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Article history:Received 9 June 2013Received in revised form 24 July 2013Accepted 30 September 2013Available online 9 October 2013

phenomenon of the removal of material is followed by the appear-

chined surface (uncut bers, thermal and/or mechanical degrada-tion of the matrix) [1,2]. When machining is conducted with acutting tool such as in the case of conventional machining, the de-fects localized at the free edges are mainly inuenced by the cut-ting forces. These forces are strongly affected by the cutting

emergence of these damages and the different mechanisms leadingbeen carried out.rthogonalining aretween the

speed direction and the bers orientation. In this case, themum damage is observed when this angle (H) is at 490. In addition, the severity of these defects increases with the in-crease of the wear of the cutting tool [2,610], that is why toolsmade of diamond and carbide are highly recommended formachining composite materials [1].

Several studies in the literature have mentioned that thegenerated defects are strongly related to the bers direction[29]. This result is well observed during the orthogonal cutting[35]. With the increasing of the tool wear, the propagation of

Corresponding author. Tel.: +33 562258872; fax: +33 562258747.E-mail address: redouane.zitoune@iut-tlse3.fr (R. Zitoune).

1 Address: 133c, avenue de Rangueil. Dpart. GMP de lIUT-A de luniversit de

Composites: Part B 57 (2014) 136143

Contents lists availab

Composite

evToulouse, INSA, UPS, Mines dAlbi, ISAE, ICA (Institut Clment Ader), France.ance of damaged zones. This can lead to the non-respect of themachining quality (according to imposed industrial standards).These damaged areas are located on the free edges of the machinedsurface (uncut bers/aking/delamination) and/or on the ma-

to their formation, various research studies haveThese studies, which are mainly based on the o[35], have shown that damages due to machinuenced by the relative angle (H) measured be1359-8368/$ - see front matter Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.compositesb.2013.09.051cuttingmainlycuttingmaxi-5 and1. Introduction

Trimming is the rst machining operation performed on com-posite structures after demoulding. However, the anisotropy andthe highly heterogeneous nature of composite materials maketheir machinability very complex. In addition, regardless of themachining process used (conventional or non-conventional), the

tools geometry, the cutting parameters, the tool wear, etc. Con-cerning the damages located on the machined surface, they aremainly affected by the relative angle between the cutting speeddirection and the ber direction of the machined composite mate-rial, the cutting parameters and also by the wear of the cutting tool[1,2].

In order to better understand the phenomenon behind theKeywords:A. Carbon bersB. DefectsB. DelaminationB. FatigueB. Mechanical propertiesThe main focus of this paper is to investigate the defects generated by different machining processes(namely burr tool machining, abrasive water jet machining AWJM and abrasive diamond cutter ADS)and their impact on the mechanical behavior of CFRP in quasi-static (compression and inter-laminarshear) and tensiletensile fatigue tests. The cutting conditions are selected so that different levels of deg-radation can be obtained. The machined surface is characterized using roughness measuring devices withand without contact and SEM observations. The results show that the defects generated during the trim-ming process with a cutting tool are bers pull-out and resin degradation. These defects are mainlylocated in the layers with the bers oriented at 45 and 90. However, when using abrasive water jetand abrasive diamond processes, the defects generated have the form of streaks and are not dependenton the ber orientation. Furthermore, the results of quasi-static tests performed on specimens machinedby cutting tools show that AWJ specimens offer a better resistance in compression but the ADS samplesoffer higher inter-laminar-shear strengths. Moreover, the results of fatigue tests show that specimensmachined with a burr tool offer higher endurance limit. Finally, it is concluded that the type and themode of the mechanical loading (quasi-static fatigue) affect the mechanical response of CFRP and favora given machining process.

Published by Elsevier Ltd.a r t i c l e i n f o a b s t r a c tStudy of trimming damages of CFRP struof the machining processes and their imp

M. Haddad a,1, R. Zitoune a,,1, H. Bougherara b, F. Eyma IUT-A GMP, 133 c, Avenue de Rangueil, 31077 Toulouse, FrancebDepartment of Mechanical and Industrial Engineering, Ryerson University, Toronto, ONc IUT de Tarbes, Dept. GMP, 1, rue Lautreamont, 65000 Tarbes, Franced INSA, 135 Avenue de Rangueil, 31077 Toulouse, France

journal homepage: www.elsures in functiont on the mechanical behaviorc,1, B. Castani d,1

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have an important inuence on quality of the machined surface.In order to reduce damages during trimming of compositemate-

tes:rial processes of machining with the diamond cutter (ADS) and theabrasive water-jet machining (AWJM) are recommended [1719].When AWJM is considered, the defects observed are striations onthe exit of the water jet and craters on the machined surface. Chenand Siores [20] have mentioned that the defects generated duringthe machining of metallic materials by AWJ process are mainly af-fected by the magnitude and the distribution of the kinetic energy.Other experimental studies [1719] have shown that the size ofstreaks increases with the distance from the water jet source.

Currently in the industrial eld, the parameter used for qualify-ing the machined surface of a composite material is the arithmeticaverage roughness (Ra). It is important to note that the roughnessparameter (Ra) is initially developed to qualify the machined sur-faces of metallic materials. When considering composite materials,there is a controversy about the use of this parameter. In fact, dif-ferent studies have shown contradictory results. For instance, theresults of mechanical tensile tests carried out on unidirectional(UD) samples made of glass bers and epoxy resin and orientedat +45 relative to the axis of loading [6] have shown that the ten-sile strength increases with the increase of the average roughness(Ra). Conversely, the results of compressive mechanical tests con-ducted on UD specimens oriented at 0 (ber direction in the samedirection of the loading) [21] have shown that the stress failure de-creases with the increase of the surface roughness. Eriksen [22] hasinvestigated the effect of the surface roughness on the quasi-staticand fatigue behavior of short ber reinforced plastics and foundthat the mechanical behavior (at the macro scale) of the compositepart made of short bers is not affected by the surface roughness.

For Multidirectional CFRP, a comparative study between abrasivewater jet (AWJ) machining, cutting with abrasive diamond cutter(ADS) and edge trimming with polycrystalline diamond (PCD) wasconducted by [23,24]. The study has shown that the abrasive dia-mond cutter provides better results in terms of surface roughnessand bending mechanical resistance. However, the PCD specimensgave a good surface roughness (Ra) and a poorest bending mechan-ical resistance. The study also suggested, without providing impor-tant details that the bending mechanical resistance of AWJ samplesdecreases with the increase of the average surface roughness (Ra).

Therefore, the aim of this study is twofold: rst, to investigatethe inuence of the trimming processes (conventional vs. non-con-ventional machining) on the induced damages and, second, toexamine the impact of these damages on the mechanical behaviorof the composite specimens. For this purpose, quasi-static tests(compressive and interlaminate shear tests) combined with fatiguetests (tensiontension) have been conducted on different compos-ite specimens obtained with trimming using a burr tool and by theADS cutting process. Quasi-static tests are curried out on speci-mens obtained by a standard cutting tool (burr tool), an abrasivewater jet machining (AWJ) and an abrasive diamond cutter(ADS). During fatigue tests, an Infrared (IR) camera is used to quan-tify the damage and estimate the endurance limit of the differentcomposite samples used. In addition, the effect of rectication onthe quasi-static and fatigue responses is also investigated.

2. Experimental procedures

2.1. Material preparationthe delamination is commonly observed during the drilling of com-posite materials [1013]. Based on previous studies [1416], the -ber content and the manufacturing process of the composite part

M. Haddad et al. / ComposiCarbon-ber reinforced plastic (CFRP) composites of 5.2 mmthickness (20 layers) were used for conducting the trimming study.CFRP composite plates were made using unidirectional prepregssupplied by Hexcel Composite Company referenced under T700/M21-GC. In order to get a multidirectional laminate, a staking se-quence corresponding to [90/90/-45/0/45/90/-45/90/45/90]S ischosen. This stacking sequence was chosen because it was demon-strated in a previous work [25] that the edge delamination phe-nomenon occurs during compression tests. The curing process forthese CFRP specimens was carried out at 180 C for 120 min duringwhich the pressure was maintained at 7 bars in an autoclave whilethe vacuum pressure was set to 0.7 bar. The prepregs are charac-terized by a ply thickness of 0.26 mm, a ber content Vf = 59%, alongitudinal Young modulus El = 142 GPa, a transversal Youngmodulus Et = 8.4 GPa, a shear modulus: G12 = 3.8 GPa and a glasstransition temperature: Tg = 187 C.

2.2. Samples preparation

The composite specimens were prepared using three cuttingprocesses, an abrasive water jet (AWJ), a diamond cutter (ADS)and a standard cutting tool. JEDO technologies company con-ducted the abrasive water jet machining using an abrasive mesh# 220 which had a mean diameter of 67 lm with a ow rate of300 g/min, a nozzle diameter of 0.25 mm and a pressure of3600 bar. To generate two different surface qualities two feedspeeds were chosen, namely 100 mm/min and 500 mm/min. A to-tal of ve specimens were prepared for each cutting condition andfor each quasi-static and fatigue mechanical test.

The ADS samples were obtained by a diamond saw (cutter) ref-erenced under DIAMFORCE JANTE CONTINUE-D100-AL19-GR427. In order to study the impact of the rectication process onthe mechanical behavior, a thickness of 0.5 mm was removed fromeach side of some CFRP samples initially machined by ADS cutting.The standard cutting tool machining experiments were performedusing a DUBUS 3-axis milling machine. These experiments wereconducted using a full experimental design; with three cuttingspeeds (350 m/min, 700 m/min and 1400 m/min) and three feedspeeds (125 mm/min, 250 mm/min and 500 mm/min). Howeverin this paper only specimens having similar roughness values asthe abrasive water jet machining and ADS cutting plus a cuttingcondition where the surface roughness is very high (poor surfacequality) are considered. It is important to notice that samples withsimilar roughness values are generated by different combinationsof cutting speed, feed speed and cutting distance (Lc). For fatiguetests, two specimens were prepared using a burr tool. The rstone is machined with a new tool with a cutting speed of 700 m/min and a feed speed of 500 mm/min in order to generate a goodsurface quality. The second specimen is machined by a used toolafter a machining distance of 2.6 m. A cutting speed of 1400 m/min and a feed speed of 125 mm/min were chosen to generate apoor surface quality.

2.3. Surface defects characterization

The machined surfaces were analyzed using two measurementdevices. First, a surface roughness tester Mitutoyo SJ 500 wasused to measure the surface roughness, the total measuring lengthwas set to 5 mm to avoid any overowing. The second device is a 3Dtopographer Altisurf 520 was used to perform the 3D measure-ments and surface roughnesses without contact. The Altisurf 520uses the principle of optical microscopy with confocal white lightsource. The wavelength was analyzed by a focused spectrophotom-eter analysis that measures the distance between the lens and thesurface of the object. The measuring step was set to 4 lm on both

Part B 57 (2014) 136143 137directions x and y. The results obtained with both measurementtechniques were then correlated with a scanning electron micro-scope (SEM) images obtained using a scanning electron microscope

JEOL. The average surface roughness Ra which measures theaverage vertical distance of the mean roughness prole line fromall measured data points is chosen, since higher surface irregulari-ties increase the probability of nucleation sites for cracks. So it is agood indicator for mechanical performances.

2.4. Mechanical experiments

2.4.1. Static loadingMechanical quasi-static tests were performed at room temper-

ature using an MTS 322 tester with hydraulic jaws (Fig. 1). Thecompressive, tensile, and inter-laminar shear tests were conductedfollowing AFNOR NF T 51-120-3, NF EN ISO 527-4 and AFNOR NF T57-104 standards.

2.4.2. Fatigue loadingFatigue tests were conducted an MTS machine equipped with

hydraulically operated wedge grips. The tested specimens havebeen instrumented on the surface by an extensometer which was

cycles). This endurance limit can also be obtained from the temper-ature stabilization curves [29,30] by intersecting the two straightlines that interpolates the stabilization temperature and the corre-sponding stress level.

3. Results and discussion

3.1. Quasi-static tests

3.1.1. Inuence of machining process on the quasi-static mechanicalbehavior

Fig. 2 shows the results of quasi-static, compressive and inter-laminar shear stresses tests curried out on specimens obtainedby the three machining processes and characterized by identicalsurface roughness devices. From these gures, it is noticed thatthe samples obtained by AWJ are characterized by higher compres-sive stresses than those of specimens machined by conventionalmachining processes (i.e., cutting tool or ADS). For instance, forspecimens with an average surface roughness Ra of 6.4 lm, thecompressive strength of specimens machined by AWJ are 15%higher than those of specimens machined by the ADS cutting (cf.Fig. 2a). For the same surface roughness value, it is observed thatthe discrepancy between the compressive stresses of specimensmachined with AWJ and those with burr tool is around of 21%.

However when considering inter-laminar shear strength, ADSsamples offer better results than the two other machining pro-cesses. For instance, for the specimens machined by ADS process,the inter-laminar shear stress is around 57 Mps and for theAWJ process is around 53 Mps. It is then concluded that themechanical proprieties of specimens are strongly inuenced by

rla

138 M. Haddad et al. / Composites: Part B 57 (2014) 136143used to monitor the local strain allowing the calculation of thestiffness degradation of the specimen during the cyclic loading. AFLIR SC5000 infrared camera with a pixel resolution of 320 240and a temperature sensitivity of

the mode of the machining process and the mode of loading: ten-sion/compression/bending/shear, etc.

Also, it is noticed that the compressive and the inter-laminarshear strengths of specimens obtained by AWJ are decreasingwhile increasing the surface roughness (cf. Fig. 2b). For instancethe average roughness Ra varies from 6 lm to 10 lm, the failurestress in compression obtained decreases from 320 MPa to285 MPa. However, for specimens obtained with a conventionalmachining (burr tool) a random evolution of the compressive andthe inter-laminar strength are observed when increasing the sur-face roughness value (cf. Fig. 2a and b).

It is also observed that, smaller standard deviations (around6 MPa) are obtained when considering samples machined withthe AWJ process. This reects the good repeatability of the ma-chined surfaces with the AWJ process.

The differences in the mechanical behavior could be related tothe form of the defects generated during the trimming process of

the entire machined surface (Fig. 5b) and the distinction of the dif-

explains the high standard deviation of the results. The depth ofthese defects is around 30 lm. This value is smaller than those ob-tained on the trimmed surface by burr tool.

For the AWJ, it was observed that the defects generated by thisprocess have the form of streaks and craters. 3D topography andSEM observation indicate that streaks defects appear at the exitof the machined surface however the craters cover the entire ma-chined surface (Fig. 6). Based on the literature review, the lengthand the width of the streaks defects decrease with the increase (de-crease) of the jet pressure (feed speed) [1719]. The cutting pro-cess by AWJ occurs by the erosion mechanisms and the produceddamages are independent of the bers orientation (cf. Fig. 6a)and appear periodically all over the surface. Getting repeatable de-fects all over the surface explains the small standard deviation ob-tained after mechanical tests.

3.1.3. Effect of rectication on the mechanical behaviorTo investigate the inuence of the wrenched damages, ten spec-

imens machined by ADS (ve for each quasi-static mechanical test)are rectied in order to remove the wrenched damages and to get a

Fig. 4. Evolution of the roughness valley (Rv) vs. the surface roughness (Ra).

Streaks

600 m

(b)

5 mm

112.9 mm

1 mm

(a)

Fig. 5. Machined surface obtained by ADS cutter. (a) 3D topography. (b) SEMobservation.

M. Haddad et al. / Composites:ferent ber orientations is impossible. The nature of these damagesdepends mainly of the feed speed (in this case, it is controlledmanually by the operator) and the size of diamond grains which

Wrenched areasUncut fibres

Uncut fibres

600 m

(b)

328.6 m

4 mm

1 mm

(a)

Fibre pull outs

Wrench areasthe composite specimens. In fact, some shapes of damages can pro-mote an important stress concentration that leads to the deteriora-tion of composites mechanical properties.

3.1.2. Generated defects during trimmingFig. 3 shows the defects obtained after trimming with conven-

tional machining using a burr tool. It is observed that defects in-duced by the cutting tool are located mainly at the plies orientedat 45 (Fig. 3). These defects have the form of wrenched areaswith different depths varying from 30 lm to 70 lm (Fig. 4) whenthe average roughness Ra varies from 6.4 lm to 19.8 lm. Thesedefects can also be assimilated to craters or even cracks. The pres-ence of these cracks induces areas of stress concentration which re-sult on the relatively small values of the compressive strength andinter-laminar shear strength when considering conventionalmachining with a burr tool.

The defects generated after machining with the ADS cuttingprocess have mostly the form of streaks (Fig. 5). These streaks rep-resent wrenched areas and follow the tool trajectory. They arecaused by the random distribution of diamond grains with differ-ent sizes and shapes on the cutting face of the abrasive diamondcutter (ADS). The trajectory of the defects is strongly linked tothe cutting tool trajectory, while the ber directions have no effecton these damages. For this reason, the ADS defects are observed onFig. 3. Form of the trimmed surface after machining with burr tool. (a) Imagetopography 3D. (b) SEM observation.0

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Abrasive water jet machining (AWJ)Standard cutting tool machining Abrasive diamond cutter (ADS)

Part B 57 (2014) 136143 139continuous surface, and tested in quasi-static loading.Fig. 7 represents the inuence of the rectication process on the

failure stresses of compressive and inter-laminar shear tests. It is

es:(a)191.5 m (a)

140 M. Haddad et al. / Compositobserved that, after the compressive tests, the rectied specimenspresent a failure stress of 10% more compared to specimens with-out rectication. Hence, the failure stress during compressive testsis strongly inuenced by the size of the damages (depth ofwrenched areas) induced by the trimming process.

3.2. Fatigue tests

Streachs (exit of the water jet) 600 m

(b)

600 m5 mm 2 mmStreaks (exit of

the water jet)

Fig. 6. Machined surface obtained by abrasive water jet. (a) 3D topography. (b) SEMobservation.

100

150

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ADS specimens with rectification ADS specimens without rectification

Fig. 7. Inuence of the rectication process of specimens trimmed by ADS processon the mechanical behavior. (a) Compressive test. (b) Inter-laminar shear test.3.2.1. Damage analysisAfter the fatigue tests the damage accumulation (D) which rep-

resents the change in the ratio of dynamic stiffness (Ei) to staticstiffness (E0) is calculated by the following equation:

D 1 EiE0

1From Fig. 7b, it is noticed that the rectication process has noinuence on the inter-laminar shear strength. It can be assumedthat this type of solicitation is not affected by the wrenched areasdefects. This may explain the fact that ADS machining process pro-duces specimens with higher inter-laminar shear strength evenwith the presence of wrenched areas defects on its machiningsurfaces.

Normalized cycle (N/Nf)

Dam

age

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

36 kN 32 kN 28 kN 24 kN 20 kN 16 kN 12 kN 8 kN 4 kN

Fig. 8. Accumulation of damage obtained for a burr tool specimen under differentloading amplitudes vs. the normalized cycles (N/Nf).

Part B 57 (2014) 136143The damage proles vs. the normalized cycles (N/Nf, whereNf = 10,000 cycles) obtained for a burr tool (good surface quality:Vc = 700 m/min, Vf = 500 mm/min) are shown in Fig. 8. From thisgure it is noticed that the accumulated damage is inferior to 5%for loads below 24 kN (50% of UTS).

When the load reaches 32 kN (67% UTS) the accumulated dam-age does not exceed 7%. However, when the applied load reaches36 kN (75% UTS) the accumulated damages is equal to 12.5%. Sim-ilar trends were observed when considering the other specimensobtained by ADS, by rectication and by edge trimming (poor sur-face quality, Vc = 1400 m/min, Vf = 125 mm/min).

Fig. 9 represents the accumulated damages at the end of cyclefor the different machining processes. It is observed that, whenconsidering a load of 36 MPa (75% UTS) the specimen obtainedby ADS and rectication processes have the same rate of damageaccumulation (around 8%) at the end of the loading cycle. However,with the same condition, the trimmed specimens with a burr toolpresent more important accumulated damages (around 12%). So, itis also noticed that, whatever the quality of the machining surfaceafter trimming by a burr tool is, the accumulated damage of spec-imens subjected to fatigue is 50% higher than the accumulateddamage of other specimens made using ADS process.

It is important to mention that although the average roughnessRa obtained by both cutting processes (ADS cutting and by trim-ming with a burr tool) are similar (cf. Table 1), the fatigue behavior

and T is the initial temperature.

0.06

0.08

0.1

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0.14

lativ

e da

mag

e

Burr tool-good surface qualityBurr tool-poor surface qualityADS cuttingRectified specimen

M. Haddad et al. / Composites: Part B 57 (2014) 136143 141for these specimens is totally different. In fact, it can be concludedthat the criteria used nowadays to quantify the quality of the ma-chined surface (e.g., Ra, Sa, etc.) is not suitable for machined com-posite materials.

3.2.2. Thermography analysisFig. 10 shows the evolution of temperatures for different loads

during fatigue tests on specimens machined with burr tool (surfaceroughness Ra of 8.89 lm). It is observed that the temperature re-mains constant throughout the surface for loads less than 16 kN(DT = 02 C). With the increase of loading and until 28 kN, thetemperature increases moderately for each load step (DT = 313 C). However, when the loading is increased by 4 kN to reach32 kN an important increase of the temperature is noticed(DT = 23 C). This temperature continues to substantially increaseuntil the nal rupture of the specimen (DT = 43 C). This variationon the temperature prole is due to the thermo-elasticity of thematerial and the friction between the layers (i.e. bers/bersand/or bers/matrix). Three stages for the temperature evolutionwere distinguished. In the rst stage, an important increase is ob-served, in the second stage, the temperature reaches a balance dueto the saturation in the damage. With the increasing loading, therate of the damage and frictions become more important. This sta-bility is followed by an abrupt increase of the damage and temper-

0

0.02

0.04

0 4 8 12 16 20 24 28 32 36 40Load (kN)

Cum

u

Fig. 9. Evolution of the damage at the end of the loading cycles as a function of thedifferent loading charges.ature of the specimen corresponding to the rupture [26]. For thenal load, the saturation is not reached and an important increasein the temperature is observed. This increase represents the thirdand nal step before rupture. From the literature works [3638],the augmentation of temperatures while loading is related to sev-eral factors (matrix cracking, delamination, bers breakage, etc.).In the rst stage, matrix cracking occurs at weak points of thematerial. In the second stage a debonding and ber matrix delam-ination take place. In the nal phase an abrupt temperature due tober breakage and continues until the total failure of the speci-mens [37].

Table 1Surface roughness of all specimens.

Surface Roughness

Burr tool good surface quality Burr tool

Ra (lm) 8.89 1.71 38.64 6.Rp (lm) 31.34 5.99 127.95 24Rv (lm) 31.66 8.41 86.88 16Rz (lm) 63.0 13.04 214.83 37Rt (lm) 86.92 23.13 265.69 39The evolution of the temperatures obtained at the end of theloading cycle as a function of different load steps and for differentspecimens is presented in Fig. 11. It is noticed that specimens ob-tained with a burr tool have the same behavior and that their max-imum temperatures before the total fracture are around 70 C.These temperatures are higher compared to those recorded duringfatigue tests on specimens machined by ADS and rectication pro-cesses. A difference in temperature around 40% (23%) is obtainedwhen comparing with ADS specimens without (with) rectication.It is also noted that these temperatures increase exponentiallywhen increasing the loading charges.

3.2.3. Endurance limit investigationFrom the literature review [22], the endurance limit is obtained

by Whler curves (stress vs. cycles). This endurance limit can alsobe obtained from the temperature stabilization curves [29,30] byintersecting the two straight lines that interpolates the stabiliza-tion temperature and the corresponding stress level. The changein temperature is given by

DT T f T0 2where DT is the change in temperature, Tf is the nal temperature

Fig. 10. Evolution of temperature as a function of the normalized cycle (N/Nf) forstandard cutting tool machining sample.0

Temperature proles obtained after calculation (Eq. (2)) at10,000 cycles for different loads and for the specimen machinedby ADS process are represented in Fig. 12. In this situation, theendurance limit is estimated to 198.4 MPa. However for rectiedADS specimen the endurance limit is improved by 7.5%. Thisimprovement is even higher when considering the specimen ob-tained by a burr tool with a similar surface roughness to ADS cut-ting specimen. In this case a variation of 14% is observed. Evenwhen considering the specimen obtained by a burr tool and havinga high surface roughness (Ra = 38.64 lm) the endurance limit ishigher to that of ADS cutting (10%) (Fig. 13).

poor surface quality ADS sample Rectication

69 8.87 1.96 1.47 0.16.7 29.14 8.86 5.27 0.48.15 28.86 10.15 8.51 3.84.9 58 18.1 13.77 4.07.1 74.01 32.19 19.08 7.65

es:50

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at th

e en

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the

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C)

Burr tool-good surface qualityBurr tool-poor surface qualityADS cuttingRectified specimen

142 M. Haddad et al. / Composit4. Conclusions

In this paper the effect of machining processes using conven-tional (a burr tool and an abrasive diamond cutter (ADS)) andnon-conventional machining (an abrasive water jet (AWJ)) on themechanical behavior of composite parts made of carbon/epoxy isinvestigated. The following conclusions can be drawn:

The quasi-static tests (compressive and inter-laminar sheartests) showed that specimens machined by the AWJ processpresent a failure stress in compression 1020% more important

obtained by different machining processes, however the formof the defects is completely different from one machining pro-

20

30

40

0 4 8 12 16 20 24 28 32 36 40Load (KN)

Tem

pert

alo

a

Fig. 11. Evolution of the temperature at the end of the last cycle in function of theapplied loading.

Fig. 12. Temperature variation vs. stresses for the ADS specimen with rectication.

Rectified specimen

ADS cutting

Burr tool- good surface quality

Burr tool- poor surface quality

180

190

200

210

220

230

240

1.47 8.87 8.89 38.64Ra (m)

Endu

ranc

e lim

it (M

Pa)

Fig. 13. Endurance limit vs. surface roughness for different specimens.cess to another, these defects inuence the mechanical behaviorof the composite material under cyclic loading.

The rectication process conducted on specimens trimmed byADS process conrmed that the removal of the wrenched areasimprove the compressive strength. However, this recticationprocess does not have any inuence on the inter-laminar shearstrength.

The fatigue tests carried out on various specimens trimmed bydifferent processes of machining show that the higher endur-ance limit corresponds to those specimens trimmed by burr toolfor any machined surface quality. In addition, the recticationprocess improves the endurance limit of ADS specimens by7.5%.

Finally, it is clear that surface roughness criteria which are usedas the gold standard for metallic materials, are not recom-mended for the machining of composite materials. It had beenseen that specimens obtained by a burr tool and with deferentlevels of surface quality (Ra = 8 lm, Ra = 38 lm) offer similarresults of the endurance limit. In addition, the specimensobtained by the ADS process offer a lower endurance limit(15%) for the same surface roughness as specimens trimmedby the burr tool process.

The compressive and inter-laminar shear quasi-static testsshowed a higher mechanical strength for ADS specimens com-pared to specimens obtained by a burr tool. However, in ten-siletensile fatigue tests, the specimens machined by a burrtool have the highest endurance limit. Therefore, in additionto the type of loading, the mode of mechanical loading (quasi-static, fatigue) may affect the mechanical response of the com-posite material.

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Study of trimming damages of CFRP structures in function of the machining processes and their impact on the mechanical behavior1 Introduction2 Experimental procedures2.1 Material preparation2.2 Samples preparation2.3 Surface defects characterization2.4 Mechanical experiments2.4.1 Static loading2.4.2 Fatigue loading

3 Results and discussion3.1 Quasi-static tests3.1.1 Influence of machining process on the quasi-static mechanical behavior3.1.2 Generated defects during trimming3.1.3 Effect of rectification on the mechanical behavior

3.2 Fatigue tests3.2.1 Damage analysis3.2.2 Thermography analysis3.2.3 Endurance limit investigation

4 ConclusionsReferences