ScienceDirect. Study of acoustic emission in a compression test of andesite rock

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1 Available online at ScienceDirect Procedia Materials Science 9 (2015 ) International Congress of Science and Technology of Metallurgy and Materials, SAM CONAMET 2014 Study of acoustic emission in a compression test of andesite rock D. A. Filipussi a,b, *, C. A. Guzmán a, H. D. Xargay c, C. Hucailuk a, D. N. Torres a a International Centre of Earth Sciences (ICES) GAIyAN Centro Atómico Constituyentes (CNEA), Av. Gral Paz 1499, Bs. As. CP1650, Argentina b Universidad Nacional de San Martín (UNSAM) Escuela de Ciencia y Tecnología, Martín de Irigoyen 3100, Bs.As. CP1650, Argentina c Universidad de Buenos Aires (UBA) Facultad de Ingeniería LMNI, LAME FIUBA, Las Heras 2214 CABA, CP 1127, Argentina Abstract In the compression test of a material s sample, elastic waves produced by dynamic processes are known as acoustic emissions (AE). These waves, which propagate from the source to the material surface, arrive to the piezoelectric sensor where they are converted to electric signs and transformed into the typical AE parameters. These parameters will be employed for the evaluation of failure mechanisms present in the test. In this work, the mechanic behaviour of andesite s rocks of Cerro Blanco, San Juan, Argentina was studied. Two compression tests were performed for two cylindrical rock samples and the AE parameters were measured. The measurement configuration consisted of two syntonized sensors at 60 khz and 150 khz placed on the lateral face of the sample. The AE parameters registered were amplitude, duration, energy, RMS, etc. This data group was related to the load time curve. Different indicators like average frequency and the ratio of rise time to amplitude of the signal were calculated. These indicators were analyzed and evaluated with the purpose of identifying the damage level and the type of failure The Authors. Published by Elsevier by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license Peer-review ( under responsibility of the Scientific Committee of SAM-CONAMET Peer-review under responsibility of the Scientific Committee of SAM CONAMET 2014 Keywords: Acoustic emission; rock andesite; test compression; damage in materials 1. Introduction Acoustic emission (AE) is the phenomenon in which energy is released in the form of elastic waves within a material that is subjected to some form of external stress. AE sources has its origin in the change of the stress field inside the material which is related to the deformation processes, crack growth, dislocations movement, inclusions cracks, etc. An AE piezoelectric sensor, in contact with the material studied, detects the impulsive mechanical wave * Corresponding author. Tel.: (int.6569) address: filipuss@cnea.gov.ar The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the Scientific Committee of SAM CONAMET 2014 doi: /j.mspro

2 D.A. Filipussi et al. / Procedia Materials Science 9 ( 2015 ) and converts it into an electrical signal which is amplified and processed by the AE equipment. This wave has very low amplitude (10 μv) and high frequencies (100 khz to 1 MHz). AE is usually transitory in nature, occurring in discrete events. Detected events, called hits, are analyzed in its waveform, studying parameters such as energy, duration, rise time, amplitude, etc. The AE in the rock is produced mostly by the micro cracks growth and is a local phenomenon associated to fragile cracking that provides a great deal of information on the development of the rupture on the rock. Fracturing process involves nucleation, growth and micro cracks coalescence to the final breaking. State of stress generates a field in the specimen. This mechanism produces small cracks at the beginning and when it reaches a critical level of growth eventually causes the final crack. AE is generated, at different spatial and temporal scales, covering from breaking atomic bonds to seismic faults, so the seismic data and the AE are complementary, both in their applications as the theoretical basis by Gutenberg and Richter (1954), Lockner (1993). From this idea, to account for the processes of damage in a rocky material subjected to stresses, there are works that study the b-value, defined as the slope of the line of this linear relationship, between the cumulative frequency distribution of AE hits and the amplitude. It is found that the relationship in log log scale is linear and is known its application in seismic as Gutenberg-Richter law by Shiotani et al. (1994), Shiotani et al. (2001), Rao and Prasanna Lakshmi (2005). Other studies propose and develop models of micro cracks formation mechanisms in rock fracture by Hajiabdolmajid et al. (2002), Potyondya and Cundallb (2004) and also implements finite element in numerical simulation of progressive accumulation of damage, propagation and coalescence of micro cracks in rock material by Tang (1997), Tang et al. (1998), Tang and Kou (1998), Tang and Kaiser (1998), Kaiser and Tang (1998), Tang et al. (2000a, b), Jing et al. (2002). On the order hands, the literature presents works on digital processing images in geo materials by Chen et al. (2004), Yue et al. (2003). Latest articles study the temporal and spatial evolution of micro cracks using the location of AE sources and comparing with numerical simulations using RFPA 2D (realistic failure process analysis), PFC 2D software (particle flow code) and FLAC 3D software (fast lagrangian analysis of continua) by Wang et al. (2013), Han et al. (2014), Liu et al. (2014). In the present work, the AE process resulting from fractured andesite rocks from Cerro Blanco (San Juan, Argentina) in a uniaxial compression test is studied. The analysis of certain AE parameters aims to determine how the damage accumulates during testing of the specimen and predict the crack. Rock materials have a brittle nature and due to the process of forming micro cracks detected by this AE can be considered useful for predicting the eventual fracture in rocks. 2. Experimental development and results Two cylindrical specimens of 75 mm diameter and 150 mm of length, called P1 and P2 extracted in situ in Cerro Blanco rock, were obtained. These specimens were tested in uniaxial unconfined compressive taking them to catastrophic rupture. Atensile testing machine, model CGTS, of 100 tons of capacity, servo hydraulic type and closed loop, where a speed actuator of 0.12 mm/min was used. Fig. 1 shows the experimental arrangement of an andesite specimen with the AE sensors. Load versus time curve was obtained; see Fig. 2, where three well defined zones are distinguished. In the first zone a non linear behavior is shown due to the own accommodations of the system and pre existing closed micro crack. A linear part follows and finally, to reach the maximum load, catastrophic cracks occurs showing a typical behavior of a fragile material.

3 294 D.A. Filipussi et al. / Procedia Materials Science 9 ( 2015 ) Fig. 1. Experimental setup of the andesite specimen with AE and LVDT (linear variable displacement transducer) sensors. Break up, in both specimens, has a laminar predominance showing indirect tensile stress, see Fig. 3. The rupture load, for P1 was 509 kn and for P2 was 449 kn, with a difference that does not exceed 10%. The AE activity recorded was performed using two channels PCI 2 acquisition device of the PAC. For recording the characteristic parameters an AEWIN Software was used. The specimens were implemented through two sensors tuned R15 of 150 khz and the other R60 of 60 khz. Among the most representative AE parameters during the tests, the energy and RMS are shown in Figs. 4(a) (b). An increased activity of the energy is observed from 70% of the applied load due to the coalescence of the micro cracks. The sensitivity of the energy is greater than the RMS in the last section. Fig. 5(a) shows four stages for P1. In the first stage of the test, channel 1 (sensor of 150 khz), has a lower frequency of events arrival that channel 2 (sensor of 60 khz). In the second stage, such behavior is reversed at approximately 30% of the maximum load, achieving a maximum arrival of hits on channel 1 at 50% of it. In stage three, from 50% to 90% of the maximum load, a new change (predominance of hits arrival on channel 2) occurs. Finally, in the last stage from 90% and until the specimen rupture, hits of channel 1 prevail. Fig. 5(b) shows, for P2, a completely different behavior. Three stages are well-defined. In the first stage, up to 70% of the maximum load hits of channel 2 are predominant. Then there is a second step from 70% to 90% where an inversion occurs and prevail the arrival of hits on channel 1 up to break. The difference presented could be associated with different mechanisms of rupture. In P1 test is primarily dominated by tensile stresses, while in P2 can be observed mixed traction and shear processes. Fig. 2. Relative load versus time for P1.

4 D.A. Filipussi et al. / Procedia Materials Science 9 ( 2015 ) Fig. 3. Break shaped of specimens 1 and 2. Fig. 4. Energy and RMS versus time for P1, channel 2 (a) energy; (b) RMS. The MF (media frequency) is defined as the ratio between the number of accounts and signal duration. In Fig. 6(a) the values of the logarithm of the number of hits corresponding to a MF given are shown. This is classified into three classes: low (0 150 khz), medium ( khz) and high values ( khz) with three time intervals. During the test there is an increase (of almost two orders of magnitude) of the number of hits for each class of MF. For each time interval, low MF values correspond to a greater number of hits (of one order of magnitude). Fig. 5. Arrival of hits count per channel for each of the specimens tested (a) P1; (b) P2.

5 296 D.A. Filipussi et al. / Procedia Materials Science 9 ( 2015 ) Fig. 6. (a) Mid frequency distribution in three kinds of time interval for P1, channel 1(b) RA distribution in three kinds of time interval for P1, channel 1. The RA value is defined as the ratio of the rise time and the amplitude of the signal. This indicator versus MF realizes how the type of internal stress experienced by the material is characterized during the test, either cutting wheel or a mixed value by Grosse and (2008), Guzmán et al. (2014). In Fig. 6(b), RA behaviour is similar to the MF. It can be seen that there is an increase in the number of hits, throughout the test (of about two orders of magnitude) in the first class. A greater difference between the number of hits, of almost three orders of magnitude, of the low RA values relative to average and high RA values is appreciated. High RA values are at least 2 class times. 3. Conclusion The energy parameters are presented like a good indicator of the level of damage; the values of the energy of the detected events show greater sensitivity than the RMS. Energy and RMS of the recorded events present a higher value from 70% of the maximum load during the last stage of the test. A different behaviour for channel 1 respect of 2 could be seen counting the arrival of hits. This could be associated with different forms of rupture presented, traction, shear or mixed. Growth in the number of hits, for MF and RA in P1 and P2, are similar throughout the test. The amount of hits that is distributed among the lowest, means and high MF values and RA also follows a similar behaviour. Acknowledgements We thank the LAME FIUBA Materials and Structures Laboratory, Faculty of Engineering, University of Buenos Aires and LMNI Laboratory for Numerical Methods in Engineering from the collaboration in this work and participation of Dra. Paula Folino. References Chen, S., Yue, Z.Q., Tham, L.G., Digital image based numerical modeling method for prediction of inhomogeneous rock failure. International Journal of Rock Mechanics and Mining Sciences 41, Grosse, C.U., Ohtsu, M., Acoustic Emission Testing, basics for research applications in civil engineering. Springer Verlag, Heidelberg. Gutenberg, B., Richter, C.F., Seismicity of the Earth and Associated Phenomena. Princeton University Press, New Jersey. Guzmán, C. A., Torres, D. N., Hucailuk, C., Filipussi, D. A., Analysis of the acoustic emission in a reinforced concrete beam using a four points bending test, 13th International Congress on Metallurgy & Materials SAM CONAMET. Procedia Materials Science, in press. Hajiabdolmajid, V., Kaiser, P.K., Martin, C.D., Modelling brittle failure of rock. International Journal of Rock Mechanics and Mining Sciences 39,

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