Using AE Technique to Monitor the Fracture Behavior in Shaking Table Tests of a Scale-down Mockup of Nuclear RC Structure

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1 19 th World Conference on Non-Destructive Testing 216 Using AE Technique to Monitor the Fracture Behavior in Shaking Table Tests of a Scale-down Mockup of Nuclear RC Structure Kuang-Chih PEI Institute of Nuclear Energy Research, Taoyuan City, Taiwan Contact kcpei@iner.gov.tw Abstract. Acoustic Emission (AE) monitoring and analysis technique were introduced to evaluate the fracture behaviour of a 1/25 scale-down RC structure during the shaking table tests. The mockup, based on a simplified structure design of nuclear power plant reactor building, was composed of primary/secondary containments and floor slabs of different scale-down thickness. Serial tests with AE monitoring were conducted on the 5x5m Tri-Axial Seismic Simulator in the NCREE Lab. Micro/inner fracture due to shaking force created numerous ultrasonic AE events received/recorded through the AE instrument. The monitored results could chronicle these damages, and relate them with seismic data, such as the corresponding accelerations and displacements during the test. In this paper, the fracture behaviour and material condition of the mockup under gradually increased resonant loading was shown using AE technique. Experiment results of seismic loadings based on Chi-Chi earthquake were also presented here with AE analyses. Through the use of AE monitoring, quantitative study on shaking table test could be promised. 1. Introduction Shaking table test is an important method in earthquake engineering to approach the structural behaviour or destruction under Tri-Axial seismic or three-dimensional mechanical vibration. The relative movement of the ground and building structure under certain earthquake can be simulated to evaluate the interaction in foundation and the dynamic behaviour of structure. The evaluation can provide an assessment basis on boundary and interlock conditions for further numerical analysis or design aspect. The prototype of certain structural element, electrical equipment or machinery, whose size and weight are not over the capacity, can be tested directly to get the seismic behaviour. But, large-size structures like high-rise buildings are unable for the directly testing. Similar but scale-down mockups will be used instead. The carrying capacity and power of table affect the test scale. During the test, the response accelerations and displacements at certain points on the prototype or scale-down mockup can be measured. Through a series of tests to progressively enlarged seismic waves of certain earthquake spectrum, the increased response, weakness or damage of structure can be identified. Nonlinear response and earthquake resistance also can be studied. In this research, ABWR containment of nuclear power plant (NPP) was referred when constructing a 1/25 scale-down mockup (see Fig.1.). The simplified reinforced concrete (RC) structure was composed of primary/secondary containment walls and floor License: 1 More info about this article:

2 slabs of different scale-down thickness. When the mockup was struck by seismic waves on shaking table, the dynamic response to the kinematical energy could be: amplified movement, structural deformation and/or material fracture. The movement and deformation at certain location on the mockup can be measured by accelerometer and displacement probe. However, to real-timely detect the crack or inner damage during the test could be difficult. The fracture behavior of mockup under earthquake shock was obscure. The earliest application of acoustic emission (AE) monitoring for the cracking in concrete material was performed by Robinson [1] in AE of frequency 13k~14kHz produced within mortar and concrete cubes during fracture were received for study. His result revealed that AE technique could provide an earlier detection of structural change than other conventional methods. Noticeable advantages of AE technology could be pointed: (1) AE events can be monitored through the entire load history to appear cracking behavior (e.g., extension and expansion of cracks) and to contour the possible damage processes in complicated material; (2) Signals are directly from the cracking. No external stress wave is applied for detection. A study by Mindess [2] had stated that the count of AE events passing certain threshold increased as the loading during the pure concrete compression tests. When approaching the ultimate load, AE from the inner concrete was speeded up, indicating that inside cracks were vivified. To record and identify AE signals of the certain range, high sampling rate DAQ system and proper sensors were required. An AE system for concrete, with 4-digital-channel/ 8kHz/16bits data acquisition (DAQ) parts and real-time control programs, was used for this research (see Fig.1.). The sensors were with 4dB integral preamplifier function and 5 khz spectral response range (15kHz Max.). The received AE could be digitized/recorded as oscillatory signals and real-timely presented on screen during the shaking table test. When RC was fractured, AE events of different major frequencies would perform for the splitting over different materials such as aggregate or cement mortar. Usually, the final burst of loading test sounded loud and clear; the acoustic frequency was surely under 2kHz and available for human hearing. However, most AE events, due to minor crack, splitting, friction, de-bonding, interlocking, etc., were ultrasonic, certainly out of hearing range. Since concrete was noted as a high-decay-rate material for ultrasound, the main frequencies of most receivable or identifiable acoustics after filtering out through the concrete medium were usually under 15kHz, whose corresponding wave length was larger than most grain sizes of aggregate. Cracking in higher strength material usually creates AE of higher main frequency. The frequency band of AE by breaking gravel in RC would be higher than by cracking along Fig. 1. The settlement of AE sensors and equipment for inspection 2

3 the sand-cement mortar surrounding gravel. De-bounding and rubbing along the re-bar/gravel interface also created acoustics, but the known range could be much lower (under 4kHz). Some connections among AE performance, microscopic mechanisms, and fracture mechanism were disclosed in Pei and Kan s [3,4,5] experiments to monitor the load tests for RC slabs, beams, and columns. Crack incubation phenomenon by RC bonding control and Kaiser s effect were discussed. 2. Preparation for Experiment 2.1 Mockup Design Fig. 2 demonstrates a photo of 1/25 scale-down mockup of ABWR containment on shaking table, so well as the design plan. The dimension was 2.9m in length, width and height (i.e. rectangular), and the designed weight was about 28 tons. The simplified RC structure was composed of primary and secondary containments of different scale-down wall/floor thickness. The RC walls from outside to inside of the mockup were the side-walls of secondary containment and the inner cylindrical wall of primary containment (i.e. RCCV, Reinforced Concrete Containment Vessel). The side-wall and cylindrical wall were with 7.5cm and 1cm in thickness, respectively. The internal diameter and height of the cylinder RCCV model were 85cm and 29cm. The mockup had three stories. The floor slab (FS) of base, directly bolted on the shaking table, was 3cm in thickness; 3.3m in length and width. Both the second and third floor slabs were 3cm in thickness, and constructed from the side walls to inside of the RCCV. The roof-slab was 15cm in thickness. Three rectangular openings and box-type structures were designed on the roof-slab as the water pools for used fuel storage of NPP. The mockup was formed using high rib lath as template for superior model of scale-down containment (i.e. non-demoulding method). Premixed Self-compacting concrete (SCC, 4psi/Type I) was also used for better workability during the pouring process in construction. After curing, the mockup was installed on the table for shaking tests. 2.2 Sensors Settlement In this test, 23 accelerometers and 11 displacement transducers (LVDT) were settled at certain locations on the mockup for real-time inspection (see Fig. 3). The tri-axial accelerations and displacements (i.e. X, Y and Z directions) on base, second, third floor slabs primary containment Fig. 2. Design of the mockup and experiment site secondary containment 3

4 and roof slab would be recorded during the shaking. Four sensors were installed on the side walls for AE monitoring. Sensor #1 and #3 were set on the outer surface of side walls close to the third floor slab; sensor #2 and #4 were close to the second floor slab (see Fig. 4). The locations should be chosen near the hot spots of structure damage, such as the wall-slab joint. Crack usually initialized there when shaking. 2.3 Integration of AE monitoring/shaking Table Test and AE Analysis The sampling rate of DAQ for AE was 8kHz, which was much higher than the DAQ rate for shaking table test (2Hz). So, both readings from AE monitoring and shaking table test could be coupled after the synchronization on the starting time. After test, the AE signal oscillations would be threshold filtered, and then counted every second (or quarter-second) to create AE signal density record (i.e. number line of hit count/1 or 5 second for each sensor) over the testing period. The obtained AE density, acceleration and displacement records could be graphed in same time historical chart to reveal the fracture behaviour during the test. 2.4 Preparation of Shaking Table Test - The Resonant Frequencies of Mockup Before the seismic tests, continuous sine sweep from 1Hz to 5Hz was applied to find the resonant frequencies of mockup on tri-axial directions. The result indicated that the frequency in X-direction was 23.38Hz; 21.89Hz in Y-direction. The frequency in Z-direction Fig. 3. Locations of the accelerometers and displacement transducers A2WY Sensor #4 A2WX Sensor #3 A3WY A3WX A2NY A3NY Sensor #2 A2NX Sensor #1 A3NX Fig. 4. Locations of the AE sensors (yellow dots) 4

5 was higher than 5Hz. Since the roof structure was not axial symmetry around Z axis, the resonances in X- and Y- directions were different. The mockup performed more rigid in X-direction. 3. Experiment Case Study 3.1 AE Monitoring on Mockup Fracture Due to Time-amplified Resonant Shaking According to the resonant frequency in X-direction (23.38Hz), the shaking simulation was proposed a 25Hz time-amplified continuous sine wave in X-direction (see Fig. 5). Since the shaking wave was close to the resonances in mockup, the movement and acceleration could be enhanced during the test. The amplitude of shaking was progressively raised by seismic simulator in a rate of.9~.18g/sec during the test period. The max acceleration of the simulator output was +.676/81g, and occurred at 16.5 second. The entire test took about 148 seconds, and was shut down for the over-displacement at 18 second. Due to the weight of 28 tons, not only the bolted mockup vibrated as resonance, the amplified vibration also dragged the table with higher acceleration. The measurement indicated that the max acceleration on shaking table was +.717/-.724g in X-direction, which was higher than the original output. The max X-acceleration of the mockup was detected on the roof-slab; was +2.46/-2.58g. It was noted that no simulator loading created on Z-direction. However, the measured max Z-acceleration on the table due to the drag was /59g (containing the gravity), and was /5g on the roof-slab. Local parts of the mockup could be damaged under such violent shaking. Structures under noticeable shaking would exhibit elastic displacement, dynamic movement, plastic deformation, or fracture. If the structure material remained linear during the movement, AE events by fracture would rarely occur. Some acoustics might happen for the friction on structure interfaces or plastic deformation in material. However, AE due to both reasons were usually reduced by loading cycles for Kaiser effect. When fracture happened, AE events increased suddenly, so did the AE density. The AE densities shown in Fig. 6 and 7 may confirm the above inference. Fig. 6 shows the time historical chart obtained from AE densities and acceleration records of three sensors/probes located at: shaking table, side-wall near third floor slab, and roof-slab. AE densities for certain sensors were shown as the number lines of hit count/sec. over the testing period, and revealed the local fracture conditions. Since resonance became obvious after 2 seconds from starting, the period of 2~11 second was focused in the chart..7.6 g D_Acc_X 469D_Acc_Z 469D_Acc_Y g Sec Sec. Fig. 5 A shaking model of 25Hz time-amplified continuous sine wave in X-direction 5

6 Identified AE events initiated at 61 second, when the X-accelerations on shaking table (probe AX), third floor (A3WX), and roof-slab (A4WNX) were 63g,.651g, and.94g, respectively. Before 61 second, the mockup remained elastic and integrated; AE monitoring resulted that no fracture or rub damage occurred for structure movement. After that time, steady AE increment revealed some friction or rub damage happening. AE densities raised fast at the time periods of 72~76 and 82~85 second. Some irreversible fracture happened and caused acoustics. During 72~76 second period, the obtained X-accelerations by probes AX, A3WX, and A4WNX were 6~63g,.892~.985g, and 1.295~1.427g, respectively. During 82~85 second, the X-accelerations were 43~7g, 1.116~1.177g, and 1.617~1.74g. In Fig. 6 and 7, AE density performed staggeringly increased after 85 second. However, no burst damage occurred in the test since the shaking was shut down for the over-vibration at 18 second. Fig. 7 demonstrates the time historical chart of AE densities and displacement record at the third floor slab. The staggering movement after 72 second also revealed the damage began to disturb the structure system. 3.2 AE Monitoring on Shaking Table Test of Seismic Loading In this case, seismic loadings based on Chi-Chi earthquake were applied to study the dynamic behaviour of the mockup. The north-south, east-west, and vertical component vectors of the recorded seismic acceleration would be re-performed in X-, Y-, and Z-directions by Acc (g) / AE signal density (4 hits /sec.) A4WNX A3WX AX AE-Sensor 1 AE-Sensor 3 AE-Sensor Sec. Fig. 6 The time historical chart of AE densities and acceleration records of three sensor/probe locations.7.6 Acc (mm) / AE signal density (4 hits /sec.) D3X AE-Sensor 1 AE-Sensor 3 AE-Sensor Sec. Fig. 7 The time historical chart of AE densities and displacement record at the third floor slab 6

7 Tri-Axial Seismic Simulator. The proposed max amplitude of shaking was.6g. However, the measured max values on the third floor slab and roof-slab were.632g and.67g, respectively. Seismic amplification occurred on the higher floors. AE monitoring during the seismic test could show the exact time and condition when the fractures occurred. Fig. 8 shows the time historical chart obtained from the AE densities and X- acceleration record collected near the third floor slab (probe A3WX); Fig. 9 is for the Y-acceleration (probe A3WY). The number lines of AE density were profiled in hit count/ 5 second over the test. Fig. 1 presents the chart form AE densities and Y- displacement record of the probe D3Y on the third floor. The comparison on Fig. 8~1 indicated that the corresponding times of AE density peak-value were close to the time of high displacements. Noticeable fracture occurs when large movement happens. Acc. (g) / Signal density (unit:4 hits/5sec.) A3WX Sensor 1 Sensor 3 Sensor Time (sec) Fig. 8 Time historical chart from the AE densities and X- acceleration (probe A3WX).6 Acc. (g) / Signal density(unit:4 hits/5sec.) A3WY Sensor 1 Sensor 3 Sensor Time (sec) Fig. 9 Time historical chart from the AE densities and Y- acceleration (probe A3WY) Displacement (mm) / Signal density(unit:4 hits/5sec.) Time (sec) Fig. 1 The chart form AE densities and Y- displacement record of probe D3Y D3Y Sensor 1 Sensor 3 Sensor 4 7

8 Signal density (unit: hits/5sec.) by Sensor Time (sec) Fig. 11 AE densities (sensor #3) of g/g/.6g seismic tests.6g G G To study the repeatability of seismic test, three individual seismic tests using Chi-Chi earthquake record but different max amplitudes of shaking, g, g and.6g, were monitored. Fig. 11 presents the AE densities (sensor #3) of three tests. After synchronizing the starting time, three AE density lines were close in the occurring times of peak-values. In other words, the fracture occurrence can be definite. The same mockup and the same seismic record of arbitrary max shaking amplitude are sufficient to indicate the repeatability of the fracture mechanisms in certain seismic test. The test result also revealed that most damage mechanisms were apparent when the max amplitude of shaking force was over g. 4. Conclusion In this study, several experiments were demonstrated using the integration of AE real-time monitoring technique and shaking table test. In the case of time-amplified resonant shaking, material condition and fracture stages were detailed in the AE density record, which could be useful to study the structural damping ability to absorb the seismic energy and the anti-seismic measurment for structure. In the seismic tests, the repeatability of seismic test in fracture behavior was identified by the comparison on AE density. The AE performance of repeatability might be applied on the evaluation of scale problem of certain mockup. Finally, it can be concluded: quantitative study on shaking table test will be promised through the use of AE technique. References [1] Robinson, G. S., Methods of Detecting the Formation and Propagation of Micro-cracks in Concrete, Proceedings of International Conference on Structure of Concrete and its Behavior under Load, London, , [2] Mindess, S. Acoustic Emission and Ultrasonic Pulse Velocity of Concrete, Int. Journal of Cement Composites and Lightweight Concrete, 4, No.3, Construction Press, Aug., [3] Pei, K.C et al, A Preliminary Study on the Fracture Behaviors of Reinforced Concrete Slabs by Monitoring the Load Test Using Acoustic Emission Method, Journal of The Chinese Institute of Civil And Hydraulic Engineering, 21(2), 29, pp [4] Pei, K.C et al, Acoustic Emission Monitoring on the Fracture Behaviors of RC and Light RC Structural Elements in the Load Tests, In: CECAR5 (The 5th International Civil Engineering Conference in the Asian Region) Sydney, Australia, 8-12 Aug., 21. [5] Pei, K.C et al, A Preliminary Study on the Bond Failure of Large Size Rebars in Beam during Bending test Using Acoustic Emission Monitory, 22th International Conference on Structural Mechanics in Reactor Technology (SMiRT 22), San Francisco, USA, Aug., 213 8