13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- The magnitude of each event is obtained summing up the number of bonds broken in each non-overlapping time window, in this case equal to 300 time steps. Finally, the frequency of the magnitude events that overcame a certain value m is reported in the classical Gutember-Richter chart (Figure 7c). It is worth noting that, in agreement with the experimental acquisition [29], the computed frequencies are aligned on a straight line. The slope of this line, which corresponds to the b value, is higher than the experimental value, and equal to 1.97. Further analyses are necessary to investigate the effect of the simulation dimensionality, and to obtain a better parameter calibration of the model. 5 CONCLUSIONS The AE results obtained from the three-point bending tests, prove that the variation of the AE parameters during the loading process strictly depends on the specimen damage: a decrease in frequency may be provoked both by dominant shear cracking process and by dominant tensile cracking process. Therefore, the two different cracking modes have to be discriminated through a different AE parameter, such as the rise angle (RA), that is defined as the ratio of the rise time to the peak amplitude of each signal. Low RA values suggest a Mode I crack propagation, whereas high RA values are obtained in case of Mode II crack propagation. All the monitored damage processes display an increase in AE signal energy content approaching the final failure. The preliminary distinct element numerical simulation of the AE statistics in the three-point bending test showed a good qualitative simulation of the Gutember-Richter law. Further analyses are necessary to investigate the effect of the simulation dimensionality, and to obtain a better parameter calibration of the model. Acknowledgements The financial support provided by the Regione Piemonte (Italy) RE-FRESCOS Project, is gratefully acknowledged. References [1] A. Carpinteri, M. Corrado and G. Lacidogna, Three different approaches for damage domain characterization in disordered materials: Fractal energy density, b-value statistics, renormalization group theory. Mechanics of Materials, 53 (2012) 15–28. [2] A. Carpinteri, P. Bocca, Damage and Diagnosis of Materials and Structures, Pitagora Editrice, Bologna, Italy 1991. [3] S. Invernizzi, G. Lacidogna, A. Manuello, A. Carpinteri. AE monitoring and numerical simulation of a two-span model masonry arch bridge subjected to pier scour. Strain, 47:2 (2011) 158–169. [4] A. Anzani, L. Binda, A. Carpinteri, S. Invernizzi, G. Lacidogna, A multilevel approach for the damage assessment of historic masonry towers . Journal of Cultural Heritage, 11 (2010) 459–470. [5] A. Carpinteri, S. Invernizzi, G. Lacidogna, Structural assessment of a XVIIth century masonry vault with AE and numerical techniques, International Journal of Architectural Heritage, 1(2), (2007) 214–226. [6] A. Carpinteri, G. Lacidogna, A. Manuello, S. Invernizzi, L. Binda, Stability of the vertical bearing structures of the Syracuse Cathedral: Experimental and numerical evaluation. Materials & Structures, 42 (2009) 877–888. [7] A. Carpinteri, S. Invernizzi, G. Lacidogna, In situ damage assessment and nonliner modelling of an historical masonry tower. Engineering Structures, 27 (2005) 387–395. [8] A. Carpinteri, G. Lacidogna, N. Pugno, Structural damage diagnosis and life-time assessment by acoustic emission monitoring, Engineering Fractures Mechanics, 74 (2007) 273–289.
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