Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America
Journal of Biobased Materials and Bioenergy Vol. 5, 1–8, 2011
Experimental Analysis of Drilling Damage in Biocomposite Laminates Manufactured by Resin Transfer Molding A. Lopez-Arraiza1 ∗ , I. Amenabar1 , M. Sarrionandia2 , and J. Aurrekoetxea2 1
Ideko-IK4 Technological Centre. Arriaga industrialdea, 2, E-20870 Elgoibar (Gipuzkoa) Spain 2 Mechanical and Manufacturing Department, Mondragon Unibertsitatea. Loramendi, 4. E-20500 Mondragon (Gipuzkoa) Spain
Keywords: Drilling, Biocomposite, Delamination, Ultrasonics, X-ray Computed Tomography.
1. INTRODUCTION Fiber reinforced polymers (FRPs) are becoming widely used in the manufacture of products where a high mechanical strength must be accompanied by a low weight. Consequently, these composite materials can be found in industrial sectors such as aeronautical, automotive, railway, marine or wind-power. The fibers are usually glass, carbon or aramid, while the polymer is usually an epoxy, vinylester or polyester. However, there is a trend toward replacing synthetic fibers with natural fibers and thermosetting resins by thermoplastics or by thermosets from renewable resources. Natural fibers such as sisal, bamboo, flax, hemp, kenaf or jute have the potential to be used as a replacement for glass or other traditional reinforcement materials in composites.1 These fibers have many properties which make them an attractive alternative to traditional materials. They have high specific properties such as ∗ Author to whom correspondence should be addressed. Email:
[email protected]
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stiffness,2 impact resistance3 4 and ductility.5 In addition, they are available in large amounts,6 and are renewable and biodegradable. Other desirable properties include low cost, low density, acoustic and thermal insulation, less equipment abrasion,5 7 less skin and respiratory irritation,8 vibration damping,2 3 and enhanced energy recovery.8 9 By contrast, the hydrophilicity of natural fibers results in high moisture absorption and weak adhesion to hydrophobic matrices. The natural fibers can be treated to improve the adhesion to matrix materials.10–12 Structural applications for these materials are rare since existing production techniques are not applicable and availability of semi-finished materials with consiste quality is still a problem. Many works are focused on developing high-performance natural fiber composite systems using continuous textile reinforcements like unidirectional tapes or woven fabrics.13 14 Production techniques such as SMC or RTM with appropriate semi-finished natural materials have mainly been studied.13–15 Materials from renewable resources are being sought to replace not only the reinforcement element but also the matrix phase of composite materials. As a result of the
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doi:10.1166/jbmb.2011.1168
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The emergence of new natural fiber reinforced composites to replace glass fiber reinforced thermosets faces a new challenge: their machining. In this work, jute fabric reinforced bioepoxy resin laminates were manufactured, drilled and inspected. Different drill geometries and machining conditions were compared. Roughness, microscopy and non-destructive tests allowed determination of the hole quality as well as delamination extent. The surface tests showed the best results were achieved with the Dowel bit, a standard wood drill, at a spindle speed of 3,000 rpm and feed rate of 0.025 mm/rev. The delamination extent, characterized by means of Ultrasonics and X-ray Computed Tomography, also confirmed that the best results were achieved with the Dowel bit. Scanning electron microscope images were taken for the drilled holes to support the results. In contrast to carbon fiber reinforced thermosets, the detected delamination at high feed rate is not as extensive as expected. These results suggest that this new biocomposite could be machined at production rates without delamination damage being generated. Besides, a standard wood drill was more suitable than a drill for composite materials to obtain excellent finishes.
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Experimental Analysis of Drilling Damage in Biocomposite Laminates Manufactured by RTM
limitations of thermoplastics, there is considerable need for development in the area of thermosets from renewable resources.16–18 Vegetable oils have been used as the building blocks of naturally derived thermosetting resins. Currently, petrochemical reagents are still needed to cross-link the monomers and the aim is to maximise the proportion of renewable resources used while retaining acceptable material properties. On the other hand, the new biocomposites faces a new challenge: machining. In most cases a composite part needs to be assembled with other parts, either with a composite or with a different material (steel, aluminium alloys, wood, etc.) and mechanical joining (rods, pins, fasteners, etc.) is the solution commonly adopted. The holes required by mechanical joining are generally drilled in the semifinished composite part. Drilling is a particularly critical operation for fiber reinforced laminates because the large concentrated forces generated can lead to widespread damage. This damage causes aesthetic problems but, more seriously, may compromise the structural integrity of the finished part.19 20 The mechanics of drilling composite materials has been studied along with the quality of the hole and the effects of tool geometry, tool material and cutting conditions.21–24 The main types of structural damage generated during drilling of polymeric matrix composites are delamination, microcracks, fiber-matrix debonding, matrix cratering and thermal damage. Delamination is probably the most common and concerning type of structural damage induced during drilling of fiber-reinforced composite materials. Owing to the drill thrust force, the different layers of the composite are prone to delaminate, the exit layers being the most vulnerable. Different techniques and parameters can be used to assess the damage caused by drilling. Sophisticated nondestructive techniques25–27 can be employed in addition to low-magnification microscopy in order to identify internal defects, while destructive techniques are rarely employed. Moreover, a scanning electron microscopy can be used to observe the cut surface morphology. Ultrasonic C-scan is a widely used non-destructive inspection technology for composite delamination measurement, especially in aeronautical engineering. However, the C-scan based delamination measurement faces several hole-interference related problems.20 27–29 On the other hand, the X-ray Computed Tomography (X-ray CT) technology offers the capability to obtain a 3D image of a sample providing internal information.30 31 The resolution of the reconstructed CT image depends on the overall sample size. In order to evaluate the delamination damage in a drilled hole, a X-ray CT reconstruction with a relatively high resolution is required.32 Thus, although the X-ray CT is in essence a non-destructive technique, the preparation of specific single-hole samples is necessary. Although this decreases the applicability of the 2
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X-ray CT technique, it can be thoroughly used to provide high-resolution reference results. In this paper, a thermosetting resin from natural resource was reinforced with woven jute fabric. The laminates were manufactured by RTM and postcured in an oven. Different drill geometries and drilling parameters were investigated. The aim of the work is to determine the most accurate tool and machining conditions. In order to detect suspected delamination as well as other structural damages, traditional tests such as optical microscopy, scanning electronic microscopy and surface roughness measurement were combined with an ultrasonic based strategy for fast non-destructive measurement of hole peripheral delamination validated by X-ray CT based reference data.
2. EXPERIMENTAL DETAILS 2.1. Materials The resin used was Epobiox™ (Amroy Europe Ltd) of which 70% is made from industrially grown and harvested natural oils like, for example, epoxidised pine oil waste. The mixing ratio of the CA35 curing agent was 100:35 by weight. The jute fiber reinforcement was supplied by Tejijut Company as a balanced 0 /90 plain weave fabric of 305 g/m2 in weight. The laminates were manufactured by RTM in circular plates of 260 mm diameter and 3 mm thick consisting of 6 plies. Total fiber weight fraction was 41% and the void content was 4.6%. 2.2. Drilling Tools and Parameters Drilling tests were carried out on a vertical machining center (MML-500). The tests were run at the spindle speed of 3,000 revolutions per minute and feed rate of 0.025 mm/rev and 0.05 mm/rev. Drilling was performed in dry conditions in order to avoid moisture adsorption by the biocomposite. Although a support beneath thin laminates is usual to avoid push out delamination and poor hole roundness, the tool diameter (Dtool = 6 mm) and the span between supports were small enough to overcome such problems. Furthermore, the lack of a support beneath the specimen avoids tool blunts and chip pollution. Three different tool geometries were tested (see Fig. 1) and five holes were made for each cutting speed and drill geometry. The Reamer (Fig. 1(a)) is a very common tool in both aeronautics and automotive industries because it provides fast drilling times. It is made from solid carbide and it has two distinct cutting edges at 45 degree angle: one for drilling and one for reaming. The Dowel bit (Fig. 1(b)) is a standard wood drill bit made of high speed steel. It has a central point which enters first to prevent “skating” and two raised spurs for optimum chip ejection. The Drill-end J. Biobased Mater. Bioenergy 5, 1–8, 2011
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Fig. 1.
Experimental Analysis of Drilling Damage in Biocomposite Laminates Manufactured by RTM
Studied tool geometries (Dtool = 6 mm): (a) Reamer, (b) Dowel bit, (c) Drill-end cutter.
Cutter (Fig. 1(c)) is a special tool for cutting composite materials and provides a smooth finish. It is a solid carbide burr cutter with drill point geometry. Regarding the spindle speed used, it is very common33 to drill at 3,000 rpm which corresponds to a cutting speed of 53 m/min. Some researches indicate that drill feed should be no greater than 0.1 mm/rev to avoid delamination in the drilling of composite laminates.26 27 32 34 So in this experiment, drill feed of 0.025 mm/rev and 0.05 mm/rev were used. 2.3. Ultrasonic C-Scan Inspection
d/2 →d N = 2N · tan −6 dB → d = 1.22 mm (2)
tan −6 dB =
where is the wavelength of the ultrasound beam and D is the diameter of the transducer. An approximate diameter of 10 mm was used for these calculations. The material was scanned in a single axis at 0.1 mm/sample, whereas in the phased array axis the maximum possible resolution was fixed by the OmniScan at 0.3 mm/sample. In this way, a 20 × 15 mm2 area was scanned obtaining 200 × 50 A-Scans. J. Biobased Mater. Bioenergy 5, 1–8, 2011
2.4. X-ray CT Inspection The X-ray CT inspections were performed with a Metrotom 1500 CT machine. The X-ray tube voltage and intensity were of V = 115 kV and I = 133 A respectively and no copper filter was used. With these parameters, the samples were fully penetrated by the X-ray radiation in
Fig. 2. Back Reflection Intensity (BRI) variation around the drilled hole.
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Ultrasonic data has already been proposed for drillinginduced delamination detection and analysis.20 26 In the present work a novel strategy based on a pulse-echo approach was proposed which enabled determination of the delamination factor around the drilled holes. A portable sound encoder OmniScan® MXU-M and a two-axes encoded scanner were used to perform ultrasonic measurements with a broadband phased array probe of 64 elements (10 × 06 mm each) and a centre frequency of 5 MHz. A gel was applied to the composite and the measurements were performed at normal incidence in pulseecho mode. In order to increase the ultrasound beam energy and decrease its angle of divergence, 16 elements were used to generate each single beam and the beam was electronically focused on the material surface (N = 20 mm). The equivalent area of the transducer with 16 elements was 10 × 96 mm2 , the approximate spot size of the beam (d) and the half-beam spread angle ( = −6 dB) were calculated using the following experimental relationships: sin −6 dB = 0514 → −6 dB = 1753 (1) D
The size and form of drilling-induced delaminations are not usually large and suitable enough as to generate a clear reflection of an ultrasound beam propagating through the material. However, the delamination damage does affect the propagation of the acoustic beam, decreasing the beam intensity by increased absorption and scattering. This effect can clearly be noticed in the change of the Back Reflection Intensity (BRI) which is commonly used to characterize the overall damage along the whole thickness of the material. Characterization of delamination at a hole periphery faces another challenge. Owing to both the spot size of the beam in the material and its divergence, apart from the delamination related intensity loss, the ultrasound beam intensity may decrease due to hole wall reflections. A threshold value in the BRI must be defined to compensate the actual loss due to the hole wall and exclusively measure the delamination damaged area. In a theoretical delamination-free hole, in the boundary of the hole approximately the half of the beam would get into the material as can be seen in the Figure 2(a). Thus, the BRI would suffer a decrease of 50% compared to the intensity of a hole-free point. As the centre of the beam moves away from the hole, the BRI would increase until all the beam gets into the material and reflects (Fig. 2(b)). Thus, a threshold of 50% of the maximum BRI value was defined as a criteria for delamination detection, considering delamination every point with a BRI lower than the threshold value.
Experimental Analysis of Drilling Damage in Biocomposite Laminates Manufactured by RTM
all the projections, obtaining good contrast images at an acceptable integration time of 2 seconds. Owing to the low density of the new biocomposite, no beam hardening artifacts were obtained in the reconstruction. Each hole was measured in a single scan obtaining a voxel size after the reconstruction of Vx = 1352 m.
(a)
2.5. Hole Quality
1mm
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After drilling, the final diameters were measured by means of a traditional hole micrometer. Surface roughness measurements were conducted in a portable tester (Mitutoyo Surftest SJ-301) according to ISO 4287-1:1997. The Ra parameter was obtained under a measuring speed of 0.5 mm/s along the hole thickness and the cut-off was 0.8 mm. The optical micrographs were acquired by means of a Olympus SZX12 Microscope with a magnification from ×7 to ×100. The more significantly damaged holes were cut in half to visualize the cut surface morphology by means of Scanning Electron Microscopy. The SEM equipment used in this work was a Carl Zeiss AG – EVO® 40 and the samples were sputter coated with a thin layer of palladium. The semi-holes were inspected by SEM in order to verify delamination, fiber cracks or other defects, which were previously detected by means of Ultrasonic C-scan and X-ray Computed Tomography.
3. RESULTS AND DISCUSSION 3.1. Ultrasonic C-Scans Taking into account the 50% threshold value, the ultrasonic C-scan color images related to the BRI amplitude were converted into binary delamination images; white (delamination), black (no delamination). The resolution of the C-scan images was limited by the phased array axis maximum resolution of 0.3 mm/sample. In order to obtain better resolution images, two perpendicular scans were performed for each hole. By arranging the signals, an approximate resolution of 0.1 mm/sample in both axes was obtained. With the above presented strategy, a 20 × 15 mm binary delamination image (200 × 150 pixels) of a drilled hole was obtained in 7 seconds. In Figure 3, three binary delamination example images are presented. Based on the binary delamination images, the delamination factor (Fd for each hole can be calculated. The delamination factor (Fd is widely used as an index to evaluate the extent of damage and is defined as the ratio of the maximum diameter (Dmax of the delamination zone to the hole diameter (DHole and can be expressed as follows:27 Fd = Dmax /DHole 4
(3)
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(b) Dmax Dhole
1mm
(c)
1mm
Fig. 3. C-scan delamination image results at 0.025 mm/rev: (a) Reamer. (b) Dowel-bit. (c) Drill-end cutter.
3.2. X-ray Computed Tomography In order to further analyze the delamination damage and check the ultrasonic C-scan results, X-ray CT measurements of the holes were performed. A 3D image obtained from a X-ray CT is composed by small 3D cubes called voxels (equivalent to pixels in 2D images). Each voxel has an specific value which is related to the mean X-ray attenuation of the material at that cube volume. This way, materials with different X-ray attenuation values can be visualized and localized in 3D. In the hole samples scanned in the present work, as expected, three materials were differentiated in the 3D reconstructed images; air, jute fibers and bioepoxy resin. By defining an attenuation value threshold, the sample can be separated from air and visualized in 3D (as shown in Fig. 4(a)). Furthermore, in the surface-parallel 2D crosssections resin and fibers are also differentiated by different gray values. In the Figure 4 the surface-parallel crosssections with the maximum “air diameter” are shown. The required measurement time for each hole scan was of 56 minutes. For a comparative analysis of both the ultrasonic C-scan and X-ray CT results, the delamination factor Fd was also calculated based on the X-ray CT results. To this end, all the surface-parallel cross-section images of each hole were observed and the maximum “air” diameter was selected to calculate the Fd . 3.3. Delamination Results With the above presented Ultrasonics and X-ray CT techniques, the drilling-induced delamination factor of each J. Biobased Mater. Bioenergy 5, 1–8, 2011
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Experimental Analysis of Drilling Damage in Biocomposite Laminates Manufactured by RTM
(a)
(b)
(c)
(d)
Fig. 4. X-ray CT results: (a) 3D image, (b) maximum diameter image, Reamer at 0.025 mm/rev, (c) maximum diameter image, Dowel bit at 0.025 mm/rev, (d) maximum diameter image, Drill-end cutter at 0.025 mm/rev.
related to the cutting conditions inefficiencies than to a real delamination between two layers (Fig. 5(b)). It can be concluded that the delamination damage measurements obtained by both inspection techniques were consistent, demonstrating the reliability of the ultrasonic C-scan approach. On the other hand, both techniques have shown that no significant delamination was induced during the drilling tests performed on the biocomposites. Similar studies done on conventional composites such as carbon or glass fiber reinforced composites27 29 shown higher delamination factor (Fd > 13). 3.4. Hole Quality Table II shows hole diameters (DHole after drilling compared with tool diameters (Dtool = 6 mm). As it can be
Table I. Diameters and delamination factor obtained after drilling by means of Ultrasonics and X-ray Computed Tomography. Reamer Feed (mm/rev) DHole (mm)
0.025 6.17 ± 0.02
Dowel bit 0.05 6.27 ± 0.02
0.025 6.06 ± 0.01
Drill-end cutter 0.05 6.06 ± 0.01
0.025 5.87 ± 0.02
0.05 5.87 ± 0.02
6.11 ± 0.02 1.01 ± 0.01
6.11 ± 0.01 1.04 ± 0.01
5.96 ± 0.02 1.02 ± 0.01
6.18 ± 0.01 1.02 ± 0.01
6.05 ± 0.01 1.03 ± 0.01
5.93 ± 0.02 1.01 ± 0.02
Ultrasonic C-scan Dmax (mm) Fd
6.29 ± 0.03 1.02 ± 0.02
6.40 ± 0.01 1.02 ± 0.01
6.11 ± 0.02 1.01 ± 0.01 X-ray CT
Dmax Fd
6.42 ± 0.02 1.04 ± 0.02
6.46 ± 0.01 1.03 ± 0.02
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6.18 ± 0.01 1.02 ± 0.01
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drilling tool at each cutting speed has been measured. Table I shows the mean values and the standard deviations for five holes of the DHole , Dmax and Fd . Concerning the ultrasonic results, the measured delamination factor for all tools and conditions was Fd ∼ 1. Taking into account the resolution of the C-scan images and the intrinsic lateral resolution limitations of the ultrasonic technique due to the beams divergence, it can be concluded that actually no significant delamination was detected. In the X-ray CT results it can be seen that again a very limited delamination damage was observed. The surfaceparallel and surface-perpendicular cross-sections of the Reamer, which is the tool that resulted in the highest delamination, are shown in the Figure 5, where the measured maximum “air” diameter can be observed. It can be observed that the detected damage pattern was more
Experimental Analysis of Drilling Damage in Biocomposite Laminates Manufactured by RTM
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(c)
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Fig. 5. (a) Surface-parallel and (b) surface-perpendicular cross-sections of the Reamer at 0.025 mm/rev.
observed, the best dimensional accuracy was obtained using the Dowel bit whatever the applied feed rate. However, a clear increase in the diameters (2.5–4.5%) could be observed using the Reamer at both low and high drill feed. The Drill-end cutter obtained hole diameters smaller than the nominal diameter of the tool. This fact could be attributed to a measurement mistake due to the uncut fiber which remained in the hole. The surface roughness values (Ra along the hole thickness can also be seen in Table II. According to the resulting diameters above, the best values of roughness are found at low drill feed. The highest surface quality was obtained using the specific wood drill (Dowel bit), whilst the worst roughness was observed using the Reamer at high drill feed. The results can be explained by considering the geometry and the cutting edges of the tool. The jute fiber is brittle and its bundles are thicker than synthetics11 therefore removing the chip is key to obtain good roughness. The Dowel bit has a central point and two raised spurs that help keep the bit drilling straight. Besides, the sharp lips cut the material and the spirals along the length remove the chip from the hole and keep the cutting area clean. In contrast, both the Reamer and the Drill-end cutter have not got a parabolic flute design to evacuate a large volume of chips. Consequently, as can be observed in Table II, both tools exhibit high values of roughness (Ra , especially at high feed, due to the difficulty to remove chip, hindering a proper tool cutting.
Fig. 6. Different hole entries at 3,000 rpm and 0.025 mm/rev (×20): (a) Reamer. (b) Dowel bit. (c) Drill-end cutter.
3.5. Optical Microscopy and SEM Low magnification microscopy can be employed to identify surface defects.31 Moreover, a scanning electron microscope can be used to observe the cut surface morphology and to identify delamination.19 The hole entry after machining with different drills at 3,000 rpm and 0.025 mm/rev are presented in Figure 6. The Reamer (Fig. 6(a)) left jute fibers uncut all around the hole, that is to say, its cutting lips are not able to cut this natural fiber properly. In contrast, the Dowel bit (Fig. 6(b)) was capable of cutting though the jute resulting in a good visual appearance. Finally, after drilling with the Drill-end cutter, some burrs containing bioepoxy resin and broken fibers were observed. Working at 3,000 rpm and 0.05 mm/rev gave similar visual results. SEM images at 100× reveal different surface textures depending on drill conditions. Figure 7(a) shows hole exit surface after machining with the Dowel bit at low drill feed whereas Figure 7(b) shows the result at higher feed rate. Although no significant irregularity exists, in Figure 7(b) fiber pull-out was observed. However, Figure 7(a) shows that at a lower feed rate, the damage was less severe and more controllable thus producing a smoother surface. On the other hand, internal separation of adjacent plies, that is to say, push-down delamination was not observed. The hole exit drilled by the Reamer at 3,000 rpm and 0.025 mm/rev (Fig. 8(a)) presented accumulation of broken fiber, pull-outs and bioepoxy resin cracking. Instead
Table II. Hole diameters and roughness obtained after drilling at 3,000 rpm. Type of drill Feed (mm/rev) DHole (mm) Ra (m)
6
Reamer
Dowel bit
Drill-end cutter
0.025
0.05
0.025
0.05
0.025
0.05
6.17 ± 0.02 8.18 ± 0.85
6.27 ± 0.02 12.54 ± 1.03
6.06 ± 0.01 5.17 ± 0.49
6.06 ± 0.01 6.63 ± 0.75
5.87 ± 0.02 7.45 ± 0.69
5.87 ± 0.02 9.45 ± 0.89
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(a)
Hole exit
Hole exit
Fiber pull -out
Broken fiber
(b)
Hole exit
Hole exit
Fiber pull-out
Fiber pull -out
Broken
Fig. 8. Hole exits after drilling at 3,000 rpm and 0.025 mm/rev with (a) Reamer. (b) Drill-end Cutter.
of cutting the jute fiber, the Reamer seems to extrude and push-down material. However, the interlaminar adhesion does not seem to have been compromised. The Drill-end cutter shows a smoother surface than the Reamer but broken fibers and pull out material were also observed at the hole exit (Fig. 8(b)). Although the cutting geometry of the tool was specially designed for machining composites, it does not properly cut the jute fibers and furthermore, it does not provide a way for the chip to clear the cutting zone. As a consequence, a bad surface finishing was also observed but delamination was not remarkable. SEM images after drilling at 0.05 mm/rev are not presented since they did not show any improvements whatever tool had been used. The plies could not resist the thrust of the drill bits and started to delaminate. The cutting lips of the different drills pushe down and extruded the composite layers rather than cutting them. Push-down delamination was the most extensive phenomenon and is consequently considered to be the most dangerous in fiber reinforced polymers.22–28 It can be underlined that this phenomenon is only observed in the last ply of the biocomposite laminates at high feed rate. The Dowel bit, 3,000 rpm and 0.025 mm/rev seem to be the best conditions for drilling jute reinforced laminates.
4. CONCLUSIONS
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An experimental analysis for surface texture and delamination caused by drilling jute fiber reinforcement bioepoxy resin was developed in the present study. The physicochemical properties of the new biocomposite affected their machinability in different ways to than in metals or fiber reinforced thermosets. The results are summarized as follows: • The cutting lips of the Dowel bit, a standard wood drill, seem to be the most suitable to avoid delamination of the back face of the plate. Both jute fiber and bioepoxy resin matrix were properly machined and delamination was not observed. • A spindle speed of 3,000 rpm and feed rate of 0.025 mm/rev, were found to have been appropriate drilling parameters. • Ultrasonics was a valid non-destructive inspection technique to detect delamination in drilling jute reinforced biocomposites. • X-ray CT, despite its limited applicability due to its reduced sample size requirements, was an interesting inspection technique that provides reliable reference results. 7
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Fig. 7. Hole exit surface after drilling with the Dowel bit at: (a) 0.025 mm/rev, (b) 0.05 mm/rev.
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• Push-down delamination is mainly affected by machining at high cutting speed and using drill bits without flutes to remove the chip. • The detected delamination damage at high cutting speed was not so remarkable as expected, so this new composite could certainly increase production rate without loss of functionality. Finally, it could be interesting to comment that in practice manufactures drill directly through the composite into the metal. Placing a metal sheet beneath the biocomposite laminate will act as a support and will prevent any delamination in joining dissimilar materials.
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Received: xx Xxxx xxxx. Revised/Accepted: xx Xxxx xxxx.
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