sustainability Article Guowei Dong 1,2, * and Yinhui Zou 3,4

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1 sustainability Article A Novel Study Waveguide Propagation Rules Coal Rock AE Signal: Effects Waveguide Size Installation Method on Propagation Rules Coal Rock AE Signal Guowei Dong 1,2, * Yinhui Zou 3,4 1 School Energy Engineering, Xi an University Science Technology, Xi an , China 2 Key Laboratory Western Mine Exploitation Hazard Prevention Ministry Education, Xi an , China 3 China Coal Technology Engineering Group Chongqing Research Institute, Chongqing , China; mkyzyh@163.com 4 National Key Laboratory Gas Disaster Monitoring, Preventing Emergency Controlling, Chongqing , China * Correspondence: leng285@ xust.edu.cn Received: 13 June 2017; Accepted: 29 June 2017; Published: 10 July 2017 Abstract: The propagation acoustic emission (AE) signal in waveguide is quite important for AE-based prediction dynamic disasters in coal rocks. In this study, based on some relevant ories in wave mechanics, elastic mechanical model one-dimensional (1D) waveguide was firstly established, relationship between AE source signal signal at waveguide s receiving end was derived. On basis oretical analysis, numerical simulation laboratory test schemes were designed; additionally, using stard vibration source method, AE response in different sizes waveguides were investigated, effects waveguide size waveguide were concluded, application conditions established oretical model were clarified. Numerical simulation results fit well with laboratory test results. Meanwhile, effects sensor s installation method on propagation rules AE signal were examined appropriate installation method was determined. Keywords: coal rock; acoustic emission signal; waveguide; size; installation method; propagation rule 1. Introduction Acoustic emission (AE) techniques began at beginning 20th century. Currently, AE technique from coal rocks gradually serve as an effective forecasting early warning method in various countries all over world for ensuring safety in underground engineering mining production. In 1920s, Polish researchers applied AE signal to monitor dynamic disasters in mines; in 1936, Zuoshan, a Japanese scholar, proposed using acoustic emission to forecast gas outbursts rockbursts; in 1940s, United States Bureau Mines began to forecast rockbursts in mines using AE method; in 1970s, Australian researchers started to apply AE technique in monitoring mine dynamic disasters; meanwhile, South African researches used AE technique for monitoring rockbursts in metal mines [1 5]. Chinese scholars have used AE technique for prediction early warning gas dynamic disasters in coal rocks for more than three decades, have made great progress in monitoring devices, monitoring technology disaster identification method. China s main related research units, including Coal Science Research Institute [6 13], Chongqing Institute [14 25], Mine Tremor or Microseismic Monitoring Research Center, University Science Technology Beijing [26 31], Sustainability 2017, 9, 1209; doi: /su

2 Sustainability 2017, 9, Sustainability 2017, 9, Beijing [26 31], China University Mining Technology [32 38], Dalian University Technology Norastern University [39 43] have conducted a great deal research on AE China signal s University generation Mining mechanism, Technology propagation [32 38], rules, Dalian de-noising University methods, Technology sensor s implementation Norastern University technology [39 43] have identification conducted a great disaster deal research evolution on processes, AE signal s generation have gained mechanism, fruitful propagation achievements. rules, However, de-noising size methods, waveguide sensor s devices implementation related technology installation technology identification have been disaster poorly evolution investigated. processes, Previous have studies gained have fruitful laid achievements. emphasis on However, early warning size technology waveguide devices related instruments related installation equipment, technology have laboratory been poorly studies investigated. have mainly Previous focused studies on have AE laid characteristics emphasis on coal early rocks warning under technology uniaxial loading. Thus, related instruments waveguide devices equipment, were generally laboratory designed studies implemented have mainly by focused experience, on which AE characteristics directly affected coal receiving rocks under effects uniaxial AE loading. signals Thus, reliability waveguide devices AE signal were generally retrieval designed hindered implemented large-scale by experience, population which AE-based directly affected forecasting receiving technique. effects In this study, AE signals based on reliability related ories AE signal in wave retrieval mechanics, hindered elastic large-scale ory model population one-dimensional AE-based (1D) forecasting waveguide technique. was established, In this study, based variations on related AE ories signal s in displacement, wave mechanics, velocity elastic acceleration ory model with propagation one-dimensional distance (1D) waveguide were concluded; was established, furr, on basis variations established AE signal s model, displacement, effects velocity size acceleration implementation with propagation technology distance waveguide were concluded; on propagation furr, on AE basis signal were established elaborated model, on by effects means laboratory size implementation test, numerical technology simulations waveguide field investigation. on propagation AE signal were elaborated on by means laboratory test, numerical simulations field investigation. 2. Theoretical Basis AE Propagation 1D Elastic Waveguide 2. Theoretical Basis AE Propagation 1D Elastic Waveguide According to field implementation waveguide device in AE system wavelength According to field implementation waveguide device in AE system wavelength AE signal, some assumptions were made based on relevant ories in wave mechanics AE signal, some assumptions were made based on relevant ories in wave mechanics [44,45] [44,45] thus, a simplified mechanical model 1D waveguide was derived, as shown in Figure 1. thus, a simplified mechanical model 1D waveguide was derived, as shown in Figure 1. Figure 1. Mechanical model one-dimensional (1D) elastic waveguide, in which A Figure 1. Mechanical model one-dimensional (1D) elastic waveguide, in which A1, E1, 1, E P1 1, P 1 C01 C denote 01 denote coal rock s cross sectional area, elastic modulus, density stress wave velocity, coal rock s cross sectional area, elastic modulus, density stress wave velocity, respectively; A respectively; A2, 2, E E2, 2, P 2 C P2 02 denote finite elastic waveguide s cross sectional area, elastic C02 denote finite elastic waveguide s cross sectional area, elastic modulus, density stress wave velocity, respectively; A modulus, density stress wave velocity, respectively; A3, 3, E E3, 3, P 3 C P3 03 denote material s cross C03 denote material s cross sectional area, elastic modulus, density stress wave velocity, respectively; I sectional area, elastic modulus, density stress wave velocity, respectively; 1, R I1, 1 T R1 1 denote T1 denote coal rock s incident wave, reflected wave transmitted wave, respectively; I coal rock s incident wave, reflected wave transmitted wave, respectively; 2, R I2, 2 T R2 2 denote T2 denote wave guide s incident wave, reflected wave transmitted wave, respectively. wave guide s incident wave, reflected wave transmitted wave, respectively. The following assumptions were n made: (1) waveguide is an 1D elastic device; (2) waveguide s plane section, which had been selected before deformation, was always a plane during deformation; (3) except axial stress σ that is uniformly distributed along cross section, or components stress equal to zero; (4) propagation direction stress wave is parallel to axial direction perpendicular to interface during propagation from self-infinite waveguide to finite waveguide; (5) waveguide s body force is is not taken into account. The dynamic equilibrium equation 1D waveguide can be written as: 2 ( σ(x, t) t) u X+ X = ρ 2 u 2 x x t t 2 (1) where X denotes waveguide s along axial direction, u denotes waveguide s axial displacement ρ denotes denotes waveguide material s material s density. density. According to to Hooke s Law, Law, following expression can can be be acquired: acquired: u σ = Eε E = E u (2) (2) x x

3 Sustainability 2017, 9, where E denotes material s Young s modulus ε denotes waveguide s axial strain. Based on Equations (1) (2), we can derive that: c 2 2 u(x, t) o x 2 = 2 u t 2 1 ρ X (c E o = ρ ) (3) where C 0 is generally referred to as waveguide s wave velocity. When waveguide s body force is not taken into account, Equation (3) can n be rewritten as: c 2 2 u(x, t) o x 2 = 2 u t 2 (4) Equation (4) is applicable to waves with long wavelengths, i.e., wavelengths exceed waveguide s diameter. The waves with long wavelengths apply to waveguide devices with arbitrary shapes cross-sections. Similarly, wave s strain, stress velocity can also be written in form Equation (4). The initial-value problems infinite waveguide can be described as: c 2 o 2 u(x,t) x 2 = 2 u t 2 u(x, 0) = ϕ(x) u(x,0) t = ϕ 1 (x) ( < x < t 0 ) (5) The solution is: u = F(x c 0 t) + G(x + c 0 t) (6) where F G are determined by initial conditions can be written as: F(x) = 1 2 ϕ(x) 2c 1 x 0 a ϕ 1(ξ)dξ G(x) = 1 2 ϕ(x) + 2c 1 x 0 a ϕ 1(ξ)dξ } (7) where a denotes arbitrary a constant. By substituting Equation (7) into Equation (6), following expression can be acquired: u(x, t) = 1 2 {ϕ(x c 0t) + ϕ(x + c 0 t)} + 1 x+c0 t ϕ 2c 1 (ξ)dξ (8) 0 x c 0 t By taking reflection transmission at waveguide s interface into account, continuity condition at interface can be written as: Displacement: u 1 = u 2,u I + u R = u T (9) Velocity: Axial force: v 1 = v 2,v I + v R = v T (10) N 1 = N 2,N I + N R = N T (11) where N denotes axial force in waveguide. If incident right traveling wave can be written as: u I = F I (x c 0 t) = F I (ξ) (12)

4 Sustainability 2017, 9, where c 01 = E1 ρ 1 ξ = x c 01 t. Then, we take partial derivatives u I with respect to x t: u I t u I x = df I dξ = c 01 df I dξ According to Equation (13), following expression can be derived: } (13) u I t = c 01 u I x (14) Similarly, for reflection wave transmitted wave, following expressions can be acquired: u R t = c 01 u R x (15) u T t = c 02 u T x By substituting Equations (14) (16) into velocity continuity condition Equation (10), following expression can be acquired (v = u t ): (16) u c I 01 x + c u R 01 x = c u T 02 x (17) Since u x = ε = E σ = AE N, Equation (17) can be rewritten as: N I N R N T c 01 + c A 1 E 01 = c 02 (18) 1 A 1 E 1 A 2 E 2 According to Equation (18), following expression can be acquired: N T = α(n I N R ) (19) where α = c 01 A 2 E 2 c 02 A 1 E 1 = c 02 A 2 ρ 2 According to Equations (11) (19), following expression (i.e., stress reflection coefficient transmission coefficient) can be acquired: c 01 A 1 ρ 1. N R N I N T N I = α 1 α+1 = 2α α+1 Based on expressions displacement, strain stress, displacement s reflection coefficient transmission coefficient can be written as: λ R = u } R ui = α 1 α+1 λ T = u T u I = α+1 2 (21) According to Equation (21), stress wave s displacement at interface between coal rock finite elastic waveguide can be written as: u T1 = λ T1 u I1, stress wave s displacement at signal receiving end can be written as: u I2 = u T1. The finite waveguide s signal receiving end can be regarded as free end; accordingly, at receiving end, A 3E 3 c 03 << A 2E 2 c 02, which corresponds to condition when a 0. According to Equation (21), λ R2 = 1, λ T2 = 2, following expressions can be derived: u r = 2λ T1 u s v r = u r t = 2λ T1 v s (22) a r = 2 u r = 2λ t 2 T1 a s } (20)

5 Sustainability 2017, 9, where u r, v r, a r denotes signal s displacement, velocity acceleration at AE waveguide s receiving end, respectively; u s, v s, a s enotes AE signal s displacement, velocity acceleration at AE waveguide s source end, respectively; λ T1 denotes AE signal s transmission coefficient in coal rock. Equation (22) describes displacement, velocity acceleration stress wave that firstly arrived at finite waveguide s receiving end; n, multiple transmissions reflections would lead to attenuation AE stress wave; finally, sensor would receive no stress signal. According to 1D elastic waveguide s mechanism model, relationships displacement, velocity acceleration between AE signal s receiving end source end were derived, as shown in Equation (22). Equation (22) is applicable to condition with no attenuation in propagation AE signal; actually, AE signal undergoes attenuation during propagation in waveguide. Next, waveguide s appropriate size was determined through numerical simulation laboratory test, i.e., application Sustainability Sustainability 2017, 2017, 9, 9, condition Equation (22) Numerical Simulations on on Effects Waveguide s Size on on AE AE Signal s Propagation 3. Numerical Simulations on Effects Waveguide s Size on AE Signal s Propagation Establishment Establishment Numerical Numerical Model Model Setting Setting Mechanical Mechanical Parameters Parameters In In In this this this study, study, propagation propagation rules rules AE signals AE AE signals in waveguide in in waveguide was simulated was simulated using large-scale using large-scale dynamic finite dynamic element finite stware element ANSYS/LS-DYNA. stware ANSYS/LS-DYNA. AE signals were AE AE sine signals acceleration were sine waves, acceleration as shown waves, in Figure as as 2. shown in in Figure Figure Figure Source Source sine sine wave. wave. Figure 3 3 shows shows established established model, model, from from which which we canwe observe can can observe that end that end end waveguide was waveguide fixed at was 25 cm fixed away at at from cm cm away left from boundary left left boundary rock or rock end was or free. end end Linear was elastic free. Linear model was elastic used, model was used, material parameters material are parameters listed in Table are are listed 1. in in Table Figure Figure Arrangement Arrangement measuring measuring nodes nodes in in simulations simulations when when diameter diameter was was set set as as different different values. values. Table Table Material Material parameters parameters in in model. model. Material Elastic Modulus (E/Gpa) Poisson s Ratio Density (kg/m 3 ) Coal rock Waveguide

6 Figure 5 shows acceleration time curves different nodes when diameter = 10 mm. Due to only considering source node (A) receiving node (E, waveguide acceleration sensor installation location), without considering process nodes (B, C, D), refore, only source node (A) receiving node (E) were compared. Figure 5 shows that receiving node (E) acoustic emission signal acceleration amplitude (0.1121) attenuates approximately 74% relative to source node (A) signal amplitude (0.4308). This is due to coal rock propagation loss when diameter = 10 Sustainability 2017, 9, Table 1. Material parameters in model. Material Elastic Modulus (E/Gpa) Poisson s Ratio Density (kg/m 3 ) Coal rock Waveguide In order to examine effects waveguide s diameter length on AE signal s propagation in waveguide, following two simulation schemes were used. According to actual coal mining situation at scene, numerical test schemes are determined. Scheme 1: Coal rock size: 60 cm 60 cm 60 cm; waveguide s length (L) was fixed at 0.3 m, while waveguide s diameter was set as 5 mm, 10 mm, 20 mm 40 mm, respectively. Scheme 2: Coal rock size: 60 cm 60 cm 60 cm; waveguide s diameter was fixed at 5 mm, while waveguide s length (L) was set as 0.5 m, 1 m, 3 m 5 m, respectively. Sustainability Under 2017, se 9, 1209 two conditions, acceleration amplitude at each node on waveguide 7 19 was simulated Analysis Results Using First Simulation Scheme 3.2. Analysis Results Using First Simulation Scheme Since stress wave ( sine curve) was loaded along waveguide s axial direction, variation Since stress stress wave wave was ( greatest sine curve) along was loaded axial direction along waveguide s somewhat axial smaller direction, along or variation directions. stressthus, wave only was greatest variation along axial AE direction stress wave somewhat along smaller waveguide s along axial or directions. was Thus, taken only into account variation in this study. AE stress wave along waveguide s axial direction was taken When into account L was fixed in thisas study. 0.3 m diameter was set as 5 mm, 10 mm, 20 mm 40 mm, respectively, When L wasacceleration fixed 0.3 data m at diameter five nodes was (specifically, set as 5 mm, Node 10 mm, A, 20 Node mmb, Node 40 mm, C respectively, Node D, along acceleration waveguide s data at axial fivedirection, nodes (specifically, Node E, Node at A, waveguide s Node B, Node free Cend) were Nodesimulated, D, along as shown waveguide s in Figure axial 3. direction, Node E, at waveguide s free end) were simulated, as shown in Figure Figure 3. 4 shows acceleration-time curves different nodes when diameter = 5 mm. Due to only considering Figure 4 shows source acceleration-time node (A) curves receiving different nodes(e, when waveguide diameter acceleration = 5 mm. Duesensor to only installation consideringlocation), source without node (A) considering receiving process node nodes (E, waveguide (B, C, D), acceleration refore, only sensor source installation node (A) location), without receiving considering node (E) were process compared. nodes Figure (B, C, 4 D), shows refore, that only receiving sourcenode (E) (A) acoustic emission receivingsignal node (E) acceleration were compared. amplitude Figure (0.1122) 4 shows attenuates that approximately receiving node (E) 90% acoustic relative emission to source signal node acceleration (A) signal amplitude amplitude (0.1122) (1.1231). attenuates This is due approximately to coal rock 90% propagation relative toloss when source node diameter (A) signal = 5 mm, amplitude source (1.1231). node This (A) ishas duea to relatively coal rocklarge propagation response loss to when sine acceleration diameter = waves 5 mm, due to source smaller node (A) diameter has a relatively waveguide. large However, response toreceiving sine acceleration node (E) signal waves acceleration due to amplitude smaller diameter is close to waveguide. that However, or diameter receiving waveguide. node (E) signal acceleration amplitude is close to that or diameter waveguide. Figure 4. Acceleration time curves different nodes when diameter = 5 mm. Figure 4. Acceleration time curves different nodes when diameter = 5 mm.

7 Sustainability 2017, 9, Figure 5 shows acceleration time curves different nodes when diameter = 10 mm. Due to only considering source node (A) receiving node (E, waveguide acceleration sensor installation location), without considering process nodes (B, C, D), refore, only source node (A) receiving node (E) were compared. Figure 5 shows that receiving node (E) acoustic emission signal acceleration amplitude (0.1121) attenuates approximately 74% relative to source node (A) signal amplitude (0.4308). This is due to coal rock propagation loss when diameter = 10 mm, source node (A) has a small response to sine acceleration waves due to larger diameter waveguide, but receiving Sustainability node 2017, (E) 9, 9, signal acceleration amplitude is close to that or diameter waveguide Figure Acceleration-time curves different nodes when diameter = 10 = mm. mm. 20 to Figure Figure 6 shows 6 shows acceleration time curves different nodes when diameter = = mm. mm. Due Due to to (E, only only considering considering source source node node (A) (A) receiving node node (E, (E, waveguide waveguide acceleration acceleration sensor sensor installation location), without considering process nodes (B, (B, C, C, D), refore, only source node installation location), without considering process nodes (B, C, D), refore, only source node (A) receiving node (E) (E) were compared. Figure 6 shows that receiving node (E) (E) acoustic (A) receiving node (E) were compared. Figure shows that receiving node (E) acoustic emission signal acceleration amplitude (0.1310) attenuates approximately 48% relative to to source emission signal acceleration amplitude (0.1310) is attenuates to approximately 48% relative to source node (A) signal amplitude (0.2507), which is due to coal rock propagation loss when diameter = node (A) signal amplitude (0.2507), which is due to coal to rock propagation loss when diameter to = 20 mm, mm, source node (A) has a small response to sine acceleration waves due to larger source node (A) has a small response to (E) sine acceleration waves due is to to larger diameter diameter waveguide, but receiving node (E) signal acceleration amplitude is close to that waveguide, or diameter but waveguide. receiving node (E) signal acceleration amplitude is close to that or diameter waveguide. Figure Figure Acceleration-time curves different nodes when diameter = 20 = mm. mm. Figure Figure 7 shows 7 acceleration time curves different nodes when diameter = 40 = mm. mm. Due Due to to to only only considering source node (A) receiving node (E, (E, waveguide acceleration sensor sensor (B, C, installation installation location), location), without without considering process process nodes nodes (B, (B, C, C, D), D), refore, only only source source node node (A) (A) receiving node (E) (E) were compared. Figure 7 shows that receiving node (E) (E) acoustic emission signal acceleration amplitude (0.1113) attenuates approximately 40% relative to to source node (A) signal amplitude (0.1855), due to to coal rock propagation loss when diameter = mm. Source node (A) has a smaller response to to sine acceleration waves due to to larger diameter waveguide, but receiving node (E) (E) signal acceleration amplitude is is close to to that or diameter waveguide.

8 Sustainability 2017, 9, Sustainability 2017, 9, receiving node (E) were compared. Figure 7 shows that receiving node (E) acoustic emission signal acceleration amplitude (0.1113) attenuates approximately 40% relative to source node (A) signal amplitude (0.1855), due to coal rock propagation loss when diameter = 40 mm. Source node (A) has a smaller response to sine acceleration waves due to larger diameter waveguide, but receiving node Sustainability (E) signal 2017, 9, acceleration 1209 amplitude is close to that or diameter waveguide Figure 7. Acceleration time curves different nodes when diameter = 40 mm. Table 2 lists maxima absolute values acceleration amplitudes at A, B, C, D E when diameter was set as different values, which were n imported to EXCEL for data process, are shown in Figure 8. Table 2. Maxima absolute values acceleration amplitudes at different nodes when diameter = 5 mm, 10 mm, 20 mm 40 mm, respectively. Diameter Waveguide (mm) A B C D E Figure 7. Acceleration time curves different nodes when diameter = 40 mm lists maxima 10 absolute values acceleration amplitudes at A, Table 2 lists maxima absolute values acceleration amplitudes at A, B, B, C, C, DD E when E when diameter diameter was was 20 set set as different as different values, values, which which were were n n imported imported to EXCEL to EXCEL for data for data process, process, are shown are shown in Figure in Figure Table 2. Maxima absolute values acceleration amplitudes at different nodes when diameter = 5 mm, 10 mm, 20 mm 40 mm, respectively. Diameter Waveguide (mm) A B C D E Figure Figure Maxima Maxima absolute absolute values values acceleration acceleration amplitudes amplitudes at at different different nodes nodes when when diameter diameter = 5 mm, mm, mm, mm, mm mm mm, mm, respectively. respectively. Figure Table 2. 8 Maxima shows variations absolute values maximum acceleration amplitudes absolute at values different nodes acceleration when amplitudes diameter at = A, 5 mm, B, C, 10 D mm, 20 E mm when D 40 was mm, set respectively. as different values. It can be observed that, when waveguide s diameter remained unchanged smaller waveguide s diameter, Diameter Waveguide (mm) A B C D E greater maximum absolute value acceleration amplitude at Node A; variations maximum absolute 5 value acceleration amplitude at C, D E were insensitive to variation waveguide s diameter. Accordingly, it can be concluded that waveguide Figure 8. Maxima absolute values acceleration amplitudes at different nodes when diameter = 5 mm, 10 mm, 20 mm 40 mm, respectively. Figure 8 shows variations maximum absolute values acceleration amplitudes at A, B, C, D E when D was set as different values. It can be observed that, when waveguide s diameter remained unchanged smaller waveguide s diameter, greater maximum absolute value acceleration amplitude at Node A; variations

9 Sustainability 2017, 9, Figure 8 shows variations maximum absolute values acceleration amplitudes Sustainability at A, B, C, D2017, 9, 1209 E when D was set as different values. It can be observed that, when waveguide s Sustainability diameter2017, remained 9, 1209 unchanged smaller waveguide s diameter, greater maximum diameter absolute imposed value only a slight acceleration effect on amplitude propagation at Node A; AE variations signal within maximum range 5~40 diameter mm. absoluteimposed value only acceleration a slight effect amplitude on propagation at C, D E were AE insensitive signal within to variation range 5~40 mm. waveguide s diameter. Accordingly, it can be concluded that waveguide diameter imposed only a 3.3. slight Analysis effect on Results propagation Using Second AE signal Simulation within Scheme range 5~40 mm Analysis Results Using Second Simulation Scheme 3.3. Analysis When D was Results fixed at Using 5 mm Second L Simulation was set as Scheme 0.5 m, 1 m, 3 m 5 m, respectively, acceleration When D data was at fixed seven at nodes 5 mm (specifically, L was set Node as A, 0.5 Node m, 1 B, m, Node 3 m C, Node 5 m, D, respectively, Node E, Node F acceleration Node WhenG, data D was listed at fixed seven in atfigure nodes 5 mm 9) (specifically, were L was simulated, setnode as 0.5as A, m, shown Node 1 m, 3 in B, m Figure Node 5 m, C, 3. respectively, Node D, Node E, acceleration Node F data Node at seven G, as nodes listed in (specifically, Figure 9) were Node simulated, A, Node as B, shown Node C, in Node Figure D, 3. Node E, Node F Node G, as listed in Figure 9) were simulated, as shown in Figure 3. Figure 9. Arrangement measuring nodes in simulations when L was set as different values. Figure 9. Arrangement measuring nodes in simulations when was set as different values. Figure 9. Arrangement measuring nodes in simulations when L was set as different values. Figure 10 shows acceleration time curves different nodes when L = 0.5 m. Due to only considering Figure Figure10 10 shows shows source acceleration time node (A) curves curves receiving different different node nodes (D, nodes waveguide when whenl L = = 0.5 acceleration 0.5m. m. Due Due to to sensor only only considering installation considering location), source source without node node (A) (A) considering receiving receiving process node nodes (D, waveguide (B, (D, C), waveguide refore, acceleration acceleration only sensor source installation node sensor (A) installation location), receiving location), without considering node without (D) considering were compared. process nodes process Figure (B, nodes C), 10 shows refore, (B, C), that refore, only receiving source only node node source (D) (A) node acoustic (A) emission receiving receiving node signal (D) acceleration node were(d) compared. were amplitude compared. Figure ( ) 10 shows Figure attenuates that 10 shows receiving approximately that node receiving (D) acoustic 82% node emission relative (D) acoustic to signal emission source acceleration node signal amplitude (A) acceleration signal amplitude ( ) amplitude attenuates (1.0062), ( ) which approximately is attenuates due to 82% approximately vibration relative to loss when source 82% L relative node = 0.5 m. (A) to Source signal source node amplitude node (A) has (A) (1.0062), a signal small which response amplitude is due to (1.0062), to sine vibration which acceleration is loss due waves when to L vibration due = 0.5 to m. coal loss Source rock when propagation node L = (A) 0.5 m. has loss, Source a small but node response receiving (A) has to a node small sine (D) response acceleration signal to acceleration waves sine due acceleration amplitude to coal rock is waves a propagation slightly due to larger coal loss, than rock but that propagation receiving or loss, node length but (D) waveguide signal receiving acceleration due node to (D) amplitude signal shorter acceleration length. is a slightly amplitude larger than is a that slightly larger orthan length that waveguide or due length to waveguide shorter length. due to shorter length. Figure 10. Acceleration time curves different nodes when L = 0.5 m. Figure 10. Acceleration time curves different nodes when L = 0.5 m. Figure 10. Acceleration time curves different nodes when L = 0.5 m. Figure Figure shows shows acceleration time acceleration time curves curves different different nodes nodes when when L = 1 m. m. Due Due to to only only considering considering Figure 11 shows source source node acceleration time node (A) (A) receiving curves receiving node different (E, node waveguide nodes (E, waveguide acceleration when L = acceleration 1 sensor m. Due installation to sensor only considering installation location), without location), source considering without node (A) considering process nodes receiving process (B, C, nodes D), refore, (B, (E, C, waveguide D), only refore, source acceleration only node source (A) sensor node installation (A) receiving node location), receiving (E) were without node compared. (E) considering were Figure compared. 11process shows Figure that nodes 11 receiving shows (B, C, D), node that refore, receiving (E) acoustic only node emission source (E) acoustic node signal (A) emission acceleration signal receiving amplitude acceleration node ( ) (E) amplitude were attenuates compared. ( ) approximately Figure attenuates 11 shows 85% approximately relative that receiving to source 85% node relative node (E) (A) acoustic to source signal emission node amplitude (A) signal ( ), acceleration amplitude which ( ), amplitude is due to which ( ) vibration is due attenuates to loss when vibration approximately L = 1 loss m. Source when 85% L node = relative 1 m. (A) Source has to source a small node node (A) has (A) signal a small amplitude response to ( ), sine which acceleration is due waves to vibration due to coal loss rock when propagation L = 1 m. Source loss, node (A) receiving has a small node response (E) signal to acceleration sine acceleration amplitude waves gets smaller due to due coal to rock larger propagation length. loss, receiving node (E) signal acceleration amplitude gets smaller due to larger length.

10 Sustainability 2017, 9, Sustainability 2017, 9, response to sine acceleration waves due to coal rock propagation loss, receiving node (E) Sustainability 2017, 9, signal acceleration amplitude gets smaller due to larger length. Figure 11. Acceleration time curves different nodes when L = 1 m. Figure 11. Acceleration time curves different nodes when m. Figure 11. Acceleration time curves different nodes when L = 1 m. Figure 12 shows acceleration time curves different nodes when L = 3 m. Due to only considering Figure 12 shows source node acceleration time (A) curves receiving different node (F, nodes waveguide when L = acceleration 3 m. Dueto to sensor only installation considering location), source source without node node (A) considering (A) receiving receiving process node (F, nodes waveguide (F, (B, C, waveguide acceleration D, E), refore, acceleration sensoronly installation sensor source installation node location), (A) without location), considering receiving without node considering process (F) were nodes compared. process (B, C, D, nodes Figure E), refore, (B, 12 C, shows D, only E), that source refore, receiving node only (A) node source (F) node acoustic receiving (A) emission node (F) signal were receiving compared. acceleration node (F) Figure amplitude were 12 compared. shows ( ) that receiving Figure attenuates 12 node shows approximately (F) that acoustic receiving emission 89% relative node signal (F) to acoustic acceleration source emission node amplitude (A) signal signal ( ) acceleration amplitude attenuates amplitude ( ), approximately ( ) which is attenuates due 89% to relative approximately vibration to source loss 89% when node relative (A) L = signal 3 m. to Source amplitude source node node ( ), (A) (A) has signal a which small amplitude isresponse due to ( ), to vibration sine which acceleration loss when is due L waves to = 3 m. vibration due Source to coal node loss rock when (A) propagation has L a= small 3 m. Source loss, response node to receiving (A) sine has acceleration a node small (F) response signal waves acceleration to due tosine coalacceleration amplitude rock propagation gets waves smaller loss, due due to coal to receiving rock larger propagation length. node (F) loss, signal acceleration receiving amplitude node (F) gets signal smaller acceleration due to amplitude larger length. gets smaller due to larger length. Figure 12. Acceleration time curves different nodes when L = 3 m. Figure 12. Acceleration time curves different nodes when m. Figure 12. Acceleration time curves different nodes when L = 3 m. Figure 13 shows acceleration time curves different nodes when L = 5 m. Due to only considering Figure 13 shows source source node node acceleration time (A) (A) receiving curves receiving node different (G, node waveguide (G, nodes waveguide acceleration when L = acceleration 5 sensor m. Due installation to sensor only considering installation location), without location), source considering without node considering (A) process nodes receiving process (B, C, D, nodes E, F), refore, (B, (G, C, waveguide D, E, only F), source refore, acceleration node only (A) sensor source installation node receiving (A) node location), (G) were receiving without compared. node considering (G) Figure were 13 compared. shows process that nodes receiving Figure (B, 13 C, node shows D, E, (G) F), that acoustic refore, receiving emission only node source signal (G) node acoustic acceleration (A) emission amplitude signal receiving ( ) acceleration node attenuates (G) amplitude were approximately compared. ( ) Figure attenuates 93% relative 13 shows approximately to that source receiving node 93% relative (A) node signal (G) to acoustic amplitude source emission node ( ), (A) signal signal which acceleration amplitude is due toamplitude ( ), vibration ( ) which loss when is attenuates due L to = 5 m. approximately vibration Source node loss (A) 93% when has relative L a= small 5 m. to Source response source node tonode (A) sine (A) has acceleration signal a small amplitude response waves( ), to due tosine coal which acceleration rock propagation is due waves to loss, vibration due to coal loss receiving rock when propagation L node = 5 (G) m. Source loss, signal acceleration node receiving (A) has amplitude a node small (G) response gets signal smaller acceleration to duesine to acceleration amplitude larger length. gets waves smaller due to due coal to rock larger propagation length. loss, receiving node (G) signal acceleration amplitude gets smaller due to larger length.

11 Sustainability 2017, 9, Sustainability 2017, 9, Sustainability 2017, 9, Figure 13. Acceleration time curves different nodes when L = 5 m. Figure 13. Acceleration time curves different nodes when L = 5 m. Table Table 3 lists lists maxima maxima absolute absolute values values acceleration acceleration amplitudes amplitudes at at A, A, B, B, C, C, D, D, E, E, F GTable when when3 L lists was was set set maxima as as different different values, values, absolute which which values were were n n imported acceleration imported to to EXCEL amplitudes EXCEL for for data data A, process, B, process, C, D, ase, as F results results G when shown shown L inwas Figure in set Figure as 14. different 14. values, which were n imported to EXCEL for data process, as results shown in Figure 14. Table 3. Maxima absolute absolute values values acceleration acceleration amplitudes amplitudes at different at different nodes nodes when when L = 0.5L m, = 0.5 1Table m, m, 31 m3. m, Maxima 3 m 5 m, respectively. 5 m, respectively. absolute values acceleration amplitudes at different nodes when L = 0.5 m, 1 m, 3 m 5 m, respectively. Maximum Absolute Value Acceleration Amplitude (m/s 2 ) Length Waveguide Maximum (m) A Absolute B Value C Acceleration D Amplitude E (m/s 2 Maximum Absolute Value Acceleration Amplitude (m/s 2 ) F G Length Length Waveguide 0.5 Waveguide (m) (m) A A B B C C D D E F F G G Figure 14 shows variations maximum absolute values acceleration amplitudes Figure at 14 A, shows B, C, D, E, variations F G when L was maximum set as different values. absolute values acceleration amplitudes at A, B, C, D, E, F G when L was set as different values. Figure 14. Maxima absolute values acceleration amplitudes at different nodes when L = Figure 14. Maxima absolute values acceleration amplitudes at different nodes when 0.5 Figure m, 114. m, Maxima 3 m 5 m, respectively. absolute values acceleration amplitudes at different nodes when L = L = 0.5 m, 1 m, 3 m 5 m, respectively. 0.5 m, 1 m, 3 m 5 m, respectively. It can be observed that, when waveguide s diameter remained unchanged smaller waveguide s It can be be observed length, that, when greater waveguide s maximum diameter remained absolute unchanged value acceleration smaller amplitude waveguide s at Node length, length, A; variations greater greater maximum maximum absolute absolute value value acceleration acceleration amplitude amplitude at at D, Node E, F A; G were variations insensitive to maximum variation absolute waveguide s value length. According acceleration to amplitude at D, E, F G were insensitive to variation waveguide s length. According to

12 Sustainability 2017, 9, Sustainability 2017, 9, at Node A; variations maximum absolute value acceleration amplitude at D, E, Fnumerical G were simulation insensitive results, to waveguide variation length waveguide s imposed only length. a slight According effect on to propagation numerical simulation AE signal results, within waveguide range length 0.5~5 imposed m. only a slight effect on propagation AE signal within range 0.5~5 m. 4. Laboratory Tests on Effects Waveguide s Size on AE Signal s Propagation 4. Laboratory Tests on Effects Waveguide s Size on AE Signal s Propagation Based on AE propagation rules, we conducted excitation tests on coal rock for investigating Based on AE propagation rules, we conducted excitation tests on coal rock for investigating effects waveguide s size on AE signal s propagation, during which stard vibrator effects waveguide s size on AE signal s propagation, during which stard vibrator source source (artificially made) virtual instrument system were used. (artificially made) virtual instrument system were used Test Scheme Parameter Settings 4.1. Test Scheme Parameter Settings According to actual coal mining situation at scene, laboratory test schemes were According to actual coal mining situation at scene, laboratory test schemes were determined. Figure 15 illustrates test scheme. The laboratory test scheme related determined. Figure 15 illustrates test scheme. The laboratory test scheme related parameter parameter settings were same as those in numerical model. settings were same as those in numerical model. Figure 15. Illustration laboratory tests tests on on acoustic acoustic emission emission (AE) (AE) propagation propagation rules rules in waveguide. in waveguide Test Results Analysis According to threshold value, AE signal event can be classified into micro-event, small-event, medium-event great event, respectively, which which were were dented dented as I, II, as III I, II, III IV. Furr, IV. Furr, each level each event level includes event two includes sub-levels two (i.e., sub-levels I1, I2, II1, (i.e., II2, I1, III1, I2, III2, II1, IV1 II2, III1, IV2, III2, respectively). IV1 IV2, respectively). (1) Variation Event Number with Waveguide Length (1) Variation Event Number with Waveguide Length As shown in Figure 16, according to different threshold value, receiving node (waveguide As shown in Figure 16, according to different threshold value, receiving node acceleration sensor installation location) acoustic emission signals AE event numbers (AE III1, III2, (waveguide acceleration sensor installation location) acoustic emission signals AE event numbers IV1, IV2 events are 4, 9, 13, 17, respectively when waveguide L = 0.5 m; AE III1, III2, IV1, IV2 events (AE III1, III2, IV1, IV2 events are 4, 9, 13, 17, respectively when waveguide L = 0.5 m; AE III1, III2, are 15, 23, 26, 33, respectively when waveguide L = 1 m; AE III1, III2, IV1, IV2 events are 52, 78, 87, 101, IV1, IV2 events are 15, 23, 26, 33, respectively when waveguide L = 1 m; AE III1, III2, IV1, IV2 events respectively when waveguide L = 3 m; AE III1, III2, IV1, IV2 events are 34, 55, 63, 73, respectively when are 52, 78, 87, 101, respectively when waveguide L = 3 m; AE III1, III2, IV1, IV2 events are 34, 55, 63, waveguide L = 5 m) change from 76( )% to 93( )% except for 73, respectively when waveguide L = 5 m) change from 76( )% to 93( AE I1, I2, II1, II2 events, as waveguide s length increased from 0.5 m to 5 m. Basically, as wave )% except for AE I1, I2, II1, II2 events, as waveguide s length increased from 0.5 m to 5 length increases, AE events numbers slightly increase due to waveguide vibration increasing. m. Basically, as wave length increases, AE events numbers slightly increase due to However, variation range AE events numbers show only a slight effect. Laboratory results waveguide vibration increasing. However, variation range AE events numbers show only are consistent with numerical simulation. a slight effect. Laboratory results are consistent with numerical simulation. (2) Variation Event Number with Waveguide Diameter As shown in Figure 17, according to different threshold value, receiving node (waveguide acceleration sensor installation location)acoustic emission signals correspond to number AE

13 Sustainability 2017, 9, Sustainability 2017, 9, events (AE III1, III2, IV1, IV2 events are 16, 26, 31, 38, respectively when waveguide diameter = 5 mm; AE III1, III2, IV1, IV2 events are 15, 25, 30, 38, respectively when waveguide diameter = 10 mm; AE III1, III2, IV1, IV2 events are 11, 19, 24, 33, respectively when waveguide diameter = 20 mm; AE III1, III2, IV1, IV2 events are 4, 6, 9, 12, respectively when waveguide diameter = 40 mm) change from 68( )% to 77( )%, except for AE I1, I2, II1, II2 events, as waveguide s diameter increased from 5 mm to 40 mm. Basically, as wave diameter increases, AE events numbers Figure slightly 16. Variation decrease AE due event tonumber waveguide with waveguide s vibration decreasing. length. However, Sustainability variation range 2017, 9, 1209 AE events numbers show only a slight effect. Laboratory results are consistent (2) withvariation numerical Event simulation. Number with Waveguide Diameter As shown in Figure 17, according to different threshold value, receiving node (waveguide acceleration sensor installation location)acoustic emission signals correspond to number AE events (AE III1, III2, IV1, IV2 events are 16, 26, 31, 38, respectively when waveguide diameter = 5 mm; AE III1, III2, IV1, IV2 events are 15, 25, 30, 38, respectively when waveguide diameter = 10 mm; AE III1, III2, IV1, IV2 events are 11, 19, 24, 33, respectively when waveguide diameter = 20 mm; AE III1, III2, IV1, IV2 events are 4, 6, 9, 12, respectively when waveguide diameter = 40 mm) change from 68( )% to 77( )%, except for AE I1, I2, II1, II2 events, as waveguide s diameter increased from 5 mm to 40 mm. Basically, as wave diameter increases, AE events numbers slightly decrease due to waveguide vibration decreasing. However, variation range AE events numbers show only a slight effect. Laboratory results are consistent with numerical simulation. Figure Figure Variation Variation AE AE event event number number with with waveguide s waveguide s length. length. (2) Variation Event Number with Waveguide Diameter As shown in Figure 17, according to different threshold value, receiving node (waveguide acceleration sensor installation location)acoustic emission signals correspond to number AE events (AE III1, III2, IV1, IV2 events are 16, 26, 31, 38, respectively when waveguide diameter = 5 mm; AE III1, III2, IV1, IV2 events are 15, 25, 30, 38, respectively when waveguide diameter = 10 mm; AE III1, III2, IV1, IV2 events are 11, 19, 24, 33, respectively when waveguide diameter = 20 mm; AE III1, III2, IV1, IV2 events are 4, 6, 9, 12, respectively when waveguide diameter = 40 mm) change from 68( )% to 77( )%, except for AE I1, I2, II1, II2 events, as waveguide s diameter increased from 5 mm to 40 mm. Basically, as wave diameter Figureincreases, 17. Variation AE events number numbers withslightly waveguide s decrease diameter. due to waveguide vibration decreasing. Figure However, 17. Variation variation AE event number range with AE waveguide s events numbers diameter. show only a slight effect. 5. Effects Laboratory Waveguide s results are consistent Installation with Method numerical on AE Proportion simulation. Rules 5. Effects Waveguide s Installation Method on AE Proportion Rules The installation AE sensor mainly includes three methods: coal surface installation, The installation AE sensor mainly includes three methods: coal surface installation, hole-bottom installation waveguide installation. Among se three installation methods, hole-bottom installation waveguide installation. Among se three installation methods, hole-bottom installation is best in signal receiving, while or installation methods are easy hole-bottom installation is best in signal receiving, while or installation methods are easy to to use sensor is recyclable. However, for sensor that was installed on coal surface, use sensor is recyclable. However, for sensor that was installed on coal surface, received AE signals would undergo strong attenuation due to existence a loose circle in received AE signals would undergo strong attenuation due to existence a loose circle in tunnel, which would seriously affect receiving effective signals. Thus, only hole bottom tunnel, which would seriously affect receiving effective signals. Thus, only hole bottom installation waveguide installation will be detailed below. installation waveguide installation will be detailed below. (1) Hole-Bottom Installation (1) Hole-Bottom Installation Using this method, sensors were generally installed in deep coals; y exhibited high Using this method, sensors were generally installed in deep coals; y exhibited high sensitivity a great effective distance signal receiving. Figure 18 illustrates this installation sensitivity a great effective distance signal receiving. Figure 18 illustrates this installation method. A hole (ϕ42 mm) was drilled in coal rib coalface, signal line was coated with Figure 17. Variation AE event number with waveguide s diameter. rubber hose (ϕ10 mm) for preventing coals from crushing signal line after hole collapses. 5. Firstly, Effects a small amount Waveguide s yellow Installation mud was Method addedon to AE Proportion cement mortar, Rules hole bottom was sealed by 0.5 m using prepared cement mortar; n, sensor was pressed in cement mortar The installation AE sensor mainly includes three methods: coal surface installation, hole-bottom installation waveguide installation. Among se three installation methods, hole-bottom installation is best in signal receiving, while or installation methods are easy to use sensor is recyclable. However, for sensor that was installed on coal surface, received AE signals would undergo strong attenuation due to existence a loose circle in tunnel, which would seriously affect receiving effective signals. Thus, only hole bottom

14 gradually compacted by coals in a short period time, which was n in solid contact with Sustainability 2017, 9, coals. Using this installation method, after compaction drilled holes, sensor was slightly method. affected A by hole (φ42 noise mm) from was external drilled environment. in coal rib The shortcoming coalface, this method signal line lies was in coated high with cost. If rubber acquired hose safety (φ10 mm) benefit for far preventing exceeded coals invested from crushing cost during signal service line after period, hole n collapses. hole-bottom SustainabilityFirstly, installation 2017, 9, 1209 a small method amount would yellow be regarded mud was added preferred to method. cement mortar, 14hole 18 bottom was sealed by 0.5 m using prepared cement mortar; n, sensor was pressed in cement mortar at bottom hole finally sealed by approximately 1m. The sensor was at bottom hole finally1 sealed by 2 approximately 3 gradually compacted by coals in a short period time, 1m. 4 which The sensor 5 was n wasin gradually solid contact compacted with coals. by Using coals inthis a short installation period method, time, which after was compaction n in solid contact drilled with holes, coals. Using sensor this was installation slightly affected method, by after noise compaction from external drilled environment. holes, The sensor shortcoming was slightly this affected method bylies in noise from high cost. external If environment. acquired safety The benefit shortcoming far exceeded this method invested lies incost during high cost. Ifservice acquired period, safety n hole-bottom benefit far exceeded installation invested method would cost during be regarded service as period, preferred n hole-bottom method. installation method would be regarded as preferred method Figure 18. Illustration hole-bottom installation sensor, in which 1,2,3,4 5 denote Rubber hose, Signal line, Drilled hole, Cement mortar Hole-bottom sensor, respectively. Practices have proven following advantages disadvantages this installation method: great effective distance in signal receiving, high sensitivity, strong capability disturbance resistance, easy for installation implementation, but also, a difficulty in disassembly high Figure cost. 18. Illustration hole-bottom installation sensor, in which 1,2,3,4 5 denote Rubber hose, Figure Signal line, 18. Illustration Drilled hole, Cement hole-bottom mortar installation Hole-bottom sensor, sensor, in which respectively. (2) Wave-Installation 1,2,3,4 5 denote Rubber hose, Signal line, Drilled hole, Cement mortar Hole-bottom sensor, respectively. Practices Figure 19 have illustrates proven this hole-bottom following advantages installation method. disadvantages Wave-installation this installation can be regarded method: as great a kind Practices effective orifice have distance installation. proven signal following receiving, In order advantages to high improve sensitivity, disadvantages strong AE signal s capability receiving this disturbance installation effects, resistance, favorable method: great easy coupling for effective installation between distance sensor implementation, in signal coal receiving, rock but must also, high be a difficulty guaranteed, sensitivity, in disassembly strong which can capability be realized high disturbance through cost. a resistance, waveguide. easy The for one installation end waveguide implementation, is fixed with but coals also, at a difficulty bottom in disassembly hole, using some (2) high cohesive Wave-Installation cost. materials, n, signal is transmitted to sensor via waveguide. Due to metal s Figure homogeneity, 19 illustrates continuity, this hole-bottom great elastic installation modulus method. Wave-installation high stiffness, can signal be regarded was weakly as a (2) Wave-Installation kind attenuated orifice installation. waveguide In order showed to improve favorable wave AE signal s guiding receiving function. effects, favorable coupling between Figure In order sensor 19 to illustrates reduce coal rom this rock hole-bottom must noise be guaranteed, installation noise caused method. which by can Wave-installation human be realized activities through can around be aregarded waveguide. sensor, as a The kind waveguide one end orifice was installation. waveguide penetrated In is through fixed order with to st improve coals materials, such bottom AE as signal s scrapped receiving hole, using hose; effects, meanwhile, somefavorable cohesive coupling materials, sensor was between also n, coated sensor with signal coal is hose transmitted rock for must protection, tobe guaranteed, sensor so that via which effects waveguide. can caused be realized by Due todropping through metal s a waveguide. homogeneity, coals water The continuity, one end coal great seam elastic waveguide injection modulus is can fixed be with high avoided. coals stiffness, Additionally, at bottom signal st was materials weakly hole, using attenuated exhibited some cohesive remarkable waveguide materials, noise insulation showed n, favorable performances. signal wave is transmitted guiding function. to sensor via waveguide. Due to metal s homogeneity, continuity, great elastic modulus high stiffness, signal was weakly attenuated waveguide 1 2 showed 3favorable 4wave 5guiding 6 function. 7 In order to reduce rom noise noise caused by human activities around sensor, waveguide was penetrated through st materials, such as scrapped hose; meanwhile, sensor was also coated with hose for protection, so that effects caused by dropping coals water in coal seam injection can be avoided. Additionally, st materials exhibited remarkable noise insulation performances Figure 19. Illustration hole-bottom installation sensor, in which 1, 2, 3, 4 5 denote Sensor Figure 19. Illustration hole-bottom installation sensor, in which 1, 2, 3, 4 5 denote Sensor sleeve, Sensor, Drilled hole, Cement mortar Hole-bottom sensor, respectively. sleeve, Sensor, Drilled hole, Cement mortar Hole-bottom sensor, respectively. In order to reduce rom noise noise caused by human activities around sensor, waveguide was penetrated through st materials, such as scrapped hose; meanwhile, sensor was also coated with hose for protection, so that effects caused by dropping coals water in coal seam injection can be avoided. Additionally, st materials exhibited remarkable noise insulation performances. Figure According 19. Illustration to practices, hole-bottom waveguide installation installation sensor, method in which shows 1, 2, 3, 4 following 5 denote advantages Sensor sleeve, disadvantages: Sensor, Drilled great hole, effective Cement distance mortar inhole-bottom signal receiving, sensor, strong respectively. capability disturbance resistance, being simple quick in installation implementation; simple processing technology easy for disassembly for sensor; but also, angle should not be too large for inclined-hole installation, because orwise, fixation will be difficult.

15 According to practices, waveguide installation method shows following advantages According to practices, waveguide installation method shows following advantages disadvantages: great effective distance in signal receiving, strong capability disturbance disadvantages: great effective distance in signal receiving, strong capability disturbance resistance, being simple quick in installation implementation; simple processing resistance, being simple quick in installation implementation; simple processing technology easy for disassembly for sensor; but also, angle should not be too large for technology easy for disassembly for sensor; but also, angle should not be too large for Sustainability inclined-hole 2017, installation, 9, 1209 because orwise, fixation will be difficult inclined-hole installation, because orwise, fixation will be difficult. Figures compare AE signal temporal spectrum amplitude (AE signal amplitude Figures compare AE signal temporal spectrum amplitude (AE signal amplitude are 3.0, 4.0, respectively when acceleration sensor was installed using waveguide installation are 3.0, Figures 4.0, respectively compare when acceleration AE signal sensor temporal was installed spectrumusing amplitude waveguide (AE signal installation amplitude hole-bottom installation) AE event numbers between se two different installation are hole-bottom 3.0, 4.0, respectively installation) when acceleration AE event sensor numbers was installed between using se waveguide two different installation installation methods, from which we can easily see that AE signal receiving effects through waveguide hole-bottom methods, from installation) which we can easily AE event see that numbers AE between signal receiving se twoeffects different through installation waveguide methods, installation method are close to that hole-bottom installation. In or words, waveguide from installation which we method can easily are close see that to that AE signal hole-bottom receiving effects installation. throughin or waveguide words, installation waveguide installation method can replace hole-bottom installation method (acceptable deviation value based method installation are close method to that can replace hole-bottom hole-bottom installation. installation In or method words, (acceptable waveguide deviation installation value method based on field experience). can on field replace experience). hole-bottom installation method (acceptable deviation value based on field experience). Figure 20. Comparison AE signal temporal spectrum between waveguide installation Figure 20. Comparison AE signal temporal spectrum between waveguide installation hole-bottom installation. hole-bottom installation. Figure Comparison AE event number between waveguide installation hole-bottom Figure 21. Comparison AE event number between waveguide installation hole-bottom installation. installation. installation. 6. Conclusions The propagation acoustic emission (AE) signal in waveguide is quite important for AE-based prediction dynamic disasters in coal rocks. In this study, based on some relevant ories in wave mechanics, elastic mechanical model one-dimensional (1D) waveguide was first established,

16 Sustainability 2017, 9, relationship between AE source signal signal at waveguide s receiving end was derived. On basis oretical analysis, numerical simulation laboratory test schemes were designed; additionally, using stard vibration source method, AE response in different sizes waveguides were investigated, effects waveguide size waveguide were concluded, application conditions established oretical model were clarified. Numerical simulation results fit well with laboratory test results. Additionally, effects sensor s waveguide installation method hole-bottom installation method were compared appropriate installation method was determined. (1) This study firstly established ory model 1D elastic waveguide based on wave mechanics n made some relevant assumptions. According to numerical simulation results, this elastic ory model is applicable to waveguides with a length smaller than 5 m a diameter smaller than 40 mm. (2) According to numerical simulation laboratory test results, waveguide s diameter imposed only a slight effect on acceleration amplitude AE event number at waveguide s receiving end within a range 5~40 mm. (3) According to numerical simulation laboratory test results, waveguide s length imposed only a slight effect on acceleration amplitude AE event number at waveguide s receiving end within a range 0.5~5 m. (4) AE signal receiving effects through waveguide installation method are close to that hole-bottom installation, based on AE signal temporal spectrum amplitude AE event numbers. Therefore, waveguide installation method can completely replace hole-bottom installation method. Acknowledgments: This work was financially supported by National Key Research Development Program China (Grant No.: 2016YFC ), National Science Technology Major Project Ministry Science Technology China in 13th Five-Year Period (Grant No.: 2016ZX ), One Hundred Youth Talents Project Shaanxi Province China,National Natural Science Foundation China (Grant No.: ) Shaanxi Province Natural Science Foundation(Grant No.: 2017JM5115). Author Contributions: Dong, G. Zou, Y. conceived designed experiments; Dong, G. performed experiments; Dong, G. Zou, Y. analyzed data; Dong, G. wrote paper. Conflicts Interest: The authors declare no conflict interest. References 1. Zou, Y.H. Monitoring Coal Gas Outbursts Using AE Technique; China Coal Technology Engineering Group Chongqing Research Institute: Chongqing, China, Yang, M.W. Acoustic Emission Detection; China Machine Press: Beijing, China, 2004; pp Katsuyama, K. Application Acoustic Emission (AE) Technique; Metallurgical Industry Press: Beijing, China, 1996; pp Peng, X.M.; Sun, Y.H.; Li, A.N. Application Present Rock Acoustic Emission Technique. World Geol. 2000, 19, Yuan, Z.M.; Ma, Y.K. Introduction Acoustic Emission Technique Applications; China Machine Press: Beijing, China, 1985; pp Jiang, F.X.; Ye, G.X.; Wang, C.W.; Zhang, D.Y.; Guan, Y.Q. Application High-precision Microseismic Monitoring Technique to Water Inrush Monitoring in Coal Mine. Chin. J. Rock Mech. Eng. 2008, 27, Song, Y.L.; Wang, C.P.; Deng, Z.G.; Wang, Y.J. Study on Spreading Route Micro-Seismic Initial Arrival Wave in Rock Strata. Coal Sci. Technol. 2012, 6, Wang, Y.J.; Deng, Z.G.; Wang, C.P. Research on Improving Precision Seismic Event Location for Deep Well Mining. China Coal 2011, 12, Xia, Y.X.; Kang, L.J.; Qi, Q.X.; Mao, D.B.; Ren, Y.; Lan, H.; Pan, J.F. Five Indexes Microseismic ir Application in Rock Burst Forecasting. J. China Coal Soc. 2010, 35,

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