## 1 Introduction
Furthermore, it is highly versatile and accurate, capable of accommodating complex geometries with superior dimensional precision (up to \(\pm\)0.01 mm) and surface quality (up to Ra 0.2 \(\upmu\)m) [9; 17]. Figure 1: The AWJC principal and kerr characteristics: (**a**) the AWJC principle, (**b**) the kerr geometry, and (**c**) the cut surface characteristics. AWJ is used for machining a lot of different parts and components in the aerospace and automotive industries. AWJ cutting is the ideal tool for trimming (external counter cutting) complex parts made from CM. For example, in the case of the Airbus 350 and Boeing 787, the composite stringers used for the T-Beam and I-Beam stiffeners are trimmed using AWJ [16], and different body components of the BMW i3 are machined using AWJ. Machining with AWJ is used in the manufacturing of various products including medical devices, sporting goods, marine components, or wind turbine blades [18]. Machining composite materials with AWJ can lead to various defects, including delamination (cracking) [19], fiber pull-out, abrasive embedment [5], poor surface quality, or low accuracy [10]. The material delamination resulting from the shock created by the high-velocity water jet hitting the material is a significant problem in machining with AWJ [19]. The delamination of the CM layers compromises its structural integrity, resulting in rejected parts, rework, and additional expenses due to material wastage and production delays. That is why avoiding CM delamination during AWJ machining is important [20; 21]. When trimming, this is avoided by selecting a starting point outside the workpiece [22]. The challenge is to machine different features like holes, slots, or internal contours, where a piercing (initial material penetration) in the CM is required [21]. In aerospace, it is prohibited to pierce the CM with an AWJ [23]. Typically, holes are drilled at the starting points before engaging AWJ for further cutting, to avoid material delamination [5]. Many studies have found that the optimal combination of process parameters, including the water pressure, standoff distance, abrasive mass flow, etc., have an influence on decreasing/avoiding delamination during the piercing of CM [5; 19; 21]. Water pressure is particularly important as it directly influences the energy of the AWJ [19]. By employing a lower water pressure, the energy with which the AWJ impacts the CM decreases, consequently reducing the delamination [21]. The homogeneity and stability of the AWJ flow also affect delamination during piercing. Using a homogeneous AWJ and a proper combination of process parameters, CM delamination during piercing could be avoided [5]. The CM delamination phenomenon typically occurs at the beginning of the piercing process but can also manifest during AWJ cutting. In the case of the AWJ cutting process, CM delamination appears rarely. The cutting process is stable, but equipment malfunctions can create problems [24]. During the AWJ machining of composite materials, equipment malfunctions may occur, such as the following:  
* The accidental reduction in the abrasive flow rate, whether partial or total, poses a significant challenge in AWJ cutting processes [19]. This reduction can occur due to various factors such as the inadvertent introduction of impurities into the abrasive delivery tank, the wear of the rubber tube used for feeding the abrasive, or the entry of water droplets into the abrasive tank. Unlike the capability to detect a complete absence of abrasive material, current AWJ equipment lacks a system capable of effectively detecting a decrease in the abrasive mass flow. * The clogging of the cutting head with abrasive grits occurs when they partially or completely block the mixing tube [24; 25; 26]. As a result, the water jet is unable to exit through the mixing tube and may flow back up the abrasive tube, potentially reaching the abrasive hopper. This situation leads to heavy splashing, and although the cutting head moves, it fails to cut effectively.