# Acoustic Emission Activity for Characterizing Fracture of Marble under Bending
## 3 Results and Discussion
### Acoustic Emission
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Type**} & **Maximum Load (kN)** & **UPV** & \multirow{2}{*}{**Fracture Stage**} & **RT** & **ENE** & **DUR** & **AMP** & **AF** & **RA** \\  & & & & **(μs)** & **(-)** & **(μs)** & **(dB)** & **(kHz)** & **(μs/V)** \\ \hline \multirow{3}{*}{MA} & \multirow{3}{*}{2.30} & \multirow{3}{*}{4798} & A & 455 & 10.9 & 1321.0 & 47.0 & 25.8 & 20305 \\  & & & B & 167 & 7.9 & 820.8 & 46.8 & 30.6 & 8222 \\  & & & C & 380 & 8.7 & 1158.7 & 46.8 & 31.0 & 15829 \\ \hline \multirow{3}{*}{MB} & \multirow{3}{*}{5.08} & \multirow{3}{*}{6383} & A & 231 & 8.6 & 832.6 & 48.3 & 35.8 & 8452 \\  & & & B & 95 & 5.8 & 508.3 & 47.9 & 41.0 & 4344 \\ \cline{1-1}  & & & C & 964 & 29.4 & 2904.5 & 50.7 & 38.2 & 22355 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of the bending tests for two types of marble. Figure 8: RA values for two specimens of marble type (**a**) MA and (**b**) MB. resonant sensors. Correspondingly, it exhibits lower duration, RT and RA values. In addition, while MA exhibits lower RA values for the last stage, type MB exhibits a strong increase. These different trends are attributed to the different fracturing behavior of the two media since the specimen geometry and the whole experimental lay-out was constant. As aforementioned changes of RA are due to the fracture mode. Although the macroscopic stress field is similar, the microstructure of the different marbles may induce different motion of the crack tips strongly influencing the AE parameters received. It has been seen that high RA values correspond to material closer to fracture while the inverse is measured for frequencies. The frequencies drop just before the final failure. It comes as no surprise therefore, that the material type with higher frequency and lower RA exhibited higher density and load bearing capacity. What should be stressed is that these differences are obvious from the initial fracture stage; MB exhibits 39% higher frequency and 58% lower RA allowing predictions as to the final capacity of the beams. By separating the activity at the different stages we can study the AE activity at low, moderate and high load. Figure 9a shows the graph of AF _vs._ RA for the first stage of fracturing (Stage A) as explained in the previous section, while the corresponding graph of the intermediate Stage B is given in Figure 9b. The whole group of points extends from frequencies of approximately 20 kHz to 45 kHz and from RA values of 3 to 30 ms/V. However, it is important to notice that the type MB occupies most of the spots of higher frequency and low RA, while the two populations (MA and MB) could even be separated by a single line (see dashed diagonal line) having only three overlapping points. For the next Stage B of moderate activity, the whole population shifts to higher frequencies and lower RA values (see Figure 9b) but there is still a substantial difference of the centers of the two populations. Marble MB again exhibits higher frequencies and lower RA values and the separation line attains a much higher slope. The separation lines are certainly not unique and successful clustering could achieved by means of pattern recognition algorithms. However, this is left for the next stage, when larger populations will be examined. Figure 9: Correlation plot between AF and RA for (**a**) the initial loading Stage A; (**b**) the intermediate loading Stage B; and (**c**) the final Stage C.  
Based on the AE activity of the preliminary loading stage it can be concluded that if the ratio of the average AF over RA is more than 1.67, the ultimate load will be higher than 4 kN (denoting type MB) while if it is lower the load capacity will be less than 3 kN (type MA). The importance of such a correlation lies in the fact that the strength capacity of the material can be assessed by a quite reliable way based on AE data resulted from loading the specimen, without however, inflicting any serious damage: the initial activity (Stage A) is recorded while the load is less than 10% of the ultimate.