# Acoustic Emission Activity for Characterizing Fracture of Marble under Bending
## 3 Results and Discussion
### Behavior of Repaired Specimens  
As aforementioned, the fractured specimens were repaired by means of adhesive epoxy, suitable for cultural heritage projects. In order to check the strength restoration they were tested again with the same bending setup. Their AE activity was once again monitored. Table 2 shows the average value of maximum load for both repaired marble types. The maximum load is approximately 1.63 kN and 2.46 kN for types MA and MB, respectively, corresponding to 70% and 48% of their original strengths. The marble with higher original strength (MB type) exhibited the higher capacity even after repair, although it suffered larger loss as a percentage of the original strength. Compared to the nearly uniform behavior of the sound specimens, the AE behavior of the repaired MA type specimens showed mixed trends. Most of the specimens (20 out of 33) followed a similar behavior to the sound (first loading) with the same stages (high initial rate-plateau-high final rate) but the rest of the specimens exhibited a more random behavior where the specific stages could not be defined. Examples are seen in Figure 10a,b for the aforementioned cases. The absence of fracture stages can be attributed to the inefficient interfacial bonding between the marble and the epoxy. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Marble Type**} & **Maximum Load (kN)** & **UPV \({}^{*}\) (m/s)** & **Fracture Stage** & **RT (\(\mu\)s)** & **ENE (-)** & **DUR (\(\mu\)s)** & **AMP (\(\mu\)s)** & **AF (\(kHz\))** & **RA (ms/V)** \\ \hline \multirow{2}{*}{MA} & \multirow{2}{*}{\(1.63\)} & \multirow{2}{*}{4798} & A & 146 & 5.6 & 629 & 47.6 & 38 & 7.33 \\  & & & B & 61 & 4.6 & 416 & 48.1 & 43.9 & 2.93 \\  & & & C & 372 & 15.2 & 1153 & 49.5 & 36.7 & 9.45 \\ \hline \multirow{2}{*}{MB} & \multirow{2}{*}{\(2.46\)} & \multirow{2}{*}{6383} & A & 179 & 7.2 & 740 & 48.8 & 37.2 & 5.60 \\  & & & B & 71 & 4.5 & 454 & 47.6 & 39.9 & 3.07 \\ \cline{1-1}  & & & C & 344 & 16.4 & 1022 & 50.9 & 43.9 & 9.08 \\ \hline \hline \end{tabular} \({}^{*}\) UPV was measured before fracture.  
\end{table} 
Table 2: Results of the bending tests for two types of repaired marble (reloading). Table 3 shows the average values of ultimate load and reload for the specimens of marble MA that exhibited the three stages at the reloading (20 specimens), the specimens with random behavior (13 specimens) as well as the average of the whole population (33 specimens). It also includes the repair factor (n) as the ratio of ultimate load of the repaired to the initial load of the intact specimens. It is worth noticing that the specimens that during reloading followed a behavior similar to the initial loading exhibit substantially higher repaired load. Specifically this group of specimens exhibited an average maximum load of 1.75 kN substantially higher than the group of the random behavior (1.46 kN). This shows that the bonding was of better quality and therefore the structural capacity, the fracture process and as a result the AE behavior was closer to the behavior of the intact material achieving a restoration factor of 76% which is quite high for similar cases [24]. Concerning type MB, nearly all specimens (38 out of 40) behaved similarly to the sound specimens. This has to be related to the bond between the epoxy and the matrix. For the case of MB, the epoxy and its bond to the marble works well and results in the same behavior like the initial monolithic material. This is confirmed by the maximum load after repair. Additionally, a general observation is that the plateaus (silent periods) are longer for the repaired specimens, while stage B is characterized by lower AE rate than the initial loading. Specifically Stage B exhibits an activity of 3.3 hits/s for marble MA and 2.2. hits/s for marble MB. For the repaired version of the same specimens, the corresponding rates was negligible with only sparse AE hits for Stage B (check again Figure 10a).