Defect suppression mechanism of ultrasonic vibration-assisted drilling carbon fiber/bismaleimide composite

Shengguo ZHANG, Wenhu WANG, Yifeng XIONG, Bo HUANG,Ruisong JIANG

a Key Laboratory of High Performance Manufacturing for Aero Engine, Ministry of Industry and Information Technology, School of Mechanical Engineering, Northwestern Polytechnical University, Xi’an 710072, China

b Engineering Research Center of Advanced Manufacturing Technology for Aero Engine, Ministry of Education, School of Mechanical Engineering, Northwestern Polytechnical University, Xi’an 710072, China

c School of Mechanical Engineering, Sichuan University, Chengdu 610207, China

KEYWORDS CFRP;Defect suppression;Rod chip;Thrust force;UVAD

Abstract Ultrasonic vibration-assisted drilling (UVAD) has recently been successfully applied in the drilling of carbon fiber reinforced polymer/plastic (CFRP) due to its high reliability.Multiple defects have been observed in the CFRP drilling process which negatively affects the quality of the hole.The carbon fiber/bismaleimide (BMI) composites is an advanced kind of CFRPs with greater strength and heat resistance, having been rapidly applied in lightweight and high temperature resistant structures in the aerospace field.To suppress the defect during the drilling of carbon fiber/BMI composites,it is necessary to comprehensively understand the defect formation and suppression mechanism at different positions.In this study,the defects formation in both conventional drilling(CD)and UVAD were observed and analyzed.The variation trend in the defect factor and thrust force with the spindle speed and feed rate were acquired.The results revealed that the UVAD could significantly enhance the hole’s quality with no delamination and burr.Meanwhile,the defect suppression mechanism and thrust force in UVAD were analyzed and verified,where the method of rod chip removal affected the exit defect formation.In summary, UVAD can be considered a promising and competitive technique for drilling carbon fiber/BMI composites.

1.Introduction

Carbon fiber reinforced polymer/plastic (CFRP) possess a variety of attractive properties, including low density, high stiffness-to-weight ratio and excellent fatigue resistance.They have been used as large-load structural parts in a wide range of contemporary applications, particularly in the aerospace field.1–5In the application of CFRP composites, a great deal of holes need to be mechanically drilled to produce riveted,bolted and screwed joints, which are used to connect CFRP components to other structures.6Nevertheless,CFRP are considered typical difficult-to-machine materials due to their heterogeneous and anisotropic structures, which have resulted in undesired drilling-induced defects during the machined drilling of CFRP, such as matrix fracture, resin-fiber de-bonding,surface irregularities,thermal damage,burr,and delamination.7–10This has led to the poor quality of machined parts and degraded fatigue performance.Consequently, suppressing the defects of CFRP hole is a significant and challenging task,which has been a research focus for decades.

To date, a variety of drill tools have been designed to suppress defects caused by the drilling of CFRP.As shown in Fig.1, the drilling tools include twist drill, step drill, dagger drill, brad & spur drill and core drill.The conventional twist drill is the most commonly used drill,there is a wide chisel edge the in centre of the tool,where the cutting linear speed is extremely low.When drilling CFRP, chisel edge presses the material rather than cutting directly, generating a large thrust force and causing serious delamination defect.11Taso and Hocheng found that with the application of the step drill,the drilling process could be divided into two steps to effectively reduce the impact of the chisel edge, and suppressing the entrance and exit delamination.12The dagger drill is a special double-point angle drill with four straight flutes, and where the small point angle of the secondary edges reams the final surface of the hole.Thus, the thrust force is lower than other drills, and the push out delamination can be reduced.11However,the special tool structure causes burrs to be easily generated at the exit, making it unsuitable for drilling large diameter holes.According to Su et al,13the brad & spur drill has excellent suppression of delamination and burr.Because the drill cuts the last ply of CFRP laminates with a sharp flank cutting edge before pushing the ply away, thereby neatly cutting the fibers alone the hole edge and reducing the thrust force.The core drill is a type of hollow grinding drill with bonded abrasives which machines the hole through abrasive cutting.Hocheng and Taso14found that when drilling using a core drill, the thrust force was lower and distributed around the hole periphery.Meanwhile, the critical thrust force (a thrust force greater than the critical thrust force is considered to form delamination) was lower and the critical thrust force was greater than that of twist drill, step drill and brad & spur drill, thereby suppressing the formation of delamination.15However, the core drill is easily blocked and face the problem of rod chip removal, which weakens the effect of defect suppression.To overcome this problem, a compound core-drill(core drill and step drill) was designed to mitigate the chip removal clog.16But the disadvantage is that the complex structure leads to high cost.

To suppress the defects during drilling, other strategies have been employed to minimize delamination, burr, and so on.Increasing the cutting speed was able to relieve fiber deformation in the affected area before fiber fracture, which could significantly improve the surface quality.17Sorrentino found that the thrust force decreased with the feed rate and the push-out delamination decreased with the thrust force, therefore, the suppression of delamination may be suppressed by employing a lower feed rate.18Backup support is one of the most effective techniques of suppressing exit burr and delamination because it provides a counter force in the opposite direction which contributes to enhancing the critical thrust force.19–21Pereszlai et al.employed tilted helical milling and found that higher tilting angles and lower pitch could reduce the burr factor and cutting force when drilling CFRP holes.22

Recently, ultrasonic vibration-assisted machining has been extensively applied in the field of CFRP machining.23Cong et al.24found that the application of rotary ultrasonic machining could reduce the cutting force, torque and surface roughness.Dong et al.applied robotic rotary ultrasonic to suppress the burr height in CFRP/Al laminates hole drilling.25Liu et al.26used longitudinal-torsional vibrations coupled rotary ultrasonic-assisted drilling CFRP and found that the thrust force reduced by 30% and the damage factor reduced by 69.67% compared to conventional drilling, which effectively improved the hole exit quality.For drilling CFRP with two-dimensional orthogonal fabrics, rotary ultrasonic drilling could be used to speed-up chip removal and reduce the thrust force of the core drill, which provide inadequate energy for delaminated fibers to reach the bundle interface.27Longitudinal-torsional coupled rotary ultrasonic drilling was introduced to improve the hole wall surface quality and suppress tear,burr,and delamination in CFRP due to large thrust force and torque exhibited during conventional drilling.28Geng et al.also successfully extended the rotary ultrasonic elliptical machining (RUEM) method to drill CFRP hole and found that RUEM could effectively suppress push-out delamination and alleviate the influence of the tearing shear effect caused by rod chip removal during delamination propagation.29

Fig.1 Various tools for CFRP drilling.

The current research on the CFRP machining performance is focused on epoxy resin matrix composites, with only a few studies for other types matrix of CFRP drilling.As an advanced resin material, bismaleimide (BMI) resin has higher strength and heat resistance.30However, carbon fiber/BMI composites has shown great brittleness and low interlamination fracture toughness due to its high crosslinking density of compounds and strong rigidity of the molecular chain, which means that delamination and tear are more likely to emerge during drilling.A few research have comprehensively considered the influence of the thrust force and rod chip removal.To further improve the carbon fiber/BMI composites machining quality of UVAD, it is essential to investigate the defect formation and suppression mechanism in UVAD.The purpose of this study is to investigate the effects of ultrasonic vibration on the hole defect suppression mechanism of carbon fiber/BMI composite.After the drilling test, a comparative observation and evaluation of the drilling-induced defects of hole in both UVAD and CD.Subsequently, the effect of ultrasonic vibration and drilling parameters on defect formation during variable speed cutting and rod chip removal was analyzed.Ultimately, the defect suppression in UVAD were proved according to the rod chip removal method and thrust force.

2.Experimental procedures and materials

2.1.Kinematic analysis of UVAD process

As shown in Fig.2(a),the core drill is rotating feed with small amplitude and ultrasonic frequency vibration along axis direction in UVAD.The schematic illustration of CD and UVAD method is shown in Fig.2(b)respectively.In CD,the abrasives have continuous contact with the workpiece and the trajectory of a single abrasive is an isometric helix.During the UVAD process,the trajectory of the abrasives is governed by periodic contact-separate pulsed dynamic cutting composed of the rotation of the core drill,the feed movement of the tool relative to the workpiece along the axial direction,and the high frequency and small amplitude of ultrasonic vibration,and it is a periodic contact-separate pulsed dynamic cutting.According to Fig.2(b), the motion paths of single abrasive in UVAD is a threedimensional spiral curve with sine wave vibration, and the motion trajectory can be calculated as.

where R is the distance of the abrasive relative to the axis of the tool(mm), nsis the spindle speed(r/min),f is the feed rate(μm/r), A and F represent the frequency of the amplitude of vibration (μm) and the ultrasonic vibration frequency (kHz),respectively.In Fig.2(a), vfis the feed speed along axis direction (mm/min).

2.2.Workpiece and tool

The workpiece used in this test, as illustrated in Fig.3, consisted of a 2D plain woven structure and bismaleimides resin matrix.It was composed of 16 plies of fiber reinforced laminates (the thickness of a single ply was 0.125 mm) laid in a designed unidirectional, and a carbon fiber yarn in a woven structure with an orientation of 0°/90°.The weft yarn in the longitudinal direction had more fiber bundles was indicated by the direction ‘1’, and the warp yarn in the transverse direction with less fiber bundles was indicated by the direction ‘2’.The workpiece had a size of 200 mm × 90 mm × 2 mm.The specific properties of the workpiece material are listed in Table 1.

The core drill is fabricated using electroplated abrasives on the end, inner surface and external surfaces of a hollow cylinder, as shown in Fig.4, and the detailed tool parameters are listed in Table 2.

2.3.Experimental conditions and measurement procedure

As shown in Fig.5, a precision three-axis milling machine(CY-VMC850) was selected as the drilling platform with a UVAD unit mounted on its spindle.The ultrasonic amplitude and frequency were measured by the LK-H020 non-contact vibrometer measurement system at the end of the core drill,with an amplitude of 8 μm (peak-to-peak) and a frequency of 20.26 kHz.

Fig.2 Illustration of UVAD process and abrasive trajectory.

Fig.3 Illustration of fiber structures in carbon fiber/BMI composites.

Table 1 Mechanical properties of carbon fiber/BMI composites.

Fig.4 A appearance of core drill.

Table 2 Specifications of core drill.

Fig.5 Experimental setup.

The UVAD and CD methods can be implemented by repeatedly turning on and off the ultrasonic power supply.Dry cutting was adapted during drilling.Table 3 shows the single factor test drilling parameters used with feed speeds of 18 mm/min, 27 mm/min, and 36 mm/min (as shown in Fig.6), the test was repeating 3 times for each group of parameters.

The hole drilling-induced defects occurs not only at the entrance and exit of the hole, but also in the CFRP internal structure due to the laminated structure.The utilization of non-destructive visualization detection technology for the accurate quantification of the internal delamination of CFRPs has proven a challenging task.In the past few decades,optical,X-ray, digital photography, ultrasonic C-scan and other nondestructive testing have been used in the detection of delamination caused by drilling.31–35Low density CFRPs allow X-rays to easily penetrate, leading to distinct contrast resolutions between delamination and intact region.Therefore, according to the defect test requirements,an optical microscope,which is a conventional and economical method, was used for the detection of the entrance and exit defect in CFRP drilling holes(as shown in Fig.7(a)).However, if the optical microscope could not judge whether there was delamination, micro-CT was used to detect possible hidden delamination inside the composite laminates in the holes.

Based delamination factor proposed by Chen which is most used conventional defect quantification method,36using the one-dimensional defect factor Fdto characterize the exit defect,Fdis defined as the ratio of the maximum diameter of thedefect area (Dmax) to the nominal diameter of the hole (Dnom)in two concentric circles,as shown in Fig.7(b).The maximum diameter of the defect area containing delamination, tear, and crack.The formula of Fdis expressed as.

Table 3 Drilling parameters and levels.

Fig.6 Single factor test drilling parameter combination.

Fig.7 Defect detection and evaluation method.

3.Results and discussion

3.1.Comparison of the hole defects in CD and UVAD

3.1.1.Surface quality of the hole wall

To compare the surface quality of the hole wall in CD and UVAD, the micro structures of the hole surface at the 16th ply (near exit) was observed via scanning electron microscope(SEM).In Fig.8(a), the chip wrapped in broken fibers are pressed into the hole wall due to chip piles up in CD, which deteriorates the surface quality.Simultaneously, BMI resin is used as the interlayer adhesive resulted in the anisotropy of the hole wall surface,the region of BMI resin matrix is prominent than other regions.In contrast, the surface is smooth in UVAD (as shown in Fig.8(b)), including the interlaminar adhesive regions.

Fig.8 Comparison of the surface quality in CD and UVAD.

Numerous studies have shown that the angle between the cutting direction and fiber direction in the clockwise direction has a significant influence on the cutting properties of the fiber reinforced composites.11Therefore, the fiber fracture morphology at four typical fiber cutting angles (i.e, θ = 0°, 45°,90°,135°)were observed.In Fig.9,it can be observed that the surface is rough and some fibers are crushed at θ=0°in CD.However, the variable speed abrasive cutting in UVAD does not cause excessive pressure on the fibers and the surface integrity can be improved.Furthermore,at θ=45°,90°,and 135°,the higher cutting speed is beneficial for shear and compression fractures due to the fiber removal mechanism.In summary,the hole wall in UVAD is relatively smooth, the fiber fracture is flat, and the surface quality is significantly better than that of CD.

3.1.2.Defects at the entrance and exit

When drilling composite laminates, the most important drilling stages occur near the entrance and exit of the drill hole.Due to the peeling and push-out effects, these two positions are prone to large-scale defects.Fig.10(a) and (b) show the hole entrance morphologies obtained with ns= 3000 r/min,f= 9 μm/r during the drilling of carbon fiber/BMI composite under CD and UVAD, respectively.It can be observed from Fig.10(a) that the defects including peel-delamination are few due to the unique bit structure of the core drill.However,the hole edge integrity was not high, and there were tiny defects such as tears and cracks in CD.Meanwhile, it can be seen that the hole edge is fairly smooth with few defects in UVAD, and the quality of the hole entrance is much better than that in CD.

Fig.9 Fiber fracture morphologies at surface of hole wall both in CD and UVAD.

Fig.10 Edge morphologies of hole entrance in CD and UVAD.

Compared to the entrance,more defects are observed at the exit of hole where is no support at the exit during drilling.From Fig.11(a),a push-out delamination region exists at hole exit due to the thrust force, based on previous research that high thrust force induces delamination.Nevertheless, there were also tears and burrs at the edge of the hole exit.As show in Fig.11(b), the torque of rotation rod chip generates outplane shear stress on the uncut residual laminate ply.Due to the strength of the fibers in the longitudinal direction is much stronger than that in transverse direction(as shown in Table 1),it was difficult to completely cut off the fibers, and simultaneously pull out the last ply of the rod chip by via rotation,which was the main reason for the formation of the exit burrs.

From Fig.12(a),it was apparent that the defects at the exit reduced significantly in UVAD, particularly delamination.In Fig.12(b),the last laminate ply is not cut entirely,rather than being directly broken before core drill penetrates the ply,showing where the rod chip fell down naturally during drilling rather than blocking core drill in CD.Simultaneously,there is rapid impact on the last ply, which can be shown by the serrated rod chip edge.As shown in Fig.12(c), there is a pushout delamination between the 15th ply and 16th ply,and some transverse cracks occur in the 16th ply.

When the feed rate gets decreased, the exit defects are also reduced.Fig.13(a) and (b) show the exit morphologies and broken rod chips in UVAD at ns= 3000 r/min, f = 6 μm/r.few defects are generated at the exit, and the edge was relatively smooth and flat.Due to the friction of core drill inner wall abrasive, the rod chip is broken into many pieces.Fig.13(c)shows a micro-CT detection section,where the inner laminate had no delamination, and the exit near the 15th and 16th ply exist residue due to rod chip removal.

Fig.11 Hole entrance morphologies and rod chip at ns = 3000 r/min, f = 9 μm/r.

Fig.12 Hole exit morphology and rod chip at ns = 3000 r/min, f = 9 μm/r, F = 20.26 kHz, A = 8 μm.

Fig.13 Hole exit morphologies and rod chip at ns = 3000 r/min, f = 6 μm/r, A = 8 μm, F = 20.26 kHz.

Fig.14 shows the defect factors at the hole exit for a spindle speed at f = 9 μm/r and feed rate at ns= 3000r/min in both CD and UVAD.Each item indicates the average result of three holes, and the error bars represent the obtained data set standard deviation of three holes drilled by the same parameter.According to the definition of the defect factor, no defects occur when Fd= 1.Fig.14(a) shows that both defect factors in CD and UVAD increased with an increasing spindle speed,because increase of the spindle speed cause increase in the core drill cutting and drilling speeds along the axial direction,which would lead to more defects due to the larger thrust force.Moreover, form Fig.14(b), the defect factors at the hole exit with regard a feed rate at ns= 3000 r/min in both CD and UVAD show that increase with feed rate where a high feed rate means a high thrust force.

Comparing Fig.14(a) and (b), it can be concluded that the feed rate has a greater influence on the defect factor than the spindle speed in both CD and UVAD.Therefore, a low feed rate should be employed instead of large cutting to improve the quality of hole.In other words,the best machining parameters to suppress defect at the same drilling efficiency without considering tool wear are a higher spindle speed and low feed rate.Meanwhile, the decrement in the defect factor is 2.2%-5.7% in the test, which indicates that a higher quality hole can be obtain in the industry for UVAD compared to CD.

3.2.Defect suppression mechanism in UVAD

3.2.1.Accelerated cutting of the core drill in UVAD

In UVAD, the instant cutting speed of the core drill had chanced periodically, the equation of instant cutting speed in the X, Y and Z directions (i.e, vx, vy, vz) can be obtained by deriving t using Eq.(1), which can be expressed as.

Fig.14 Drilling hole defect factor at different drilling parameters for both in CD and UVAD.

From Eq.(3), the instant resultant velocity vUVADof the abrasive at any point on the core drill in UVAD can be obtained as.

When the values of A and F are 0, the instant resultant acceleration aCDis a constant value.For example, when the cutting parameter was ns= 3000 r/min, f = 9 μm/r, the vCDand aCDof the abrasive on the outside edge are 56.5 m/min and 1381.7 m/s2.The value of vUVADis 56.5–84.5 m/min,and the maximum value of aUVADcan reach 1.4 × 105m/s2.This indicates that the abrasive with ultrasonic vibration has a higher cutting speed which accelerates cutting.According to Fig.9,a higher cutting speed is beneficial for shear and compression fracture during chip removal at θ = 0°, 45°, 90° and 135°.Fig.15 shows that the path taken by abrasive passed is longer in UVAD for the same time.

The abrasive’s cutting trajectory is related to the spindle speed, feed rate, frequency and amplitude.The abrasive’s cutting path in UVAD was significantly larger than that in CD,the rate of material removal from the hole wall increased,and the hole wall surface quality is improved.The trajectory is denser with a lower feed rate,and a higher spindle speed,frequency and amplitude.The trajectory of a single abrasive at different positions with core drill is also different, which can be expressed using by angle γ:

Fig.15 Schematic of the abrasive 2D-trajectory in both CD and UVAD.

where Riis the distance of the different abrasive positions relative to the axis of the tool.The angle γ is positive when correlated with the feed rate f,and it is negative when correlated with Ri.The γ of the abrasive in the outer wall is smaller than the abrasive in the inner wall,and the cutting path of the abrasive on the hole wall surface is different from the surface of the rod chip.The smaller angle γ means more stable cutting,but the cutting time becomes longer in the case of the same spindle speed.

3.2.2.Defect formation with rod chip removal

The delamination formation during drilling CFRP is caused by the debonding failure between the plies due to the stress between the lower plies exceeds the interlayer bonding strength during drilling.The energy balance equation of material at the crack propagation position can therefore be calculated as37.dUd=dW-dU （8)where dW is the virtual elementary work of the external forces,dU is the strain energy stored when material deforms,and dUdis the variation in the energy absorbed by the delamination crack extension of incremental area.

Geng used a high-speed camera to record the formation of hole exit delamination,where the delamination at the hole exit was not only caused by the thrust force, but also by the tear shearing effect.Due to the weak back support, the last uncut plies were squeezed and pushed toward the exit when the core drill was about to drill through the laminate.29As shown in Fig.16, Model Ⅰvertical stress-induced opening cracks and Model III out-plane shear stress-induces tearing cracks.The push-out delamination caused by Model Ⅰeventually occurs when the value of vertical stress surpasses the bonding strength between two plies.When the rotary rod chip shears the residual fibers on the last ply,the Model III tearing shear fractures generally occur the shear delamination.Therefore, the formation of delamination is related to the two forces acting on the uncut plies,one is the cutting force of the abrasives on the end face of the core drill, Ft.The other is the friction of the abrasive on the cylindrical surface of the core drill between hole wall and rod ship, Ff1, Ff2.

As shown in Fig.17, the torque of the rotating rod chip generated out-plane shear stress on the uncut residual laminate ply.The rod chip wrapped by the core drill does not begin to rotate until most of the carbon fibers had been cut off and the residual fibers could not restrain rod chip.Due to the strength of fibers in the longitudinal direction is stronger than that in the transverse direction (as the Table 1 shown).It is difficult to completely cut off the carbon fibers in the longitudinal direction, and rod chip simultaneously pulled out the last ply of via rotation to generate burr at the exit.BMI is a type of brittle material, the rod chip removal has a tendency for the machined rod to break through the workpiece before the tool reached the end surface of the workpiece.As shown by the edge topography of the rod chip in Section 3.1.2, this resulted in exit tear and crack.Hence,the friction between the abrasives and rod chip is essential inhibiting the defects caused by rod chip rotation.Since the thrust force is distributed around the hole, the sliding friction Ffin CD between the rod chip and abrasives in the drill inner surface can be expressed by the Coulomb friction law29,38, as Eq.(9).

Fig.16 Formation of delamination and tear during core drilling.

Fig.17 Schematic illustration of rod chip rotation and burr formation.

where μCDrepresents the sliding friction coefficient between the rod chip and drill inner surface, qN2is the pressure load between the rod chip and abrasives in the drill inner surface,LTis the length of the cutting edge on the drill inner surface and mg is the gravity of the rod chip, and R0is the rod chip radius.

In UVAD, the abrasives in the inner edge of the core drill reciprocate up and down with trajectory to cut the rod chip surface(shown in Fig.16),where the gap between the core drill and the rod chip is wider.In the meantime,the separated intermittent pulse cutting mode is beneficial for small chips removal.The axial sliding friction exists in the cutting region and changes periodically with the ultrasonic vibration.There was less friction between the drill and rod chip due to the wider gap.According to the relative motion between the abrasives and rod chip,the sliding friction F′fin UVAD can be expressed as Eq.(10).

It has been reported that the sliding friction coefficient μ can diminish with ultrasonic vibration (μCD> μUVAD).38,39The torque MUVADchanges with the acceleration az(az?g)and the tearing shear caused by the rod chip can be significantly suppressed.As shown in Fig.12(a), the delamination decreased in UVAD as the thrust force decreased and the tear was reduced.However,burr and tear still occurred at the same position on the hole exit.Similarly,there is rapid impact on the last ply when vz<0(as Fig.16 illustrated), which accelerated the fracture of the laminate resulting in rod chip ejection.To some extent, the defects has been suppressed in UVAD.

In Fig.13(b),the rod chip is broken into many pieces by the friction of abrasive with ns= 3000 r/min, f = 6 μm/r in UVAD.When the sliding friction in the opposite direction of feed enhanced vertical stress,the internal plies of rod chip were prone to generate delamination cracks,because the tiny size of rod chip owing low interlaminar bonding strength.Meanwhile,angle γ(γ=arctan f/2)was small enough to ensure stable cutting and friction motion time.When m reaches a certain value,the friction is greater than the critical push-out force(opposite of the critical thrust force in the axial direction),and delamination cracks(Mode I)occurs in the interlaminar of rod chip.As shown in Fig.18, the cracks propagate with the cyclical friction motion of abrasives on the rod chip surface,until the sliding friction works make rod chip break.When drilling with a core drill,the thickness of the rod chip increases,which means that the weight of the rod chip (m) increased.The sliding friction with ultrasonic vibration in the Z direction (F′f2) can be expressed as.

During the drilling process, the rod chip has been broken several times.Therefore,the weight(m)of the residual rod chip is smaller after multiple fractures, which makes the torque MUVADof the residual rod chip decrease sharply,thereby suppressing the tear and burr by restricting rod chip rotation.

3.2.3.Thrust force analysis

The thrust force measurement and analysis would assist in comprehending the damage suppression mechanisms.The trajectory of a single abrasive on the core drill in UVAD is described using Eq.(1).During periodic motion, ultrasonic vibration is the main form of abrasive motion,and the periodic motion diagram of the single abrasive is shown in Fig.19.Brittle fracture is the mechanism of material removal in UVAD.Radial,median,and lateral cracks form and propagate during the indentation of CFRP, and the material is then removed from the workpiece when lateral cracks form.40The abrasive on the tool end moved followed a sine-wave-like.During the time Δt, the indentation in the abrasive increases from 0 to δ and then decreases form δ to 0 due to the ultrasonic vibration.Simultaneously, due to the rotation of the tool, the effective cutting distance L of the abrasive increases on the workpiece surface.Therefore, the length and width of the lateral fracture zone also increase from 0 to the maximum, and then decrease to 0.

The indentation depth of the abrasive into workpiece can be calculated by41.

Fig.18 Schematic illustration of rod chip removal.

Fig.19 Calculation of cutting time and fracture zone of single abrasive on CFRP during vibration periods.

where Fiis maximum impact force between tool and workpiece, N; n is the number of active abrasive on the end face of cutting tool; d is the size (diameter) of abrasive, mm; E is the elastic modulus of workpiece material, MPa; v is the Poisson’s ratio of workpiece material.In Fig.19, CLis the lateral crack length; CHis the lateral crack depth; L is the effective cutting distance that a single abrasive travels during effective cutting time, and the cutting time Δt of single abrasive penetrates into the workpiece is.

where Cais the abrasive concentration;ρ is the density of abrasive material,g/mm3,ρ=3.22×10-3g/mm3for SiC;A0is the area of the cutting tool end face, mm2, A0=π（D21-D22/4).The maximum impact force Fsof a single abrasive is.

where Edis the elastic modulus of abrasive grain, MPa.And the cutting force Ftof the abrasive in the end face of the core drill is.

As illustrated in Fig.17,the mean thrust force Fnof UVAD can be expressed as.

As shown in Fig.20, the thrust force in UVAD is lower than that it in CD.The drilling process can be divided into three stages according to thrust force signal: drilling in (StageⅠ),stable drilling(Stage Ⅱ),and drilling out(Stage III).Carbon fiber cutting generates a much higher thrust force than resin cutting because the carbon fiber in CFRP has a higher elasticity modulus,as shown by Eq.(13).The structure of the 16 laminated plies causes continuous thrust force signal fluctuations.Consequently, the cutting of individual ply may be monitored in real time by force variation.

Fig.20 Thrust force curve obtained in both CD and UVAD at ns = 3000 r/min, f = 9 μm/r.

With ns= 3000 r/min, f = 9 μm/r, it would take approximately 6.7 s for the core drill to drill from drilling out, and thrust force can be combined with the machine time for analysis together.In stage I, the 1st and 2nd ply are penetrated,and the thrust force climbs rapidly, resulting in an only one signal variation.In stage Ⅱ,the core drill starts to steadily penetrate the laminates layer-by-layer, and while drilling the 3rd ply,the thrust force is at its highest.Subsequently,due to fewer uncut plies sustaining the thrust force,the workpiece’s support force and thrust force are decreased.As stage III approaches,the thrust force again rises, which is compensated by the fixture,but there is no support at the exit.The signal of the thrust force in stage III is complex.In CD, high thrust force exceeds the internal bond strength, resulting in push-out delamination and a rapid decrease in thrust force.There is only one signal fluctuation in stage III, the ply bents downward and rotation of the rod chip caused the last ply to be torn from the workpiece, and was not directly cut.However, in UVAD, there are two signal fluctuation in stage III, demonstrating that the 16th ply is not entirely cut and pulled off by rod chip rotation.

On the other hand, with ns= 3000 r/min, f = 9 μm/r in UVAD, the 16th ply is completely cut, as evidenced by the 15 signal fluctuations of thrust force in the drilling process(as shown in Fig.21.).Therefore, the core drill can penetrate the 15th and 16th plies at the corresponding time.Additionally, there is no delamination and the broken rod chips prevents rotation, suppressing the tear and burr at exit.In Fig.21, there is a higher thrust force and less machine time when the feed rate is at 12 μm/r.The 16th ply is not cut but torn directly because there are only 14 signal fluctuation of thrust force.High feed rate generates a high thrust force,which is an important factor for the formation of delamination.The last ply is pushed out to form delamination with the rod chip rotates.As a consequence, a low feed rate can not only reduce thrust force to entirely suppress delamination,but also restrict rod chip rotation to fully cut the 16th ply,and suppress tear and burrs.

Fig.22(a) and (b) show the average maximum thrust force(FN) for the three drilling holes with respect to spindle speed and feed rate in both CD and UVAD.It can be seen that UVAD can significantly reduce thrust force.Compared to CD,a decrement of 19.2%-24%is observed in Fig.23(a)and a decrement of 15.3%-26% in Fig.22(b).The decrement of thrust force can be attributed to the intermittent cutting at the end face of core drill in UVAD.

Fig.21 Thrust force curve obtained at ns = 3000 r/min,f = 6 μm/rev and f = 12 μm/r in UVAD.

Fig.22 Thrust force of CFRP hole at different drilling parameters both in CD and UVAD.

At the hole exit, push-out delamination is the primary defect.It has been reported that excessive tool thrust force causes delamination, and the delamination factor has a positive linear relationship with the thrust force29,36.Fig.23 illustrates the relationship between defect factors Fdand thrust forces Fn, for selected values of Fnare not the maximum or the average thrust force, but the instantaneous thrust forces at the exit according to the machine time.

Fig.23 Relationship between defect factors and thrust forces at hole exit.

Table 4 Model coefficient for determining the relation between Fd and Fn.

As shown in Fig.23, the relationship between the defect factor Fdand thrust force Fnat hole exit is linearly positive when there is delamination at the exit.Fdcan be approximately expressed as a function of Fnby Eq.(19).

As shown in Table 4,the coefficients of Eq.(19)were identified via regression analysis.

It can be observed from Fig.23 and Table 4.that the slope for the line of CD is significantly greater than that of UVAD,which means that the defects are less sensitive to thrust force in UVAD, especially delamination.Due to the reduction in the thrust force in UVAD, it can be concluded that there is no delamination, but a certain degree of tear was observed in the optical microscope and CT results.Consequently, there are different types of defects at the hole exit, where delamination is the main type of defect.Single defect evaluation method cannot accurately characterize all defects,it is therefore necessary to use different evaluation methods comprehensively.

4.Conclusions

The main purpose of this study is to investigate the effects of ultrasonic vibration on the hole defect suppression mechanism of carbon fiber/BMI composite.The defects formation and the thrust force in both CD and UVAD were compared.The effect of ultrasonic vibration and drilling parameters on defect formation during variable speed cutting and rod chip removal was analyzed.The rod chip removal method and thrust force demonstrated that UVAD could greatly suppress defects.Based on the analysis of the calculation and experimental results, the following conclusions were drawn.

(1) In UVAD, variable speed cutting is conducive for fiber removal, resulting in a smoother fiber fracture and higher surface quality of the hole wall,and smaller angle γ means more stable cutting.

(2) Observation of the defects showed that the hole exit is the most prone to form defects, including push-out delamination, tear and burr, where delamination is the primary defect.Increases in the spindle speed and feed rate led to a rise in the defect factor.Comparing to CD, the exit defect factor was reduced by 2.2%-5.7%in UVAD.

(3) Exit defect formation is directly affected by rod chip removal methods.UVAD makes rod chips fall off naturally without blocking the core drill.Under low feed rate(f = 6 μm/r) in UVAD, the broken rod chip can suppress the tearing shear and burr caused by rod chip rotation.Enabling the drilling of carbon fiber/BMI composites with no delamination, no burr and few tear,which significantly enhances the hole’s quality.

(4) In UVAD,the defects suppressed due to the sliding friction between core drill and rod chip decreases and restricts rod chip rotation.variable sliding friction would also break the rod chip to further suppress defect.And the thrust force could be employed to predict the size of the exit delamination, and delamination does not occur when the thrust force is less than the critical thrust force.Compared to CD, the thrust force is reduced by 15.3%-26%,and the rise in the spindle speed and feed rate result in an increase in the thrust force.

(5) In UVAD, the exit defects are sensitive to the thrust force, particularly for delamination.Moreover, the defect detection methods and evaluation models have a substantial effect on the test results.Therefore,different types of assessment methods should be used to comprehensively evaluate different defects in carbon fiber/BMI composite drilling.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was co-supported by the Sichuan Science and Technology Program(Grant No.2020YFG0109)and the NSAF of China (Grant No.U1830122).