rockstabilityassessmentbasedonthechronologicalorderof...

Research ArticleRock Stability Assessment Based on the Chronological Order ofthe Characteristic Acoustic Emission Phenomena

Penghai Zhang 1 Tianhong Yang 1 Tao Xu 1 Qinglei Yu 1 Jingren Zhou2

and Wancheng Zhu 2

1Center for Rock Instability and Seismicity Research Northeastern University Shenyang China2State Key Laboratory of Hydraulics and Mountain River Engineering College of Water Resources and HydropowerSichuan University Sichuan China

Correspondence should be addressed to Qinglei Yu yuqingleimailneueducn

Received 31 May 2018 Revised 11 October 2018 Accepted 14 October 2018 Published 15 November 2018

Academic Editor Giosue Boscato

Copyright copy 2018 Penghai Zhang et al-is is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Sudden inelastic deformations in rock are associated with acoustic emission (AE) -erefore AE monitoring technique can beused to study the fracture processes of rock In this paper AE tests were conducted on the granitic gneiss specimens under theuniaxial compressive loading conditions -e temporal changes in AE hit parameters and spatial-temporal evolution of AE eventsduring the failure process of the granitic gneiss specimens were studied and several characteristic AE phenomena (ie dramaticincrease in dominant frequency AE energy and hit rate the AE event with a high energy level and the through-going distributionof the AE events with intermediate energy levels) were statistically analyzed before the failure occurred It was found that thechronological order of the characteristic AE phenomena was relatively certain and correspondingly had a close relationship withthe crack development stage Because of the difference of the stress level at each crack development stage the stability at differentcrack development stages is different -erefore a rock stability assessment approach was established based on the chronologicalorder of the characteristic AE phenomena and then the rock stability was assessed using the proposed approach

1 Introduction

As a brittle material rock will experience sudden inelasticdeformations before failure such as the initiation andpropagation of cracks when it is subjected to external loads-ese deformations are associated with acoustic emission(AE) which is defined as a transient elastic wave generated bythe rapid release of strain energy [1] -erefore AE mon-itoring technique can be used to study the fracture or failureprocess of rock

Since the 1960s considerable efforts [2ndash6] have beenmade to analyze the evolution characteristic of AE andrecognize the characteristic AE phenomena to assess therock stability and forecast its failure

-e number of AE hits is most widely analyzed among allthe AE parameters because the AE hit is easy to acquire andits value has a positive correlation with the crack numberMany previous studies have shown that when approaching

the peak stress the AE hit rate increases significantly in therock whose plastic deformation is not obvious while the AEhit rate might decrease in the rock whose plastic de-formation is obvious [7] -ese results indicate that the AEhit rate is significantly influenced by the rock deformationcharacteristic

-e AE energy is another AE parameter that is widelyanalyzed the significant increase of AE energy before rockfailure is observed in a large number of rock types such asgranite [8 9] gneiss [10] and tuff [11] indicating that theincrease of energy is a reliable precursor of the rock failure

In addition research studies on the spectral analysis of AEhave been conducted in the past decades For exampleSpetzler et al [12] found that as failure was approached morepower was recorded at the lower frequency in large graniteand basalt specimens He et al [13] observed that there weremuch lower frequency AE events near the bursting failure ofthe limestone specimens However Li et al [14] observed the

HindawiShock and VibrationVolume 2018 Article ID 6863925 10 pageshttpsdoiorg10115520186863925

increase of the higher frequency component before rockfailure under cyclic loading and multistage loading Ji et al[15] have also observed the similar phenomena in granite andmarble -e conflicting experimental results indicate therelationship between the spectrum and fracture in rock is toocomplex to be well summarized

Further detailed analysis of the space-time distribution ofAE events can help us better understand the rock fractureprocess [1] For example if the failure was induced by through-going shear fault or compaction band then before the failureoccurred the AE events tended to gather around the futuremacroscopic fracture plane [16ndash18] -us the spatial distri-bution of AE events can be used for not only failure warningbut also macroscopic crack shape forecasting Nevertheless insome relatively homogeneous rock AE events exhibited dis-persion when failure occurred and the macroscopic crackshape cannot be forecasted based on the spatial distribution ofAE events before the failure occurred [19]

Based on the above introduction it can be found thatmany characteristic AE phenomena have been recognizedHowever the temporal relation between each characteristic AEphenomenon has rarely been reported in the literature-erefore in the present study AE tests were conducted ongranitic gneiss specimens under uniaxial compressive loadingconditions and then the chronological order (temporal re-lationship) of characteristic AE phenomena was studied Onthis basis a rock stability assessment approach was established

2 AE Tests of Granitic Gneiss Specimens

21 Experimental System -e experimental system consistsof the load device and the monitoring apparatus -e loaddevice used the computer control electrohydraulic servopress TAW-2000KN PCI-2 AE test apparatus was applied tocollect the AE signals produced by granitic gneiss In thisexperiment eight Nano30 sensors were arranged on thespecimen surface (the small grey cylinders) 45 dB thresholdwas selected for all sensors and the preamplifier gain was40 dB Each waveform was digitized into 1024 samples ata sampling rate of 1MHz

22 Rock Specimen and Loading Control -e granitic gneissspecimens (Φ50mm times 100mm) used in the AE experimentwere collected from the Shirengou iron mine Tangshan cityHebei province China -e relatively detailed introductionabout the mine can be found in the literature [20] -egranitic gneiss consists of approximately 40 plagioclase32 alkali feldspar 8 quartz 5 biotite and 15 horn-blende [21] Uniaxial loading was conducted on 12 speci-mens (No Sim1sim12) and the loading rate was 0003mms

3 Characteristic Parameters of AE

31 AE Hit Parameters A typical AE hit is shown inFigure 1 -e ldquothresholdrdquo is a preset voltage value ie onlythe signal whose voltage value is higher than the thresholdcan be detected by the AE sensor -e ldquohitrdquo is a detectedsignal -e ldquohit raterdquo is the number of hits per second -eldquoenergyrdquo is based on the sum of the squared voltage readings

divided by a token resistance R (R is equal to 10 kΩ) -eenergy is reported in attojoules (aJ 1 aJ 10minus18 J)

Using the fast Fourier transformation (FFT) the AE hitcan be converted from the time domain to the frequencydomain and the dominant frequency is the correspondingfrequency of the maximum voltage (Figure 1)

32 AE Event Parameters One of the major strengths of AEtechnique is its ability to locate an active source in threedimensions when enough hits are available from multi-sensors-e located source is called an AE event in this paper

-e energy of each AE event is determined as follows

Es 1n

1113944

n

i1

r2i102

middot Ei1113888 1113889 (1)

where Ei is the absolute energy of the AE hit detected by eachsensor ri is the distance between the AE event and the ithsensor in millimeters and n is the total number of sensorsused for the energy calculation -e computed value is anaverage energy for the whole array assuming elasticallypropagating spherical waves of a point source corrected forgeometrical spreading on a 10mm reference sphere

-e energy level of each AE event is determined asfollows

Em lgEs (2)

4 The Change of AE Hit ParametersExperimental Results

To seek the change characteristic of AE during the fractureprocess of rock specimens the temporal changes in pa-rameters of AE hits received by a single sensor are studied

According to the difference in the change characteristicof the AE hit parameters the specimens can be divided intotwo categories One category includes the majority speci-mens and can be represented by specimen Sim2 Anothercategory includes a minority of the specimens and can berepresented by specimen Sim10 In this section the speci-mens Sim2 and Sim10 were taken as examples to illustratethe change characteristic of the AE hit parameters of thegranitic gneiss specimens It should be noted that thechange characteristics of the AE hit parameters of the otherspecimens were similar with specimen Sim2 or Sim10 be-sides the ranges of the AE hit parameter values beingsomewhat different To avoid the redundancy of illustrationthe change characteristics of the AE hit parameters of thespecimens have not been discussed one by one

41 Hit Rate As shown in Figure 2(a) the specimen Sim2experienced the crack closure stage (I 0ndash17 peakstrength) the elastic stage (II 17ndash51 peak strength) thestable crack growth stage (III 51ndash77 peak strength) andthe unstable crack growth stage (IV 77ndash100 peakstrength) successively -e hit rate buildup commenced at230 sndash240 s which corresponded to the transition from theelastic stage to the stable crack growth stage and became

2 Shock and Vibration

noticeable at approximately 300 s when the rock entered theunstable crack growth stage

For the specimen Sim10 (Figure 2(b)) a dierent trendin the AE hit record was observed a noticeable increase ofthe hit rate not only occurred at a high stress level (such asthe unstable crack growth stage) but also occurred at a lowstress level of only 6ndash15 peak strength We interpret thisspecial AE response at low stress level as unstable crackgrowth in a local region of the specimen which will beconrmed by the spatial distribution of AE events in Section53 erefore the special AE response stage was namedldquolocal unstable crack growth stage (L)rdquo in this paper Fur-thermore it can be found that the maximum hit rate might

be reached at the local unstable crack growth stage or theunstable crack growth stage

42 Dominant Frequency e changes of dominant fre-quency shown in Figure 3 were calculated using a movingwindow approach For both the calculation window and thesliding window 100 hits were adopted e arrival time ofthe last hit in the calculation window was used as the time ofthe calculated result

As shown in Figure 3 for both Sim2 and Sim10 thedominant frequency increased signicantly with stress in thelow stress level Before specimen failure occurred dierent

0

200

400

600

800

1000

1200

020406080

100120140160

0 100 200 300 400

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

Time (s)

StressHit rate

I II III IV

(a)

0

100

200

300

400

500

600

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

Time (s)

I L II III IV

StressHit rate

(b)

Figure 2 Changes of stress and AE hit rate with time (a) Sim2 (b) Sim10

ndash004

ndash002

0

002

004

0 200 400 600 800 1000A

mpl

itude

(V)

Time (μs)

reshold

0

02

04

06

08

0 100 200 300 400 500

Am

plitu

de (V

)

Frequency (KHz)

Dominant frequency

Figure 1 Schematic of AE hit in time domain and frequency domain

Shock and Vibration 3

change trends of the dominant frequency such as decrease(Figure 3(a)) and uctuation (Figure 3(b)) were observed indierent specimens In addition no obvious correlation wasobserved between the change trend before the failure andwhether the specimen experienced the local unstable crackgrowth stage

43 Energy e changes of energy were calculated using thesame approach in Section 42 As shown in Figure 4(a) theenergy of specimen Sim2 uctuated in a low level at the crackclosure stage (I) the elastic stage (II) and the stable crackgrowth stage (III) In contrast a drastic increase of energyoccurred at the unstable crack growth stage (IV) For thespecimen Sim10 (Figure 4(b)) the energy increased dra-matically at the local unstable crack growth (L) and theunstable crack growth stage (IV)

5 The Spatial-Temporal Evolution of the AEEvents Experimental Results

According to the dierence in the spatial-temporal evolution ofthe AE events the specimens can be divided into three

categories In this section the specimens Sim2 Sim9 andSim10 were taken as examples to illustrate the three categories

51e General Spatial-Temporal Evolution of the AE Eventse evolution characteristic of the AE events in a majority ofspecimens is similar to that of specimen Sim2 As shown inFigure 5 the spheres represent the AE events and the dif-ferent colors indicate the dierent energy levelse top 20of the energy level scale is dened as the high energy level(red and orange) the bottom 45 of the energy level scale isdened as the low energy level (blue and purple) and theintervening 35 is dened as the intermediate energy level(yellow and green)

According to the location results (Figure 5) the AE eventstrend from the top and bottom to the middle of the specimenBefore 318 s AE events with low energy level and in-termediate energy level were concentrated in the top andbottom of the samples and there were noAE events located inthe middle of the sample (Figure 5(a)) At 3237 s the rst AEevent with a high energy level occurred (Figures 5(b) and 6)corresponding to approximately 85 of the peak strength(Figure 2(a)) From 318 s the number of AE events with an

150

200

250

300

350

020406080

100120140160

0 100 200 300 400

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

Time (s)

StressDominant frequency

I II III IV

(a)


150

200

250

300

350

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

Time (s)

I L II III IV

(b)

Figure 3 Changes of stress and dominant frequency with time (a) Sim2 (b) Sim10

StressEnergy

2

4

6

8

020406080

100120140160

0 100 200 300 400

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)

I II III IV

(a)

3

4

5

6

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)

StressEnergy

I L II III IV

(b)

Figure 4 Changes of energy with time (a) Sim2 (b) Sim10


intermediate energy level began to increase in the middle ofthe specimen and formed a through-going distribution(Figures 5(b) and 5(c)) At 326 s an AE event with a highenergy level occurred in the middle of the specimen(Figures 5(c) and 7) After approximately 18 s specimenfailure occurred (Figure 5(d))

From the general spatial-temporal evolution of the AEevents which can be represented by Sim2 it can be inferredthat the three phenomena the first AE event with the highenergy level the through-going distribution of AE eventswith the intermediate energy level and the AE event with thehigh energy level in the area which was the last through byAE events with the intermediate energy level occurred insequence at the high stress level

-e phenomenon that the number of AE events in themiddle of the sample was still few at the high stress level(Figure 5(b)) might be mostly attributed to the loadingboundary effects such as friction and uneven stress on thespecimen ends -e friction was due to elastic parametermismatch between the loading platen and the specimen andthe uneven stress was due to the unflatness of the specimenends -is observation is consistent with experimental ob-servations made in [17 22 23] Stress concentration at thespecimen ends is believed to be the consequences of thefractures generated at the top and bottom of the specimenAnd as the load increases the fractures propagated from thetop and bottom to the middle of the specimen

In addition the phenomenon that the spatial distribu-tion of AE events exhibited dispersion before the failureoccurred was mainly due to the relatively homogeneousstrength distribution in the specimens -e main mineral

components of granitic gneiss specimens used in this paperare plagioclase alkali feldspar and hornblende which ac-count for more than 85 -e hardness of plagioclase alkalifeldspar and hornblende is about 6 and the minerals withsimilar hardness are generally similar in strength -ereforegranitic gneiss specimens are relatively homogeneous inspatial distribution of strength As the experimental resultsshown in the literature [19 24 25] in some relatively ho-mogeneous rock the spatial distribution of AE eventsexhibited dispersion and cannot forecast the macroscopiccrack shape before the failure occurred

52 0e Gap of AE Event with Intermediate Energy Level-e evolution characteristic of AE events at the high stresslevel in some specimens is different from the general evo-lution characteristic as illustrated by specimen Sim9

-e spatial-temporal distribution of AE events in Sim9 isshown in Figure 8 At 344 s two AE events with a high energylevel occurred in succession at the top of the specimen(Figures 8(a) and 9) corresponding to approximately 88 peakstrength -e phenomenon ie the AE events with a highenergy level occurred at a high stress level in agreement withthe general evolution characteristic is described in Section 51However when the failure occurred at 364 s there was stilla gap of the AE event with an intermediate energy level and inthe gap the AE events with low energy level were few (outlinedby dashed lines in Figure 8(c)) -is gap suggested that themicrofractures in the gap were few when approaching failureand that the macroscopic crack (Figure 8(c)) passed throughthe gap suddenly with an obvious brittle failure characteristicwhen failure occurred

Occurrence time 3237 sEnergy level 696 aJ

Stress level 85 peak strengthCrack development stages IV

Figure 6 -e spatial distribution of the AE events in the areaoutlined by the solid lines in Figure 5(b) at 3237 s



Figure 7 -e spatial distribution of AE events in the area outlinedby the solid lines in Figure 5(c) at 326 s

Low Intermediate High

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

33 82

Figure 5 -e spatial-temporal distribution of the AE events in Sim2 (a) 0ndash318 s (b) 0ndash324 s (c) 0ndash328 s (d) 0ndash346 s


53 e AE Events with a High Energy Level at a Low StressLevel e evolution characteristic of AE events at low stresslevels in some specimens is dierent from the generalevolution characteristic e specic evolution characteristiconly appears at the local unstable crack growth stage asillustrated by specimen Sim10

e spatial-temporal distribution of AE events in Sim10is shown in Figure 10 e local unstable crack growth stagelasted from approximately 128 s to 231 s corresponding toFigures 10(b) and 10(c) At 163 s the rst AE event witha high energy level occurred at approximately 82 peakstrength (Figures 10(b) and 11) which was much lower thanthe majority specimens In the meantime the AE eventsgathered in a band shape around the AE event with a highenergy level suggesting there was a weak area where thestrength was signicantly lower than that in the other areaand the fracture of the weak area induced the high energylevel AE event In addition as the load applied on thespecimen was still low during the local unstable crack growthstage the density and energy level of AE events were low inthe area far away from the weak area

6 Chronological Order of the CharacteristicAE Phenomena

Based on the above analysis several characteristic AEphenomena during the failure process of the specimenscan be observed such as the obvious increase in the hitrate dominant frequency and energy e chronologicalorder of these AE phenomena and the corresponding crackdevelopment stage are summarized in Table 1

As shown in Table 1 the obvious increase in dominantfrequency always occurred rst among all the characteristic

AE phenomena and it occurred before the specimens en-tered the unstable crack growth stage

e obvious increase in the energy and the hit rate andthe rst AE event with a high energy level (short for ldquorsthighrdquo in Table 1) tended to occur simultaneously or atshort intervals In consideration of the observation that theenergy and hit rate have positive correlations with themicrofracture scale and crack number respectively and it





Figure 9 e spatial distribution of the AE events in the area outlined by solid lines in Figure 8(a) at 3441 s

Low Intermediate High33 77

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

Figure 10 e spatial-temporal distribution of the AE events inSim10 (a) 0ndash128 s (b) 0ndash192 s (c) 0ndash231 s (d) 0ndash535 s


Stress level 82 peak strengthCrack development stages L

Figure 11 e spatial distribution of the AE events in the areaoutlined by solid lines in Figure 9(b) at 1631 s

34 77Low Intermediate High

(a) (c)(b)

Figure 8 e spatial-temporal distribution of the AE events in Sim9 and the failure mode (a) 0ndash351 s (b) 0ndash364 s (c) the failure mode


can be inferred that the crack area and crack number tendto increase simultaneously or at short intervals

For most of the specimens the first AE event with a highenergy level might occur either at the local unstable crackgrowth stage or at the unstable crack growth stage If the firstAE event with a high energy level occurred in the unstablecrack growth stage there are two types of AE event dis-tribution characteristics (1) AE events were few and scat-tered such as for Sim8 (Figure 12) which indicated that therock was relatively uniform and there were few newmicrofractures formed under the high stress level and (2)many AE events with intermediate energy levels occurredwhich indicated that the rock was relatively nonuniform andthere were many new microfractures formed under the highstress level such as for Sim2 (Figures 5(b) and 6) and forSim9 (Figures 8(a) and 9)

If the first high energy level AE occurred in the localunstable crack growth stage it can be found that someAE events with intermediate energy levels gatheredaround the AE event with a high energy level and thedensity and energy level of the AE events were low in thearea far away from the AE event with the high energylevel (Figure 10(b)) -us the spatial distributioncharacteristic of the AE events can help distinguish

between the local unstable crack growth stage and theunstable crack growth stage

-e through-going distribution of AE events (short forldquothrough-going distributionrdquo in Table 1) and the AE eventwith a high energy level in the area that was the last toundergo AE events with intermediate energy levels (short forldquohigh in last to undergordquo in Table 1) would occur at theunstable crack growth stage (IV) and after all the charac-teristic changes of the AE hit parameters For some speci-mens such as Sim8 Sim9 (Figure 8(b)) and Sim12 thefailure occurred before the through-going distribution of theAE events with intermediate energy levels However if thethrough-going distribution of the AE events with intermediateenergy levels occurred then an AE event with a high energylevel would be certain to occur in the area that was the last toundergo AE events with intermediate energy levels

7 Rock Stability Evaluation Based on theChronological Order of the CharacteristicAE Phenomena

71 Rock Stability Evaluation Approach Based on the sta-tistical result (Table 1) and analysis above the general

Table 1 Statistics of characteristic AE phenomena

NoAE hit AE event

Dominant frequency Energy Hit rate First high -rough-going distribution High in last to undergoSim1 1-III 2-IV 2-IV 5-IV 4-IV 6-IVSim2 1-I 3-IV 2-IV 3-IV 5-IV 6-IVSim3 1-III 2-III 4-IV 2-IV 5-IV 6-IVSim4 1-II 3-IV 2-IV 3-IV 5-IV 6-IVSim5 1-II 2-III 4-IV 2-III 5-IV 6-IVSim6 1-II 2-IV 2-IV 2-IV 5-IV 6-IVSim7 1-II 2-IV 2-IV 4-IV 5-IV 6-IVSim8 1-II 2-III 3-IV 3-IVSim9 1-II 2-III 3-IV 3-IVSim10 1-I L 2-L 2-L 4-L 5-IV 6-IVSim11 1-II L 2-L 3-L 2-L 5-IV 6-IVSim12 1-I 2-L 2-L 2-LNoteArabic numerals before ldquo-rdquo represent the chronological order of the characteristic AE phenomena roman numerals and letter ldquoLrdquo after ldquo-rdquo represent thecrack development stage at which the characteristic AE phenomena occurred



Figure 12 -e spatial distribution of the AE events from 0 s to 2728 s in Sim8


chronological order of the characteristic AE phenomena andthe corresponding crack development stage are shown inFigure 13 Because of the dierence of the stress level at eachcrack development stage the stability at dierent crackdevelopment stages is dierent us based on the chro-nological order of the characteristic AE phenomena the rockstability can be assessed

e rock stability evaluation approach is as follows

(1) e obvious increase in dominant frequency in-dicates that the specimen has not entered the un-stable crack growth stage ie the stability is high tomedium

(2) When the obvious increase in energy and hit rateand the rst AE event with a high energy leveloccurred if the AE events gathered in a band shapearound the AE event with a high energy level andthe density and energy level of AE events were lowin the area far away from the AE event with a highenergy level then the rock has entered the localunstable crack growth stage Because the stress levelis still low during the stage the stability can beassessed as high

(3) When the obvious increase in energy and hit rate andthe rst AE event with a high energy level occurred ifthere were rare AE events or many AE events withintermediate energy levels then the specimen hasentered the stable crack growth stage or the unstablecrack growth stage indicating that the rock stabilityis low

(4) AE events with intermediate energy level forming thethrough-going distribution indicate that themicrofractures have fully developed and the rockstability is very low

(5) e occurrence of the AE event with high energylevel in the area that was the last through by AEevents with intermediate energy levels is the lastcharacteristic AE phenomenon before the rockfailure ie failure is approaching

72 Rock Stability Evaluation Result Taking Sim2 as anexample the rock stability evaluation can be illustrated Asshown in Figure 14 the dominant frequency began to

increase at 30 s and the stability was assessed as high From3001 s to 3242 s obvious increases were observed in energyand hit rate and the rst AE event with high energy level

I II III IV

L

Gathered around the first highFirst high

Hit rateDominant frequency

Energy

Through-goingdistribution

High in last through

Figure 13e general chronological order of theAEdata and the corresponding crack development stage and rock stability ( represents high tomedium stability represents high stability represents low stability represents very low stability represents the failure is approaching)

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


150

200

250

300

350

020406080

100120140160

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

0

200

400

600

800

1000

1200

020406080

100120140160

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

2

4

6

8

00 100 200 300 400

20406080

100120140160

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)


StressHit rate

StressEnergy

Figure 14 Stability assessment for specimen S2


occurred successively ie the stability was low From 3243 sto 3259 s the number of AE events with intermediate energylevel increased in the middle of the specimen and formeda through-going distribution (Figure 14(c)) suggestingthat the rock stability was very low At 326 s an AE eventwith high level occurred in the middle of the specimen(Figure 14(d)) ie failure was approaching

8 Conclusions

AE monitoring was used to study the fracture process ofgranitic gneiss under the uniaxial loading condition A rockstability assessment approach was established based on thechronological order of the characteristic AE phenomena-e following conclusions can be drawn

(1) Under the uniaxial loading condition severalcharacteristic AE phenomena such as dramatic in-creases in dominant frequency energy and hit ratethe AE event with a high energy level and thethrough-going distribution of AE events with in-termediate energy levels were observed beforefailure occurred thus indicating that AE monitoringhas the potential to assess the rock stability

(2) For most of the granitic gneiss specimens thechronological order of the characteristic AE phe-nomena was certain and had a corresponding re-lationship with the crack development stage Becauseof the different stress levels at each crack developmentstage the stability at different crack developmentstages is different -us based on the chronologicalorder of the characteristic AE phenomena the rockstability can be assessed

Data Availability

-e AE monitoring data used to support the findings of thisstudy are available from the corresponding author uponrequest

Conflicts of Interest

-e authors declare that they have no conflicts of interest

Acknowledgments

-e work was financially supported by the National KeyResearch Project (2016YFC0801607) the National NaturalScience Foundation of China (51604062 and 51574060) andthe Science and Technology Major Project of Anhui Prov-ince (17030901023) -e authors are thankful to the refereesand editors for their valuable comments and suggestionsdevoted to improving the quality of our manuscript

References

[1] D Lockner ldquo-e role of acoustic emission in the study of rockfracturerdquo International Journal of RockMechanics andMiningSciences vol 30 no 7 pp 883ndash899 1993

[2] C H Scholz ldquo-e frequency-magnitude relation of micro-fracturing in rock and its relation to earthquakesrdquo Bulletin ofthe Seismological Society of America vol 58 no 9pp 1909ndash1911 1968

[3] J L Knill J A Franklin and A W Malone ldquoA study ofacoustic emission from stressed rockrdquo International Journalof Rock Mechanics amp Mining Sciences amp Geomechanics Ab-stracts vol 5 no 1 pp 87-88 1968

[4] C H Sondergeld and L H Estey ldquoAcoustic emisson study ofmicrofracturing during the cyclic loading of Westerly gran-iterdquo Journal of Geophysical Research Solid Earth vol 86no B4 pp 2915ndash2924 1981

[5] P G Meredith and B K Atkinson ldquoStress corrosion andacoustic emission during tensile crack propagation in WhinSill dolerite and other basic rocksrdquo Geophysical Journal of theRoyal Astronomical Society vol 75 no 1 pp 1ndash21 1983

[6] T Hirata T Satoh and K Ito ldquoFractal structure of spatialdistribution of microfracturing in rockrdquo Geophysical JournalInternational vol 90 no 2 pp 369ndash374 1987

[7] X Yin S Li H Tang and J Pei ldquoStudy on quiet period and itsfractal characteristics of rock failure acoustic emissionrdquoChinese Journal of Rock Mechanics and Engineering vol 28pp 3383ndash3390 2009 in Chinese

[8] R Prikryl T Lokajicek C Li and V Rudajev ldquoAcousticemission characteristics and failure of uniaxially stressedgranitic rocks the effect of rock fabricrdquo Rock Mechanics andRock Engineering vol 36 no 4 pp 255ndash270 2003

[9] X G Zhao J Wang M Cai et al ldquoInfluence of unloading rateon the strainburst characteristics of Beishan granite undertrue-triaxial unloading conditionsrdquo Rock Mechanics and RockEngineering vol 47 no 2 pp 467ndash483 2014

[10] H Zhang Y Yan H Yu and X Yin ldquoAcoustic emissionexperimental research on large-scaled rock failure undercycling loadmdashfracture precursor of rockrdquo Chinese Journal ofRock Mechanics and Engineering vol 23 pp 3621ndash3628 2004in Chinese

[11] S A Hall F D Sanctis and G Viggiani ldquoMonitoring fracturepropagation in a soft rock (Neapolitan tuff) using acousticemissions and digital imagesrdquo Pure and Applied Geophysicsvol 163 no 10 pp 2171ndash2204 2006

[12] H Spetzler C Sondergeld G Sobolev and B Salov ldquoSeismicand strain studies on large laboratory rock samples beingstressed to failurerdquo Tectonophysics vol 144 no 1ndash3pp 55ndash68 1987

[13] M C He J L Miao and J L Feng ldquoRock burst process oflimestone and its acoustic emission characteristics under true-triaxial unloading conditionsrdquo International Journal of RockMechanics and Mining Sciences vol 47 no 2 pp 286ndash2982010

[14] N Li E Wang E Zhao Y Ma F Xu and W QianldquoExperiment on acoustic emission of rock damage andfracture under cyclic loading and multi-stage loadingrdquoJournal of China Coal Society vol 35 pp 1099ndash1103 2010 inChinese

[15] H Ji H Wang S Cao Z Hou and Y Jin ldquoExperimentalresearch on frequency characteristics of acoustic emissionsignals under uniaxial compression of graniterdquo ChineseJournal of Rock Mechanics and Engineering vol 31pp 2900ndash2905 2012 in Chinese

[16] P M Benson B D -ompson and P G Meredith ldquoImagingslow failure in triaxially deformed Etna basalt using 3Dacoustic-emission location and X-ray computed tomogra-phyrdquo Geophysical Research Letters vol 34 no 3 2007


[17] S Q Yang H W Jing and S Y Wang ldquoExperimental in-vestigation on the strength deformability failure behaviorand acoustic emission locations of red sandstone under tri-axial compressionrdquo Rock Mechanics and Rock Engineeringvol 45 no 4 pp 583ndash606 2012

[18] J Fortin S Stanchits G Dresen and Y Gueguen ldquoAcousticemissions monitoring during inelastic deformation of po-rous sandstone comparison of three modes of deformationrdquoPure and Applied Geophysics vol 166 no 5ndash7 pp 823ndash8412009

[19] M H B Nasseri S D Goodfellow L Lombos andR P Young ldquo3-D transport and acoustic properties ofFontainebleau sandstone during true-triaxial deformationexperimentsrdquo International Journal of Rock Mechanics andMining Sciences vol 69 pp 1ndash18 2014

[20] P Zhang T Yang Q Yu et al ldquoMicroseismicity induced byfault activation during the fracture process of a crown pillarrdquoRock Mechanics and Rock Engineering vol 48 no 4pp 1673ndash1682 2015

[21] P Zhang Study on precursory law prior to rock failure basedon acoustic emission time order PhD Dissertation De-partment of Mining Engineering Northeastern UniversityBoston MA USA 2015

[22] M J Heap N Brantut P Baud and P G MeredithldquoTimemdashdependent compaction band formation in sand-stonerdquo Journal of Geophysical Research Solid Earth vol 120no 7 pp 4808ndash4830 2015

[23] X Zhao Y Li J Liu J Zhang and W Zhu ldquoStudy on rockfailure process based on acoustic emission and its locationtechniquerdquo Chinese Journal of Rock Mechanics and Engi-neering vol 27 pp 990ndash995 2008 in Chinese

[24] L Liu S Ma M A Jin X Lei K Kusunose andO Nishizawa ldquoEffect of rock structure on statistic charac-teristics of acoustic emissionrdquo Seismology and Geology vol 21pp 377ndash386 1999 in Chinese

[25] X U Jiang S Li X Tang Y Tao and Y Jiang ldquoInfluentialfactors of acoustic emission location experiment of rock underuniaxial compressionrdquo Chinese Journal of Rock Mechanicsand Engineering vol 27 pp 765ndash772 2008 in Chinese


International Journal of

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increase of the higher frequency component before rockfailure under cyclic loading and multistage loading Ji et al[15] have also observed the similar phenomena in granite andmarble -e conflicting experimental results indicate therelationship between the spectrum and fracture in rock is toocomplex to be well summarized

Further detailed analysis of the space-time distribution ofAE events can help us better understand the rock fractureprocess [1] For example if the failure was induced by through-going shear fault or compaction band then before the failureoccurred the AE events tended to gather around the futuremacroscopic fracture plane [16ndash18] -us the spatial distri-bution of AE events can be used for not only failure warningbut also macroscopic crack shape forecasting Nevertheless insome relatively homogeneous rock AE events exhibited dis-persion when failure occurred and the macroscopic crackshape cannot be forecasted based on the spatial distribution ofAE events before the failure occurred [19]

Based on the above introduction it can be found thatmany characteristic AE phenomena have been recognizedHowever the temporal relation between each characteristic AEphenomenon has rarely been reported in the literature-erefore in the present study AE tests were conducted ongranitic gneiss specimens under uniaxial compressive loadingconditions and then the chronological order (temporal re-lationship) of characteristic AE phenomena was studied Onthis basis a rock stability assessment approach was established

2 AE Tests of Granitic Gneiss Specimens

21 Experimental System -e experimental system consistsof the load device and the monitoring apparatus -e loaddevice used the computer control electrohydraulic servopress TAW-2000KN PCI-2 AE test apparatus was applied tocollect the AE signals produced by granitic gneiss In thisexperiment eight Nano30 sensors were arranged on thespecimen surface (the small grey cylinders) 45 dB thresholdwas selected for all sensors and the preamplifier gain was40 dB Each waveform was digitized into 1024 samples ata sampling rate of 1MHz

22 Rock Specimen and Loading Control -e granitic gneissspecimens (Φ50mm times 100mm) used in the AE experimentwere collected from the Shirengou iron mine Tangshan cityHebei province China -e relatively detailed introductionabout the mine can be found in the literature [20] -egranitic gneiss consists of approximately 40 plagioclase32 alkali feldspar 8 quartz 5 biotite and 15 horn-blende [21] Uniaxial loading was conducted on 12 speci-mens (No Sim1sim12) and the loading rate was 0003mms

3 Characteristic Parameters of AE

31 AE Hit Parameters A typical AE hit is shown inFigure 1 -e ldquothresholdrdquo is a preset voltage value ie onlythe signal whose voltage value is higher than the thresholdcan be detected by the AE sensor -e ldquohitrdquo is a detectedsignal -e ldquohit raterdquo is the number of hits per second -eldquoenergyrdquo is based on the sum of the squared voltage readings

divided by a token resistance R (R is equal to 10 kΩ) -eenergy is reported in attojoules (aJ 1 aJ 10minus18 J)

Using the fast Fourier transformation (FFT) the AE hitcan be converted from the time domain to the frequencydomain and the dominant frequency is the correspondingfrequency of the maximum voltage (Figure 1)

32 AE Event Parameters One of the major strengths of AEtechnique is its ability to locate an active source in threedimensions when enough hits are available from multi-sensors-e located source is called an AE event in this paper

-e energy of each AE event is determined as follows

Es 1n

1113944

n

i1

r2i102

middot Ei1113888 1113889 (1)

where Ei is the absolute energy of the AE hit detected by eachsensor ri is the distance between the AE event and the ithsensor in millimeters and n is the total number of sensorsused for the energy calculation -e computed value is anaverage energy for the whole array assuming elasticallypropagating spherical waves of a point source corrected forgeometrical spreading on a 10mm reference sphere

-e energy level of each AE event is determined asfollows

Em lgEs (2)

4 The Change of AE Hit ParametersExperimental Results

To seek the change characteristic of AE during the fractureprocess of rock specimens the temporal changes in pa-rameters of AE hits received by a single sensor are studied

According to the difference in the change characteristicof the AE hit parameters the specimens can be divided intotwo categories One category includes the majority speci-mens and can be represented by specimen Sim2 Anothercategory includes a minority of the specimens and can berepresented by specimen Sim10 In this section the speci-mens Sim2 and Sim10 were taken as examples to illustratethe change characteristic of the AE hit parameters of thegranitic gneiss specimens It should be noted that thechange characteristics of the AE hit parameters of the otherspecimens were similar with specimen Sim2 or Sim10 be-sides the ranges of the AE hit parameter values beingsomewhat different To avoid the redundancy of illustrationthe change characteristics of the AE hit parameters of thespecimens have not been discussed one by one

41 Hit Rate As shown in Figure 2(a) the specimen Sim2experienced the crack closure stage (I 0ndash17 peakstrength) the elastic stage (II 17ndash51 peak strength) thestable crack growth stage (III 51ndash77 peak strength) andthe unstable crack growth stage (IV 77ndash100 peakstrength) successively -e hit rate buildup commenced at230 sndash240 s which corresponded to the transition from theelastic stage to the stable crack growth stage and became







0

200

400

600

800

1000

1200

020406080

100120140160

0 100 200 300 400

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

Time (s)

StressHit rate

I II III IV

(a)

0

100

200

300

400

500

600

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

Time (s)

I L II III IV

StressHit rate

(b)


ndash004

ndash002

0

002

004

0 200 400 600 800 1000A

mpl

itude

(V)

Time (μs)

reshold

0

02

04

06

08

0 100 200 300 400 500

Am

plitu

de (V

)

Frequency (KHz)

Dominant frequency










150

200

250

300

350

020406080

100120140160

0 100 200 300 400

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

Time (s)


I II III IV

(a)


150

200

250

300

350

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

Time (s)

I L II III IV

(b)


StressEnergy

2

4

6

8

020406080

100120140160

0 100 200 300 400

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)

I II III IV

(a)

3

4

5

6

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)

StressEnergy

I L II III IV

(b)

















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

33 82
















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






(a) (c)(b)











NoAE hit AE event















I II III IV

L



Energy




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


150

200

250

300

350

020406080

100120140160

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

0

200

400

600

800

1000

1200

020406080

100120140160

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

2

4

6

8

00 100 200 300 400

20406080

100120140160

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)


StressHit rate

StressEnergy




8 Conclusions




Data Availability




Acknowledgments


References






























RoboticsJournal of




VLSI Design



Shock and Vibration







Journal of



Volume 2018



Volume 2018


Journal of




SensorsJournal of



RotatingMachinery





Propagation






Hindawi


Advances in

Multimedia







0

200

400

600

800

1000

1200

020406080

100120140160

0 100 200 300 400

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

Time (s)

StressHit rate

I II III IV

(a)

0

100

200

300

400

500

600

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

Time (s)

I L II III IV

StressHit rate

(b)


ndash004

ndash002

0

002

004

0 200 400 600 800 1000A

mpl

itude

(V)

Time (μs)

reshold

0

02

04

06

08

0 100 200 300 400 500

Am

plitu

de (V

)

Frequency (KHz)

Dominant frequency










150

200

250

300

350

020406080

100120140160

0 100 200 300 400

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

Time (s)


I II III IV

(a)


150

200

250

300

350

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

Time (s)

I L II III IV

(b)


StressEnergy

2

4

6

8

020406080

100120140160

0 100 200 300 400

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)

I II III IV

(a)

3

4

5

6

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)

StressEnergy

I L II III IV

(b)

















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

33 82
















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






(a) (c)(b)











NoAE hit AE event















I II III IV

L



Energy




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


150

200

250

300

350

020406080

100120140160

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

0

200

400

600

800

1000

1200

020406080

100120140160

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

2

4

6

8

00 100 200 300 400

20406080

100120140160

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)


StressHit rate

StressEnergy




8 Conclusions




Data Availability




Acknowledgments


References






























RoboticsJournal of




VLSI Design



Shock and Vibration







Journal of



Volume 2018



Volume 2018


Journal of




SensorsJournal of



RotatingMachinery





Propagation






Hindawi


Advances in

Multimedia









150

200

250

300

350

020406080

100120140160

0 100 200 300 400

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

Time (s)


I II III IV

(a)


150

200

250

300

350

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

Time (s)

I L II III IV

(b)


StressEnergy

2

4

6

8

020406080

100120140160

0 100 200 300 400

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)

I II III IV

(a)

3

4

5

6

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)

StressEnergy

I L II III IV

(b)

















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

33 82
















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






(a) (c)(b)











NoAE hit AE event















I II III IV

L



Energy




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


150

200

250

300

350

020406080

100120140160

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

0

200

400

600

800

1000

1200

020406080

100120140160

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

2

4

6

8

00 100 200 300 400

20406080

100120140160

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)


StressHit rate

StressEnergy




8 Conclusions




Data Availability




Acknowledgments


References






























RoboticsJournal of




VLSI Design



Shock and Vibration







Journal of



Volume 2018



Volume 2018


Journal of




SensorsJournal of



RotatingMachinery





Propagation






Hindawi


Advances in

Multimedia
















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

33 82
















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






(a) (c)(b)











NoAE hit AE event















I II III IV

L



Energy




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


150

200

250

300

350

020406080

100120140160

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

0

200

400

600

800

1000

1200

020406080

100120140160

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

2

4

6

8

00 100 200 300 400

20406080

100120140160

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)


StressHit rate

StressEnergy




8 Conclusions




Data Availability




Acknowledgments


References






























RoboticsJournal of




VLSI Design



Shock and Vibration







Journal of



Volume 2018



Volume 2018


Journal of




SensorsJournal of



RotatingMachinery





Propagation






Hindawi


Advances in

Multimedia















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






(a) (c)(b)











NoAE hit AE event















I II III IV

L



Energy




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


150

200

250

300

350

020406080

100120140160

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

0

200

400

600

800

1000

1200

020406080

100120140160

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

2

4

6

8

00 100 200 300 400

20406080

100120140160

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)


StressHit rate

StressEnergy




8 Conclusions




Data Availability




Acknowledgments


References






























RoboticsJournal of




VLSI Design



Shock and Vibration







Journal of



Volume 2018



Volume 2018


Journal of




SensorsJournal of



RotatingMachinery





Propagation






Hindawi


Advances in

Multimedia










NoAE hit AE event















I II III IV

L



Energy




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


150

200

250

300

350

020406080

100120140160

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

0

200

400

600

800

1000

1200

020406080

100120140160

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

2

4

6

8

00 100 200 300 400

20406080

100120140160

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)


StressHit rate

StressEnergy




8 Conclusions




Data Availability




Acknowledgments


References






























RoboticsJournal of




VLSI Design



Shock and Vibration







Journal of



Volume 2018



Volume 2018


Journal of




SensorsJournal of



RotatingMachinery





Propagation






Hindawi


Advances in

Multimedia











I II III IV

L



Energy




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


150

200

250

300

350

020406080

100120140160

Dom

inan

t fre

quen

cy (k

Hz)

Stre

ss (M

Pa)

0

200

400

600

800

1000

1200

020406080

100120140160

Hit

rate

(sndash1

)

Stre

ss (M

Pa)

2

4

6

8

00 100 200 300 400

20406080

100120140160

Ener

gy (l

g(aJ

))

Stre

ss (M

Pa)

Time (s)


StressHit rate

StressEnergy




8 Conclusions




Data Availability




Acknowledgments


References






























RoboticsJournal of




VLSI Design



Shock and Vibration







Journal of



Volume 2018



Volume 2018


Journal of




SensorsJournal of



RotatingMachinery





Propagation






Hindawi


Advances in

Multimedia



8 Conclusions




Data Availability




Acknowledgments


References






























RoboticsJournal of




VLSI Design



Shock and Vibration







Journal of



Volume 2018



Volume 2018


Journal of




SensorsJournal of



RotatingMachinery





Propagation






Hindawi


Advances in

Multimedia














RoboticsJournal of




VLSI Design



Shock and Vibration







Journal of



Volume 2018



Volume 2018


Journal of




SensorsJournal of



RotatingMachinery





Propagation






Hindawi


Advances in

Multimedia


rockstabilityassessmentbasedonthechronologicalorderof...

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