More info about this article: http://www.ndt.net/?id=21866 Abstract IX th NDT in PROGRESS October 9 11, 2017, Prague, Czech Republic APPLICATION OF ACOUSTIC EMISSION METHOD DURING CYCLIC LOADING OF CONCRETE BEAM Lubos PAZDERA, Libor TOPOLAR, Karel MIKULASEK Faculty of Civil Engineering, Brno University of Technology; Brno, Czech Republic E-mail: pazdera.l@fce.vutbr.cz, libor.topolar@vutbr.cz; Concrete structures are loaded during their lifetime. These structures are loaded and unloaded i.e. loading stressed structure as cyclically loaded. This paper is concerned with the acoustic emission method used to detect the forming cracks. The detection by the acoustic emission method is based on recording the elastic waves formed by active cracks. The loading frequency of the concrete beams tested was 2 Hz. Measurement started with a loading force of 30 kn being increased in 5 kn steps to the final value of 70 kn for 1 hour. The whole experiment contains about 72,000 cycles, i.e., the loading and unloading of the concrete structure. The paper describes the application of the acoustic emission method to this concrete beam at cyclic loading. Keywords: acoustic emission, concrete, beam, cyclic loading, stress, 1. Introduction The Acoustic Emission Method is being described in many ways. Topolar et al. wrote: The Acoustic Emission Method, which is part of the non-destructive testing techniques, records the propagated elastic waves generated from the place of an active crack. This method is more sensitive than visual observation because it makes it possible to continually monitor the acoustic emission activity during loading. [1] or The method of acoustic emission is an experimental tool suitable for monitoring the failure processes in materials. The typical parameters of the acoustic emission signal were identified during the acoustic emission records for different concrete specimens to further describe the under-the-stress behaviour and failure development. The amount of crack growth was continuously monitored using four acoustic emission sensors mounted on the specimens. [2] or Acoustic emission in nondestructive testing is physically similar to an earthquake in seismology. That is a phenomenon mostly associated with a destruction process. The acoustic emission events can be analyzed either qualitatively or quantitatively. [3] A similar definition can be found in Mazal et al: In the signal monitored on the surface of the material, it is possible to identify the course of a number of processes that appear both on the surface and inside the material of the construction. From this point of view, we can see that this method could provide us with information about the course of fatigue damage and it could help specify its residual fatigue life. [4]. Other interesting definitions can be found in Pazdera et al: The acoustic emission method detects the transient elastic waves within the material caused by the release of cumulated stress energy, which can be mechanical, thermal, or chemical. Hence, the cause is a phenomenon that releases elastic energy into the material, which then spreads in the form of an elastic wave. [5] The acoustic emission principles are very clearly described by Ohtsu: Acoustic Emission is applied to a variety of fields related to concrete engineering. The basic principle is demonstrated in Fig. 1. [6] - 51 -
Figure 1 Basic acoustic emission principle [6] The evaluation of the data obtained by the acoustic emission method is very complicated and requires sufficient experience of the operator of the measuring device. For example, Ohtsu evaluates by: A parameter Relaxation Ratio is determined from the ratio of average energy during unloading phase to average energy during the loading phase. A relaxation ratio greater than one implies that the average energy recorded during the unloading cycle is higher than the average energy recorded during the corresponding loading cycle and, therefore, the relaxation is dominant. [7] or Pazdera et al evaluate by The time series of acoustic emission events may be analyzed in the time, frequency, and/or time-frequency domains. The selection is based on the type of the signal in question, on the type of analysis to be used or the result achieved. In many applications, direct evaluation of the time-amplitude representation is neither easy nor advantageous. [8] Sagar et al applies the method of acoustic emission to dynamic loading: Acoustic emission testing is a well-known method for damage identification of various concrete structures including bridges. This article presents a method to assess damage in reinforced concrete (RC) bridge beams subjected to incremental cyclic loading. [9]. The Acoustic Emission Method is interesting for Panjsetooni et al: The reinforced concrete beams were tested under the loading cycle and simultaneously monitored using AE. The AE test data were analyzed using Relaxation, Load and Calm ratio. [10]" Park et al uses nonlinear ultrasonic techniques: "On the other hand, other studies have monitored the stress-dependent characteristics of concrete by using nonlinear ultrasonic methods, such as the higher harmonics method, the time-shift method, and the nonlinear resonant ultrasonic method, for measuring ultrasonic nonlinearity, which is based on the nonlinear acoustic behaviour of concrete caused by its stress-strain nonlinearity. [11]" Tsioulou et al uses: ultra-high performance fibre-reinforced concrete with different amounts of steel fibres has been examined. Compressive and tensile tests have been conducted alongside with ultrasonic pulse velocity and rebound hammer measurements and the development of appropriate empirical non-destructive models has been examined. [12] - 52 -
2. Experimental set up The reinforced concrete beam with a length of 5 m, width of 2 m and depth of 0.2 m was loaded in the centre by a sinusoidal force with different amplitudes. The loaded frequency was 2 Hz and the specimen was pre-stressed by a force of 10 kn. In total, nine load steps with maximum load forces of 30 kn, 35 kn, 40 kn, 45 kn, 50 kn, 55 kn, 60 kn, 65 kn and 70 kn were performed. Each cycle was applied during two hours, i.e. 7,200 loaded cycles per the maximal force. The ranges of forces FC and their amplitudes for each single cycle nc are clearly shown in Fig. 2. Therefore, the minimum load amplitude of cyclic loading was 10 kn while the maximum was 30 kn, however, it should be noted that the central forces were from 20 kn up of 40 kn and forces stressed the beam only in one direction. Figure 2 Bending force F C of monitored cycles mc. Eight acoustic emission sensors were placed on the surface of the specimen and two others on parts of the loading device. The acoustic emission device called Dakel XEDO was used to monitor the acoustic emission activities. Photos of the experiment are shown in Fig. 3 Figure 3 Photos of the experiment - 53 -
3. Results The number of acoustic emission counts was used as a description of the acoustic emission activity in the structure monitored. In the first part of the analysis, the cumulative number was evaluated of acoustic emission activities depending on the number of cycles at constant loading amplitude. A linear approximation in all cases was first assumed and then computed with high precision (see Tab. 1 and Fig. 4). In the second part, the dependence of the slope of linear approximation on the monitoring cycle was evaluated. Monitored cycle (load amplitude) Table 1 Correlation coefficient depending on the monitoring cycle number Interval of loading force [kn] Correlation coefficient between cumulative acoustic emission activity and cycle number 1 10 30 0.9897 2 10 35 0.9915 3 10 40 0.9965 4 10 45 0.9932 5 10 50 0.9996 6 10 55 0.9989 7 10 60 0.9993 8 10 65 0.9986 9 10 70 0.9995 Figure 4 Dependence of the number of acoustic emission activity n AE depending on the number of cycles at constant loading amplitude n C (left). Dependence of the cumulative number of acoustic emission activity cum AE on the number of cycles at constant loading amplitude (10 kn 40 kn) nc (right) - 54 -
Figure 5 The dependence of the slope of linear approximation s AE on the monitored cycle mc In our case, the dependence the slope of the cumulative number of acoustic emission activities on a number of loading cycles can be approximated by equation (1) with the determination coefficient equal to 0.9964. =48.4 0.19 50 (1) where sae is the slope of the cumulative number of acoustic emission activities depending on a number of loading cycles, mc is the value of loading amplitude used in the cycle (see Fig. 5). 4. Conclusion The micro cracks in the concrete beam in a three-point bending test were monitored by an acoustic emission system. Acoustic emission activities increase with an increasing number of cycles linearly. A similar result can be observed with an increasing amplitude of force. Thus, the slope of the curve of a cumulative acoustic emission activity dependence on the number of monitored cycles increases with the increasing amplitude exponentially. The results obtained show that structural changes and defects in the structure increase even during the cyclic loading even without changing the size of the loading amplitude. Also, the change in load amplitude accelerates the structural changes, thus increasing the acoustic emission activity. 5. Acknowledgement This paper has been worked out under the project No. LO1408 "AdMaS UP - Advanced Materials, Structures and Technologies", supported by Ministry of Education, Youth and - 55 -
Sports under the National Sustainability Programme I" the project GAČR No.16-02261S supported by Czech Science Foundation References [1] L Topolar, L Pazdera, V Bilek, L Dedeckova, 'Acoustic Emission Method Applied on Four Point Loading of Concrete Structures with and without Small Wires', Proceedings of the 50th Annual Conference on Experimental Stress Analysis, pp 477-+, 2012 [2] L Topolar, B Kucharczykova, D Kocab, L Pazdera, The Acoustic Emission Parameters Obtained during Three-point Bending Test on Thermal-stressed Concrete Specimens, Procedia Engineering, Vol 190, pp 111-117, 2017 [3] L Topolar, L Pazdera, B Kucharczykova, J Smutny, K Mikulasek, 'Using Acoustic Emission Methods to Monitor Cement Composites during Setting and Hardening', Applied Sciences-Basel, Vol 7, Iss 5, Art Num 451, 2017 [4] P Mazal, L Pazdera, L Kolar, 'Advanced Acoustic Emission Signal Treatment in the Area of Mechanical Cyclic Loading', 8th International Conference of the Slovenian Society for Non-Destructive Testing, Conference Proceedings: Application of Contemporary Non- Destructive Testing in Engineering, pp 283-292, 2005 [5] L Pazdera, L Topolar, J Smutny, K Timcakova, 'Nondestructive Testing of Advanced Concrete Structure during Lifetime', Advances in Materials Science and Engineering, Art Num 286469, 2015 [6] M Ohtsu, 'The History and Development of Acoustic Emission in Concrete Engineering', Magazine of Concrete Research, Vol 48, Iss 177, pp 321-330, 1996 [7] M Ohtsu, M Uchida, T Okamoto, S Yuyama, 'Damage Assessment of Reinforced Concrete Beams Qualified by Acoustic Emission', ACI Structural Journal, Vol 99, Iss 4, pp 411-417, 2002 [8] L Pazdera, L Topolar, P Danek, J Smutny, K Mikulasek, 'Evaluation of Acoustic Emission Events Generated at Three Point Bending of Different Concrete Specimens by Spectral Analysis', Solid State Phenomena, Vol 258 SSP, pp 485-488, 2017 [9] RV Sagar, BKR Prasad, R Sharma, 'Evaluation of Damage in Reinforced Concrete Bridge Beams using Acoustic Emission Technique', Nondestructive Testing and Evaluation, Vol 27, Iss 2, pp 95-108, 2012 [10] A Panjsetooni, NM Bunnori, 'Fracture Formation Evaluation of Reinforced Concrete Beam Subjected to Cycle Loading using Acoustic Emission Technique', International Journal of Scientific Research in Knowledge (IJSRK), Vol 1, Iss 4, pp. 51-59, 2013 [11] SJ Park, GJ Kim, HG Kwak, 'Characterization of Stress-Dependent Ultrasonic Nonlinearity Variation in Concrete under Cyclic Loading Using Nonlinear Resonant Ultrasonic Method', Construction and Building Materials, Vol 145, pp 272-282, 2017 [12] O Tsioulou, A. Lampropoulos, S Paschalis, 'Combined Non-Destructive Testing (NDT) Method for the Evaluation of the Mechanical Characteristics of Ultra High Performance Fibre Reinforced Concrete (UHPFRC)', Construction and Building Materials, Vol 131, pp 66-77, 2017-56 -