Composites Science and Technology 71 (2011) 1719–1726

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Composites Science and Technology

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Occurrence and propagation of delamination during the machining of carbonfibre reinforced plastics (CFRPs) – An experimental study

Wolfgang Hintze ⇑, Dirk Hartmann, Christoph SchütteInstitute of Production Management and Technology, Hamburg University of Technology, 21073 Hamburg, Germany

a r t i c l e i n f o

Article history:Received 15 March 2011Received in revised form 27 July 2011Accepted 9 August 2011Available online 22 August 2011

Keywords:A. Carbon fibresA. LaminateB. DelaminationB. MachiningC. Damage mechanics

0266-3538/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compscitech.2011.08.002

⇑ Corresponding author. Tel.: +49 40 42878 3051; fE-mail addresses: w.hintze@tu-harburg.de (W.

burg.de (D. Hartmann), christoph.schuette@tu-harbur

a b s t r a c t

The machining of carbon fibre reinforced plastics (CFRPs) is often accompanied by delamination of thetop layers of the machined edges. Such damage necessitates time-consuming and costly post-machiningand in some cases leads to rejection of components. The work described in this paper systematicallyinvestigates the occurrence of delamination of the top layers during the machining of CFRP tape, withthe focus being on the process of contour milling. The occurrence and propagation of delamination werestudied by milling slots in unidirectional CFRP specimens having different fibre orientations and mainlyanalysing the slot tip. This allowed the key mechanisms to be clarified. The results show that delamina-tion is highly dependent on the fibre orientation and the tool sharpness. The experiments allow deriva-tion of a novel system for describing the occurrence and propagation of delamination during milling.Furthermore, the principles also apply for drilling. The results allow customisation of the machining pro-cedure to reduce and in some cases totally avoid delamination, leading to a significant increase in thequality of components.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

During the manufacture of components from carbon fibre rein-forced plastic (CFRP), it is usually necessary to carry out a post-machining step after curing in order to meet the required toler-ances and to manufacture fitting and joining surfaces. Classicalproduction processes such as milling and drilling are mainly usedfor this.

Regardless of the production process, damage in the form ofdelamination can occur during the processing of CFRPs. This char-acteristic production defect in CFRPs is particularly prevalent in thetop layers of the laminate as these are only supported on one side.Top and bottom plies of a multiaxial CFRP tape are even more crit-ical than those of woven fabric, where crossing fibres are mutuallysupported. The fibres are cut by the tool in an undefined way,deflect under the action of the cutting edge and consequentlydelamination occurs in the form of fibre overhang and fibre break-out at the cut edges. Such damage must be absolutely avoided, be-cause this requires time-consuming and costly post-machining torectify and in some cases leads to rejection of components.

Although delamination during the drilling of CFRPs has alreadybeen the subject of numerous publications [1,2,6–8,10,13,14,16,

ll rights reserved.

ax: +49 40 42878 2295.Hintze), d.hartmann@tu-harg.de (C. Schütte).

21–23,25–27,29,30], little attention has been put on millingprocesses up until now. Colligan and Ramulu [3,4] characterisedtypical forms of delamination during contour milling and devel-oped a classification system. They also established that delamina-tion during machining mainly results from poor support of thetop laminate layers.

Rummenhöller [24] and Hohensee [15] identified tool sharp-ness as a decisive factor for the cutting process. They also observedthat fibres, in particular in the top layers of the laminate, delami-nate. Davim [5] analysed the effect of the cutting parameters onthe component quality when milling CFRPs. They came to the con-clusion that increasing the feed per tooth leads to increased dam-age in the form of delamination, although the authors gave noreason for this.

The mentioned study highlighted the significance of fibre orien-tation as an important factor when machining CFRPs. Zhang et. al.[30] investigated damage that occurs when drilling CFRP compo-nents. They found that delamination depends on the anglebetween the cutting direction and fibre direction. However, theauthors did not show there to be a systematic relationship be-tween this angle and the delaminated regions. For contour milling,Hohensee [15] called this angle the fibre cutting angle h (Fig. 1b). Incontrast to the fibre orientation angle U (Fig. 1a), which is mea-sured between the feed direction and the fibre orientation, thefibre cutting angle changes continuously during the engagementof the cutting edge (Fig. 1c). The reason for this is the rotary cuttingmotion of the tool.

Fig. 1. Difference between the fibre orientation angle and the fibre cutting angle.

Fig. 2. Experimental set-up for slot milling.

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A major reason for the occurrence of delamination whenmachining FRPs is tool wear [4,6,9,15]. Due to the increasing cut-ting edge radius rn, which is the main type of wear in CFRP machin-ing [7,15,18,20], the machining forces increase and defined cuttingof the fibres becomes more difficult [11].

With regard to the drilling of CFRP fabric, Faraz et al. [7] re-cently characterised the extend of delamination by the ratio ofthe delaminated area around the drilled hole and the nominal areaof the drilled hole. They found correlations between this parameterand the maximum thrust force for each type of drill, which do notindicate a unique thrust force limit for delamination occurrence asstated in [13] but a strong influence of the drill geometry.

From drilling with twist drills it is well known, that delaminationat the exit side of the workpiece is initiated already by the chiseledge [13,14,21,23]. Subsequently, during exit of the major cuttingedges delamination propagates until the cutting edge corners haveleft the workpiece. In edge milling, such history of delaminationpropagation of the top layers is not considered in the scientific liter-ature up to now [15,17,19,24,28]. These studies mostly deal withdamage of the inner laminate plies at the machined surface.

This experimental work focuses on delamination of the lami-nate top layers in edge milling. It highlights for the first timedelamination propagation effects, which are essential for theunderstanding of frequently observed damage at finished work-piece edges. For that, the motion of the cutting edge along thewhole engagement angle has to be considered.

The work reported mainly investigates the milling of CFRPs as afunction of increasing tool wear. Using new and worn tools, theeffect of fibre orientation on the delamination is studied. Incontrast to previous studies, the interplay between wear and fibreorientation on the delamination is systematically investigated andis represented in a novel system for describing delamination.

The knowledge that has been gained has high practical relevanceas it allows reduction or avoidance of delamination despite progres-sive tool wear when trimming CFRP components by contour milling.

2. Materials and methods

In order to evaluate the delamination, slots were milled inunidirectionally reinforced CFRP specimens (prepreg, HT fibre,

65% fibre volume fraction, epoxy resin Cycom� 977-2). This proce-dure provided information about the location of delamination andalso about the extent of the delamination because the slot end re-mained in tact. By deliberately aligning the fibres at U = 0�, 45�, 90�and 135� to the feed direction (Fig. 1), the effect of fibre orientationwas systematically studied.

The machining tests were carried out using a milling machinemade by Röders (Röders RFM 600) with a spindle power of10 kW and a maximum speed of 42,000 rpm. For carrying out thetests, a cutting speed (vc) of 800 m/min and a feed (f) of 0.06 mmwere chosen. The work pieces were secured with clamping claws.Typical double-edged PCD end mills (diameter = 12 mm, angle oftwist = 0�) were used. A coolant was not used. Fig. 2 shows theexperimental set-up.

As tool wear has a major effect on delamination, tools wereused in the new state and in a state of defined wear. By trimmingthe CFRP test specimen over a longer feed path, a defined cuttingedge radius of rn = 45 lm and rn = 90 lm could be set at which

Fig. 3. Delamination as a function of tool wear.

W. Hintze et al. / Composites Science and Technology 71 (2011) 1719–1726 1721

delamination occurs under the selected cutting parameters. Thecutting edge radius rn was measured with an optical 3D measuringunit made by Alicona (Alicona Infinite Focus). The wear measure-ments were repeated five times. The drilling tests were performedusing the same CNC-machine, CFRP tape and a twist drill with a tipangle of r = 85�.

3. Results and discussion

3.1. Identification of the critical cutting angle range

Knowledge from research studies and practical experienceindicates that tool wear is a key factor for the occurrence of delam-ination. Fig. 3 highlights this effect. Using a sharp tool (rn = 9 lm),the fibres can be cut cleanly and no delamination occurs. Using atool having average wear (rn = 45 lm), there are small fibre over-hangs. Using a tool having high wear, namely rn = 90 lm, verymarked delamination in the form of fibre overhangs and fibrebreakouts occurs on the top layers of the cut edges.

When slot milling different fibre orientations using a very worntool (rn = 90 lm), as can occur in practice, it was found that delam-ination only appears in certain regions on the machined edges and/or in the slot tip (Fig. 4). Fibre overhangs indicate that the delam-ination not only occurs on the machined edge, but rather before-hand in the region of the slot tip. The shape and length of thedelamination vary considerably depending on the milled fibre ori-entation. Furthermore, delamination does not occur in the entireslot tip, but only in the regions that are marked red1 (Fig. 4,bottom). The rest of the machined edge is free of delaminationdespite the very high tool wear.

For a fibre orientation of U = 0�, there is only delamination inthe form of long fibre overhangs in the region of the down-milledslot tip. These fibre overhangs extend far into the machined slot,but the edges of the component are free of delamination.

When milling a fibre orientation of U = 45�, delamination on theup-milled edge extends far into the machined slot, whilst thedown-milled edge shows no delamination. For a fibre orientationof U = 90� there is marked delamination on the up-milled edge.The maximum length of the overhanging fibres corresponds to halfthe slot width. The down-milled edge once again shows no delam-

1 For interpretation of color in Fig. 4, the reader is referred to the web version ofthis article.

ination. At a fibre orientation of U = 135� both machined edgesshow delamination in the form of short fibre overhangs, whilstthe slot tip is free of delamination. It can be concluded from theseexperiments that top layer delamination does not primarily de-pend on the fibre orientation angle but rather on the fibre cuttingangle h.

Considering the fibre orientation of U = 90� in Fig. 4 it becomesclear that delamination occurs in a critical fibre cutting angle rangeof 90�6 h < 180�. At the same time, there are regions where there isabsolutely no delamination. This finding also holds for the other fibreorientation angles that were considered. At U = 0� delamination alsoappears in the slot tip in the down-milled region for fibre cutting an-gles of 90�6 h6 180�. At U = 135� delamination is present on bothedges of the component. This involves cutting angles of 135�6h6 180� for the up-milled edge and cutting angles of 90�6 h6 135�for the down-milled edge. As a result, delamination when millingCFRPs only occurs within a critical fibre cutting angle range of90�6 h6 180�. Only at a fibre orientation of U = 45� is delaminationalso found outside the critical range on the up-milled edge.

3.2. Occurrence and propagation of delamination

In order to investigate the mechanisms responsible for theoccurrence and propagation of delamination, up-milling wasundertaken at an orientation of U = 45� with variable workingengagement using a very worn tool (rn = 90 lm) (Fig. 5). Whenthe working engagement ae corresponds to the tool diameter (ae/d = 1), delamination occurs on the up-milled edge despite the factthat this region lies outside the critical fibre cutting angle range. Ifae is chosen such that the fibres are only cut at a fibre cutting angleh < 90� (namely ae/d 6 0.13), then no delamination occurs. Increas-ing the working engagement ae beyond a fibre cutting angle ofh = 90� leads to delamination with fibre overhangs which dependon the working engagement ae.

Important for the occurrence and propagation of delaminationhere is the chronological order of the fibre cutting angle h the cut-ting edge passed through. Fig. 6 shows, for a selected fibre, thechange in the fibre cutting angle with increasing feed path lf. Withthe exception of a fibre orientation of U = 0�, the cutting edge con-tacts the fibre for the first time at a fibre cutting angle of 0� and180�, providing the working engagement corresponds to the tooldiameter (ae = d).

Based on first contact at h = 180�, the delamination only propa-gates during the further feed motion if the subsequent cutting an-

Fig. 4. Delamination when milling using a worn tool (rn = 90 lm).

Fig. 5. Delamination at different working engagement ae for U = 45�.

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gle lies in the critical range, namely the critical cutting angle rangeis passed through in reverse (h = 180� to h = 90�).

Fig. 7 clarifies this situation using the example of a fibre orienta-tion of U = 45�. The critical cutting angle range is travelled from thefirst contact point at h = 180� backwards to h = 90�. Delaminationcan only occur on the edge machined by up-milling, because thefibres there are continuously cut at a critical angle h. Noteworthy

here is that the delamination due to the tool feed motion also prop-agates beyond the critical cutting angle range in the direction ofangles smaller than h = 90�.

When the working engagement ae corresponds to the tool diam-eter (ae = d), then the length of the fibre overhangs in the region ofthe slot tip corresponds to the distance of the up-milled edge to thepoint at which the cutting edge cuts the fibres for the first time

Fig. 6. Change in the fibre cutting angle as a function of the feed path lf.

Fig. 7. Delamination when slot milling at a fibre orientation of U = 45�.

W. Hintze et al. / Composites Science and Technology 71 (2011) 1719–1726 1723

Fig. 8. Delamination when slot milling at a fibre orientation of U = 135�.

Fig. 9. Systematic scheme for describing the occurrence of delamination in milling.

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below a fibre cutting angle of h = 180� (see Fig. 7). This correlationholds for all fibre orientations U. This effect is generally not evi-dent on trimmed component edges, because some of the overhang-ing fibres are broken off or shortened due to subsequent multiplecontact of the cutting edge.

At a fibre orientation U = 135�, delamination occurs on both theup-milled and down-milled edges of the component because thefibre cutting angle assumes the critical value of h = 135� on bothcomponent edges (see Fig. 6). Fig. 8 shows that the critical cutting

angle range on the two component edges is equally large (h = 135–180�, and h = 90–135�) and hence the fibre overhangs on the twocomponent edges are equally long.

With further increasing fibre orientation angle (U > 135�) thelength of the fibre overhangs increases on the down-milled edgeand decreases on the up-milled edge, until at U = 180� delamina-tion only occurs in the down-milled region. Propagation of thedelamination beyond a fibre cutting angle of h = 0�/h = 180� intothe non-critical range was not observed in the experiments, and

Table 1Active force in planning of UD-CRFP-laminate, vc = 10 m/min, f = 0.03 mm, ap = 4 mm,cf = 0�, af = 12�, PCD [11].

Cutting edge radius rn Fibre cutting angle h

0� 65� 90� 135�

8 lm 152 N 112 N 247 N 169 N70 lm 357 N 173 N 350 N 538 N

W. Hintze et al. / Composites Science and Technology 71 (2011) 1719–1726 1725

consequently no delamination took place on the down-milled edgefor fibre orientations of 0� 6U 6 90�.

Independent of the working engagement ae and the fibre orien-tation angle U delamination of the top layers occurred as soon asthe cutting edge contacted the fibres for the first time within thecritical cutting angle range. The length of the fibre overhang corre-sponds to the distance of the milled edge to the first contact pointof the fibres and cutting edge in the critical cutting angle range.

3.3. Systematic scheme for describing the occurrence of delamination

The knowledge we have acquired allows a systematic scheme tobe drawn up to describe the occurrence of delamination. Fig. 9shows this scheme for typical fibre orientations U. Region A refersto the critical fibre cutting angle range in which delamination oc-curs. Region B is where the described propagation of the delamina-tion occurs. Region C is the cutting angle range where there is nopropagation of delamination. On the one hand this concerns theup-milled side of the U = 0� orientation. When the fibres are orien-tated at 0� (i.e. parallel) to the feed direction, each individual fibreis cut at the same cutting angle h during the whole machining pro-cess, and no propagation can occur. On the other hand, no delam-ination occurs in the slot tip at a fibre orientation of U = 135�. Thisis due to the fact that the fibres there are exclusively cut in thenon-critical range 0� 6 h 6 90� and only with increasing feed pathnear the milled edge in the critical cutting angle range. Different tothe situation at U = 45�, propagation whereby the fibres are firstcut in the critical range is hence not possible.

For delamination-free machining, the process must be carriedout such that the fibre cutting angle h on the relevant component

Fig. 10. Delamination at the dr

edge is in the 0� 6 h 6 90� range and down-milling must beundertaken.

The findings reported in this study are based on experimentalobservations. They correspond to former force measurements con-ducted in planning of this unidirectional laminate [11]. These re-sults clearly indicate the increase of the active force (F2

c +F2f )0.5

with the cutting edge radius rn and a higher active force level inthe so called critical fibre cutting angle range, see Table 1. Ofcourse, the stress exerted on the workpiece surface during machin-ing must be considered in order to assess delamination rather thanonly the active force. This is realised by calculating the stress dis-tribution and taking into account the static failure modes for ortho-tropic materials. The basic concept is reported in [12].

3.4. Delamination when drilling

The proposed system for describing delamination when millingcan also be used to describe the situation when drilling CFRP tapewith spiral drills (point angle of the drill r� 180�). Even when thefeed per tooth fz is perpendicular to the laminate surface there is aradial feed component fr = fz tan(r/2) which at a point angle ofabout a half of r = 90� corresponds to the feed per tooth fz anddue to the conical geometry of the drill bit is tangential to the lam-inate surface. Accordingly, there is a significant radial force compo-nent that acts in addition to the axial feed force and cutting forceand corresponds to the passive force. Such passive forces act oneach of both major cutting edges. They are the reason why similardelamination processes are observed when drilling as are whenmilling. Because of their opposite direction the passive forces arebalanced and therefore difficult to measure during drilling.Fig. 10 shows the tool exit side of a CFRP specimen (unidirectionaltape). As in milling, the individual fibres are first of all cut at a cut-ting angle of h = 0� or h = 180� (Fig. 10b). Subsequently, non-criticalcutting angles of 0� < h 6 90� are travelled through in two oppositequadrants of the hole and no delamination results. In the other twoquadrants, critical cutting angles of 90� 6 h < 180� are travelledthrough and delamination occurs (see Fig. 10a).

To explain the situation, consider the fibre marked by a fullblack line in Fig. 10b). This fibre is cut in either a cutting anglerange of 0� < h 6 70� or in a range of 180� > h P 110�. In contrast,

ill exit when drilling CFRP.

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the fibre marked by a dashed black line, which runs through thecentre of the hole, has a constant cutting angle of h = 90�. Propaga-tion of the delamination into the non-critical cutting angle range,as occurs when milling at a fibre orientation of U = 45�, is thereforenot possible. Hence, the depicted ‘‘delamination pattern’’ inFig. 10a, which is well known in drilling of CFRP tape at the exitside but is not sufficiently explained by former models [13,14],matches the empirical findings from the milling experiments sum-marised in Section 3.3.

4. Conclusions

Delamination is a major issue when machining fibre reinforcedplastics and avoiding this would bring considerable time and costsavings. This paper presented a systematic scheme for describingthe delamination that occurs when milling CFRPs. Systematic pre-diction of delamination allows machining procedures to beadapted in order to avoid delamination when contour milling.

It was demonstrated that two mechanisms are of key impor-tance for describing delamination: occurrence and propagation.The results can be summarised as follows:

1. The occurrence of delamination and fibre overhangs during themachining of CFRPs generally depends on the condition of thetool (tool wear) and the fibre cutting angle h on the top laminatelayers.

2. Even when using a very worn tool there is a preferred fibre cut-ting angle range which results in no delamination and no fibreoverhangs (0� < h < 90�). Furthermore, there is a critical fibrecutting angle range (90� 6 h < 180�) in which delaminationand fibre overhangs occur.

3. Delamination occurs where fibres are initially cut in the criticalcutting angle range. When the working engagement equals thetool diameter, the fibres are always initially cut at h = 180�.

4. Delamination can propagate from the critical cutting anglerange to the component edge, provided the fibres are initiallycut at a cutting angle of 90� 6 h < 180� and at the componentedge with a cutting angle of 0� < h < 90�.

5. When milling below the critical cutting angle range in whichdelamination occurs (90� 6 h < 180�), the length of the fibreoverhang equals the distance of the component edge to thepoint at which the cutting edge cuts a fibre at a critical cuttingangle for the first time.

6. Empirical findings of milling and drilling experiments matchwith regard to the critical and uncritical ranges of the fibre cut-ting angle thereby indicating their overall basic validity.

The results of these experiments have high practical relevance.The findings allow milling processes to be configured such thatdelamination is largely avoided, meaning considerable time andcost savings for the manufacture of composite components.

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