istet

CChi

Unidirectional carbon ber reinforcedpolymer (UD-CFRP)Orthogonal cuttingAnisotropyCutting forceSpecic cutting energy

al cear

laminates, and investigated machining mechanism by studying cutting mechanics characteristics in cut-

anced

Generally, large CFRP components can be manufactured directlythrough material molding. But in order to allow the CFRP compo-nents to meet the geometric dimension, shape accuracy and sur-face quality required for nal components, secondary machiningis often needed after the material molding, and common process-ing methods include edge trimming and hole machining.

ting cutting forceuality. Thegime durinasy forma

machining defects in terms of material composition and comproperties.

Studies on machining CFRPs mainly focus on the defects,force and tool performance when operations such as drilling,milling and trimming are involved [611]. These machining char-acteristics are always related to the material removal mechanismduring the cutting process. To investigate the cutting mechanismof CFRP, orthogonal cutting of unidirectional carbon ber rein-forced polymer (UD-CFRP) laminate is often used to observe the Corresponding author. Tel.: +86 21 34206824; fax: +86 21 34206317.

E-mail address: qlan@sjtu.edu.cn (Q. An).

Composite Structures 131 (2015) 374383

Contents lists availab

Composite S

sevload-bearing components, fuselage, wing and other large primaryload-bearing structures [3,4]. As an example, currently, CFRP hasbeen fully applied to the large primary load-bearing structures ofadvanced large civil aircrafts like B787, A380, A350 and A400M.

force inside each layer, and signicantly uctuaoverall, ultimately affecting the machining qCFRPs bring about complex material removal reting process, relatively poor machinability and ehttp://dx.doi.org/10.1016/j.compstruct.2015.05.0350263-8223/ 2015 Elsevier Ltd. All rights reserved.refore,g cut-tion ofposite

cuttingmaterial, carbon ber reinforced polymer (CFRP) has been usedin the eld of aviation manufacturing since the 1970s owing toits advantages such as high specic strength, high specic stiffness,corrosion resistance and strong designability [1,2]. CFRP has grad-ually replaced traditional metal materials like aluminum alloy andhigh-strength steel to become an important aviation structuralmaterial, such as bearing fairing, empennage-level secondary

on machining of CFRP materials is mainly manifested by signicantdirectionality. For example, inter-layer bonding strength of CFRPlaminates is only 520% of its tensile strength along the ber direc-tion, which will easily lead to interlayer delamination under theaction of cutting force. Besides, during CFRP cutting process, mate-rial removal mechanism differs completely at different ber orien-tation angle conditions, which results in anisotropic internal actingMachinability

1. Introduction

As the most representative advting process, in order to provide the basis for improving their machinability. Experimental results showedthat the cutting force and specic cutting energy of T700 and T800 UD-CFRP laminates were all signi-cantly directional; and at the conditions of same ber orientation angle, greater cutting force and speciccutting energy were found for cutting of T800 UD-CFRP than that for cutting of T700. As the cutting speedincreased, main cutting force and radial thrust force both decreased and specic energy map shrankrapidly, indicating reduced cutting energy consumption, and improved machinability of CFRPs. Withthe increase in cutting depth, main cutting force and radial thrust force both exhibited increasing trends,but specic cutting energy map narrowed, indicating improved machinability.

2015 Elsevier Ltd. All rights reserved.

resin-based composite

The characteristics of anisotropy and heterogeneity for CFRPmake its machining process differ from metal materials [5], whichalso make the machining face new challenges. Effect of anisotropyKeywords:also ever increasing. Owing to its prominent anisotropy and heterogeneity, UD-CFRP laminate has ratherpoor machinability. This paper conducted orthogonal cutting tests on T700/800 high-strength UD-CFRPStudy on the cutting mechanics characterlaminates based on orthogonal cutting m

Qinglong An a,, Weiwei Ming a, Xiaojiang Cai b, Minga School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PRb Shanghai Aerospace Control Technology Institute, Shanghai 201109, PR China

a r t i c l e i n f o

Article history:Available online 16 May 2015

a b s t r a c t

High-strength unidirectionmaterial for primary load-b

journal homepage: www.elics of high-strength UD-CFRPhod

hen a

na

arbon ber reinforced polymer (UD-CFRP) has gradually become one majoring structural components of aircrafts, and related machining demands are

le at ScienceDirect

tructures

ier .com/locate /compstruct

fracture and separation forms of carbon bers, elasticoplastic stable. Then, he works out that cutting force increases gradually

Nomenclature

ac cutting depthCFRP carbon ber reinforced polymerCVD chemical vapor depositionUD-CFRP unidirectional carbon ber reinforced polymerFc main cutting forceFp radial thrust forceFr resultant cutting force

re rounded edge radiust cutting widthu specic cutting energyvc cutting speeda clearance anglec0 rake angleh ber orientation angle

Q. An et al. / Composite Structures 131 (2015) 374383 375deformation of resin matrices, as well as chip formation duringUD-CFRP cutting process [1214]. Fiber orientation angle h is anintersection angle between the direction of cutting speed and thedirection of bers on non-cut layers, which is considered as themost inuential factor in CFRP cutting process. As shown inFig. 1, Sreejith et al. [13] divided the material removal mechanismduring orthogonal cutting of CFRP into three categories: (1) layeredpeeling fracture mechanism when the direction of cutting speed isconsistent with the ber direction; (2) extrusion shear fracturemechanismwhen the direction of cutting speed is in an acute anglewith the ber direction; and (3) bending shear fracture mechanismwhen the direction of cutting speed is in an obtuse angle with theber direction. High strength and modulus of bers, as well as verypoor cutting performance could easily lead to fracture or severewear of tools. As the standards-setter for aerospace grade carbonber materials, tensile strengths at 0 direction of Torays carbonbers range from 3.5 Gpa for baseline T300 ones to nearly7.0 Gpa for T800, T1000 high-strength ones. The cutting perfor-mance will differ greatly, because the strength difference betweendifferent types of enhanced carbon bers can double.

During the cutting process, cutting force is the root of a varietyof physical phenomena. Cutting force generates cutting heat, whilethe force and heat jointly inuence the machined surface quality.Therefore, to study the cutting mechanism of a new CFRP material,the mechanics behaviors in cutting process must be studied rst,which is also the basis of cutting tool design and CFRP moldingfor machining process. Wang et al. [14,15] systematically studiedthe material removal mechanism of CFRP unidirectional laminatesbased on orthogonal cutting, compares main cutting force andradial thrust force at four typical ber orientation angles, 0/45/90/135, through orthogonal cutting tests, and nds signicantdifference in cutting force of UD-CFRP laminates under differentdirections. Cutting forces in 0 and 135 directions uctuate sub-stantially, while those in 45 and 90 directions are relatively(a) Layered peeling fracture (b) Extrusion sh

Fig. 1. Material removal mechanismwith the increase in ber orientation angle based on the compar-ison of average force. Zhang et al. [16,17] constructtwo-dimensional orthogonal cutting force model of CFRP with berorientation angle less than 90 on the basis of orthogonal cuttingtests, as shown in Fig. 2. According to the prediction results ofthe model, main cutting force basically shows a linear proportionalrise with the increase in ber orientation angle, reaching its max-imum at 90 direction; radial thrust force, on the other hand,reaches its maximum at 30 direction, then drops rapidly withthe increase in ber orientation angle, and reaches its minimumat 90 direction. The experimental results also show that 90 is acritical angle, beyond which severe subsurface damages will occurand the deformation mechanisms in the cutting zone will change.Arola et al. [18,19] studies the orthogonal cutting process of CFRPusing cutting tools with a combination of different rake and clear-ance angles taking into account factors such as tool parameters,workpiece material directionality and cutting parameters.Research results indicated the inuences of these factors onmechanical behaviors and chip formation during cutting process,in a descending order, as follows: direction of material itself, toolgeometry and cutting parameters. Zhang [20,21] proposed ahypothesis that fractures of CFRP occur under the action of toolcutting edge are all due to shearing stress in the direction perpen-dicular to ber exceeds the shear strength, and derived a simpliedCFRP orthogonal cutting force model under ber direction condi-tions. Qi et al. [22] established a force prediction model for orthog-onal cutting of UD-CFRP by taking slipping, peeling, and boundingmechanism in three different deformation areas into consideration.Karpat et al. [23,24] proposed a mechanistic cutting force modelfor milling and drilling of UD-CFRPs based on experimentally col-lected cutting force data using two different polycrystalline dia-mond cutters. The mechanistic model is also shown to becapable of predicting cutting forces during milling and drilling ofmultidirectional CFRP laminates. The experimental milling forceear fracture (c) Bending shear fracture

of CFRP in cutting process [13].

the experiment had a rake angle of 25, clearance angle of 15,and a rounded edge radius of 15 lm, as shown in Fig. 4.

2.2. Experimental conditions

Schematic diagram and test-site photo of orthogonal cuttingtests of CFRP unidirectional laminates are shown in Figs. 5 and 6,

376 Q. An et al. / Composite Structures 131 (2015) 374383measurements and predicted milling forces agree well with eachother.

In summary, the signicant directionality of mechanical behav-iors during CFRP cutting process has gained the most attention.Research ndings in the literature are mainly concentrated onthe processing along the ber direction; moreover, explanationson cutting mechanical behaviors of CFRP under continuous varia-tion of ber orientation angle within a 0180 range still diverge.This paper will adopt orthogonal cutting tests to study the inu-ences of cutting parameters on mechanical behavioral characteris-tics like cutting force and specic cutting energy during UD-CFRPmaterial removal process.

2. Experimental procedures

Orthogonal cutting method used in this paper is a basic exper-imental approach commonly used in the research of cutting mech-anisms of materials [25]. During cutting process, only one maincutting edge is involved, which is perpendicular to the directionof cutting speed. So it can simply and intuitively reect the inter-action between workpiece and cutting edge.

2.1. Workpiece materials and cutting tool

Workpiece materials used in this paper were T700/LT-03A and

Fig. 2. A schematic of the two-dimensional orthogonal cutting of UD-CFRP [16].T800/X850 CFRP unidirectional laminates, which are isotropicallylayered (all 0 direction) with 40 layers and thickness of 5 mm,and 32 layers and thickness of 6.08 mm, respectively. Carbon bervolume fractions of both materials were 60%; relevant mechanicalproperties are shown in Table 1.

In the orthogonal cutting tests, CFRP laminates were cut intosmall sheets with dimension of 25 mm 30 mm, and orthogonalcutting tests were performed on unidirectional laminates withber orientation angles of 0, 15, 30, 45, 60, 75, 90, 105,120, 135, 150 and 165, respectively, obtained by cutting 0CFRP laminates in different directions, see Fig. 3. Chemical vapordeposition (CVD) diamond-coated carbide cutting tool used in

Table 1Mechanical properties of unidirectional CFRP laminates.

Material Tensile strength(MPa)

Tensile modulus(GPa)

Compressivestrength (MPa)

Compressmodulus (

T700 2450 125 1430 T800 2840 168 1570 145

Note: Unspecied mechanical property parameters all represent the properties in 0 dirrespectively. It can be seen from Fig. 5 that the orthogonal cuttingwas achieved by cutting CFRP laminates at different ber orienta-tion angles. The experiments were completed on aKENT-KGS-1020AH surface grinder, where the grinding wheelwas replaced by a specially designed ywheel installed with cut-ting tool. The working table of surface grinder remained xed,and orthogonal cutting motion was accomplished by spindle rota-tion; besides, cutting depth was achieved by Z direction feedmotion. Cutting forces were collected with a Kistler-9272dynamometer, the charge signals obtained by the dynamometerwere amplied through Kistler-5017B amplier; nally, acquisi-tion of cutting forces was completed using computer signal acqui-sition system. Single-factor experimental method was adopted.Cutting speed vc and cutting depth ac both had ve levels, whichwere 100, 150, 200, 250 and 300 m/min, and 0.005, 0.01, 0.015,0.020 and 0.025 mm, respectively.

3. Results and discussion

3.1. Effects of ber type and ber orientation angle on cutting force

Fig. 7 shows the effects of ber type and ber orientation angleh on cutting force under conditions of vc = 200 m/min, ac = 20 lm.As can be seen from the gure, main cutting force Fc and radialthrust force Fp both have signicant uctuations under the inu-ences of different ber types and ber orientation angles. Thereexist the largest values with h= 90 for both Fc and Fp, which almostshows the same as Zhang observations [26]. It also can be seenfrom Fig. 7 that the inuence of ber type on cutting force was rel-atively stable and direct. T800, whose ber strength was higher,had Fc and Fp both about 1030 N greater than T700 at the sameber orientation angle.

As can be seen from Fig. 7(a), when ber orientation angle hchanges between 0 and 75, main cutting force Fc basicallyincreases linearly with the increase of h. From 75 to 90, Fc soaredquickly. After reaching the peak at 90, Fc quickly fell to a lowerlevel as h became larger, except for a small peak in the 165 direc-tion, Fc uctuated rather slightly within a 105180 range. It canthus be seen that in the forward ber direction of h< 90, Fc showedlinear growth as h increased, the smaller the forward ber orienta-tion angle, the smaller the value of Fc, whereas the larger the for-ward ber orientation angle, the greater the value of Fc. In thereserve ber direction of h > 90, Fc was maintained at a relativelylow level, which did not change markedly with changes in ber ori-entation angle, except for a slight rise in the direction of h = 165.

It can be seen from the above analysis that the effect of ber ori-entation angle h on main cutting force Fc exhibited a strong aniso-tropy. Fc of T700 and T800 CFRPs both presented a signicantlysoaring peak at h = 90, which was, Fc was the largest at a vertical

iveGPa)

In-plane shearstrength (MPa)

90 tensile strength(MPa)

Bending strength(MPa)

92 70 1580

98 80 1670

ection.

ructuQ. An et al. / Composite Stcutting direction, which could reach 96.3 N and 72.9 N, respec-tively, for T800 and T700. The position with the best machinabilitywas the reverse ber direction of 120150, where Fc values ofT800 and T700 were the lowest, which were 27.2 N and 17.9 N,respectively. In addition, the value of Fc was also relatively lowwhen the angle was 0 or 180, which was 32.3 N for T800, and26.7 N for T700, respectively.

As to the radial thrust force Fp, it can be seen from Fig. 7(b) thatin the forward ber direction with h < 90, Fp showed piecewiselinear growth as angle h increased; the growth was faster within

Fig. 3. Workpiece material

Fig. 4. Orthogonal cutting tool in experiments.a 030 range, and slower within a 3090 range, which peakedin the vertical direction of h = 90. Afterwards, as h continued togrow into the reserve ber direction of h > 90, Fp showed a signif-icant decline, which further decreased with the increase of h.Similar to Fc, Fp also had a slight rise in the direction of h = 165.

It can be seen from the above analysis that radial thrust force Fpalso had signicant directionality, but it was somewhat different

s (0 CFRP laminates).res 131 (2015) 374383 377from Fc. Because UD-CFRP laminate was a material with very sev-ere bouncing-back of machined surfaces, Fp was always greaterthan Fc at the same ber orientation angle [27]. Moreover,bouncing-back resilience differed distinctively in different direc-tions. Maximum of Fp also appeared in the direction of h = 90,reaching 148.8 N and 119.7 N for T800 and T700, respectively;however, the peaks did not soar signicantly as Fc but were rela-tively stable. Minimum values of Fp appeared only in the paralleldirection of h = 0, which were 55.3 N for T800 and 42.9 N forT700. This is because the bers have the highest longitudinalstrength, which will lead to a high stiffness in theparallel-to-ber direction, and hence the resistance to the verticalcompression of the cutting tool becomes higher when h changes to90. Meanwhile, the material was removed by shearing fracture ofbers, and there will be largest resistance when cutting a bundle ofwell-bonded bers with h = 90 [26].

3.2. Effects of ber type and ber orientation angle on specic cuttingenergy and specic energy map

Specic cutting energy refers to the energy consumed forremoval of unit volume of material in orthogonal cutting [25],which can be expressed as:

u Fctac

1

where Fc is the main cutting force, and t and ac are cutting widthand cutting depth in orthogonal cutting, respectively. Specic cut-ting energy is an important parameter for machinability of

ming

uctuData acquisition and analysis

Dynamic cutting force acquisition syste

Amplifier

Fig. 5. Scheme of orthogonal cutt

378 Q. An et al. / Composite Strmaterials, which directly reects the difculty of chip separation incutting.

Fig. 8 shows the specic cutting energy u and resultant cuttingforce Fr of T700 and T800 CFRPs. As can be seen, T800 performed apoorer machinability than T700; and under the same cutting con-ditions, T800 CFRP consumed more energy. Machinability of bothCFRPs exhibited signicant directionality. With changes in the berorientation angle, energy consumed during machining differed dis-tinctly. There existed a critical angle of h = 90, beyond which sev-ere subsurface damages will occur and the deformationmechanisms in the cutting zone will change. The removal of bermaterial in UD-CFRP cutting was mainly completed by shear frac-ture and extrusion fracture in 90 ber orientation. There would bea larger resistance when cutting a bundle of well-bonded berswith h = 90. Therefore, machinability was the poorest, and chipseparation was most difcult in the vertical direction of h = 90.In the forward ber direction of h < 90, machinability becamepoorer linearly with the increase in h. However, in the reverse berdirection of h > 90, machinability was superior to that in the for-ward ber direction, and chip separation became easier. The paral-lel direction of h = 0 (or 180) was also a direction where materialremoval was rather easily achieved.

In the aeronautical application of CFRP, drilling is the most com-mon processing method of secondary machining. This paper wouldstudy specic cutting energy during drilling process based on

Workpiece

Cutter

Fig. 6. Photo of orthogonal cutting oCFRP laminate

Kistler dynamometer

Spindle

Flywheel

Cutter

of CFRP unidirectional laminates.

res 131 (2015) 374383orthogonal cutting test combining CFRP unidirectional laminatedrilling principle by establishing cutting specic energy map.Specic steps were: if drill bit made a 360 circumferential rotarymotion, the intersection angle between drill edge and direction ofbers in a certain layer during cutting would also change withinthe range of 0360. Taking into account that all of the intersectionangles between cutting direction of tool and ber direction couldbe described by ber orientation angle h, intersection angles withina 0180 range could be described directly by ber orientationangle h, while the intersection angles ranging 180360 could bedescribed by ber orientation angle h after subtracting 180.Specic energy map of T700 and T800 unidirectional CFRP lami-nates during circumferential cutting can be obtained based onthe test results in Fig. 8, as shown in Fig. 9. As can be seen fromthe gure, the specic energy of monolayer CFRP laminate is signif-icantly directional within a single cutting cycle. The entire mapwas in a pigeon-shape: specic cutting energy was smaller inthe second and fourth quadrants where cutting was performed inthe reverse ber direction, and larger in the rst and third quad-rants where cutting was performed in the forward ber direction;maximum specic energy appeared when cutting was performedin the perpendicular to ber direction (h = 90/270), which wasabout 3 times that of the minimum value, forming the wings ofthe pigeon-shaped map; there were slight specic energyincreases in the positions close to the horizontal axis

vc

Fp

Fc

ac

0

r

f CFRP unidirectional laminates.

Q. An et al. / Composite Structu(h = 165/345) in the second and fourth quadrants, which formedthe head and tail of the pigeon-shaped map. The pigeon shapein the specic energy map of monolayer CFRP laminate dependedon the properties of CFRP material itself, including the fractureproperty of carbon bers, cutting performance of epoxy resins,and anisotropy of long-ber-reinforced composites. The shapes ofspecic energy maps for monolayer T700 and T800 CFRP laminatesof different ber types were basically the same.

Fig. 7. Effects of ber type and ber orientation angle on cutting force in horizontaldirection (a) and vertical direction (b).

Fig. 8. Effects of ber type and ber orientation angle on specic energy andresultant cutting force.3.3. Effects of cutting speed on cutting force

Fig. 10 shows the effects of cutting speed on cutting force forT700 CFRP. Overall, with an increasing cutting speed, main cuttingforce Fc and radial thrust force Fp both decreased to some extent, Fcwas rather obviously affected by vc, while Fp was relatively insen-sitive to vc, and exhibited mild overall changes.

Within the test range of cutting parameters, the inuence ofcutting speed vc on main cutting force Fc was divided into twostages: (1) within the vc range of 100200 m/min, Fc declinedrapidly with changes in vc; increased cutting speed signicantlyfacilitated the cutting, and cutting of carbon bers also becamemore smooth; (2) within the vc range of 200300 m/min, Fc nolonger changed obviously with changes in vc, and showed a slightupward trend, that was, after the cutting speed exceeded200 m/min, increased cutting speed was longer able to furtherreduce the main cutting force Fc. It was also investigated bySreejith et al. [13] that the range of 200300 m/min of cuttingspeed is the most suited for the machining of CFRP based on thesteady specic cutting pressure criterion.

Taking into account different ber orientation angles, main cut-ting force Fc in these two stages also had regularities: (1) within thevc range of 100200 m/min, the anisotropy of Fc was more promi-nent at different ber orientation angles, which was higher forh = 45/90 than h = 0/135/165; (2) within the vc range of 200300 m/min, although Fc maintained the anisotropy, difference ofFc at different ber orientation angles was no longer prominent,that was, under high-speed cutting conditions, anisotropy ofCFRP had a gradually weakening trend. This was because high cut-ting speed would lead to cutting off the bers easily. The high cut-ting temperature caused by high-speed cutting would also result insoftening the resin base material and reducing the cutting forces.These all helped to weaken the anisotropy of UD-CFRP.

In contrast, cutting speed vc had smaller inuence on radialthrust force Fp. In the directions of h = 45/90 with larger Fp, Fpexhibited a downward trend with the increase of vc, but attenedafter 200 m/min. In the directions of h = 0/135/165, the inuenceof vc was rather small. Fp maintained a stable level under variouscutting speed conditions, except at the minimum cutting speedvc = 100 m/min, where the value of Fp was slightly larger. Radialthrust force Fp was insensitive to changes of cutting speed vc inthe reverse ber and parallel to ber directions. In addition, theanisotropy of CFRP also showed a gradually weakening trend underhigh-speed cutting conditions according to the changing regularityof thrust force.

3.4. Effects of cutting depth on cutting force

Fig. 11 shows the effects of cutting depth on cutting force forT700 CFRP. Overall, with the increasing of cutting depth ac, maincutting force Fc and radial thrust force Fp both showed upwardtrends, which were directly related to the growth in volume of cut-ting materials.

Under different ber orientation angle h conditions, the regular-ity of inuence of ac on cutting force was also slightly different. Inthe ve ber directions shown in Fig. 11(a), main cutting force Fcall exhibited an increasing and attening trend (except for h= 90 direction) after cutting depth ac was increased to 15 lm,which can be considered as a threshold. When ac was smaller thanthis value, Fc grew faster as ac became larger; while after acexceeded this threshold, the growth of Fc signicantly sloweddown. It was because the cutting tool had a rounded edge radiusof re= 15 lm, and there was a size effect for the inuence of cutting

res 131 (2015) 374383 379depth on main cutting force during UD-CFRP cutting process [28].The critical value of ac was close to the rounded edge radius re.When ac < re, Fc was more sensitive to the changes in ac, and grew

Fig. 9. Specic energy map in cutting T700/T800 CFRP of one uniform unidirectional laminate.

Fig. 10. Effects of cutting speed on cutting force in horizontal direction (a) and vertical direction (b).

Fig. 11. Effects of cutting depth on cutting force in horizontal direction (a) and vertical direction (b).

380 Q. An et al. / Composite Structures 131 (2015) 374383

faster with increase in ac; when ac > re, Fc had reduced sensitivity tochanges in ac, and grew slower with an increasing ac. Size effecthad certain directionality, which appeared in the directions ofh = 0/45/135/165, but was not obvious in h = 90 direction. Inaddition, directional difference of main cutting force Fc did notchange with ac, and the difference in Fc between different berdirections almost had no change (except for h = 90) under differ-ent cutting depths.

The effects of cutting depth ac on radial thrust force Fp, in con-trast, was relatively simple, without size effect near the roundededge radius re. Fp roughly showed a proportional growth trend withthe increasing ac in different ber directions. Meanwhile, the ani-sotropy of Fp was also amplied with the increase of ac. When cut-ting UD-CFRP, it would result in more pronounced directionality ofFp with increasing ac. Maximum value of Fp occurred in the h = 90direction, while the minimum value occurred in the h = 0 direc-tion. The difference between maximum and minimum would beamplied in proportion with the increase of ac.

3.5. Effects of cutting speed on specic energy map

Fig. 12 shows the maps of specic energy for T700 CFRP at dif-ferent cutting speeds. As can be seen from the gure, cutting speed

vc has rather signicant effect on specic cutting energy u; with theacceleration of vc, the specic energy map shrank quickly, indicat-ing reduced cutting energy consumption, and improved machin-ability of CFRP. As can be seen from Fig. 12, specic energy mapall maintained a pigeon-like shape within the range of vc= 100200 m/min; the pigeon area decreased in the same pro-portion as vc increased. This indicated that the inuence of cuttingspeed vc on the machinability of CFRP in various ber directionswas uniform at low cutting speeds; besides, cutting speed had littleinuence on the anisotropy of CFRP. However, at the high-speedrange of vc = 200300 m/min, the shape of pigeon-like mapbegan to change. With an increasing of vc, map area had an ampli-fying trend, which indicated that at the high-speed range, themachinability of CFRP in various ber directions gradually becameconsistent, and anisotropy was gradually weakened during cuttingprocess. It shows a similar trend with the effect of vc on the cuttingforces. On one hand, high cutting speed would lead to cutting offthe bers easily, on the other hand, high-speed cutting wouldresult in high cutting temperature that will soften the resin basematerial and help reduce the overall chip separation energy. Butthere existed a critical temperature for stable machining when lessspecic energy was consumed [13]. It may lead to the little differ-ence of specic energy map between 100 m/min and 200 m/min.

Q. An et al. / Composite Structures 131 (2015) 374383 381Fig. 12. Effects of cutting speed on specic energy map of T700 CFRP.

uctu382 Q. An et al. / Composite Str3.6. Effects of cutting depth on specic energy map

Fig. 13 shows the maps of specic cutting energy for T700CFRP under different cutting depth conditions. As can be seenfrom the gure, the shape and numerical range of specic energymap shrunk with the increase of ac. This was also observed bySreejith et al. [13]. At the minimum cutting depth of ac = 5 lm,the specic energy map was most ample, and the speciccutting energy u in various ber directions was the highest,showing relatively strong difculty in machinability. It is becausethe tested T700 carbon ber had a lament diameter of about 68 lm, and cutting may occur inside the bers with ac = 5 lm,which would consume more energy. Within a cutting depthrange of ac = 1025 lm, specic energy map roughly maintainedthe relatively regular pigeon-like shape. It could be describedas follows: in the forward ber direction of h < 90 and thereverse ber direction of h > 90, specic energy map narrowedas ac increased, indicating improved machinability; in a h = 90direction, specic energy was less affected by ac, which main-tained stable after ac exceeded the ber diameter, indicating thatthe machinability in this direction is basically not affected by thecutting depth.

Fig. 13. Effects of cutting depth on spres 131 (2015) 3743834. Conclusions

This paper studied the effects of ber orientation angle on cut-ting force characteristics of UD-CFRP laminates using the orthogo-nal cutting test, and reached the following conclusions.

Cutting force and specic cutting energy of T700 and T800CFRPs were all signicantly directional. At the same ber orienta-tion angle conditions, T800 always had greater cutting force andspecic cutting energy than T700.

Fiber orientation angle had signicant effects on main cuttingforce Fc and radial thrust force Fp. In the forward ber directionof h < 90, Fc and Fp increased linearly as the angle h increased; inthe reverse ber direction of h > 90, Fc and Fp maintained at rela-tively low levels except for a slight rise in the h = 165 direction.Fc and Fp had the peak value in the vertical direction of h = 90.

As the cutting speed increased, Fc and Fp both decreased; underhigh-speed cutting conditions, the cutting anisotropy of CFRPshowed a weakening trend. With the increase of cutting depth, Fcand Fp both exhibited upward trends; Fc has a size effect when cut-ting depth is close to the rounded edge radius.

With the increase of cutting speed, specic energy map shrankquickly, indicating reduced cutting energy consumption, and

ecic energy map of T700 CFRP.

improved machinability of CFRPs. Specic cutting energy was lessaffected by ac in h = 90 direction, while in the forward ber direc-tion of h < 90 and reverse ber direction of h > 90, specic cuttingenergy would decrease with an increasing of ac. Specic energymap narrowed, which indicated improved machinability.

According to the above analysis about cutting mechanics char-acteristics of UD-CFRP, a suitable high cutting speed with appropri-ate large cutting depth is helpful for stable machining of CFRP. Toget a comprehensive study on the cutting mechanism of UD-CFRP,more experiments should be carried out in terms of tool parame-ters, cutting temperature and machining quality. In the futurework, an optimal model will be built and used in design of cuttingtool and improvement of machining quality.

Acknowledgement

This research is supported by National Natural ScienceFoundation of China (Nos. 51475298 & 51105253).

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Study on the cutting mechanics characteristics of high-strength UD-CFRP laminates based on orthogonal cutting method1 Introduction2 Experimental procedures2.1 Workpiece materials and cutting tool2.2 Experimental conditions

3 Results and discussion3.1 Effects of fiber type and fiber orientation angle on cutting force3.2 Effects of fiber type and fiber orientation angle on specific cutting energy and specific energy map3.3 Effects of cutting speed on cutting force3.4 Effects of cutting depth on cutting force3.5 Effects of cutting speed on specific energy map3.6 Effects of cutting depth on specific energy map

4 ConclusionsAcknowledgementReferences