# Acoustic Emission Activity for Characterizing Fracture of Marble under Bending
## 4 Discussion  
The reason why AE inspection acquires even more significance is that ultrasound cannot reliably assess the condition of the repaired specimen. As shown in Figure 4, the nominal wave path is 160 mm. In the repaired material, the layer of epoxy resin is approximately 1 mm corresponding to less than 1% of the total. Therefore, the stiffest marble (in this case type MB) would always exhibit much higher velocities and the sensitivity to the epoxy layer and its adhesion to the matrix would be negligible. Despite the fact that ultrasound shows very good correlation to the material properties and the final strength, it would not be possible to characterize by the same test the capacity of the repaired specimens. It should also be mentioned that although AE gets signals from fracturing of the material it is not necessarily \"close to failure\", as micro-cracking activity is induced very early in the loading. There are several other examples in literature where AE starts in a small fraction of the ultimate load [7; 11; 12; 25; 26]. Therefore, AE can be used in the concept of \"proof loading\". This kind of test is a usual application in different types of structures (e.g., pressure vessels). The structure is loaded up to a safety point and the AE is monitored. According to the received AE the structure can be categorized either as safe for continuous use, as safe for a period of time before another monitoring, or unfit and should be replaced. AE, due to the sensitivity of the sensors records the very early fracturing incidents in the micro-scale before the structural capacity of the member/specimen is compromised. In the specific case, the loading rate was constant. Taking into account the accumulated AE activity for marble MA, the initial AE stage is completed well before 15 or 20 s in a total of 50 to 60 s until fracture (check Figure 7a). This means that the load until that moment was approximately one-third of the maximum load. For marble MB, the initial activity (first stage) was completed at about 30 s while the duration of the loading until fracture was more than 100 s (Figure 7b). Again this means that the loading for the first AE stage is well below one-third of the maximum. Therefore, it can be supported that the AE data used for correlation with the final load capacity, as presented in Figure 9a correspond to a stage when the material is still macroscopically intact since the load was about 30% of the ultimate. For the repaired specimen, the initial stage of AE represents approximately half the duration of the experiment, so we can say that the \"early\" AE data were recorded until 50% of the capacity (Figure 10a). After the initial stage, the specimen continued to receive increasing load within a period of acoustic silence before it was fractured 20 to 40 s later. Therefore, it can again be argued that the early loading did not effectively compromise the load bearing capacity of the specimens. Concerning the localization of the AE sources, the present setting with two sensors enables only linear location. More sensors would not add much to the accuracy of the AE events. This is because the accuracy is compromised by different factors. The specific type of sensors is widely applied in practice due to their high sensitivity but on the other hand they are far from being considered point sensors, as their diameter is approximately 16 mm. This is comparable to the propagation distance between the crack (middle of specimen) and the 1 st sensor and also comparable to the specimen dimensions (thickness of 40 mm). Therefore, AE location would enable to separate events that occurred centimeters away but within a single damage zone it would be troublesome to make more accurate statements. As an example, in our study the thickness of the epoxy in the repaired specimens is approximately 1 mm. AE location results would not be reliable enough to check if an event occurred within this mm (crack in epoxy) or 0.5 mm to the right or left (marble crack), as the typical error is certainly higher.