CHARACTERIZATION OF FRACTURE MODE IN MASONRY BY ACOUSTIC EMISSION

Transcription

1 More info about this article: h Czech Society for Nondestructive Testing 3 nd European Conference on Acoustic Emission Testing Prague, Czech Republic, September 7-9, 16 CHARACTERIZATION OF FRACTURE MODE IN MASONRY BY ACOUSTIC EMISSION Abstract G. LIVITSANOS 1, N. SHETTY, E. VERSTRYNGE, M. WEVERS 3, D. Van HEMELRIJCK 1, D. G. AGGELIS 1 1 Vrije Universiteit Brussel Department Mechanics of Materials and Constructions (MEMC) KU Leuven Building Materials and Building Technology division, Civil Engineering Department 3 KU Leuven Department of Materials Engineering Damage development in masonry is particularly complex. Acoustic Emission (AE) is a powerful technique for detection and analysis of the elastic waves produced by the failure mechanisms. However, in-situ characterization of such structures is complicated due to the uncertainties caused by heterogeneity which hinders the accurate localization of damage as well as the source identification. In this study the different fracture mechanisms are studied in relation to the properties of the bricks, mortar and their interface. The aforementioned fracture process is being evaluated using the acoustic emission (AE) technique. The isolation of the AE signatures of different types of failure mechanisms is feasible by testing unique bricks in flexure and brickwork couplets in compression. All the AE data received are analyzed with a simplified AE parameter-based analysis. It is the first time that such a direct correspondence between the fracture process and the results of a monitoring technique emerge for masonry. This procedure offers new insight in the individual mechanisms and the material's behavior and especially in relation to different kind of bricks and mortars where the dominant stress mode is not known a priori. Also this leads to a better interpretation of results for large scale experiments in future research. Keywords: masonry, acoustic emission, fracture mechanisms, simulation, wave propagation 1. Introduction A large number of historical structures are built up in masonry. Characterization of its material properties and structural condition is necessary for repair and conservation aiming to a long and safe service life. The damage initiation and crack propagation in masonry under stress is particularly complex. Several collapses of historical masonry structures in the past have taken place due to timedependent deformations caused by high sustained load. For instance, some wellknown examples are the Maagden Tower at Zichem, Belgium which partially collapsed 6 [1] and the cathedral of Noto, Italy in 1996 []. In these cases the self-weight of the structure was nonnegligible and acted as a high sustained compressive loading. Therefore, studying the behavior of masonry under compression is of paramount importance. This study will focus on isolated fracture mechanisms and the interface which is generally considered to be the weakest zone in historical (unreinforced) masonry. Masonry components and structures present high heterogeneity which hinders the accuracy of many non-destructive techniques such as the acoustic emission (AE) technique. The distortion and the high attenuation of the elastic waves which are emitted passively by cracks pose specific difficulties in masonry. Up to now the experimental characterization of the fracture modes in masonry has been based on monitoring the deformations of the masonry s surface. Such techniques are conducted using strain gages for local or using digital image correlation 3 nd EWGAE 87

2 for full field strain measurements. By the aforementioned procedures, a macroscopic observation of the surface cracks is only achieved. On the other hand, AE is a powerful technique for detection and analysis of the elastic waves produced by the failure mechanisms. The localization of the cracks as well as the source identification of the micro cracks and the AE parameters-based analysis give a clear aspect of the crack initiation and propagation. Studies such as AE damage evaluation in masonry under persistent loading have been carried out [3]. However, due to the heterogeneity there is limited knowledge on the accuracy of the AE evaluation in masonry. More research has been done on other materials fields such as metals [4], concrete [5], composites [6], and wood [7]. This paper presents an analysis of AE results obtained during laboratorial flexural tests as well as compression of couplets. Firstly two different types of bricks and three types of mortar were chosen with the aim of studying different properties of the materials. Ultrasonic measurements were performed measuring the Ultrasonic Pulse Velocity. Next, three point bending as well as compression tests were carried out for correlating the mechanical fracture properties with the AE characteristics of each material. Consequently, as a step up, compression tests on brickwork couplets were done which also contain the mortar-brick interface. Parameters of AE such as RT, AF, cumulative activity, amplitude are analyzed and discussed showing that based on single element tests, the behavior of more complex elements can be studied.. Experimental details.1 Materials Two types of bricks were used (Fig. 1 [a], [b]). The Spanish Red Clay brick (RCB) with a density 1653 kg/m 3 and dimensions 188x88x63 mm³ is the weakest one. The second with the high compressive strength is the White Clay Brick (WCB) with a density 1916 kg/m 3 and dimensions 194x88x65 mm³ (Table 1). Furthermore, three different types of mortar were chosen, the properties of which depend on the different compositions (Fig. 1 [c]). As a result a cement based mortar, a limecement and a hydraulic lime mortar were composed (Table ). The preparation of the sand, the adding order of the components and the mixing is important. The sand is dried in 8 O C for three days for all compositions. In the case of Hydraulic Lime mortar (HL) the grade of NHL5 is used. The NHL stands for Natural Hydraulic Lime. The number relates to compressive strength in N/mm². In case of Lime-Cement mortar (LC) Supercalco9 is mixed with cement and it acts as a plasticizer. It is a building lime according to EN and it is recommended for all types of masonry. It is a workable binder suitable for more general purposes. It has a good adhesion, and excellent early strength. Spanish Clay Brick (with cavity) White Clay Brick (with cavity) Factory Terca Factory Desimpel Dimensions 188x88x63 (mm) Dimensions 194x88x65 (mm) Weight 175 g Weight 1931 g Density 1653 Kg/m 3 Density 1916 Kg/m 3 Table 1: RCB WCB properties 88 3 nd EWGAE

3 [a] [b] [c] Figure 1: Visual aspect of [a] Red Clay Brick (RCB) and [b] White Clay Brick (WCB), [c] Mortar specimens (CM LC HL) Composition Cement (CM) Lime Cement (LC) Hydraulic Lime (HL) River Sand (g) Binder [g] CEMENT %*57.5 CALCO %*57.5 CEMENT 57.4 NHL5 Water ( o C) Volume density (kg/m 3 ) Table : Composition of Mortars [8] [a] [b] [c] Figure : [a] Brick directions A,B,C, [b] Brick cubes 4x4x4 mm, [c] Ultrasonic (UT) measurements [a] [b] [c] [d] Figure 3: Test setup for [a] mortar beams under three point bending (side view), [b] bricks under three pointbending (16 mm span) (top view), [c] Brick and mortar cubes - compression tests, [d] brick couplets under compression For the first series of experiments, bricks and mortars were prepared for Ultrasonic measurements (UT) and for three point bending and compression tests. The bricks were cut on all sides for standardizing the dimensions for three point bending tests. They were cut on one side to discard the cavity for preparation of masonry couplets. In total three bricks of each type were subjected to bending with span 16 mm. The loading was displacement controlled at a constant rate of.5 mm/s. Furthermore five other bricks of each type were applied for ultrasonic testing (UT) in three different directions (Fig. [a]). Next, each brick was cut in 8 cubes which were measured using UT and tested in compression again in the three directions (Fig. [b]). The mortar beam specimens were made according to the standards with dimensions 4x4x16 mm. After UT (Fig. [c]) and AE measurements during three point bending tests (Fig. 3 [a], [b]) they were tested in compression (Fig. 3 [c]). 3 nd EWGAE 89

4 As for the couplets (Fig. 3 [d]) the preparation was specific. Before applying the mortar layers they were hydrated for two minutes in order to be saturated and as a result avoid absorption of the mortar s water into the pores of the bricks.. Sensor setup (AE and UT) Acoustic Emission monitoring took place by piezoelectric sensors (R15α, Mistras with 4dB preamplifier). The computer is equipped by PCI/DSP-4 Data Acquisition Boards with up to 8 parametric / channel inputs with sampling rate up to 1 MHz. The resonant frequency of the sensors is 15 khz. Before the fracture tests, ultrasonic measurements were conducted on the samples, according to Fig. [c]. They were conducted using the portable Ultrasonic Pulse Analyzer apparatus by Controls Group, with MHz sampling rate and 5 V transmitter pulse. The transmitting transducer (pulser) was of 5 khz frequency. The measurement corresponds to the longitudinal waves which are the fastest type. Moreover, pulse velocity was calculated by the length of the specimens over the wave transit time. Pulse velocity measurements were performed on the specimens (Bricks Mortars Couplets) in order to correlate the velocity with the mechanical properties. Furthermore the initial estimated velocity value was used as an input for the AE software for source location. The position of the AE sensors on the specimens are shown for each set up in Fig. 3 [a], [b], [d]. The threshold was 35 db. In the case of three point bending of the mortar beams, two sensors were placed at a distance of 8 mm for acquiring the total activity and for localizing the cracking linearly (Fig 3 [a]). In the case of three point bending of brick samples, four sensors were placed aiming to apply linear localization in two directions within the bricks, as well as planar localization, as the area of interest is larger. The distance between the sensors was 1 mm. All the dimensions and the details are shown in Fig. 3 [b]. Finally, in the case of couplets, planar location was also applied where each couple of sensors was placed on either sides of the mortar joint (Fig. 3 [d]). Acoustic coupling was improved by Vaseline (petroleum jelly) between the sensors face and the specimens' surface. Some of the main features are the maximum amplitude, A (usually in db), and the duration (period between the first and the last threshold crossing). The rise time, RT (which is the time between the first threshold crossing and the point of peak amplitude in μs) is related to the fracture mode of the crack [9]. Frequency content can be measured by AF (average frequency), which is the total number of threshold crossings divided by the duration [9]. 3. Results 3.1 Ultrasonic wave velocity and mechanical properties Red White CM LC HL CM LC HL a 11 m/sec Compressive Strength-Velocity Mortars b 15 m/sec 4 c 14 m/sec 35 a 45 m/sec 3 b 7 m/sec 5 c 55 m/sec 35 m/sec 151 m/sec m/sec 1 RED WHITE RED WHITE RED WHITE 1 m/sec 8 m/sec 14 m/sec 18 m/sec 85 m/sec m/sec Table 3: Mean values of UT Velocities m/sec strength (MPa) CM LC HL Figure 4: Compressive strength-velocity for mortars 9 3 nd EWGAE

5 Five specimens of each type of bricks, RCB and WCB were subjected into velocity measurements in the three different directions A, B, C (Table 3). All the bricks were cut for removing the cavity on the bottom. The size of RCB was 188x88x4 and WCB 194x88x4. Next, each brick was cut into 8 cubes and for each cube velocity measurements in three directions were conducted for confirmation as the length of the specimen may influence the result. The directions are shown in Fig. [a]. Different velocities in the three different directions were found which is consequence of the heterogeneity of the bricks (Fig. 5). For this reason, compression tests were done in each direction of the cubes for investigating the strength alteration in different directions and for correlating it with the corresponding velocity value. For reliability and statistical aspects 5 bricks of each type were used and as a result a strength velocity correlation was achieved by testing approximately twelve cubes in each direction (Fig. 5). Finally, the heterogeneity plays a significant role in the properties. Classifying the strengths according to the brick directions the following rule is observed: strengthb > strengthc > strengtha. The velocity follows the same trend. This means that in both types of bricks the longitudinal direction exhibits higher strength and stiffness properties. This anisotropy may be related to the heating and the overall manufacturing process of the bricks. As it is shown from the graphs, comparing the inclination of the trendiness it is notified that in the case of RCB the empirical trend estimation of the values presents stronger correlation between wave velocity and strength rather than in case of WCB. This remark is also confirmed by the correlation coefficient R. The closer the coefficient is to 1 the better fit of the curve to the data and the less declination there is. Consequently, in the case of RCB the stronger correlation is obvious. In case of mortars, triplets of each type were casted with dimensions 16x4x4 mm 3. In each specimen velocity measurements were conducted in the longitudinal direction and other in the transversal. Thus in total xx3(specimens of each type) 1 measurements were done in each type of mortar. Also the velocity measurements are correlated with the compressive strength (Fig. 4). The mean value of each type of them is presented in Table 3. Finally, concerning the couplets 1 UT measurements were done in each couplet in the vertical direction so as the signal transmission takes place through the bricks and the interface brick mortar layers. All the mean values are also presented in Table 3. Figure 5: Correlation of compressive strength with Ultrasonic pulse velocity for RCB and WCB, Correlation velocity-strength by correlation coefficient R 3. Acoustic Emission Parameter Analysis - Three-point bending tests on bricks and mortars Three-point bending tests were performed on brick and mortar specimens with the test set-up as shown in Fig. 3 [a], [b]. The load was applied using the Instron 5885 testing machine with a 5 KN capacity load cell at. mm/min loading rate under displacement control. In total 3 bricks of each type and 3 mortar specimens of each type were subjected in three point bending 3 nd EWGAE 91

6 tests. In the following graphs (Fig. 6 [a], [b], [c], [d]) the flexural load and the cumulative hits over the time for bricks and mortars are presented. These curves are the representatives of each type of specimen for gaining a better overview and understanding about the mechanical characteristics and related AE signals during three point bending tests. The total AE activity is separated in load stages in order to get into a more detailed insight in the results (Fig. 7 [d]). As a result, stage 1 is representative for the early loading time, stage corresponds to a middle period of time, in stage 3 all the AE activity before cracking is isolated and stage 4 stands for AE activity after in the post-peak loading stage. After this classification, mortars and bricks are compared in terms of AE parameters such as RT and AF (Fig. 7 [a], [b]) and in the AF-RA axes (Fig. 7 [c]) as proposed by the relevant Rilem recommendation [1]. It is seen that the activity of bricks presents higher AF and lower RT for all loading stages. This is encouraging as it shows that fracture of the individual constituents has distinctly different characteristics. In this case this is important as this may enable characterization in the case of masonry components made up by these constituents. It is also interesting to be mentioned that cement mortar (CM) follows the same behavior as bricks in terms of RT and AF and the opposite with the other types of mortars. Also the compressive strength of CM is closer to both brick types. Thus they are not presented in the graphs (Fig. [a], [b], [c]) for avoiding confusing results and for providing the most comparable ones. 4, 5 4 3, 5 3, 5 1, 5 1, 5 3, 5, 5 1, 5, 5 [a] 3 1 Flexural load - Time Time (Sec) RCB CM LC HL 4 6 Flexural load - Time Time (Sec) WCB [b] Hits - Time [c] [d] Figure 6: load time [a] bricks [c] mortars, Cumulative AE hits time [b] bricks [c] mortars Time (Sec) RCB WCB CM LC Hits - Time HL Time (Sec) 9 3 nd EWGAE

7 [a] [b] [c] [d] Figure 7: Acoustic Emission parameters analysis for bricks and mortars. [a] RT number of stages, [b] AF number of stages, [c] AF RT correlation graph, [d] Separation of total AE activity in stages 3.3 Acoustic emission activity-compression tests on masonry couplets Compression tests were performed on couplets with the test set up as shown in Fig. 3 [d] AF - RT Bricks - Mortars - Couplets 4 Mortars Bricks 1 4 Couplet RT Red White LC Mortar HL Mortar Couplet HL Red Figure 8: Acoustic Emission parameters analysis for brick couplets. [a] RT number of stages, [b] AF number of stages 3 nd EWGAE 93

8 The same AE parameters (AF and RA) are presented for the case of couplets. Only one type of couplet is presented as a representative, the couplet composed by the Hydraulic Lime mortar. Also the LC mortar follows the same trend. As it is notified by the graph (Fig. 8), the range of RT and AF values is closer to the corresponding mortar values rather than the bricks. Indicatively, by checking the couplet composed by the white bricks and HL mortar the RT varies between 15 and 3 μs and the AF around 35 khz. The mortars RT and AF vary between 1 to 16 μs and from 5 to 7 khz respectively. In contrast the bricks RT and AF vary between 4 to 6 μs and from 75 to 1 khz respectively. Consequently, it can be reasonably argued that the main fracture activity comes from the mortar in the case of LC and HL mortars. However, in case of couplet composed by CM mortar the trends of AF and RT follow the same behavior as bricks. As also previously mentioned CM mortar s AE and mechanical characteristics are similar to the bricks and as a result not easily comparable, thus more study has to be conducted also combined with other Non-Destructive Techniques such as Digital Image Correlation for the crack characterization. 4.Discussion In this section, some specific issues are discussed after the basic results were presented. The first aspect concerns the heterogeneity which affects many parameters. This important factor must be taken into account for larger laboratorial experiments and further analysis. It has been proven that there is orthotropic differentiation of the bricks mechanical and AE properties with the longitudinal direction being stronger and stiffer. The second point concerns the characterization of the fracture mode. The different porosity of the bricks which assists or complicates the brick-mortar adhesion, the different mortar properties and the high heterogeneity of the bricks determine the universal behavior of the brickwork couplet and even further the whole masonry. Up to now the trends show that the failure can be distinguished depending on the preliminary isolated measurements. With the specific combination of mortar and brick used for couplet manufacturing in this study, it seems that the AE of the whole component during loading is closer to the activity of the mortar. Therefore, even if the accuracy of AE localization cannot be high enough to indicate events in between the 1 mm of mortar or the surrounding brick it is still possible to characterize the nature of the fracture. In any case it must be stated that the study is ongoing to examine the effect of wave propagation through the different interfaces. It is highly possible that the AE signals undergo strong changes in their propagation path from the source to the receiver and therefore numerical simulations are in order to examine the change of the waveform shape after propagation in masonry components. Even though more types of mortars have been tested the most indicative differentiations are presented in this paper. 5.Conclusions This initial study indicates that passive monitoring by AE could provide information on the failure mechanisms in masonry which cannot be provided by other non-invasive techniques. The research continues with other types of mortars as well as other set ups. This research is also situated in a framework of round robin testing for comparison of the results and improvement of the techniques that are used. In further work, the effect of attenuation due to damping and reflection will be studied thoroughly via numerical simulation, which will help to upgrade the tests to larger geometries nd EWGAE

9 Acknowledgements The Research Fund - Flanders (FWO) is acknowledged for funding the FWO project AEFracMasS: advanced Acoustic Emission analysis for Fracture mode identification in Masonry Structures (G.C38.15). References [1] Verstrynge, E., Schueremans, L., Van Gemert, D., Wevers, M., (9). Monitoring and predicting masonry s creep failure with the acoustic emission technique, NDT & E International, Vol. 4 (6), pp [] Tringali, S., De Benedictis, R., Gavarini, C., La Rosa, R., (, September). The cathedral of Noto: from the analysis of the collapse to the restoration and reconstruction project, In Proceedings of the UNESCO/ICOMOS international millennium congress archi, Paris (France). [3] Verstrynge, E., Ignoul, S., Schueremans, L., Van Gemert, D., (8). Damage accumulation in masonry under persistent loading evaluated by acoustic emission technique, 14 th International Brick & Block Masonry Conference, February 17-, Australia, Sydney. [4] Han, Z., Luo, H., Zhang, Y., & Cao, J. (13). Effects of micro-structure on fatigue crack propagation and acoustic emission behaviors in a micro-alloyed steel, Materials Science and Engineering: A, 559, [5] Aggelis, D. G., (11). Classification of cracking mode in concrete by acoustic emission parameters, Mechanics Research Communications, Vol. 38, pp [6] Eaton, M., May, M., Featherston, C., Holford, K., Hallet, S., & Pullin, R, (11). Characterisation of damage in composite structures using acoustic emission, In Journal of Physics: conference series (Vol. 35, No. 1, p. 186). IOP Publishing. [7] Lamy, F., Takarli, M., Dubois, F., Angellier, N., Pop. I.-O., (13). Acoustic Emission Technique (AET) for failure analysis in wood materials, 13th International Conference on Fracture, June 16-1, Beijing, China. [8] Hendrickx, R., (9). The adequate measurement of the workability of masonry mortar, PhD Thesis, KU Leuven, Arenberg Doctoral School of Science, Engineering & Technology. [9] Grosse, C. U., and Ohtsu, M., (8). Acoustic emission testing basics for research applications in civil engineering. Springer, 8. [1] Ohtsu, M.: Recommendations of RILEM Technical Committee 1-ACD: acoustic emission and related NDE techniques for crack detection and damage evaluation in concrete: 3. Test method for classification of active cracks in concrete structures by acoustic emission. Mater. Struct. 43(9), (1). 3 nd EWGAE 95

10 96 3 nd EWGAE