13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- (a) (b) Figure 3. Three-point bending test: (a) load vs. time; (b) cumulated AE events vs. time. The RA values obtained from the three-point bending test are considerably higher than what is obtained in case of small specimens (e.g. compression tests on cylindrical specimens [16]): as a matter of fact the additional propagation distance from AE sources to sensors, considering attenuation mechanisms, involves an increase in AE signals rise time [13]. Since the longitudinal waves are the fastest type, the delay in each AE signal between longitudinal and shear waves grows with the increasing of the propagation length, and consequently the RA rises from lower to higher values. (a) (b) Figure 4. Three-point bending test: (a) AF values vs. time; (b) RA values vs. time;
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