A review on machinability of carbon fiber reinforced polymer(CFRP)and glass fiber reinforced polymer(GFRP)composite materials

Meltem Altin Karataı,Hasan Gökkaya

aAbant˙Izzet Baysal University,Gerede Vocational School,Machinery and Metal Technology Department,Bolu 14900,Turkey

bKarabük University,Engineering Faculty,Mechanical Engineering Department,78050 Karabük,Turkey

Keywords:Composite materials Fiber reinforced polymer composite materials CFRP GFRP Machining Wear Surface damage

A B S T R A C T Fiber reinforced polymer(FRP)composite materials are heterogeneous and anisotropic materials that do not exhibit plastic deformation.They have been used in a wide range of contemporary applications particularly in space and aviation,automotive,maritime and manufacturing of sports equipment.Carbon fiber reinforced polymer(CFRP)and glass fiber reinforced polymer(GFRP)composite materials,among other fiber reinforced materials,have been increasingly replacing conventional materials with their excellent strength and low specific weight properties.Their manufacturability in varying combinations with customized strength properties,also their high fatigue,toughness and high temperature wear and oxidation resistance capabilities render these materials an excellent choice in engineering applications.In the present review study,a literature survey was conducted on the machinability properties and related approaches for CFRP and GFRP composite materials.As in the machining of all anisotropic and heterogeneous materials,failure mechanisms were also reported in the machining of CFRP and GFRP materials with both conventional and modern manufacturing methods and the results of these studies were obtained by use of variance analysis(ANOVA),artificial neural networks(ANN)model,fuzzy inference system(FIS),harmony search(HS)algorithm,genetic algorithm(GA),Taguchi's optimization technique,multi-criteria optimization,analytical modeling,stress analysis, finite elements method(FEM),data analysis,and linear regression technique.Failure mechanisms and surface quality is discussed with the help of optical and scanning electron microscopy,and profilometry.ANOVA,GA,FEM,etc.are used to analyze and generate predictive models.

1.Introduction

More than fifty thousand material types have been used in the design and production of a wide range of engineering applications[1,2].These materials range between those which were available even centuries ago(copper,cast iron,brass,etc.)and the recently developed advanced materials(composites,ceramics and high performance steels,etc.)[2].Composite materials are defined as a combination of two or more synergic micro-constituents,which differ in physical form or chemical composition[3,4].The structure of composite materials consists of two components,namely matrix and reinforcement,and the three dimensional region with specific characteristics between these two constituents is known as the interphase region.The interface,on the other hand,constitutes the boundary between the constituents with its two-dimensional structure(Fig.1)[5].The two-phased structure of composite materials,consisting of the reinforcement phase surrounded with the matrix phase,enables utilization of the superior characteristics of both materials[6,7].Matrices involve metallic,polymer or ceramic materials whereas reinforcements are in the form of fibers,particles or crystal filaments(whiskers)[2,6].The matrix of fiber reinforced materials are chosen among different kinds of resins(epoxy,phenolic,polyester,vinyl ester,etc.)while the reinforcement is selected among glass,carbon or aramid(kevlar).In general,reinforcements( fibers)act as the main load bearing element,whereas the matrix encloses the fibers and protects them in the desired direction.Matrices act as load transfer elements between the fibers and protect the structure against harsh environmental conditions such as high temperature and humidity[8].

Carbon fiber reinforced composite materials,in which carbon fiber is used as the reinforcement element,can involve polymer matrix,metal matrix,ceramic matrix or carbon matrix.Carbon and glass fiber reinforced polymer composites have been commonly preferred in the space and aviation industry[9,10].Increasing number of aircraft components involve CFRP composite constituents due to their superior characteristics such as high strength and stiffness,low weight and high fatigue resistance[11-18].These applications may involve small components such as doors and clips as well as large ones as wing flaps and the main body.The components made of carbon fiber reinforced composite materials used in Airbus 350 aircraft are shown in Fig.2[11].

The failures arising from the machining of CFRP composite materials were found to reduce the strength and fatigue life of the components[3,21].Occurrence of varying failure mechanisms such as fiber pull-out, fiber break,matrix smearing and delamination result in rejection of numerous components(Fig.3)[22].The dominant failure mechanism during the drilling of composites is reported as delamination[23,24].

Researchers in general have sought to determine the optimum cutting parameters to avoid the failures such as fiber rupture,resin fiber de-bonding,stress concentration,micro-crack formation and deformations around drilling region,that occur during the drilling or cutting of GFRP and CFRP materials.In the present review study,the machinability characteristics and approaches for GFRP and CFRP materials were addressed and the outcomes of the studies conducted in this respect were compared.

2.Machinability of fiber reinforced composites

Fiber reinforced polymer composite materials have been applied in several fields for years due to their high specific strength and modulus[26,27].Because of the strength and stiffness of a composite buildup depends on the orientation sequence of the plies,the layer orientation of fiber reinforced polymer composite materials needs to be designed correspondingly.While the fibers in a unidirectional material run in one direction and the strength and stiffness is only in the direction of the fiber;the fibers in a bidirectional material run in two directions and the strength and stiffness is in two direction of the fiber.The layers should require 0°plies to respond to axial loads,±45°plies to react to shear loads,and 90°plies to react to side loads(Fig.4).Since the strength design requirements are a function of the applied load direction,ply orientation and ply sequence have to be true[28].

In aircraft industry,carbon fibers are widely used to reduce the weight of the structural components,to reduce emissions,to improve the fuel efficiency,and the load bearing capacity of the airplanes[30].It is a known fact that in the aircraft industry that there are more than hundred thousand mounting holes on a single small aircraft and more than a million holes on larger ones[31-34].Thus,from manufacturers' point of view,drilling process constitutes the 40%of all machining operations during the assembly(riveted,bolted)of components[24,34-36].However,failures such as fiber rupture,resin- fiber de-bonding,surface irregularities,micro-crack formation and deformations around drilling regions are commonly encountered during the machining of CFRP composite materials due to the presence of two or more phases[32,37].Accordingly,the machinability of composite materials has been addressed differently from the machinability of conventional materials[3,22,38,39].Such surface failures may have significant adverse effects on the product surface quality,which prompts the researchers to conduct continuous studies for their elimination or mitigation[32,34-36,40-43].It is reported in the conducted studies that the surface quality depends on the cutting parameters,tool geometry and cutting forces[23,24].Therefore,correct selection of cutting parameters is essential in the machining of polymer matrix composites[24,32,35].

The studies on CFRP composite materials revealed that the failures that arise during their machinability reduce the strength and fatigue life of the material[3,21].Moreover,the drilling process becomes a challenging issue during assembly[44].The most serious failure arising from the drilling of composite materials is reported to be delamination on hole surfaces(Fig.5)[23,24,45-58].Theoretical and experimental studies reveal that,hole entry and exit regions are the most delamination-sensitive areas[54,57,59-65].Thrust force is regarded by some of the researchers as the underlying reason for emergence of this failure mechanism[38,66].

2.1.Machinability of CFRP and GFRP composite materials with conventional manufacturing methods

Composite materials are regarded as difficult-to-machine materials due to their heterogeneous structures.Conventional machining methods such as turning,milling,planning,drilling,etc.,are typically used in the machining of these type of materials[69].Due to anisotropic and heterogeneous structures of composites,machining of such materials with conventional machining processes often results in material failures such as matrix cracking,fiber pull-out,swelling and delamination(hole surface failure)[3,18,69-77].Failure behaviors do not only arise from the heterogeneous and anisotropic structure,but also from the machining methods and their interactions[78-80].In addition;due to their heterogeneous structure,machining of polymer composite materials with conventional methods gives rise to structural and health related issues such as delamination,reduced tool life, fiber pull out,matrix smearing and unhealthy dust formation[45,57,81,82].Despite their high hardness and abrasiveness(at times even harder than some of the tool materials),due to their brittle nature,crushing of fibers is implemented via conventional machining methods,to avert the plastic deformation of the tool[45,83,84].The low machinability of CFRP composite materials generally leads to various machining failures including delamination,burrs,and subsurface failures[13,85-94].

Typical finishing and surface integrity-related problems are commonly encountered during the machining of CFRP composite materials with conventional solid machining tools.Occurrence of various failure types such as fiber pull out, fiber break,matrix smearing and delamination end up with rejection of a vast number of work pieces[22].High rejection rates for airplane components reaching 60%arising from delamination related failures have been reported in the aircraft industry[3,15,22,34,57,59,95-97].Also,narrow working spaces cannot be reached with conventional solid tools due to the spindle size of machine tools,and tool changing times for worn out milling and drilling tools result in extended machining times[11].

2.1.1.Drilling,cutting and milling of CFRP and GFRP

In their study on drilling-induced surface failures on CFRP and GFRP composite materials and the effects of drill bit geometry and cutting parameters,Durãoet al.reported that,low feed rates reduce the axial forces,which in turn reduces the delamination initiation risk,thus proving to be suitable for drilling of composite layers.They also reported that delamination results are affected by the tool geometry,and accordingly twist drills with 120°point angle should be used for minimum delamination(Fig.6)[36].

In their study on measurement of wear criteria with regard to cutting temperatures,hole surface topography and cutting forces during drilling,Ramirez et al.reported flank wear and burr formation as a result of the conducted drilling process[98].Eneyew and Ramulu stated in their study,in which they used PCD drill for the drilling process,that the compressive force increases with increasing feed rate and decreases with increasing cutting force.Various researches reveal that a good hole surface quality is obtained with high cutting speeds and low feed rates[99].Gaitonde et al.used cementite carbide(K20)twist drill in their high speed drilling process and reported a decrease in delamination tendency as a result of increasing cutting speed.They also suggested the use of a low feed rate-point angle combination[3].Grilo et al.applied the drilling process with different drill bits(SPUR,R950,R415)and observed no delamination on the entry-surfaces of the holes,whereas uncut fibers were found on the hole-exits.Additionally;the lowest levels of delamination were obtained with SPUR drill bit[100].Kılıçkap stated that,during drilling,delamination on the hole exit was higher than the one on the hole entry at a rate of 13-30%,and reported that the lowed delamination factor was observed with low cutting speed and low feed rate values[32].According to the test results obtained by Ekici and Iıık,the failure factor was reduced after the use of high cutting speed and low feed rate values.The results of their study also indicate that increasing values of cutting tool point angle and the number of cutting edges also increased the failure factor.The lowest failure factor was observed with 90 m/min cutting speed and 0.06 mm/rev feed rate with a drill having two cutting edges with 60°point angle[101].Abrˋao et al.reported that thrust force was increased with increasing feed rate,while cutting speed barely influenced the thrust force,and that tool wear resulted in increasing levels of thrust force[34].

As for the milling of CFRP and GFRP composite materials,Karpat et al.attempted to mill CFRP composite materials with differing fiber orientations(0°,45°,90°ve 135°)with a PCD milling tool,and according to the test results,the radial forces emerging in the milling of composite materials with 0°orientation were higher than those emerging in the milling of composites with 45°fiber orientation.In this research the highest tangential forces were found to be those observed in the milling of composites with 135°fiber orientation;whereas the lowest ones were those observed during the milling process of composites with 45°orientation(Fig.7)[102].

Surface measurement results of Erkan and Iıık's work indicate that surface roughness was improved with increasing cutting speed whereas it deteriorated with increasing feed rate[103].In their another work,in which they conducted surface roughness measurement with varying cutting directions,Erkan and Iıık reported that the surface roughness values obtained from the channels milled with 45°machining direction were higher than those obtained after milling with 90°machining direction.In these studies,the average surface roughness values were reported to be increasing with increased feed rate,while it was reported to decrease with increasing cutting speed.The change in cutting speed was found to have no effect on the average surface roughness[104].After their contour milling process Takmaz et al.reported that the most effective parameter on average surface roughness was the number of the cutting edges,which was followed by the cutting speed and the cutting depth.In their work,the lowest average surface roughness was obtained as 2.14μm with 4 cutting edges at 60m/min cutting speed,0.08 mm/rev feed rate,6mm cutting depth[105].Wang et al.investigated that mechanisms of orthogonal cutting in conventional edge trimming of unidirectional Gr/Ep using PCD tools with various geometry.They stated that chip formation,cutting forces,and the surface morphology in edge trimming of unidirectional Gr/Ep were highly dependent on fiber orientation[106].

2.2.Machinability of CFRP and GFRP composite materials with nontraditional manufacturing methods

Damage-free machining of polymer matrix fiber reinforced polymer composite materials with conventional machining methods such as drilling,cutting,milling,grinding,etc.is a highly challenging process even under proper conditions,due to the issues such as heterogeneity and thermal sensitivity[107].Regardless the type of the used manufacturing method,CFRP composite materials,like all layered(laminated)composites,undergo numerous failures such as matrix defects(gap,porosity), fiber cracks,interface cracks,delamination,impurities,etc.as a result of their machining with conventional(traditional)(turning,milling,drilling,etc.),or nontraditional(water jet machining(WJM),abrasive water jet machining(AWJM),ultrasonic machining(USM),electrochemical machining(ECM),electrical discharge machining(EDM),laser machining(LJM),chemical machining(CHM),photochemical machining(PCM),etc.).In general,the working principle of modern manufacturing methods is characterized with their high specific energy and low chip formation rate.The advantages of modern manufacturing methods over traditional methods can be listed as high precision,high surface quality for complex geometries,no requirement for work tools,burr-free surfaces,etc.[108].

2.2.1.WJM,AWJM,LJM and EDM of CFRP and GFRP

Many studies have been carried out on water jet(WJ)and abrasive water jet(AWJ)machining of CFRP and GFRP composite materials[11,44,95,107,109-125].The experimental results of WJ and AWJ applications implemented by Shanmugam et al.indicate that,an increase in the cutting speed of water jet induces an increase in the maximum crack length;while an increase in the jet pressure decreases the maximum crack length(Fig.8)[109].

According to Hashish;kerf width on the machined material increased with increasing cutting speed,and cutting speed did not have any effect on the upper surface(compared to lower).Also;jetlag angle was reported to be increasing with increasing AWJ cutting speed[11].Phapale et al.stated in their study that,no delamination was observed after the use of low water pressure,abrasive-mass flow rate and stand-off distance;and high values for these parameters resulted in higher levels of delamination[110].The experimental results of Mayuet et al.showed that SEM/SOM analyses were applicable for determination of delamination formation mechanism;that the type of used abrasive is likely to be the most effective parameter in delamination formation;that thicker layers could be machined by use of higher pressures;and that,higher abrasive-mass flow rate with average flow range is likelyto resultin less damage[111].In their experimental study,Alberdi et al.applied abrasive water jet machining(AWJM)to machine two different types of CFRP composites(M1 and M2),and the experimental results showed that M1(6mm)could be machined faster than M2(12mm),which was attributed to the fiber/volume ratio and/or the stress module[112].Ibraheem et al.reported that;traverse feed rate,stand-off distance,AWJM pressure and abrasive-mass flow rate are the effective parameters in the drilling of CFRP composite materials;that,AWJM pressure has considerable effect on the material strength;and that,AWJM pressure should be reduced as a means to avert the adverse effects of the pressure on material strength[113].The experimental results obtained by Doreswamy et al.showed that,jet pressure,stand-off distance and feed rate have more effect on upper(top)kerf width(TKW)as compared to lower(bottom)kerf width(BKW).It was also concluded in their research that,kerf width increased with increasing jet pressure and stand-off distance,whereas it decreased with increasing feed rate.They also reported that abrasive concentration was not effective on the kerf width,and that no delamination was observed on AWJ machined surfaces with optimized machining parameters[114].Lemma et al.applied vibrating and non-vibrating cutting processes with an AWJ machine tool and investigated the effects of these two processes on average roughness of CFRP composite materials.The results of their study revealed that the surface quality was improved at a significant rate of 20%by use of a vibrating cutting head as compared to the use of a non-vibrating head;and they also reported that the highest roughness value was obtained with 6 Hz vibrational frequency and 2°vibration angle[115].Azmir and Ahsan stated that,the lowest surface roughness value was obtained with 22.5°cutting direction,276 MPajet pressure,1.5 mm stand-off distance,7.5 g/s abrasive-mass flow rate,1.5 mm/s traverse rate,and by use of aluminum oxide abrasive.They also stated that, fiber/volume ratio did not have a significant effect(8%)on the average surface roughness value[116].In another study of Azmir and Ahsan,increasing jet pressure and decreasing stand-off distance were found todecrease the surface roughness values;the average surface roughness was increased to a certain limit with the increase in abrasive-mass flow rate;low traverse feed rate would yield a better surface quality;and the cutting direction comparatively affected the surface roughness.After the tests conducted to determine the effect of machining parameters on kerf width,researchers reported that abrasive particles with high hardness were likely to cause lower kerf widths,and upper kerf width was in general larger than lower kerf width.They also proposed that increasing jet pressure would induce formation of a wider channel which would in turn result in larger upper and lower kerf widths.The researchers concluded that,kerf width increased with increasing stand-off distance;kerf width converged to 1 with increasing abrasivemass flow rate,and as in the case of average surface roughness,lower traverse feed rate also resulted in lower kerf width.They determined that,differing cutting directions,as surface roughness,have negligible effect on kerf width[117].Miller et al.carried out a research on the difficulties and failures encountered in vertical milling(PCD(polycrystalline diamond),DA(diamond abrasive)and carbide tools)AWJ cutting and drilling of CFRP composite materials under dry conditions.Results of their study indicate that,in the drilling of CFRP composite,compressive force and torque increased with increasing feed rate and decreased with increasing cutting speed.They also reported that,AWJ cutting time and cutting depth were primarily dependent on feed rate;and the combined use of high feed rate with low abrasive-mass flow rate yielded a comparatively uneven surface finish quality[118].Miron et al.obtained a high dimensional accuracy of±0.05 and 7243μm average surface roughness in the drilling of CFRP composite with AWJ machining and observed abrasive residuals in the material(Fig.9)[119].

CFRP composite specimens were subjected to AWJ cutting operation by Unde et al.and the effect of machining parameters on material removal rate(MRR),delamination factor,kerf width and average surface roughness(Ra)were investigated.Following the tests,the resultant delamination factor after machining with 45°fiber orientation was found to be higher than those obtained with 60°and 90°fiber orientations[95].Arisawa et al.carried out a study on availability of a more practical method in terms of machining efficiency and tool life for machining of CFRP composite materials by any machining technology(AWJM,end mill,electroplated diamond tool)with varying machining parameters.In end milling operation the average surface roughness value was observed to gradually deteriorate and exceed 3μm after an increase in feed rate from 200mm/min to 1000mm/min.Also they achieved an average surface roughness of 1.5μm with 2000 mm/min feed rate using the tool they developed in their study.In the same study a fine surface quality with 4μm average surface roughness and without delamination was reported after the performed AWJM operation.The optimized drill geometry developed during the research was reported to yield machined surfaces with up to 22 times better surface quality as compared to prior operations.It was also determined that it was possible to extend the tool life as much as the number of holes which is at least four times higher than other tools[44].Kakinuma et al.performed an experimental analysis on machinability with ultra-fast feed drilling(UFFD),ultrasonic vibration assisted drilling(UVD)and AWJ drilling of CFRP composites in terms of material properties;measured the cutting forces with a 3-component dynamometer and measured the delamination damage with an optical microscope.The results obtained after the fast drilling process indicated that it was possible to yield a holeexit surface with significantly reduced delamination by setting a feed rate higher than 3000 mm/min.In the preliminary drilling tests,delamination and burr formation were found to occur on hole exit surfaces rather than the hole entries.The researchers applied axial ultrasonic vibration for the drilling of CFRP composite material and reported a reduced friction between the work piece and tool.Results of the AWJ drilling operation on CFRP composite material indicated that the use of high water pressure was likely to result in severe failure[120].As for average surface roughness and machining time,UFFD yielded a better surface quality in a shorter machining time as compared to AWJ machining.They concluded that UFFD method yielded better results in terms of overall surface quality,geometric accuracy and machining time for machining of CFRP composite materials[120].Patel and Shaikh conducted a review study on AWJ machining of CFRP composite materials.Due to its main characteristics,they evaluated the use of AWJM method for machining of polymer matrix composite materials which have been used in a wide range of industrial and domestic applications.Despite being regarded as the best alternative in machining of FRP materials,the AWJ technology also results in formation of undesired conical and rough kerf walls,which however can be minimized through selection of optimal AWJ parameters for machining[107].

Several studies have been carried out on laser machining of CFRP and GFRP composite materials[126-140].Leone et al.investigated that laser cutting of 0.5 mm thickness CFRP laminates using multi-passes scanning technique with the aim to obtain the maximum cutting speed together with a narrow kerf and a limited HAZ(Heat Affected Zone).They pointed out that the effective cutting speed depends on scanning speed and pulse power and they indicated that for the adopted source and the selected process parameters,cutting speed varies in the range 5.6-11.5 mm/s[129].Takahashi et al.had an experimental investigation of CFRP composite processing with a high-power pulsed fiber laser was conducted.They indicate that the cutting quality mainly depends on the hatching distance and the processing quality was improved with an increase of the hatching distance for each scanning speed.Also they concluded that the hatching distance and scanning speed have significant effects on the cutting quality and processing rate[128].

Chaudhury and Samantaray presented the review article on role of Carbon Nano Tubes(CNT)for enhancing surface quality through EDM.They stated that since many authors have studied the feasibility of machining CNT composites through EDM,the performance of such machining process is found to be higher in terms of surfacefinish and controlled MRR which will be helpful for application of CNT in engineering application[141].

3.Conclusions

Researchers have reported that,in the machining of CFRP and GFRP composite materials with conventional machining methods(turning,milling,drilling,etc.),increasing feed rate resulted in higher compressive forces;and higher hole surface quality could be obtained as a result of increased cutting speed and reduced feed rate.Some other researchers,on the other hand,obtained the lowest delamination factor with low cutting force and low feed rate.In general,average surface roughness was found to be reduced with use of high cutting speed and low feed rate.

It is stated in various researches on the machining of CFRP and GFRP composites with non-traditional manufacturing methods(WJM,AWJM,LJM,EDM,etc.)that,increasing cutting forces,in WJ and AWJ method,caused an increase in the maximum crack length;whereas increasing jet pressure resulted in reduced maximum crack lengths;jet-lag angle was increased with increasing cutting speed and a better surface quality was obtained with increased cutting speeds.Conducted researches revealed that,kerf width increased with increasing jet pressure and stand-off distance,and decreased with increasing feed rate.It was determined that the use of high pressure and low feed rate values were required to minimize the kerf angle and roughness values.It was also reported in other researches that,the surface quality of holes drilled with AWJ method were improved by use of low water pressure,stand-off distance and abrasive-mass flow rate.Researchers obtained better surface quality within shorter machining periods with UFFD method as compared to AWJ machining.