Drilling Woven CFRP Composites

7/27/2019 Drilling Woven CFRP Composites

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DOI:10.1002/adem.201200342

In Situ Qualitative Inspection of Hole Exit Delamination at Bottom-Ply during Drilling of Woven CFRP Epoxy Composite LaminatesByAli Faraz* and Dirk Biermann

It is commonly known that drilling is mostlyperformed as apost-manufacturing operation for carbon ber reinforcedplastics (CFRP) parts andcomponents, mainlyfor their further joining and assembling. Sometimes, hundreds of thousands of holes are drilled on a complete aircraft unit manufacture, forriveting and bolted-joints. The delamination phenomenon

during drilling of CFRPs has been recognized as one of thecritical problems by all of the researchers. It is mainly denedas an inter-laminar or -ply failure behavior of the CFRPcomposites. At the hole entrance periphery, it is called aspeel-up delamination, or simply hole entry delamination. Itoccurs more severely at the bottom-most surface or ply of CFRP materials and is called as push-out or -downdelamination, or simply hole exit delamination. [1] Theresearchers do have a major consensus in regarding orconsidering the hole exit delamination in drilling CFRPcomposites as very critical, with respect to the nal quality of the components and their (sub)assemblies. Therefore, aconsiderable amount of research has been carried out by

[*] Dr. A. Faraz, Prof. D. Biermann Institute of Machining Technology (ISF), Baroper Strae 301Technische Universita t Dortmund, 44227 Dortmund,GermanyE-mail: [email protected]

Exploiting very high speed digital videography, an in situ examination of the hole exit delamination atthe bottom-most ply during drilling holes in the selected woven CFRP epoxy laminates is presented. Atthe beginning, a rotating elastic bulge of the carbon bers at the bottom-ply, which is just theimpression of the protruding drill chisel edge, was always observed. Following the elastic bulging, a few, initial cracks along the weak ber/matrix interfaces appeared. Thereafter, tensile failures in thecarbon bers were seen. The exact location of the initiation of these ber failures specically depends onthe actual drill-hole position with respect to the woven conguration of the bottom-ply. A visual model for the weak interstitial or undulated regions at the bottom-ply is also proposed in this paper, showingthe undulating bers, which are susceptible to mostly tensile failures under the drilling loads. During asub-completion drilling-phase at the bottom-ply, various cracks were seen to be propagating mostly vialinear paths. Also, the exit delamination at the bottom-ply during a sub-completion drilling-phase wasalways observed as to be divided into various small, independent localized contourseach of whichpropagated almost independently through within several individual warps/wefts during drill-feed. Also, the shape of each such tiny contour within a single warp/weft was identied as elliptic, which isobserved around an entire drill-hole in unidirectional (UD) composites as reported in literature. It wasalso observed that the overhanging cantilever-like bers at the bottom-ply are really difcult to cut, oncetheir base-location or their exit delamination contour reaches outside the hole nominal diameter. Moreover, by referring to some very basic cutting angle congurations for the orthogonal trimming of UD-composites as found rarely in archival literature, an illustrative model diagram is also proposed forthe drilling of the selected laminate material. This idea is also approximately validated via a few visualobservations. The proposed visual model is generally an attempt in correlating the observed peripheralhole quality (delamination) with various instantaneous tool/ber engagement congurations occurringacross the entire drill-hole periphery at the bottom-ply, during the very last drilling-phase.

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determine, analyze, and to control the inevitable problem of the hole exit delamination damage in drilling various types of CFRP composites.

The work of DiPaolo et al. back in 1996[2] perhaps isthe very rst known attempt for capturing the initial creation

and accruement of hole exit delamination cracks duringdrilling of unidirectional (UD) CFRP epoxy laminates byvirtue of video techniques. They identied three signicanttypes of the observed cracks and interpreted them via some basic principles of fracture mechanics. They recorded drillingloads in order to correlate them with fracture growth and toinfer the cause and extent of delamination damage at the bottom-most ply of the selected UD laminates. Shape of thehole exit delamination in their observations appeared to be of typical form, which is usually elliptical in case of drilling UDber reinforced plastics (FRPs). A contrast exists among thevarious approaches by many different researchers, whohave modeled analytically the critical drilling forces (mostlythrust) for UD FRPs so far, in assuming the shape of the holeexit delamination. A circular shape is mainly assumed forsimplicity. Like various other researchers, Hocheng andTsao [3] also assumed a circular shape of delamination formodeling critical thrust force, which is required for the onsetof delamination, for various designs of drill bits. They lateron, however, incorporated an ellipticity-ratio in anotherresearch [4] in order to address material anisotropy issues of the UD laminates.

Earlier, Jain and Yang [5] also employed linear elasticfracture mechanics (LEFM) to analytically predict the criticalthrust force, as required for theonset of hole exit delamination

crack. According to them, in an ideally isotropic laminate,the shape of delamination would be circular. Whereas, forUD laminates, the observed delamination shape will beellipticalits principal directions parallel and transverse tothe ber orientation within the laminate as shown in Figure 1.The elliptical shape of the hole exit delamination wasexperimentally veried by the same authors in anotherresearch, [6] later on.

In the same context, the main ndings of Singh andBhatnagar [7] in drilling UD glass ber reinforced plastics(GFRP) were also the same. Singh et al.,[8] in the results of one of their experimental and FEA study, also observed thesame shape.

One of the major in situ observations made in this paper,during drilling the chosen woven epoxy CFRP laminates, wasthat the hole exit delamination damage at the bottom-mostlaminate always appeared as divided into several small,localized, independent macroscopic contours which werespotted within a single warp, or similarly, within a single weftof the woven CFRP fabric used. When carefully observed,which will be evidenced in the results later on, the shape of each such individual localized delamination contour couldroughly be identied as elliptical within a single weft or warpof the chosen fabric laminates. Therefore, a schematic sketchor an illustrative model for the said localized delamination

contours or regions across a hole exit periphery is beingproposed and presented [9] in this paper, in advance inFigure 2, for a better understanding of the relevant results(visual observations) discussed later on. It is also asserted herethat due to a relatively broader warp or weft size value of 5 mm 2.5 mm (approximately) of the selected woven CFRPlaminate material conguration, the straight carbon bers,which are oating freely within a single warp/weft of the bottom-most ply, might also have almost similar character-istics as that of the straight oating bers at the bottom-mostply in a UD laminate.

Regarding the extent or spread of the hole exit damage indrilling CFRPs, Ko nig and Gra [10] wrote that that in drillingthe UD laminates, the delamination damage or crack(s) canpropagate as wide as up to a location where the adhesion between the ber and matrix at their mutual interface, couldsustain the occurring drilling loads. On the other hand, in caseof a woven fabric laminate, the corresponding cracks might betraced up to next available crossing bers (ber-crossovers)that could block or resist their propagation. We may presumethat the extent or the width of the hole exit damagepropagation may strongly depend on the oat-length or-width of the concerned bers of the weft and warp regions.Shin and Jang [11] studied various possible paths of thedelamination cracks in their fractographical analysis of the woven epoxy CFRP laminates. Two different possiblecharacteristic paths for crack propagation in the warp(s) of the selected woven composite were identied by them. Firsttype of crack was proposed as to be propagating alone in thematrix, owing to the brittle fracture of the epoxy matrix resinof the warp, until it reached the adjacent or the next weft.Whereas, the other crack was traced as to be propagatingalong the mutual ber/matrix interface of the warp,translating till the next weft. They, however, did not reportanything concrete about the failures (break) of the carbonbers in their aforementioned research.

A. Faraz and D. Biermann/In-Situ Qualitative Inspection of Hole Exit Delamination. . .

Fig. 1. Schematic sketch of elliptical shape of hole exit delamination inUD-composites.[5]

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The woven fabric (plain, twill, or satin) composites aregiven preference over the UD ones in those industrialapplications where complex part surfaces and geometriesare involved. The formers are more drapeable as compared tothe latter ones. On behalf of the in situ observations of this

paper, which will be shown and explained in detail later on, aschematic sketch (model) as given in Figure 3 is also beingproposed and presented here in advance. The major inferenceof this visual model is that that an interstitial as well as anundulated region (carbon bers in the form of a bend) in the


Fig. 2. Proposed illustrative model for several tiny, individual, or localized exit delamination contours present within a single warp/weft. The linear paths of crack propagation at thebottom-most ply during drilling the chosen woven CFRP are also illustrated.[9]

Fig. 3. (a) Sketch of a woven fabric[11] (b) proposed illustrative model sketch (based on results of this paper) for carbon ber failures at an interstitial or undulated interface ina wovenCFRP fabric laminate.[9]

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region, when compared to theonewith straight, freelyoatingbers. Figure 3 illustrates the aforementioned regionsschematically. Figure 3(a) is a reproduction of a gure takenfrom ref. [11] of this paper. Whereas, Figure 3(b) illustrates thevisualmodel which is being proposed anew in this research.A

typical, schematic carbon ber undulation (bend) is marked by a dashed-box at the shown interstitial region in Figure 3(b).It is much likely that these undulating carbon bers having bends are already prone to residual tensile stresses due tolaminate curing; which makes them more susceptible totensile fractures or failures under the applied drilling loads asshown, compared to those bers which are oating straightand freely within a single weft or a warp. [9] As discussedabove, Konig and Gra [10] also quoted the possibility of hindrance offered by the ber-crossovers to crack propagationon its way in a woven laminate. Therefore, the basicassumption in this paper is that the hindrance caused bythe intersecting ber-crossovers gives rise to the tension,resulting in their eventual fractures, as illustrated inFigure 3(b).

Valuable research, however in a very few amount hitherto,for investigating cutting congurations or the resultingmechanisms with respect to various ber orientations inorthogonal trimming of FRPs has been added to archivalliterature. Sakuma and Seto [12] related the inuence of theber orientation on thecutting loads, thesurface nish andthetool wear in turning the selected GFRP composites. Later on,the work of Wang et al.[13] may be considered as of primeimportance in introducing andcategorizing three very basic or

the principal orthogonal cutting mechanisms in edge-trimming of the selected UD graphite epoxy laminates. Themechanisms were identied and explained mainly on behalf of the dened three different ranges for various cutting tool/ber orientation (conguration) angle values in a laminaterelative to the tool feed direction. A similar approach for

determining the chip formation mechanism in orthogonalmachining of UD CFRP laminates can also be found in thework of Bhatnagar et al.[14] They performed the Iosipescushear tests, in parallel to their machining investigations, tocharacterize shear properties of the carbon bers havingdifferent ber orientations.

It is noteworthy here that during the drilling process of CFRPs, or even that of FRPs in general, the ber orientationvaries with every innitesimal instant of a single revolution of the main cutting edge of a drill bit. The cutting angleaninstantaneous angle subtended between the orientation of thestraight oating bers and the instantaneous velocity vectorof the revolving main cutting edgecontinuously varies duringa single drill bit revolution. Via the experimental observationspresented at the end of this paper, the three principalorthogonal cutting congurations along with their angleranges as given in ref. [13] are also examined for the drillingprocess of the selected woven CFRP laminates in the currentwork. Therefore, another illustrative model sketch is beingproposed here in this paper as given in Figure 4. This visualmodel approximately identies and correlates the threeprincipal instantaneous cutting anglecongurations or rangesacross the entire hole exit periphery during drilling theselected woven CFRP composite, while presuming the shown


Fig. 4. (a) A proposed visual model in this paper for various instantaneous cutting angle congurations at the bottom-ply in drilling the selected woven CFRP assuming the showndrill hole location.[9] (b) Three principal orthogonal cutting congurations as proposed in ref.[13] .


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hole-position relative to the rotating drill bit cutting edgesright at the bottom-most ply during the very nal drilling-phase. The supposed instantaneous points (A) and (E)correspond to the Case-I; whereas, the point (F) relates tothe Case-II; and similarly, (B)(D) belong to the Case-III typeorthogonal cutting mechanism [13] as reproduced in

Figure 4(b).The aimof this paper is to analyze, in situ, some qualitativeaspects of the hole exit delamination during drilling through-holes in the selected woven epoxy CFRP fabric laminates,using uncoated cemented carbide twist drill bits, on ahorizontal spindle CNC machining center. A modern digital(CMOS-sensor) high speed video camera was utilized tomonitor the creation and propagation of cracks and the exitdelamination damage at the bottom-most laminate-ply of theselected woven (twill 2/2) CFRP epoxy laminate material. Apure elastic behavior (elastic bulging) of carbon bers at the bottom-most laminate-ply, prior to their initial fractures, wasalways observed, initially, in the acquired high speedfootages. The initiation and propagation of very initialmacrofailures at the bottom-ply were examined. Variouspossible paths for cracks in linear form and carbon berfailures are also examined with respect to the nature andconguration of the selected woven fabric CFRP composite.The exit delamination observed to be divided in various tiny,localized individual contours and their individual propaga-tion, all is also explained during a sub-completion drilling-phase. The difcult-to-cut exible or elastic behavior of somespalled cantilever-like overhanging carbon bers and their bundles across the hole exit periphery is also described.Finally, an attempt is made for generally identifying

the three principal orthogonal cutting angle congurations,which were described for the orthogonal trimming of UD laminates in ref. [13] , and roughly examining andcorrelating the resulting delamination quality at the bottom-ply of the selected woven CFRP material, during the very naldrilling-phase.

1. Experimental Techniques

1.1. Workpiece Material and the Utilized Cutting ToolsThe selected workpiece material was woven CFRP epoxy

laminates with a thickness of 10 mm. Sixteen plies of theSigratex CE 8204-650-42 prepegs having twill 2/2 weavepattern, were stacked up together to form one cured laminate.The binding matrix was epoxy type E201 (hot cured)thermoset resin. The post-process ber content of thecomposite laminates reached a maximum limit of 60% byvolume. Some of the key mechanical properties of the usedwoven epoxy CFRP composite laminates are listed in Table 1.

Two totally different geometries of uncoated cementedcarbide twist drill bits (details given in Table 2) having anequal diameter of 8 mm were used in this study. The uncoatedcarbide tools are much suitable when compared to the TiN- orTiAlN-coated ones, because of the inappropriate tribological behavior in cutting, the lower fracture toughness and the

relatively weaker interlayer adhesion of these coatings to thecarbide substrate. [15] The selected drill bit T1 has a very neand tough grain-structure K30F, which is why it is usuallyrecommended for the applications with abrasive workpiecematerials like CFRPs. T2 is also a twist drill bit with two utes.However, it is not suitable for making holes in FRPs, owing toits near-to-zero geometry and a relatively larger chisel-edge.

1.2. Experimental Setup, Cutting Conditions, and Videography

DetailsDrilling tests were carried out on the horizontal-spindle

CNCmachining center, called Ixion (model: TLF1004). Thisis a very robust 4-axis machine-tool designed specically forthe deep-hole-drilling operation, and has the maximumspindle speed and powerof 6000 rpmand 11 kW, respectively.The cutting conditions implemented in this study are listed in

Table 3. No coolant was used during the experiments.Figure 5(a) illustrates schematically the used experimental

setup for the drilling tests performed in this research, whereasa small glimpse of the actual experimental ambience is alsodepicted in the portion (b) of the same gure. The workpiecewas clamped rigidly and vertically on a xture as shown,which was mounted onto the machine-table directly, such thatthe exit delamination could easily be visually recorded. Thiswas achieved by milling a circular cavity into the vertical/


Table 1. Mechanical properties of the CFRP laminates used in this study.

Prepeg name Sigratex CE 8204-650-42Resin type Epoxy E201 (hot-cured)Weave type Twill 2/2Laminate density, r [gcm

3] 1.55Fiber content [% by volume] 5560Tensile strength [MPa] 760Youngs modulus, E [GPa] 70Flexural strength [MPa] 780Shear modulus, G [GPa] 55Resin glass transition temperature, T g [

8 C] 140Carbon ber type HT (high tenacity) berYarn type 12K (DIN 65184)

Table 2. Drill bit utilized in this study.

Drill bit

No. of lips 2 2Point angle [deg] 118 85Rake angle [a] [deg] 29 0Clearance angle [deg] 8 18

[a] Measured right at the corner of the main cutting edge.



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upright link-plate of thexture, which theworkpiecelaminatewas clamped onto.

High speed video footages of the hole exit delaminationduring drilling the selected woven CFRP epoxy laminateswere recorded using a commercial digital high speed camera,of themake PhotronFastcam SA3(Model: 120K-M2). This isalso a very robust, modern, and sophisticated device which is based on the CMOS-sensor technology and is mostly used inthe crash-tests and various other similar critical industrialapplications. This device was provided with a detachable,supplementary set of optical objective-lenses and a collimatorthat could help in proper focusing, sharpening the tinydelamination contours across an entire hole exit periphery.The camera was installed rigidly in order to avoid smallvibrations during the tests. This whole camera setup,however, allowed for necessary positional changes (panand tilt, etc.) which were required after every single test-run,i.e., for the adjacent hole, to be drilled next. The use of photographical zooming objective lens and collimatorseverely reduces the intensity of the captured light. Therefore,three additional external spotlight lamps had to be alsointegrated into this setup, in order to properly illuminate the

hole exit peripheral zone of the selected black-coloredworkpiece material.

The video camera was linked directly to a laptop PC via avery fast Photron Gigabit Ethernet communication inter-face, allowing for a recording of round about 4 GB of data pertest within a few seconds. The laptop was installed with the

supplementary commercial software for the acquisition,storage, editing, or post-processing and for the analysis of the acquired digital video footages data. The footage data wasstored in the AVI-format, at a resolution of 512 256 pixels(monochrome at 12-bit depth) and at a very high acquiringframing frequency of exactly 7500 frames-per-second (fps).

2. Results and Discussion

2.1. Elastic Bulging, Initial Macrocracks, and Fiber Failures atthe Bottom-Most Ply

Figure 6 shows a few images taken out and compiledtogether from the acquired high speed digital video footagesat various drilling time instants, t, under the given drillingconditions. A pure elastic behavior of the carbon bers at the bottom-most ply of the selected workpiece material wasvisible at t 0.0692s. This is similar to what was also seen byDiPaolo et al.[2] It is a dynamic impression of the protrudingdrill chisel edge on the bottom-most ply of the selectedlaminate when thechiseledge is aligned almostperpendicularor transversely to theoating orientation of theshown straightwarp bers. At the following time instant of t 0.07333 s, thesame chisel edge has twisted further clockwise and is nowaligned parallel or longitudinally to the aforementioned warpbers direction. The bulge also appeared as to be changing its


Table 3. Cutting conditions used in the experiments.

Drill diameter, D [mm] 8Cutting speed, vc [mmin

1] 80, 150Feedrate, f [mmrev

1] 0.05, 0.18, 0.35Laminate thickness [mm] 10Dry cutting

Fig. 5. (a) Schematic illustration of experimental setup. (b) Actual photograph of the setup.


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shape and was revolving along with the revolutions of theprotruding shape of the chisel edge, continuously. Nomacrocracks were observed until then, and the bers werecontinuously being pushed down, under the tension exerted by the distributed load of the drill chisel edge during its feed.The hole exit delamination in drilling CFRP laminates

may be considered as analogous to the problem of burrformation during the drilling or machining of the metals andother conventional engineering materials. A comprehensiveaccount on burr formation and its control has been gathered by Aurich et al.[16] Categorizing various burr formationmechanisms, analytical models, nite-element-method (FEM)analyses, and simulations etc. for machining metals, it can beconstrued from the aforementioned authors review that thereis always an elastic/plastic deformation zone right ahead of the tip of a cutting tool during its action, and also around itstip. Contrary to that, there is a clear evidence (Figure 6) thatthe carbon bers at the bottom-ply exhibited only a pureelastic behavior under the protruding tool-tip or the so-calleddrill chisel edge in case of drilling the selected CFRP material.The elastic bulge was seen continuously revolving i.e.,changing its shape in-phase with the drill revolutions duringits feed.

Finally, at around t 0.08480 s, the very rst macrocrackswere observed along the shown intersticea slit of resin between the shown two adjacent warps. An interstice isalways a matrix- or resin-rich region, and therefore, it is tooweak to sustain drilling loads, when compared to that borne by stronger carbon bers prior to their (ber) own failure. Thistype of crack was proposed by authors DiPaolo et al. aspossibly of Mode-I type. [2]

The observation made in the footage, as compiled inFigure 6 above, preliminarily revealed that the location of thevery initial macrocracks at the bottom-most ply of the selectedwoven CFRP laminate depends very much on the actual orexact chisel edge position relative to any nearest neighboringinterstitial region etc. This argument will be further supported

with the evidence taken from another video footage, results of which have been compiled in Figure 7 as follows. In Figure 7,under the given drilling conditions, in between the referencedrilling time instant t 0 and 0.02467 s, a very small, cyclic,and partial appearance, as marked by the white arrow, of thecutting edge was observed. Black arrows mark the elastic bulging/impression of the main cutting edge on the carbonbers at the bottom-ply. When noticed, this location of theinitial macrofractureswhere that small part of the cuttingedge seems to be protruding through as shown by the whitearrowis quite offset to the actual or ideal central position of theshown drill-hole, as shown via thedashedblack circlewithtwo white cross-diagonals depicting the ideal hole center-point location. In other words, the true center of the attackingchisel edge itself could not appear breaking through theshown bottom-most ply during several drill revolutionsduring the aforementioned time-span. It was merely due tothe actual presence or the exact true location of the initialmacrofailures in the carbon bers or their bundles which wereoverhanging like small cantilevers and were covering the saidcentral region of the drill chisel edge, during the shownsub-completion drilling-phase. The reason is explained asfollows. These cracks or fractures were initially seen along theber/matrix interface similarly to what has been discussed just previously as for Figure 6. These cracks spalled the shown


Fig. 6. Elastic bulging and appearance of very initial macrofractures at ber/matrix interface at an interstice at the bottom-ply of CFRP/laminate under protruding drill chiseledge.



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warp bers longitudinally, propagating to the nearestsituated, neighboring ber-crossovers, or the so-calledundulated regions. As asserted already above in this article

[i.e., theproposed visualmodel as illustrated in Figure3(b)], atan undulated or interstitial region of a cured bulk laminate,ber-bundles are already under residual tensile stresses,supposedly owing to their bends or the undulation. Therefore,it was observed in the footage and as shown in Figure 7 thatthe carbon bers or ber-bundles of the shown warp regionfailed right at the neighboring interstitial region which wassituated slightly offset to the true or ideal center of thatdrill-hole, unlike there in case of the drilling of metals andother conventional materials. The push-down forces exerted by the drill-feed motion possibly gave rise to the tensionwhich the carbon bers were already suffering from, at thatinterstitial region, leading to their eventual failures.

In addition, another common observation (Figure 7) is thatthat a majority of the failures in the ber-bundles at the bottom-ply covered within the drill-hole nominal area duringa sub-completion drilling phase, i.e., when the hole is not fullydrilled or nished yet, appeared to be propagating roughly intrue linear tracks, as can be seen at t 0.12827 s, and onwardsas well. The cracks propagated initially along the ber/matrixinterfaces (spalling) and translated further to the neighboringmain interstitial and undulated regions with ber-crossovers,which were intersecting their way there. This macroscopiccrack propagation pattern is more clearly evident in theobservations from the interval t 0.21493 s and onwards until

t 0.33733 s. Some of them are pointed out by the white and black arrows in the same gure. The macrocracks wereobserved as propagating further via next neighboring weaker

paths available to them, quite in a similar fashion within thenominal drill-hole area, the visual model for which hasalready been proposed, explained and illustrated as above inFigure2. Thecracks were observed propagatingfurther until awhole warp or a weft ber-bundle got detached, evidence forwhich will also be provided later on in Figure 9. Generally, inthe acquired video footages, main macrofracture types wereidentied as: Mode-I at an interstice and that observed incarbon bers due to dominant tensile loading, and Mode-IIIthrough a ber/matrix interface, details of which may beconsulted in the work of DiPaolo et al.[2] This entirephenomenon heavily depends on the actual or true locationor position of thedrill-hole relative to thewoven congurationof the bottom-most plyof theselected composite laminate, i.e.,the alignment and the geometry of its various warps andwefts, etc.

In another case, as shown in Figure 8, at t 0.09973 s, thelinear-shaped macrofailures in the carbon bers and alsoalong their ber/matrix interfaces emphasized via theshown white-colored vertical dotted-line at the shown point(a) within the marked white rectangular box covering theentire perimeter of the shown weft (or say ll) region understudyare notemanating from anyneighboring interstitialorundulated regions. This is mainly due to a relatively greaterdisplacement between the nearest neighboring interstitial/


Fig. 7. Location of the initial carbon ber failures (dependency of their vicinity to a nearest neighboring interstitial or undulated region).


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undulated region and the actual breakthrough-point positionor location of the drill chisel edge which, in this case, is almost

at the true central portion of the said weft. The tensilemacrofailures in bers traversed again in a linear track alongthe marked vertical dotted-line to reach the two intersticesacross, i.e., situated at the two horizontal banks of the shownweft, as marked by the white and the black arrows,respectively, at the same drilling time instant. From t 0 suntil t 0.05600 s, the same pure elastic bulging behavior of the carbon bers, the occurrence of the very rst macrober/matrix interface cracks are also discernible, as discussedpreviously for Figure 6. In Figure 8, at t 0.25013 s, relatively bulkier powdery chips were also observed, mainly due to thechosen higher feedrate value.

In Figure 9, at t 0 s, the main drill cutting edge revolvescounter-clockwise to approach and to attack theuncut, spalledrectangular-shaped weft ber-bundles, which are seen over-hanging like a real cantilever as marked by the white arrowthere. The gure shows the same sub-completion drilling-phase, during which the hole is not nished yet. Att 0.00120 s, the approaching cutting edge just touchesthe said cantilever weft ber-bundles. At t 0.00200 s, theattacked weft is seen as bent and pushed away from theposition where it was initially attached to. This position is aninterstitial point region. At t 0.00227 s, the weft bundle ispushed away further and is just about to release from itsattachment (base) point, i.e., the said interstitial point.

Onwards at t 0.00253 s, the said weft got already separatedfrom its base and can be seen ying away as captured in

the white circles, shown also at t 0.00280 s. This was also amajor observation of the chipping-off or the separation of theber-bundles of the warps/wefts bundles at the bottom-most ply of the selected woven CFRP laminate, during asub-completion drilling-phase. The said spalled, overhangingcarbon ber-bundle (chip) was attacked periodically by therevolving two cutting edges within the hole nominal-area aslong as it was chipped off from its base, i.e., after t 0.00227 s.

2.2. Tiny Localized Delamination Contours, and theCantilever-Like Flexibility of the Uncut Fiber-Bundles underCyclic Impacts of the Drill Bit Main Cutting Edges

Figure 10 depicts the preliminary results of the variousindividual tiny, localized delamination patterns or contoursandalso their respective propagation arounda drill-hole at the bottom-most laminate-ply, at various drilling-time instants,during the shown sub-completion drilling-phase, under thegiven drilling conditions. At t 0.06747 s, the two whitearrows mark the two of the aforementioned localizeddelamination contours. They propagated further as marked by the two white and one black arrows, respectively, att 0.09267 s, with the drill-feed to translate to the nextavailable interstitialor undulated regions. By referring back toFigure 8, the presence or initiation of such elliptic-shapeddelamination, localized within a single warp or a weft, is also


Fig. 8. Transverse occurrence of macrober-bundle failures when the chisel edge breakthrough-point position is nearly at the center of the marked weft (i.e., far from an interstitial/undulated region nearby).



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Fig. 9. Separation of a whole rectangular weft from an interstitial point within the nominal drill-hole area (sub-completion drilling phase).

Fig. 10. Various tiny, localized exit delamination contours at the bottom-most p ly during a sub-completion drilling-phase.


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Fig. 11. A spalled, overhanging carbon ber-bundle being removed from its initial position (A) by the cutting edge corner (nal drilling phase).

Fig. 12. The detached bundle of the previous Figure 11 now chipped off from its previous position (A) but still attached to its adjacent uncut warp bundle at position (B)localizedelamination forms are also marked by the white and black arrows.



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of t 0.09973 s.The aforementioned observations were made in the

footages when the drill had not completed or nished thehole nominal diameter at the bottom-ply. Now, anotherimportant observation concerning the mutual bonding of the

two different but adjacent carbon ber-bundles (say warps) atthe bottom-most laminate-ply during the ending drilling-phase will be discussed via Figure 11 and 12, respectively. InFigure 11, at the considered very initial drilling time referenceof t 0 s, the white arrow marks the spalled warp ber-bundleoverhanging at its very initial or base position, marked at thepoint (A). At t 0.00040 s, the attacking outer cutting edgecorner of the drill bit touches that spalled overhangingber-bundle. At t 0.00080 s, the revolving cutting edge ispushing or bending it. At t 0.00133 s and onwards untilt 0.00173 s, that ber-bundle is detached and separated fromits base or origin, being sheared away by the cutting edgecorner as shown at t 0.00213 s.

Figure 12 follows Figure 11 in the sequence of incidencesfor the same drilling test-run. In Figure 12, at t 0.00267 s, thewhite circle marks the new position of the same warpber-bundle seen fully bent by the revolving main cuttingedge. This is due to the contact friction or reaction of thesecondary (minor) cutting edge passing over it. Onwards, i.e.,from t 0.00307 s until t 0.00347 s, respectively, the sameber-bundle springs back toward the drill-hole area due to itscantilever-like elasticity effect. It is seen at a new position, because by then, i.e., at t 0.00427 s, the secondary cuttingedge has had already traveled past it. Finally, fromt 0.00453 s until at t 0.00600 s, the same warp-bundle can

be seen still attached at a new position (B) and is nowoverhanging inward the hole again, marked by the dashedwhite circles. The drill is retrieving back at t 0.00600s, asshown.

The difference between both of its locations can becompared now by referring back to the previous gure (i.e.,at t 0 s in Figure 11). The former ber-bundle underconsideration did not y off, yet being chipped off; as itwas seen at a new position being still bonded to its adjacentwarp ber-bundle which itself was attached to the bottom-plyat (B). This latter bundle could not actually fully be separated by the main cutting edges of the shown drill bit, hence bothwere hanging freely inwards as shown, in the form of asingle lump being attached at the new base-point (B), i.e.,the base-position of the latter one which is in fact beyond thehole-nominal diameter. In drilling GFRP epoxy laminates, theimportance of a drill cutting edge and corners with regard toits delamination results has quite recently been highlighted byFaraz et al.[17] Konig and Gra [10] earlier also stated that thatduring the drilling process of FRPs, owing to a greater cuttingedge rounding magnitude of a drill bit when compared withthat of the individual bers, bending of bers takes place,instead of their pure shearing or cutting. Therefore, itis asserted here too that the observed failures of carbon berswere predominantly due to the tensile and the exural loads

(not shearing) exerted by the drill main cutting edges and itscorner. To some extent, the evidence in Figure 11 and 12 alsosupports this argument. Had the selected tools been madesharp enough, which is not the usual case because the chosencutting material is cemented carbide, they would havesheared the individual carbon bers straight away, instead

of pulling and dragging (due to contact friction) the wholeber-bundle that remained attached to its neighboring bundlewhich itself could also not be cleanly cut and henceremained attached to its base-position (B). In another research,Faraz et al.[18] have also stressed the need and importance of quantifying the cutting edge rounding magnitude of thecarbide drill bits, in the same context, during drilling of thesame CFRP workpiece material. [9]

It also depends on the exact geometry and, similarly, theexact location of any particular spalled or frayed ber-bundle,which is under attack, as to whether it would be removed(cut), or otherwise, would remain intact to its base, afterwardsstill. Its true location stands for its relative displacement fromany nearest neighboring interstitial or the undulated region.And this is mainly because the carbon bers are morevulnerable to failure at those regions, as explained alreadyabove via the proposed visual model [Figure 3(b)] of thecurrent research. Figure 9 showed one of the related examplesof the spalled carbon ber-bundle which was nally detachedfrom its interstitial regionwhere it was initially attached to. Asobserved in Figure 10 above, the same localized, tiny contoursof the divided exit delamination and their individualpropagation are also clearly depicted in Figure 12. The whiteand black arrows (Figure 12) mark the said tiny contourswithin theshown twoadjacent wefts, at thegiven drilling time

instants.Another similar example of thecyclic impacts of thecutting

edge on some uncut, spalled groups of carbon bers in thecaptured footages will be discussed via theresults as compiledin Figure 13. They were observed as to be actually over-hanging (delaminated) from a position that was outside thehole nominal diameter, during drilling at the bottom-mostlaminate-ply at the very nal drilling-phase. These cantilever-like, frayed carbon ber groups were observed as swinging back and forth, periodically along with the drill-revolutions,and could not be sheared or cut due to their base-position beyond the hole nominal diameter. In Figure 13, att 0.00093 s, the overhanging frayed bers, as marked bythe white arrow, are attacked by the revolving cutting edgeapproaching toward them. At t 0.00213 s, they are bent andpushed outwards, partly being rubbed by the minor cuttingedge of the drill bit. At t 0.00307 s, the cutting edge has gonepast the rst group of the frayed bers which can be seen asnow springing back toward their initial position, owingto their inherent elasticity. Meanwhile, the same cuttingedge now approaches to engage the next group (marked bythe second white arrow below). And at t 0.00440 s, the same behavior was revealed for the second group of bers in thefootage, too. Until at t 0.00573 s, both groups of the frayed,overhanging carbon bers are seen back at their respective



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initial overhanging state (stationary), remained uncut still.This exible, cantilever-like behavior of the uncut bers was

repeatedly observed onwards in the footage with severaldrill revolutions, until the drill retrieved back nally, afteraccomplishing its feed-stroke.

2.3. Analysis of Various Cutting Angle Congurations at theBottom-Most Ply

In this section, the observations of exit damage around adrill-hole periphery concerning the three principalorthogonalcutting angle ranges, [13] at the bottom-most ply during thevery nal drilling-phase, will be discussed. Ko nig and Graalso related this aspect, generally, for the drilling process of FRPs.[10] But, their limited explanation cannot be comparedwith the reference work of Wang et al.[13] DiPaolo et al.correlated instantaneous loads (thrust and torque) withvarious ber congurations during drilling of the selectedUD CFRP laminates. [2] But they did not also draw anyconcrete conclusion regarding the hole exit delaminationquality and the instantaneous cutting angle, during drillingUD reinforced composites. As asserted earlier in this research,where the relevant visual model was proposed and described[Figure 4(a)], the nature of the exit delamination or suchdamages in any particular hole-exit peripheral zone at the bottom-most ply of woven CFRP laminate might also stronglydependon the classication of the cutting angle range, and theresulting cutting mechanism occurring over there. The very

nal drilling-phase is the moment during which the outercutting edge corners of a drill bit are nishing the nominal

hole diameter and are revolving almost at the same level asthat of the bottom-ply of the workpiece. Therefore, referring back to Figure 13 and correlating it generally with Figure 4,the frayedber-overhangs were found approximately at thoseradial hole-exit peripheral locations where a cutting anglerange of about 90 8 u 1358 ( 458 ) might roughly be identi-ed. This range corresponds to the Case-III cutting mechan-ism of the reference Wang et al.,[13] as reproduced alreadyabove in Figure 4(b). This particular type of the cutting anglerange or the resulting cutting mechanism produced severetrimming damages on the edge surface of the selected UDgraphite epoxy laminates in their work, too. On the otherhand, relatively much cleaner cuts were observed across thoseradial, hole peripheral regions in this research, where acorresponding range of about 0 8 u 458 could approximately be identied. This range corresponds to the Case-I and -II of the orthogonal cutting conguration of ref. [13] ; for which,much cleaner cuts were observed.

It was also examined as illustrated in Figure 14 that thesame range(s) for the instantaneous cutting angle u , asdescribed just above for Figure 13, hold also true for theobserved many different spalled overhanging ber-bundles,and also similarly for theobserved neatlycut radial peripheralregions of the shown drill-hole. On behalf of this crudeanalysis, it may be inferred here that in drilling holes in the


Fig. 13. Elastic cantilever-like, spalled/frayed carbon bres overhanging into the hole, while being attached from beyond the hole nominal diameter, i. e. the radial locations where thcutting angle range is approximately: 908 u 1358 ( 458 ).



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COMMUNICATION

selected woven epoxy CFRP, apart from the chosen drillingconditions, the nal quality of the hole exit delamination oruncut bers at the bottom-ply observed at any particularradial peripheral region may also signicantly depend on thetype and nature of the actual tool/ber mutual contactsituation, or in other words, the occurring instantaneouscutting angle conguration, and its resulting cutting mechan-ism involved therein.

Various instantaneous cutting angle magnitudes or theirranges, and the resulting cutting mechanisms across thehole-exit periphery at the bottom-ply of a woven composite(especially like the one as selected in this research), do heavily

depend on onemajor fact: theactualor true geometric locationand positioning of the drill-hole, relative to the wovenconguration of the bottom-ply, i.e., the geometry andalignment of all the warps and the wefts relative to thatdrill-hole. In other words, the occurrence or the presence of a particular instantaneous cutting angle value and theresulting conguration/mechanism present at a particularperipheral region of one hole (say the reference hole) mightnever be guaranteed again in the identical correspondingperipheral region of any other hole drilled in the sameworkpiece, unless and until the same hole-positioning, withrespect to the woven conguration of the workpiece materialat the bottom-ply, is absolutely identically ensured, orrepeated ideally.

It is equally important to emphasize here at the end, as alsoquoted in ref. [9] , that the macroscopic determination orapproximation of a particular instantaneous cutting angleconguration at a particular radial peripheral region of thedrill-hole right at the bottom-ply, and the determination of the instantaneous alignment and tool/ber mutual contact andengagement condition with respect to the woven congura-tion of the bottom-ply, this all demands immense attentionand care. No major contrary variations were observed in theresults regarding this particular nding, in this research. Aninvestigation on the chip formation and that on the process

mechanical loads, similar to what was carried out by Wanget al.,[13] in order to draw some concrete conclusions regardingthe proposed visual model i.e., the examined instantaneouscutting angle ranges and their resulting effect on drill-holequality (delamination) during the drilling process of theselected CFRP laminates etc., is beyond the scope of this

research.

3. Research Summary, Conclusions, and Outlook

In this paper, an in situ inspection of the initiation and theonset of the hole exit delamination at the bottom-most plyduring drilling the selected woven epoxy CFRP laminates ispresented. Contrary to what is usually found in case of thedrilling of metals, a fully elastic behavior (revolving bulge) of the carbon bers at the bottom-ply of the selected laminateswas always initially observed. The protruded elastic bulgewas always observed as to be revolving along with the drillchisel edge revolutions, prior to the very initial macrofrac-tures. Two types of the preliminary macrocracks wereidentied at the bottom-ply: initial fractures produced alongthe ber/matrix interface, i.e., longitudinally between the freeoating bers of a single weft or warp, founded on theirmutual weaker adhesion; followed by the macrotensilefailures of the carbon bers, traveling in linear paths,transverse to their own alignment within the same warp orweft. The initiation of thevery rst tensile failures/fractures of the carbon bers at the bottom-ply strongly depends on theirexact vicinity or the neighborhood, i.e., the relative displace-ment between the crack initiation location and the nearestneighboring interstitial or the undulated region. It is asserted

in this research that the interstitial and the undulated regionsare usually the matrix-rich weaker regions, where, presum-ably, the carbon bers are already suffering from residualtensile loads, apparently due to their undulation state and bend-geometry within the selected, cured woven fabriclaminate. Basically, linear-shape (slit-like) cracks were mostlyobserved, which propagated through various linear, weakerpaths available to them, during a sub-completion drilling-phase. The cracks were seen translating further to variousneighboring interstitial or undulated regions in the samepattern until a complete or a spalled warp or weft ber-bundlegot detached or chipped off from the bottom-most ply. It wasalso observed that some ber-bundles remained attached totheir adjacent bundles which were actually found still intactto the bottom-ply being uncut, mainly depending on theirgeometry and their radial location across the drill-holeperiphery.

A very common observation made in the footages was thatthat the delamination, in general, always appeared to bedivided into several tiny, localized independent portions, allof which traveled or propagated through various individualwarps or wefts almost independently, until reaching the nextencountering carbon bers-crossovers, or the so-calledundulated regions intersecting their way. Due to a relativelylarger size of the individual warp or weft of the selected


Fig. 14. Uncut, overhanging weft-bundle identied at instantaneous cutting angle of about u % 458 .


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woven CFRP composite, shape of an individual delaminationcontour was practically identied as elliptic, as usually foundfor the case of the UD FRP composites. A spalled or frayedber-bundle may completely be removed all of a sudden; orotherwise, it may remain partially attached to the bottom-most ply at an undulated region, apparently depending on its

locality and also on the geometry and strength of the ber/matrix interface present over there. Moreover, the spalledoverhanging ber-bundles, if remained uncut, were alwaysseen swinging back and forth like a pure exible cantilever beam, continuously, due to the cyclic collisions of the tworevolving drill lips. And this was just because their origin(attachment- or base-point of such overhanging ber-bundles)was observed beyond the nominal diameter of the hole beingdrilled, at the bottom-ply, making them really hard to cut,owing to their exible cantilever-like behavior.

It was also examined at the end of the presentedexperimental observations that the occurrence or presenceof the hole exit delamination or any such damage atany particular radial location of a drill-hole at the bottom-plymay also strongly depend on the nature and characteristicsof the instantaneous cutting angle range present orinvolved over there. The involved cutting mechanism is based on a process variable, which has been dened as theinstantaneous cutting angle in this researchthe values orthe range of which might also approximately reect the holequality. After having a crude macroscopic judgement, the exitdamage (spalling and ber-overhangs, etc.) was found to bemore severe across those radial peripheral regions of adrill-hole at the bottom-ply of the selected woven CFRPlaminate, where an unsuitable cutting angle conguration/

mechanism for the orthogonal trimming of UD laminatesis reported in the reference literature. However, it is alsostrongly recommended that a separate study is absolutelycompulsory whichshould address some substantial analyticaland experimental aspects of this particular nding. Thiswould help researchers, in future, to comprehend thequalitative aspects of the hole exit delamination in drillingthe selected woven CFRP composites. It would certainly helpthem also further in dening some novel, suitable strategiesand approaches as essential for the analytical modeling andFEM simulations, etc. for the drilling processes of the chosenwoven CFRP types.

Received: November 10, 2012 Final Version: December 21, 2012

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Drilling Woven CFRP Composites