## 4 Results and Discussion
### Validation of the Proposed Method for Monitoring AWJC Malfunctions  
The proposed monitoring technique was validated using an experimental study. To exemplify the Acoustic Emission (AE) monitoring technique within the framework of the AWJC process, a scenario was investigated involving the accidental reduction in the abrasive flow rate. The accidental reduction in the abrasive flow rate, whether partial or total, poses a significant challenge in AWJ cutting processes. It can lead to a compromised cutting performance and potential damage to the CFRP workpiece. Typically, the optimal value of the abrasive flow rate is determined based on the cutting head dimensions, often through initial testing. As an example, for an Omax cutting head with an orifice diameter of 0.35 mm and focusing tube dimensions of \(df\) = 0.76 mm and \(f\) = 101 mm, an abrasive mass flow \(Ma\) of 0.45 kg/min (using garnet mesh 80) is utilized. In this case, the abrasive flow rate \(Ma\) decreased by approx. 50% from 0.45 kg/min to 0.23 kg/min. The \(AE_{RMS}\) signal analyzed in this scenario is plotted in Figure 9.  
The initial experimental runs were conducted with an abrasive mass flow rate (\(Ma\)) of 0.45 kg/min. For the specified parameters (with \(P\) = 350 MPa, \(V\) = 3000 mm/min, and \(T\) = 4 mm), the calculated values for the \(AE_{RMS}\) at the cutting head (CH) were 3.22 V (calculated) and 3.31 V (measured), while for the workpiece (WP), they were 0.62 V (calculated) and 0.58 V (measured). When the abrasive flow rate was reduced to 0.23 kg/min while maintaining the other operating parameters as constant, the \(AE_{RMS}\) at the cutting head (CH) was calculated to be 2.3 V, with a measured value of 2.106 V. Similarly, for the workpiece (WP), the calculated \(AE_{RMS}\) was 1.112 V, with a measured value of 1.089 V. Figure 9 presents the \(AE_{RMS}\) signals acquired during the experiments, with the \(AE_{RMS}\) obtained under decreased abrasive flow rate conditions (the malfunction) highlighted in red. In the case of the signals measured at the cutting head (Figure 9b), the \(AE_{RMS}\) value decreased from the normal value of 3.31 V (\(Ma\) = 0.45 kg/min) to 2.106 V (\(Ma\) = 0.23 kg/min) in the event of an accidental drop in the abrasive mass flow. A 48% decrease in the abrasive mass flow corresponds to a 33% decrease in the \(AE_{RMS}\). Conversely, for the signals measured at the workpiece (Figure 9a),  
Figure 8: The influence of the process parameters on the AE signal: (**a**) the AE signal measured at the cutting head, (**b**) the AE signal measured at the CFRP workpiece. the \(AE_{RMS}\) value increased from the normal value of 0.58 V (\(Ma\) = 0.45 kg/min) to 1.089 V (\(Ma\) = 0.23 kg/min) under the same conditions of reduced abrasive mass flow. In this case, a decrease of 48% in the abrasive mass flow corresponds to an 87% increase in the \(AE_{RMS}\). This equipment malfunction introduced during the AWJ cutting of CFRP results in kerf geometry modification and a decrease in the cut surface quality. Figure 10 presents the results obtained in the analyzed scenario. The dimensions of the kerf profile were measured using the PG 2000 microscope with a 9.5\(\times\) zoom. The cut surface was scanned with a 3D measurement system, model usurf Focus, and analyzed with usoft surface analysis software. When the equipment malfunction was introduced by reducing the abrasive flow rate from the normal value of 0.45 kg/min to 0.23 kg/min, the kerf dimensions decreased. The top kerf width (\(W_{t}\)) decreased from 1.33 mm to 1.26 mm, and the bottom kerf width (\(W_{b}\)) decreased from 0.98 mm to 0.87 mm. In the case of the cut surface topography, this malfunction resulted in an increase in the surface roughness from (\(R_{a}\)) 1.56 up to 2.22 \(\upmu\)m. In the 3D topography, valleys were observed in the direction of the AWJ's movement. Additionally, the valley depth and width increased as the abrasive flow rate decreased. Similar characteristics of the surface topography were observed by [PERSON] et al.