# Acoustic Emission Analysis of Fracture and Size Effect in Cementitious Mortars
## 4 AE Analysis: Results and Discussion
### Localization of AE Sources  
The accuracy of the AE source localization is initially investigated using pencil lead break (PLB) tests. These tests were part of the calibration test conducted on all samples. Six to eight locations of the PLBs are used on each sample, and the average localization error for the x-, y-, and z-axis of the larger, medium, and small samples are (2.59 mm, 4.32 mm, 4.2 mm), (1.74 mm, 1.77 mm, 2.73 mm), and (0.71 mm, 1.52 mm, 1.13 mm) respectively. Also note that for the fracture process zone, AE sources are located with respect to the X-direction, which has the lowest error for all sample sizes.  
In Figure 8, the located AE sources during the Brazilian splitting tests are shown in three directions for a representative large sample. The color of the located AE events indicates the load ratio at which the event was detected, and the size of the dots is relative to the AE amplitude.  
Initially, AE sources are located at the bottom contact surface, occurring at around 20% of the peak load (see Figure 8). Despite using packing strips, local crushing due to concentrated compressive stresses and a non-fully smooth sample surface occurred. This causes early friction events that are observed at the start of the test (see Figure 6a). As the load reaches the peak value, clusters of AE sources are detected along the vertical center line of the sample (at X = 53 mm) that goes along the direction of the loading (from \(Z=-53\) to \(Z=53\) mm).  
The unsymmetrical nature of crack propagation is captured and is shown in Figure 8. This is observed in the top view; the splitting crack initiated at one end of the sample (at Y = 100 mm) and propagated to the other side (Y = 0). This type of crack propagation has long been identified, and for this reason, most researchers working with splitting tests monitor crack propagation from both directions [49; 50; 51; 52]. [PERSON] et al. [53] also observed similar crack propagation patterns using AE localization plot on a splitting test of concrete cubes. This can be due to the compression surfaces not being fully parallel and/or one end of the sample being slightly weaker due to the casting process. Cast  
Figure 8: Projections of AE source locations on the top (**left**), front (**middle**), and side (**right**) plane, at stages of a monotonic Brazilian splitting test on a large sample (\(L_{3}\)). Size of the circular marker indicates the AE event amplitude.  
that make the sample slightly conical (one side diameter larger than the other) can also cause the crack to initiate on the side with a larger diameter. Overall, the 3D located AE sources clearly represent the cracking process during the splitting tests.  
The localization plots for three representative, different-sized samples are presented for subsequent loading steps in Figure 9. For the larger sample, it can be observed that there is a lack of AE activity in the post-peak period. This is because, at peak load, there is a rapid crack propagation that immediately splits the sample apart. During the test, the larger sample failed in a very brittle (almost \"explosive\") manner, making a loud noise. On the other hand, the smaller sample failure was hardly noticeable, and only a small crack was formed, allowing for post-peak crack bridging and friction events. In addition, there are hardly any low-amplitude AE events located for the larger samples. This was attributed to the lack of micro-cracking before the development and propagation of the main macro-crack.