELE signal is shown. AE and ELEsignals differ by about two orders of magnitude inthe signal duration, 0.025 and 3.5 ms, respectively.These two detected signals are in accordance tothe frequency range of PZT (50–500 kHz), andaccelerometer transducers (1–10 kHz) used duringthe data acquisition. Furthermore, the AE signal hasa mean frequency of about 200 kHz, while the ELEsignal is characterized by a frequency of about 6 kHz.

Considering that the velocity of AE and ELEpropagation in quasibrittle materials like concrete androcks6 is about of 4000 m/s, the signal wavelengthsof AE and ELE are 0.02 and 0.66 m, respectively. Inthe case of ELE the signal wavelength is greater thanthe specimen dimension.

Models for EME

An explanation of the EME origin was related to dis-location phenomena,11,12 which, however, are notable to explain EME from fracture in brittle materialswhere the motion of dislocations can be neglected.13

The weakness of the ‘‘dislocation movement hypoth-esis’’ was confirmed in some experiments showingthat the EME amplitude increased with the brit-tleness of the investigated materials.14 In brittlematerials the fracture propagation occurs suddenlyand it is accompanied by abrupt stress drops in the

sensor Micro-crack

AE, high-frequency waves

Am

plit

ude

[arb

itra

ry u

nits

]

Time (ms )

sensor Macro-crack

ELE, low-frequency waves

Am

plit

ude

[arb

itra

ry u

nits

]

Time (ms )

F

F

F

F

Particle and specimen vibration

Particle vibration

3.5 ms

0.025 ms

(b)

(d)

(a)

(c)

Figure 1 (a) Typical AE signal detected during compressive test.

(b) Schematic representation of AE due to particle vibration around their

equilibrium position. (c) Typical ELE signal detected during compressive

test. (d) Schematic representation of ELE due to relevant oscillations of

the entire specimen.

stress–strain curve related to sudden loss in the spec-imen stiffness.

Another relevant attempt to explain the EME originwas made through the ‘‘capacitor model,’’ whereEME is assumed to be caused by net charges ofopposite sign appearing on the vibrating facesof opening cracks.15,16 This model is not widelyaccepted since an accelerated electric dipole createdby charged opening cracks apparently does notexplain EME from shearing cracks, which indeedare experimentally observed.13 However, the generalvalidity of this model can be maintained, sincea certain separation between charged faces isguaranteed even in shear fractures.

In Fig. 2(a), a typical EM signal detected duringa compression test is shown. The analyzed timeduration of the EME is almost similar to the AEduration. The main frequency is close to 160 kHzaccording to the working frequency range of theadopted device Narda ELT-400, that is 10 Hz and400 kHz.

54 Experimental Techniques 36 (2012) 53–64 © 2011, Society for Experimental Mechanics

A. Carpinteri et al. Mechanical and Electromagnetic Emissions

EME, Electtromagnetic Emission

X

Y

X

YZ

Direction of crack propagation

Hypothetical crack width

Layer of oscillating atoms

Am

plit

ude

[arb

itra

ry u

nits

] 0.035 ms

(a)

(b)

(c)

Time (ms)

Figure 2 (a) Typical EM signal

detected during a compressive test.

(b) Crack surfaces are in the xz

plane and the crack propagates in

the x direction. (c) Schematic

representation at a specific time of

surface waves propagating on the

two newly formed crack surfaces.

Layers of atoms move together

generating surface vibrational

waves on each face, where positive

charges vibrate in opposite phase

to the negative ones.

Frid et al.13 and Rabinovitch et al.17 recentlyproposed a model of the EME origin, where, followingthe rupture of bonds during the cracks’ growth,mechanical and electrical equilibrium are broken atthe fracture surfaces with creation of ions movingcollectively as a surface wave on both faces. Linesof positive ions on both newly created faces (whichmaintain their charge neutrality unlike the capacitormodel) oscillate collectively around their equilibriumpositions in opposite phase to the negative ones(see Fig. 2(b and c)). The resulting oscillating dipolescreated on both faces of the propagating fracture act asthe source of EME. According to this model, the EMEamplitude increases as long as the fracture propagates,since the rupture of new atomic bonds contributes tothe EME. When fracture stops, the waves and theEME decay by relaxation.

Since larger fracture advancements produce largerstress drops, this model agrees with the resultsin compression tests on rock specimens obtainedby Fukui et al.,18 which establish a relationship ofproportionality between stress drop and intensityof related EME.

Four specimens made of different brittle materi-als (Concrete, Syracuse Limestone, Carrara Marble,and Green Luserna Granite) were examined in thisstudy (Fig. 3).2 They were subjected to uniaxial

(a) (b)

(c) (d)

Figure 3 Views of test specimens: (a) Concrete, (b) Syracuse Limestone,

(c) Carrara Marble, and (d) Green Luserna Granite.



compression using a Baldwin servo-controlledhydraulic testing machine with a maximum capac-ity of 500 kN and load measurement accuracy of±1.0%. This machine is equipped with control elec-tronics which makes it possible to carry out tests ineither load control or displacement control. Each testwas performed in piston travel displacement controlby setting constant piston velocity. The test specimenswere arranged in contact with the press platens with-out any coupling materials, according to the testingmodalities known as ‘‘test by means of rigid platenswith friction.’’ The tested materials, shapes and sizesof the specimens, and the employed piston velocitiesare listed in Table 1.

AE and EME Measurements

The AE emerging from the compressed specimenswas detected applying to the sample surface a PZTtransducer, sensitive in the frequency range from50 to 500 kHz for detection of high-frequency AE. Asstated in the introduction, during the test on specimenP1 (see Table 1), a PZT accelerometric transducer,sensitive in the frequency range from 1 to 10 kHz,was also used for detection of low-frequency AE(ELE).1

The EM emission was detected using an isotropicprobe calibrated according to metrological require-ments at the National Research Institute of Metrology(Turin, Italy) for measuring the magnetic componentof EM fields. The adopted device (Narda ELT-400exposure level tester) works in the frequency rangebetween 10 Hz and 400 kHz, the measurement rangeis between 1 nT and 80 mT, and the three-axialmeasurement system has a 100 cm2 magnetic fieldsensor for each axis. This device was placed 1 m awayfrom the specimens.

The outputs of the PZT transducer and themagnetic tester were connected to a DL708 Yokogawaoscilloscope (10 MSa s−1) in order to acquiresimultaneously AE and EME signals associated with

Table 1 Materials, shapes, sizes of the tested specimens, and piston

velocities

Specimen Material Shape Volume (cm3)

Piston velocity

(m s−1)

P1 Concrete Cubic 10 × 10 × 10 0.5 × 10−6

P2 Syracuse

Limestone

Cylindrical π × 2.52 × 10 1.0 × 10−6

P3 Carrara Marble Prismatic 6 × 6 × 10 0.5 × 10−6

P4 Green Luserna

Granite

Prismatic 6 × 6 × 10 2.0 × 10−6

the same fracture event. Data acquisition of theEME signals was triggered when the magneticfield exceeded the threshold fixed at 0.2 μT afterpreliminary measurements to filter out the magneticnoise in the laboratory. The recorded AE and EMEsignals were related to the time history of the loadapplied to the specimens.

Test Results

All specimens were tested in compression up tofailure, showing a brittle response with a rapiddecrease in load-carrying capacity when deformedbeyond the peak load (Figs. 4–7). Experimentalevidence indicates the presence of AE and EME:an increasing AE activity is always detected as theload increases, while EME is only observed whenabrupt stress drops occur. In the following we focuson specimens P1 and P2 (Figs. 4 and 5), representingtwo typical examples of catastrophic and quasibrittlebehaviors.19–22

The Concrete specimen P1 exhibits a very steepsoftening branch (descending part of the stress–straincurve), which is called ‘‘snap-back’’ or catastrophicbehavior (Fig. 4).20–22 On the other hand, the Syra-cuse Limestone, specimen P2, retains considerablestrength beyond the peak load. Despite their differentmechanical behaviors, specimens P1 and P2 both gen-erate EME during sharp stress drops: P1 only at thepeak load, whereas P2 even at the two intermediatestress drops occurred before the peak load. No furtherEME signals were detected in the post-peak region(Fig. 5).

The Concrete specimen P1, subjected to AE, ELE,and EME monitoring, is characterized by a load versustime diagram almost linear up to failure. At 70% ofthe peak load, we observed a significant increasein the AE rate (the slope of the dashed line inFig. 4), and the appearance of ELE (the dotted linein Fig. 4). At 90% of the peak load, the ELE rateincreases dramatically while the AE rate suddenlydrops down. This evidence clearly indicates thetransition from a microcracking-dominated damageprocess (revealed by the AE occurrence) to aprocess dominated by the propagation of a fewlarge cracks and ELE. Therefore, AE and ELE areboth fracture precursors, while an EME event (withmagnetic component of 2 μT) was detected just incorrespondence with the abrupt stress drop occurredat the specimen collapse (i.e., at the peak load).

The Syracuse Limestone specimen P2, subjectedto AE and EME monitoring, is characterized bya more complex load versus time diagram due



Time (s)

0

100

200

300

400

500

0 500 1000 1500 2000 2500 3000 3500 4000

0

200

400

600

800

1000

Cumulated ELE

Load

Piston velocity 0.5×10−6 ms−1

Concrete specimen 100x100x100 mm3 EME (2.0 μT)

Cumulated AE

Cu

mu

late

d A

E e

ven

ts

Lo

ad (

kN)

Figure 4 Load versus time curve of the Concrete specimen P1 (bold line). The dashed line and the dotted line represent the cumulated number of,

respectively, AE (high-frequency) and ELE (low-frequency). The star on the graph shows the moment of EME event with a magnetic component of

2.0 μT.

to the heterogeneity of limestone. We observedthree stress drops followed by as many drops inthe AE rate, suggesting momentary relaxation aftersudden crack advancements (Fig. 5). Even in thiscase, we detected three EME events (with magneticcomponents ranging between 1.4 and 1.8 μT) incorrespondence with each observed stress drop untilthe peak load is reached. The first stress dropoccurred at 70% of the peak load, and the last oneoccurred at the peak load. It is worth noting thatthe EME intensity apparently does not depend, ordepends weakly, as observed in Fukui et al.,18 onthe stress drop. In fact, similar magnetic componentsare associated to different stress drops. In particular,

the first stress drop is clearly the smallest. Duringthe post-peak stage, that is, softening branch in theload versus time diagram, no further EME signalswere detected. In fact, at the peak load the fractureis completely formed and the subsequent stages arecharacterized only by opening of the fracture surfaces.According to the model proposed by Frid et al.13 andRabinovitch et al.,17 this means that no newly brokenatomic bonds can contribute to EME. Summarizing,the AE activity behaves as fracture precursor since itprecedes EME events, which accompany stress dropsand related discontinuous fracture advancements (seealso the response of specimens P3 and P4, reported inFigs. 6 and 7).

Cu

mu

late

d A

E e

ven

ts

0

20

40

60

80

100

0 1000 2000 3000 4000 5000 6000

0

500

1000

1500

2000

2500

3000

Lo

ad (

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)

Piston velocity 1× 10-6 m s-1

AE

Figure 5 Load versus time curve of the Syracuse Limestone specimen P2 (bold line). The dashed line represents the cumulated number of AE. The

stars on the graph show the moments of EME events with magnetic component comprised between 1.4 and 1.8 μT.



0

50

100

150

200

0 200 400 600 800 1000 1200 1400 1600

0

200

400

600

800

1000

Load

Cumulated AE

Cu

mu

late

d A

E e

ven

ts

EME (1.8 μT) Carrara Marble specimen 60x60x100 mm3

Piston velocity 0.5×10−6 m s−1

Time (s)

Lo

ad (

kN)

Figure 6 Load versus time curve of the Carrara Marble specimen P3 (bold line). The dashed line represents the cumulated number of AE. The star on

the graph shows the moment of EME event (magnetic component of 1.8 μT).

Detailed AE Data Analysis on Specimen P1

First of all, the global level of the background-noisevibration in the low-frequency range, between 1 and10 kHz, has been evaluated in the laboratory. In thisfrequency range, the mechanical noise has an averagevalue of about 62 ± 2 dB (referred to 1 μm/s2). Inparticular, it is possible to distinguish three differentpeak levels corresponding to 2.5, 5.3, and 5.8 kHz,with an almost constant peak level between 52 and56 dB (Fig. 8). These signals can be recognized asdue to the servo-hydraulic press noise. Nevertheless,it is possible to note that the background noise hasa negligible influence compared to ELE signals. TheELE acceleration spectral levels are between 80 and120 dB.

In Fig. 9(a and b) the amplitude-level time historiesof AE and ELE on Concrete specimen P1 are reported.AE have been detected since the beginning of the test(Fig. 9(a)), while the early ELE have been detected2200 s later (Fig. 9(b)). The late appearance of ELE,their increasing rate (number of ELEs per unit time),and the increase in the amplitude levels are signaturesof specimen damage evolution. In Fig. 9(c) load versustime, together with cumulative AE and ELE counts(for each minute of the testing time), are depicted.

Cumulative AE counts increase, very slowly at first,then proportionally to the load up to 2500 s. After thistime, AE rate increases notably reaching a peak in theproximity of the highest load (3500 s). The AE rateincrease (observed in the time window: 2500–3500 s)

0

100

200

300

400

500

0 500 1000 1500 2000

0

500

1000

1500

2000

2500

Load

Cumulated AE

Cu

mu

late

d A

E e

ven

ts

EME (1.9 μT)Green Luserna Granite specimen 60x60x100 mm3

Piston velocity 2.0×10−6 m s−1

Lo

ad (

kN)

Time (s)

Figure 7 Load versus time curve of the Green Luserna Granite specimen P4 (bold line). The dashed line represents the cumulated number of AE. The

star on the graph shows the moment of EME event (magnetic component of 1.9 μT).



Figure 8 Global level of the background-noise vibration.

is in correspondence with a dramatic increase in ELEcounts. In this phase (3500–3700 s), just before thecollapse condition, ELE counts grow more quicklythan AE counts and a larger number of macrocracksis created (Fig. 9(c)).

Figure 10(a) represents the initial phase of ELEactivity characterized by sporadic signals with energycontent concentrated in a narrow frequency interval.Similarly in Fig. 10(b) the phase just before the spec-imen collapse is represented. In this case a greaternumber of ELEs is detected with more significantenergy content in a wide frequency range, proba-bly extending beyond the observation window size.Figure 11 shows the spectral contents of two ELEsat 2757 and 3424 s. In Fig. 11(a) the spectrum has apeak level equal to 62 dB and a global level of about77.1 dB. In Fig. 11(b) the spectrum has a peak levelequal to 100 dB and a global level of about 115.0 dB.Based on these data, it is possible to describe macro-crack effects in terms of released energy, measuringthe local acceleration in the accelerometer point ofapplication.

In Fig. 12, the peak frequency and amplitude ofeach ELE signal are reported. By means of thesediagrams, it is possible to investigate the specimendamage evolution. The time dependence of the peakfrequencies and amplitudes are split into two partsidentifying the two different stages in the damageevolution.

In the first part, the peak frequencies arebetween 4.8 and 5.8 kHz (Fig. 12(a)), while the peakamplitudes are between 60 and 75 dB (Fig. 12(b)).Though the number of macrocracks increases,macroscopic collapse is not reached.

On the contrary, in the second phase, a suddendecay in the frequency domain and an abrupt increasein the amplitude levels indicate that macrocracks coa-lesce to generate the final rupture surfaces (Figs. 12(a

and b)). Therefore, the peak frequencies of ELE signalsdecrease with increasing damage level. These resultsimply that the ELE frequency decay can be assumedas a valid indicator of the damage evolution.

AE and ELE Frequency–Magnitude Statistics

By analogy with seismic phenomena, the magni-tude of AE and ELE events can be defined asfollows6,8,23–26:

m = Log10Amax + f (r) (1)

where Amax is the amplitude of the signal expressedin dB (referred to 1 mV for the AEs and to 1 μm/s2 forthe ELEs), and f (r) is a correction term which accountsfor the amplitude attenuation with the distancer between the source and the sensor. In seismology,the Gutenberg–Richter (GR) empirical law27

Log10N(≥m) = a − bm or N(≥m) = 10a−bm (2)

is one of the most widely used statistical relationshipsto describe the scaling properties of seismicity.

In Eq. 2, N is the cumulative number of earth-quakes with magnitude ≥m in a given area and withina specific time period, whilst a and b are positive con-stants depending on the considered area and timeperiod.

Equation 2 has been successfully used in the AEfield to study the scaling laws of the amplitudedistribution of AEs. This approach emphasizes thesimilarity between damage phenomena in the mate-rials and the seismic activity in a given region ofthe Earth Crust, extending the applicability of theGR law to Damage Mechanics. The b-value in Eq. 2changes systematically during the different stages ofthe damage process and therefore can be used todetect the evolution of damage.6,8,23–26



60

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0 500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Am

plit

ud

e (d

B)

50

60

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80

90

100

110

0 500 1000 1500 2000 2500 3000 3500 4000

0

100

200

300

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500

0 500 1000 1500 2000 2500 3000 3500 4000

0

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400

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Cumulated ELE

Load

Concrete specimen 100x100x100 mm3

Cumulated AE

Am

plit

ud

e (d

B)

Time (s) (a)

(b)

(c)Time (s)

Lo

ad (

KN

)

Cu

mu

late

d A

E e

ven

ts

Figure 9 (a) AE and (b) ELE time history on Concrete specimen P1. (c) Load versus time together with cumulative AE and ELE counts for each minute

of the testing time.



(a)

Frequency (kHz)

Am

plitu

de (

dB)

2500 s

2440 s 10

100

(b)

3440 s

3500 s

Frequency (kHz)

Am

plitu

de (

dB)

10

100

Figure 10 (a) An initial and (b) a final phase of ELE activity during the

test.

The GR law can be written in an alternative form28:

N(≥L) = cL−2b (3)

where N is the cumulative number of AE eventsgenerated by cracks having a characteristic length≥L, c is a constant of proportionality, and 2b = D isthe fractal dimension of the damage domain. It hasbeen pointed out that this interpretation rests on theassumption of a dislocation model for the seismicsource and requires that 2 ≤ D ≤ 3, that is the cracksare distributed in a fractal domain comprised betweena surface and the volume of the analyzed region.28

The cumulative distribution of Eq. 3 is substantiallyidentical to the one proposed by Carpinteri,20,29

according to which the number of cracks with length≥L contained in a body is given by the following:

N∗(≥L) ∼ NtotL−γ (4)

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

(b)

Frequency (kHz)

Ampl

itude

(dB)

Signal Time: 3424 s Total level: 115.0 dB Measured acceleration: 0.56 m/s2 ELE signal energy: ~3.10-9 J

1.0

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

(a)

Ampl

itude

(dB)

Figure 11 Spectral contents of two ELEs measured at (a) 2757 and

(b) 3424 s.

In Eq. 4, γ is a statistical exponent reflectingthe disorder, that is, the dispersion of the cracklength distribution, and Ntot is the total number ofcracks contained in the body. From Eqs. 3 and 4,we find that 2b = γ . During the formation of thefinal fracture, cracks concentrate in a narrow bandto form the final fracture surface. In this case, asshown by Carpinteri20 and Carpinteri et al.,6,8,25,26

the self-similarity condition entails γ = 2.0. Thisexponent corresponds to the value b = 1.0, whichis experimentally approached in structural elementsduring the final crack propagation.

Subdividing the loading process into different stagesand calculating the related b-values, the relationD = 2b permits to explain the evolution of damagein terms of progressive microcracks localization ontopreferential domains. The trends of the b-value dur-ing the test on Concrete specimen P1 are shown inFig. 13 for both AE and ELE events. These trendsare obtained by partitioning all detected events intogroups of 100 events, and for each group the b-valuehas been calculated. With this method, already



Figure 12 (a) Time history of peak frequencies and (b) peak amplitudes

of ELE signals.

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-v

alu

e

Time (s)

AE

ELE

b-value = 1.5 Peak Load

Figure 13 Trends of the b-value computed for AE and ELE signals.

adopted in other damage analyses in structural con-crete elements,23,30 the testing time was subdividedinto 6 and 10 intervals (600 and 1000 events) for ELEand AE time series, respectively.

Figure 13 shows that AEs generated during theearly stages of loading give high b-values (b > 1.5). Inparticular, the b-values obtained by AE signals resultto be greater than 1.8 during the first 2500 s. After3000 s, the b-values reach 1.5 and tend to 1.0 at theend of the loading process.

When the b-value is ∼=1.5 (Fig. 13), the cracksrevealed by the AE signals are likely to be uniformlydistributed throughout the specimen volume (D =2b ∼= 3). During the subsequent stages, microcrackscoalesce to form macrocracks and the b-value dropsbelow 1.5 for both AE and ELE signals. The rapiddecay of the b-value observed close to the peak load

is a further confirmation to the catastrophic behaviorof the concrete specimen.

Conclusions

It is widely reported that changes in geoelectricpotential and anomalous radiation of geoelectromag-netic waves, especially in low-frequency bands, occurbefore major earthquakes. At the laboratory scale,similar phenomena have also been observed on rockspecimens under loading. In this case, crack growthis accompanied by AE ultrasonic waves and by redis-tribution of electric charges.

We investigated the mechanical behavior of rocksand concrete samples loaded in compression upto their failure by the analysis of AE and EMEsignals. In all the considered cases, the presenceof AE events has been always observed during thedamage process. Moreover, it is very interesting tonote that the EME was generally observed only incorrespondence with the sharp stress drops in the loadversus time diagrams. While the mechanism of AE iswell understood, we adopted the model proposed byFrid et al.13 and Rabinovitch et al.17 to explain theEME origin, according to which EME is generated byoscillating dipoles created by ions moving collectivelyas a surface wave on both faces of the crack. Thismodel accounts correctly for the occurrence of theabrupt stress drops in load versus time diagramsbecause of sudden loss of specimen stiffness whichaccompanies discontinuous crack propagation.19,21,22

Finally, the damage process occurring in a concretespecimen under compression was carefully studiedby means of elastic wave propagation induced bycrack growth. In particular, ELE in the frequencyrange 1–10 kHz were detected and analyzed. Inthe last phases of the test, ELE count grows morequickly than AE count, revealing that a largenumber of macrocracks is generated just beforethe collapse condition. These results imply thatthe sudden increase in the number and amplitudeand the simultaneous frequency decay of ELEs canbe assumed as valid indicators of the impendingfailure. Furthermore, the achievement of the criticalcondition is also investigated through a syntheticparameter, the b-value. The rapid decay of b-valueis a further confirmation to the steep decay in theload versus strain curve after the peak load.

Acknowledgments

The financial support provided by the RegionePiemonte RE-FRESCOS Project is gratefully acknowl-edged. Special thanks are due to Mr. V. Di



Vasto from the Politecnico di Torino for his col-laboration in the execution of mechanical com-pressive tests. The authors are also grateful toDr. Eng. Pavoni Belli of the National Research Insti-tute of Metrology—INRIM for his valuable assistancein the AE signals elaboration process.

References

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