# Acoustic Emission Activity for Characterizing Fracture of Marble under Bending
## 3 Results and Discussion
### Acoustic Emission  
Figure 7a,b show typical examples of cumulative AE activity as received by both sensors for marble type MA and MB, respectively. In all of the cases, three distinct stages of activity can be defined. At the start of the experiment a quite high rate of events is recorded (Stage A) followed by a long time window of moderate activity (Stage B). At the end and quite close to the macroscopic fracture of the material, the rate increases again (Stage C). Similar AE activity patterns have been seen recently in similar specimens during compression. The initial saturation was attributed to the depletion of a \"weakpoint reservoir in the deforming material\", while the increase at the final stage was attributed to clustering of the individual defects due to decreased distance among the sources [12]. It seems that after application of load, formation of micro-cracks is quite fast. However, coalescence of them into larger ones that lead to final fracture takes more time and accumulated energy. This energy is released at the end with another \"avalanche\" like activity. Although the number of AE hits of Stage C are just a small fraction of the total activity, their energy is much higher. Figure 6: Correlation between pulse velocity and maximum load for the marble specimens. Figure 7: Cumulative AE activity history for marble type (**a**) MA and (**b**) MB. The behavior of the different marble types was quite repeatable. Apart from the three indicative cases included in the figures, nearly all specimens exhibited the three stages of high, moderate and high activity again. However, in addition to the total number of hits, high importance is carried by the AE waveform parameters. To give an idea of the transient changes of AE characteristics during loading, Figure 8 shows the RA values for two representative specimens of type MA marble (Figure 8a) and other two from type MB marble (Figure 8b). The data corresponding to the last Stage C are included in a transparent pattern since it is intended to focus on the behavior prior the macroscopic failure. It is obvious that the RA initial level is at much higher levels for type MA than type MB, since values up to 50 ms/V are quite frequent, while for MB specimens values are almost restricted below 30 ms/V. As loading continues high values become scarce resulting in a decreasing trend for RA for both types. At the end of Stage B, and just before the macroscopic failure, the RA values of both types of specimens seem to deepen below 10 ms/V. Concerning the whole population of specimens (more than 30), the average values are shown in Table 1.  
From the above table, a general conclusion is that the AE features differ for the successive stages. As an example, the rise time and duration of the second stages (Stage B) is clearly lower than the first (Stage A), while the final stage (Stage C) produces again higher values. Inversely, the frequency content as measured by AF increases for Stage B relative to Stage A. Duration of AE signals is related to the intensity of the source and it shows again the same transition as the total AE activity. It would be premature to interpret these fluctuations of AE parameters in terms of shift between shear and tensile phases, as it should be combined with the knowledge of the microstructure. However, this lowering of RA in the intermediate stage, follows the rate of damage accumulation, which is slower in Stage B, as revealed by the low AE activity rate denoting the creation of micro-cracks, the coalescence of which leads to the final fracture at Stage C.  
Apart from the differences between the successive stages, there are considerable differences in between the different marble types. Type MB exhibits for the first two fracturing stages, AF higher by as much as 10 kHz than marble MA. This difference is quite high considering that it was recorded by