an optical method for measuring surface roughness of machined carbon fibre-reinforced plastic...

JOURNAL OFC O M P O S I T EM AT E R I A L SArticle

An optical method for measuringsurface roughness of machined carbonfibre-reinforced plastic composites

N Duboust1, H Ghadbeigi1, C Pinna1, S Ayvar-Soberanis2,A Collis3, R Scaife2 and K Kerrigan2

Abstract

Characterization of the damage induced by machining of fibre-reinforced composites is usually performed by measuring

surface roughness. Contact-based surface profilometers are the most used equipment in industry; however, it has been

found that there are performance limitations which may result when used to measure machined heterogeneous com-

posite surfaces. In this research, surface roughness is characterised using a commercial non-contact optical method, and

compared with a conventional stylus profilometer. Unidirectional and multidirectional carbon fibre laminates were edge

trimmed and slot milled. The variation in surface roughness was compared using different tool types, fibre orientations

and cutting parameters. Surface damage and cutting mechanisms were assessed by using scanning electron microscope

images, and the suitability of roughness parameters were also analysed including: Sa, Skewness and Kurtosis. Using the

optical system allowed accurate roughness calculation of individual plies on a multidirectional laminate with different fibre

orientations. The research has also shown that the optical system, including the use of areal roughness parameters, can

increase the accuracy of roughness measurement for machined fibrous composite surfaces and is less sensitive to

measurement position than the stylus.

Keywords

Carbon fibre, composite machining, edge trimming, surface roughness, milling, surface defects, surface inspection,

surface integrity

Introduction

Surface roughness measurement and damage character-isation of composite materials are necessary aftermachining in order to assess surface quality.However, accurate and reliable surface roughnessmeasurement of composite materials can be challengingbecause of the non-homogeneous structure of compos-ites and the variation in damage. Ramulu et al.1 founddifficulty while measuring parallel to the fibre directionwith a stylus, this was because the stylus path may spanover a few plies or pass over multiple fibre orientations.

Research has shown that the cutting mechanism andthe surface roughness can vary depending upon fibreorientation and machining direction.2–5 Wang et al.5

reported that the surface damage and roughness oneach fibre orientation of a multidirectional laminatewere different. However, they were not able to reliablycharacterise roughness of each individual layer. Azmiet al.6 reported that the Ra roughness values may not

always reflect the surface qualities of machined fibrouscomposite materials. Two problems were raised whichrelate to stylus based quantification: (i) that the pres-ence of fibres on the machined surface can affect themovement of the stylus tip, and (ii) that variation in theroughness reading varies with measurement positionrelative to ply configuration.

During machining the surface quality and/or integ-rity can be affected by multiple factors including: toolgeometry and wear; machine vibration and fixture

1Department of Mechanical Engineering, University of Sheffield,

Sheffield, UK2Advanced Manufacturing Research Centre (AMRC) with Boeing,

Rotherham, UK3Rolls-Royce, London, UK

Corresponding author:

K Kerrigan, AMRC with Boeing, Advanced Manufacturing Park, Wallis

Way, Catcliffe, Rotherham S60 5TZ, UK.

Email: [email protected]

Journal of Composite Materials

0(0) 1–14

! The Author(s) 2016

Reprints and permissions:

sagepub.co.uk/journalsPermissions.nav

DOI: 10.1177/0021998316644849

jcm.sagepub.com

at University of Sheffield on August 22, 2016jcm.sagepub.comDownloaded from

http://jcm.sagepub.com/

rigidity; workpiece material properties and manufactur-ing process.7 In order to assess surface quality, the sur-face texture measurement is often used and roughnessparameters are calculated.1 A high surface roughnessheight parameter value is often an indicator of poormachining parameters, or a high level of tool wear.This can also lead to an increased likelihood of othercritical defects including un-cut fibres, fibre pull-out ordelamination. Haddad et al.8 have demonstrated thatvarious damage types, which are characteristics of amachined composite surface, can be correlated tomechanical performance. It is therefore important tobe able to accurately quantify surface roughness toachieve understanding of machining-induced surfacedefects. This in turn, allows accurate quantificationon the effects of these defects on the in-service perform-ance of components.

Composites can suffer from a number of differenttypes of damage due to the machining process includ-ing: fibre pull out, inter-ply delamination, matrix crack-ing, matrix burning, un-cut fibres and fibre-matrix de-bonding.3 The damage can affect the fatigue life andstrength of a component and therefore should be rep-resented reliably by a surface damage parameter.Haddad et al.8 investigated the effect on fatigue lifeand mechanical properties due to surface quality.Machining by water jet, abrasive diamond cutter anda standard burr tool were compared using contact andnon-contact roughness measurement methods. It wasshown that the surface roughness and type of machin-ing process had a strong effect on mechanical perform-ance for a fibre composite. The authors found that anincrease in the average surface roughness of samplescaused a decrease in the inter-laminar shear strengthand compressive strength. However, the failure stressof the burr tool-machined samples was less than that ofthe water jet, even when specimens had a similar Ra.Different machining processes could therefore result inthe same value for surface roughness parameter Ra,even where different machining-induced surfacedamage and mechanical properties were reported.

Research has also shown that the machining param-eters, feed rate and cutting speed will have a stronginfluence on the surface quality.6,9–11 Azmi et al.6

looked at the machinability of GFRP by end millingwith respect to surface roughness, tool life and machin-ing forces. Increasing feed rate had the most dominanteffect on increasing surface roughness and was said toincrease the uncut chip thickness. The research showedthat increasing the spindle speed generally decreasedthe surface roughness.

Nwaogu et al.13 compared tactile and focus variationoptical device for the texture measurement of castingsurfaces, using areal and profile parameters. It wasfound that the areal (Sa) parameters had less variation

in measurement than the profile parameters (Ra). Seereference [12], for a description of areal and profiletexture parameters. The Ra parameter is the arithmeticmean roughness and gives the centre line average or thedeviation of the surface profile from the centre. Theabsolute value of the peak or valley is used, so the Ra

does not distinguish between peaks or troughs.Similarly, the Sa parameter is described as the arith-metic mean of the absolute of the ordinary valuesover a definition area. The authors compared the Saparameter from the focus variation system and the Ra

from stylus measurement and found an agreement. Itwas recommended that the areal parameters (Sa) weremore useful in measuring the surface of castings due toa better repeatability and a more substantial surfacesection being measured. The similarity in terms of non-homogeneity and unreliability of measurement betweencomposite and cast surfaces indicates that the Sa param-eter has good potential for application in composite sur-face roughness quantification.

Ghidossi et al.14 also looked at the effect of machin-ing parameters on roughness and failure stresses in edgetrimming of glass fibre reinforced plastic and carbonfibre reinforced plastic (CFRP). It was found thatincreasing the cutting speed reduced the surface rough-ness. However, a surprising result was found that thelowest failure stresses were seen on specimens machinedat the highest cutting speed. Therefore, the surfaceswith the highest Ra did not necessarily have the mostlikely failure, and the Ra parameter did not give a fullindication of damage.

Another relevant issue is whether the surface rough-ness parameter (Ra) alone can give an accurate repre-sentation of the damage from machining, or do othersurface roughness parameters such as maximum peakto valley height (Rt), Skewness (Rsk) and Kurtosis(Rku) also need to be considered. Herring et al.15 stu-died the effect of manufacturing method on the surfacetexture of CFRP mould facing surfaces using a numberof profile parameters recorded from a tactile measure-ment device. The parameters identified in this investi-gation were Ra, Rt, Rsk and Rku. Rsk is a measure ofthe symmetry of the height distribution and gives infor-mation about the amount of hills or valleys on the sur-face (Figure 1). Rku, demonstrated in Figure 1,explains the distribution sharpness and highlightswhether the surface profile has peaks and valleyswhich are either steep or rounded. Their investigationindicated that the roughness parameters studied werecapable of distinguishing the surfaces generated by dif-ferent manufacturing techniques. However, Rt wasmore sensitive to individual scratches or particles onthe surface. Interestingly, the authors state that at min-imum Ra, Rsk and Rku should be calculated to achievea useful understanding of a composite surface.

2 Journal of Composite Materials 0(0)



The literature has highlighted the potential problemswith the use of a stylus for composite roughness quanti-fication for the measurement of individual fibre orienta-tions. In addition the need for roughness parametersother than Ra has been identified. The motivation forthis work is therefore to compare the advantages ofroughness measurement of machined composite surfacesmade using the new optical system with a stylus profil-ometer. Following this an assessment is made of thedamage produced by an edge trimming experiment.

Experimental procedure

In the present study, multidirectional and unidirec-tional CFRP laminates were machined by full slot-milling and edge trimming, respectively. The two testsare included in separate experiment and discussion sec-tions. The surface quality was assessed using the non-contact optical method and compared with the stylusprofilometer to find the advantages of each system.

Optical system for texture measurements

Typically, profilometers are the most widely usedequipment for assessing surface quality of components,by measuring surface texture and calculating the Ra

roughness parameter. The profilometer works by trail-ing a diamond tipped stylus in a straight line along thespecimen. A transducer then detects small deviations inprofile height. In this study, a stylus was compared withthe commercial focus variation optical system manufac-tured by Alicona. The optical system is called anInfiniteFocus SL shown in Figure 2, where a schematicdiagram is also illustrated. The optical system cancreate three-dimensional surface images which capturecolour and topographical information, which can beused to analyse form and roughness parameters.

It can also be used to find areal surface roughness par-ameters including Sa. The optical system uses focusvariation of an optic, taking multiple images at varyingvertical distances and using optical sharpness to createa full high resolution 3D surface image. Danzl et al.16,17

details the system and demonstrates its capability toachieve similar results to a calibrated stylus deviceusing a standard roughness specimen. This system hasnot been applied to the characterization of machinedfibrous composite surfaces in any known literature.

Milling of multidirectional laminate (Test 1)

A multidirectional laminate made of carbon fibre in a0,�45,90,45 stacking sequence was slot milled in thisexperiment using a CMS Ares 5-axis Machine Tool.The composite was manufactured from lay-up of pre-preg and autoclave cured. The laminate had a thicknessof 4.1mm and 22 plies in total, shown in Figure 3(a).The fibre was of type T700GC and the Matrix is anepoxy resin of type Hexply M21. In Figure 3(b), thefibre orientation definition is explained, and the relationbetween cutting direction of the tool and fibre axis isshown. The composite was slot milled with a 25mmdiameter tool holder and polycrystalline diamond(PCD) tipped insert as shown in Figure 4. Table 1describes the parameters of the tool holder and insertused. PCD inserts were used due to their good surfacequality performance and an excellent resistance towear. The feed rate and cutting speed were bothvaried at four different levels and each test was repeatedonce to give a total of 32 tests as per Table 2. The cutswere performed with full tool holder diameter width ofcut, i.e. ae¼ 25mm, and axial depth of cut ofap¼ 4.1mm. A 52mm length of cut was used for eachtest. All of the texture measurements were taken fromthe same side of the slot in each test. There will be either

Figure 1. Explanation of Skewness and Kurtosis surface profile parameters. Adapted from Herring et al.14

Duboust et al. 3



conventional milling or climb milling on either side ofthe slot and this will result in different cutting mechan-isms and therefore surface quality. In order to be con-sistent, all of the measurements were taken from theintended machined surface which would be left on thepart, which was in conventional milling.

Experiment – Milling of unidirectionallaminate (Test 2)

In test number two, a unidirectional carbon fibre lamin-ate was edge trimmed in the Ares CMS 5-Axis Machine

Tool, using two different tool types and varying feedper tooth. In Figure 5, the 9.5mm diameter 4-flutemilling tool is shown, and the 10mm diameter, 12-flute burr tool. Both tools are diamond coated, whilethe burr tool has an ultra-fine diamond coating whichhas a very small grain size. The 12-flute tool is a burrstyle router which has a segmented helical cutting edge.This tool has a primary helix which is intersected by asecondary helix to create a number of cutting teeth.Each tooth has two edges which cut the material, andtwo non-cutting edges, which allow the removal ofchips and machined material. The helical 4 flute tool

Figure 3. (a) Composite stacking sequence (SEM). (b) Fibre orientation in relation to cutting direction.3

Figure 2. (a) Optical focus variation surface measurement instrument (Alicona), with machined CFRP sample. (b) Schematic diagram

of optical system.16




has a single helix with slightly positive rake angle. Bothtool parameters are shown in Table 3. Table 4 showsthe machining parameters used in the test. Samples ofthe unidirectional laminate were edge trimmed at threedifferent fibre orientations of 0, 45 and 90�. The toolswere compared at a constant r/min and the feed pertooth was varied at three different levels. The slottingoperation was performed at full tool diameter width ofengagement which was ae¼ 9.5mm and ae¼ 10mm, fortool one and tool two, respectively. The whole thicknessof the workpiece (7mm) was selected for the axial depthof cut ap, and an 80mm length of cut was used for eachtest. Each tool was replaced after making seven cuts inorder to reduce any effects of tool wear.

Roughness measurement methods

The surface roughness was measured after machiningusing both the profilometer and optical systems on theunidirectional laminate. Figure 6 shows the unidirec-tional sample and stylus profilometer device. Whileusing the stylus, repeat roughness measurements weretaken in three different positions of the sample in orderto increase reliability. The three measurements weretaken in the feed direction with stylus on the same pos-ition of each sample at the top, middle and bottom inorder to obtain an average result. A �c cut-off wave-length, of 800mm was used with a 4mm evaluationlength and 5 mm probe diameter.

For the optical system, the Sa areal roughness par-ameter was used which takes the roughness over a scanarea rather than across a profile line. This parametercalculates the arithmetic mean roughness of the surfacedeviation, similar to the Ra parameter, but it is usefulfor composite surfaces that are heterogeneous, due tocharacterisation over a larger area of the surface. Anevaluation area of 4mm by 4mm was used in order tocalculate the Sa parameter. A cut-off wavelength of800 mm was used in all measurements made with theoptical system according to ISO 4288. The cut-offwavelength determines the amount of

Figure 4. (a) Multi-directional CFRP with milled slot and fixture. (b) PCD insert with tool holder.

Table 1. Insert and tool holder parameters.

Insert Holder

Helix angle 0�

No. of flutes 4

Tool diameter 25 mm

Material Solid carbide

Cutting direction Right hand

Insert

Clearance angle – AN 21�

Rake Angle 21�

Insert width – W1 6.8 mm

Cutting edge

effective length – LE

11 mm

Major cutting edge angle 90�

No. of inserts 1

Cutting material PCD brazed to WC

Corner radius – RE 0.4 mm

Weight 0.002 kg

Max. axial depth of cut 4.5 mm

Table 2. Parameters used in multidirectional laminate test

(32 tests).

Feed per

tooth

(mm)

Feed

(mm/min)

Cutting

speed

(mm/min)

Axial

depth of cut

(ap), mm

Width

of cut (ae),

mm

0.025 17.75 85 4.1 25

0.04 28.41 175 4.1 25

0.06 42.61 225 4.1 25

0.075 53.26 285 4.1 25

Duboust et al. 5



low-spatial frequency components or waviness of thesurface profile which is removed, so that the desiredhigh-spatial frequency roughness parameters are mea-sured. A vertical resolution of 100 nm and a lateral

resolution of 2 mm were applied using the opticalsystem which is suitable for the composite which hasa fibre diameter of approximately 5 mm. Levelling of thesurface is first performed before filtering by setting areference plane which removes any profile or form devi-ation on the surface scans.

Results and discussion

Roughness measurements for multidirectionallaminate (Test 1)

The optical system works by taking a three-dimensionalsurface image in order to calculate roughness param-eters, these are shown in Figure 7. The advantage ofthe optical system is that it is possible to measure thesurface texture along a defined path of the material; ittherefore allows the measurement of individual plylayers and different fibre orientations, as highlighted bythe red line in Figure 7. The profile path taken for theroughness measurement is shown in Figure 7(a), for the0� fibre orientation, and for the transverse to feed direc-tion shown in Figure 7(b). It can be seen that there is alarge difference in maximum profile height between thetransverse measurement and 0� fibre orientation.

Some problems were found when using the stylusprofilometer. There is a large variation in the roughnessvalue measured depending upon the position or angleof the stylus, especially when measuring parallel tothe fibres. It was also found that the stylus path maynot lie directly parallel with the fibres, or pass overmultiple laminate layers. Similarly, it is difficult toknow which laminate layer or fibre orientation isbeing measured, as each layer has a slightly varyingthickness and cannot be seen with the naked eye.Therefore, it was preferred to use the optical systemfor measurements. It was found that it can accuratelymeasure individual ply layers, unlike the profilometerwhich proved to be unreliable for measuring roughnessparallel to the fibres direction. Importantly, the rough-ness measurements made with a stylus were thereforenot considered fully dependable for a multidirectionallaminate.

Figure 8 shows the average roughness on each of thefibre orientations taken by the optical method. It wasfound that there is a large variation in roughness acrossthe surface of one laminate, and that the 135 fibreorientation had the greatest roughness, while the 45�

had the lowest. This effect can be explained by the dif-ferent cutting mechanisms across each of the fibreorientations which will be explained further in theSEM images (multidirectional laminate) section. The135� fibre orientation was found to have an averageroughness nearly a factor of 10 greater than the 45�

fibre orientation. It can therefore be realised that the

Figure 5. (a) Tool 1, helical 4-flute tool by Sandvik (9.5 mm

diameter). (b) Tool 2, 12-flute burr tool by OSG (10 mm

diameter).

Table 3. Milling tool properties. Unidirectional laminate –

Test 2.

Tool 1 – Helical

end mill Tool 2 – Burr tool

Tool diameter 9.5 mm 10 mm

No. of flutes 4 12

Coating Diamond like coating

(average thickness

7.5 mm)

Ultra-fine

diamond

coating

Rake angle 1.5� 8�

Clearance angle Primary – 11.5�

Secondary – 9�16�

Helix angle 30� 15�




Figure 7. (a) 3D surface structure images of multidirectional laminate measured by optical system. Red line shows the user selected

roughness measurement path in transverse direction and a 0� laminate. (b) Profile path for 0� fibre orientation (c) profile path for

transverse direction.

Figure 6. (a) Mitoyoto profilometer on uni-directional Laminate. (b) Uni-directional edge trimmed laminate (test 2).

Table 4. Cutting parameters in unidirectional test.

Feed per

tooth (mm) r/min

Fibre

orientation

Feed speed

(mm/min)

Tool 1

Feed speed

(mm/min)

Tool 2 ap (mm) ae (mm)

0.005 4775 0 95.5 286.5 7 Tool 1 – 9.5

0.0095 4775 45 181.4 544.3 7 Tool 2 – 10

0.014 4775 90 267.4 802.1 7

Duboust et al. 7



135� fibre orientation is critical for determining thehighest roughness and most prominent damage.

In order to assess the suitability of using the stylusfor measurement it was compared with an opticalapproach: it was found that when using the stylus formeasurement, the roughness has a high probabilityof being under-represented, if the stylus path does notfall across the 135� fibre orientation. Consequently,there is an expected uncertainty in using the stylusfor roughness measurement of a multidirectional lamin-ate due to the variation in structure at different areas onthe surface.

In Figure 9, the roughness across the 90� fibre orien-tation is plotted against feed rate grouped by all of theapplied cutting speeds. Similar trends in the data wereseen across the 0, 45 and 90� orientations. However, forthe 135� orientations, there was a higher magnitudeand standard deviation or scatter in the roughnessmeasurements. The obtained data for the latter orien-tation showed slightly different trends across therange of cutting speeds. This can be explained bythe different cutting mechanism and large variation ofthe structure due to pits and torn chunks. It can beconcluded from Figure 9 that increasing the feed ratecaused an increase in the surface roughness across all ofthe cutting speeds used.

A component of this increase in roughness will be dueto the increase in ideal roughness – which is a function ofthe square of the feed and tool nose radius.19 Idealroughness represents the best surface finish which willbe possible for a given feed and tool geometry.

Figure 10 shows a main effects plot for the feed rateand cutting speed which was produced using Minitab

software.19 A main effects plot works by creating amean response from each factor using all of the testdata. In this case, the two factors were feed rateand cutting speed and their corresponding outputeffect on Ra. Figure 10 shows that there was a meanincrease in the surface roughness due to a higher feedrate. Statistical methods were applied using regressionmodelling and it was found that the feed rate had astronger contribution to increasing the roughness thancutting speed. The feed rate had a P value of 0.03 whichindicates a statistically significant effect on the results.

Figure 8. Average roughness versus fibre orientation (multidirectional laminate), with error bars shown as standard deviation using

optical technique.

Figure 9. Roughness calculated for 90� fibre orientation,

versus feed rate at each cutting speed. Shown for multidirectional

laminate and optical technique.




It was also found that the feed rate had a more signifi-cant effect on the 135� orientation. This increase inroughness with feed can be explained by the changein the effective chip thickness of material which isremoved. Ahmad and Shahid20 have shown that thelowest surface roughness is strongly correlated withdecreasing the equivalent chip thickness, which is a geo-metric function of the feed rate and cutting speed.Therefore, in order to get the best surface quality,both the feed and cutting speed parameters must beoptimised together.

Figure 10 illustrates the effect of increasing thecutting speed from 85 to 225 (m/s) caused a meanincrease in the roughness, however, at the highestspeed of 285 (m/s) there was a mean decrease. Thecutting mechanism and the temperature of tool–work-piece interface will be affected by changes in cuttingspeed. An effect of increasing the cutting speed is thatduring each pass of the cutting edge, there will be lesstime for energy dissipation. This will lead to anincreased cutting temperature – as the tool heats dueto friction – and can cause thermal softening of thematrix and a reduction in its stiffness. As a result, thematrix would hold the fibres together less strongly andthere would be a reduction in cutting forces. Smearedmatrix on the surface could also be a by-product of anincrease in the cutting temperature and show a reduc-tion in the surface roughness. It has also been reportedthat the cutting mechanism can be changed to a ‘‘mech-anical wrenching’’ due to increases in the cuttingspeed.21

Roughness measurements for unidirectionallaminate (Test 2)

For the unidirectional test, the main effects plot forroughness with increasing feed per tooth for each ofthe different tools types as shown in Figure 11.Texture measurements were taken using the opticalsystem which enabled calculation of the Sa areal rough-ness parameter. Overall results showed that the

Figure 10. Roughness of 90� fibre orientation, against feed rate at each cutting speed. Shown for multidirectional laminate and

optical technique.

Figure 11. Plot of the mean roughness (Sa) versus feed per

tooth by optical technique for each of the two tool types uni-

directional laminate. Tool 1, 4-flute helical fluted tool. Tool 2,

burr style 12 flute.

Duboust et al. 9



minimum average roughness was again found on the45� fibre orientation laminates and roughness increasedwith feed rate. An average roughness of 2.1mm wasseen on the 0� fibre orientation, 1.85mm on the 45,and 2.0mm on the 90� fibre orientation. These trendscompare well with the results from Test 1 on the multi-directional laminate. When the optical system was com-pared against the stylus profilometer measurementmethod in this test the results showed the sametrends. The use of areal parameters (Sa) was found tobe advantageous when using the optical device becauseit will take into account the damage across a largermeasurement region and therefore the roughness wasfound to be less sensitive to measurement position thanby stylus. For this reason, the optical measurementswere presented.

Using regression methods, the tool type hadthe strongest effect on the surface roughness followed byfeed rate, while both had a P value which showed a stat-istically significant effect on the roughness. In Figure 12, acomparison of the tools is shown; the 12 flute burr toolwas found to leave a higher Sa surface roughness than the4-flute single helix tool – while running at the same feedper tooth. However, the 12-flute burr tool was also remov-ing the material at faster rate. The burr style tool had amaximum material removal rate of 56.2 cm3/min, com-pared to 17.8 cm3/min for the 4-flute helical tool.Haddad et al.21 also found that a burr style tool produceda higher surface roughness than a fluted tool. Theyreported a different cutting mechanism between the twotools and that the burr style tool was more similar to anabrasive machining process, there was closely spacedgrooves left on the surface, which were dependent on

feed rate. In agreement with our findings (Figure 11),the authors found a larger increase in the roughness dueto feed rate in the burr style tool, which was attributed toan increase in temperature with feed.

The benefits of the optical system were found to bestrongest when used to measure a multidirectional lamin-ate (Test 1). This is because the unidirectional laminatehas a lower variation in surface texture and therefore themeasurement is less sensitive to stylus path. It was foundthat the variation or scatter in measurements made usingthe profilometer on the unidirectional laminate was lessthan that seen on the multidirectional. Therefore, the useof the optical device will be beneficial on a multidirec-tional composite surface as it allows the ability to takeinto account the different degrees of damage across all ofthe fibre orientations, and this will also apply to othercommonly used fibrous composites like glass fibre. Theoptical device and areal parameters have been found tobe less sensitive to measurement position, or path, thanthe stylus and therefore will provide a more accuratemetric with which to assess machined composite surfaces.

Skewness and Kurtosis measurements (Test 1)

The Kurtosis and Skewness are not represented in theRa parameter, and it is known that two different sur-faces can have the same Ra.

7 Therefore, these add-itional parameters were investigated to see if they canbe used to more thoroughly characterise the machinedsurface and damage from machining. Histograms of theprofile at varying fibre orientations were also taken.These show the percentage of all the values at eachheight above the mean profile line.

Figure 12. 135� fibre orientation histogram on multidirectional laminate using optical system.




The average Skewness and Kurtosis across each ofthe fibre orientations was analysed as shown in Table 5.It was found that the average Skewness for the 135�

orientation was negative, and that in the histogram inFigure 12 there is a longer tail in the negative direction.A negative Skewness characterises a surface with deepcracks or voids, and this can be explained by the chunksof torn material and fibre pull-out which has beenremoved from the material, which results in pitting.On the other hand, the 45 and 90� fibre orientationstypically had a positive Skewness. This could beexplained by the lack of sub-surface damage andfibres which protrude from the surface. The 0� fibreorientation also had a positive Skewness due to thelack of cracks propagating below the surface. TheSkewness can therefore be a useful indicator of thedamage in machined composites from machining, andwhether the surface is characterised by protrudingfibres (positive Rsk) or by fibre pull-out and voids(negative Rsk). The Rt parameter was analysed alongwith the Sa and Ra parameters, and the mean Rt acrosseach fibre orientation is shown in Table 5. However, theRt parameter was not found to distinguish betweendamage on the 0, 45 and 90 plies more effectivelythan the Sa or Ra parameter. The Rsk and Rku para-meter values were therefore analysed in more detail dueto their ability to give unique information about thesurface structure on each fibre orientation.

The highest Kurtosis was found for the 90� fibreorientation. This describes a surface with steep orsharp peaks and can be explained by uncut fibres pro-truding perpendicularly from the surface. The histo-gram shows a lack of tails in the positive or negativedirection shown in Figure 13. All of the fibre orienta-tions have a relatively high Kurtosis suggesting thereare quite sharp peaks and valleys. The 135� fibre orien-tation had the lowest Kurtosis which can be explainedby the torn chunks of material and pits rather than thesharp protruding fibres on the 90� orientation. Theseparameters were analysed using the optical system, thiswould not have been possible using the stylus profil-ometer due to the difficulty in measuring one layer ofthe multidirectional laminate. Rsk was found to be themost useful parameter due to its ability to distinguish

surfaces with cracks and pits versus protruding or un-cut fibres.

SEM images (multidirectional laminate)

SEM images were used to take micrographs of the sur-face of the machined samples from the multidirectionallaminate (Test 1). These were used to identify the extentof the damage from machining. As depicted in theSEM images shown in Figure 14 and at a higher mag-nification in Figure 15, the fibre orientation had astrong effect on the machining damage and quality ofthe surface. The 0 and 45� fibre orientations were seento be relatively smooth. While for the 135� fibre orien-tation, the surface quality was significantly worse,with large pits and torn fibres on the surface as indi-cated in Figures 14 and 15(b).

The explanation for the worse surface quality for the135� fibre orientation is due to the cutting mechanism:the fibres are bent and subsequently a primary fracturewill propagate below the surface along the fibre matrixboundary. Finally, the fibres break in a combination ofbending and fibre shearing.22 This effect can be seen bythe large pits and fibres which have been bent out ofplane on the surface. A bouncing back effect has beensuggested by Wang and Zang.2 This happens afterthe cutting tool has passed and the fibres spring back.This effect would be expected to be greater in cuttingthe 135� orientation due to the large amount of bendingand buckling of the fibres.

Machining of the 0� fibre orientation causes abending and fracture type chip removal mechanism.From the SEM micrographs, it can be seen that thedamage does not propagate below the surface to thesame extent as in the 135� fibre orientation. Most ofthe fibres can be seen lying on the surface in theiroriginal orientation without damage indicated inFigures 14(a) and 15(a). Here, the fibres are removedin a bending and crushing mechanism which causesthe fibre to de-bond from the matrix and then frac-ture. This effect was also found in the work by Wangand Zang.2 and Wang et al.5

The 90� fibre orientation shown in Figure 14(b) issmoother than the 135 orientation but was generally

Table 5. Skewness and Kurtosis at each fibre orientation for the Multidirectional laminate.

Fibre

orientation (�)

Average

Skewness Rsk

Average

Kurtosis Rku

Multidirectional

laminate – mean (Ra (mm))

Multidirectional

laminate – mean (Rt (mm))

0 1.55 13.73 0.4 4.7

45 1.68 13.01 0.3 2.9

90 1.48 15 0.5 4.9

135 (�)1.25 9.25 2.7 38.4

Duboust et al. 11



found to have a slightly higher roughness than the 0 and45� orientations. However, the surface is different to thatof the 0� fibre orientation shown in Figure 14(a). Muchof the surface is covered in smeared matrix, but somefibre ends can be seen on the surface, which lie in their

original orientation. Any variation in the surface heightappears to be caused by fibres which have been shearedat different lengths. A fibre shearing cutting mechanismis suggested in accordance with reports from Sheikh-Ahmad.3

Figure 14. (a) SEM image of 135 and 0� ply comparison. (b) SEM image of 90 and 135� ply. (These tests were performed at a cutting

speed of 175 mm/min and feed of 0.075 mm/rev on multidirectional laminate.)

Figure 13. 90� fibre orientation histogram on multidirectional laminate using optical system.




The 45� fibre orientation is most similar to the 90�

fibre orientation in appearance and again much of thesurface is covered with smeared matrix and somesheared fibre ends as indicated in Figure 15(c). In thisimage it is hard to distinguish any individual fibres andthe surface looks very smooth. The machined surfacefor the 45� fibre orientation was found to be the leastrough and there appears to be very little damage orcracks propagating below the surface. This suggeststhat most of the fibres are being sheared throughtheir cross section, without being bent out of plane. Afibre crushing and cutting mechanism has been sug-gested in the literature for machining of the 45� fibreorientation.5 Pecat et al.23 found that the resultant cut-ting force was highest while machining the 45� fibreorientation. This was explained due to a strong fibretension-dominated failure mode, in contrast to a weakmatrix dominated failure mode in the 135� orientation.

Conclusions

This research used a new optical method to measuresurface roughness and damage of machined compositesurfaces. The results have confirmed that the fibreorientation plays an important role in the chip removalmechanism and surface damage. The 135� fibre orien-tation was found to be the critical fibre orientation forsurface damage and roughness measurement. Kurtosisand Skewness roughness parameters were found tobe useful and give a more thorough representation of

the surface quality and damage, including showingthe presence of cracks or pits and protruding fibres.Statistical methods showed that the feed rate and tooltype had the most significant effect on the surface qual-ity, and that the 135� fibre orientation was the moststrongly affected by changes in feed.

The non-contact optical method has been shown tobe a useful research tool for measuring surface rough-ness of machined composites. Importantly, it allows thecharacterisation of roughness or damage on individualply layers of a machined multidirectional laminatewhich is not possible with a stylus. The techniqueavoids the unreliability issues that were found to beassociated with the quantification of surface roughnessusing a stylus, and thus enables a more thorough char-acterization of the profile and the complex damagemechanisms seen in machined fibrous composite lamin-ates. It is suggested that the use of areal surface param-eters like Sa should be further adopted in roughnessmeasurement of fibrous composite surfaces and willincrease reliability of roughness measurement for multi-directional laminates.

However, there were some limitations found with thedevice, which is constrained by the distance between thestage and lens and is therefore unsuitable for measuringlarge parts. It is also less portable and more expensivethan the stylus, and it is thought that the device willbecome more appropriate to an industrial machiningapplication when combined with the use of roboticarms to make automated or in-machine roughness

Figure 15. (a) 0� fibre orientation SEM image. (b) 135� fibre orientation SEM image. (c) 45� fibre orientation SEM image. (These

tests were performed at a cutting speed of 175 mm/min and feed of 0.075 mm/rev on multidirectional laminate.)

Duboust et al. 13



measurements. The results from this work can be usedto demonstrate new opportunities for academia andindustry in understanding roughness measurementand optimising the machining of polymeric compositematerials.

Acknowledgements

The authors thank the staff at the Knowledge Transfer Center

at the AMRC and Professor Richard Leach of the Universityof Nottingham for his support.

Declaration of Conflicting Interests

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

article.

Funding

The author(s) disclosed receipt of the following financial sup-port for the research, authorship, and/or publication of this

article: This work was co-funded through the EPSRCIndustrial Doctorate Centre in Machining Science (EP/I01800X/1) and by Rolls-Royce whom the authors wouldlike to thank.

References

1. Ramulu M, Wern CW and Garbini JL. Effect of fibre

direction on surface roughness measurements of machined

graphite/epoxy composite. Compos Manuf 1993; 4: 39–51.2. Wang XM and Zhang LC. An experimental investigation

into the orthogonal cutting of unidirectional fibre reinforced

plastics. Int J Mach Tools Manuf 2003; 43: 1015–1022.3. Sheikh-Ahmad J. Machining of polymer composites.

Boston, MA: Springer US, 2009.

4. El-Hofy MH, Soo SL, Aspinwall DK, et al. Factors affect-

ing workpiece surface integrity in slotting of CFRP.

Procedia Eng 2011; 19: 94–99.

5. Wang D, Ramulu M and Arola D. Orthogonal cutting mech-

anisms of graphite/epoxy composite. Part I: unidirectional

laminate. Int J Mach Tools Manuf 1995; 35: 1623–1628.6. Azmi AI, Lin RJT and Bhattacharyya D. Experimental

study of machinability of GFRP composites by end

milling. Mater Manuf Process 2012; 27: 1045–1050.7. Davim JP. Surface integrity in machining. London:

Springer, 2010.

8. Haddad M, Zitoune R, Bougherara H, et al. Study of

trimming damages of CFRP structures in function of the

machining processes and their impact on the mechanical

behavior. Compos Part B Eng 2014; 57: 136–143.

9. Azmi AI, Lin RJT and Bhattacharyya D. Machinabilitystudy of glass fibre-reinforced polymer composites duringend milling. Int J Adv Manuf Technol 2012; 64: 247–2461.

10. Sheikh-Ahmad J, Urban N and Cheraghi H. Machiningdamage in edge trimming of CFRP.Mater Manuf Process2012; 27: 802–808.

11. Davim JP, Reis P and Antonio CC. A study on milling of

glass fiber reinforced plastics manufactured by hand-layup using statistical analysis (ANOVA). Compos Struct2004; 64: 493–500.

12. Leach R. Fundamental principles of engineering nanome-trology. Elselvier, 2014.

13. Nwaogu UC, Tiedje NS and Hansen HN. A non-contact

3D method to characterize the surface roughness of cast-ings. J Mater Process Technol 2013; 213: 59–68.

14. Ghidossi P, Mansori MEl and Pierron F. Edge machining

effects on the failure of polymer matrix composite cou-pons. Compos Part A Appl Sci Manuf 2004; 35: 989–999.

15. Herring ML, Mardel JI and Fox BL. The effect of mater-ial selection and manufacturing process on the surface

finish of carbon fibre composites. J Mater ProcessTechnol 2010; 210: 926–940.

16. Danzl R, Helmli F and Scherer S. Focus variation – a

robust technology for high resolution optical 3D surfacemetrology. Strojniski vestn. J Mech Eng 2011; 57:245–256.

17. Danzl R, Helmli F and Scherer S. Comparison of rough-ness measurements between a contact stylus instrument andan optical measurement device based on a colour focussensor. Vol III 284, Boston: Nanotech, 2006, pp.284–287.

18. Groover M. Fundamentals of modern manufacturing:materials, processes, and systems. John Wiley & Sons,2007.

19. Minitab. Minitab support – what is a main effects plot?n.d. http://support.minitab.com/en-us/minitab/17/topic-library/modeling-statistics/anova/basics/what-is-a-main-

effects-plot/ (accessed 21 November 2014).20. Ahmad JS and Shahid AH. Effect of edge trimming on

failure stress of carbon fibre polymer composites. Int J

Mach Mach Mater 2013; 13: 331.21. Haddad M, Zitoune R, Eyma F, et al. Study of the sur-

face defects and dust generated during trimming ofCFRP: influence of tool geometry, machining parameters

and cutting speed range. Compos Part A 2014; 66:142–154.

22. Arola D and Ramulu M. Orthogonal cutting of fiber-

reinforced composites a finite element analysis. Int JMech Sci 1995; 39: 597–613.

23. Pecat O, Rentsch R and Brinksmeier E. Influence of

Milling process parameters on the surface integrity ofCFRP. Procedia CIRP 2012; 1: 466–470.



http://support.minitab.com/en-us/minitab/17/topic-library/modeling-statistics/anova/basics/what-is-a-main-effects-plot/




an optical method for measuring surface roughness of machined carbon fibre-reinforced plastic...

Documents