Avoiding CFRP Delamination During Abrasive Water Jet Piercing: A New Piercing Method

Carbon Fibre Reinforced Polymer (CFRP) is used in top industries like aerospace, automotive or medicine. Abrasive water jet (AWJ) technology has demonstrated its capacity in machining CFRP parts with a high dimensional accuracy due to its low mechanical loading, reduced machining temperature, high productivity, reduced tooling, and environmental friendliness. An important challenge when machining composite materials with AWJ is material delamination, determined by the high-speed water jet hitting the material during the piercing process. It is the ideal tool for cutting complex CFRP parts, in cases where the piercing point is outside of the workpiece. The challenge lies in machining features where material piercing is required, like holes, slots or internal contours. This paper presents a method of piercing the composite materials with abrasive water jet, that can avoid delamination. The method requires adding the abrasive particles in the water jet at the very beginning of jet formation, thus obtaining a mixed abrasive water jet during the rst impact with the composite workpiece. A new cutting system was designed and set up based on the proposed piercing method and was compared with a conventional AWJ cutting system. The insertion of the abrasive particles into the water jet was monitored by using acoustic emission (AE). An analysis of the inuence of piercing parameters (water pressure, standoff distance, abrasive inlet angle and abrasive delay time) on the delamination was conducted. The process outcomes such as hole surface integrity, delamination, particles embedment, uncut bers and dimensional characteristics, were evaluated. The results show that the method is promising in reducing delamination.

The material deterioration due to the shock created by the high-velocity waterjet hitting the material is an important problem in composites machining with AWJ [2,6,[13][14][15][16][17][18]. This phenomenon takes place at the beginning of the piercing process. This is avoided in case of trimming or external contour cutting, by selecting a starting point outside of the workpiece. The challenge is to machine different features like holes, slots or internal contours, where a piercing of the composite material is required [19].
The AWJ process consists of a water jet (WJ) at high pressure mixed with abrasive particles (Fig. 1a), in order to process a variety of materials. Water is pumped at high pressure through a small ori ce to create a high speed WJ, that accelerates the abrasive particles at a high speed, generating kinetic energy. The WJ stream reaches the mixing chamber where abrasive particles are mixed in. The abrasive particles reach the mixing chamber through an inlet tube and are pulled in gravitationally and by the suction generated by the WJ. The resulting AWJ is oriented towards the material through a focussing tube [7,10,11].
a. The AWJ working principle; b. The mechanism of composite material delamination.
Piercing composite materials with AWJ can create several defects like delamination, cracking, ber pullout or abrasive embedment. The most common defect while machining composite material with AWJ is delamination of the layers [13][14][15][16][17][18]20]. Delamination can appear in all sections of the material (Fig. 1b): the upper (peel-up delamination), middle or bottom sections (push-out delamination) [21]. Several methods are used to evaluate the level of delamination: the maximum crack length [14], the delamination planar area [2], the delamination factor [21] or the delamination extent [13,18]. Delamination often occurs when piercing composite material is required, like when machining features such as holes, slots or pockets [19].
Shanmugam et al. [14] noted that delamination takes place in two stages: the initial generation of cracks and the propagation of cracks. The initial cracks occur due to the shock wave caused by the high-velocity WJ impacting the material. This happens at the beginning of the piercing process before any abrasive particles are introduced. The propagation of the cracks is a result of penetration of the water at a high pressure, into the initial cracks, causing a water-wedge action. AWJ characteristics like the kinetic energy rate, jet velocity and jet-workpiece interaction time, are directly corelated with the delamination mechanism [14,17].
AWJ piercing parameters (water pressure, standoff distance, abrasive mass ow, abrasive inlet angle, delay time, material thickness, ori ce diameter, focusing tube diameter) can in uence CFRP delamination [2,[13][14][15][16][17][18]. Shanmugam et al [14] researched the in uence of AWJ piercing parameters on CFRP delamination. They proposed an energy conservation model to predict the extent of delamination. Experimental results indicate that delamination could be reduced by decreasing the jet diameter and water pressure. Phapale et al. [13] observed that, by increasing water pressure, the kinetic energy of the AWJ raises, resulting a higher delamination. A low standoff distance affects the AWJ divergence and can reduce the delamination. Experimental studies also showed that abrasive mass ow has less of an effect on delamination than water pressure and standoff distance. It was observed that by increasing the abrasives mass ow, delamination slightly decreases [2,13,14]. Garam et al. [2] recommends using low pressure (under 148 MPa) for piercing, to decrease the delamination area, and switching to a higher pressure for the cutting process itself. Delamination could be considerably reduced by using an optimum set of process parameters, but it cannot be fully avoided [22].
Homogeneity and stability of the AWJ ow also affect delamination during piercing [2,14]. In the early stage of the AWJ formation, abrasive particles can reach the mixing chamber before the WJ [14] and can clog the mixing chamber. This can be prevented by delaying the abrasive ow [14] with 0 to 2 seconds [2]. Garam et al. [2] demonstrated that delamination could be reduced or even eliminated if the mixture of abrasive particles and WJ is stabilized at the beginning of the piercing process.
There are several types of cutting head designs with different values of abrasive inlet angle (Ψ) which has a direct in uence in the AWJ formation process. This angle can have different values depending on the equipment manufacturer: 90° for KMT and Omax, 60° for Flow, and 40° for AccuStream. Nevertheless, there are a few studies regarding the effect of the Ψ on AWJ formation and the process quality characteristics.
The three main types of piercing methods used in AWJ machining are: stationary piercing, dynamic piercing and drilling [2,[23][24][25][26]. Stationary or static piercing is the most basic piercing method and can be used for a large variety of materials. The nozzle remains xed in the hole centre while the AWJ pierces the material [2,25]. This piercing method takes a long time, especially in thicker materials but it has the advantage of small hole diameter and that it is easy to program [25]. Dynamic piercing methods involve the AWJ moving relative to the hole centre, during the piercing process. This piercing method avoids the interference of the incoming jet with the re ected one during piercing. The jet movement is linear or circular. In linear piercing, the water jet moves in a straight line (1-3 mm) along the cutting path while during circular piercing, the AWJ moves in a 2-3 mm diameter circle around the hole centre [2,[23][24][25][26]. Dynamic piercing is the main type of piercing used in manufacturing because it is much faster than stationary piercing. Thongkaew et al. [6] suggested that pre-drilling a starter hole could eliminate the composites material delamination. Some AWJ machines can be accessorised with an extra spindle, used for drilling [23] but it can result in a higher machining time and costs.

Purpose Of The Research
The present paper proposes and demonstrates a new method of AWJ piercing of advanced composite materials, that reduces or eliminates delamination.
To this purpose, a cutting system was designed based on the proposed method and it was compared with a conventional AWJ cutting system within an experimental study.
It was followed by an analysis of the in uence piercing parameters (water pressure, standoff distance, abrasive inlet angle and abrasive delay time) on the delamination using the Response Surface Methodology (RSM).
Finally, an evaluation of the surface integrity of the holes was made to study delamination, particles embedment, uncut bers and dimensional characteristics.

The Proposed Method For Abrasive Water Jet Piercing
The problem addressed by the new method is the lack of abrasive particles in the WJ as it hits the material in the beginning of the piercing process. The concept consists of adding the abrasive particles into the mixing chamber at the very beginning of the jet formation. Obtaining a homogeneous AWJ on the early stages of the piercing process can reduce or avoid the composite materials delamination.
In conventional cutting applications, the abrasive particles are introduced into the mixing chamber after the WJ start, with a delay, that is introduced through the cutting system design or intentionally, to avoid the clogging of the mixing chamber [2]. The WJ hits the composite material without any abrasive material and the resulting shock creates delamination [14].
A cutting system must be designed to add the abrasive particles into mixing chamber through gravitational ow. Figure 2 shows the proposed cutting system for composite materials piercing.
Opposite to the conventional system, in the proposed design, the abrasive particles are introduced in the mixing chamber through an abrasive inlet tube oriented at an angle causing the abrasive particles to be added to the jet from the beginning. After jet stabilization the suction generated by the WJ and gravitational ow provide a constant mass ow rate of abrasive particles.
The main component of the system is the cutting head. It is required to use a cutting head with an abrasive inlet tube orientated at Ψ = 40-60 deg. The abrasive delivery system is another key component. An electronic system must be installed on the upper part of the cutting system and it must accurately control the abrasive ow rate and the abrasive start moment. Cutting head and abrasive delivery system are connected in a vertical line through a tube. The internal diameter of the abrasive delivery system must be at least 3 mm to be able to provide the abrasive particles ow and to eliminate the air from the mixing chamber.
An e cient AWJ piercing process of composite materials depends on: 1. The cutting system design. The method requires to use a cutting system which can introduce the abrasive particles into the mixing chamber through a ow, based on the gravitational force. 2. The synchronization of the start of the WJ with the start of the ow of abrasive. If the WJ starts before the abrasive, it hits the material without abrasive, causing delamination. If the abrasive ow starts before the WJ, it can clog up the mixing chamber. 3. The selection of suitable piercing process parameters (P, m a , SOD) is also paramount.

Experimental Setup
To validate the proposed AWJ piercing method, a set of experimental trials have been conducted. The research uses the Response Surface Methodology (RSM) for the analysis, as it is recognized as a proper solution for studying AWJ processes [13,17,20,27]. Central Composite Design (CCD) is a method that was selected as the experimental design which is suitable in RMS because of its good statistical properties. The water pressure (P) and the stand-off distance (SOD) are independent variables (k = 2) while the delamination extent (D ext ) is the output variable. Thirteen experimental runs (with three repetitions) were conducted and the results are presented in Table 1. Each factor is varied over 5 levels: 2 k = 4 axial points, 2 k = 4 factorial points and the centre points 2k = 6. The composite ber reinforced polymer (CFRP) was used in this experimental study. The workpiece, with a 3 mm thickness, was manufactured through a heat pressing process. The prepreg was made from a 3kfabric style 452 in twill weave 2/2 and a quantity of 364 g/m² epoxy resin. The specimens were trimmed by AWJC at 100 x 70 mm.
The cutting was performed with a Omax 2626 machine (Fig. 3a) equipped with an ultra-high pressure (UHP) cutting system. The water pressure (P) can be varied between 100 and 350 MPa, using a direct UHP pump. The machine is equipped with an Al abrasive delivery system which can automatically adjust the abrasive mass ow rate (m a ) ranging from 0.05 to 0.6 kg/min. It is controlled with an OMAX CNC control system, which can move the cutting head in 3 axis simultaneously. A clamping system (Fig. 3b) was manufactured to x the specimens on the worktable. Table 2 presents the speci cations of the two cutting systems designed for these experimental trials. The systems can be seen in Fig. 4. The conventional cutting system (CS90), recommended by Omax, has the abrasive delivery system xed on the right side of the cutting head, as it is shown in Fig. 4a. In the case of the proposed cutting system (CS40), the abrasive delivery system was xed above the cutting head, aligned with it (Fig. 4b).
The abrasive ow and WJ start times were synchronized through an experimental study. Omax software (Omax Make) offers the possibility to set the abrasive ow start in relation to the pump start. The abrasive start delay time was varied between 0 and 2 s, with 0.1 increments while the rest of the cutting parameters were held constant (P = 100 MPa, SOD = 1 mm, m a = 0.45 kg/min). There is no need for complex monitoring techniques or equipment to detect the clogging of the mixing chamber, as the WJ ows through the abrasive inlet tube instead of the focusing tube when this happens.
In case of the conventional cutting system (CS90) the mixing chamber clogging phenomenon was not detected. The abrasive delivery system was xed on the right side of the cutting head (Fig. 4a) and the abrasive particles were owing downwards in the abrasive tube and stopping close to the cutting head. The abrasive grains are added in the cutting head through the suction generated by the water jet.
For the new proposed cutting system (CS40) the mixing chamber clogging phenomenon was avoided by selecting 0.2 s as the abrasive start delay time. By xing the abrasive delivery system vertically above the cutting head and an abrasive inlet tube oriented at 40 deg. (Fig. 4b), the abrasive particles were owing directly to the mixing chamber at the same time with the water jet.
After determining the optimal synchronization time for the abrasive ow and water jet, the 13 experimental trials were carried out. The delamination extent was determined by cross-sectioning each specimen (Fig. 5.a) and observing it with two optical microscopes Optika B-1000 and Guhring PG 2000 ( Table 3).

Results And Discussions
The specimens were analysed from the perspective of the machined surface morphology. The piercing process was monitored using Acoustic Emissions (AE) and the resulting signals were analysed. Finally, the effects of the piercing parameters on material delamination were determined.

Morphology of the machined surfaces
The machined surfaces were analysed under a microscope to understand the CFRP behaviour during the piercing process (cracks, delamination, particle embedment, uncut bers or plastic deformation of the matrix). The main machined zones were analysed: initial damage zone (IDZ), smooth cutting zone (SCZ) and rough cutting zone (RCZ) [10].
Delamination was observed in all 13 specimens pierced with the conventional system (CS90) while this phenomenon was observed for only 11 (out of 13) trials for the proposed system (CS40). A typical delamination of CFRP during the piercing process looks like in Fig. 6, that was obtained with the conventional system (CS90) during trial run no. 8 (P = 350 MPa, Sod = 3.5 mm, m a = 0.45 kg/min). The delamination appears in all zones.
In Fig. 6b is shown the delamination of the bottom bre layer (RCZ) and some pulled out or uncut bres. It was observed also on some specimens in IDZ and it appeared in the experimental trials where a high pressure of over 300 MPa was used. In Fig. 6c the top bre layer delamination is shown. It is also noticed that, in a few cases, abrasive particles are embedded inside the delamination cracks.
In 70% of the experimental trials the delamination appears in SCZ and RCZ.
No delamination was present in two experimental runs on the CS40, namely: run number 4 (P = 136.61 MPa, SOD = 1.73 mm) and run number 12 (P = 100 MPa, SOD = 3.5 mm). Both experimental trials were made using a low water pressure (under 137 MPa). This observation indicates that composite material delamination is corelated with the process parameters. The hole obtained in experimental run number 12 (Fig. 7) shows a good quality, smooth surface (Ra = 3.1 µm). The hole diameter varies from Ø1.37 mm at the top of the hole to Ø1.21 mm at the bottom. On the top part of the pierced hole (IDZ) a llet was created with a radius of 0.21 mm (Fig. 7a, c) while the bottom edge (RCZ) remained sharp and the bottom bre layer is not delaminated (Fig. 7b).

The piercing process analysis via acoustical emissions AE
To validate this concept an experimental study was made. Acoustic Emission (AE) was used to detect the moment when the abrasive particles are added in the WJ. The new proposed cutting system CS40 was compared with a conventional cutting system CS90, evaluating the abrasive particles adding time in the WJ.
This non-destructive testing is based on monitoring the transient elastic waves inside the material called acoustic emissions [27]. This technique is a proper solution for monitoring AWJ technologies [28,29] because the AE signals can offer information about the changes of AWJ characteristics, cutting system malfunction, crack formation/propagation on the workpiece or material removal [27][28][29].
The AE signal acquisition system employed was composed of a data acquisition (DAQ) board, preampli ers and the AE sensors (Fig. 8). The DAQ board, a PCI-6110 from National Instruments, can acquire data through four channels, at a sample rate of up to 5M samples/s. The AE sensors were connected to the DAQ board through a NI 2110 connection box. The data acquisition program, developed in NI LabView 2015 and NI DIAdem 2015, was used for signal analysis. The AE-sensor Vallen VS900-M that was used is a passive piezoelectric sensor with a response frequency (fPeak) between 100 and 900 kHz. The Physical Acoustic preampli er 2/4/6 was selected to amplify the signal and a gain of 40 dB was selected to acquire a clear AE signal.
Three cases were monitored using AE: Each experimental trial was repeated 5 times to ensure consistency and the same 3 mm thickness CFRP workpiece material like in the main experiment was used.
The AE signal acquired from the cutting head shows the moment when the abrasive particles get added to the WJ (Fig. 9). The peak amplitude and signal duration were evaluated. In all cases the WJ was started at about 1.2 s from process start.
In the rst case (piercing without abrasive on the CS40), the average height of the peak amplitude was around 0.6 V for the whole process (Fig. 9a). The resulting delamination was high (D ext > 40 mm) and the specimens were almost separated in two pieces.
In the second case (piercing with abrasive on the CS90), the abrasive and water are mixed 0.4 s after the start of the water jet (Fig. 9b). The medium height of the peak amplitude of the signal rises from 0.7 V (water jet only) to 5.6 V (water jet with abrasive). The material did show delamination, but to a smaller extent (D ext = 7 mm).
In the last case (piercing with abrasive on the CS40), the abrasive particles were added into the water jet almost immediately after its start (0.039s), as can be seen in Fig. 9c. No delamination was present in this case.
In the conventional system, the abrasive particles are pulled into the water jet through suction. This creates almost a 0.5 s delay, leading to delamination. By using the proposed system, the particles are gravitationally pulled into the WJ and they are added almost instantly (0.039 s). By decreasing the adding time of the abrasive particles in WJ the material delamination could be reduced or avoided.

Effects of piercing parameters on material delamination
An analysis of the piercing process parameters (water pressure, standoff distance, abrasive inlet angle or abrasive delay time) was also made as they affect the composite material delamination.
Effects of water pressure and standoff distance on material delamination Based on RSM, the effects of the independent variables (P and SOD) on the output variable (D ext ) were analysed. This method was involved in building the response surface model for predicting the delamination extent. The experimental data for the proposed cutting system CS40 was analysed with the Design Expert 2019 software.
Multiple models were t (linear, two factor interaction, quadratic and cubic) and the lack of t test was used to determine how well each model ts the data ( Table 4). The α = 0.05 level of signi cance was chosen for the test. Some signi cant models were found (p < 0.05) but the quadratic model t the data best (R 2 adj = 0.977, p = 0.012).  The delamination extent is mainly in uenced by the AWJ pressure (P) as seen in Fig. 10a. An increase in pressure from 136 to 313 MPa, increases the deamination extent (D ext ) from 3 to 27 mm. The stand-off distance (SOD) has a smaller in uence on the delamination extent (Fig. 10b). Increasing the SOD from 1.73 to 5.27 mm results in an increase in D ext from 9.3 to 15.4 mm. By reducing both AWJ pressure and the standoff distance, the delamination extent can be reduced or even eliminated (Fig. 10c) Effects of abrasive inlet angle and delay time on material delamination In these experimental studies the abrasive inlet angle Ψ (Fig. 2) has a signi cant in uence on the abrasive particles owing into the WJ. Two cutting systems with 40 deg. and 90 deg. abrasive inlet angle were analysed.
During the trials made by using the cutting system CS90 (Fig. 4a) with Ψ = 90 deg., the abrasive particles were owing downwards in the abrasive tube and stopping close to the cutting head. The abrasive was added in the cutting head through the suction generated by the waterjet. Due to this reason, the mixing chamber clogging phenomenon did not appear. This results in the water jet hitting the part without any abrasive, causing delamination in the material. In the experimental trials with this cutting head, the delamination was not fully avoided.
In the experimental trials done with the cutting system CS40 (Fig. 4b), with Ψ = 40 deg., the abrasive particles are owing gravitationally into the mixing chamber in a very short interval. If the abrasive arrives in the mixing chamber before the water jet, it can clog up the mixing chamber. This problem is solved by introducing abrasive with a delay time. The start of the WJ was synchronized with the start of the ow of abrasive particles that was added into the WJ in the early stages of the jet formation. By contrast with CS90, delamination did not appear while using the CS40 system when using low AWJ pressures.
Using the optimal process parameters and a low value of the abrasive inlet angle the material delamination was avoided.

Conclusions
The composite material delamination mechanism takes place during the shockwave generated by the high-speed water jet impacting the material.
This paper presents a new method of piercing composite materials (CFRP) with abrasive water jet that reduces or avoids delamination.
The new method of adding the abrasive particles into the mixing chamber at the very beginning of the jet formation, showed better results, using an abrasive inlet angle of 40º, instead of 90º.
The moment when the abrasive particles should be added was identi ed by using Acoustic Emission. By reducing the time when the abrasive particles were added to the water jet from approximately 0.4 s to 0.04 s, delamination was reduced and for some parts even avoided.
The most signi cant parameter for the composite delamination is the water pressure. A low water pressure and low standoff distance are recommended, to minimize the delamination. The experimental results showed that, during the composite material piercing process it is recommended to use a water pressure lower than 136 MPa and a standoff distance lower than 3.5 mm.
Optimizing the process parameters is recommended to avoid delamination, but the homogeneity of the abrasive water jet on the early stages of the piercing process is also important.
The delamination was observed in a large number of specimens through the analysis of the holes surface morphology. In 70% of the experimental trials the delamination appears on the middle section and push-out delamination on the bottom. In a few cases, uncut bbers and abrasive particles embedded inside the cracks were observed. In the experimental trials where the delamination was avoided, a wellde ned hole and good quality surface was obtained.
The proposed method is simple, quick setup and no special equipment is required. It can be implemented on any kind of AWJ industrial machine tool, used for manufacturing composite parts. Availability of data and material -Not applicable.
Code availability -Not applicable.
Ethics approval -Not applicable.
Consent to participate -Not applicable.
Consent for publication Figure 1 The working principle of composite material piercing through AWJ: a. The AWJ working principle; b. The mechanism of composite material delamination.  The cutting systems used in the experimental study: a. The conventional cutting system (CS90); b. The proposed cutting system for composite materials piercing (CS40).

Figure 5
Experimental results evaluation: a. The position to cutoff the specimens, b. The delamination area evaluation.   The experimental setup for AWJ piercing monitoring via AE. Figure 9