Composites: Part A 41 (2010) 1403–1408

Contents lists available at ScienceDirect

Composites: Part A

journal homepage: www.elsevier .com/locate /composi tesa

Study on UV laser machining quality of carbon fibre reinforced composites

Z.L. Li a,*, H.Y. Zheng a, G.C. Lim a, P.L. Chu a, L. Li b

a Singapore Institute of Manufacturing Technology (SIMTech), 71 Nanyang Drive, Singapore 638075b Laser Processing Research Centre, School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Oxford Road, Manchester M139PL, United Kingdom

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 November 2009Received in revised form 17 May 2010Accepted 22 May 2010

Keywords:A. Carbon fibreA. Polymer–matrix compositeD. Mechanical testingE. Cutting

1359-835X/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.compositesa.2010.05.017

* Corresponding author.E-mail address: zlli@SIMTech.a-star.edu.sg (Z.L. Li)

The large differences in material properties of the carbon fibre and the epoxy resin in carbon fibre rein-forced plastic (CFRP) composite make laser processing very challenging. The heat affected zone (HAZ) hasbeen the major obstacle for wide industry applications of laser machining of CFRP composites. This paperinvestigates the quality of CFRP machined by a diode pumped solid state (DPSS) UV laser. The resultsshow that minimum HAZ (about 50 lm) is achievable in machining of CFRP composite by using shortpulsed UV laser. The study found that heat is easily accumulated in the material during laser processing,especially when the carbon fibres are sliced into small pieces. The paper discusses how to make use of theheat accumulation and how to avoid potential damage by the heat accumulation. Bearing strengths testand fracturing mechanisms study were conducted. Method of characterization of thermal damage inpolymer matrix is developed.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The use of CFRP materials in aerospace, automotive and marineindustries is rapidly growing due to their lighter weight and supe-rior performance. Therefore, CFRP composites have become majorstructural materials and are considered as substitutes for metals inmany weight-critical components. For instance, Airbus and Boeingrecently announced two new aircrafts, the A350 and the B787,with composite content anticipated to be over 30% and 50%,respectively [1]. Although the mechanical drilling of the CFRP lam-inate is common practice in industry, the mechanical force appliedonto the laminate can cause serious material damage such asdelamination, fibre pull out and inadequate surface roughness ofthe cut walls. Among the defects, delamination appears to be themost critical one [2]. Tool wear is another problem with themechanical drilling due to abrasive nature of the carbon fibres[3]. Furthermore, the introduction of coolant in mechanicalmachining can cause further long term damages to the composite.The use of lasers as an alternative non-contact, dry and abrasion-less machining tool imparts several unique advantages in materialprocessing [4–7]. These include the ease of automation, no cuttingforce and no abrasive or liquid media. Therefore lasers have oftenbeen proposed as alternative tools for machining of compositesover the past 30 years [8–10].

There is however challenges in laser processing of CFRP wherethe goal is to minimise or eliminate excessive HAZ in the polymermatrix and to maintain a high processing speed. The properties of

ll rights reserved.

.

conventional materials such as metals, ceramics and polymers areisotropic so that the machining quality is the same in all directions[11–14]. However CFRP composite material is laminated with dif-ferent fibre orientation bound together in a polymer matrixaccording to its application. Each constituent retains its own chem-ical, physical and mechanical properties and therefore poses achallenge in laser processing due to the large differences of mate-rial properties of the two constituents at elevated temperatures.For example, vaporisation of fibre and matrix occur at very differ-ent temperatures as shown in Table 1. During laser processing, thetemperature at the ablation front may not exceed the vaporisationtemperature of carbon fibres, but it is significantly higher than thedegradation or decomposition temperature of the polymer. This re-sults in the surface deteriorated – polymer burnout [15]. Aniso-tropic heat conduction at different fibre orientation directions isanother problem that characterizes the HAZ. The heat conductionparallel to the fibre axis is faster than in the transverse directions,which results in a non-uniform HAZ size as a result of the differentfibre orientations. Furthermore the difference in thermal conduc-tivities between carbon fibres and polymer matrices makes it moredifficult to achieve uniform, high quality machining for CFRP usinga laser as heat conduction along the fibres is much faster than thatin the polymer matrix. Consequently the polymer matrix is furtherheated up by the hot fibres resulting in an extended polymerremoval or degradation around the fibres. Fibre extruding is ageneral problem of laser machining of CFRP composite wherethe length of fibre extruding from the matrix often characterizesthe degree of the heat damage [16–18]. Fibre extruding from thematrix seriously affects the laminate’s properties since the regularload transfer from matrix to fibres does not occur [19], which has

Table 1Thermal properties of fibres and matrix materials.

Material Conductivity (W/m/K) Density (g cm�3) Specific heat Vaporisation temperature (�C) Heat of vaporisation

J kg�1 K�1 J cm�3 K�1 J g�1 J cm�3

Polyester 0.2 1.25 1200 1.5 350–500 1000 1250Graphite 50.00 1.85 710 1.3135 3300 43,000 79,550

1404 Z.L. Li et al. / Composites: Part A 41 (2010) 1403–1408

been a major obstacle for wide industrial applications of lasermachining of CFRP composites in the past [20]. However recentdevelopment in laser technology have opened up new opportuni-ties; Denkena and co-workers [21] successfully avoided the fibreextruding during laser machining of CFRP composites using a solidstate UV laser. Traditional characterization criteria of HAZ in CFRPcomposites are no longer appropriate in this case, consequently thepolymer degradation induced by thermal energy and its effects onmechanical properties need to be investigated for such atechnique.

This paper focuses on the machining qualities of UV laser ma-chined CFRP composite. Laser processing parameters are optimisedto minimise thermal damage. Bearing strengths test and fracturingmechanisms study were also conducted. The characterizationmethod of thermal damage in polymer matrix is developed byusing a bearing fracture test.

2. Experimental

CFRP composites with thicknesses of 0.3, 1, 3.1 and 7 mm,obtained from Robot Market Place, were used for the study. AQ-switched third-harmonic Nd:YVO4 laser with a wavelength of355 nm and pulse width of about 25 ns was used in the experi-ments. The maximum average output power of the laser was10 W at a pulse repetition rate of 40 kHz. The beam had a Gaussianprofile and was delivered to the CFRP substrate using a galvanom-eter scanner and focused using an f-theta lens with effective focallength (EFL) of 108.3 mm. The effective beam focus size (measuredkerf width) was 35 lm at the scanning speed of 100 mm/s. The fo-cal depth (5% spot size change) of the lens was measured as0.3 mm. The samples were mounted on a CNC X–Y–Z controlled ta-ble. Since the thickness of the CFRP substrates was much greaterthan the focus size and focal depth, circular holes were cut by mul-tiple-circle trepanning with successively smaller circles in order togenerate larger cut kerfs for the beam to cut into the substrates.The spacings between laser traces during multi-circle trepanningwere varied as 75, 100, 150 and 200 lm, which were greater thanthe effective beam size. The advantages of the process conditionsare described in detail in Section 3.1.2 and Ref. [22]. A number ofrepeated trepanning passes were performed until the hole wascut through. The cutting quality was examined using opticalmicroscopy and scanning electron microscopy (SEM). Mechanical

25 mm

140 mm

6 mm

Laminate

Pin

Fig. 1. Bearing test sample geometry and pin-load setup.

drilling was performed manually using a bench drill to comparewith the laser cut samples. An Instron 505 tensile testing machinewas used for bearing test. The test was run at a displacement con-trolled rate of 1 mm/min. The bearing test sample dimensions andpin-load setup are shown in Fig. 1 as referenced in previous study[23].

3. Results and discussions

3.1. UV laser cutting holes

3.1.1. Morphology of UV laser drilled laminateAn array of nine holes was cut in a 1 mm thick CFRP laminate.

The hole diameter and pitch spacing were 2 mm and 4 mm, respec-tively. Three-circle trepanning with 100 lm spacing was cut ineach hole. Fig. 2a shows typical SEM cross-sectional micrographsof above array before process optimisation. It was found that theinner wall surface was not even along the entire circumference.A periodic stripe structure was observed from top to bottom. Itwas likely to be caused by the effect of the hot plume generatedduring laser processing. There was no serious fibre extruding atthe cut edge but heat damages of polymer matrix were foundin the hole as well as the area between the holes. It is believed thatthe heat accumulation between holes during the array cutting wasresponsible to the thermal damage. Further discussion on the HAZduring laser perforation of CFRP composite is in Section 3.3. Fig. 2bshows that partial polymer decomposition took place during thelaser process, which generated small voids between the fibresand gaps at the boundary of the fibres with different orientation.

For the cutting of holes in thick CFRP substrates, e.g. 3.1 mmand 7 mm, the laser beam was first scanned along the outer circleat a speed of 100 mm/s. The spacing between the multi-circle tre-panning was 100 lm. The scanning speed was gradually increasedat 20 mm/s interval towards the inner circles. It was observed thatthe surface quality was significantly affected by the scanning speedof the outer circle while only being slightly affected by the scan-ning speed in the inner circles. It should be noted that by graduallyincreasing the scanning speed of the internal circles, the cuttingefficiency was improved. The focal plane was set at the substratesurface at the beginning of the experiments. The X–Y stage wasmoved up to raise the substrate by 0.5 mm after every 10 passes.This ensured the beam focus followed the depth as the cutting pro-gressed deeper into the laminate. Fig. 3 shows the cross-sectionalmicrographs of a 6 mm diameter holes cut in the 3.1 mm lami-nates. The right side of Fig. 3a is laser drilled surface. Different con-trast shows different the fibre orientations. No obvious fibreextruding or delamination was observed at the edge of drilled hole.Fig. 3b is an enlarged picture which shows individual fibres weresurrounded by a polymer matrix. Similar results were obtainedin cutting the 7 mm laminates.

Above results indicated that high machining quality of FRPcomposite is achievable using a pulsed UV laser. In contrast withthe machining quality of FRP composites using a continuous modeCO2 laser, polymer burn out and large dimension HAZ was ob-tained as earlier studies by Mathew et al. [9] and Davim et al.[15]. The long wavelength of CO2 basically produces thermal effectbecause the photon energy is only sufficient to excite molecular

(b) Voids

20 µm

Gap

(a)

200 µm

Laser drilled hole

Mechanical sawed area

Fig. 2. Cross-sectional SEM micrographs of laser drilled samples.

10 µm

Laser drilled hole surface

(b)

200 µm

Laser drilled hole surface

Edge of laser drilled hole

(a)

Fig. 3. Cross-sectional SEM micrographs of laser drilled 3.1 mm sample under optimised conditions. No obvious HAZ and delamination were observed at the edge of laserdilled hole.

Z.L. Li et al. / Composites: Part A 41 (2010) 1403–1408 1405

vibrations on the surface. It results in significant thermal damageespecially in the CW mode that the long laser–material interactiontime has more effect on HAZ size [24]. Therefore, the CW CO2 laserhas poor workability for CFRP composite [15]. Our experimentalresults suggest that short laser–material interaction time, such asshort pulsed laser and fast scanning speed, is primarily needed toreduce HAZ during laser machining of CFRP composite. We intro-duced a new method to further reduce the total laser–materialinteraction time by increasing the spacing between adjacent laserbeam scan traces as discussed in following section.

0

10

20

30

40

50

60

70

80

90

100

0 0.1 0.2 0.3

Laser beam trace spacing (mm)

Num

ber

of p

asse

s dr

illi

ng th

roug

h

0

4

8

12

16

20

24

28

32

36

40

Tim

e of drilling through (s)

2-circle

3-circle

2-circle

3-circle

Fig. 4. Number of passes and time required to cut through 1 mm thick CFRP bycutting traces separated by different spacing.

3.1.2. Optimised laser–material interaction time and laser energy forcutting holes

It should be noted that the spacing of 100 lm between adjacentbeam scan lines in the above experiments were larger than theeffective focused beam size of 35 lm. In other words, the laserbeam did not overlap during the cutting process. In conventionalmaterial laser processing, a laser beam path spacing greater thanthe effective beam focus size will cause un-irradiated area or‘‘island” between the laser traces. However, in cutting fibre rein-forced composite laminate, the fibres were sliced into small piecesbetween the laser traces if the cutting direction was not parallel tothe fibre orientation. Our FEM simulation results showed clearlythat the laser energy delivered to the material in the experimentalcondition does not lead to serious HAZ in the material. However,the temperature within the ‘‘island” could be sufficient high dueto the heat being constrained and accumulated within the ‘‘island”which led to local temperature rise. The raised temperature mightnot cause fibre to decompose or vaporise directly but was suffi-ciently high to cause polymer decomposition [25]. As the results,the fibres were fragmented and ejected due to the thermochemicalforce [19].

We studied the optimisation of total time to cut a 2 mm diam-eter hole with good quality. We made two and three circular cutson 1 mm thick CFRP with the cutting track separation as variable.

Fig. 4 shows the total number of repeated passes and the corre-sponding time required to cut through the laminate. It is clear thatthe optimum (minimum) cut occurred at a track separation of100 lm. This optimum cutting parameter occurred when the laserbeam energy was sufficient to vaporise all polymer and carriedaway fragmented graphite fibres from the cut slots in every pass.When the track separation was wider than 100 lm, the ‘‘islands”between the tracks were too wide for the polymer to be completelydecomposed in every pass. Hence additional passes were requiredto remove the residue fragments of graphite and polymer. It isnoted that if the beam track separation is less than 100 lm, a larger

Table 2Calculation of laser energy required for decomposing the polymer and graphite in the cut slot. Three tracks were used to cut a 2 mm hole. The track diameters were 2, 1.8 and1.6 mm, giving an average diameter of 1.8 mm and an average circumference of 3.14 � 1.8 = 5.65 mm or 0.565 cm. For the polymer material, the volume of material decomposedwas 0.565 � 0.0235 � 0.1 (total track width was 0.0235 cm which included the total separation of the three tracks and the focus beam diameter, the composite thickness was0.1 cm), giving a volume of 0.0013 cm3. For the graphite, we assumed only 30 lm (0.003 cm) of the material under the focused beam spot size of 35 lm was decomposed, hencethe total volume was 0.565 � 0.003 � 3 � 0.1 � 0.0005 cm3. The decomposition temperature and specific energy were derived from Table 1.

Volume of material removed fromthe track (cm3)

Temperature ofdecomposition (�C)

Energy to raise to decompositiontemperature (�C)

Decomposition energy(J)

Total energyrequired (J)

Polymer 0.0013 � 40% = 0.0005 450 1.5 � 0.0005 � 450 � 0.34 0.0005 � 1250 � 0.63 0.34 + 0.63 = 0.97Graphite 0.0005 � 60% = 0.0003 3300 1.313 � 0.0003 � 3300 � 1.30 0.0003 � 79,550 � 23.87 1.30 + 23.87 = 25.17

1406 Z.L. Li et al. / Composites: Part A 41 (2010) 1403–1408

number of passes are required to cut through the composite. This isbelieved to be due to the fact that a cut slot opening of below235 lm (the total width of three tracks is 200 lm together withthe beam focus diameter of 35 lm) was very narrow and the debrishas difficulty in being expelled from it when the cut was penetrat-ing the 1 mm thick laminate. The same reasoning could be used toexplain the fact that if only two tracks were cut, the cutting timewas longer than those with three tracks, as shown in Fig. 4.

To estimate the theoretical minimum energy required to cut thehole with three-circle traces separated by 100 lm each, it is as-sumed that (a) all polymer within the cut slot of 235 lm (the totalwidth of three tracks separated by 100 lm is 200 lm together withthe beam focus diameter of 35 lm) were decomposed, (b) mostgraphite under the focused beam spot size of the three tracks weredecomposed, and (c) there were 40% polymer and 60% graphite byvolume in the composite. Table 2 shows the calculations. It showsthat the total energy to decompose both the polymer and graphitewas about 26.14 J. The laser power used was 10 W, and assumingthe absorption efficiency of the laser beam by the composite is70% (take off reflection, plasma absorption, etc.), this would requirea total time of 3.7 s. This compares very well with the actual timeof 4 s taken in our experiment as shown in Fig. 4.

3.2. Laser cutting quality at the edge

Laser cutting quality is significantly affected by the relative an-gle between the cutting direction and fibre orientation [26]. This isespecially so at the edge of a cut, where the relative angle plays asignificant role on the edge finishing. The following two examplesshow different effects at the edge quality when cutting the 0.3 mmthick CFRP laminate with 0o/90o fibre orientation. If the cut direc-tion is at 0� or 45� to the first layer fibre orientation, the secondlayer fibre orientation relative to laser beam cutting direction mustbe 90� or 45�, respectively.

Fig. 5a and b shows the micrographs of the laminate samplewhich were cut in directions of 0�/90� and 45�/45� relative to fibreorientations, respectively. The samples were cut out by scanningthe focused laser beam in parallel traces separated by 100 lm. Itis seen from the figures that different lengths of fibre materials

(a) Cutting direction

Heat conduct into surrounding

Fig. 5. Cutting edges of laser cutting direction: (a) 0�

were left hanging on the edges depending on the direction of laserscanning with respect to the fibre orientation. This phenomenoncould be attributed to the differences in heat conduction paths be-tween the two cases. When the laser cutting direction is parallel tothe fibre orientation, as in the 0�/90� cut, heat conduction to the‘‘island” between the laser cut lines was poor due to poor conduc-tivity of the polymer matrix transverse to the fibre direction. Fur-thermore, the heat conducted into the island was dissipatedthrough the fibres into the surrounding material. Hence the‘‘island” temperature at the edge might not be sufficiently highto decompose the polymer which helped to retain the fibre with-out being fractured into small pieces. The leftover fibre at the edgecould be as long as 1.3 mm (Fig. 5a) in this experiment conditions.In contrast, in the case of cutting direction at 45� relative to the fi-bre orientation, the fibres were sliced into small pieces. The heatconducted to the ‘‘island” material was retained. The temperaturein the ‘‘island” material was sufficient to decompose the polymer,in turn the island was removed. Short leftover fibres at the edgewere observed as shown in Fig. 5b. The above observation demon-strates the complex effects of machining CFRP composite as a re-sult of different fibre orientations and heat conduction pathswithin the machining area and the surrounding materials.

The effect from heat constrained in a small area is revealed inFig. 6. In this experiment, the laser cut through the CFRP compositeat the edge of a sample. The laser cut direction relative to fibre ori-entation was 60�/30�. The figure shows the peeling off of fibres atthe corner on the top portion of the cut line. The fibre ejectioncould be explained as the heat accumulated in the short piece offibres was sufficiently high to cause the polymer matrix todecompose.

The studies shown above suggest that the quality of machiningcomposite is not only affected by laser parameters and fibre orien-tations, but also affected by the machining ‘‘environment”, whereheat might be conducted away or being constrained. Poor cuttingedge or serious HAZ may occur if these are not taken into consid-eration. Furthermore, high laser power is generally used in order toablate carbon fibres. The risk of heat damage to polymer matrix isthus significant increased. Heat conduction in vertical direction(between fibres and ply stacks) is not effective which favours heat

(b)

Cutting direction

Heat retain in the island

/90� and (b) 45�/45� relative to fibre orientation.

Fibres peel off at the edge

Fig. 6. Cutting qualities at edge of a sample.

0

1

2

3

4

5

6

7

0 0.5 1 1.5 2 2.5

Displacement (mm)

Loa

d (k

N)

5.3 kN

Fig. 8. Load displacement curves of 3.1 mm thick pined laminates (bold red solidline: laser, fine dashed blue line: mechanical). (For interpretation of the referencesto colour in this figure legend, the reader is referred to the web version of thisarticle.)

Heat may be accumulated between the perforation

Perforated holes

Heat conduction is not effective in this direction

CFRP

(a)

(b) Laser first cut outline

Heat conduct to CFRP base is blocked by the first cut

Heat accumulation in the islands

Laser subsequent inward cut lines

CFRP

Fig. 7. Illustration of results of heat accumulation in CFRP during laser machining:(a) cross-sectional view of perforation – potential thermal damage between holescaused by heat accumulation; (b) top view of laser scan traces in drilling a hole –enhancement of cutting efficiency due to heat accumulation.

Z.L. Li et al. / Composites: Part A 41 (2010) 1403–1408 1407

accumulation and further increase the potential thermal damageto polymer matrix. These phenomena are not very critical as inmachining of isotropic materials, because: (i) the heat in the‘‘island” can be easily dissipated away in all directions in metals,therefore less heat accumulation, (ii) high ablation threshold ofmetal. Therefore, specific properties of composite materials shouldbe considered during component design and laser machining strat-egy planning. For instance, as shown in Fig. 7a, pitch size and holearrangement should be considered during high density perforationof composites. The heat between holes is constrained and accumu-lated due to lack of heat dissipating path, which may cause thermaldamage (degradation) to the polymer matrix if the pitch size is toosmall. The thermal damage to polymer matrix was observed byYoung et al. when they investigated the performance of CFRP afterNd–YAG laser perforation [27]. They found that the fibres betweenadjacent holes were removed in the rain erosion testing. Fig. 7bshows cutting of a large hole in a thick laminate when multi-circlelaser scanning strategy has to be used in order to produce a widecut kerf for the focusing beam to go deeper into the laminate. Itis wise to cut the outline first and gradually cut inward with

progressively smaller circles. The advantages of such machiningstrategy are: (1) block heat conduction to surrounding materialand hence limit the heat damage to the component only by the firstoutline cutting but not the subsequent inward cutting; (2) heatis accumulated within the ‘‘island” (to be removed) whichenhances the cutting efficiency. In our experiments, we graduallyincreased the scanning speeds of the subsequent inward cuttingto increase the cutting efficiency. This strategy is more promisingin laminated composites but not to isotropic metals as describedbefore.

3.3. Mechanical property test

Mechanical fastening is a common method to join compositeswith other materials, such as metals. The compressive stresses actson the contact area of fastener and laminate may cause bearingfailure. Bearing strength tests were conducted on the 3.1 mm thicksamples. Several laser and mechanical-drilled samples were testedto investigate the process repeatability. The laminates were pinloaded under quasi-static conditions until the load displacementcurve displays a sudden drop in loading. Typical load displacementcurves of the pined laminates are shown in Fig. 8. In the case of la-ser drilled samples, the load curves were relatively confined andessentially follow that of the mechanically drilled hole, indicatingthat the laser cutting process was consistently repeatable androbust. The average maximum load was 5.3 kN at a 1.5 mmdisplacement. In the case of mechanical-drilled samples, the loaddisplacement curves were widely spread, and best result is shownin Fig. 8. The average maximum load was about 5.8 kN at a dis-placement of 1.9 mm.

Fig. 9 shows SEM micrographs of the pin-laminate contact sur-face after the bearing failure. Microcracks in polymer matrix wereobserved in the mechanical-drilled laminates, where the fibre ori-entation was normal to the contact surface, as shown in Fig. 9a. Thecompressive loading promoted polymer matrix cracks. This obser-vation is consistent with earlier investigation [23], where interlam-inar matrix shear cracking was observed. Fig. 9b shows the failuresurface of laser drilled sample. It is seen that fibres at the orienta-tion normal to the load contact surface are broken and reveals thefracturing root. It is well known that one of polymer matrix func-tions in a composite is to hold the fibres in position and transferloads between fibres [20]. When a load is applied to the surface,

20 µm

(a)

20 µm

(b)

Fig. 9. Cross-sectional SEM micrographs of fracture surface: (a) mechanical drilled and (b) laser drilled.

1408 Z.L. Li et al. / Composites: Part A 41 (2010) 1403–1408

the stress may not be evenly distributed to the contact surface.Some of portions may receive higher stress than others. If polymermatrix is damaged due to extensive heat during laser processing,e.g. degradation or decomposition, the support provided by poly-mer matrix to fibres under a load is weakened or is not exist. Thefibres are fractured if the stress is higher than their strength. Thelength of fibre fracture may be characterized as HAZ where poly-mer matrix was degraded or decomposed during laser processing.In our experiments, the HAZ is about 50 lm.

4. Summary

This paper studies the quality issues during UV laser machiningof CFRP composite. The hole with minimised HAZ (50 lm) has beenachieved through parameter optimisation. The study suggests thatthe short laser–material interaction time is primarily required inorder to reduce HAZ, short pulse and high scanning speed is recom-mend. A new method of further reducing laser–material interac-tion time is introduced – optimizing the spacing of adjacent lasertraces. At the optimum spacing, minimum energy or time isneeded to cut through a hole.

Heat accumulation is easier during laser machining of CFRPlaminate than metal, especially in the situation of the carbon fibresare sliced into small pieces, because heat conduction in the verticaldirection (between fibres and ply stacks) is not effective. The heataccumulation can be used to increase material removal, however,it also can significantly affect the quality of the laser machinedCFRP. Attention must be paid to the cutting ‘‘environment”, suchas high density perforation in laminated composite or machiningat the edge. Sufficient heat dissipation path or time should be pre-served to avoid heat accumulation between holes or the edge.

The bearing strength tests show that a laser drilled sample hassimilar failure strength as that of the mechanical-drilled samples.The failure in mechanical-drilled laminate is initiated by polymermatrix cracking. In contrast, fibre cracking was found at the rootof HAZ in the laser drilled samples. The length of the fibre fracturedcan be characterized as HAZ.

References

[1] Berges DE. Hexcel Corporation annual report; 2004.[2] Abrão AM, Faria PE, Campos Rubio JC, Reis P, Paulo Davim J. Drilling of fiber

reinforced plastics: a review. J Mater Process Technol 2007;186:1–7.[3] Korn D. Machining composites by conventional means. Modern Mach; 2005.

<http://www.mmsonline.com/articles/machining-composites-by-conventional-means.aspx>.

[4] Bäuerle D. Laser processing and chemistry. 2nd ed. New York: Springer; 1996.p. 4–12.

[5] Tagliaferri V, Di Ilio A, Visconti C. Laser cutting of fibre-reinforced polyesters.Composites 1985;16(14):317–25.

[6] Fenoughty KA, Jawaid A, Pashby IR. Machining of advanced engineeringmaterials using traditional and laser techniques. J Mater Process Technol1994;42:391–400.

[7] Shanmugam DK, Chen FL, Siores E, Brandt M. Comparative study of jettingmachining technologies over laser machining technology for cuttingcomposite materials. Compos Struct 2002;57:289–96.

[8] Griffis CA, Masumura RA, Chang CI. Thermal response of graphite epoxycomposite subjected to rapid heating. J Compos Mater 1981;15(5):427–42.

[9] Mathew J, Goswami GL, Ramakrishnan N, Naik NK. Parametric studies onpulsed Nd:YAG laser cutting of carbon fibre reinforced plastic composites. JMater Process Technol 1999;89–90:198–203.

[10] Al-Sulaiman FA, Yilbas BS, Ahsan M, Mansoor SB. Laser hole drilling ofcomposites and steel workpieces. Lasers Eng 2006;16(1–2):105–20.

[11] Davim JP, Oliveira C, Barricas N, Conceição M. Evaluation of cutting quality ofPMMA using CO2 lasers. Int J Adv Manuf Technol 2008;35(9–10):875–9.

[12] Caiazzo F, Curcio F, Daurelio G, Minutolo FMC. Laser cutting of differentpolymeric plastics (PE, PP and PC) by a CO2 laser beam. J Mater Process Technol2005;159:279–85.

[13] Zhou BH, Mahdavian SM. Experimental and theoretical analyses of cuttingnonmetallic materials by low power CO2 laser. J Mater Process Technol2004;146:188–92.

[14] Rooks B. Laser processing of plastics. Ind Robot: Int J 2004;31(4):338–42.[15] Davim JP, Barricas N, Conceição M, Oliveira C. Some experimental studies on

CO2 laser cutting quality of polymeric materials. J Mater Process Technol2008;198(1–3):99–104.

[16] Caprino G, Tagliaferri V. Maximum cutting speed in laser cutting of fibrereinforced plastics. Int J Mach Tools Manuf 1988;28(4):389–98.

[17] Cenna AA, Mathew P. Evaluation of cut quality of fibre-reinforced plastics—areview. Int J Mach Tools Manuf 1997;37(6):723–36.

[18] Yung KC, Mei SM, Yue TM. A study of the heat-affected zone in the UV YAGlaser drilling of GFRP materials. J Mater Process Technol 2002;122:278–85.

[19] De Iorio I, Tagliaferri V, Dillio AM. Cut edge quality of GFRP by pulsed lasers:laser–material interaction analysis. In: Proceedings of LAMP, Osaka, May;1987. p. 279–84.

[20] Campbell FC. Manufacturing processes for advanced composites. Oxford: Elsevier;2004. p. 442 and 9.

[21] Denkena B, Volkermeyer F, Kling R, Hermsdorf J. Novel UV–laser applicationsfor carbon fibre reinforced plastics. In: Proceedings of APT, Bremen,September; 2007. p. 17–9.

[22] Li ZL, Chu PL, Zheng HY, Lim GC, Li L, Marimuthu S, et al. Process developmentof laser machining of carbon fibre reinforced plastic composites. In:Proceeding of ICALEO, Temecular, October; 2008. p. 222–30.

[23] Kelly G, Hallstrom S. Bearing strength of carbon fibre/epoxy laminates: effectsof bolt-hole clearance. Compos Part B: Eng 2004;35(4):331–43.

[24] Aoyama E, Inoue H, Hirogaki T, Nobe H, Kitahara Y, Katayama T. Study on smalldiameter drilling in GFRP. Compos Struct 1995;32:567–73.

[25] Negaestani R, Sundar M, Sheikh M, Mativenga P, Li L, Li ZL, et al. Numericalsimulation of laser machining of carbon fibre reinforced composites. In:Proceeding of IMechE. J Eng Manuf, in press. doi:10.1243/09544054JEM1662.

[26] Uhlmann E, Spur G, Hocheng H, Leibelt S, Pan CT. The extent of laser-inducedthermal damage of UD and crossply composite laminates. Int J Mach ToolsManuf 1999;39:639–50.

[27] Young T, Mahony B, Humphreys B, Totland E, McMlafferty A, Corish J.Durability of hybrid laminar flow control (HLFC) surfaces. Aerosp Sci Technol2003;7:181–90.