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Abstract

In modern manufacturing sectors, mechanical drilling of high-strength carbon fiber–reinforced polymer represents the most challenging task as compared to conventional low-strength carbon fiber–reinforced polymer drilling due to the extremely superior mechanical/physical properties involved. The poor machinability of the composite usually results in serious geometric imperfection and physical damage in drilling and hence leads to a large amount of part rejections. In this article, an experimental investigation concerning the cutting-induced damage when drilling high-strength carbon fiber–reinforced polymer laminates was presented. The studied composite specimen was a newly developed high-strength T800S/250F carbon fiber–reinforced polymer composite. A special concentration was made to inspect and characterize the phenomena of various cutting-induced damage promoted in the material drilling. The work focused on the study of the influence of cutting parameters on the distribution and extent of hole damage formation. The experimental results highlighted the most influential factor of feed rate and tool wear in affecting the final extent of induced hole damage when drilling high-strength T800S/250F carbon fiber–reinforced polymer. For minimizing the various damage formation, optimal cutting parameters (high spindle speed and low feed rate) and rigorous control of tool wear should be seriously taken when drilling this material.

Keywords

High-strength carbon fiber–reinforced polymer drilling damage distribution geometric defects hole diameter delamination

Introduction

The emergency of carbon fiber–reinforced polymer (CFRP) has significantly revolutionized the conventional material usage in modern industrial fields. Owing to its superior mechanical/physical properties and flexible structural functions, CFRP has gradually replaced traditional metal materials including aluminum alloy and titanium alloy in various aircraft applications. As an example, currently, CFRP has been fully applied to the primary load-bearing structures of advanced large civil aircrafts like B787, A380 and A350.¹

Prior to its post-application, the CFRP composite is often manufactured in the near-net-shape products in order to achieve dimensional tolerance and assembly requirement. Among the available machining processes, mechanical drilling is the most important and frequently used operation in the industry for finishing composite structures. Previous research work has shown that drilling CFRP laminate represents the most challenging task in modern manufacturing sectors due to the anisotropic and heterogeneous nature of the fiber/matrix system and their relatively poor machinability.^2–6 Special issues may arise from the serious subsurface damage, irreparable inter-ply delamination, rapid tool wear and heat generation.^7–10 Especially, the severe hole damage and defects produced in CFRP drilling could be considered as the key contributor to the large amount of all part rejections during machining. In CFRP drilling, the machined hole surfaces comprise both conventional hole defects as occurred in metal drilling (e.g. hole size error, roundness error, position error and verticality error) and unique damage for composite drilling including delamination, interlaminar cracks and fiber/matrix debonding.^11,12 The machining-induced damage usually results in serious detrimental consequences of the CFRP-made components, for example, structural strength reduction, poor assembly tolerance and in-service performance deterioration.¹³

The drilling-induced damage, however, exhibits strong sensitivity to the used cutting parameters, tool material/geometry, cutting environment and so on and most significantly, the machinability level of the workpiece specimen. The machinability of CFRPs depends greatly on the mechanical/physical properties of the fiber/matrix system, especially the presence type of the reinforcing fibers. Generally, the CFRPs can be classified into three categories, that is, low-strength CFRPs, medium-strength CFRPs and high-strength CFRPs.^5,11 The high-strength CFRPs commonly exhibit much higher mechanical/physical properties than the conventional low-strength CFRPs, for example, much higher tensile strength and tensile modulus, which result in consequently much poor machinability. Due to their superior properties, the applications of high-strength CFRPs are mainly focused on manufacturing the key load-bearing structural components of large commercial aircrafts in modern aerospace industry.⁵

In spite of their wide usage, the research findings concerning the drilling studies of high-strength CFRPs are still significantly lacked. Current investigations of CFRP drilling are primarily focused on the low-strength composite laminates. Rubio et al.¹⁴ investigated the parametric effects on the cutting-induced damage when drilling low-strength HY956/Araldite-M CFRP laminates (tensile strength of 420 MPa and elasticity modulus of 78.2 GPa). Special focuses were made on the study of the influence of high-speed cutting (HSC) on delamination damage. The authors pointed out that the use of HSC could greatly reduce the delamination extent and significantly increase the material removal rate. Krishnamoorthy et al.¹⁵ carried out the delamination analysis when drilling standard low-strength CFRP laminates (reinforcement: standard grade of carbon fiber; matrix: EPON Resin 8132) using response surface methodology (RSM). Results confirmed the predominant role of feed rate in affecting the delamination extent and the favorable effects of high spindle speed and low feed rate on minimizing delamination. Iliescu et al.¹⁶ studied the parametric effects and damage modes when cutting low-strength T300/914 CFRP laminates using different tool geometries and feed rates. The experimental observations revealed that the fiber orientation was a key factor affecting the machined surface quality (surface roughness), and abrasive wear and adhesion wear were the key wear modes governing the tool wear progression. Furthermore, Krishnaraj et al.¹⁷ studied the machinability issues involved in drilling low-strength T300 woven CFRP laminates where different cutting conditions were employed. The influences of cutting parameters on various drilling responses (e.g. thrust force, push-out delamination, hole diameter and hole circularity) were carefully addressed. It was found that the feed rate had significant influences on the thrust force, push-out delamination and hole diameter, while spindle speed was the key factor influencing the circularity of the drilled holes.

In sum, the above reference analysis showed that the previous research focus was primarily made on the conventional low-strength CFRP drilling, and relatively few experimental studies were reported on high-strength CFRP drilling. Since the high-strength composite laminates exhibit extremely poor machinability, revealing the cutting responses and physical phenomena activated in drilling would provide sufficient benefits for both their current and their future industrial applications. On one hand, despite the fact that Xu et al.^5,11 and An et al.^18,19 had attempted to undertake series of fundamental studies on the machining issues when drilling high-strength CFRP, a thorough work concerning the drilling-induced damage assessment and analysis is still significantly lacked. On the other hand, hole-damage formation plays a key role in determining the final CFRP-part acceptance in a real production. Revealing the mechanisms dominating the hole-damage formation and distribution would offer a beneficial guideline for the damage control and elimination and hence maximize the industrial application of the composite. This is the key incentive that motivates this work to make an attempt to clarify the mentioned issues. On this basis, the main objective of this research aims to study the distribution law of the drilling-induced damage and to correlate them with the used cutting conditions when drilling high-strength T800S/250F CFRP. Besides, the effects of cutting parameters on the drilling-induced hole damage extent were also precisely studied, and several key conclusions were drawn from this investigation.

Experimental details

The studied high-strength CFRP was a newly developed T800S/250F laminate fabricated by carbon/epoxy prepreg with a total thickness of 2.6 mm and dimensions of 200 mm × 300 mm. The high-strength T800S/250F CFRP was made of seven layers following the stacking sequence of (45°/90°/135°/0°/135°/90°/45°). The basic composition of T800S/250F and the main properties of the T800S reinforcement are summarized in Tables 1 and 2, respectively. A property comparison is made among the T800S reinforcement and some other reinforcements, that is, T300 and T800H widely used in low-strength and medium-strength CFRPs, respectively. It can be estimated that the tensile strength and tensile modulus of T800S fiber increase approximately by 66.5% and 27.8%, respectively, than that of conventional T300 fiber used in the low-strength CFRP, potentially resulting in much poorer machinability in drilling.

Table 1.

Composition and dimension of T800S/250F CFRP laminates.

Matrix	Reinforcement	Fiber volume fraction	Tow	Thickness
Toray-250F	Toray-T800S	60%	5 µm, 24 K	2.6 mm
Schematic dimension of the drilled CFRP laminate

CFRP: carbon fiber–reinforced polymer.

Table 2.

Comparative mechanical properties among T800S, T300 and T800H reinforcing fibers.

Carbon fiber grade	Tensile strength (MPa)	Tensile modulus (GPa)	Single-beam intensity (g/1000 m)
T800S	5880	294	1030 (12 K)
T300	3530	230	800 (12 K)
T800H	5490	294	445 (12 K)

The drilling trials were performed on a high-speed, five-axis vertical computer numerical control (CNC) machining center DMU 70 V with a spindle range of 20–12,000 r/min and a maximum power of 22.5 kW under dry cutting condition, as shown in Figure 1. A standard carbide twist drill with the diameter of 4.9 mm was utilized. The drill bit has two flutes: a point angle of 118° and a helix angle of 35°. A full-factorial methodology including 25 parametric values (spindle speed (n) of 2000, 4000, 6000, 8000 and 10,000 r/min and feed rate (f) of 0.002, 0.006, 0.010, 0.014 and 0.018 mm/rev) was adopted for the experimental arrangement, as shown in Table 3. It should be noted that the feed rate values were selected lower than conventionally used range reported in the open literature^20,21 because the conventional feed rate range (probably 0.01–0.05 mm/rev) is inappropriate (probably higher) for the high-strength CFRP drilling cases especially from the viewpoint of better cutting-parameter effect inspection and excellent machined hole-quality pursuit. The selected cutting-parameter values were adopted based on the optimization of the authors’ previous research work^5,11 in order to better inspect the parametric effects on the induced damage when drilling high-strength T800S/250F CFRP.

Figure 1.

Experimental setup for high-strength T800S/250F CFRP drilling.

Table 3.

Summary of the used cutting parameters in this study.

Cutting parameters	Values
Spindle speed, n (r/min)	2000, 4000, 6000, 8000 and 10,000
Feed rate, f (mm/rev)	0.002, 0.006, 0.010, 0.014 and 0.018

With respect to the drilling-result measurement, the hole-exit surface morphologies were recorded by using a Keyence digital microscope with ×30 magnification and 1-µm resolution. Hole diameters were measured on the circumference of the holes at 1 mm from the upper (onset hole) and lower (exit hole) free surface, respectively, by using a Talyrond 300 instrument equipped with a 3-mm diameter balls stylus. The measurements were conducted by palpating 12 points, with a probe of 3-mm diameter. In addition, scanning electron microscope (SEM) analyses (JSM 7600F mode-Japan) were also utilized to facilitate the distribution characterization of the drilling-induced hole damage. Prior to the SEM shooting, the acetone cleaning treatment and spraying gold on the sample hole surfaces were also conducted in order to improve the imaging quality of the damage morphologies. During the drilling process, tool flank wear was measured several times by built-in software in a Nikon toolmaker’s microscope according to the ISO 3685 standard (1993) with average flank wear width (VB) used for carbide tools. Each measurement was replicated at least three times to ensure the sufficient credibility of the obtained results.

Results and discussion

Hole-damage distribution and characterization

In composite machining, the cutting-induced damage is often characterized by the extent of geometric imperfections, thermal injuries and physical damage. The key mechanisms controlling the damage formation can be attributed to the brittleness and heterogeneity of the fiber/matrix system and the specific brittle-fracture chip-separation modes governing machining. In drilling cases, the changeable fiber breaking mode versus fiber cutting angle (χ) (the intersection angle between the fiber orientation (θ) and the cutting speed direction (v_c)) is also a key contributor to making the local defect distribution, for example, fiber pullout, delamination, and tearing and exhibiting regional symmetrical characteristic. Figure 2 shows a global view of the drilled hole wall surface under the cutting conditions of n = 2000 r/min and f = 0.002 mm/rev. The micro observations indicated that the examined internal hole area was mainly composed of the outcropping fiber surface (the roughness region) and the coated resin surface (the smooth region). Severe fiber pullout, inter-ply delamination, resin loss and micro-crack were detected on the drilled hole wall surface, and more pronouncedly, focused on the hole-exit side. This was because the uncut composite layers in the hole-exit side usually exhibited weak rigidity owing to the progressively thinner material layer governing the chip removal process. In such case, the uncut fibers would exhibit strong recession to the drill cutting edges, which resulted in various carbon fibers prevented from cutting off and consequently severe subsurface damage formation, as depicted in the magnified view of region A in Figure 2. In contrast, the middle region of the hole inside surface primarily consisted of the coated resin surface and less outcropping fiber surface.

Figure 2.

Global observation of hole wall surface under the cutting conditions of n = 2000 r/min and f = 0.002 mm/rev.

In order to specifically clarify the defects and imperfections generated in T800S/250 drilling and to correlate them with the used cutting parameters, special investigations were made on the macroscopic observation of the hole-exit side (the fiber orientation of hole-exit layer was 45°; as presented in Figures 3 and 4). In Figures 5 and 6, damage extent has been represented as a function of input cutting parameters and the fiber cutting angle (χ). As can be seen in Figures 5 and 6, the damaged peripheries were signified as the red shaded areas, while non-shaded areas denoted the straight cut fibers and the areas absence of defect.

Figure 3.

Macroscopic observation of hole-exit surface versus feed rate (f) (n = 10,000 r/min) (×30).

Figure 4.

Macroscopic observation of hole-exit surface versus spindle speed (n) (f = 0.014 mm/rev) (×30).

Figure 5.

Defect distribution at hole-exit surface versus feed rate (f) (n = 10,000 r/min).

Figure 6.

Defect distribution at hole-exit surface versus spindle speed (n) (f = 0.014 mm/rev).

As depicted in Figure 3, severe overhanging fibers, tearing and delamination were noticeably produced around the hole circumference, especially under the higher feed rate conditions. The damage distribution typically exhibited symmetrical regional characteristic on the hole periphery, which might be caused due to the fiber-orientation symmetry relatively to the hole center. In addition, the feed rate was confirmed to have intense effects on the formation of the drilling-induced damage (overhanging fibers) by evidence that a small increase in feed rate typically promoted a larger occurrence of overhanging fibers concentrated on offset hole periphery. Such phenomenon apparently indicated that serious hole damage would be produced in drilling action when higher feed rate was employed. The key mechanism controlling the phenomenon could be explained by the fact that when feed rate increased, the drill bit was required to cut off more chip volume per revolution, which consequently resulted in higher cutting resistance during the material removal process. As such, a large bundle of carbon fibers would survive from being cutting off due to the enhanced fiber recession to the drill cutting edges.

Moreover, as illustrated in the quantitative analyses in Figure 5, the occurrence of the hole-exit damage primarily dominated on areas where the angular positions were obtuse. The reason could be attributed to the anisotropic machinability of the CFRP laminate related to the changeable chip-separation modes versus χ during the circumferential cutting. Specifically, in obtuse fiber cutting direction (90° < χ < 180°), for example, χ = 135°, the chip separation occurs through severe compressive crushing coupled by extensive degree of out-of-plane shear with the tool advancement, which then promotes severe extent of sub-layer damage beneath the cutting plane, as interpreted in Figure 7(a).²² In contrast, when χ becomes an acute angle (0° < χ < 90°), for example, χ = 45°, the key chip removal mechanism is governed by compressive shear and interfacial shear as shown in Figure 7(b).²² In such case, cracks appear in fibers above and below the cutting plane. Then, the interfacial shear takes place at the end of the primary shear fracture and propagates toward the fiber orientation direction until the complete chip separation, which results in a little bit smooth machined surface as compared to the cases under obtuse χ angles.

Figure 7.

Schematic illustration of the fiber-separation modes when cutting composite laminates versus χ: (a) χ = 135° and (b) χ = 45°.²²

Furthermore, the increased red shaded areas (defect zone) in terms of feed rate also strongly demonstrated the positive impact of the feed rate on the hole-damage generation. It could be observed that when f was elevated, the red shaded area experienced gradual evolution with a special focus on the direction of χ = 90°. Figures 4 and 6 illustrate the influences of spindle speed on hole-exit damage under the fixed feed rate of 0.014 mm/rev. It was apparent that, under lower spindle speed (e.g. n = 2000 r/min), severe defects were produced focused on the hole circumference. When spindle speed was elevated, the damage extent decreased slightly until the highest spindle speed (n = 10,000 r/min) was utilized. The phenomenon implied that the high-speed drilling might facilitate the reduction in hole-exit damage extent when drilling high-strength T800S/250F CFRP.

Through the above analysis, it can be concluded that the feed rate was the predominant factor that significantly influenced the damage extent when drilling high-strength T800S/250F CFRP, while the effect of spindle speed was minor and slight. The defects produced in high-strength CFRP drilling mainly took place around the obtuse fiber cutting angle (χ). The anisotropic machinability and the changeable chip-separation modes versus χ were the key contributors responsible for the distribution law of the drilling-induced damage on the hole circumference.

Parametric effects on drilling-induced damage

In CFRP drilling, the induced damage usually exhibits strong sensitivity to the input parameters, for example, spindle speed (cutting speed), feed rate and tool wear (drilled hole number). The analyses and studies of parametric effects on hole damage can provide a beneficial role in further parameter selection and optimization when drilling high-strength CFRP. In this section, particular concentrations were made on the investigations of hole-onset/exit diameters, overhanging defect analyses and delamination damage versus used parameters.

The evolution of diameters measured at both hole onset (Φ_o) and hole exit (Φ_e), as a function of feed rate (f), for various spindle speeds (n) is presented in Figures 8 and 9, respectively. It was noticeable that all the produced hole-onset/exit diameters (Φ_o and Φ_e) were lower than the nominal hole diameters (Φ_n) of 4.9 mm, irrespective of the used cutting parameters. The phenomena, however, could be attributed to two key factors, that is, the bouncing-back effects of uncut-off fibers and the unavoidable tool wear, which significantly influenced the drilling process. The bouncing-back effects pointed out by Wang and Zhang²³ revealed that when cutting laminated composites, the uncut-off fibers existing on the machined surfaces would recover back elastically when the cutting tool passed through the region, which would make the real cutting depth lower than the nominal one (i.e. the real hole diameter lower than the nominal diameter in drilling cases). Another important factor was the tool wear. This was because during the drilling process, the fresh tool inevitably suffered gradual wear progression dominating the chip removal process. Specifically, the tool wear exacted on the drill-edge margins made the tool diameter much smaller. As a result, the finally produced hole diameter became smaller than the nominally generated one. Moreover, when the feed rate became high, the hole-exit diameters exhibited slightly lower than those at the onset sides. This was because in drilling the last piles were easy to be bent and delamination was accentuated. Hence, the phenomenon promoted the occurrence of uncut-off fibers which disturbed the measurements. In addition, through the comparison between Figures 8 and 9, it can be concluded that with the elevation of feed rate, all the produced hole diameters deviated far away from the nominal hole diameter (Φ_n). The increase in feed rate absolutely gave rise to the noticeable decrease in hole diameters, irrespective of Φ_o and Φ_e. The phenomena could be associated with the enhanced bouncing-back effects and accelerated tool wear when feed rate was elevated during drilling high-strength T800S/250F CFRP. In contrast, as depicted in Figures 8 and 9, the drilled hole diameters (both Φ_o and Φ_e) suffered slight increase when spindle speed was elevated, which emphasized the favorable effects of high-speed drilling on aiming to produce nominal hole diameters. The reason could be ascribed to the fact that HSC facilitated the reduction in hole-onset/exit damage (especially the overhanging fibers) during the drilling process. Through the hole diameter analysis, it can be confirmed that for producing accurate hole diameter, cutting parameters consisting of lower feed rate and higher spindle speed should be preferentially adopted.

Figure 8.

Influence of cutting parameters (n and f) on hole-onset diameter (Φ_o).

Figure 9.

Influence of cutting parameters (n and f) on hole-exit diameter (Φ_e).

As discussed above, in high-strength T800S/250F CFRP drilling, typically serious hole defects (damage) were generated focused on the hole circumference. To objectively evaluate the real extent of hole-exit defects (especially the overhanging fibers), a new indicator, namely, the burr area (A), was introduced for the inspection in order to exclude the error effect of a single maximum uncut-off fiber length. Figure 10 then shows the variations of burr area (A) and hole-exit morphologies in terms of tool wear under the fixed cutting conditions of n = 10,000 r/min and f = 0.01 mm/rev. In addition, Table 4 also summarizes the drilled hole number (N) in accordance with the VB values in Figure 10. The tool wear was measured as the flank wear land width (VB) by adopting average flank wear land width, VB = 0.25 mm, as the wear criterion of the used drills.

Figure 10.

Effect of tool wear (VB) on burr area (A) and hole-exit morphologies (n = 10,000 r/min and f = 0.01 mm/rev).

Table 4.

Drilled hole number (N) in accordance with the VB values in Figure 10.

	VB = 0.012 mm	VB = 0.151 mm	VB = 0.250 mm	VB = 0.306 mm
Drilled hole number (N)	3	29	83	96

VB: average flank wear land width.

As shown in Figure 10, globally the tool wear exhibited significant effects on the produced exit-hole quality, that is, accelerated tool wear promoted dramatically decreased exit-hole quality. The burr area (A) basically showed a linearly increasing trend with slow evolution rate before VB exceeding 0.25 mm, which referred to the wear criterion of the used drill bit. As an example, when VB was low, for example, VB = 0.01 mm, the drill bit generated near-net hole surface nearly without the appearance of overhanging fibers, and the accurate aperture roundness was obtained. However, with the advances of the ongoing drilling process, the exit-hole morphologies were deteriorated significantly, especially when VB exceeded 0.15 mm. Meanwhile, serious tearing-shaped fibers became pronounced concerning the hole circumference of obtuse χ angles. With the further exacerbated tool wear progression (i.e. VB = 0.25 mm), the drill cutting edges would then become blunt completely, which led to poor edge-forming precision dominating the drilling operation. The key manifestations of the exit-hole morphologies were degraded hole roundness and oval-shaped aperture. Moreover, when VB reached over 0.30 mm, the twist drill failed completely and lost its cutting ability fully due to excessive drill flank wear over the tool wear criterion (VB = 0.25 mm). As such, the exit overhanging fibers experienced substantial increase, and the worst hole surface was produced. In such condition, the burr area (A) also reached its maximum value of 5.897 mm² simultaneously. Through the above analysis, the tool wear was confirmed to have a pivotal role in affecting the finally generated hole-surface quality. For minimizing the destructive effects arising from tool wear, strict control should be put on the monitoring of drill wear during drilling high-strength T800S/250F CFRP in order to achieve excellent hole-surface quality.

With regard to the delamination damage, it is always identified as the most critical mode of failure in composite drilling, which accounts for an estimated 60% of all part rejections.^11,24 Delamination is an interlaminar and inter-layer failure mode in laminated composite materials that may arise due to the low resistance of the thin resin-rich interface existing between adjacent layers, under the action of impacts, transversal loads or free-edge stresses. Besides, since there exist two types of delamination damage, that is, the peel-up delamination and the push-out delamination, in laminated composite drilling, and among which, the push-out delamination is always considered as the most serious one as compared to its counterpart (peel-up delamination).^25,26 On this basis, the following study was performed solely on the analysis of the push-out delamination. To characterize the extent of delamination damage, the most-used one-dimensional factor (F_d) was adopted in this study, which is defined as the ratio of the maximum diameter (D_max) of the delamination area to the hole nominal diameter (D_nom), as illustrated in equation (1)²⁷ and shown in Figure 11, respectively.

F_{d} = \frac{D_{m a x}}{D_{n o m}}

(1)

Figure 11.

Schematic diagram showing the measurement of the delamination factor (F_d).

Figure 12 depicts the evolution of the calculated delamination factor (F_d) in terms of spindle speed (n) and feed rate (f). Results showed that the feed rate usually exhibited pronounced effects on the delamination formation in such manner that a slight increase in feed rate usually gave rise to higher delamination factor (F_d). In contrast, the impact of the spindle speed was slight and probably negative, that is, the increased n led to decreased push-out F_d. Such findings also agreed well with the experimental observation of Karnik et al.²⁸ when drilling CFRP laminates. The key mechanisms controlling the parametric effects on delamination extent could be associated with the variation of thrust forces generated in drilling. It was believed by some researchers^29–31 that the thrust force was the key factor that directly contributed to the formation of delamination damage when drilling laminated composites. The delamination took place mainly when the generated thrust force exceeded the critical thrust force (CTF) of the used composite-tool configuration, and its extent depended heavily on the gap between the actual thrust force and CTF. Furthermore, as revealed in our previous research work,^5,11 the feed rate was confirmed to have remarkable effects on the generated thrust forces when drilling T800S/250F CFRP, while the influence of spindle speed was negative. In such case, the elevation of feed rate would inevitably promote highly increased thrust force and hence lead to accelerated delamination damage. Analogously, an increase in spindle speed favored the reduction in the thrust force and thereby resulted in the decrease in delamination extent.

Figure 12.

Effect of spindle speed (n) and feed rate (f) on push-out delamination factor (F_d) when drilling high-strength T800S/250F CFRP.

Conclusion

In this work, one type of high-strength CFRP (T800S/250F) was precisely investigated with a special concentration on the drilling-induced hole damage. Based on the results acquired, some key results can be drawn as follows:

In high-strength T800S/250F drilling, severe cutting-induced damage including fiber pullout, delamination and micro-crack were detected on the hole wall surface. The specific studies focused on the hole-exit damage analysis demonstrated that the damage formation (especially the overhanging fibers) depended greatly on the anisotropic machinability of the CFRP laminate relative to the changeable chip-separation modes versus χ during the circumferential cutting. In addition, the occurrence of hole damage primarily existed on the obtuse χ angles due to the poor brittle-fracture chip removal mechanism governing the case drilling. Moreover, the hole-damage distribution commonly exhibited symmetrical regional characteristic around the hole periphery due to the symmetry feature of fiber orientation relative to the hole center.

The key mechanisms controlling the variation of hole diameter could be attributed to the bouncing-back effects from uncut-off carbon fibers existing on the machined hole wall surfaces and the unavoidable tool wear generated in the drilling process. The parametric investigations confirmed the significant role of tool wear in affecting the final exit-hole quality. To minimize the exit-hole damage, and in order to achieve excellent aperture accuracy, particular attention should be paid to the controlling and monitoring of the tool wear when drilling high-strength T800S/250F CFRP. In addition, the results also proved the predominant effect of feed rate on push-out delamination formation and the beneficial role of HSC in reducing the delamination extent.

The global hole-damage studies demonstrated the significant role of feed rate in affecting the cutting-induced damage extent and the negative impact of spindle speed on T800S/250F drilling. For minimizing the drilling-induced damage, optimal cutting parameters in terms of lower feed rate and higher spindle speed should be adopted when drilling high-strength T800S/250F CFRP.

Based on the examined parametric range, the optimum spindle speed and feed rate for drilling high-strength T800S/250F CFRP were found to be 10,000 r/min and 0.002 mm/rev, respectively.

In this article, a preliminary study focused on the cutting-induced damage when drilling high-strength T800S/250F CFRP was performed by using one standard drill bit. However, the drill type (core drill, step drill, etc.), drill geometry (point angle, helix angle, etc.) and tool material (various coating constituents) also have significant influences on the induced damage extent of the high-strength CFRP. In the future, several systematic studies concerning the evaluation of various types of drill bits and also the cutting-parameter optimization for high-strength T800S/250F CFRP drilling will be precisely performed in order to greatly improve the machinability of the composite material.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research work is supported by the China Scholarship Council (CSC; Contract No. 201306230091) and the National Natural Science Foundation of China (No. 51475298 and No. 51105253).

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An experimental investigation on cutting-induced damage when drilling high-strength T800S/250F carbon fiber–reinforced polymer

Abstract

Keywords

Introduction

Experimental details

Results and discussion

Hole-damage distribution and characterization

Parametric effects on drilling-induced damage

Conclusion

Footnotes

Declaration of Conflicting Interests

Funding

References