Comprehensive Review of Strategies to Mitigate Delamination and Uncut Fibers in CFRP Drilling

Article information

Abstract

1 Introduction

Composite fiber materials have emerged as indispensable components in contemporary manufacturing due to their exceptional properties. These materials, known for their nonmetallic, nonmagnetic, and corrosion-resistant attributes, offer a unique combination of high strength, low weight, stiffness, and toughness [1–3]. As illustrated in Fig. 1(a), a global market report [4] highlights the sustained growth in the demand for carbon fiber materials, with statistical forecasts indicating a continued upward trajectory. Carbon fiber has become pervasive across diverse sectors, ranging from sports equipment manufacturing to high-speed automotive and aerospace industries [5,6], significantly enhancing the performance of the products in which it is incorporated. Fig. 1(b) provides a detailed breakdown of global carbon fiber usage percentages across various sectors as of 2021. For instance, carbon fiber accounts for nearly 50% of the materials used in the aerospace sector, contributing to aircraft construction. This integration reduces fuel consumption by 57% compared to traditional metal structures and improves maneuverability by reducing inertia. Consequently, industry leaders such as General Electric have leveraged composite fiber materials to develop high-performance aircraft engines and various components [7,8] Carbon fiber composite materials typically consist of a matrix of carbon fiber and epoxy, meticulously blended in specific proportions to achieve exceptional strength and mechanical properties.

Fig. 1

(a) Global demand for carbon fiber [4], (b) Global carbon fiber demand by applications in 2021 [17], and (c) Used material volume percentage in Boeing 747 and 787 [17] (Adapted from Refs. 4,17 with permission)

This epoxy matrix effectively supplements metals in various applications due to its influential properties and lightweight nature. Within carbon fiber materials, composite fiber-reinforced polymers (CFRPs) hold a prominent position, employing a series of fiber sheets. These sheets are created by grouping straight fibers into specific patterns and arranging them into single plies [9]. The strength of these plies, and consequently the composite fiber sheets and products, is enhanced by stacking multiple plies in precise orientations [10–12]. A high-strength epoxy material blended with a specific hardener is employed at carefully calibrated ratios to ensure optimal adhesion between the plies. Typically, the epoxy-to-hardener ratio and volume percentage of carbon fibers are meticulously adjusted to achieve the desired properties in CFRP composites [13,14]. The orientation sequence of fiber plies plays a pivotal role in the superior mechanical strength of fiber-reinforced polymer (FRP) composites, aligning with established failure theories. These properties make composite materials highly applicable in various industrial sectors, with CFRP being the most commonly used composite material in the manufacturing of different parts and products [15].

The production of functional products or assemblies often requires several machining operations, including milling, turning, and drilling. Among these, drilling is particularly critical for creating holes necessary for attaching parts using rivets and bolts. For instance, Kong et al. [16] reported that Boeing 747 and 787 aircraft use approximately 50% composite material for their bodies and other components, as shown in Fig. 1(c). The assembly of aircraft components requires millions of connecting holes, with the Boeing 747 needing approximately 3,000,000 holes and the F-16 fighter jet requiring 240,000 holes.

Drilling operations in CFRPs depend on two fundamental machining parameters: the feed rate in the Z-direction and the rotational speed of the tool. Additionally, the geometry of the tool, including its diameter, rake angle, clearance angle, and point or cone angle, significantly influences the characteristics of the drilled surface hole. The fiber sequence angle of the plies also plays a critical role in CFRP drilling operations. The unique properties of composite materials, characterized by anisotropy and nonhomogeneity, differentiate their machining processes from those of metals [18]. However, the specific attributes of CFRPs present various challenges, including higher tool wear, fiber peel-up, push-up, uncut fibers, and delamination. These issues can lead to decreased structural strength, resulting in the rejection of up to 60% of parts during assembly. Among these problems, uncut fibers and delamination are considered the most serious forms of damage to FRP parts [19,20].

Delamination refers to the development of interlaminar fractures, which can cause the separation of neighboring composite fiber plies from the bottom ply [21,22]. Peel-up occurs when the chisel edge of the drilling tool contacts the top ply surface, causing significant fluctuations. Conversely, push-up occurs when the tool exits the bottom ply, as the continuous cutting feed force compresses the adjacent ply and suddenly loses contact with the bottom, causing fluctuations on the surface and edge of the hole [23–25]. Uncut fibers and delamination often occur on the circumference of the bottom ply of the hole due to the lack of support from the last ply and fluctuations in the thrust force during the exit [26]. Push-out delamination can be effectively controlled by optimizing the machining feed rate. Implementing efficient machining strategies offers a practical means to increase throughput and tool lifespan while maintaining component quality.

In this study, we aim to comprehensively cover the significant contributions made over the past decade in the field of delamination and uncut fiber formation during the drilling of CFRP composites. The review focuses on understanding the underlying mechanisms behind these defects based on previously published research, including both experimental and modeling studies. By analyzing the root causes and influencing factors, this paper provides critical insights into the challenges and advancements related to CFRP drilling quality and damage mitigation.

The primary contribution of this study is to enhance tool performance over its service life without increasing the likelihood of push-out delamination or compromising overall quality. This advancement not only improves repeatability but also contributes to enhanced product quality and economic feasibility. Fig. 2 presents a comprehensive flowchart that elucidates the process and performance analysis associated with composite drilling. This visual representation serves as a roadmap, highlighting the key elements involved in this complex machining procedure. Emphasis is placed on critical machining parameters, the properties of FRP sheets/products, and the detailed characteristics of tool geometry.

Fig. 2

Process of composite material drilling and evaluation

Monitoring drilling operations in composite materials is a multifaceted task that involves both online and offline evaluation techniques. During online monitoring, real-time measurements of drilling thrust force and torque are captured using a dynamometer. Concurrently, machining vibrations are monitored via an accelerometer. These real-time assessments are essential for evaluating the immediate performance of the drilling process. Offline monitoring is conducted after drilling and involves detailed post-process analysis. Microscopic imaging is employed to examine the condition of the drilled holes, allowing for the evaluation of key attributes such as surface quality, delamination area, uncut fiber length, fiber distribution, and diametral error. Various magnification levels are used to accurately measure cut fiber lengths, providing critical insights into the structural integrity of the holes. Furthermore, a correlation chart is meticulously developed by integrating data from both online and offline monitoring. This chart serves as a powerful analytical tool, offering a comprehensive understanding of the drilling process. It plays a pivotal role in optimizing hole quality by revealing the relationships between real-time process parameters and post-drilling evaluation metrics.

2 Impact of Ply Sequence on Drilling Operations

The precise drilling of CFRP remains challenging due to its anisotropic and heterogeneous fiber structure, which makes the material sensitive to machining-induced damage such as delamination, uncut fibers, and burr formation [33]. To overcome these challenges, researchers have investigated both conventional and nontraditional machining methods, including water jet machining (WJM), abrasive water jet machining (AJM), and electric discharge machining (EDM) [34–36]. However, while these methods can reduce certain thermal and mechanical stresses, they often introduce other forms of damage, particularly in aerospace applications where high dimensional accuracy is critical [37–39]. CFRP drilling defects, including uncut fibers and burrs, are highly dependent on machining parameters, fiber ply orientation, and tool geometry [40]. For instance, Wang et al. [41] observed that fiber overhang beyond the machined edge toward the chisel tip is heavily affected by fiber direction, leading to variations in uncut fiber length and spatial distribution [42,43].

Fig. 4 presents the relative orientation between fiber sequence and spindle rotation direction, which determines the critical cutting angle (Ø). This angle dynamically changes during drilling because the tool rotates while the fiber orientation remains fixed, directly impacting the mechanism of fiber separation and uncut fiber formation [43,44].

Fig. 4

Fiber sequence and tool spindle rotation direction

Figs. 5(a) shows the critical cutting zone where uncut fibers typically form, while 5(b) provides a comparative experimental image confirming the defect accumulation in those regions. When the cutting angle lies between 90°–180° or 270°–360°, the fiber orientation tends to resist the tool’s movement, thereby increasing the probability of uncut fibers [45,46]. Su et al. [47] further emphasized that initial fiber orientation has a pronounced effect on both uncut fiber distribution and delamination. Specifically, fiber orientations in the range of 30°–60° result in reduced uncut fiber formation and smoother slot edges. These findings highlight the critical role of ply sequence in determining machining outcomes. To mitigate these issues, researchers have explored customized drilling techniques and tool path strategies, as summarized in Table 1. These studies employed a variety of ply orientations, tool geometries, and cutting parameters to minimize defects. The diversity in outcomes emphasizes the need for tailored drilling strategies based on specific CFRP layup designs. Moreover, integrating numerical models that predict delamination, burr formation, and uncut fiber length particularly those incorporating ply-by-ply mechanical behavior would significantly enhance our predictive capabilities. A more comprehensive review of such modeling frameworks is warranted to fully understand and control defect mechanisms in CFRP machining.

Fig. 5

(a) Critical cutting zone for generating the uncut fiber [7] and (b) Comparative experimental image of the uncut fiber zone (Adapted from Ref. 7 with permission)

Table 1

Different CFRP drilling and fiber ply sequences

3 Tool Geometry Optimization

Achieving a smooth, defect-free hole in CFRP material is highly dependent on the geometry of the drilling tool, which is a crucial parameter in the drilling process. Researchers have investigated various tool geometries to minimize common drilling defects, such as uncut fibers and delamination. The geometry of the tool directly influences key factors, including feed force, temperature at the tool-chip interface, and drilling accuracy. Moreover, an optimized tool structure not only helps prevent delamination but also reduces tool wear, thus extending the tool’s lifespan [53]. Unlike conventional drilling tools, modified geometric drill bits have been studied to enhance performance. Tool geometry modifications typically involve adjustments to parameters such as the rake angle, helix angle, point angle, and clearance angle [54,55]. Fig. 6 illustrates various modified tool geometries that have been employed in recent studies to improve drilling precision and minimize defects. These tools are often constructed from durable metal materials and may be coated with specialized substances to enhance their mechanical properties. Common coatings include diamond, tungsten, and cemented carbide, all of which significantly improve tool strength and durability while reducing uncut fibers and delamination [56,57].

Fig. 6

Optimized drill geometry (a) Twisted drill, (b) One-shot drill, (c) Double point angle drill, (d) Brad point drill, (e) Step drill, and (f) Diamond core drill

Fig. 6(a) presents a conventional two-flute twist drill bit, commonly used in CFRP drilling. The tool lip, or flute edge, is designed to create a sloping cutting surface [58]. The web, situated between the two flutes, is relief-ground at an angle of 59°, typically ranging between 118° to 120° to form a point. However, due to limited space for debris evacuation and a lower surface speed at the point, conventional twist drills are less effective and often cause delamination and uncut fibers at the drill exit [59,60]. To mitigate these issues, drill bits can be coated with diamond, tungsten, or cemented carbide to improve tool durability and cutting efficiency. Fig. 6(b) depicts the one-shot drill bit, designed with multiple cutting edges specifically for CFRP drilling. This design effectively reduces thrust force, uncut fibers, and delamination while improving hole quality [61]. Additionally, tool wear can be assessed by examining the development of cutting-edge roundness, sharpness loss, and secondary cutting-edge dullness [62,63]. Fig. 6(c) shows a modified double-point angle drill, a variation of the conventional twist drill bit. This tool features two-point angles instead of one, improving mechanical properties while maintaining economic feasibility. The maximum drilling temperature generated by this tool increases with spindle speed and decreases with feed rate [64,65].

The brad point drill bit, shown in Fig. 6(d), features a uniquely pointed tip that allows for precise cutting while minimizing delamination, resulting in defect-free holes. The specially designed flutes and brad point tip facilitate efficient chip evacuation and help prevent drill bit wandering [66]. This geometry includes four cutting edges with three-pointed cone angles: two inward cutting edges, two outward cutting edges, and one inner drill tip, along with two outer drill tips. The rake and clearance angles further contribute to its enhanced cutting performance [67,68]. An experimental study by Grilo et al. [69] demonstrated that the brad point drill bit outperforms both twist and one-shot drill bits by minimizing delamination, uncut fibers, and peel-up effects during CFRP drilling. Fig. 6(e) illustrates a step drill bit, which combines the characteristics of a twist drill with a two-step cutting mechanism. As pressure is applied, the hole diameter gradually increases, reducing cutting force and torque while improving the stage ratio [70]. Studies have shown that step drill bits, especially at low feed rates, produce lower delamination factors and thrust forces compared to conventional twist drills [71,72]. Fig. 6(f) shows diamond core drill bits, which are used in CFRP drilling through grinding processes. A hybrid machining technique known as rotary ultrasonic machining, which combines ultrasonic and diamond grinding methods, is employed for material removal. However, chip evacuation remains a challenge when using diamond core drills [73]. The use of core-step drills has improved chip evacuation, though drill lifespan varies, and chip entrapment can occur at lower spindle speeds [74–76].

The oblique breaking mechanism has demonstrated a significant reduction in cutting torque and forces [76]. Additionally, diamond core drill bits can minimize delamination at the hole exit and enhance surface quality [77]. To further reduce uncut fiber and delamination, brad-spur and dagger drill bits have been explored, with the dagger drill proving particularly effective in lowering thrust force and damage, making it suitable for initiating reaming in CFRP drilling [78,79]. These tool geometry modifications highlight the ongoing efforts to improve CFRP drilling processes by optimizing tool selection based on specific drilling objectives.

3.1 Modification of Tool Geometry

Researchers have investigated various modifications to conventional drilling tools due to their limitations in producing error-free and precise holes. These modifications primarily target the side cutting edge, chisel edge, and overall tool geometry. Fig. 7 provides an overview of recently developed modified drilling tools derived from conventional designs. Notably, modifications to the side edge and sharp chisel edge have shown promising results. However, experimental results indicate that some residual uncut fiber may persist [80]. To achieve multiple cuts with specific angles and distances along the cutting edge, fracture-inducing cuts have been introduced. This technique effectively reduces heat generation at the tool-chip interface, enhances chip evacuation, lowers power consumption, and improves cutting efficiency [81]. Additionally, incorporating secondary cutting-edge grooves creates a multi-groove drill bit that reduces feed force (push force) during drilling. The groove design significantly influences the cutting mechanism and direction of the drill bit [82].

Fig. 7

Modified tool geometry [63,80–84] (Adapted from Refs. 63,80–84 with permission)

The introduction of staged grooves enables more efficient drilling by reducing thrust force. Compared to conventional drill bits, multi-geometry groove drilling tool bits provide enhanced drilling performance due to their ability to facilitate step drilling mechanisms [83]. The second-stage design of multi-geometry grooves gradually increases in width from the starting position, allowing for controlled depth of cut [15,16,83]. Experimental results from Jia et al. [84] confirm that multi-geometry grooves effectively reduce delamination in CFRP drilling, as well as uncut fiber and exit burrs in stacked Ti/CFRP materials. The formation of burrs depends on material properties; for example, titanium produces segmented chips [85], while CFRP contains carbon microparticles. Furthermore, the multi-margin construction of drill bits ensures that hole sizes remain within tolerance limits. Compared to conventional step drill bits, multi-geometry groove modifications have demonstrated a fourfold improvement in the number of high-quality holes produced. These advancements underscore the commitment of researchers to refining tool geometries for CFRP drilling, ultimately enhancing efficiency and reducing errors in hole creation.

4 Analytical Analysis of the Uncut Fiber

In the production of CFRP sheets, multiple groups of carbon fiber threads form plies with specific ply sequences. Achieving a smooth and efficiently drilled hole depends heavily on tool geometry and machining parameters. If these parameters are not optimized, errors such as uncut fiber and delamination may appear, often due to inadequate bottom-ply support [86]. Even with optimized parameters, these issues can persist. When the drilling tool engages the bottom ply of the CFRP sheet, different theories explain the cutting phenomena. Phadnis et al. [87] utilized the Hashin and Puck failure criteria to analyze the cutting failure of unidirectional CFRP sheets. They used Puck’s failure model to describe matrix failure and Hashin’s model for overall failure estimation. The general-purpose finite element program Abaqus/Explicit was employed to simulate hole creation, incorporating these failure models with user-defined material properties. Fig. 3, Phase 3, illustrates the tool’s penetration through the bottom ply.

Fig. 3

CFRP drilling with delamination and uncut fiber [27]; CFRP hole mechanism of the last ply (a) Line diagram of point load through wedge angle, (b) Deflection of last ply, (c) Maximum deflection, (d) Break of the last ply, and (e) Final shape of the last ply with error (Adapted from Ref. 27 with permission)

For failure of the carbon fiber (Hashin’s criteria)

For failure of the epoxy in the matrix (Puck’s criteria),

Where, σ₁₁, σ₂₂, σ₁₃ represent stress tensors, and s₁₁, s₁₂, s₁₃, x₁t, x₂t, x₂c denote shear, tensile, and compressive failure parameters in different planes.

In a study conducted by Xu et al. [88], the cutting of the final ply in a fiber bundle was analyzed after bonding, requiring an analytical approach based on bending moment theory. The researchers derived an equation for a single fiber within the contact region between the tool and the workpiece. This deflected fiber was segmented into three distinct zones for detailed analysis. The first zone begins above contact point A in the tool–fiber region, as illustrated in Fig. 8. The second zone spans from point A to point E, while the third zone lies below point E. The lengths of the three contact zones were calculated using the expressions z < a_c−r_e, z ≤ a_c+ h − Δt and z > a_c + h − Δt, respectively. The study further evaluated the cutting behavior with and without tool tip vibration during the drilling of unidirectional FRP. The results demonstrated that introducing tool vibration significantly reduces cutting forces, minimizes fiber deflection, and promotes cleaner fiber fracture near the tool–fiber contact zone.

Fig. 8

Mechanism of the tool and workpiece contact area and its cutting phenomenon [27,88] (Adapted from Ref. 27 with permission)

Su et al. [47] conducted milling experiments, while Jaiswal et al. [27] focused on drilling. Both studies identified uncut fiber forming in the critical cutting angle region. To define these regions, an analytical equation was established for two cutting scenarios: (i) 0 < Ø ≤ + π/2 and (ii) γ₀+ π/2 < Ø, where Ø represents the fiber orientation angle and γ₀ denotes the tool rake angle. The analysis of this cutting model is based on the behavior of a single fiber, specifically the deflection, bending, and cutting that occur in the last ply before the fiber is cut. The deflection of each fiber element during contact comprises two portions. The first portion involves initial fracture, whereas the expanded crack, as depicted in Fig. 8, represents hogging of one tool edge and sagging of the opposite side, resembling a cantilever beam. The fiber deflection model assumes an initial fracture followed by crack expansion, resembling a cantilever beam. The critical uncut fiber angle is calculated using Eqs. (3) and (4) [27], which are derived based on the bending moment experienced by the fiber.

Where,

In that study, various parameters and equations were considered to analyze the behavior of the CFRP composite during drilling operations. The bending stress of the fiber (σ), the Moment of Inertia (MOI) of a single fiber (I_f), the maximum deflection on one side (L₁), the maximum deflection on the other side (L₂), and the radius of the fiber (r_f) are crucial factors in the analysis. In a certain area of the final ply in the FRP sheet, the uncut fibers exhibit approximately half of the maximum deflection of a single fiber. While the length of an uncut fiber cannot be precisely predicted, it is typically found to be maximum in the middle region. The calculation of the maximum length of the uncut fiber is based on the critical cutting thrust force in the critical cutting angle region.

A review of the literature identifies three primary methods for estimating the cutting thrust force responsible for delamination and uncut fibers:

5 Analysis of the Maximum Uncut Fiber Length

Estimating the uncut fiber length in CFRP drilling poses a significant challenge due to the inherent variability in fiber properties. Even under identical machining parameters and conditions, consecutive holes often display differing lengths of uncut fiber [103,109]. Nonetheless, averaging the uncut fiber lengths across multiple holes can yield a reasonably accurate estimate, typically within a 10–15% error margin. In a study by Jaiswal et al. [27], the maximum uncut fiber length was estimated using Timoshenko’s beam deflection theory. By analyzing the influence of cutting forces, the researchers established a reliable method for predicting the maximum uncut fiber length. This estimation was expressed using Eq. (5), where l represents the deflected length of the deformed fiber, L_lf denotes the length of the fiber deflected by the drill bit, L_df represents the total deflected fiber length, and r is the radius of a single fiber.

6 Estimation of Drilling Hole Quality

Achieving high-quality drilled holes in CFRP typically requires precision equipment such as computer numerical control (CNC) machines or highly accurate, vibration-free drilling setups, along with optimized tools and parameters. During drilling, the applied forces on the CFRP sheet can be measured using a dynamometer, providing real-time data on thrust and torque. Vibration levels and temperatures are commonly monitored using accelerometers or vibration sensors and thermocouples, respectively, as illustrated in the flowchart in Fig. 2. This real-time data acquisition is known as online monitoring. After drilling, hole quality is evaluated through offline monitoring. This involves capturing microscopic images of the drilled holes using digital microscopes and analyzing them with image processing techniques, custom-developed codes, or specialized software. Some studies have incorporated artificial intelligence to improve hole quality assessment and optimize process parameters [126–131]. Offline monitoring is particularly valuable for evaluating defects such as delamination and uncut fibers. By integrating both online and offline monitoring methods, researchers and manufacturers can gain a comprehensive understanding of hole quality, enabling continued advancements in CFRP drilling technologies.

Digital image processing is used to analyze hole characteristics captured via microscopy. Researchers have utilized various image processing techniques, often leveraging algorithms written in programming languages such as Python, MATLAB, and C# [132,133]. The process involves several steps. First, the captured image undergoes preprocessing, where it is converted into a binary format, followed by noise reduction and reflection removal to enhance clarity. Next, the shape contour of the hole is extracted, enabling the calculation of critical parameters such as delamination, uncut fiber length, area, and orientation [134]. Finally, automated defect characterization is performed using techniques introduced by Hrechuk et al. [52], which rely on mathematical assessment and non-destructive evaluation of drilled hole images. Automated contour generation is employed to outline delamination zones and uncut fibers, with the maximum uncut fiber length at the hole exit and the area factor serving as key metrics for assessing hole quality [116,135]. Fig. 9 illustrates the image processing workflow for evaluating uncut fibers and delamination in unidirectional CFRP drilling. The input data includes the microscopic image, applied scale, actual hole diameter, and center coordinates of the hole perimeter [45]. The output consists of segmented images obtained through image linearization, mirror image elimination, delamination measurement, uncut fiber length assessment, and hole contouring [136–138]. Delamination is typically observed along the outer circumference of the hole, while uncut fibers are found on the internal side, both of which are considered critical defects in CFRP drilling. The preprocessing module filters out irrelevant image regions while establishing precise contours for uncut fibers and delamination zones [139]. Contour analysis enables accurate measurement of defect distribution within the hole.

Fig. 9

Image processing for evaluating of uncut fibers and delamination [52] (Adapted from Ref. 52 with permission)

In 2D digital image processing, the quality of drilled holes is assessed by detecting circumferential errors, including delamination and uncut fiber presence. Fig. 10 illustrates the analysis of uncut fiber contours, with measurement errors represented by deviation lines. Factors such as intensity adjustment, image enhancement, brightness control, edge detection, and noise suppression play crucial roles in ensuring accurate image analysis [140–142]. Due to the complex geometric variations in CFRP-drilled hole defects, optimizing image-based quality assessments remains a challenge [32,69,128,143,144]. Ongoing research aims to refine this approach for different CFRP types, correlating cutting parameters and material constraints to optimize drilling outcomes. To enhance production efficiency and avoid wastage caused by delamination defects detected during industrial assembly, researchers have focused on developing predictive models based on historical sensor data. These models correlate real-time sensor readings with predicted delamination severity [121,145].

Fig. 10

(a) hole microscopic image and circumferential profile length distribution of (b) uncut fiber and (c) delamination fiber [52] (Adapted from Ref. 52 with permission)

By establishing such a pattern, production efficiency can be improved, time wastage reduced, and delamination issues that fall outside the tolerance range minimized. This approach offers a systematic and proactive solution, as opposed to relying solely on random inspections during industrial assembly [146–150]. Residual porosity in prepreg fabric plies complicates the delamination process, leading to variations in CFRP workpiece performance. Developing an adaptive model that accounts for porosity-related effects is essential for optimizing CFRP manufacturing.

7 Modification of the CFRP Drilling Process to Minimize Error

In current machining operations, particularly conventional drilling of CFRP materials, various types of errors frequently occur. Researchers have explored alternative and modified processes to minimize or eliminate these errors. Some studies have investigated the use of aluminum and titanium in combination with CFRP, which can reduce errors but may also increase material costs and weight [104,151–154]. Additionally, modified drilling techniques specifically tailored for CFRP materials have been examined. For example, Rubio et al. [155] explored the reduction of drilling errors such as peel-up, push-down, delamination, and uncut fibers by coating the top and bottom plies of CFRP with epoxy. These epoxy coatings provide support to the plies, resisting the compressive forces exerted by the drilling tool on the CFRP surface. While coated plies may still incur defects during drilling, they help protect the CFRP plies, ensuring proper hole formation with minimal errors. Damage intensity and characteristics after CFRP drilling are assessed by examining the entry and exit ply features. The dual coating of the CFRP sheet on both the top and bottom plies is shown in Fig. 11(a). Additionally, Scotch tape can be applied to the top surface of the CFRP sheet to reduce top ply errors such as peel-up, as investigated by Prakash et al. [156]. Their study evaluated the effect of varying Scotch tape ply counts on thrust force and torque, noting that these parameters did not exhibit significant fluctuations, as shown in Fig. 11(b). Changes in the number of Scotch tape plies had minimal impact on thrust force and torque.

Fig. 11

(a) CFRP drilling coating with epoxy ply top and bottom ply [155], (b) the top ply coated with the number of scotch tape [156], (c) CFRP drilling with the attachment of a damper [158], and (d) CFRP drilling with bottom ply supported through wet tissue and ice ply in cryogenic conditions with ultrasonic vibration [51]

Thrust force plays a critical role in CFRP drilling, as it can contribute to drilling errors. When the chisel edge of the drilling tool interacts with the first top ply surface of the CFRP, a substantial force is applied to the workpiece surface. This thrust force can lead to errors such as peel-up, push-down, delamination, and uncut fibers [157]. Wang et al. [158] conducted an experimental study to mitigate the impact of thrust force by using dampers. Four effective dampers were placed beneath the CFRP drilling jig, as shown in Fig. 11(c), to absorb the impact force and stabilize the forces, thereby reducing or eliminating errors in CFRP drilling. Park et al. [51] investigated methods to minimize or eliminate CFRP delamination and uncut fibers by providing temporary bottom support. Their experimental study employed four different setups involving water, ice, wet tissue, and liquid nitrogen, combined with ultrasonic vibration (U/V) CFRP drilling. The setups were as follows: (i) U/V and liquid nitrogen, (ii) wet tissue and liquid nitrogen, (iii) wet tissue, liquid nitrogen, and U/V, and (iv) wet tissue, liquid nitrogen, U/V, and bottom ply support via an ice plate. The results showed that the fourth combination yielded the best outcomes, as shown in Fig. 11(d). This setup involved creating a backup ice ply at the base of the CFRP sheet, followed by U/V treatment to remove uncut fibers. By facilitating bottom support with ice ply and wet tissue, the freezing time of the water was reduced. This fourth combination successfully eliminated all uncut fibers. In CFRP drilling, using different metals, wet tissue, and liquid nitrogen to create a bottom ply supported by ice inside and beneath the drilled hole generates opposing forces against the cutting and thrust forces, allowing for the removal of uncut fibers over the cutting edge. Components at the specimen’s base, instead of using the hybrid cryogenic deburring approach, could be highly beneficial [159,160].

However, when the surface of the CFRP specimen is irregular, it becomes challenging to create a backup ply plate using materials such as aluminum, titanium, steel, or others [127,161–165]. Nevertheless, creating a bottom ply support through the rapid solidification of water to form an ice ply in the hybrid liquid nitrogen deburring process has proven effective. The ice ply on the exit surface generates a reaction force that counteracts the thrust force [119]. The ability to deburr uncut fibers, resulting in a thin, curved, or intricate shape, offers significant advantages in drilling operations.

8 Conclusion

The growing adoption of CFRP materials across diverse industries is largely attributed to their exceptional strength-to-weight ratio and superior mechanical properties. However, machining CFRP-particularly drilling-remains a significant challenge due to common defects such as uncut fibers, delamination, and burr formation. To mitigate these issues, researchers have proposed several effective strategies. Optimizing machining parameters, including cutting speed, feed rate, and tool geometry, has been shown to significantly reduce uncut fiber formation and enhance hole quality. Specialized drill bits designed specifically for CFRP applications have also proven effective in minimizing uncut fiber defects, although they may require customization based on fiber orientation and stacking sequence. Another approach involves bonding CFRP sheets to a rigid metal substrate during drilling, which stabilizes the material, reduces fiber pull-out, and improves dimensional accuracy. Temporary bottom-ply support methods-such as using ice, wet tissue, or liquid nitrogen-have been employed to counteract thrust forces and reduce uncut fiber formation. Additionally, incorporating dampers beneath the drilling setup helps absorb impact forces and maintain stable cutting conditions. Advanced digital image processing techniques are increasingly used for post-process evaluation, offering detailed insights into defect formation and enabling further process optimization. While these developments have significantly improved the understanding and control of CFRP drilling, ongoing research is essential to enhance efficiency, minimize defects, and develop scalable, cost-effective solutions for broader industrial implementation.

Article information Continued