## 2 Numerical Methods and Experimental Procedure
### Source Modelling  
For the validation of the FEM calculations, we simulated a pencil lead break, PLB (Hsu-Nielsen source), on the surface of the sample (Figure 2a) [45]. Figure 2b shows the temporal evolution of the point force used to simulate the lead break according to the work of [PERSON] [20]. Figure 2c represents the frequency content of the source. The modes are excited using a monopole point source located on the surface of the specimen, acting in the out-of-plane direction in the centre of the specimen surface at following location (x = \(-\)36 mm, y = 0, z = 1.85 mm). The origin of the coordinate system is situated at the centre of the specimen.  
For the modal analysis, in order to identify the excited modes, we used a chirp signal (Figure 3a), which rather equally stimulates all frequencies up to 1.2 MHz, as shown by its frequency content (Figure 3b). The chirp is applied as a point force on the surface of the material (x = \(-\)36 mm, y = 0 mm, z = 1.85 mm).  
Assuming that the physical dimensions of the AE sources are much smaller than the distance to the point of observation, we chose a point source. In contrast to complex models, force dipoles are computationally more efficient to investigate the effect of propagation. According to [PERSON] [15; 16; 17], we model two sources corresponding to two different damage mechanisms as buried point sources. To model buried sources in the continuous mesh domain, two closely spaced, self-equilibrating monopoles are simultaneously applied along opposite directions. The distance between monopoles, 800 \(\upmu\)m, is assumed to be as small as possible, considering the element size in the FEM calculation. Sources are denoted by \(D_{i}\) (\(M_{a}\), \(z\), \(RT\))), where i = 1, 2, \(\alpha\) = \(x,y,z\), \(M_{\rm a}\) is the intensity of the force dipole (in N.\(\upmu\)m) in the \(\alpha\)-direction, z is the distance from the midplane (xoy) of the specimen, and \(RT\) is the source rise time. The two sources shown in Figure 4 represent the fracture-opening mode (D\({}_{1}\)) and the fracture in-plane shear mode (D\({}_{2}\)). For the source D\({}_{1}\), the strongest dipole is in plane and aligned along the x-direction. The source D\({}_{2}\) represents a slip, which occurs in a direction in the plane (x, y) at 45\({}^{\circ}\) to the x-axis. The dipole moments are determined based on the literature [17]. The order of magnitude of the dipole moments is 100 N.\(\upmu\)m. The dipole moment for the source D\({}_{1}\) is assumed to be 5 times larger. The source rise times are 0.1 \(\upmu\)s, 1 \(\upmu\)s and 10 \(\upmu\)s. These values are also determined based on the literature [16; 21]. The temporal shape of the source rise functions considered are linear. The dipole forces reach their maximum value after the rise time and persist at a constant value. The sources are located in the gauge section at three different values of z (z = 0 mm, 0.25 mm, 0.95 mm) and at (x = \(-\)15 mm, y = 0).  
Figure 1: The three specimen geometries (EPL3; EPL16 and EPL80) used in all studies of this paper (thickness of the samples 3.7 mm).  
_Appl. Sci._**2018**, \(8\), 2557  
Figure 2: (**a**) Main characteristics of the pencil lead-break procedure (PLB); (**b**) temporal evolution of the applied force exerted by a 2H lead break of 0.5 mm diameter, 4 mm length and with 45\({}^{\circ}\) inclination with respect to the surface; and (**c**) amplitude in the frequency domain.  
Figure 4: The modelled sources: fracture opening mode (D\({}_{1}\)) and fracture in-plane shear mode (D\({}_{2}\)) (values of the applied loading M\({}_{\alpha}\) are indicated for each source. The values of the source rise time and of the location d of the source are also pictorially represented).  
Figure 3: Amplitude of the chirp source in the (**a**) time domain and (**b**) frequency domain.